lecture 1a.3-introduction to structural steel costs

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ESDEP WG 1A:

STEEL CONSTRUCTION:

ECONOMIC & COMMERCIAL FACTORS

Lecture 1A.3: Introduction to StructuralSteel Costs

OBJECTIVE/SCOPE

To introduce the different factors affecting the cost of a steel construction. To show how the factors areconsidered in developing a design taking into account technical and recurrent and environmental costs.

PREREQUISITES

None.

RELATED LECTURES

Lecture 1A.1 : European Construction Industry

Lecture 1B.1 : Process of Design

SUMMARY

Total costs of a steel construction are affected by technical and environmental factors and recurrent costs.The steel costs, the energy, maintenance, adaptability and end of life costs must all be appreciated from thevery beginning of a project in order to lead to a well designed construction, which meets the requirementsof the client. Different parameters such as speed of construction, or choice of foundations are studied so

that they may be taken into account in the design of the construction and in determining its cost. Secondaryactivities such as erection, fabrication and protection against corrosion and fire complete this analysis.

1. INTRODUCTION

Costs of construction works can be considered in various categories. Technical costs, relating to materialand labour in completing the project, are those which can most easily be quantified. Recurrent costs shouldalso be considered in studying whole life economics, and again these can be estimated. Environmental costsare more difficult to establish; there are signs that an environmental audit will increasingly be required as apart of the consideration given to proposed projects. Environmental aspects can be considered in terms of local and global effects and include issues such as appearance, safety, local economics, use of naturalresources, and energy consumption.

This lecture concentrates on the technical costs of steel construction. It deals with the topic in a broad way.Whole life costing is dealt with first before examining the costs of execution, which are concerned initiallywith total construction, leading onto structural costs and finally economic considerations applied toindividual activities such as fabrication and erection. This sequence has been chosen deliberately toemphasise the need to examine overall costs in an integrated manner.

2. LIFE CYCLE COSTS

2.1 Attitude

Traditionally designers have considered only the initial cost of that part of the project for which they are

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responsible and have sought the most cost-effective solution for it. There is increasing recognition tha t thesum of optimum cost components do not necessarily lead to the most economic solution overall. However,there is still relatively little regard given to whole life costs. This is in marked contrast to the consideration

given to running costs when purchasing a new car, for instance. Then fuel consumption, likely service costs,repair costs and depreciation are often carefully accounted for alongside initial price, when making costcomparisons between different models.

2.2 Cost Elements

Initial costs of execution, including the fabric, structure, foundations and services, are an important andimmediate consideration. In addition to these items, the need to finance the construction, and the

associated costs, should be quantified and included as part of the debate on the form of the design. The wayin which the cost of finance influences the project is discussed in more detail in Section 3.2. In some cases,it might be a major factor.

Other recurrent costs, which should be considered in the overall economic discussions surrounding aproposed project, include maintenance, future alterations and running costs associated with environmentalcontrol of building interiors (heating, ventilation and lighting). Another factor which should be considered isthe likely benefit or financial return. For instance, projected traffic density will clearly be an importantinfluence on required highway bridge capacity, whilst clearer floor areas and greater flexibility in the use of floor space may attract higher revenue in commercial developments. These factors are discussed in moredetail in Section 2.6.

2.3 Energy Costs

Energy costs for lighting, heating and ventilation remain a significant recurrent cost item. For buildings,initial expenditure related to energy requirements is concerned with such aspects as the balance of provisionof natural and artificial lighting, and the heating/ventilation requirements, which are clearly affected by theinsulation specification for the external skin.

Artificial lighting represents a surprisingly high proportion of energy consumption in commercial andresidential buildings. Adequate provision of windows and rooflights can therefore mean significant long-termsavings. These provisions need to be assessed against higher initial expenditure, and secondaryconsiderations, such as security. Heat gain and glare are two potential problems which should also beconsidered since they may effectively neutralise any potential savings.

Space heating and ventilation are both related to insulation levels and the volume of air to be treated. One of

the reasons why low-pitch portal frames have become more popular than the traditional column and trussconstruction for industrial applications is that the enclosed building volume is reduced and includes very littlewasted space.

2.4 Maintenance

All structures (buildings, bridges and others) should be inspected and maintained on a regular basis. Thereis often a trade-off between costs associated with these activities and initial costs. Areas which are difficult

or impossible to inspect need careful treatment. In many cases there is a trade-off between capitalexpenditure and life expectancy/maintenance requirements.

For steels, corrosion and its prevention is a major concern. Cost factors associated with corrosionprevention relate to exposure conditions, planned inspection and maintenance, design detailing, the

protection specification, and the quality of the first application.

Good detailing in fact has very little cost implication and is an important part of all designs. For instance,arrangements which allow water to collect should be avoided and inaccessible areas sealed (Figure 1).


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The specification for the corrosion protection system should be appropriate to the exposure conditionsexpected. Although some extremely good systems are now available, there is little point in using suchsystems where corrosion risks are low. This point is discussed in more detail in Section 4.3.

2.5 ADAPTABILITY

Although it is not always possible to predict future client requirements, alterations and extensions toprojects are often carried out subsequent to the initial development. Such projects can be disproportionately

expensive. Specific provision for future alterations can only be made if details are known at the outset, butsignificant savings are possible if the original design takes account of possible changes. Although the initialdevelopment costs may be marginally increased, long-term costs overall can be reduced.

The building life is always longer than the life expectancy of services, so the construction should be able toaccommodate likely changes in use. This capability can be provided by adopting a "loose fit" approach,

giving additional space without disproportionate increase in cost.

Shell and core construction, in which the building consists of the structure and major services only, with themore specific services for floors installed by the tenant, is becoming increasingly popular for speculativecommercial developments. In such cases there is an even clearer need for a "loose fit" approach from the

outset.


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It is very common for the use of building to change. Change of use may require upgrading floor loadings,modifications to floor layouts, installation of new lifts, or extending the structure to provide more usablespace. Allowing for such developments in the original design could lead to significant subsequent savings.

Steel structures can be adapted or extended without great difficulty. The potential exists for making

connections to the existing frame, and the strength of the existing structure, and any new attachments to it,can be determined with confidence. Nevertheless, where future changes are envisaged, it is often moreefficient to provide for these at the outset. Where future extension is planned, simple modifications to the

fabrication details and appropriate sizing of critical members for the new conditions should be incorporated.For instance, pre-drilling of steelwork for new connections at the interface with the possible extension andsizing columns for increased loads facilitates later construction.

If unforeseen changes arise, it is not difficult to strengthen individual beams and columns, for instance, byattaching flange plates to the existing section. Strengthening connections is very much less simple, andsome designers therefore overspecify the shear capacity for connections to minimise the need forstrengthening in subsequent alterations.

2.6 Benefits and Financial Return

Costs should not be considered in isolation but set against perceived benefit. The benefit may be clearlyquantifiable in terms of rental income or relate to the provision of additional facilities. In either case usablefloor area is a key factor. This might suggest fewer, smaller columns, and should certainly encourage thedesigner to avoid unusable space, for instance between columns and adjacent perimeter walls. Minimisingthe thickness of partition walls and the external skin may also yield an increased floor area, but theirperformance should not be compromised in doing so.

2.7 End of Life Costs

For many structures there comes a time when demolition is necessary. The cost associated with this activitycan be offset by income from the sale of recyclable materials. For steel structures the material can bere-used either as scrap in the manufacture of liquid steel or as secondhand products which can be re-usedin new structures. The nature of steel construction lends itself to dismantling rather than demolition.

Some structures take this principle further and are designed as demountable. Such structures are generallyfor short-term use such as exhibition facilities, temporary car parks and highway crossings. With carefuldesign the complete structure can be dismantled and erected elsewhere.

3. TOTAL CONSTRUCTION

Total building cost is a complex issue due to the interaction of various elements. Usually the best design of one aspect (e.g. structure) conflicts with others (e.g. services or cladding). It is not, therefore, simply acase of optimising each to achieve an optimum solution for the whole building, but rather the costingsshould be examined in an integrated, holistic manner.

Buildability is also important. It is concerned not simply with the development of new details or erectionsystems which might facilitate work on site, but with an understanding of how design and construction canbe dealt with in an integrated fashion to produce a building which is simple, quick and cost-effective toexecute and maintain. This approach involves harmonisation of structural, service and planning grids.

At a more detailed level, standardisation, particularly of connection and fixing details, can lead to significanteconomies, even if it implies some apparent wastage of materials. Co-ordination between different

elements, such as cladding and structure, achieved by simplicity of the interfaces between them, isparticularly important. Non-typical areas such as corner panels for cladding and edge details for floors needspecial consideration. All too often these areas are ignored until execution is well under way and last minute

solutions can be both inelegant and costly.

Bridges and other structures are much less sensitive than buildings to interactions between structural andnon-structural components. However, for offshore structures for instance, aspects such as appropriateconstruction sites and transparency of the structure to wave loadings, installation procedures and fitting out

all influence the total costing.


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3.1 Typical Breakdown of Costs and Interactions

An important cost element in a steel framed building is that of the frame itself. Other major items includefoundations, flooring, cladding/external finishes and services. The relative contributions of these items varyconsiderably from one project to another, but typical cost proportions are 9% for the steel frame compared

with 25-35% each for cladding and services. Land prices can sometimes be as high as the direct costs of the construction, in which case speed of execution becomes a predominant factor as discussed in Section3.2.

For multi-storey buildings, the importance of lateral bracing becomes a primary consideration. For low-riseconstruction, a rigid jointed frame is often an economic solution, but as overall height increases, this systembecomes too flexible. Cross-bracing or shear walls might then be preferred, despite possible restrictions oninternal planning imposed by the location of the bracing. With greater building heights, more sophisticatedlateral bracing systems become necessary. These systems include derivatives of both rigid frames and

cross-bracing such as outrigger trusses, braced tubes and facade frames. Some of these systems areshown in Figure 5 which includes indications of their appropriate ranges of use.

A similar discussion of the use of rigid and simple frames can be held in relation to gravity loads. The effect

of rigid frame action is typically to reduce beam sizes but increase column sections. In general, whilst thetotal weight of steel is less in rigid construction, savings are often more than offset by the increasedcomplexity of the connections. However there may be other considerations - reduced structural depth, the

undesirability of bracing (if rigid frame action is not used to provide lateral stability) and reduced deflectionsresulting from improved stiffness. For longer spans, material savings for rigid construction are likely to begreater. Not only does the increased rigidity become more important in controlling deflections, but therelative saving in steel weight of beams compared with the increased weight of columns becomes moresignificant.


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Important advantages of steel construction are speed of execution, prefabrication and lightness. Tomaximise the advantages the concepts must be followed through in the design of the building as a whole,including cladding, finishes and services. For example the use of smaller foundations can only be achieved if

the lightness of the structure is reflected in appropriate design of other building components. This exampleagain emphasises the need for co-ordinating the design of services, cladding and structure and associatedwith this approach, the discipline of producing the final design at an early stage.

This approach also implies that the steelwork contractor should be involved at the earliest opportunity as

part of the project team and it also places additional responsibilities on other members of that team to avoidlate changes.

3.2 Speed of Execution

The costs of financing a project may be a major consideration. High land prices and staged payments to thecontractor mean that the client may have to sustain a high borrowing requirement throughout the period of execution without any return in terms of either rental income or use of the building. With high interest ratesthe borrowing requirement can represent a major element in the total project cost and, under thesecircumstances, the speed with which the project can be completed becomes an extremely important factor.

The importance of speed has been highlighted in a comparison of costs for typical 3, 7, and 10-storey officebuildings using three different building systems (steel frame and precast concrete floor, steel frame andcomposite floor, and in-situ reinforced concrete). Construction programmes and cost estimates preparedindependently for each building clearly showed the importance of execution time. The study alsodemonstrated the benefits of steel deck composite flooring in avoiding the need for temporary propping orscaffolding, the advantages of simplicity, and the need to ensure that various trades are able to completework at one level in a single operation. Providing access by programming staircases to rise with the frameand installing floors as quickly as possible after the frame also streamlines site work.

The trend towards greater prefabrication and sub-assembly with reduced site work has spread from simplythe steel frame to include other elements such as cladding panels and accommodation modules.Prefabrication all helps to save execution time but it places more pressure on the design phase. It alsoimproves quality, reduces reliance on a skilled, mobile workforce, and enables deficiencies to be rectified

more easily than on site. Precommissioning should also produce a greater awareness of, and provision for,future maintenance requirements.

Late changes and traditional reliance on resolving problems on site may be suitable for insitu construction.

However, more emphasis on prefabrication in modern construction means that the site becomes anassembly shop where the components must fit first time if expensive delays and corrections are to beavoided.

Fast build rather than fast track is perhaps the optimum solution. In the latter the construction programmeoverlaps with the design phase, implying incomplete information. In contrast fast build construction does

not start until all design is complete, and embodies the best features of efficient building.

3.3 Weather

Any construction can be affected by adverse weather. Execution programmes and methods themselves are

generally organised with this in mind. For instance when industrial sheds are built it is normal practice tocomplete the frame and envelope at the earliest opportunity, with the concrete ground slabs subsequently

being cast within a relatively controlled and sheltered environment. Multi-storey construction utilisingcomposite floor decks offers similar advantages of rapid isolation from the worst effects of adverse weather.

Some building systems have been developed on the basis of providing a dry envelope for the work of execution.

3.4 Services, Cladding and Structure

The greatest cost interactions between building components are probably those between structure, mainservices and cladding. The total floor depth includes the structure (slab and beam) and services. The greaterthis depth the greater will be the total height of the building, increasing the area of cladding. Even for simple

enclosure systems, increased costs will result. For sophisticated curtain walling systems these increases


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could be very high. In extreme cases, where planning constraints are particularly severe with regard to totalbuilding height, it is possible that the selection of a shallow floor zone could result in the inclusion of anadditional storey compared with the case for a deeper floor construction depth.

Smaller scale services (electrics, telecommunication wiring) can be accommodated within raised floors, or in

trunking set in the screed or within the structural concrete slab. They have little implication for the structure.Large ducted air conditioning systems involve greatest interaction. Here the objective is to produce anefficient floor structure, which can accommodate the required size of ducts (including cross-overs), and

which also allows the addition or increase of sizes as servicing needs change.

Possible strategic solutions are separation or integration. Separation gives greatest flexibility and provisionfor future changes. Allowing the services to pass through the structure may result in some savings in overallconstruction depth but installation may be difficult and cause possible damage to paint and fire protection;future changes may also be limited.

Structural forms to facilitate accommodation of services relate particularly to different arrangements of beams. There is generally much more space available between beams where only slab depth contributes toconstruction depth. Possible solutions (Figure 2) include standard I beams, castellated beams (which,although deeper, provide limited opportunity for accommodating service ducts), trusses and stub girders(both deeper again but with greater provision for ductwork). The parallel beam arrangement, whichseparates on two levels both structure and service runs in two directions, has proved to be a successfulsolution for a number of projects (Figure 3). Other possibilities include various forms of tapered andhaunched beams, used to optimise overall depth and structural efficiency, but at the expense of greater

fabrication costs (Figure 4).


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3.5 Foundations

Foundation costs are an important factor in the overall economics of building construction. For small scalebuildings on sites with good foundation conditions, simple foundation solutions are likely to be suitable.Where foundation loads are high and/or foundation conditions are poor, more sophisticated and expensive

solutions such as piling may be necessary, Figure 6. In such circumstances the weight of the superstructuremay be critical and suggest a lighter, possibly less efficient form. For instance closely spaced beams toreduce the thickness of floor slabs, which might themselves be constructed using lightweight concrete canreduce foundation loads considerably. Steel as a structural material is also lighter than other structuralmaterials for a given load resistance.


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4. STEELWORK COSTS

At a more detailed level the economics of steelwork construction can be affected by decisions regarding theprecise form of element, type of steel used and the method of connection. Some of these decisions are

influenced by the purchase route for the steel itself. For large projects, steel can be purchased directly fromthe mill in the exact lengths required and in the desired grade. The price of individual structural productsvaries not only with type (hollow sections are generally more expensive than open sections such as I-beamsand H-columns) but also within a product range, with little apparent rationale behind the pricing policy. Thusselecting a section of minimum weight does not guarantee an optimum solution in terms of cost, even for anindividual element. Specifiers should therefore be aware of pricing policies.

Small orders cannot be processed in this way and the steel is then purchased from stockholders. In thiscase the steel is only available in a limited range of grades, (probably only mild steel) and a premium is

payable to the stockholder. In addition certain sizes of standard section may not be stocked and thesections will only be available in a limited range of lengths.

These considerations clearly have important implications for the specifier.

Where higher grades of steel are available they may offer the opportunity for improved efficiency. Forinstance, high yield steel has a yield strength approximately 25% higher than normal mild steel yet costsonly about 10% more. However, where strength is not a critical design condition, for instance in the case of very long span beams where deflection control may be dominant, the use of high grade steel may simply bewasteful.

A breakdown of costs for structural steelwork in a multi-storey building might typically be as follows:


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steel 47%

corrosion protection 5%

fabrication 22%

erection 8%

fire protection 18%

Clearly optimising the cost of the steelwork (within an optimum solution for structure and construction as awhole) is dependent on minimising the total cost of these contributory elements, rather than optimising

each independently. A balance is needed between structural efficiency, simplicity of construction andbuilding use.

It is clear that there is more potential for reducing costs in fabrication and erection than in the steel itself. Inthis respect, work on site is of most importance - easier assembly is likely to lead to overall economy.

Transport is also important, not as a cost item in itself but as an aid to more efficient erection.

4.1 Erection

Because erection is carried out in the open, often under difficult conditions, and it is the essential interfacewith other construction trades, it is in many ways the most important part of the design and executionprocess for a steel structure. Problems at this stage can be costly to rectify and involve long delays to the

programme. Apparently trivial issues, such as steelwork delivered out of sequence, lack of bolts or fittings,

long lead times for minor additional items, extensive double handling of materials and misfit of members,can cause significant reductions in construction efficiency.

Much depends on good planning. Preparation of an erection scheme should be made on receipt of first

construction issue drawings prior to detailed drawing office work by the fabricator. At this stage items fordelivery as sub-assemblies can be identified and the need for temporary bracing assessed. Attachments forbracing should be incorporated within initial fabrication drawings to avoid double handling of both drawingsand materials.

The need for safe access for erectors must be recognised. Time can be saved and material re-used if temporary stagings are pre-engineered and delivered with steelwork rather than relying on makeshiftmethods on site.

Loose temporary landing cleats under major beams and girders, shop-bolted to columns greatly facilitate

erection. Erection drawings should be clear, unambiguous and complete, including on a single drawing alldetails such as bolt sizes, weights of members, presence of fittings, etc.

4.2 Fabrication

The size of individual components is limited by the lifting capacity of available cranes and transportation.This applies also to other parts of the construction such as finishes and cladding. Within these constraintshowever the general principle is to maximise work at the fabrication stage and minimise work on site,pre-assembling units in sections which are as large as possible.

Connection design and detailing which standardises details, bolt diameters and lengths (HSFG and 8.8 boltsof same diameter should never be used on the same job) simplifies erection and minimises risk of error.Although material and fabrication costs may be increased marginally, savings on site far outweigh such

increases. Simplicity and repetition of frame components is related to design; for instance special fabricatedsections such as tapered beams become more economic in larger numbers.

Preferred details should be incorporated to facilitate site erection. For instance fin plates are preferable toend plates or cleats since they enable beams to be swung directly into position (Figure 7). Momentconnections should be avoided if possible, but where necessary the erector should be consulted with regardto the preferred type of detail.


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A loading schedule should be prepared by the erector showing when steel is to be delivered, how it is to bebundled, where to be placed on the trailer for optimum off-loading and where to be set down in its correctlocation on the building frame. Lack of vigorous production control often requires a buffer store on site tomaintain the erection programme. This procedure is inefficient in terms of storage space, cranage and multi-

handling. Programmed site erection should be the control and 'pull' on fabrication, with delivery scheduled toaccord with the daily erection programme. One of the most notable examples of erection programming wasthe construction of the Empire State Building in New York.

There is a clear need for a production engineering philosophy in the design office, factory and on site. Forexample, a careful study of the fabrication process can significantly reduce material handling. Theproductivity of the most efficient shops is based on a labour content of 2 man hours per tonne for simplemulti-storey construction compared with more than 20 man hours per tonne average for all steelwork.

Some structurally efficient solutions, even based on standard rolled sections, may be less efficient in termsof fabrication. Column sections with bigger overall dimensions generally have larger radii of gyration andhence, for a given application, have lower slenderness ratios and higher buckling strength; they maytherefore be lighter in weight than a comparable section of more compact shape. Where these sections are

used as part of a moment resisting frame, however, the reduced flange thickness of the bigger section maywell mean that local stiffening is required, increasing fabrication costs.

Computer controlled cold sawing, punching and drilling machines mean that bolting for low to medium riseconstruction is often cheaper than welding which involves more labour, cost and time. This is particularly so

on site where special access, weather protection, inspection and temporary erection supports are required.

Linking of CAD/CAM and management information systems avoids transcription of information, saving timeand eliminating possible errors.

4.3 Corrosion and Fire Protection

The cost of initial corrosion protection is unlikely to be greatly influenced by the steelwork details, although

maintenance costs and performance can be significantly affected. Appropriate specification of the corrosionprotection system is important. Steel within a heated building is unlikely to need any long term protection at

all, whereas exposed steel or steel within the external envelope may need a high level of protection. Detailedadvice is available. Painting costs are partly dependent on the area to be painted, whilst galvanising costsare related to the weight of steelwork. The latter therefore becomes a more attractive alternative forlightweight structures with a large surface area such as trusses and lattice girders.

Regulations relating to fire protection requirements now allow calculation methods to prove reduced orindeed the elimination of such protection. A range of relatively cheap, lightweight proprietary systems is alsoavailable and, if these systems are adopted, performance, appearance, and wet or dry application influencethe final selection. Some structural solutions such as slimfloor beams offer the potential for adequate fireresistance without protection. Although the weight of steel is greater than for conventional systems, theoverall effect may be some savings. In addition, slimfloors offer a reduction in structural depth and are,


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therefore, attractive in terms of accommodation for services.

5. CONCLUDING SUMMARY

Costing of construction projects is a complex issue and should include all aspects in an integratedfashion.Evaluation of whole life costs should be encouraged rather than focusing only on initial construction

costs.Buildability and good planning are important aspects in minimising costs.

Efficient integration of structural and non-structural items is dependent on detailed information beingavailable at an early stage, but is essential if efficient construction is to be achieved.

6. ADDITIONAL READING

British Steel Corrosion Protection Guides.1.Brett, P. Design of Continuous Composite Beams in Buildings; Parallel Beam Approach. The SteelConstruction Institute, 1989.

2.

Owens, G.W. An Evaluation of Different Solutions for Steel Frames, ECCS International Symposium,"Building in Steel - The Way Ahead", No: 57 September 1989, pp 6/1 - 6/28.

3.

Glover, M.J. Buildability and Services Integration, Ibid.4.Horridge, J.F. and Morris, L.J. Comparative Costs of Single-Storey Steel Framed Buildings, The

Structural Engineer, Vol. 64A, No. 7, July1986, pp. 177-181.

5.

Iyengar, H. High Rise Buildings, ECCS International Symposium, "Building in Steel - The Way Ahead",No: 57 September 1989, pp 1/1 - 1/30.

6.

Copeland, B., Glover, M.J., Hart, A., Haryott, R. and Marshall, S. Designing for Steel, ArchitectsJournal, 24 & 31 August 1983.

7.

Hayward, A.C.G. Composite Steel Highway Bridges, Constrado.8.Customer Led - Construction Led, Steel Construction, Vol 7, No. 1, (BCSA), February 1991.9.Horridge, J. F. and Morris, L. J., "Comparative Costs of Single Storey Steel Framed Structures", TheStructural Engineer, Vol 64A, No. 7, July 1986.

10.

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