The Institution of

Founded 1908 Incorporated by Royal Charter 1934

From the PresidentP L Campbell |1> D i e C E n n F lS l ru c tE KICK F IM arE F IH T FIDE FASCF. MConsK

The Institution of S tructu ral Engineers 11 Upper Belgrave S treet London SW lX BBH

01-235 4535

Fax: 01-235 4294

PLC/CC

18 April 1989

e Rt Hon Douglas Hurd MP The House of Commons LONDON SW1A.0AA

Dear Home Secretary

THE HILLSBOROUGH PUBLIC ENQUIRY

I wish to convey my deep concern about the tragedy that occurred in Sheffield last weekend, and express my support for the Public Enquiry you have initiated.

There will, I am sure, be a peed for an input by a structural engineer or engineers, and I write offering my services either personally, or as the President able to nominate distinguished members with the special expertise that will be required.

Please le t me know if I can be of service to you and Lord Justice Taylor.

Yours sincerely

PETER CAMPBELL

THE INSTITUTION OF STRUCTURAL ENGINEERS

Appraisal of

existing structures

THE INSTITUTION OF STRUCTURAL ENGINEERS

Appraisal of

existing structures

JULY 1980

The Institution of Structural Engineers11 UPPER BELGRAVE STREET, LONDON SW1X 8BH

Constitution of the ad hoc Committee

Professor E. Happold, BSc, CEng, FIStructE, FICE, FIOB, ChairmanMr. A.P. Backler, BSc(Eng), DLC, CEng, MICEMr. J.A. Baird, CEng, MIStructEMr. P.R. Bartle, CEng, FIStructEMr. P. Beckmann, CEng, FIStructE, MICEMr. G.A. Bettany, MSc, CEng, MIStructE, FRICSDr. J.L. Clarke, MA, CEng, MICEDr. M.S.G. Cullimore, BSc, CEng, FIStructEMr. W.G. Curtin, MEng, CEng, FIStructE, FICEMr. D.L. EckettMr. K.W. Gibson, BSc, CEng, MIStructE, MICEMr. J.H.R. Haswell, BSc, CEng, FIStructE, FICEMr. R.A. Heaton, CEng, FIStructE, FICEMr. P.K. Jaitly, BSc, MA, LLB, CEng, MIStructEDr. J.B. Menzies, BSc(Eng), CEng, FIStructEMr. F. Myerscough, CEng, MIStructEMr. A.L. Randall, CEng, FIStructEMr. W.H. Sharp, CEng, FIStructEMr. A. Stevens, CEng, FIStructE, MICEMr. R.J.M. Sutherland, BA, CEng, FIStructE, FICE, FIHEMr. C.J.K. Williams, MA

Miss Margaret Law, BSc, FIFireE prepared the draft for Appendix 5 (Fire).

Ex officio during Presidential year:1976-771977-781978-791979-80

Dr. W. Eastwood, BEng, FEng, FIStructE, FICEMr. Peter Dunican, CBE, FEng, FIStructE, FICEProfessor Sir Alan Harris, CBE, BSc(Eng), FEng, FIStructE, FICEMr. J.A. Derrington, BSc(Eng), DIC, FEng, FIStructE, FICE, FCIArb

® 1980: The Institution of Structural Engineers

This publication is copyright under the Berne Convention and the International Copyright Convention.All rights reserved. Apart from any copying under the UK Copyright Act 1956, part 1, section 7, whereby a single copy of an article may be supplied, under certain conditions, for the purposes of research or private study, by a library of a class prescribed by the U K Board of Trade Regulations (Statutory Instruments, 1957, no. 868), no part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without prior permission of the copyright owners. Permission is not, however, required to copy extracts on conditions that a full reference to the source is shown. M ultiple copying o f the contents o f the publication without perm ission contravenes the aforementioned Act.

Contents

FOREWORD 5

1 INTRODUCTION 7

2 PROCEDURES 82.1 Terms of reference 82 .2 P o in ts to be es tab lish ed 82.3 Responsibilities 82.4 Reporting 9

2.4.1. General considerations 92.4.2 Principal parts o f the report 10

3 PROCESS OF APPRAISAL 113.1 General H3.2 The path of assessment: the flow charts 13

3.2.1 Introduction 133.2.2 Gathering o f information 143.2.3 The initial assessment 143.2.4 Improving the assumptions 163.2.5 Further stages o f assessment 163.2.6 Load testing 21

4 METHODS 234.1 Survey 23

4.1.1 Introduction 234.1.2 Documents 234.1.3 Dimensions 234.1.4 Loadings and environm ent 244.1.5 Condition 26

4.2 Testing 274.2.1 Determination o f testing requirements 274.2.2 Materials testing 284.2.3 Load testing 28

APPENDIXES1 Sources of information on design, construction and history 352 Survey report 363 Survey of condition: observations record sheet 374 Types of defect 385 Fire 506 Methods of test 54

Reprinted December 1982

Foreword

In 1742 Pope Benedict XTV, concerned with the state of the dome of St. Peters, requested three men, Le Seur, Jacquier and Boscowich to carry out a structural survey to determine the causes of distress and to devise remedial measures. The report, published the following year, was prefaced by an apology that said they had assessed it with theoretical mathematical reflection only because the building was so unique. Then followed a detailed survey of the dimensions and a discussion on possible explanations for the damage and named the yielding of the tie rings at the circumference as the cause. But the interesting part of this report was the second part because an attempt was made to calculate the horizontal thrust and to prove that the two tie rings built in at the time of erection were no longer able to carry this thrust.

The report caused a furore. One comment at the time stated: ‘If it were possible to design and build St. Peter’s dome without mathematics and especially without the new fangled mathematics of our time, it will also be possible to restore it without the aid of mathematicians and mathematics... Michelangelo knew no m ath em atics and yet was able to build the dome... Heaven forbid that the calculation is correct. For, in that case, not a minute would have passed before the entire structure would have collapsed.’ Certainly the analysis contained some errors. But in spite of disagreements as to the causes of the damage most people were agreed on the measures to be taken, and in 1743 five additional rings were built in the cupola.

The importance of this event was that, contrary to tradition, the stability of a structure had not been based on empirical rules and opinion but on a detailed survey and mathematical analysis.

Today we are even more interested in developing the art of structural appraisal. We have a large stock of structures and buildings representing successive deposits of human imagination, which we are reluctant to discard for emotional or hard economic reasons. Urban renewal is a rapidly expanding exercise.

The art of appraisal of structures is different from design. In design the forces follow the choice of form and the analysis follows that. In appraisal the engineer is left face to face with an existing structure of definable qualities and must determine its condition and suitability of use. This is not an easy task. In defining the structures qualities the engineer may gain from the experience of other engineer’s methods, available testing procedures and current develop­ments in analytical techniques, and this report hopes to assist him.

5

The group who wrote the report are not the fount of all current wisdom. The report will hopefully be amended and improved, and it is our wish that engineers who read it will comment, draw the Institution’s attention to omissions and add to the useful references.

The Chairman thanks the members of the committee and those other engineers who commented on this report.

The committee enjoyed and learnt from their meeting, and we all are grateful for the experience. But our respect and thanks go most of all to Mr. R.J.W. Milne, Assistant Secretary (Technical), whose constant attendance and help gave much to us all.

E. HAPPOLDChairman

6

1 Introduction

Checking the adequacy of an existing structure sometimes becomes necessary as a result of:

• defects in design and construction• deterioration with time or in service• accidental damage or collapse• for purchase, insurance, or legal purposes• change of use• future safety or serviceability.

Recent experience in the appraisal of buildings has led to developments in methods of assessing the strength and future serviceability of existing construction. This has required consideration of the levels o f safety appropriate to the future use of the construction; the assessm ent of loading; the evolution of methods for determining the strength of a structure, its components and constituent materials; and the derivation of suitable methods for calculating structural behaviour.

The aim of this report to consolidate this experience by providing general guidance for engineers.

This report is particularly concerned with the structural appraisal o f buildings, but it is also relevant to the appraisal o f other constructions such as bridges, masts, chimneys, cooling towers, cranes and gantries, docks and harbour works, underground structures, retaining structures, pipelines and the like. Structures being appraised exist physically as opposed to being designed, and therefore much of the uncertainty present at the design stage is absent. This greater certainty can be taken intoaccountinthe appraisal.

O n the basis o f the Institution report Criteria for structural adequacy o f buildings1 1. Criteria for structural adequacy of, , , . j « , buildings, Institution of structural

S tr u c tu r a l sdC (JU 2C y C3H DC e x a m in e d u n a e r . Engineers, London, 1976

• overall stability• strength• geometric permanence (freedom from creep and other long-term

deformations)• stiffness• dynamic response• fire resistance• durability• impermeability• appearance

There is no absolute measure of adequate safety and even less of serviceability. There does, however, exist a generally accepted level of safety provided by design and construction in accordance with current regulations and codes of practice. This level of safety should be used as a datum, but when assessing existing structures, engineering judgment should take precedence over compliance with the detailed clauses of codes.

The requirements of serviceability should, for existing structures, be stipulated by the user in consultation with the engineer.

7

2 Procedure

TERMS OF REFERENCEThe first stage is the receipt in writing of a brief from the client in which he states his requirements, dates for reporting and confidentiality. It is essential to find out at the start exactly what the client needs, how detailed the appraisal is to be and to what use the results are to be put. A n appraising engineer must recognize that he may be called in by someone who does not understand or define his problem. Thus it may fall to the engineer to develop his own brief, together with the client.

The client will probably be in one of the following categories, each of which may require a different approach to the appraisal and a different type of report:

• government department• national corporation• local authority• industrialist• property company• private owner• corporate or private tenant• architect, consulting engineer or other designer• contractor or supplier• civic group• insurance company

POINTS TO BE ESTABLISHEDThe engineer will need:

• to clarify the brief and its implications with the client and to state these in writing with any qualifications. This should be done before accepting the commission or as soon after as is practicable. The client must know clearly what is being done and on what he will get a report

• to define the line of responsibility especially when making an appraisal jointly or on behalf o f more than one party

• to isolate and state the factors that are central to the issue as soon as possible and to let the client know immediately when any new problems or factors are found in the appraisal, giving, if appropriate, an interim report and an updating of the brief.

• to state in general terms how he will proceed with the appraisal and what he will aim to establish, and to work within these terms

• to agree the extent o f the appraisal and to give the client an initial estimate of its cost

• to advise on whether other experts or testing houses are to be employed and, if so, how they are to be paid

• to establish the time available• to keep the client fully informed of all continuing financial implications and

to get approval of any special expenditure (tests, scaffolding, etc.)• to advise the client on the need to seek legal advice• to reach definite conclusions and to state these clearly in a written report;

recommendations should be included in the report only if it has first been agreed with the client that these are required.

RESPONSIBILITIESN o engineer can avoid responsibility for his actions. However, if the appraising engineer states his opinions and advice clearly, gives the bases for these and can be seen to have acted wholly objectively and prudently, he should have no need to fear legal action against him even if his opinions are later proved wrong or his advice unsound.

The engineer should:• decide whether the brief is morally and professionally acceptable; decide

whether to accept it, reject it or persuade the client to alter it

• remain objective at all times• be aware o f his responsibility for public safety and decide whether to report

factors or findings not covered by the original brief

• respect the reputations and feelings of others involved; in particular to avoid defamation by irrelevant or derogatory statements even if this is apparently in the client’s interests

• avoid playing too safe in the appraisal or giving undue thought to personal responsibilities rather than the client’s best interest

A note2 by Lord Justice G raham on duty of care and professional responsibility states:

‘I t is not possible nor appropriate. . . to define duty of care and professional responsibility in any detail. A part from cases where there can be shown to be breach of contract, this is a m atter largely based on the Common Law o f negligence, which is being developed from day to day by the Courts. The following are some general principles of the law, as it stands at present, which may help to clarify the position in regard to negligence.

Duty o f care

The professional person has a duty, ju st as much as anyone else, in the particular circumstances to exercise reasonable care to avoid acts and omissions which can be reasonably foreseen to be likely to cause physical or financial injury to persons or property. Negligence is failure to exercise that care.

The practice of a profession, art or calling which, by its nature, demands some special skill, ability or experience, carries with it a duty to exercise, to a reasonable extent, the amount of skill, ability and experience which that practice demands. If a person professing to practise such a profession, art or calling fails to possess that amount of skill, ability and experience which is usual in that profession, art or calling or if he neglects to use the skill, ability and experience which he possesses or which is demanded or professed, he will be liable for breach of duty. This duty is owed not only to those parties with whom he has a contractual arrangement, e.g. by whom he has been employed or consulted, but also to all persons who are so closely or directly affected by the negligent act that they ought reasonably to have been foreseen as likely to suffer from that act.

D uty of care thus arises where there is such proximity between two persons that the negligence of one is likely to affect the other injuriously.

Liability for damages

Liability attaches to negligence not only where it is the sole effective cause but where it is also a contributory cause of an injury.

Negligence of a professional person thus gives rise to liability under Common Law in tort, apart from the more obvious liability for breach of contract. W here an employee is negligent, both he and his employer will normally be liable. A person who has been sued for negligence, although liable himself, may in some circumstances, be able to pass on his liabilty to someone else (e.g. an independent contractor or a professional man who is actually.responsible for the particular negligent act).

Jn all cases, it is for the courts to decide who, if anyone, is liable. The position in any given case will depend on the particular facts and circumstances of that case, and the taking of legal advice should be considered as soon as any serious question of liability seems likely to arise . . .

Further information can be found in H alsbury’s Laws o f England: Negligence (Vol. 28), 3rd Edition, and 1977 Supplement.’

2.4 REPORTING

2.4.1 General considerationsThe writing of the report requires much careful thought, review and redrafting. The meaning of each sentence must be examined critically. The report will often be used by non-technical readers and lawyers, and it must therefore be simple and clear. A t the same time it must be technically accurate and precise, and proper weight must be given to interpretation. Symbols and abbreviations must be generally understod or be given precise meaning to avoid confusion.

The report should be logical, have continuity and be easy to follow. The contents must be relevant to the brief, nothing of importance should be left out or any unnecessary materials included. The report must be based on factual data and should be objective in approach. Any reservations or limitations implicit in the method, technique employed or interpretations of results should be clearly stated.

2. D uty o f care and professional respon­sibility, S tructura l Engineer, 57A , no. 5, M ay 1979, p.168

9

Conclusions should be explicit and the use of vague generalities avoided. W here either there is insufficient information or the results of the survey and/or tests are inconclusive, this should be clearly stated and the conclusions qualified accordingly. Any recommendations should include the appropriate engineering solutions clearly defining disadvantages and advantages. The engineer should indicate his preferred solution.

The client must be made aware of any risk to public safety.

2.4.2 Principal parts of the report1. Title

This should normally read:

A structural appraisal o f . . . ’

• describe whether building or structure, type and use• give location (address)• say for whom prepared (name of the client)• state by whom (engineer and/or the firm).

It should be dated and may also have a serial or other identification number. It may also state the status o f the report, e.g. whether confidential, interim, etc.

2. Synopsis

This should contain a brief summary of the significant information presented in the report (e.g. the reason for the appraisal), stating the problem, and the investigations that were carried out, including significant features and principal conclusions and recommendations. The synopsis should be written after the completion of the report and must not contain anything not included in the main body of the report.

The historical background, description of routine procedures and discussion of results is not normally given in this part.

3. List o f contentsThis should be given if required

4. Introduction

This part should introduce the history and subject-matter of the report, the original brief and later amendments, and the scope and limits o f the work. The limits may arise from:

• the need for a quick decision• limits on the cost of the appraisal• the client’s restrictions regarding the type and extent of tests that can be

made, e.g. minimal damage to finishes etc.• restricted access within the building, especially if it is in use• difficulty in obtaining information regarding the construction

5. The body o f the report

This should quantify the problem and state what was done, how it was done and what were the results that form the basis for the subsequent conclusions and recommendations.

This part may include some of the following:

5 .1 The background and history of the structure should give information regarding:

• the property, its site and location• the age of the property• past and present use• any previous structural alterations• the availability of drawings• the availability o f calculations• specialist or test reports.

5.2 D etails o f inspections carried out, including names, and datesshould be reported. These should include visual inspections, physical surveys and more detailed investigations.

W here materials and/or load testing had been carried out, the details should be given. A brief description of the methods used should be included, the reason for their choice highlighted, and any limitations clearly stated.

5.3 the assessment of the investigations should now be described. All

10

results and their bases should be given in full. I f descriptions or calculations are lengthy and laborious they should be given in a appendix and a summ ary only given in this part.

6. Conclusions These should be firm reasoned judgm ents reached after careful consideration of the information assessed. I t is prudent to discuss briefly the accuracy of the methods employed and the true significance of the findings. Every conclusion should be based on the information contained in the body of the report.

Consideration of the available information should lead to one of the following conclusions:• the structure is adequate for current use for its normal life provided that

it is maintained properly• the structure, although adequate at present, may not remain so in future• the structure is inadequate for the current use but may or may not be

adequate for alternative uses• the structure is inadequate and needs remedial measures• the structure is unsafe and beyond normal repair• the information is not sufficient to reach a definite conclusion.

7. Recommendations These should be in harmony with the rest o f the report and firmly based on the conclusions. They may include proposals for remedial work, regular maintenance inspections or inspections to detect further deterioration.

3 Process of appraisal

3.1 GENERALStructural appraisal is a different activity to structural design. It is aimed at assessing the real condition of an existing structure.

The adequacy of a structure is assessed by the exercise of engineering judgm ent on information obtained from the study of drawings and calculations, the results of surveys, inspections and possibly testing. Each of these activities should be taken no further than is necessary for a definite for a definite conclusion to be reached.

The methods used for assessm ent are largely those used for the justification of a design. The starting points of calculations differ as does the form in which the results are presented, but the basic theoiy is the same.

Codes of practice are intended for use with present-day materials and construction methods and therefore may contain implicit or explicit assumptions that are not valid for the structure under consideration. It may therefore occasionally be necessary to take a code formula and work back using first principles to find the assumptions made in its derivation. Even if a structure is constructed using present-day methods, certain code assumptions may not be valid. Codes must, for practical reasons, be limited in length and complexity and cannot allow for the infinite number of possible variations in structural layout.

It is sometimes relevant to make use of out-of-date codes of practice that were current at the time of construction, but if this is done it should be remembered that an old code is more likely to contain information that is now considered inappropriate. The combined use of old and new codes should be avoided unless absolutely necessary. I f they are combined special care should be taken with safety factors and the way that they are used in calculations.

Certain phenomena require extra care in the formulation of the assumptions for their calculation and in the interpretation of the results, for instance if the mode of failure of a structure or member is likely to be sudden (e.g. failure of cast iron beams or over­reinforced concrete beams). Similarly, the collapse of a member that could lead to loss of life should be treated with an entirely different level of concern than structural distress merely affecting serviceability.

Serviceability is very much ‘in the eye of the user’. Calculating deflections and comparing calculated values with code recommendations is rarely of much value when appraising an existing structure, especially as deflection calculations are notoriously unreliable.

For an existing structure with unchanged use it is the magnitude of the existing deflections that is im portant and whether they are likely to increase. F or a change of use an estimate should be made of the deflections to be expected, and the effect on the intended use should be assessed. Care should be taken to distinguish between deflections caused by permanent load and those arising from fluctuations in imposed load. Similarly, the effects of creep deflections on finishes should be considered.

The importance of assessing loads adequately is vital especially if there is risk of overloading arising from change in use.

The total number of manhours available for assessm ent will generally be limited, and it is important that the time should be spent as effectively as possible. It is easy to get so involved with the details of a particular calculation that other equally important calculations are given only a small amount of time or completely forgotton For example, a considerable amount of time and effort might be put into a computer analysis o f a steel-framed building, and the joints then analysed using a highly simplified method. The time would have been far better spent o b tain ing references to establish how such joints really behave and then checking the joints using forces and

12

oments from a simplified but reasonably accurate analysis of the building as a whole. Tt mav therefore be useful to set out a list of the calculations as envisaged and check the refinement o f each operation against that of the others before carrying out any

numerical work.

W hen failures are investigated they are usually found to be caused by combinations of several factors. It is rare for failures to be attributable to shortcomings that would be c h n w n u d bv a conventional design calculation. This should be borne in mind when Pan n in g the calculation checks.

THE PATH OF ASSESSMENT: THE FLOW CHARTS

IntroductionThe process of appraisal is cyclical as shown in the flow charts (see Figs. 1 and 2). Information is collected and assessed. If the result shows that the structure is adequate the process can stop there. If inconclusive more information can be collected, assessed more thoroughly and so on. The action required should be taken in stages, each stage depending on the findings of the previous one.

The stages can be as given in clauses 3.2.2 to 3.2.6 below.

Define the structure

1. Process of appraisal

t s it a mechanism a s defined?

Canthe s t r u c tu r e be

redefined

Has it co l lapsed

A ssess forces on structure a s a whole and check stability

S horeCan forces \ or sa fe ty f a c to r s be

^ re co n s id e red[10]

I s stability adequate ? Investigate

other possible causes of collapse

Wtil removal* of

single element(s) endanger stability of

whole structure [3] p a r t s ?

Cancri t ical e lement(s) r es is t acc identa l

fo rce s ?

Protect or No I s t r en g th en

critical elements

A s s e s s ac tual toads on each element an d in situ s treng th (s) of materials

R e a s s e s s loads on element considered

R e a s s e s s s t r en g th (s) s i z e s a n d /o r as s u m p t io n s

Simple check calculation

Do in Situ c o n d i t i o n s just i fy a tt

a s s u m p t i o n s

Element isCheck sa t i s f ied ? adequate

Can e lement shed

load to others

Are real conditions a s bad a s

assum ed

[6]

deficiencydrastic?

Element is inadequate

Canother ^ No

elem ents ca rry their share

Will early yield of

e lement a f fec t stobitity of o ther p a r ts

rgi 'v . of s truc tu re

Recheck stability of s tructure a n d /o r p a r t s

Are pas t a s s u m p t io n s too

conservativeStabili ty still

ad e q u a te?

More ref ined a n a ly s is required More refined analysis may be helpful

2. R efinem ent o f process

/ I s \ '"mathematica l model t o o c r u d e , a s so far N . def ined ? /

Refine mathematical model reconsider assumptions. Make accura te ana lysis of redefined structure

Make accurate analysts ofelement (s) and sec t ions

calculated capac

adequate

No

Measure dead loads and m a te r ia ls s t ren g th s in situ

a t e rchecked

Areloads and/or

s t reng ths accurately known now

Reconsider safety factors w here ap p rop r ia te d e q u a te

Final ultimate ana lysis

I sca lculated

capacity (ies) ad equa te

Willyield of e lem e n t

ad e qu a te

S . a f f e c t stabili ty ot o t h e r s x ^\ ? y /

No \

rYes [ s X .

\ 0 . K . ?

ly es

R ejec t or t e s t loadelem ent

3.2.2 Gathering of informationInformation should be gathered about the design of the structure, its construction, histocy and present appearance. Documents containing this information may not still be in existence, but an attem pt should be made to locate and examine what is available. Suggestions for this are given in sub-section 4.1 and Appendix 1.

The structure should be visited to make a survey of its condition (see Appendixes 2 and 3). Precisely what is done will depend on the reason for the appraisal. W here purchase or change of use is being considered the dimensions of individual members, accuracy in construction and the type and quality of the materials may be relevant. It may also be necessary to determine what loadings the structure has been subjected to as well as the intended or probable future loading. This is partly determined visually or by asking people, although some people’s statements need to be treated with reservation. W ritten confirmation of verbal statements should be obtained. In the case of structural defects, damage or lack of serviceability it will also be necessary to determine the nature of these situations and the parts affected. The condition will first be assessed visually. Advice is given on this in Appendix 4.

3.2.3 The initial assessmentThe information to hand should now be studied and analysed. Checks should be made of the load-carrying capacity of the structure and the margins of safety by calculation using the available information on actual loads and on the size and strength of the materials and components. In particular, the inherent stability and the adequacy of the construction should be looked at. In these calculations it will be necessary to make assumptions about the distribution of loading and the strength of materials. Such assumptions should be conservative.

The first calculation will practically always be a simple conventional design calculation. This may lead to one of three possible conclusions:

14

[1] The calculations show that the structure has an adequate margin of safety according to the relevant code of practice.

If the engineer is satisfied that the Code recommendations are adequate for the likely use of the structure, and if the visual examination of the structure has not revealed any signs of distress, it should be necessary only to re­check the assumptions to guard against gross error before pronouncing the structure safe. The possibility of such modes of failure as fatigue or unseen corrosion should be considered. If the structure however does show signs of distress a more careful recheck of the survey and the calculation must be undertaken.

(ii) The calculations indicate that the structure is grossly overloaded to the extent that the calculated overall factor of safety is unity or less.If the structure nevertheless is earring its load without any signs of overstress and generally appears in good order the basis of the calculation must be examined for error.

If the structure has been seen to be badly cracked, grossly deflected or collapsed, the type of failure should be compared to that predicted by the calculations.

(iii) The calculations indicate a factor of safety greater than unity but less than that recommended by the codes and the structure shows little if any indication of overload.

In this case a revised calculation is called for using a refined mathematical model that takes account of diverging (alternative) load paths and secondary load-carrying mechanisms.

The flow charts (Figs. 1 and 2) indicate the general sequence that the process will follow and show the cyclic paths along which such assessments usually proceed. They will not apply in toto to all appraisals and may not be complete for certain situations. They are, however, considered to provide to provide useftil food for thought.

There will obviously be occasions when deviations from the sequence will be beneficial. F or example, when a member shows visible distress, a simple check on this member should be carried out first.

W eak point are as common at connections as in members. W here the flow charts read ‘element’, this should therefore be interpreted as ‘member and/or connections’.

[ 1 ] A mechanism is a system that, because of the disposition and number of members and/or the freedom of the joints to deform without increase in moment, is inherently unstable.

[2] A t this stage the check is for overall stability.

[3] This is equivalent to the check for Building Regulations D 17 and D 18 (‘the 5th Amendment’). These are legally applicable only to buildings of over four storeys for which building regulation approval was sought after the 5 th Amendment came into force in 1970.

[4] ‘Simple’ refers to the absence of assumptions and/or procedures beyond& those normally used in initial design. The ‘frame analysis’ may at first be

[5] no more than reasonable estimates of support moments, but a proper analysis may be necessary when ‘recycling’. ‘Check satisfied’ means that the calculation indicates (possibly by inference) that the recommend­ations of the relevant Code of Practice could be shown to be observed.

[2] may depend on the outcome of some of the calculations referred to under [4] and [5] and these latter are obviously repeated for each element in turn.

[6] A visit to the site should be made at this stage to confirm that the parameters used in the calculations are realistic.

[7 ] D rastic deficiency means that the calculated overall load factor is less than1.1 for dead load only.

[8] If, for example, the element under consideration provides lateral restraint on which a compression member (or just a compression flange) relies to prevent it buckling, then early yielding (say, as prerequisite for load sharing) could deprive the possibly more essential compression member of some of its lateral restraint.

[9] A n optional path for marginal elements (see (iii) above), working in parallel with others.

[10] See clause 3.2.5.

[11] Refinement of the mathematical model may involve the basic arrangement

of the structural system and its mode of behaviour as well as its geometric dimensions. It may also include reassessment of load paths and loading sharing. This may lead to several cycles of trial and error which for reasons of space and because they will be different for each structure have not been shown in the flow charts. It may also at this stage be worth checking whether the results of previous calculations are sensitive to changes in the assumptions of strength and stress/strain relations.

3.2.4 Improving the assumptionsSo far the assessment has been based on conventional design assumptions. I f the calculations indicate a moderate shortfall in loadbearing capacity it may be worth while to improve the accuracy of the assumptions by measuring the exact structural geometry, by measuring the actual thickness of and determining from samples the densities o f the materials that make up the dead loads and by carrying out tests to ascertain the strengths of the structural materials on site or in the laboratory.

3.2.5 Further stages of assessment

3.2.5.1 Introduction

W ith these more accurate data a second series of calculations can now be carried out.

The most common design calculations use very simplified (mostly 2-dimensional) models, and the mechanical properties of the materials are simulated by fairly coarse approximations. A s a consequence the secondary contributions to the load-carrying capacity of a member and the reductions in the loads acting on a perticular member or part of member, which arise from static indeterminacies, are ignored.

On the other hand, at the time of design the actual dimensions and material properties of the structure to be built are, to some degree, uncertain, and the calculations have to include an allowance for this. However once a structure has been built much of the uncertainty present at the design stage has been removed (e.g. more is known about the strength of the materials), and when the strength has been assessed itshould be possible to adjust the safety factors because more is known about the strength of the materials used and the condition of the construction. In relation to loads, for example, appraisal allows a more accurate estimate of dead load to be made, and hence the normally accepted level of real overall safety will be achieved with a lower nominal value for the factor of safety being used in the assessment calculation.

The use of factors different from those normally used in design should however be considered with care and resorted to only when the additional information is adequate to demonstrate that the resulting level o f safety is comparable to that which results from sound building practice.

W hen reconsidering the safety factors ([ 10] on the flow chart) it may be useful to adopt the now more common approach of partial factors for loads and materials. As an aid to relating the further appraisal calculations to the currently accepted safety datum (i.e. the Codes of Practice) the following considerations below may be used.

According to the partial safety factor format used in C P I 103 and in revisions to other structural codes the basic design equation is written:

. j cc . ^ structural resistanceYf x load effects < — -----------

7 f and ym between them cover the seven partial safety factors listed in ISO 23944.

3.2.5.2 The load factors, yfG EN ER A L

The yf -factors in C P1103 and similar codes are made up of three factors:

7 fj Load variation factor. This takes account of the possibility of unfavour­able deviation of the various loads from the values considered in deriving the characteristic loads.

7 f2 Load com bination and sensitivity factor. This takes account of the reduced probability that various loads acting together will all simultane­ously be at their characteristic value and also the increased safety margin which is required for load combinations in which the forces act in opposition.

7 f3 S tructural perform ance factor. This takes account of possible inaccurate assessment of the overall effects of loading, unforeseen stress redistribu­tion within the structure, variations in the dimensional accuracy achieved

3. C P 110: The structural use o f concrete, British Standards Institution, London, 1972

4. ISO 2394 : 1973, General principles fo r the verification o f the safety o f structures

16

in the structure in as far as they affect its response, and the importance of the lim it state being considered.

F or design of ordinary structures for strength (‘the ultimate limit state’) a value of yf} =1.2 is implied so that the design loads in C P110 clause 2.3.3.1 can be written approximately:

(i) Dead and imposed loads1.2(1.0 x 1.15G + 1.0 x 1.35Q) or 1.2(1.0 x 0.85G + 1.0 xO x Q)

i.e. for dead loads acting adversely Yf, — 1.15 } for combination offor dead loads acting beneficially Yf, = 0.85 } two loads Yf2 — 1.0

for imposed loads acting adversely Yf, = 1.35 ) for combinationfor imposed loads acting beneficially Yfi = 0 } of two loads Yf2 — 1.0

Yf, = 1.2

(ii) Dead loads and wind loads1.2(0.9 x 0.85G + 1.0 x 1.15W)

i e for dead loads acting beneficially Yf, = 0.85 } for m o loads combinedyfj =0.9 for dead load

for wind loads acting adversely yc = 1.15 } for two loads combined Yf2 =1.0 for wind load

rf, = 12(iii) Dead loads imposed loads and wind loads

1.2(0.9 x 1.15G + 0.75 x 1.35Q + 0.9 x 1.15W)

i e for dead loads acting adversely Yf, = 1.15, for three loads combined y f2 =0.9 for dead load

for imposed loads acting adversely Yf, = 1.35, for three loads combined Yf2 =0.75 for imposed load

for wind loads Yf, = 1.15. ** t<hree }” * c°rabined =0.9 for wind load

Vf, = 12

T H E LO A D V AR IA TIO N F A C T O R Yf,

D ead loads

It may sometimes be feasible, particularly in buildings, to measure stnjctural dimensions and densities accurately so that it is possible to calculate the weight of the structure with an accuracy that would justify the reduction of the load variation factor Vfj from 1.15 to 1.05. A possible exception may be thin slabs, where 1.10 may be appropriate for thicknesses of 100mm or less.

I f thicknesses and densities of screeds and partitions are measured, Y f,= 1 05 may be apropriate for these finishes provided that the actual partition loads are used and not a ‘blanket allowance’. I f screeds and/or partitions are to be renewed the normal design value of Yf, = 1.15 should be used.

Im posed loads

If proper ‘characteristic’ values were used for imposed loads there would be little justification for reducing y q for separate elements. F or multi-storey columns reduction factors such as those in CP3, Chapter V, P art 15 should take adequate care of the lower probability of all floors being fully loaded simultaneously.

It is the experience of most engineers that floors are very rarely subjected to the imposed loads stipulated by C P3, Chapter V, P art 1. G ross overloading does however occur occasionally, particularly on floors originally intended for dwelling use. The engineer must assess the suitability of the floors for its intended use.

Even in situations where the statutory aspects of the CP3, Chapter V, P art 1, la d in g s do not apply, extensive safeguards on the possible future use (as opposed to the immediately intended use) would be needed, before one contemplated reducing the imposed loads from the values in the Code. Similar reservations obviously apply to the alternative approach of reducing y fl as the probability of exceedmg the load usually remains the same. In some circumstances it may even be necessary to increase the

imposed loads and/or Yf,.

Liquid pressures in storage tanks can be calculated as accurately as the weight of the structure; the factor 1.05 as used for dead loads would be appropriate if applied to the pressure corresponding to the highest possible head.

Similar considerations might apply to the weight of existing earth fill but not to earth

pressures.

W ind loads

C P 3 Chapter V Part 26‘wind loads do not conform to the definition of characteristic’. It appears, however, that by multiplying the wind load corresponding to S j = 1 b y ™ - 115 a wind load corresponding to a return period of 140 years is obtained. This s probably a reasonable ultimate value for most buildings, and only for special elements

5. C P 3. Code o f basic data fo r design o f buildings: Chapter V. Loading: Part 1. 1967, D ead a n d im posed toads, British Standards Institution, London

6. CP 3. Code o f basic data fo r design o f buildings: Chapter V. Looking: Part 2. 1972, W ind loads, British Standards

Institution, London

17

or structures that cannot cause injury and therefore may be allowed to fall at less exceptional winds should yf, be reduced.

Experience suggests th a t the wind loads derived from CP3, Chapter V, P art 2 are reasonable for design of cladding and particularly cladding fixings. F or brickwork parapets and panel infill walls they appear sometimes to be excessive, and for the assessment of such features it may be preferred to refer to the sizes permitted by the London Building By-Laws7rather than using reduced loads and/or y ft — factors in calculations according to BS5628, P art l 8. Care must be taken, however, in areas where wind loading is high.

T H E L O A D C O M B IN A T IO N A N D S E N S IT IV IT Y F A C T O R , yfj

A s the considerations that govern this factor are not affected by the difference between a design and an existing structure yf2 remains unaltered for appraisal calculations.

T H E STR U C T U R A L P E R F O R M A N C E F A C T O R , y fj

The design factor yf3 = 1.2 covers inaccuracies of construction, inaccuracies of analysis and the severity of the consequences of collapse.

I f measured dimensions, including eccentricities caused by building inaccuracy, are used in an assessment of an existing structure and realistic or conservative assumptions are made about the mechanics of load transfer the following values o f yf3 could be used:

Yf3 = 1 -05 for secondary elements, failures of which will not lead to progressive collapse.

Yf3 = 1.15 for primary members supporting other parts o f the structure and for secondary members failures of which might cause loss o f life and/or substantial m aterial damage.

3.2.5.3 The material factors, ym R E IN F O R C E D C O N C R E T E

The factor ym = 1.5 in C P I 10 includes the factor ymi in ISO 23944 which allows for the difference between in situ strength and the strength of the test cubes. The value of 1.5 has been chosen largely because of uncertainties in the quality of materials and workmanship, compaction, curing, etc. I f concrete strengths are ascertained by tests on cores from the actual structure supplemented by ultrasonic pulse velocity or rebound hammer measurements to assess the variability, it would be reasonable to reduce the overall value of ym 9.

W here the failure mechanism is well understood and'/or ductile (e.g. bending of underreinforced beams and slabs) consideration could be given to reducing ym to 1.25. It should also be remembered that the factor 0.67 which is used in design to translate cube strength to beam ‘design strength’, increases with decreasing concrete strength. Conversely, there are failure modes that are not clearly understood, (e.g. shear), and there are members that may fail suddenly without warning (e .f . columns). In both cases caution is appropriate, and ym = 1.35 may be more appropriate. F or slender columns that cannot be safely cored even higher values of ym may be required.

The value of y ^ = 1.15 for steel given in CP 110 is based on a single tensile test by the manufacturer for every 25t (40t for large diameter bars) and has, in addition, to cover rolling tolerance.

A s sampling from a primary member will usually lead to substantial loss of structural resistance, it is usually impractical to extract bar samples for testing. If samples have been obtained from a number of representative members and tested and the consistency of the mechanical properties of the other bars checked using non-destructive means there could be a case for reducing ysteel to 1.05 provided that measured effective depths are used in the calculations and provided that the full stress/strain curve has been obtained from the tests and shows ductility and reserve of strength beyond the yield point.

STR U C TU R A L S T E E L

The 1977 proposed revision of B S 44910 follows the pattern of partial safety factors covering variations in material quality, structural performance and applied loads laid down in C P I 10. A material factor (ym) of 1.07 has been adopted for structural steel covering variation in strength only. This has been incorporated in the draft by multiplying the yield strength (Min) by a factor of 0.93 (i.e. 1/1.07). The structural performance factor, yf3 includes for variations intolerances on rolled steel, fabrication and erection, together with allowance for design and detailing inconsistencies. As far as loading is concerned, the principle of C PI 10 has been adopted but with a different format.

7. G reater London Council, Constructional by-laws, London Building A cts 1930-1939, London Building (Constructional) By-laws 1972, London, G L C , M arch 1973

8. B S 5628: The structural use o f masonry. Part 1; 1978 Unreinforced masonry, British Standards lnsitution, London

4. ISO 2 3 9 4 : \§ l% r G eneral principles fo r the verification o f the sa fe ty o f structures

9. D D 00: Assessm ent o f concrete strength in structures, British Standards Institution, London (to be published)

10. D raft Standards Specification fo r the structural use o f steelwork in building Part I: S im p le construction and continuous construction, 7 7 /13908DC, British Standards Institution, London, November 1977

18

W here, in existing structures, steels to BS 1511, BS 54812 and BS 96813 have been used, the strength may be assumed to be comparable to the equivalent grade of BS 43 6 0 14. In these acircumstances ym may be assumed to have a value of 1.07. In other cases the strength of the m aterial and value of ym should be determined, if possible, from the relevant specifications and tests. If the manufacturing specification cannot be ascertained and only one or two specimens can be tested, conservative values should be adopted e.g. ym = 1.25.

W here welding is to be undertaken on existing structures care m ust be taken to ensure that the original material is of a suitable quality, and any necessary adjustments are made to the safety factors.

F or concrete-encased members some reserve strength may be available, particularly if composite action can be substantiated.

Care must be taken in 19th Century structures since wrought iron may have been used instead of structural steel, the appearance being similar. The strength of wrought iron is directional varying between 310 and 360N /m m 2 in the line of rolling, while at right- angles it is only about % to % of that value. The m aterial can delaminate fairly readily, and corrosion is easily recognizable since it occurs in a flaky manner.

M A SO N R Y

The ym -factors in BS5628, P art 18 depend on the brick m anufacturer’s quality control, workmanship and the site control o f the mortar. W hile these factors are difficult to quantify after the event it is however usually possible to assess the variability of the brick strength and m ortar composition by sampling and testing, and one can assume that the ym values include some inbuilt allowances for deviations from the design such as laying bricks ‘frog’ down, cutting of horizontal chases and raking out of joints prior to pointing with soft mortar. D etailed site information on this point may make some reduction of ym acceptable.

The very low stresses which codes of practice allow in brickwork laid in lime mortar may be motivated by a requirement to limit the squeezing-out o f the mortar and the resulting settlement of the brickwork in the period following the construction. This consideration is not relevant when assessing an existing building. The increase in stress that m ight be allowed is however difficult to assess in the absence of further information, such as may sometimes be obtained by sawing out large prisms of brickwork and testing them intact.

T IM B E R

If the markings from machine grading can be found, the mechanical properties and appropriate safety factors can be deduced from BS 5268, P art 2 .

In the majority of existing buildings the best that can be done is to identify the species from samples and to stress grade a representative number of members according to visible defects as laid down in BS 4 9 7 8 16 for softwood and BS 575617 for tropical hardwoods. Once species and grade has been assessed, mechanical properties can be found in BS 5268, P art 2 15.

Certain members such as floor joists in old houses are sometimes undersized by today’s standards and frequently notched for service pipes. Some 18th Century joint details in roof trusses are not sound engineering. In both cases calculations are likely to prove unhelpful, and subject to careful inspection of bearings, and joints, the engineer has either to accept ‘structural adequacy by force of habit’, provided that the load is not increased, or to recommend remedial works.

.4 Calculations requiring special consideration

G E N E R A L

W hile most engineering calculations do not in any way represent the real structural behaviour they do however in most cases, when used with the customary safety factors, result in structures that are safe and serviceable. Certain types of structural behaviour are however not amenable to accurate prediction by calculation so that safety factors used in calculations will not necessarily ensure safety. Unfortunately the types of failure that are most difficult to predict accurately also tend to be the most sudden and therefore present the most danger to life.

Perhaps the greatest danger is the use of a formula in a situation where it is not directly applicable. If the formula is obtained from a recognized code or standard there should be no danger provided that the formula is used exactly as intended in the document.

Form ulas from textbooks have usually been derived for idealized, simplified conditions and will give values applicable for those conditions only. The Euler formula, for example will give the exact elastic critical load for a straight pin-ended bar, but does not include parameters such as yield strength and initial bow which are necessary to

11. BS 15: 1961. M ild steel fo r general structural purposes, British Standards Institution, London

12. BS 548: 1974. H igh tensile structural steel fo r bridges etc. an d general building construction. British Standards Institution, London

13. BS 968:1962. High yie ld stress ( welding qua lity) structural steel, British Standards Institution, London

14. BS 4360: 1979, Specification fo r weldable structural steels, British Standards Institution, London

8. BS 5628: The structural u s e o f masonry, P art 1; 1978 Unreinforced m asonry, British Standards Insitution, London

15.BS 5268: The structural u s e o f limber, P a rt 2; British Standards Institution, London

16. BS 4978: 1973, Tim ber grades fo r structural use, British Standards Insitution, London

17. BS 5756: Specification fo r grading o f tropical hardwoods fo r structural purposes, British Standards Institution. London (tobe published)

calculate the buckling load of a real strut. A textbook formula will not, generally, contain safety factors. These must be chosen to take account of the applicability of the

formula.

B R IT T L E -F R A C T U R E F A IL U R E IN STR U C T U R A L ST E E L W O R K

Brittle-fracture failures in structural steelwork occur only under a combination of factors. This combination includes:

• tensile stresses• crack-like defects or severe stress concentrations• material of poor notch ductility or fracture toughness at service

temperatures.

In carrying out appraisals o f risks of brittle-fracture failure it is important to know the notch ductility or fracture toughness o f the steel, the stress levels present, the presence of any general stress concentrations or risks of crack-like defects, and the previous loading history. F or structures subject to loading in one direction only, successful carrying of high loads previously indicates that the structure will not suffer brittle fracture failure before this stress level is reached, provided that there has been no deterioration of the structure by fatigue crack growth or material changes.

It should be noted that no indication of the notch ductility is obtained from the ordinary

tension test.

It may be that the structure being appraised is found to have cracks. Fracture mechanics techniques are now available to predict the residual strength of such structures. In these circumstances specialist advice should be sought. Brittle fracture and fracture mechanics are discussed in references 18, 19 and 20.

B R IT T L E M A T E R IA L S

Some materials, e.g. concrete, are inherently brittle and fail with little or no plastic deformation. The energy-absorbing capacity of a brittle component is much lower than that of a similar component made from a ductile m aterial, and care should be taken in the consideration of dynamic and accidental loads. Imposed displacements such as temperature movement and foundation settlements are unlikely to cause collapse of a ductile structure, whereas the reverse is true of a brittle structure or component.

In some cases it may be impossible to carry out calculations that have any confidence of prediction, e.g. the strength of fixings cast into or drilled into concrete can be ascertained only from test data.

C O M B IN E D S T R E S S E S

The basic states o f stress are uniaxial tension, uniaxial compression and shear. The strengths of a material in these modes may be found by careful tests. Loading in practice may not be unidirectional, direct and shear stress components being present two or three planes. In such cases the principal stresses must be calculated, and the failure criterion appropriate to the material, often expressed in terms of an ‘equivalent stress’, applied to determine the limiting load.

It follows, therefore, that in quoting the results o f tests made to determine the ‘strength’ of a material the stress conditions in which the data were obtained must be stated

precisely.

F A T IG U E

F atigue calculations will be necessary when it is required to calculate the remaining life of a structure that is and has been subjected to repeated fluctuating loads. These calculations will entail an analysis o f the loading to which the structure has already been subjected and that anticipated in the future, together with an analysis to obtain the stress spectrum for the areas of stress concentration. Existing fatigue test data may then be used for the damage calculation. M ethods of carrying out these calculations are set out in references 18, 21, 22, 23 and 24.

B U C K L IN G

For most practical structures, buckling will involve material yield or fracture. M aterial strength must therefore be taken into account, together with imperfections such as bow in a column or twist in a beam.

Questions that need to be asked before starting a buckling calculation include:• W ill the strength of the structure drop suddenly after buckling? If so, initial

imperfections will have a greater effect on the collapse load.• Is the material subject to creep? If so this is equivalent to a reduction in

stiffness and should be taken into account.• If the component is assumed to be restrained by adjacent elements are

these elements stiff and strong enough?

18. Suh, N .P ., and Turner, A .P.L .'.Elements o fih e m echanical behaviour o f lids, Scripta Book Co. and M cGraw-Hill, 1975

19. Boyd, G .M .: Brittle fracture in steel structures, Butterworths, London, 1970

20. K nott, J .F .: Fundam entals o f fracture mechnaics, Butterworths, London, 1973

21. D raft BS 5400. Steel, concrete an d com posite bridges. P art 3: Code o f practice f o r design o f steel bridges, 79 /1314D C , British Standards Institution, London

22. BS 3518: Parts 1 to 5: 1962-1966, M ethods o f fa tig u e testing, British Standards Institution London

23. G um ey, T.R.: Fatigue o f welded structures, University Press, Cambridge,

1968

24. F rost, N .E ., M arsh, E .L ., and Pook, L.P.: M eta l fa tigue, University Oxford,

Press, 1974

20

• If a Code of Practice or British Standard formula is used to check buckling does the accuracy of construction of the structure corresponded to that assumed in the Code or Standard?

C O N N E C T IO N S

The rigorous analysis of connections is extremely difficult, and it is often only the ductility o f most engineering materials that enable calculations to give results that predict actual failure loads.

Certain connections have to be checked for imposed displacements caused by temperature movements, foundation settlement, etc. This applies particularly when the connection is the ‘weakest link in the chain’ so that movement is concentrated at the connection.

If, as is often the case connections are hidden, so that their condition cannot be fully ascertained by inspection, no reduction of safety factors should be comtemplated.

B O L T E D C O N N E C T IO N S

Bolted and riveted connections should be checked to ensure that the fasteners are in good condition (e.g. not loose) and of a type and number adequate to transm it the service loading.

High-strength friction-group bolted joints should also be inspected for satisfactory contact o f the faying surfaces and absence o f corrosion between them. A check should also be made with a torque wrench for the existence of adequate shank tension.

W E L D E D JO IN T S

These should be checked to ensure that the welds are of a size and type adequate to transm it the required loads and inspected for cracking or other signs of deterioration.

3.2.6 Load testingThe real behaviour of certain structural arrangements is sometimes not amenable to calculation. I f a structure appears to be adequate but the calculations at this stage fail to demonstrate an acceptable margin of safety, it may be worth while to:

• load test the structure within the elastic range to examine the manner in which it actually distributes loading

• remove components from the structure to test to failure in the laboratory

• isolate part of the structure and test it to failure• test the complete structure under an overload.

Advice on the decision as to testing requirements is given in sub-section 4.2.

Calculations should be carried out to predict as far as possible the results before the tests are carried out. This is to establish approximately the amount of loading required and the magnitude o f deflections to be measured. A calculation that predicts the result before the test is carried out wil also carry more weight than a similar calculation performed after the load test.

The procedure to interpret the test results should be decided before the tests are commenced so that the instrumentation can be arranged to providethe relevant data necessary for the calculations.

If calculations to predict test results are based on Codes o f Practice, partial factors of safety should be assumed equal to 1.0 and m aterial strengths and stiffness adjusted to expected rather than minimum values.

Load tests have the effect of reducing uncertainty. Safety factors in calculations subsequent to load tests can be adjusted accordingly. C are must be taken to ensure that only the correct part o f the safety factor is reduced, e.g. if a water-retaining structure is filled with water and the induced stresses are measured accurately in various components then if there is no possibility o f a denser liquid ever being stored it could be argued that the factor y f could be reduced to 1.0 for the assessment o f those components. This is incorrect since yf also has to account for the importance of the limit being considered, and this portion of y f must remain.

There will be occasions when it is impossible to carry out reliable calculations or impractical to perform load tests to assess directly the level of safety of a structure. In such situations any other information that can lead to a conclusion regarding adequacy is of great use. Other data that could be used include:

• an estimate of the ratio between the maximum load taken by the structure in the past and that likely in the future (=

• an estimate of the ratio between the load at which signs of distress would become apparent and the load to cause collapse (=-Rs); if brittle failure is possible then R s must be taken as 1.0.

21

If the structure is practically in ‘as good as new’ condition, if further deterioration is unlikely, and if

past maximum load x collapse load

Rs future maximum load load for apparent distress

is considered sufficiently greater than 1.0 then the structure can be deemed adequate. If such procedure is adopted, then the structure should be monitored over a period of time.

4 Methods

4.1 SURVEY

4.1.1 IntroductionThe information required for the first stage of a structural appraisal includes a comprehensive knowledge of both the construction and the conditions of service towhich it is now or is likely to be subjected.

The character and scope of the survey will be limited by the dictates of time and money, but it must include as far as possible all relevant information about geometry and condition. The survey may range from a brief visit to examine one building component to a complete survey o f condition and dimensions, including a programme of materials testing.

Before a major survey is put in hand, the engineer should make a reconnaisance to establish conditions generally and to decide which finishes have to be removed to expose the structure. I t may be necessary to arrange the attendance of a contractor who can provide the means of access, carry out the necessary work and make good after the

survey.

The information required is in three parts:

• The dimensions of the structure are required. They may be found on existing drawings and must then be checked on site for accuracy.

• The loadings and other conditions of service at present and in the future must be established.

• The materials of construction, the physical condition and the visual defects must be ascertained.

Before visiting the site the engineer should study any evidence initially available to determine which information has to be obtained.

4.1.2 DocumentsValuable information on the design, construction and history of the structure can often be obtained from documents prepared for the original design and construction and for subsequent modifications. The engineer should be aware that drawings and calculations may not be the latest revision nor be exactly as the structure was built. Information on other construction in the locality may also be useful. Possible sources of such information are listed in Appendix 1.

4.1.3 DimensionsThe reconnaissance and available documents provide the basis for a dimensional survey which should measure the site, building, level, room and structural elements as necessary. The standard grid and level datum, national or local, to which the survey is to be referred may need to be specified.

The form and content of the final record should be agreed in advance. This agreement should include the size and scale of the survey drawings, the features to be measured and the accuracy required.

A diary should be kept during the survey.

It may be necessary to establish and protect survey stations and level points for later re­

use.

F or surveys aimed at monitoring structural movement,the frequency, accuracy and form of readings should be agreed with the survey team in advance of work starting on site Attention should be drawn to items of special interest in the survey and to specific points and surfaces on which readings are to be taken. The engineer should indicate

23

where the timing of the readings is important and where more than one observation will be needed. Requirements for specialized information (e.g. changes in slope on a surface) should be noted.

4.1.4 Loadings and environment

IntroductionThe existing loadings and environment may be different from those foreseen at the time of design or envisaged in Standards and Codes of Practice.

Dead loadsDead loads will initially be estimated in accordance with statutory requirements7-25-26-27 and Codes of Practice5. Differences between design and actual loads may arise from variations in the dimensions, the density and moisture content of building m aterials28.

Imposed loadsSince the imposed loads depend on the use of the building a full specification of current and proposed usage should be obtained from the building user. This should be confirmed by the survey. The appropriate loads can then be derived from statutory requirements,7l25,26’27 Codes of Practice5 or results of loading surveys.29"31

Warehouse loading and storageAttention should be paid to the current and proposed methods and patterns of storage. M echanical stacking may induce dynamic effects and increase loading.

It is necessary to confirm that materials stored in structural containers are of similar characteristics to those assumed in the original design. Overloading is common in stockpiling periods. A bibliography on the design of hoppers and silos is given in reference32, while references33' 37 should prove useful.

Loads arising from machineryStatic and dynamic loadings applied by plant and equipment to the structure may be obtained by reference to the user or the manufacturer. Attention should paid to the loads applied during the installation, relocation or replacement of plant and equipment. The size, location and direction of application of point loads from lifting equipment may be of significance.

Dynamic effects of mobile equipment, e.g. forklift trucks, should be investigated. Account should be taken of impact from presses, hammers, compressors and similar equipment producing cyclic loads that may induce a dynamic response of the structure. It may prove necessary to measure dynamic effects and to assess the fatigue properties of the materials.

Service loading from pipework, valves and ductwork, should be examined in order to confirm that the allowances used in the appraisal are adequate.

Explosion and impactGeneral provisions for accidental damage are covered by statutory requirements and Codes of Practice. The resistance required for specific explosions or impact loads must be determined from first principles.

Highway and railway loadsThe frequency, weight and distribution of actual road and rail traffic, particularly on private roads and sidings may have to be determined.38 The dynamic effects of road and rail traffic may be significant.

Snow and ice loadingsConsideration should be given to the build-up of snow and ice particularly in roof valleys and on cables. It may be desirable to take account of variation in snow loading with geographical locations (see Appendix F , Ice formation on structures, of reference 6 and reference 39).

Wind loadsThere have been changes in design wind loadings6 on structures in recent years. Structures designed before 1972 may be found to have different factors of safety than are currently required. Additional information on loading is available in the literature from the Building Research Establishment and the Meteorological Office, where information exists on the variation of wind speed with direction.39-40-41-42

7. G reater London Council, Constructional by-laws, London Building Acts 1930-1939, London Building (Constructional) By-laws 1972, London, G L C , M arch 1973

25. The B u ild ing Regulations, 1976, no. 1676, London, H M SO , 1976

26. B uild ing S tandards (Scotland) (C on ­solidation) Regulations, 1971, SI 1971, no. 2052, London, H M S O , 1971

21. Building Regulations (Northern Ireland), 1977, Stat. Rules o f N I, 1977, no. 149, Belfast, H M S O , 1977

28. BS 648: 1 964, Schedule o f weights o f building materials. British Standards Institution London

29. M itchell, G .R ., and W podgate, R.W .: Floor loadings in office buildings the results o f a survey, BRS Current paper C P 3 /7 I , BRS, G arstone, 1930.

30. F loor loadings in retail premises, BRS Current Paper C P 25 /71 , BRS, Garston,

1971

3 1 Floor loadings in domestic premises, BRS Current Paper CP33/76, BRS, Garston, 1976

32. A selective bibliography on hoppers and silos. C IR IA Bibliography 1/1970

33. Codes of Practice — G erm an D IN 1055 P art 6 , Design loads fo r buildings, loads in silo bins

34. Codes of Practice — R ussian C H 302- 65, Instructions fo r design o f silos fo r granular materials

35. Paterson, W .J.: ‘Pressures in silos — a review o f some structural aspects’, C ivil Engineering, 65 , no. 766, M ay 1970, p, 497

36. Particulate Review, Jan. 1972, B ulk storage o f granular materials. A review o f som e structural aspects

37. B ins and bunkers f o r handling bulk materials — Practical design a n d techni­ques, by W . Reisner and M.V. Eisenhart Rothe. T rans Techn Publications, 1973

38. BS 5400, Steel, concrete a n d composite bridges,: P art 2. 1978, Specification fo r loads, British Standards Insitution, London

39. Mitchell, G .R.: Snow loads on roofs. BRS Current Paper C P 33/76

40. Newbury, C. W ., and Eaton, K.J.: W ind loading handbook, BRS Report, London, H M SO . 1974

41. Penwarden, A .D ., and Wise, A .F .E .: W ind environm ent around buildings. BRS Report. London, H M SO , 1975

42. The m odern design o f wind sensitive structures, proceedings of the seminar held on 18 June 1970 at the Institution o f Civi! Engineers, London, C IRIA , 1971

24

The dynamic effects of wind on structures such as masts, towers43 and buildings of flexible construction will require special consideration.

Earthquake loadsThe zones in which earthquakes are likely to occur and the design criteria are usually given by local building regulations. A refinement of the dynamic loading may be achieved by the application of methods of analysis that combine a characteristic ground disturbance with the dynamic response of the structure.44 45

Soil pressures and ground movementLoadings from soils depend on the nature of the material, its condition and ground movements.46-52 The engineer should look for movements in the ground that m ay be caused by:

failure in loadbearing soil strata soil consolidationvariation in moisture content o f soilsoil compactionslope instabilityheave due to frostsoil shrinkage and swellingearth tremors, earthquakessoil shrinkage due to heatingsoil swelling due to freezingmineral extraction, tunnellingsoil compaction due to vibrationsettlement due to the collapse of cavitiesmovement due to construction on the site or in the localitysettlement due to combustion in coal seamssub-surface erosionsoil swelling in clays caused by moisture soil erosion from faulty drains or w ater supply swelling of unsuitable hardcore

FireConsideration should be given to surveying the compartmentation, means of escape, fire load and the provisions for fire detection and fire-fighting in order to determine the requirements for protection of the structure against fire. It should be noted that structural modifications that may have a negligible effect on strength may considerably reduce resistance to fire.

Advice on inspecting fire damaged structures53 is given in Appendix 5.

Atmospheric conditionsNorm al atmospheric conditions give rise to corrosion at an acceptable rate for structures designed, constructed and maintained in accordance with the relevant Codes of Practice. Account should be taken of the proximity of moisture, corrosive materials, gases, discharges, etc. The rate or incidence of corrosion may be affected in some materials by the level of stress.

Adverse conditions54 may be indicated by the deterioration of materials in the locality if not on the building itself. Investigation of possible local sources of pollution may indicate the corrosive agent. Further information may be available in the U K from offices of the Health and Safety Executive or the Alkali Inspectorate.

Thermal effectsThe effects o f diurnal and seasonal variations in ambient temperature on the structure cause expansion and contraction. The temperature range to which the structure is subject may be determined by reference to the records of the Meteorological Office. The thermal effects of heat gain and loss caused by radiation may be established by transient heat flow analysis and by reference to published solar radiation data.

Significant temperature variations56 in the structure arising from building use, inadequately insulated furnaces and cold rooms and stores, kitchens in large hotels, etc. may need to be measured on site. Extremes of temperature may affect the performance of materials, e.g. brittle fracture of steels,57 strength of concrete58 and steel, etc.

Variations in moisture content in some building materials give rise to dimensional changes that can affect structural conditions.

A change in temperature, moisture content or humidity may increase the rate of corrosion or the deterioration in building materials59-62.

43. D raft Code o f Practice — Lattice towers a n d m asts — loading, 7 8 /13085 , British Standards Institution, London, Septem ber 1978

44. Recom mended lateral force require­ments, and com m entary Seismology Com m ittee, Structural Engineers, A ssocia­tion o f California, San Francisco, 1975

45. Dowrick, D .J .: E a rthquake resistant design. London, W iley, 1977

46. BS C P 2004:1972, Foundations, British Standards Institution, London

47. S o ils an d fo u n d a tio n s: /, B R E Digest63, H M S O , O ctober 1965

48. S o ils an d fo u n d a tio n s: 2, B R E Digest64, H M S O , Novem ber 1965

49. So ils an d fo u nda tions: 3, B R E Digest 67, H M S O , February 1966

50. Subsidence engineers’ handbook. National Coal Board, M ining Departm ent, 2nd ed., 1975

51. D esign an d construction o f deep base­ments, Institution o f Structural Engineers, London, 1975

52. Structure-so il interaction, Institution o f Structural Engineers, London, 1978

53. Ashton, L.A.: Fire an d protection o f structure, S tructural Engineer, 46 , no. 1, January 1968, p. 5

54. W ilson, J.G .: Concrete in the weathering and perform ance o f building materials, (Ed. by Simpson, J .W ., and Horrobin, P.J.), Medical and Technical Publishing

Co., 1970 p. 41

55. The Institution o f Heating and Ventila­ting Engineers, G uide A6: S o la r data, IH V E , 1975

56. Temperature an d concrete, Committee on the Effect o f Tem peratuire on Concrete, A C I SP25, 1971, p. 1

57. W ells, A.A.: N otched bar tests,fracture mechanics a n d the brittle strengths o f weldedstructures, British W eldingJoumal, January 1965

58. Building Research Establishment Current Paper l /1 0 ,E v iro n m en t changes tempera­ture, creep an d shrinkage in conrete structures, H M SO , 1970

59. Building Research Establishment Digest no. 75 (2nd series), Cracking in buildings, H M SO , October 1966

60. BuildingResearchEstablishmentDigest no. 164, Clay brick work: /, H M SO , April 1974

61. Building Research Establishment Digest no. 165, Clav brick w ork: , H M SO , May 1974

62. The m ovem ent o f timbers, Technical Note no. 38, BR E, Princes Risborough Laboratory, August 1975

Abrasion, erosion and deteriorationIn time a structure may be eroded and a significant reduction in strength and serviceability may result. A brasion from vehicles or equipment and erosion by wind and water are examples. Vandalism is a further example.

The structural properties of some building materials deteriorate with time, e.g. high alumina cement. The engineer should look for signs of such deterioration.

Fungal and insect infestationOrganic materials are susceptible to fungal and insect infestation. Possible forms of attack which the engineer should look for are described in the literature

CreepC reep _ the increase in strain with time under constant load — can occur with certain materials. The level of stress and the period of its application are important factors. >

Aggressive constituentsSubstances included in certain structural materials can cause deterioration, such as undispersed calcium chloride in concrete etc. Different metals separated on the galvanic scale but in close contact in the works will corrode in the presence of a suitable electrolyte, e.g. water.67’68

Aggressive ground conditionsCertain constituents in the ground may degrade foundations, e.g. sulphate attack on concrete in footings and piles, the effects of buried industrial waste, etc.oy

Aggressive conditions caused by the use o f a buildingIt may be necessary to look for aggressive conditions of service deriving from the use of the building. Reference should be made to the manufacturer, to process management or to independent expert advice.

The effects o f vibrationVibrations70 may affect the use or performance of a structure. The effects may be

show y^ caused by resonance in components with insufficient damping• fatigue failure, usually in metals, where the number and amplitude of

vibration cycles are excessive• effect of frequency and amplitude of vibration on people.

Strains induced in a structure by manufacture and assemblyBadly fitting components forcibly assembled or local deformation induced in manufacture and assembly (e.g. caused by welding or by excessive local deflection of temporary works) may cause strains and forces within the structure that can affect its

strength and serviceability.71

FloodingDamage may be caused by flooding. Guidance on repair of flood damaged buildings is

given in reference 72.

4.1.5 ConditionA n important part of the survey is to determine the physical condition of the construction The task of determining the condition of a structure should be approached with an open mind without prejudging the cause of any a p p a r e n t defects since the complexity of each structure’s history prevents all possible combinations of defects and their causes being defined. There is always a danger that new defects that are outside of previous experience will be missed, and much effort may go into trying to find a well known type of defect that is not present. It is then necessary to describe the conditions adequately so that the situation can be reviewed objectively back in the office . However previous experience of defects, such as those described in Appendix 4, should be borne inmind and checks made for them.

Lack of integrity and stability of wall cladding73 is a common form of defect often traceable to faults that have been present since construction. W hile fixings tie back* and supports are of prime interest, inspection should consider that the life of most sealants is less than that of the cladding. It is important to see whether any defect repeats in a pattern. Differential movement, particularly that caused by creep in in situ concrete construction, is potentially damaging.

63. D ecay in buildings: recognition, pre­vention an d cure, Technical note no. 44, BR E, Princes Risborough Laboratory,

September 1977

64. Tim ber decay and its control, Technical note no. 53, BRE, Princes Risborough Laboratory, N ovem ber 1977

65. Ills ton, J .A ., and England, L.: Creep an d shrinkage o f concrete and their infl uence on structural behaviour, S tructura l Engineer, 48 , no. 7 , July 1970, p. 283

66. The creep o f structural concrete, Report o f a Concrete Society working party, Technical paper no. 101, Concrete Society, 1974

67. Concrete in the M idd le East, R eprint of five articles from Concrete, orginally pub­lished 1975-76, C & C A , 1977

68. Building Regulations Advisory Committee, Report by Sub-Committee P, High a lum ina cem ent concrete, D epart­ment o f the Environm ent and W elsh Office, London, August 1975

69. Building Research Establishm entDigest no. 174, Concrete in su lphate bearing soils and ground water, H M S O , February 1975

70. Steffens, R .J.: S tructural vibration and damage, Building R esearch Establishm ent Report, H M SO , 1974

71. Proceedings o f conference on fatigue of welded structures, July 1970. W elding Institute, Cambridge. 1970

72. Repair and renovation o f flo o d - dam aged buildings, BRE Digest 152, H M SO , April 1972

73. W all cladding defects and their diagnosis. BRE Digest 217, September 1978

26

The engineer should trust only the evidence of his own eyes. H e m ust therefore be dressed so as to be able to inspect all spaces and equipped with a torch, tape, hammer, ladder, notebook, etc. The engineer m ust have the courage to expose members or connections where he judges that defects m ay occur, e.g. fixings of precast cladding panels, connections in prefabricated structures, etc.

Clear notes together with sketches and photographs should be kept; a possible form is given in Appendix 3.

A list of common defects, together with possible causes and suggested further investigation, is given in Appendix 4.

4.2. TESTING

4.2.1 Determination of testing requirementsThe requirements for testing will depend on the outcome of the survey.

There is no need for testing where the survey provides sufficient information to enable the engineer to complete the appraisal and reach conclusions and recommendations with confidence. This situation will occur where:

• the structure is clearly in a sound condition without defects and the physical dimensions found in the survey allow calculations to confirm the suitability of the structure for its intended future use

• visual defects or the poor condition of the structure point to obvious conclusions that meet the requirements of the brief.

Requirements for testing will arise in other situations where, for example:

• there is a lack of information on what materials are present in the structure

• the presence of deteriorated or deleterious materials is suspected.

W here testing is needed it is necessary for the engineer to make an assessm ent of what further information is needed. The purpose o f particular tests and the information that they can provide m ust be understood so that the right tests are carried out.

Available test techniques cover a wide range of cost and complexity, the engineer should proceed in a logical way. Some tests lead to little disturbance. A t the other extreme it may be necessary to remove the structure from service while tests are undertaken. In some circumstances the cost of testing may be such that immediate remedial action will provide the m ost economic solution. Appropriate experience may be necessary to see that some types of test are carried out successfully, and specialist advice m ay be needed to interpret the results.

Types of test can be divided into those that give a local measurement of a dimension, material integrity or material condition, composition or property and those that give a direct indication of the performance of a structure or structural element. It is often necessary to use a combination of testing techniques.

Tests providing local measurement are described in clause 4.2.2 and Appendix 6. Since these tests are local it is necessary for the engineer to decide on the number of tests and their locations so that representative samples are obtained. U sually there are typical dimensions or material characteristics in structural elements. I f not the dimensions of the complete structure must be established, which can be used later in the appraisal. Minimum values or average values may be required. In either case, several tests will be needed.

The numbers of tests and location will depend on the particular circumstances and should be decided by the engineer remembering that:

(i) the likely variation in m aterial properties within and between parts of the structure

(ii) the probable critical locations

(iii) the possible errors in the test procedure

(iv) different techniques may measure properties o f different volumes, e.g. in concrete ultrasonic-pulse-velocity measurements indicate the average quality through the depth whereas core tests measure to only 100-300 mm depth.

These factors will affect the consistency (scatter) of the results and hence the strength obtained by standard statistical procedures, the confidence level appropriate to which will increase with the number of results. Guidance on interpretation of the results of tests which give an indication of strength is available in some cases (e.g. for concrete structures see reference 9).

Tests that give a direct indication of structural performance are discussed in clause

4.2.3.

9. D D 00: Assessm ent o f concrete strength in structures, British Standards Institution, London (to be published)

27

4.2.2 Materials testingThe engineer carrying out an appraisal will rarely carry out the tests himself, bu t he should be present to observe and supervise the work. Thus this clause does not give detailed information on test techniques but identifies the types of test that are available, what information they give, and their advantages and limitations.

I f a sample of concrete is being obtained it may be possible to carry out a complete chemical analysis of the constituents. However, this analysis can be broken down in separate parts depending on what information is required. F or example the main interest would n o r m a l ly be the cement content of the concrete, the water/cem ent ratio and the presence of deleterious materials such as chlorides, reactive aggregates, etc. In some cases there are tests that can be performed in the field, but these will be indicative tests. F or accurate quantitative results the use of proper laboratory facilities is needed.

W here steel samples are required and can be removed from a structure, all the tests described (including non-destructive tests on material not removed from structure) can be carried out on a comparatively small sample of material. The largest piece required is likely to be for a standard tensile test specimen. W hen it is possible to obtain only non-standard size test specimens (i.e. miniature pieces of different shape) the results must be carefully considered since they may not be representative nor give a true indication of actual properties.

Available test techniques are summarized in Tables 1—5, with further details of the techniques in Appendix 6.

4.2.3 Load testing

4.2.3.1 IntroductionA fter the survey and local tests on the materials the engineer m ay still have doubts whether the structure can perform satisfactorily under the loads likely to be imposed in

TA BLE 1 — S T E E L A N D O T H E R M ET A LS

Information Techniques Remarks Referencesought available (see Appendix 6)

Dimensional Tapes, callipers, clauses 4.1.2micrometersOptical (eg lasers) T17Radiographic Specialist T2

essential

Integrity (eg presence Ultrasonic T5band degree of cracking Radiographic Specialist T2—surface and internal essential— lamination, porosity inclusions

Presence of cracks Ultrasonics T5Radiographic Specialist T2D ye penetrants essential T18Magnetic particle T19

M echanical Tensile T20Properties Im pact T21

Bend T22H ardness T23

Composition M etallography T24Spectrography Specialist T25Chemical analysis essential

W rought iron and Visual T27cast iron Drilling for small

samples

Aluminium and Chemical analysisaluminium alloys

Corrosion

Fatigue

28

TA B L E 2 C O N C R E T E — P L A IN , R E IN F O R C E D A N D

Information Techniques Remarkssought

Dimensional

available

Tapes, callipers, micrometers

Optical (eg lasers) Radiographic Specialist

essential

Cover and position of steel reinforcement

Degree of corrosion of steel reinforcement

Com paction and quality of concrete

Strength of concrete

E xtent of cracking

Permeability

Cem ent content

Cement/aggregateratio

W ater/cem ent ratio

Presence and type of admixtures

Covermeter Radiographic

Physical exposure of reinforcement

Electrical Physical

exposure of reinforcement

Endoprobe Radiographic

Ultrasonic pulse velocity

Rebound ham mer Coring

Internal fracture Crushing of cores

D ye and ultraviolet light

Absorption test on cores

Surface absorption

Chemical Analysis

Petrographic analysis

Absorptiontechniques

Chemical analysis

Specialistessential

Free-lime content

D epth of carbonation

Type of cement

Chemical analysis

Petrographic analysis

Phenolphthalein test

Chemical analysis Petrographic analysis

Radioisotope

Specialistessential

Specialistessential

Specialistessential

Specialistessential

Specialistessential

Specialistessential

Specialistessential

SpecialistSpecialist

essentialSpecialistessential

P R E S T R E S S E D

Reference (see Appendix 6)

clause 4.1.2

T17T2

T1T2

Specialist essential

M ay give some indication of strength

T3

T4T2

T5

T6T7

T8T7

T9

TlO aTlO b

T i l

T i l

T12

T13

T14

T14

T14

T15

Type of aggregate Chemical analysis Specialistessential

Petrographic analysis Specialist essential

T16

TA B LE 3 — T IM B E R

Informationsought

Dimensional micrometers

Optical (eg lasers)

Type of timber or plywood

M ethod of assembly for built-up units

(eg glued, nailed, nailed and glued, etc.)

Checking and/or delamination of plywood.

Integrity and properties

Techniquesavailable

Tapes, callipers,

Radiographic

Laboratoryexamination

Analysis of sample (to determine glue)

Physical means such as a knife

blade or similar small tool

Physical tests

Remarks

Specialistessential

Specialistessential

Reference (see Appendix 6)

clause 4.1.2

T17T2

T34

F or plywood the type should be established

Refer to C P I 12 for permissible stresses

M oisture content M oisture meter Drying or sample

Type of preservative Visual with sample testSpecialistessential

Insect attack Visual Specialistessential

T30

T31

T32

D ry rot/w et rot Laboratory examination for early signs

Specialistessential

T33

the future. Calculations based on the information to hand may suggest that the structure may have become unserviceable or that there may be an inadequate margin of safety. In these circumstances consideration should be given to carrying out load tests on structural elements or on the complete structure.

Clause 3.2.6 outlines the circumstances in which load testing is considered as a step in the process of assessing the safety of a structure. Load testing may also be indicated when, because of past deterioration of materials and/or change of use, there are doubts about the ability of the structure to perform satisfactorily under the loads that are likely to be imposed in the future.

A load test of a full-scale structural element or o f a complete structure is a costly and time-consuming operation that generally cannot be repeated. I t should be used only as a last resort, and then it requires careful planning and design to ensure that all the desired information is obtained.

4.2.3.2 Types o f load testThe appropriate type of test should be chosen. There are four main types, three of which may be performed with the same testing arrangement. The four types are:

(i) S tatic service load testThe purpose of this type of test is to check for serviceability (e.g. deflections, crack widths, etc.) and/or to investigate the behaviour of the

30

T A B L E 4 — M A S O N R Y (IN C L U D IN G B R IC K W O R K , B L O C K W O R K A N D P R E C A S T C O N C R E T E C L A D D IN G )

Informationsought

Dimensional

Quality

Strength

Permeability

Chemical attack

M ortar check

Integrity of wall

Techniquesavailable

Tapes, callipers, micrometers

O ptical (eg lasers) Radiographic

U ltrasonic pulse velocityRebound hammer Coring

Internal Fracture Crushing of cores Crushing of blocks

Remarks

Specialistessential

Reference (see Appendix 6)

clause 4.1.2

T17T2

M arker tests for w/c ratio

Surface absorption

Chemical analysis

Chemical analysis

ties Inspection of cavity by endoprobe (or similar device)

T5

T6T7

T8The compressive T7 strength of bricks or blocks in masonry may be assessed by removal o f individual units and carrying out standard crushing tests

T i l

T10

T4

TA B LE 5 — P LA ST IC S

Informationsought

Dimensional

Techniquesavailable

Tapes, callipers, micrometers Optical (eg lasers) Radiographic

Remarks

Specialistessential

Reference (see Appendix 6)

clause 4.1.2

T17T2

Identification of type of plastics

Visual inspection Simple hardening Effect of flame Chemical analysis

Trade marks oftenT35 give a good guide to chemical composition and spread-of-flame characteristics

structure or part of the structure (e.g. load distribution between beams in a floor) under a load equal to the known design service load.

(ii) Overload test to assess safetyThis is a test in which the structure or part of the structure is tested under an overload to confirm that an adequate margin of safety exists under service loads. W here there has been no deterioration of the structural materials this test will not normally lead to collapse.

31

W here deterioration is apparent, the test may be used to assess the working load corresponding to a required margin of safety. W hen incipient failure can be recognized, i.e. if the mode of failure is sufficiently ductile, it will not normally be necessary to continue the test so as to produce actual collapse.

(iii) S tatic load test continued to collapseThis test would be earned out on a mass-produced structure or structural element, or on a part o f a complete single structure, in order to investigate the mode of failure or to obtain an estimate of the failure load of the remaining similar structures or s ■ ctural elements. Any such estimate must take account of the inherent variation between structures and the variability of the strength of materials, arising from the processes of manufacture and construction. Non-destructive measurements, as des­cribed in clause 4.2.2 and Appendix 6, can assist in this.

Such a test may be carried out by isolating the structure or element and testing it to failure in situ or by removing it to the laboratory. The choice will be governed by relative costs and consideration of the practical difficulties.

(iv) D ynam ic testingIt is sometimes necessary to ascertain the dynamic behaviour of a structure or to assess its remaining fatigue life. The dynamic behaviour can be assessed on the basis of measurements of vibration amplitudes at different frequencies in service conditions or under imposed dynamic test loads. Fatigue life may be assessed, when many identical components have been subjected to similar fatigue loading, by removing a number of components for fatigue testing to failure in a laboratory.

Alternatively, the stresses at the stress raiser in the component may be calculated and the fatigue life estimated from data on specimens with the same stress concentration factor.74

Specialist advice should generally be sought about the likely usefulness of the results.

(v) Soil testingThe identification and classification of the subsoil and the determination of its strength and deformation properties aimed at assessing possible causes of foundation failure is a specialist subject. Descriptions of methods of investigation of subsoil conditions including in situ measure­ments of soil properties are given in references 52, 75, 76 and 77.

4.2.3.3 Planning, preparation and precautionsThe planning should consider every step in the test and should allow for a course of action to follow situations that may arise. If the planning reveals that the proposed test may not be interpreted so as to yield the essential information required for the appraisal, the decision to test should be reconsidered.

The testing and acceptance clauses included in several Codes of Practice, e.g. BS 449, are intended primarily as guidance forjudging prototypes. They do not apply directly to appraisal of existing structures. The engineer may refer to the testing clauses in the relevant Code but must interpret them with care taking into account their relevance to the structure being appraised. 78 79

A n element of danger is inherent in all load testing and particularly so for existing structures whose behaviour and load paths are not clearly discernible beforehand. An experienced engineer should be appointed to be in charge of the entire preparation and execution of the test and particularly to be responsible for all aspects o f safety. He must acquaint himself with, and look out for, signs of impending collapse. The contractor’s representative responsible for the personnel engaged in the testing must be under the direction of this engineer. The engineer should fully inform the contractor of the nature of the work and of the quality of the personnel required for it. Casual labourers will not

normally be suitable.

Free-standing structures, e.g. unclad frames,must be stayed against lateral collapse. Provision must be made to restrict movement at failure. This may be done with chocks or layers of timber, which can be removed progressively as the test proceeds.

For load tests on beam or slab structures, scaffolding or loose props that do not interfere with the observed deflections but which are strong enough to withstand the impact o f the structure in the event o f collapse should be provided.

As the collapse load will usually exceed the safe capacity of the floor below this will have to be ‘propped through’ to the next and perhaps the third floor b e l o w (If the test load is applied by dead weight the impact load of the collapsing floor could be about

twice its total static load.)

74. E S D U data item 74016, Fatigue and sub-series

52 Structure— soil interaction. Institution of Structural Engineers, London, 1978

75. C P 2001: 1957, S ite investigations, British Standards Insitution, London

76. Scott, C.R.: A n introduction to so il mechanics an d foundations, 3rd edit.. Applied Science Publishers, 1980

77. Lam be, T .W ., and W hitm an, R.V.: So il m echanics, John W iley & Sons, 1979

78. M enzies, J.B.: L o a d testing o f concrete building structures, Structural Engineer. 56A . no. 12, Decem ber 1978. p. 347

79. Jones, D .S ., and Oliver, C.W .: The practical aspects o f load testing, Structural Engineer, 56A, no. 12, D ecem ber 1978. p.

353

32

The possibility o f damage to adj acent parts o f the structure or to adj acent property must be considered, and bracing may have to be provided to ensure stability o f the part of the structure remaining after a collapse of the tested part.

Preparatory work will normally include exposing and cleaning areas where gauges are to be fixed and measurements made. It m ay be possible in some cases to isolate parts o f existing structures, e.g. floor beams that have to be tested. This should be done by non- percussive means, e.g. sawing or core drilling.

W hen a load test on a structure of unknown capacity is in progress o r when the test load exceeds the service load no one must be allowed under the area of possible collapse. Measurements m ust be made with instruments that can be read from a safe position.

Appropriate protective clothing m ust be worn: ‘hard hats’ are ‘de rigueur’ anywhere near the te st area, hard-toed boots should be available at least for personnel carrying kentledge, and safety goggles are desirable in case concrete or m asonry fragments start flying immediately prior to collapse.

4.2.3.4 InstrumentationInstrumentation should be limited to the minimum that will provide adequate information for the proper interpretation of the test in the context of the overall appraisal of the structure. Instruments that give interesting but inessential information can distract attention from the observations that are crucial to the purpose of the test.

The measurements generally required are those of displacement (including rotation) and cracking. Electronic devices (e.g. resistance strain gauges, displacement trans­ducers) m ay enable large numbers of observations to be made and recorded rapidly remote from the immediate test area, but expensive installation costs are involved especially on sites exposed to the weather where elaborate precautions will be necessary.

On site, mechanical devices although slower in use will generally be more reliable. Deflections up to 50mm may be measured with dial gauges to an accuracy within 0.1mm. Larger deflections may be measured with scales, or observed remotely with telescope if necessary. It is important to provide measuring devices at the supports as well as at midspan of flexural members in order to eliminate errors from column shortening and bearing bedding-in.

W hen necessary rotations can be measured by clinometers or, in the case of large expected movements, by pointers attached to the structure and moving over independently supported scales.

W hitewash applied after the initial survey may facilitate the observation of fresh cracking of concrete. G raduated magnifying glasses are available for measurement of crack widths, but for safety reason their use is restricted. W hen attempting measurement of crack movement, great care must be taken to ensure that the crack width is measured at exactly the same position every time.

M echanical strain gauges such as the Demec can be used for measuring both strains and crack opening. Their gauge lengths are generally not small enough, however, to obtain adequate resolution in areas o f rapid change of strain. If this is necessary electrical-resistance strain gauges will be required. In biaxial stress fields where the directions of the principal stresses are unknown strain gauge rosettes will be required to obtain the stresses.

Local yielding of metal structures may be detected by coating the surface with a brittle lacquer (of which there are several proprietary brands).

Thermometers should be provided to read the ambient temperature. F or slabs it should be read above as well as below the test area, and for roof slabs subject to intermittent sunshine, the presence or absence of sunshine must be recorded for each deflection observation. Alternatively, thermocouples may be used to measure temperature differences between members.

4.2.3.5 LoadingThe test load may be in the form of dead weight or may be applied mechanically.

D ead weight requires labour for handling and is consequently usually slow and expensive to use. The means, however, are fairly readily available (e.g. sand, bricks, precast concrete blocks), and a uniformly distributed load can be obtained.Bricks or blocks should be stacked in separate piles to avoid arching effects. A means of weighing samples of these materials should be provided on site. Precise measurement of load may be obtained by using cast-iron weights, but costs of hiring these may be substantial. Because dead load persists when failure is reached the precautions to restrict movement referred to above are even more essential as accelerating motion may occur aggravating the effect of the collapse. Even small amounts of tilting may result in piles of blocks being upset.

33

W ater contained in dustbins, proprietary or purpose-made plastic tanks forms a convenient dead load. I f loads are to be maintained for any appreciable period it should be noted that only rubber provides a wholly impervious lining. W ater is fairly easily handled by pumping. In the event o f sudden failure water may be harmlessly dispersed, although care has to be taken to avoid damage to finishes.79

Hydraulic o r screw jacks or cables tensioned by jacks, winches or ‘pull-lifts’ are rapid and convenient mechanical means of loading. The load may be measured with pressure gauges in the case of hydraulic jacks, although a load cell or proving-ring at the point of application of the load to the structure is preferable. Loads should always be applied axially to load cells and to jacks to ensure accuracy and prevent damage to the equipment.

These mechanical means of loading will generally provide some restraint to movement o f the point o f the structure at which it is applied. To ensure that the desired loading conditions are obtained this restraint should be minimized, e.g. by using ball seatings or rollers. D istributed loading may be simulated by using a number of jacks or by single jacks with load-spreading beams. Adequate means for the applied load to react against adjacent parts o f the structure or a suitable anchorage must be provided.

4.2.3.6 Procedure and recordingBefore any load is applied a carefully annotated sketch and photograph of the testing arrangements should be made, and record sheets should be drawn up on which all the observations can be noted, including the temperature, weather and all other relevant data.

The load should be applied incrementally at a controlled.rate without impact. A full set of observations should be recorded at each increment, and graphs should be plotted of observations at critical points on the structure as the test proceeds, to obtain an assessment of the response and the possible onset of failure. Once any non-linearity is observed the structure should be unloaded and the recovery recorded. A t this stage it may be desirable although expensive to unload and reload after every few increments. In any case a careful inspection of the structure should be made at each increment of load and the progress of any local damage noted.

W here tests are continued to collapse carefully dimensioned sketches should be made after failure of the configuration of the collapsed structure, supplemented by photographs.

The report must state explicitly all the conditions of the test. I t may not be necessary to present all the data obtained, but the full record should be retained. It is essential to distinguish in the report between observed and deduced values.

4.2.3.7 Criteria for acceptance

There will usually be several criteria for accepting that the structure has performed satisfactorily. Codes of Practice and, for concrete structures, reference 79 give some guidance.

79. Jones, D .S ., and Oliv practica l aspects o f load tes Engineer, 56A , no. 12, Dec 353

34

APPENDIX 1

Sources of information on design, construction and historyBuilding owner — current/previous

as-built drawings legal representatives

Original structural designersconsulting engineers, contractors

Original design teamarchitects, services engineers, quantity surveyors

Soils Investigation contractor records, reports

Building surveyors change of ownership

Adjoining owners party-wall agreements

Constructorscontractors, specialist sub-contractors

Routine maintenance informationbuilding owner’s staff, contractors

Design teamalterations, refurbishment architects, consulting engineers, quantity surveyors

Insurers

Ordnance survey

Geological survey

Mineral valuer/National Coal Board

Statutory undertakersrailways, Post Office tunnels, underground services

Local authoritiesbuilding control, planning, district surveyors (in. Inner London)

Central and local Government departments, agencies

Technical institutions, societies, journals, proceedingsrecords, libraries

Local musems,public libraries, local societies

/

35

APPENDIX 2

Survey reportStandard Record Sheet: General Information Survey to be carried out by: (firm, names)

Premises to be surveyed: (title)Address:OS map reference:

Authority, client for survey:Address of client:Name, telephone number of client contact:

Name, occupier:Nature of businessName, telephone number of occupier contact:

Name, address of building owner:Name, telephone number of owner contact:

Current use of building(s)Proposed future use:Brief statement of reason for survey:Reference to client’s brief:

Scope of survey:

Brief description of buildings(s):Approximate age, history of buildings:

Name, address of original designer, constructors (if known):

Titles, addresses of planning, building control authorities:

Dates of survey: by areas/buildings:

36

APPENDIX 3

- •

Survey of condition: observations record sheet

subject of survey: building, level componentchecklists

observations: considerations checklist

loading current use adequacy for use etc.

observationschecklist

observations by: date:

Types of defect APPENDIX 4

BUILDING COMPONENT: Loadbearing brick walls (solid) /Brick or block boundary walls

VISIBLE D E F E C T

POSSIBLE

LO A D BEA RIN G BRICK OR BLOCK W A LL

C A U SE

BR ICK /BLO CK BO UNDARY W A LL

IN VE STIG A TIO I

LO A D BEA RIN G BRICK OR BLOCK W A LL (SOLID )

sTS SU G G E ST E D

BRICK O R BLO CK BOU N DA RY W A LL

wall out o f plumb rotation of foundation lack of lateral

restraint exertion of

horizontal forces weathering effects

(see parapets) sulphate attack on one side overloading cutting of chases

thermal/moisture movement presence of lateral

forces

examination of foundation and subsoil — presence of trees — lack of depth below ground of foundation

adequacy of wall — floor connections and roof ties (in case of pitched roofs)

ascertain presence or otherwise of expansion joints (particularly necessary where walls are curved on plan)

ascertain whether wall is ‘surcharged’ by soil or other materials

wall fractured vertically — cracks tapering or diagonal following vertical and horizontal brick joints

differential foundation or support settlement

heave in clay soils roof movement too strong mortar overloading cutting of chases

differential foundation settlement

heave in clay soils

as for loadbearing cavity walls

test mortar

effects o f adjacent tree roots in clay soils

presence of water adequacy o f foundation

and subsoil

wall bulged inadequate lateral restraint

inadequate ‘bond’ overloading horizontal chases sulphate attack on mortar

physical growth of tree trunk

pressure of local lateral forces

adequacy of wall — floor connection

use of ‘snapped headers’ especially in Flemish bonded walls

loading and design appraisal

presence of trees immediately adjacent to wall

walls fractured vertically and/ or horixontally — cracks not tapering

movement/corrosion of embedded structural steelwork

overloading thermal movement

ascertain presence or otherwise of structural steelwork in wall and adequacy or omission of concrete casing

,

I

' ■ - r - •■

splitting and/or bulging of brick piers — fractures tapering off to zero at top and bottom of pier

overloading fault in bond

loading and design check 1

further examination

- V - n U

wall oscillates under application of intermittent load

inadequate thickness or stiffening

plane of weakness in the bed joints above foundation

design check using lateral wind loading

presence of a dpc incapable of transmitting tensile stresses

OJ

BUILDING COMPONENT: Brick parapet walls

VISIBLE D E F E C T POSSIBLE CA U SE IN V ESTIG A TIO N S S U G G E ST E D

parapet ‘bowed’ on plan or leaning inwards or outwards

climatic effectsrot in timber bonding wall plates below parapet torsional uplift at comers of rectangular concrete roof slabs sulphate attack

evidence of similar defects in adjacent buildings in same situation

examination of condition of timber plates built into walls look for horizontal cracks at perimeter of building

parapet brickwork saturated

faulty construction porous brickwork no dpc under coping

evidence of dpc under coping stone (if any) and its condition

suitability of bricks used for purpose

diagonal cracks along brick courses and down vertical joints at each end of parapet

moisture/thermal movement porosity of brickwork used and likelihood of expansion because of absorption of moisture.

\effects of temperature — location (i.e. south facing)

BUILDING COMPONENT: Loadbearing brick or block external cavity walls/Brick or block external cavity panel walls

VISIBLE D E F E C T POSSIBLE CA U SE IN V ESTIG A T IO N S SU G G E ST E D

LO A D BEA RIN G CAVITY W A LL

CAVITY PA N E L W A LL LO A D BEA RIN G CAVITY W A LL

CAVITY PA N E L W A LL

bowing of inner or outer leaf inadequate lateral restraint from floors

inadequate tying of leaves unsuitability of materials used overloading inadequate design

creep, shrinkage and elastic shortening of building frame

moisturemovement (expansion of brickwork)inadequate tying of leaves

compliance (or otherwise) with BS 5628

omission or inadequate number of embedment ties, corrosion of ties - strength of brickwork and mortar used for purpose

loading and design appraisal

provision (or otherwise) of ‘compression’ joints at the top of panel walls and their effectiveness

adequacy and condition of ties between leaves the spacing and embedment into

each leafsuitability of materials used conditions of supporting nibs

vertical tapering cracks and/or diagonal cracks following horizontal and vertical brick joints in zig-zag pattern

differential foundation movement and/or heave in clay soils

too strong mortar

adequacy o f foundation provided depth of foundation below

surface of ground presence of water (cracked drains) ground-water movement following

adjacent earthworks proximity of trees in clay soils mining subsidence removal of trees prior to

construction of foundations (clay soils)

test

vertical hairline fractures at vertical joints with some evidence of hairline crack along bed joints (not zig zag)

shrinkage of bricks used

omission of movement joints

evidence of no vertical differential movement symptomatic of foundation

failure

horizontal fractures in flank walls at concrete floor levels

thermal presence of underfloor heating

adequacy or omission of joint between slabs at expansion joints

dampness on inside face of inner leaf

water bridging of cavity condensation

existence and size of cavity (50mm minimum recommended)mortar droppings on tiesother bridging of cavity — wood, bricks, etc.omission of dpc trays or drainage joints in bottoms of outer leafbridging of cavity by faulty filling of cavity with thermal insulation material

cracking (random pattern) of internal leaf and plaster finish

thermal shrinkage of blocks — central heating inadequate mechanical tying of inner leaf to column

at edges of panel

BUIJLDIMG C O M P O N E N T: R C general, RC columns and walls, nibs and corbels

VISIBLE D E F E C T POSSIBLE CA U SE IN V ESTIG A TIO N S SU G G ESTED

cracking of concrete cover/ exposure of reinforcment

very fine cracks are inherent in reinforced concrete

rust staining on concrete surface

corrosion of reinforcement (e.g.by CaCl2) nails and wire ties left in formwork firecorrosion of tying wires or chairs etc. presence of iron containing compounds

in the aggregate at surfaces

adequacy of concrete cover to reinforcement having regard to quality o f concrete and severity of exposure

visual examination — concrete usually white, straw or pink

strength teststest for carbonisation (phenolphthalein) analysis of concrete samples

fine hair crazing of surface construction fault

vertical fractures at intervals along rc walls

shrinkagemoisture movement

provision or omission of movement joints amount and spacing of distribution reinforcement

diagonal fractures in rc walls

differential foundation or support settlement

adequacy of foundation on stiffness of support presence of water — ground movement

spalling of concrete from face or top of nibs or corbels

cracking of ribs vertically

corrosion of reinforcement inaccurate positioning of top

reinforcementinadequacy of top reinforcementlack of anchorage of top reinforcementoverloadingexpansion of infilling brick panels elastic shortening and shrinkage of

building frame frost attack shrinkage

adequacy of concrete cover in relation to severity o f exposure

cover meter checks design checkexamination of detail drawings (where possible)loading assessment and comparison with loading capacityprovisions (or otherwise) o f compression joints in panel walls

\diagnonal spalling of front edge

of nib

presence of horizontal and vertical forces on nib

thermal effects other horizontal effects

BUILDING COMPONENT: RC beams and slabs

VISIBLE D E F E C T POSSIBLE CAUSE IN V ESTIG ATIO NS SU G G ESTED

vertical or slightly inclined cracks overloadingunder-reinforcement, inadequate depth fault in design thermal movement shrinkage around stirrups

ascertain actual load being carried compared with design load

compare span/depth ratio with Code requirements

ascertain existence or otherwise of uneven temperature gradient

diagonally inclined hair cracks generally at or near supports

of beams

overloading in shear thermal effects

ascertain shear capacity and compare with shear forces

compare span/depth ratio with Code requirments

ascertain existence or otherwise of uneven temperature gradient

presence of diagonal cracks in beams on the faces of and extending around the perimeter

of section

torsional shear stresses adequacy of designcause and magnitude of torsional moment

excessive deflection (damage to partitions/glazing below) in slabs and beams

inadequate depth overloading (long-term) fault in construction: tensile

reinforcement out o f position fault in design deterioration of materials

compare span/depth ratio with Code recommendations

ascertain calculated theoretical deflections

check loading history test concrete cover metre checks

cracking of slab and/or finishes over supports of slabs

slabs designed simply supported although continuous

anti-crack steel at supports inadequate o r incorrectly positioned

excessive relaxation of support moments in original design

cover meter checks for presence of ‘top’ steel examination of design

,

/

s. . - . t - . - / ■ * *. <■ y 'r}1 ?" ' * ~i ' i*>~!'M j£ ~ — - . » . . v .;

VISIBLE D E F E C T POSSIBLE CA U SE IN V ESTIG A TIO N S S U G G ESTED

random diagonal cracking or lateral cracking across bays at near-even spacing

inadequate provision for contraction/shrinkage over-rich concrete mix

check amount of reinforcement and spacing of joints analysis of concrete samples

breaking up or spalling of surfaces

excessive wear due to traffic (forklift trucks, steel-tyred vehicles, impact from mechanized plant)

poor quality concrete chemical attack due to spillage frost attack

ascertain plant manufactures loadings test and analyse concrete check use — history

wet and damp areas and deterioration of applied finishes

random cracking due to shrinkage of concrete inadequate daywork/expansion joints honeycombed or badly compacted concrete faulty filling of movement joints bad mix design inadequate reinforcement

examine spacing and detailing of joints cover metertest and analyse concrete

repetitive vertical fractures excessive spacing of movement joints shrinkage

examine details

local settlements combined with diagonal cracks poor compaction of sub-grade/inadequate reinforcement ground movement (due to water movement, erosion,

mining or other outside causes)

removal of small areas to investigate conditions below slab

BUILDING COMPONENT: Post-tensioned externally or internally stressed concrete beams

VISIBLE D E F E C T POSSIBLE CA U SE IN V ESTIG A TIO N S SU G G ESTED

cracking (1) or spalling of precast concrete with or without rust staining (2)

corrosion of secondary reinforcement check condition of secondary steel at cracked locations check cover, carbonation and chloride content of concrete if

corrosion is confirmedwhere chloride contents are high consider checking

tendon condition (especially if environment is wet) and seeking specialist advice

rust staining (2) below mortar covering of externally-stressed beams

corrosion of tendons ascertain location of corrosion and dampness

excessive deflections anchorage loose or fracturedseparation at joints in beams of segmental construction tendons visibledisturbance of mortar covering tendons of

externally-stressed beams shear cracking of diagonals of beam segments

fracture of tendons overloading inadequate prestress

structure may be unsafe; assess and consider closure and propping

investigate condition of other tendons if tendon fracture obvious check design against existing loading conditions

BUILDING COMPONENT: pretensioned c

VISIBLE D E F E C T

oncrete beams or columns

POSSIBLE CA U SE IN V ESTIG A TIO N S SU G G ESTED

transverse cracking of concrete (1) or excessive deflection overloading inadequate pretension

check design against existing loading conditions

cracking (1) or spalling of concrete with or without rust staining (2) (cracking generally parallel to to the direction of the steel)

corrosion of tendons or secondary steel reinforcement

check condition of embedded steel check cover, carbonation and chloride content of

concrete if corrosion is confirmed

bowing of columns distortion during erection creep and shrinkage of concrete fracture of tendon if concrete

cracked longitudinally

check conditions of loadingcheck adequacy, of designcheck tendon condition at cracked locations

Note (1) Very fine cracks may be of no significance in reinforced concrete structure but in prestressed concrete they indicate that conditions in the structure are substantially different fromthose assumed in design.

Brown rust staining on the concrete surface may be indicative of corrosion of embedded steel, but it may also arise from other causes such as iron-containing aggregate or

reinforcement tying wire.

_____ — — ..

BUILDING COMPONENT: Stone or precast concrete cladding

VISIBLE D E F E C T POSSIBLE CA U SE IN V ESTIG A TIO N S SU G G ESTED

spalling of vertical joints between stones and/or outward ‘bowing’ of stonework in the

horizontal direction

thermal movement provision (or otherwise) of expansion joints and their adequacy adequacy of restraint fixing adequacy of stonework behind fixing adequacy of support for stonework thickness, strength and durability of stonework used conditions of fixing cramps used and suitability of material

for purpose (i.e. ferrous, non-ferrous) evidence of stress corrosion (high tensile brass)

spalling of horizontal joints between stones and/or outward ‘bowing’ of stonework in the vertical direction

creep, shrinkage and eleastic shortening of building frame thermal movement

as aboveprovision (or otherwise) o f soft ‘compression’ joints

between stones at storey-height intervals and their adequacy

differential ‘face’ movement of one stone with another

fixing fault frost movement

failure or omission of fixingscondition of joints — likelihood of water penetration,

lack of drainage in ‘cavity’ behind stone

■t*

vertical fractures in stonework corroded structural steelwork or reinforcement in wall behind cladding

shrinkage thermal movement

removal o f one or more stones to ascertain cause

rust stains on surface, random occurence

units unreinforced likely to be iron bearing aggregate

if a problem analyse concrete

rust stains on surface of precast concrete units suggest a pattern

corrosion of reinforcementpossible use of calcium chloride in manufacture

check cover, carbonisation and analyse

\

BUILDING COMPONENT: Structural steelwork

VISIBLE D E F E C T

excessive deformations resulting from: out-of-plumb columns floors or supporting beams or

column joints not at correct levels

distorted or buckled members bowing of beams, ties and

bracings

lack of fit at connections

damage to windows or cladding

POSSIBLE CA U SE

poor fabrication or erection

overloadingwrong grade of steel, or size or

stiffness of members inadequate bracing or lateral

support poor fit, slip or failure of

supports or connections torsional effects not accounted for

in design unsatisfactory base or holding-

down material or grouting impact; due to vehicles, forklift

trucks, etc. elevated temperature effects not

allowed for in design

INV ESTIG A TIO N S SU G G ESTED

structure may be unsafe, therefore assess conditions and give consideration to closure and temporary support

check: original member designfor under-size and/or lack of stiffness;

existing load conditions against design and verify adequacy for load trans­ference;

effectiveness of connec­tions, i.e. bolts, rivets or welds;

line and level of beams and columns with original general arrangement;

foundations for inadequacy or movement

check whether structural arrange­ment is able to accommodate forces imposed by thermal movements

unacceptable flexibility of a complete structure or of indivi­dual members

effects from wind or rotating machinery not adequately allowed for in design

vortex shedding on circular structures such as chimneys (see also item above)

check when wind is not blowing or machines are not working

check if machinery is unbalanced or if anti-vibration mountings are deficient

member or members removed from a framework causing:

partial failure of remaining structure;

instability or overstress­ing of individual members;

collapse

slight waviness or localised buckling or bulging generally at or near points of support or concentrated loads

members removed by owner, maintenance engineers, local build, etc. to provide opening or

access

local yielding due to overload caused, e.g. by high shear or buckling through an inadequate bearing length

check effects of changed structural arrangement and assess if rein­statement of removed member is sufficient remedy

check actual load, shear and bearing capacities against applied loads

absence of welds poor appearance of welds

oversightunsatisfactory welding

test for adequacy

hairlines or slight gaps on edges of material often revealed by flame cutting (generally only critical in members or parts of members in compression), or near weldments

laminations lamellar tearing

non-destructive testing

surface appearance of fractured material which may vary from smooth to crystalline

cracks on surface of welds or heat-affected zones

cracks initiated at stress raisers such as punched holes or sheared or flame-cut edges not subse­quently machined or ground

poor weldingfatigue caused by repetition of

cyclic loading having reached the fatigue life of the structure or structural element,

brittle fracture, caused by low temperature combined with a tensile stress and notch effect, not allowed for in the design and selection of the steel

cause of metallurgical failure, indicated by the appearance of the fractured surface, is best determined by a expert.

slipping of joints and connections tearing or distortion of metal

adjacent to holes

overloadingincorrect type of fastener, e.g.

undersize, wrong grade, etc. inadequately or over tightened bolts flame cut or poor punched holes hole drifting during erection

check loading, size and effective­ness of fasteners

open holes indicating missing fasteners

rivets with loose or missing heads loose bolts, loose nuts (wrong size

nuts)bent or distorted fasteners uneven seating of head or nut

fasteners badly fitted or never installed at time of original erec­tion, through inadequate super­vision or poor workmanship

misaligned or misplaced holes failed fasteners because of over­

load or defective fastener material

check load adequacy and need to replace defective items

corrosion-varying between light rusting on previously protected surfaces to pronounced rusting on unprotected surfaces.

_ . . .

environmental attack presence of moisture failure of protective system

ascertain causeif moisutre is present — where is it coming from — can it be prevented? is structural performance likely to be afFected i.e. is sufficient sound material remaining to support the applied leading?

Note: corrosion may be detri­mental if present between surfaces of joints, due to the products of corrosion causing expansion.

BUILDING COMPONENT: Timber pitched and flat roofs

VISIBLE D E F E C T POSSIBLE CAUSE IN V ESTIG A TIO N S SU G G E ST E D

PITC H ED ROOFS FLA T RO O FS PIT C H E D RO O FS FLA T RO O FS

sagging of roof excessive span/depth ratio of roof joists

inadequacy of any main supporting beam or joist

change or roof covering or addition to existing covering

inadequate or lack of bracing heavy items on roof

such as water storage tanks, ventilating plant etc.

‘ponding’ (i.e. water lying) inherent defects in the actual structural timber such as wet/dry rot. splits shakes and insect infestation

determine size and spacing of roof joists and compare with span and

check adequacy and condition of any supporting beam(s) and that additional layer(s) of asphalt have not been provided since completion of construction

check for any ‘plant’ placed on the roof and the fall of roof and and any evidence of ‘ponding’

visual examination for rot or infestation

ridge sagging horizontal movement of feet of rafters

removal of purlins and/or struts, of wall in topmost storey supporting the feet of the struts, earring the purlins

recent change in the roof covering causing an increase in loading

examine verticality o f external walls in the topmost storey; extent and efficiency of ties provided between the feet o f rafters (i.e. ceiling joists); condition and spacing of purlins and struts; support for struts in topmost storey; roof for any recent changes in coverings

valley beam sagging removal or settlement of central support (usually timber stud partition)

valley beam split or rotten where built into external walls or because of leaking gutter

insect infestation

visual examination of support provided and its effectiveness

remove part of ceiling for visual examinationditto - particularly at ends where beam is built into walls

visual examination for beetles or wood boring insects, their holes and wood particles resulting therefrom

examine gutter

disturbance of tile coursing roof inadequately bracedexamine construction

---------------------- -- -------------------------------

1 i

VISIBLE D E F E C T POSSIBLE CA USE IN V ESTIG A TIO N S S U G G E ST E D

cracks in rendering, tiling or floor screeds

wall cracks, horizontal, up to about lm above floor level

cracks in asphalt roof finishes

if associated with cracks in parent structure see cracks brickwork in earlier table

if not associated with parent structure — differential shrinkage between finish nd structure

incompatible materials with ineffective bonding agent

deflection of floor supporting bottom courses of brickwork/blockwork

damp areas of plaster on inner leaf ofbrick/block wall:near ground level;adjacent to gills, lintels;behind precast concrete beams or lintels

thermal movement (plus shrinkage) of roof structure (some lightweight decks very susceptible)

torsional movement of areas of rectangular simply supported slabs relative vertical movement of adjacent precast units

(NB prestressed units may be more susceptible) crushing of insulation

blocked or partly filled cavityfaulty dpc , . ■ ,structural cracking due to one of the causes listed under brickworkfaulty detailing of dpc shrinkage causing opening of joints condensation burst pipes

remove local areas of finishes to inspect the structure: then process as for cracks in brickwork or concrete walls or concrete floors

check whether finishes properly bonded to structure: tapping will generally reveal ‘hollow’ areas)

ascertain expansion of roof under known environmental conditions examine

external walls for horizontal cracks inspect joints from below and check deflection of units inspect total construction

partial removal of brickwork to inspect dpc

investigate detail

Fire

APPENDIX 5

A5.1

A5.2

A5.2.1

A5.2.2

INTRODUCTIONBuilding fires, which normally reach temperatures of the order of 1000°C, can affect the loadbearing capacity of structural elements in a number of ways. Apart from such obvious effects as charring and spalling there can be a permanent loss of strength of the remaining material and expansion may cause damage in parts of the building not directly affected by the fire.

Criteria for the behaviour of structural elements exposed to fire are derived from the standard fire resistance test of BS 476: Part 880. This has been criticised in (at least) three ways: first, that in practice the variation of fire temperature with time may be different; secondly, that the effects of rapid cooling by fire hoses are not reproduced; and thirdly, that the test does not show the effects of structural restraint or composite action. These criticisms are, of course, of some importance when the structure is being designed. However, in this Appendix, the main emphasis is on estimating the residual load carrying capacity and hence assessing the remedial measures, if any, that are needed to restore the building to its original design for fire resistance and other requirements. Obviously, if some weaknesses in the original design have been exposed by a fire these should be put right.

EFFECTS OF HIGH TEMPERATURE ON MATERIAL PROPERTIES

GeneralThe effects of temperature on thermal, strength and deformation properties are published in a range of papers and an excellent summary is given by Lie81. It should be noted that building materials, except timber, heated above 250°C are likely to show significant loss of strength which may or may not be recovered after cooling.

ConcreteFor concrete, the compressive strength varies not only with temperature but also with a number of other factors including the rate of heating, the duration of heating, whether the specimen was loaded or not, the type and size of aggregate, percentage of cement paste and the water/cement ratio. In general it may be said that concrete heated by a building fire always loses some compressive strength and continues to lose it on cooling. However, where the temperature has not exceeded 300°C most of the strength will eventually be recovered.8182

Because of the comparatively low thermal diffusivity of concrete (of order 1 mm2/s) the 300°C contour may be at only a small depth below the heated face. The elastic modulus also decreases with temperature, although it is believed that it will recover substantially

86. BS 476: Fire tests on building materials and structures. P a rt 18. 1972 Test methods and criteria fo r the fire resistance o f elements o f building construction, British Standards Institution, London

81. Lie. T .T . : Fire a n d bu ild ings. A pp l ied

S cien ce Pub li she rs Ltd. . Lon d o n , 1972

82. G r e e n . J .K .: 'R e in s ta te m e n t of concre te

st ru ctures a f ter f ; r e \ A rch i tec ts Journa l,14 a n d 21 Ju ly 1971

50

A5.2.3

A5.2.4

with time provided that the concrete has not been heated above 500°C. The coefficient of thermal expansion of the concrete is of the order of 1(T5 per °C but varies with aggregate. Creep becomes significant at quite low temperatures, being of orders 10-4 to 10~3 per hour over the temperature range 250°C to 700°C, and can have a beneficial effect in relaxing stresses. More details of the properties of concrete exposed to fire are given in the Report83 of a Joint Committee of the Institution of Structural Engineers and the Concrete Society.

SteelF or steel, the yield strength is reduced to about half at 550°C ,and at 1000°C it is 10% or less. Because of its high thermal conductivity the temperature of unprotected internal steelwork will normally be little different from the fire temperature so that structural steelwork is usually insulated. A part from losing practically all its loadbearing capacity unprotected steelwork can give considerable expansion, the coefficient being of order 10~5 per °C. Young’s modulus does not decrease with temperature so rapidly as yield strength.

Cold-worked bars, when heated, lose their strength more rapidly than hot-rolled high- yield bars and mild-steel bars. The properties after heating are of interest from the point of view of reinstatement. The original yield stress is almost completely recovered on cooling from temperatures of500°-600°C for all bars, but on cooling from 800°C it is reduced by 30% for cold-worked bars and 5% for hot-rolled b a rs . '

F o r prestressing steels, the loss of strength occurs at lower temperatures than for reinforcing bars. Cold-drawn and heat-treated steels lose a part of their strength permanently when heated to temperatures in excess o f about 300°C and 400°C, respectively.

The creep rate* of steel is very sensitive to higher temperatures; it becomes significant for mild steel above 450°C and for prestressing steel above 300°C. In fire resistance tests, the rate of temperature rise when the steel is reaching its critical temperature is fast enough to mask any effects of creep, but where there may be a long cooling-down period, as in prestressed concrete, subsequent creep may have some effect in an element .hat has not reached the critical condition.

Further information on the properties of steel exposed to high temperature is available from the British Steel Corporation Teesside Laboratories.

TimberTimber ‘browns’ at about 120°-150°C, ‘blackens’ around 200°-250°C and evolves combustible vapours at about 300°C. Above a certain temperature 400°—450°C, or 300°C if a flame is present) the surface of the timber will ignite and char at a steady rate. A ny charred part o f a section must be assumed to have lost all strength, but any timber beneath the charred layer may be assumed to have no significant loss of strength because the thermal conductivity of timber is low. The T able below gives notional rates of charring.84

Table: Notional rate of charring for the calculation of residual section

SpeciesChat

30 minTing in

60 min

mm mm

(a ) all structural species listed in Table 1 of CP 112: Part 2: 20 40

1971,85 except those noted in items (b ) and (c)

(b ) western red cedar 25 50

(c) Oak, utile, keruing (gurjun), teak, greenheart. jarrah 15 30

A column that is exposed to the fire on all faces should be assumed to char equally on all faces 1.25 times faster than the rates given in the Table. Reference should be made to BS 5268: Part 4 for further details84.

•The creep rate referred to is secondary period creep.

83. F ire resistance o f concrete structures. Report o f a jo in t Committee o f the Institu­tion o f Structural Engineers and the Concrete Society, Institution o f Structural Engineers, London 1975

84. BS 5268: Code o f practice fo r the structural useo ftim ber, Part 4: 1978, Fire resistance o f tim ber structures, British Standards Institution, London

85. C P 112: The stm ctura l use o f timber, Part 2 :1971 , Metric units, British Standards Institution, London

51

A5.2.5 BrickworkThe physical properties and mechanisms of failure of brickwalls exposed to fire have not been analysed in detail. As with concrete there is a loss of compressive strength, unequal thermal expansion of the two faces, and behaviour is influenced by edge conditions86'87. For solid bricks, fire resistance is directly proportional to thickness. Perforated bricks and hollow clay units are more sensitive to thermal shock; there can be cracking of the connecting webs and tendancy for the ‘leaves’ to separate. With cavity walls the inner leaf carries the major part of the load. They can be subjected to more severe forces by heated and expanding floor slabs than an internal wall surrounded by structure. All types of bricks give much better performance if plaster is applied, thus giving improved insulation and reduction of thermal shock.

A5.3 ESTIMATION OF FIRE BEHAVIOURIt is useful to estimate the maximum temperature attained in building fires. Parker and Nurse88 give the following guidance: moulded glass objects soften or flow at 700° or 800°C. Metals form drops or have sharp edges rounded as follows: 300°— 350°C for lead, 400°C for zinc, 650°C for aluminium and alloys, 950°C for silver, 900° to 1000°C for brass, 1000°C for bronze, 1100°C for copper and 1100° to 1200°C for cast iron. There are also the well known colour changes in concrete or mortar investigated by Bessey89. The development of the red or pink colouration in concrete or mortar containing natural sands or aggregates of appreciable iron oxide content occurs at 250°— 300°C, and normally 300°C may be taken as the transition temperature. For the standard fire and various periods of heating Bessey gives the following results for slabs:

Heatingperiod

h

Maximu-risurface

temperature'

°C

M aximum depth of concrete showing characteristic change, mm

Pink or red

fading of red, friability

buff sintering

300°C 600°C 950°C 1200°C

1 950 56 19 0 0

2 1050 100 38 6 0

4 1230 140 63 25 3

6 1250 170 90 38 6

The temperature within a slab may continue to rise after the fire is ended, and some of the above maxima were attained after the end of the heating period.

A5.4 ANALYSIS AND REPAIR OF CONCRETE STRUCTURES

A5.4.1 Effective cross-sectionRemoval of the surface material down to the red boundary will reveal the remaining cross-section that can be deemed effective. Compression tests of cores can indicate the strength of this concrete, giving a value to be adopted in calculations.

A5.4.2 CracksM ost fine cracks are confined to the surface. Major cracks that could influence structural behaviour are generally obvious. A wide crack or cracks near supports may mean there has been a loss of anchorage of the reinforcement.

A5.4.3 ReinforcementProvided that mild steel or hot-rolled high-yield steel is undistorted and has not reached a temperature above about 800°C, it may be assumed to have resumed its original properties, but cold-worked bars will have suffered some permanent loss.

A5.4.4 Prestressing steelIt is likely that prestressing steel will have lost some strength, particularly if it has reached temperatures over 400°C. There will also be a loss of tensile stress. These effects can be assessed for the estimated maximum temperature attained.

86. M alhotra, H.L.: Fire resistance of brick and block walls, Fire note no. 6 , HM SO, London 1966

87, F isher, K.: ‘The performance of brick­work in fire resistance tests, Jubilee con­ference — Structural design for fire re­sistance, 9 - 1 1 September 1 9 7 5 ,Midlands Branch, Institution of Structural Engineers

88. Parker, T .W ., and Nurse, R.W .: The estim ation o f the m axim um tem perature attained in buildingfires from examination o f the debris, Investigations on building fires Part 1, National Building Studies technical paper no, 4 , HM SO , London, 1950

89. Bessey. G .E: T he v isib le changes in concrete or m ortar exposed to high tem per­atures, ibid. P art 2

52

?*T“

A5.4.5

A5.5

A5.6

A5.7

A5.8

Remedial measuresIn some situations, replacement of a damaged member may be the most practical and economic solution. Elsewhere, reinstatement will be justified in order to avoid inconvenience and loss or damage to other structural members.

W here new members are connected to existing ones, monolithic action must be ensured; this calls for careful preparation of the concrete surfaces and continuity of steel. F or reinstatement, the removal of all loose friable concrete is essential, to ensure adequate bonding. E xtra reinforcement should be fixed by experienced welders.

New concrete may be placed either by casting with formwork or by the gunite method;90 with the latter it may be possible to avoid increasing the original dimensions of the member. The choice of method will depend on the thickness of the new concrete, the surface finish required, the possibility of placing and compacting concrete in the formwork and the degree of importance attached to an increase in size of section.

Large cracks can be sealed by injection o f a latex solution or resin. Various washes or paints are available to restore the appearance of finely cracked or crazed surfaces.

Further guidance is given in reference 91. A n interesting analysis of fire effects and design guidance is given by Kordina et a f 1.

ANALYSIS AND REPAIR OF STEEL STRUCTURESIn general, a structural steel member remaining in place, with negligible or minor distortions to the web, flanges or end connections should be considered satisfactory for further service. The exception will be for the relatively small number of structures in cold-worked or tempered steel where there may be permanent loss of strength. The change in strength may be assessed using estimates of the maximum temperatures attained or on-site tests; if necessary, the steel should be replaced. Microscopy can be used to determine changes in microstructure. Since this is a specialized field, the services of a metallurgist are essential.

ANALYSIS AND REPAIR OF BRICK STRUCTURESIt will be possible to determine from the colour change of the mortar or of the bricks themselves the degree of heating of the wall. F or solid brickwalls without undue distortion the portion beyond the pink or red boundary may be considered as serviceable and calculations made accordingly. Perforated and hollow brick walls should be inspected carefully for the effects of thermal shock and may need replacement. Plastered bricks sometimes suffer little damage and may need replace­ment of the surface treatment only.

ANALYSIS AND REPAIR OF TIMBER STRUCTURESBS 5268: Part 4: Section 4.1 can be used to analyse the residual strength of members that have been subjected to a fire. Generally, any wood that has not charred could be considered to have full strength. It may be possible to show by calculation that a timber section or structure which has been subjected to fire still has adequate strength once the affected surfaces are cleaned off (by planing, sanding etc.). W here additional strength is required it may be possible to provide this by gluing on strengthening pieces or otherwise replacing lost strength. Joints that may have opened and metal fixings that may have conducted heat to the interior are points of weakness that should be carefully examined.

TEST LOADINGAnalysis of the damage and assessment ofthe necessary repairs may be possible within a reasonable degree of accuracy, but final acceptance may depend on proof by load test. Performance is generally judged in terms of recovery of deflection after removal of the load.

90. G reen . J .K ., and Long, W .B.: ‘Gunite repairs to fire damaged concrete structures’. Concrete. 1971

91. A ssessm ent o ffire -d a m a g ed concrete structures an d repair by gunite , Concrete Society Technical report no. 15, London, 1978

92. Kordina, K., Krampf, L , and Seiler, H .F .: 'An exam ination o fth e effects o f a big f ire in som e concrete buildings', F ire Pre­vention Science a n d Technology no. 14., (reprinted from Beton and Stahibetonbau, 67, nos. 5 and 6, p. 108 and p. 129

53

Methods of test APPENDIX 6

T1 Cover meter

This is a simple electromagnetic technique for measuring the depth to the steel and to determine its orientation and distribution. Response differs for different diameter bars and will be influenced by multiple layers and or close pitch. M odem equipment is claimed to identify the diameter of bar. Particular care is needed where the meter is used on lightweight concrete or concrete containing crushed rock aggregate.

T2 Radiographic techniques

y - and X -rays may be used to examine the interior of relatively thick concrete members to check for the presence of voids, poor compaction, continuity of grouting in prestressing ducts, layout of reinforcement, etc. It is an expensive specialist technique suitable for the survey of relatively small areas of concrete. Special care has to be taken to ensure personnel are not exposed to harmful radiation. The test is probably best used in conjunction with other non-destructive tests.

Radiographic techniques may also be used to determine the quality and integrity of steel, i.e. to ascertain the presence and degree of cracks, laminations, porosity and inclusions, both in parent materials and welds, y - and X -rays will highlight defects and lack of continuity (see also T l) .

T3 Electrical techniques

M easurement of the electric potential between parts of a concrete member can be used to give an indication of corrosion of the reinforcement. Similarly, the resistivity of the concrete is lowered by the presence of corroding steel. Neither technique will give an accurate measure of the degree of corrosion.

T4 Endoprobe

Prestressing tendons in ungrouted or poorly grouted ducts can be inspected using this device. A small diameter hole (say 10 mm) must be drilled through the concrete and into the duct: extreme care will be necessary to avoid damaging the prestressing steei. An endoprobe (or similar device) may also be used to inspect the integrity of wall ties.

T5 Ultrasonics

(a) ConcreteThe quality of concrete can be assessed by measuring the velocity of ultrasonic pulses through it. The method may be used to determine the presence of voids, cracks or other imperfections in a member, and to compare the strength of the concrete in different

BS 4408: R ecom m endations fo r non­destructive m ethods o f tex t fo r concrete. Part 1: 1969, Electrom agnetic cover measuring devices, British Standards Institution, London

BS4408: R ecom m endations fo r non-des­tructive m ethods o f test fo r concrete, Part 3: 1970, G am m a radiography o f concrete, British Standards Institution, London

BS 3683: Glossary o f term s used in non­destructive testing. Part 3: 1964 R adio ­logical f la w detection, British Standards Institution, London

BS 2600 : 1973: M ethods fo r radiographic exam ination o f fu sion welded butt jo in ts in steel, British Standards institution, London

G erw ety ,M .W .,ei al: Causes and repair o f deterioration to a California bridge due to corrosion o f reinforcing steel in a marine environment. Highway Research Board Bulletin no 182, California. 1957

BS 4408: Recom m endations fo r non­destructive methods o f test fo r concrete. Part 5: 1974, M easurem ent o f the velocity o f ultrasonic puise velocities in concrete, British Standards Institution. London

BS 3683: Glossary o f terms used in non­destructive testing. Part 4 :1976 Ultrasonic f la w detection, British Standards Institu­tion, London

54

members or along a given member. Unless the instrument can be calibrated against concrete of known strength, it cannot be used to give an accurate measure of the strength of the concrete in the member. The technique requires precise measurement of the distance between the two ultrasonic heads. It is suitable for use only by experienced persons.

(b) Steel and other metalsF or steel and other metals, ultrasonics, while providing similar information to radiographic techniques, will, in addition, indicate the presence o f laminations in parent materials. Lamellar tearing will also be revealed.

T6 Rebound hammer

This is a simple technique for assessing the quality of concrete by testing its surface hardness. Strength can be determined by calibrating against test cubes. As the impact on the surface of the concrete is over a small area the readings are susceptible to local variations. A t least nine readings should be taken over a given area to obtain a representative mean.

T7 Coring

Standard cores of 100 or 150 mm diameter may be cut from suspect concrete for measurement o f the actual strength and density. They may also be used to indicate the distribution of materials in the concrete, the concrete quality (voids, honeycombing, etc.) and may be used to obtain a measure of the shrinkage and absorption properties of the concrete. As they must be drilled to a depth of at least 150 mm they are likely to pass through the reinforcement: the cores can thus be used to give an accurate measure of the cover to the reinforcement and to determine the type and size of steel used. The consequence of cutting the reinforcement should be considered.

F or situations where standard cores cannot be obtained, for example in small beams, smaller cores can be taken. The results obtained should be subjected to specialist interpretation since the relationships to cube strength will not be the same as for standard cores.

T8 Internal fracture test for concrete

This test (sometimes called ‘pull-out test’) measures the strength of concrete by pulling on an anchor in the concrete until internal fracture occurs. It is a simple technique requiring only light equipment. Unlike the two proceeding tests it results in some slight damage to the concrete, but this will require only minor cosmetic repair work. I t is unlike core cutting (see T7).

T9 Detection of cracks using ultraviolet light

Very fine cracks in concrete may be detected using ultraviolet light if the surface is specially treated with a fluorescent flow detector. Extreme care must be taken in interpreting the observed pattern to differentiate between shrinkage cracks and those caused by applied loads.

T10 Absorption

(a) Absorption testTests are made on small cores, 75 mm in diameter, cut from the concrete. Considerable care needs to be taken in making these tests. Absorption limits for concrete at different ages are specified in some British Standards for precast concrete products.

(b) Initial surface absorption test (ISAT)This test measures the surface absorption of the concrete. Again considerable care needs to be taken. It does not damage the structure but is most suitable for comparative purposes. The results are affected by the nature o f the surface, and there are little data on tests on in situ concrete.

T il Cement content and cement/aggregate ratio

The determination of the cement content o f a hardened concrete requires the facilities of an analytical laboratory. The techniques used depend on whether or not the aggregate grading and content is to be established as well as the cement content, and the type of cement aggregate used. If, by chance, samples of the materials used in the concrete are still available the inherent errors in concrete analysis are considerably reduced compared with those in the results when assumptions about the materials have

D D 21 :1 9 7 2 Q uality grading o f steel p la te fro m 12 m m to 150 m m th ick by m eans o f ultrasonic testing, British Standards Instit­ution, London

BS 2704: 1978: Specifica tion fo r calibra­tion b lo cksfo ru se in u ltrasonic f la w detec­tion, British Standards Institution, London

BS 3923: M ethodsforu ltrason ic exam ina- tin o f welds, Part 1: 1978 M a n u a l exam ­ination o f welds in ferritic steels , Part 2; 1972 A utom atic exam ina tion o f fu sio n welded butt jo in ts in ferritic steels, and Part 3: 1972 M a n u a l exam ina tion o f nozzle welds, British Standards Institution, London

BS 3889: M ethods fo r non-destructive testing o f pipes a n d tubes, P art I A: 1965 Ultrasonic testing o f ferro u s p ip es (ex ­cluding cast), British Standards Insitution. London

BS 4124: Non-destructive testing o f steel forgings, Part 1: 1967 U ltrasonic f la w detection, British Standards Institution, London

BS 4408: Recom m endations fo r non- destrutive m ethods o ftestforconcrete, Part 4: 1971 Surface hardness measurements. British Standards Institution, London

BS 1881: M ethods o f testing concrete, Part 4: 1970 M ethods o f testing concrete fo r strength, British Standards Institution, London

Concrete core testingforstrength, Concrete Society Technical Report no . 11, London, M ay 1976

Chabowski, A .J., and Bryden-Smith, D ..A sim ple pull-ou t test to assess the in-situ strength o f concrete, Precast Concrete, M ay 1971, p. 243 (reprinted as BRE Curren Paper C P 25/77)

BS 1881: M ethods o f testing concrete. Part 5: 1970 M ethods o f testing hardened concrete fo r other than strength, British Standards Institution, London

Figgs, J.W ., and Bowden, S.R.: The analysisofconcrete, HM SO ,London, 1971

Determ ination o f chloride and cement content in hardened P ortland cement concrete, BRE leaflet IS 13/77

55

to be made.

Requirements for tests are indicated by suspected low strength, poor durability, permeability and concrete failure.

T12 Water/cement ratio

W hen the cement content has been determined, a knowledge of the original water content enables the water/cement ratio to be calculated and an estimate of the concrete strength obtained. A comparison can also be made with the original mix design if known. The methods of determining the original water content are usually based on saturation techniques and will give only approximate answers. The methods cannot be used with damaged, poorly compacted or aerated concretes.

Requirement for tests are indicated by suspected low strength, poor durability, permeability, concrete failure.

T13 Admixtures and contaminants

There are many varieties of admixtures that could have been used in a concrete, but the most likely one to be considered in a structural appraisal would be the presence of chloride. The amount in a sample can be easily determined in the laboratory or a close estimate can be obtained by a field test with strip indicators. The presence of sulphate is another factor that could be considered in an appraisal. Again this can be determined quite readily in the laboratory.

Other admixtures and contaminants, e.g. organic admixtures, sugars, metals, etc., can be determined by laboratory techniques such as X -ray fluorescent spectroscopy, infra­red absorption, etc.

Requirement for tests are indicated by concrete failure, e.g. cracking and spalling, corrosion of steel, etc.

T14 Free lime content — depth of carbonation

The alkaline environment in concrete acts as an inhibitor to the rusting of embedded steel, and if corrosion has taken place or is suspected, it may be necessary to find out whether or not free lime is present and also the depth to which the concrete has been affected by carbonation from the exposed surfaces. Determination of free lime in concrete requires a laboratory technique if quantative results are needed, but for an indication of the depth of carbonation and lack of an alkaline environment, a newly broken surface can be treated with a phenolphthalein solution when a purple red colouration will appears where the concrete is still alkaline.

Requirement for tests is indicated by concrete failure, e.g. cracking and spalling by corrosion of steel etc.

T15 Type of cement

M ost concretes use the Portland type of cement — ordinary, rapid-hardening, sulphate- resisting or low-heat — and these are not normally distinguishable by the analysis of concrete. A compelte chemical analysis of the fine material from a sample of concrete can be compared with typical analyses of various types of cement, and also a petrological examination can be helpful. Portland blastfurnace cement and high- alumina cement may be distinguishable visually by the colour o f the matrix, but this may also be affected by the aggregate used, and extreme care is necessary.

Requirements for tests are indicated by suspected high-alumina cement, sulphate- resisting cement specified and concrete failure.

T16 Type of aggregate

The types of aggregates used in concrete under examination may be immediately apparent, but if not then a petrological examination of a cut slice will identify them. It may also be necessary to examine the aggregate for alkali reactivity.

Requirements for tests axe indicated by concrete failure, cracking and spalling, staining of surface, etc.

T17 Optical methods

To verify dimensions of members visual measurements obtainable with rules, tapes,

Figgs, J .W ., and Bowden, S.R.; The ana lysis o f concrete. H M S O , London 1977

BS 1881: M ethods o f testing concrete. Part 5: 1970 M ethods o f testing hardened concrete fo r o ther than strength, British Standards Insitution, London

BS 1881: M ethods o f testing concrete, Part 6: 1971 A n a lysis o f hardened concrete, British Standards Institution, London

Figgs, J .W ., a n d Bowden. S .R .: The analysis o f concrete, H M S O , London 1977

Sim plified m ethod fo r the detection and determ ination o f chloride in hardened concrete, BRE leaflet IS 12/77

BS 1881: M ethods o f testing concrete. Part 6: 1971 A n a lysis o f hardened concrete, British Standards Institution, London

Figgs. J .W ., a n d Bowden, S .R .: The analysis o f concrete, H M S O , London, 1977

Carbonation o f concrete, BRE leaflet (to be published)

Figgs. J .W .. an d Bowden. S .R .: The analysis o f concrete, H M SO . London. 1977

B S 1881: M ethods o f testing concrete, Part 6: 1971 A n a lysis o f hardened concrete, British Standards Institution. London

R apid chem ical test fo r the detection o f high-alum ina cement concrete. BRE leafletIS 15/74

Figgs. J.W ., and Bowden, S .R .: The analysis o f concrete. H M SO . London. 1977

A n n u a l book o f A S T M standards Part 14: Potential reactivity o f aggregates (chem ical method). C 289-71 , 1976

BS 1881: M ethods o f testing concrete. Part 6: 1971 A n a lysis o f hardened concrete, British Standards Institution, London

Figgs. J .W ., a n d Bowden, S .R .: The analysis o f concrete, H M SO . London 1977

56

callipers and even micrometers will usually suffice. Sophisticated optical methods are available however, such as lasers, X - and y-rays, which may be used to check on dimensions (but see T2).

T18 Dye penetrants

These methods will provide information on surface condition of steel in regard to presence of cracks and surfaces imperfections, particularly when associated with

welding.

T19 Magnetic-particle crack detection

This is an important inspection tool used during fabrication and erection of steelwork. Although it is considered to be a simple technique for smooth flat-plate surfaces with flush welds it is less simple with more complex structural arrangements, and is sometimes not applied effectively.

T20 Tensile tests

The testing of steel by measurement o f the tensile load required to rupture standard size specimens is most probably the oldest form of test used for the selection o f materials. This type of test is more widely employed than any other single test and is frequently used to obtain measurements o f the ductility of the material in addition to the tensile strength. Other properties obtained include elastic limit, yield point, proof stress and modulus of elasticity. Standard size test pieces are usually required whose cross- section may be circular, square, rectangular or in special cases of some other form. The test pieces should generally be machined to the dimensions given in the various Standards, but some sections, bars, tubes, etc. may in certain circumstances be tested without being machined.

T21 Impact tests

Both standard tests — the Izod (cantilever) and Charpy (beam) — measure the energy required to fracture a standard notched specimen with a blow from a pendulum. The Charpy test is the more versatile as it enables results to be obtained over a range of temperatures. The results enable comparisons of the notch ductilities of different materials to be made and to ensure compliance with standard requirements for adequate resistance to brittle failure obtained from service experience on actual steel structures,

T22 Bend tests

This is used to determine the ductility of steel. Steel that has been shaped into its final form by bending must have ductility, and therefore bend tests are useful for any subsequent investigation. Typical examples are reinforcement bars for concrete and sheet and strip formed into profile shapes. The test piece must withstand, without fracture, being bent round set formers through a given angle (generally 90° or 180°). Some standards call for reverse bend tests, and in these cases the test specimen is bent through a given angle and then back to the original position, which constitutes one bend test.

T23 Hardness tests

The hardness number of a material is determined from the size o f an indentation made on its surface, whose properties may differ from those within the body of the material. An empirical relation exists for steels between the hardness number and ultimate (not yield) strength which should be regarded as a guide only. If a strength is required for calculations a tensile test (T20) should be made wherever possible. The standard laboratory hardness tests are the Brinell (ball), Vickers (pyramid diamond) and Rockwell (ball or diamond cone); these give much more reliable results than on-site tests using portable testing apparatus. The ranges of tensile strengths of the various grades of structural steel specified in BS 4360 overlap, so that it may not be possible to identify positively the grade of the steel from its hardness number. Because of these limitations, the cost of exposing a sample in a structure (e.g. steel reinforcing bars in concrete) and the subsequent reinstatement should be carefully considered against the value of the results to be obtained.

T24 Metallography

Metallographic examination can give fuller information on the internal structure of the

BS 3683: G lossary o f term s used in non­destructive testing, P art 1; 1963 Penetrant f la w detection, British Standards Institution, London

BS 3889: M ethods o f non-destructive testing o f p ipes an d tubes, P art 3A: Penetrant testing o f ferrous p ip es an d tubes, British Standards Institution, London

BS 4124: N on-destructive testing o f steel forgings. Part 3: Penetrant f la w detection, British Standards Institution, London

BS 4416: 1969 M ethods fo r penetrant testing o fw elded or braced jo in ts in metals, British Standards Institution, London

BS 3683; G lossary o f term s used in non­destructive testing, P art 2: 1963 M agnetic particle fla w detection. British Standards Institution, London

BS 3889: M ethods ofnon-destructive testing o f p ipes a n d tubes, P art4A ; 1965 M agnetic particle fla w detection: ferrous pipes and tubes, British Standards Institution, London

BS 4124: N on-destructive testing o f steel forgings, P art 2: 1968 M agnetic particle f la w detection, British Standards Institution, London

BS 4397: 1969 M ethods fo r m agnetic particle testing o f welds, British Standards Institution, London

BS 18: M ethods,for tensile testing ofm etals, Part 2; 1972 S tee l (general), P art 3: 1971 S tee l sheet an d strip (less than 3 m m an d not less than 0 .5 m m thick), and P art 4: 1971 S teel tubes, British Standards Institu­tion, London

BS 1639:1964 M ethods fo r bend tes tin g o f metals, British Standards Institution, London

BS 240: M ethod fo r Brinell hardness test, P art 1: 1962 Testing o f metals, British Standards Institution, London

BS 4 2 7 -.Methods fo r Vickers hardness test, Part 1: 1961 Testing o f metals, British Standards Institution, London

BS 709: 1971 M ethods fo r testing fu sio n welded jo in ts a n d weld m etal in steel, British Standards Institution, London

BS 860: 1967 Tables fo r comparison o f hardness scales, British Standards Institu­tion, London

BS 891: M ethod fo r Rockw ell hardness test. P art 1; 1962 Testing o f metals, and Part 2: 1964 Verification o f the testing machine, British Standards Institution, London

BS 417 5 M ethod fo r Rockw ell superficial harness test (N a n d TSca les). P art 1:1967 Testing o f metals, British Standards Institution, London

57

material; for instance it might indicate some chemical segregation that would explain anomalous response to welding.

T25 Drillings and spectrographic analysis — ‘spot’ tests

These tests will enable material composition to be ascertained. A small amount of drillings can usually be taken from non-critical areas in most steel plates, sections and, with care, even reinforcement. These can be submitted for full chemical analysis by wet methods, or for selective analysis by spectrography.

In situ ‘spot’ tests of an elementary nature can sometimes identify some elements, but this is a crude method and not recommended.

T26 The crack opening displacement test

BS 5762: 1977 M ethods fo r crack opening displacem ent (C O D ) testing, British Standards Institution, London

BS 5447: 1977 M ethods o f test fo r plane strain fracture toughness (K o f metallic materials. British Standards Institution, London

T27 Wrought iron and cast iron

The presence of wrought iron and cast iron should be expected in all 19th Century structures. The structural use of these materials diminished with the introduction of steel in the 1880s and effectively ceased by the 1930s.

(a) W rought ironAlthough not easily distinguishable chemically from a low-carbon steel, wrought iron may readily be identified by its characteristic laminated structure. I f a small patch is ground on the surface to remove scale and expose the clean metal, which is then polished with fine abrasive paper, the filaments of slag, which are visible through a hand magnifying glass, will identify the material as wrought iron. In fonged elements — such as clevises, brackets, eye-bars — the direction of the lamination should be checked to identify areas in which tension stress occurs normal to the plane of lamination.

Forge welding was a common practice in fabricating structural elements. The presence of welds in, for example, eye-bar ends, large bars with threaded ends, or where there is a marked change in section, should be suspected and non-destructive tests made to verify the soundness of the weld. A s the laminations o f wrought iron constitute internal flaws, crack detection techniques are of little use in determining its integrity. Visual examination, if possible supplemented by tension test samples, must be used. The surface of the sample should also be examined for stamp marks that identify its quality.

(b) Cast ironCast iron may sometimes be identified by the apearance on its surface of evidence of the casting process — sand m arks, mould joints. The reaction of the metal to cutting with a sharp file will normally distinguish it from steel. I t may also be identified by drilling, which will result in fragmented swarf. Beams with unequal flanges, or of unusually thick section, should be suspected as being made of cast iron.

Cast-iron members must be examined for cracks and for soundness (i.e. casting defects) by any of the N D T methods previously described. Additionally, dimensional accuracy (e.g. constancy of wall thickness) should be checked.

The objective of this test is to determine the value of the critical crack opening displacement at the tip o f the defect at the onset of stable and or unstable crck extension, displacement at the tip o f the defect at the onset o f stable and/or unstable crack extension. This test is not suitable for general application in structural appraisal, but in exceptional circumstances, the engineer may wish to seek specialist advice on its use.

T28 Cables

The comments below refer to structural cables and not to those used in lifting gear or other mechanical equipment.If the original specification is available a check should be made to see if the cable was prestretched (normally by the manufacturer). Subsequent stretching in service, which may be checked by measuring the sag or alignment o f adjacent structural elements and comparison with the original design, will normally indicate significant damage.

Appraisal is generally by visual examination, bearing in mind that:

• damage is most likely to occur at the ends (terminations), the outer wires generally failing before the inner ones

• examination of the ends for signs of relative movement between the cable and socket or end connection, which indicates pulling out, is necessary

58

; ) ■

corrosion is more likely to occur at the lower end of a cable caused by rainwater running down it; particular attention should be paid to the cable/socket junction where there is frequently a groove in which water can collect

if the outside of a cable is painted the rpesence of a broken wire in the outer lay will be indicated by a spiral crack in the paint caused by the relative movement between adjacent wires when the tension in the broken wire is released

areas where the lay of the cable or individual strands are disturbed are potential failure sites

T29 Mechanical properties of timber

To determine the mechanical properties of timber the engineer should first try to establish the stress grade of the timber. W ith machine stress graded timber or timber v i s u a l l y stress graded to BS 4978, BS 5756 or the Canadian N L G A rules the tim beror component should have been marked. If no marking is visible the engineer should measure defects and relate this to BS 4978 to establish the stress grade. Reference should then bd made to CP 112 for permissible stresses.

To arrive at the permissible stress of plywood the type of plywood should be established (either by reference to makers or to specialist organisations) and reference made to CP

112 .

T 30 Moisture content of timber

Authority, Vancouver, 1 July 1978

The moisture content of solid untreated timber can be determined usually with standard grading rules fo r Canadian

sufficient acuracy by a portable battery-operated moisture meter. D eep probes should lumber■ Natl0nal Lumber Gradespreferably be used, and the moisture should be checked at several points including, in particular, those where ventilation is poor. W hen using a moisture meter to take moisture readings of plywood or preserved timber, reference should be made to a correction factor for the particular meter being used. In extreme cases it may be necessary to remove part of the timber for a more accurate test, drying the timber in an oven.

If the moisture content is about 15-16% it may prove valuable to check the temperature and humidity of the air (with a dry bulb/wet bulb hygrometer) to relate (by available tables) to the moisture content.

T31 Identification of chemical preservatives

A green tinge to the timber would suggest copper/chrome/arsenate salts had been used.M ost reputable manufacturers of preservatives have a small kit that can be used to check if their preservative has been applied. However if the engineer is not aware which preservative may have been used it may be necessary to send a small sample to a laboratory or to call in an expert.

T32 Identification of insect attack

There are two main pests that infect constructional timber — the common furniture beetle and the house longhorn beetle. The latter is prevalent only in a small area of south-east England.

It is necessary to distinguish between live attack, attack that has died out and emergency holes associated with attack on the timber in the forest, e.g. pinhole borer (Ambrosia beetle) which dies out on conversion.

It is recommended that identification is carried out by a specialist, but the engineer should be able to distinguish live attack by evidence of bore dust around the exit holes.

T33 Identification of dry rot/wet rot

The early signs of fungal attack are not easy to spot without specialist laboratory equipment. However, one can forecast with some degree of certainty that given a sufficiently long exposure to damp conditions any unpreserved perishable wood will decay. Therefore, the conditions are easier to identify than the fungus in the early stages, i.e. prolonged moisture content in excess of 20%.

The two main types of fungii — dry rot and wet rot — are easier to diagnose in the later

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stages of development when the fruiting body appears on the surface of the timber.

Earlv signs of dry rot are cracks along the grain, and a sharp pointed knife can easily be pushed into the tim ber. D ry rot usually gives off a smell of mouldy cheese.

W et rot will continue to develop only on timber that is wet, whereas dry rot having established itself on wet timber will spread to otherwise sound dry timber.

The treatment of the two types of decay are different, and therefore, accurate

identification is important.

T34 Identification of glues

Although a chemical analysis would be necessary to establish exactly the glue that had been used there are guides that the engineer can use. I f the glue is dark brown it is likely that the glue used is weather and boil proof glue such as resorcinol. If it is white then it could be either moisture resistant, such as a urea glue, or an ‘interior glue such as

casein.

T35 Identification of type of plastics

A variety of rigid plastics materials are used in construction including poly- vinylchloride, acrylonitrile butadiene styrene, acrylics, polystyrene, polypropylene, polyesters and epoxides. Tests to identify whether the plastics is a rubber, a flexible thermoplastic, a rigid thermoplastic or a thermosetting plastic are by appearance, bounce, odour, feel, colour, specific gravrty and the Beilstein test which detects halogens Simple heating tests can often determine the class of plastics and are usually carried out before an elemental test for nitrogen, sulphur, chlorine, bromine and fluorine. Infrared spectrographic analysis can also be of use in identifying plastics and

their degradation products.

Requirement for test indicated by deterioration of the plastics caused by degradation by ultraviolet and infrared radiation, moisture, etc., and giving nse to crazing, resin-g lass interface failure, colour fading and a subsequent deterioration of structural properties.

Saunders, K.J.: The identification o f plastics and rubber, Chapm an & Hall, London, 1966

Kluckow, P.: Rubber and p lastics testing, Chapm an & Hall, London. 1963

A ids to the iden tifica tion o f p lastics — explanatory notes with p la stic samples, Building Research Establishm ent, January

1977

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