ch 1b_1 process of design
DESCRIPTION
A detailed view design, production, and erection of steel structures according to the new European code EC3TRANSCRIPT
STEEL CONSTRUCTION: INTRODUCTION TO DESIGN
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STEEL CONSTRUCTION:
INTRODUCTION TO DESIGN
Lecture 1B.1: Process of Design
OBJECTIVE/SCOPE
To introduce the challenge of creative design and to explain approaches by which it may
be achieved.
PREREQUISITES
A general knowledge of basic applied mechanics is assumed and prior encouragement
should be given to read J E Gordon's three books [1,2,3].
RELATED LECTURES
Since this lecture deals with the process of design in general terms almost all other
lectures are related to it in some way. Those sections which are most closely associated
with it are 1B: Introduction to Design, 14: Structural Systems: Buildings, 15A: Structural
Systems: Offshore, 15B: Structural Systems: Bridges, and 15C: Structural Systems:
Miscellaneous.
SUMMARY
The lecture begins by considering a definition of design and some objectives. It discusses
how a designer can approach a new problem in general and how a structural designer can
develop a structural system. It concludes by considering differences of emphasis in design
approach for different classes of structure.
1. DESIGN OBJECTIVES
The results of successful design in structural engineering can be seen and used by
everyone, see Figure 1.
The question is: how can professional designers be developed and eventually produce
better designs than those previously encountered, to benefit and enhance the performance
of human activities? In particular how can steel be utilised effectively in structures for:
travelling more easily over awkward terrain, requiring bridges.
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enabling basic industrial processes to function requiring, for example, machinery
supports, docks and oil rig installations.
aiding communications, requiring masts.
enclosing space within buildings, as in Figure 2.
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Design is 'the process of defining the means of manufacturing a product to satisfy a
required need': from the first conceptual ideas, through study of human intentions, to the
detailed technical and manufacture stages, with the ideas and studies communicated with
drawings, words and models.
'Designers'? All people are capable of creative conceptual ideas - they are continuously
processing information and making conscious imaginative choices, e.g. of the clothes they
wear, of the activities they engage in, and the development of ideas they pursue, causing
changes.
In structural design, prime objectives are to ensure the best possible:
unhindered functioning of the designed artefact over a desired life-span.
safe construction system, completed on time and to the original budget cost.
imaginative and delightful solution for both users and casual observers.
These points could possibly be satisfied by either:
simply making an exact copy of a previous artefact, or,
're-inventing the wheel', by designing every system and component afresh.
Both these extreme approaches are unlikely to be entirely satisfactory. In the former case,
the problem may well be slightly different, e.g. the previous bridge may have stimulated
more traffic flow than predicted, or vehicle weights may have increased. Economic and
material conditions may have changed, e.g. the cost of labour to fabricate small built-up
steel elements and joints has increased compared to the production cost of large rolled or
continuously welded elements; also, corrosion resistant steels have reduced maintenance
costs relative to mild steel. Deficiencies of performance may have been discovered with
time, e.g. vibrations may have caused fatigue failures around joints. Energy consumption
conditions may have changed, e.g. relating to the global discharge of certain chemicals,
the cost of production of certain materials, or the need for greater thermal control of an
enclosed space. Finally, too much repetition of a visual solution may have induced
boredom and adverse cultural response, e.g. every adjacent building is produced in the
"Post Modern Style".
With the latter approach, 'life is often just too short' to achieve the optimal solution whilst
the client frets.... Civil and structural engineering projects are usually large and occur
infrequently, so a disenchanted client will not make a second invitation. Realisation of
new theoretical ideas and innovations invariably takes much time; history shows this
repeatedly. Thus methodical analysis of potential risks and errors must temper the
pioneering enthusiast's flair.
Positive creative solutions must be achieved for all aspects of every new problem. The
solutions will incorporate components from the extremes above, both of fundamental
principles and recent developments. However, throughout the Design Process it is prudent
to maintain a clear grasp of final objectives and utilise relatively simple technical means
and solutions.
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2. HOW DOES THE DESIGNER APPROACH HIS
NEW TASK?
At the outset of a new task an "instant of blind panic" may occur. There are a variety of
Design Methods to help progress [4, 5] with the new task, but the following methodical
approach is suggested:
1. Recognise that a challenge exists and clearly define the overall objectives for a
design, see Figure 3.
2. Research around the task and investigate likely relevant information (Analysis).
3. Evolve possible solutions to the task (Synthesis).
4. Decide on, and refine, the best solution (Evaluation), establishing clear priorities
for action (in terms of manufacture, construction, operation and maintenance).
5. Communicate decisions to others involved in the task.
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At the outset, these five phases appear as a simple linear chain; in fact the design process
is highly complex, as all factors in the design are interdependent to a greater or lesser
degree. Hence there will be many steps and loops within and between the phases, as seen
in Figure 4. The first rapid passage through phases 1, 2 and 3 will decide if there is 'any
problem', e.g. is the likely traffic flow adequate to justify a convenient but high cost
bridge?
All factors and combinations must be explored comprehensively from idea to detail, with
many compromises having to be finely balanced to achieve a feasible solution. Ideas may
be developed: verbally, e.g 'brainstorming' or Edward de Bono's 'lateral thinking'
approaches [6], graphically, numerically or physically. Always qualitative assessment
should proceed quantitative evaluation.
The starting point for Analysis may thus be the designer's current preconceived notion or
visual imagination, but the Synthesis will reveal the flexibility of his mind to assimilate
new ideas critically, free of preconception.
A designer can prepare himself for the compromises and inversions of thought and
interaction with other members of the Design Team leading to successful synthesis,
through 'Roleplay Games', e.g. see 'The Monkey House' game, in Appendix 1.
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3. HOW DOES THE DESIGNER DEVELOP HIS
STRUCTURAL SYSTEM?
An example of structural design, and the various decision phases, will be briefly
considered for a simple two-lorry garage building with an office, toilet and tea room,
shown completed in Figure 2. It is assumed in this hypothetical case that an initial decision
has already been made by the client to have this set of requirements designed and built.
3.1 Pose an Initial Concept that may well Satisfy the Functions
It is invariably the best idea to start by looking at the functions (performance) required and
their relationships. Make a list of individual functions; then generate a 'bubble' (or flow)
diagram of relationships between different functional areas to decide possible
interconnections and locations, see Figure 5. Find, or assume, suitable plan areas and
minimum clear heights of each three-dimensional 'volume of space'. A possible plan
layout may then be indicated, noting any particular complications of the site, e.g. plan
shape, proximity of old buildings, slope or soil consistency.
Many other plan arrangements will be possible and should be considered quickly at this
phase.
The requirements of each 'volume of space' and its interfaces must be examined for all
functional, cost and aesthetic criteria, e.g. what structural applied live loads must be
resisted; what heating, ventilating, lighting and acoustic requirements are likely to be
desired, see Figure 6.
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The main criteria can easily be recognised and then followed up and tested by numerical
assessment. Incompatibilities may be 'designed out' by re-arranging the planned spaces or
making other compromises, see Figure 7, e.g. would you accept an office telephone being
very close to the workshop drill or lorry engine, without any acoustic insulation?
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Prepare a set of initial assumptions for possible materials and the structural 'Frame',
'Planar' or 'Membrane' load-bearing system [7] that might be compatible with the 'volumes
of space' as shown in Figure 8. These assumptions will be based on previous knowledge
and understanding of actual constructions[8-13] or structural theory, see Figure 9 a, b, as
well as the current availability of materials and skills. Initial consultations may be needed
with suppliers and fabricators, e.g. for large quantities or special qualities of steel.
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Steelwork, with its properties of strength, isotropy and stiffness, and its straight and
compact linear elements, lends itself to 'Frame' systems, see Figure 9 c-e, which gather
and transfer the major structural loads as directly as possible to the foundations, as a tree
gathers loads from its leaves through branches and main trunk to the roots.
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Next (and continuously) elucidate and test your ideas by making quick 3D sketches, or
simple physical models, to explore the likely compatibility and aesthetic impact.
A range of stimulating evocative patterns viewed at different distances from, all around,
and inside the buildings must be developed:
Long range the skyline silhouette or "landscape" pattern
Middle distance when the whole built object can be seen
Close up when a detail is clearly seen
Very close when the texture of the materials can be seen.
All these conditions should be satisfied, and especially for very large buildings for most of
the time. Deficiencies may be made up in some people's minds if their social conditions
change for the better or natural or changing phenomena occur, e.g. the rays of the setting
sun suddenly give a completely different colour appearance or after sunset the interior
lighting creates patterns previously unnoticed.
Form, colour, warmth and definition can be achieved with skilful use of steel, especially
with "human scale" elements though repetition will soon induce boredom; but only as part
of the complete sensory experience which must include elegant solutions to all aspects -
especially those easily visible - of the total building design.
It is very important that all principal specialists (architects, engineers for structure and
environmental services, and also major suppliers and contractors who should all have
common education and understanding of basic design principles) collaborate and
communicate freely with each other - also with the client - at this conceptual design phase.
Bad initial decisions cannot subsequently be easily and cheaply rectified at the more
detailed design phases.
Be prepared to modify the concept readily (use 4B pencils) and work quickly. Timescale
for an initial structural design concept: seconds/minutes. But hours will be needed for
discussion and communication with others in researching an initial complete design idea.
3.2 Recognise the Main Structural Systems and Contemplate the Necessary
Strength and Stiffness
Consider the applied live loads from roofs, floors or walls, and trace the 'load paths'
through the integral 3D array of elements to the foundations, see Figure 10.
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If the roof is assumed to be profiled steel decking, the rainwater should run to the sides,
and a manufacturers' data table will indicate both the slope angle to be provided (4 - 6
minimum) and the secondary beam (purlin) spacing required, e.g. commonly 1,4m - 2,6m.
The purlins must be supported, e.g. commonly 3m - 8m, by a sloped main beam or truss,
usually spanning the shorter direction in plan, and supported by columns stabilised in three
dimensions.
Wind loads on the longer side of the building can be resisted by cladding that spans
directly to the main columns, or onto sidewall rails spanning between columns. The
columns could resist overturning by:
cross-bracing (in this case the large entry door would be impeded).
or rigidly fixing the columns to the foundation bases ("linked cantilevers"); can the
soil resist the extra overturning effect at the base?
or rigidly fixing the tops of the columns to the main beams (creating 'portals') and
giving smaller, cheaper "pin" base foundations.
Wind loads on the open short side of the building can be resisted by the opening door
spanning top or bottom, or side to side. At the closed short side the wind loads can be
resisted by cladding that either spans directly between secondary end wall columns, or
onto rails to these columns.
At both ends of the building, longitudinal forces are likely to be induced at the tops of the
columns. Trussed bracing can be introduced, usually at both ends of the roof plate, to
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transfer these loads to the tops of a column bay on the long side - which must then be
braced to the ground.
Identify the prime force actions (compression C; tension T; bending B) in the elements
and the likely forms of overall and element deflections for all applied loadings both
separately and when combined.
It is always useful to have the elements drawn to an approximate scale, which can be done
using manufacturers' data tables for decking and cladding, from observations of existing
similar buildings, or using 'Rules of Thumb', e.g. the span/depth ratio for a simply-
supported beam equals about 20 for uniform light roof loading, see Figure 11.
At this phase the structural design becomes more definite (use B pencil) and takes longer.
Timescale: minutes.
3.3 Assess Loads Accurately and Estimate Sizes of Main Elements
Establish the dead load of the construction and, with the live loads, calculate the
following, see Figure 12:
beam reactions and column loads (taking half the span to either side of an internal
column).
maximum bending moments, e.g. wL2/8 for a simply supported beam, under
uniform load.
maximum shearing forces in beams.
deflection values, e.g. 5/384 wL4/EI for a simply supported beam with uniform
load.
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The size of columns carrying little moment can be estimated from Safe Load Tables by
using a suitable effective length. Significant bending moments should be allowed for by a
suitable increase, i.e. twice or more, in section modulus for the axis of bending.
Beam sizes should be estimated by checking bending strength and stiffness under limiting
deflections. Structure/service duct or pipe integration may require beams to be as shallow
as possible, or deeper and with holes in the web.
Likely jointing methods must be considered carefully: is the beam to be simply supported
or fully continuous and what are the fabrication, erection and cost implications?
Structural calculations are now being performed (use HB pencil with slide rule, simple
calculator or computer) and the time involved is more significant. Timescale:
minutes/hours.
3.4 Full Structural Analysis, using Estimated Element Sizes with Suitable
Modelling of Joints, Related to Actual Details
Carry out a full structural analysis of the framework, either elastically or plastically. A
computer may well be used, though some established 'hand' techniques will often prove
adequate; the former is appropriate when accurate deflections are required, see Figure 13.
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For the analysis of statically indeterminate structures, an initial estimate of element
stiffnesses (I) and joint rigidity must be determined by the third phase above, before it is
possible to find the disposition of bending moments and deflections. If subsequent
checking of the design of elements leads to significant changes in element stiffness, the
analysis will have to be repeated. The role of the individual element flanges and web in
resiting local forces within connections must also be considered very carefully when
determining final element sizes. Excessive stiffening to light sections can be prohibitively
expensive.
The analysis cannot be completed without careful structural integration and consideration
of the compatibility of the entire construction system including its fabrication details.
Element joints will usually be prepared in the factory using welding, with bolts usually
completing joints of large untransportable elements at site. Bracings, deckings and
claddings will usually be fixed on site with bolts or self-tapping screws. It is important to
remember that failures most frequently arise from poor jointing, details and their
integration.
The structural calculations and details are now progressing (use HB pencil with slide rule,
calculators and computers). Timescale: hours/days.
Iteration of phases 1-4 above will undoubtedly be required, in particular to ensure that the
early structural decisions are compatible with the subsequent investigations concerning the
functional, environment, cost and aesthetic aspects. The effect of any change must be
considered throughout the complete design. Changes usually necessitate a partial 're-
design'.
3.5 Communicate Design Intentions through Drawings and Specifications
Prepare detail drawings and specifications for contractors' tenders, see Figure 14. Iteration
of the design may again be necessary, due to variations in contractors' prices and/or
preferred methods, e.g. welding equipment available, difficulties in handling steelwork in
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the fabricating shop or for transportation and erection. Changes and innovations in the
design must be communicated and specified very carefully and explicitly.
In many cases it is common practice for a Consulting Structural Engineer to prepare
preliminary designs with choice of main sections, leaving a Steelwork Fabricator to
complete the detailed design and jointing system, before checking by the Consultant.
The structural design is now being finalised (use 2 to 4 H pencils and pens, or computers).
Timescale: days/weeks.
3.6 Supervise the Execution Operation
Stability of the structure must be ensured at all stages of the execution, see Figure 15.
High quality components and skilled erectors must be available at the right place and time,
calling for very careful organisation. If 'all goes to plan' every piece will fit into the
complete jigsaw.
The design ideas are now being put into operation (use gumboots). Timescale:
weeks/months.
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3.7 Conduct Regular Maintenance
Only regular maintenance already thoroughly planned into the design will be needed, with
occasional change and renovation needed with change of use or occupation. Correction of
design faults due to innovation and errors should not be needed.
This is the operation phase. (Use a serene outlook on life!) Timescale: years/decades.
3.8 Differences of Emphasis in Design Approach Compared to that of a
Medium Sized Building
3.8.1 Single houses
Most "traditionally" built timber and masonry houses include some standard steel
elements, e.g. hot-rolled steel beams to span larger rooms and support walls, hollow
section columns for stair flights, cold-rolled lintels over window openings, stainless steel
wall ties and straps, also nails, screws and truss-rafter nail plates.
Cold-rolled galvanised or stainless steel sections can be made up into truss-rafters and
replace timber in repetitive conditions. Similar sections can be made up as stud walls, but
fire protection of the thin-walled sections will require careful attention, especially for
multi-storey houses.
A main steel structural frame may be used for houses, but integration of services, thermal
control, fire protection in multi-storeys, corrosion and fabrication costs of elegant jointing
must be designed appropriately. Various types of profiled or composite panel cladding can
be used for the exterior.
3.8.2 Bridges
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The magnitudes of gravity loading are often relatively greater in bridges, and particular
load patterns need to be assessed; also trains of moving wheel loads will occur giving
marked dynamic effects. Dynamic effects of wind loading are significant in long-span
structures. Accessibility of site, constructability of massive foundations, type of deck
structure and regular maintenance cost will govern the system adopted. Aesthetics for
users and other observers are important; long distance scale should be appropriately
slender but psychologically strong; careful attention is needed for fairly close viewing of
abutments and deck underside.
3.8.3 Offshore oil rigs
The scale of the whole operation will be very many times that of an onshore building.
Gravity loading, wind speeds, wave heights and depth of water are significant design
parameters for structure size and stability (here larger elements cause larger wind and
wave loads). The scale of the structure also poses special problems for fabrication control,
floating out, anchorage at depth by divers and, not least, cost, see Figure 1. Later when the
design life is complete, the problems of dismantling should be easy, if considered during
the initial design.
4. CONCLUDING SUMMARY
This lecture introduces the challenge of creative design and suggests a holistic
strategy for designing structural steelwork. It seeks to answer questions about what
a designer is trying the achieve and how he can start putting pen to paper. It
illustrates how a successful design is iterated, through qualitative ideas to
quantitative verification and finally execution.
Creative and imaginative design of structures is most challenging and fun - now try
it and gain confidence for yourself. Do not be afraid of making mistakes. They will
only be eliminated by repeating and exploring many other solutions. Make sure the
design is right before it is built, using your own personal in-built checking
mechanisms.
5. REFERENCES
[1] Gordon, J. E. 'The New Science of Strong Materials', Pelican.
[2] Gordon, J. E. 'Structures', Pelican.
[3] Gordon, J. E. 'The Science of Structures and Materials', Scientific American
Library, 1988.
[4] Jones, J. C. 'Design Methods', Wiley 2nd Edition 1981.
A good overview of general design methods and techniques.
[5] Broadbent, G. H. 'Design in Architecture', Wiley, 1973.
Chapters 2, 13, 19 and 20 useful for designing buildings.
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[6] De Bono, E. eg: 'Lateral Thinking' or 'Practical Thinking' or 'The Use of Lateral
Thinking', Pelican.
[7] LeGood, J. P, 'Principles of Structural Steelwork for Architectural Students', SCI,
1983 (Amended 1990).
A general introduction and reference booklet to buildings for students.
[8] Francis, A. J, 'Introducing Structures, Pergamon, 1980.
A good overview text, especially Chapter 11 on Structural Design.
[9] Lin, T. Y. and Stotesbury, S. D, 'Structural Concepts and Systems for Architects
and Engineers', Wiley, 1981.
Chapters 1-4 give a very simple and thoughtful approach to total overall structural
design, especially for tallish buildings.
[10] Schodek, D. L, 'Structures', Prentice Hall, 1980.
Good clear introductory approach to structural understanding of simple concepts, also
especially chapter 13 on structural grids and patterns for buildings.
[11] Otto, F, 'Nets in Nature and Technics', Institute of Light Weight Structures,
University of Stuttgart, 1975.
Just one of Otto's excellent booklets which observe patterns in nature and make or suggest
possible designed forms.
[12] Torroja, E, 'Philosophy of Structures', University of California Press, 1962.
Still a unique source book.
[13] Mainstone, R. J, 'Developments in Structural Form', Allen Lane, 1975.
Excellent scholarly historical work, also chapter 16 on 'Structural Understanding and
Design'.
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APPENDIX 1
'The Monkey House' roleplay game for a group of students at a seminar, Figure 16
Between 10 and 12 acting roles are created, one for each student in the group, to consider
design requirements and interactions. Each actor sees an outline sketch plan of a possible
building and has about 3 minutes to prepare his role's requirements, likes and dislikes.
These requirements are propounded for about 2/3 minutes to his uninterrupting fellow
participants, who note points of agreement/disagreement. When all actors have spoken, the
many conflicts are then generally discussed and explored by the actors for about 30
minutes. Then the chairperson seeks a conclusion - who is The Monkey House really for?