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ESDEP WG 1B

STEEL CONSTRUCTION:

INTRODUCTION TO DESIGN

Lecture 1B.5.1: Introduction to the Design

of Simple Industrial Buildings

OBJECTIVE/SCOPE

To describe the reasons for the use of steel and to present common forms of structure for industrial

buildings.

PREREQUISITES

None.

RELATED LECTURES

Lecture 1A.1: European Construction Industry

Lecture 1B.2.1: Design Philosophies

Lecture 1B.3: Background to Loadings

Lecture 7.12: Trusses and Lattice Girders

Lecture 14.1.1: Single Storey Buildings: Introduction and Primary Structure

Lecture 14.1.2: Single Storey Buildings: Envelope and Secondary Structure

Lecture 14.2: Analysis of Portal Frames: Introduction and Elastic Analysis

Lecture 14.3: Analysis of Portal Frames: Plastic Analysis

SUMMARY

The reasons for the wide use of steel for industrial buildings are discussed. The advantages of steel include

its high strength-to-weight ratio, speed of erection and ease of extension. Steel is used not only for

members but also for cladding.

Common types of structure are described. These types include portal frame, lattice girder and truss

construction. It is shown that overall stability is easily achieved. The wide variety of sections used in

industrial buildings is presented. Possible approaches to global analysis are identified.

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1. TYPES OF INDUSTRIAL BUILDING

A wide variety of building types exists, ranging from major structures, such as power stations and process

plants, to small manufacturing units for high quality goods.

The most common type is the simple rectangular structure (Figure 1), typically single-storey, which

provides a weatherproof and environmentally comfortable space for carrying out manufacturing or for

storage. First cost is always an overriding consideration, but within a reasonable budget a building of good

appearance with moderate maintenance requirements can be achieved. While ease of extension and

flexibility are desirable, first cost usually limits the provisions which can be usefully included in the design

for these potential requirements. Although savings in the cost of specific future modifications can be

achieved by suitable provisions, for example by avoiding the use of special gable frames (Figure 2),

changes in manufacturing processes or building use may vary the modifications required.



When, for reasons of prestige, the budget is more liberal, a complex plan shape or unusual structural

arrangement may provide a building of architectural significance.

While many features are common to all industrial buildings, this lecture deals mainly with single-storey

buildings of straightforward construction and shape.

2. STRUCTURAL STEEL FOR INDUSTRIAL BUILDINGS

Compared to other materials, particularly reinforced or prestressed concrete, steel has major advantages. Its

high strength-to-weight ratio and its high tensile and compressive strength enable steel buildings to be of

relatively light construction. Steel is therefore the most suitable material for long-span roofs, where self-

weight is of prime importance. Steel buildings can also be modified for extension or change of use due to

the ease with which steel sections can be connected to existing work.

Not only is steel a versatile material for the structure of a building, but a wide variety of cladding has been

devised utilising the strength developed by folding thin sheets into profiled form (Figure 3). Insulated

cladding systems with special coatings are now widely used for roofing and sidewall cladding. They have

good appearance and durability, and are capable of being speedily fixed into position.



The structure of a steel building, especially of an industrial building, is quickly erected and clad, providing

a weatherproof envelope which enables the floor and installation of services and internal finishes to

proceed at an early stage. Since the construction schedule is always tied to the earliest handover date fixed

by production planning, time saved in construction is usually very valuable.

In a dry closed environment steel does not rust, and protection against corrosion is needed only for the

erection period. For other environments protection systems are available, which, depending on cost and

suitable maintenance, prevent corrosion adequately.

Single storey industrial buildings are usually exempt from structural fire protection requirements. Spread of

fire beyond the boundary of the building must not occur as a result of collapse of the structure. This

requirement can be met by the provision of fire walls and through the restraint which arises in practice

between the bases and the columns which they support.

3. CHOICE OF INDUSTRIAL BUILDING

A prospective owner may have a fully detailed design brief derived from the construction of industrial

plants elsewhere. More usually the owner is assisted in the choice of a suitable building by the completion

of a detailed list of requirements so that a design brief can be prepared. Initial options in respect of

preferred location, site acquisition and environmental needs must first be decided. Then main dimensions,

process operation, plant layout, foundation needs, handling systems, daylighting, environmental control,

service routes, staffing level and access all require definition.

The preliminary selection must be made between a building specially designed for the owner, a new factory

largely built of standard structural components, or the adaptation of an existing building. The latter may be

either an advance unit built as a speculative development, or a unit which has been vacated.

The location of internal columns and the internal headroom are always important, and consideration of

these requirements alone may determine the choice. The advantage of freedom to plan the building to suit

requirements closely and allow for future development is very valuable. However, unless there are

exceptional reasons such as permanence of specific use, it is unwise to design an industrial building

exclusively for a single process, since special features appropriate to the process may make redevelopment

difficult.



4. SHAPES OF INDUSTRIAL BUILDINGS

Because of its economy, the most widely used building shape is the pin-based single or multi-bay pitched

roof portal frame, typically of 20-30m span at 6m centres (Figure 4). Hot-rolled I, welded or cold-formed

sections are usually used for the members.

During recent years an increasing use of welded sections has occurred. This increase is the result of

progress achieved in making welding automatic and the ability to adapt the cross-section to the internal

forces.

Since internal columns sterilise an appreciable space around them, their spacing may be increased by using

spine I-beams to support the portal rafters. For this type of roof the cladding is usually insulated metal

decking, which may also be used for the upper sidewalls. Daylight is provided by profiled translucent

sheeting in the roof.

When hot-rolled sections are used, haunches (Figure 5) are usually provided at the eaves and the ridge.

These haunches deepen the overall section, thereby reducing bolt forces. By extending the haunched

regions along the rafter the frame is also strengthened and stiffened.



Lattice girders (Figure 6) are lighter than portal frame rafters for wider spans, but the additional

workmanship increases fabrication costs. Based on structural requirements alone, lattice systems are likely

to be cost-effective for spans above 20m. Roof trusses may also be used for structures which support heavy

cranes (Figure 7).



A wide variety of structural sections may be used for lattice girders and roof trusses, including single

angles, angles back- to-back, tees, H-sections or hollow sections (Figure 8). For light loading, cold-formed

sections may be used as booms, with reinforcing bars as the web members (Figure 9).



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The disadvantages of multi-bay pitched roofs are that internal gutters and rainwater disposal are required,

which are a possible source of leaks, and access to plant mounted externally on the roof is difficult.

The most versatile roof shape is the nominally flat roof, covered with an insulated membrane on metal

decking (Figure 10). This shape allows wide freedom in plan form, and eliminates the need for internal

gutters, although some internal rainwater disposal may be necessary if the extent of the roof is large. The

mounting and weather protection of external plant on the roof is simply achieved, and access can readily be

provided.

Flat roofs can be supported by rolled or cold-formed purlins on main I-beams or lattice girders. For smaller

structures the deck may span directly from one frame to another, without the need for purlins.



When services are extensive and there are many external plant units on the roof, castellated beams or

double-layer grid space frames (Figures 11 and 12) can be very suitable for flat roofs. The two-way grid

distributes local loads better than any other structural form. The support for the roof deck is provided

directly by the top layer and support for the services by the bottom layer of the grid. Castellated beams have

a much higher moment of resistance than I-beams.



The provision of daylighting in flat roofs is expensive, since either dome or monitor lights must be used.

Flat roofs are most common for industries where daylighting requirements are minimal.

5. STABILITY OF INDUSTRIAL BUILDINGS

It is essential to ascertain the loads applied to the structure and to determine the load paths from the

cladding to the purlins and side rails, through the main frames to the foundations. The loads may arise from

dead load, wind load and snow load, and sometimes from cranes or impact caused by fork-lift trucks.

The overall resistance of simple single-storey industrial buildings to horizontal loading is usually easy to

achieve. One of the attractions of portal frame buildings is that in-plane stability follows from the rigidity

of the frame connections. Stabilising bracing between the portals is therefore only required in line with

corresponding rafter bracing in the roof plane.

For short buildings, bracing in one end bay may be sufficient. For longer buildings, bracing of two or more

bays may be necessary.

The rafter bracing itself provides restraint to the heads of the gable stanchions. The braced end bays provide

anchor points to which the longitudinal rafter stabilising ties, which are usually the purlins, are attached.

During erection, bracing facilitates plumbing and squaring of the building, as well as providing essential

stability.

For frames with lattice girders (Figure 6), in-plane stability can be provided by connecting both top and

bottom booms to the column.



If the building has roof trusses (Figure 7), or if only the top booms of the lattice girders are connected to the

column (Figure 13), the frame is effectively pinned at eaves level. To provide in-plane stability, either the

column bases should be fixed or longitudinal girders should be provided in the plane of the roof (Figure

14). These girders span between the gable ends, which must be braced appropriately. If the building is long,

or is divided by expansion joints, longitudinal bracing may not be practicable and the columns must have

fixed bases.



Buildings using lattice girders or truss roofs also need bracing to provide longitudinal stability.

Bracing members for industrial buildings commonly use circular hollow sections, rods or angles.

When cranage is provided the stability requirements need further examination, since longitudinal and

transverse surge from the crane increases the forces in the bracing systems.

6. GLOBAL ANALYSIS

The structure may be treated either as a 2-D or 3-D system.

Bracing systems are analysed as if pin-jointed. When cross-bracing is used, for example in vertical bracing,

only the members in tension are assumed to be effective (compression members are assumed ineffective

because of buckling).

The choice of the method of global analysis, either plastic or elastic, of portal frames at the ultimate limit

states depends on the class of the cross-section.

An example of the plastic collapse mechanism of a frame with haunches is given in Figure 15. Buildings

with cranes should always be analyzed elastically. Elastic analysis should always be used to determine

deflections under service loading.



7. CONCLUDING SUMMARY

Steel construction is widely used for industrial buildings, including structural members (like frames,

purlins, side rails) and cladding systems.

Overall stability is obtained from the rigidity of connections and the use of bracing systems.

The buildings may be analyzed using 2-D or 3-D modelling and elastic or plastic analysis, depending

on their cross-sections.

A wide variety of hot-rolled shapes are available for structural members. More flexibility can be

obtained using welded sections. Purlins and side rails may be formed from cold-rolled sections.





ESDEP WG 1B

STEEL CONSTRUCTION:


Lecture 1B.2.1: Design Philosophies

OBJECTIVE/SCOPE:

To explain the objectives of structural design and the uncertainties which affect it; to outline how different priorities might influence the

design, and to describe different approaches to quantifying the design process.

RELATED LECTURES:

Lecture 1B.1: Process of Design


Lecture 1B.8: Learning from Failures

Lecture 2.4: Steel Grades and Qualities

Lecture 2.5: Selection of Steel Quality

SUMMARY:

The fundamental objectives of structural design are discussed. The uncertainties associated with designing structures in terms of loading and

material properties are considered. The development of structural design methods for strength and resistance is reviewed briefly and the

importance of achieving structural stability is explained. Other design considerations such as deflections, vibration, force resistance and

fatigue are discussed. Matters of construction and maintenance are included. The importance of considering these aspects and others, such as

accommodating services and cladding costs, in developing an efficient design is emphasised. The responsibilities of the designer and the

need for effective communication are considered.

1. INTRODUCTION

The precise objectives of structural design vary from one project to another. In all cases, the avoidance of collapse is an important - if not the

most important - requirement and an adequate factor of safety must be provided. In this context, the structure must be designed in order to

fulfil both strength and stability requirements. These concepts are illustrated in Figure 1 in which a long thin rod is subject to tension (Figure

1a) and compression (Figure 1b). In the case of tension, the load resistance of the rod is governed by strength, that is the ability of the

material to carry load without rupturing. The rod can only carry this load in compression if it remains stable, i.e. it does not deform

significantly in a direction perpendicular to the line of action of the applied load. The stiffness of the structure is yet another important

characteristic, concerned with resistance to deformation rather than collapse. This is particulary important in the case of beams whose

deflection under a particular load is related to their stiffness (Figure 1c). Large deformations are not necessarily associated with collapse, and

some brittle materials, such as glass, may rupture with little prior deformation. Other considerations may also need to be included in the

design process. They include: quantifiable behaviour such as deformation, fatigue, fire resistance and dynamic behaviour; considerations

such as corrosion and service accommodation which may influence both detail and overall concept, but in a more qualitative way; and

appearance, which is largely a subjective judgement. In addition considerations of economy are likely to be a significant influence on the

great majority of structural designs. In this context questions of speed and ease of construction, maintenance and running costs, as well as

basic building costs, are all relevant. The relative importance of each of these aspects will vary depending on circumstances.



The approach to structural design is dealt with in Lecture 1B.1, which describes how the designer might begin to accommodate so many

different requirements, many of which will exert conflicting pressures. In this lecture the focus is on how a satisfactory structural design can

be achieved through a rational analysis of various aspects of the structure's performance. It is worth emphasising that the process of

structural design can be considered as two groups of highly interrelated stages. The first group is concerned with defining the overall

structural form - the type of structure, e.g. rigid frame or load bearing walls, the arrangement of structural elements (typically in terms of a

structural grid), and the type of structural elements and material to be used, e.g. steel beams, columns and composite floor slabs. A high

degree of creativity is required. The synthesis of a solution is developed on the basis of a broad understanding of a wide range of topics. The

topics include structural and material behaviour, as well as a feel for the detailed implications of design decisions made at this stage - for

instance recognising how deep a beam may need to be for a particular purpose. Formalised procedures are of little use at this stage. A

satisfactory solution depends more on the creative ability of the designer.

The later stages are concerned with the more detailed sizing of structural components and the connections between them. By now the

problem has become clearly defined and the process can become more formalised. In the case of steelwork the process generally involves

selecting an appropriate standard section size, although in some circumstances the designer may wish to use a non-standard cross-section

which, for execution, would then need to be made up, typically by welding plates or standard sections together into plate girders or trusses.

Design regulations are largely concerned with this stage of detailed element design. Their intention is to help ensure that buildings are

designed and constructed to be safe and fit for purpose. Such design legislation can vary considerably in approach. It may be based simply

on performance specification, giving the designer great flexibility as to how a satisfactory solution is achieved. An early example of this is

the building laws published by King Hummarabi of Babylon in about 2200BC. They are preserved as a cuneiform inscription on a clay tablet

and include such provisions as 'If a builder builds a house for a man and does not make its construction firm and if the house which he has

built collapses and causes the death of the owner of the house, then that builder shall be put to death. If it causes the death of the son of the

owner of the house, then a son of the builder shall be put to death. If it causes the death of a slave of the owner of the house, then the builder

shall give the owner a slave of equal value'. The danger, and at the same time the attraction, of such an approach is that it depends heavily on

the ability of the designer. Formal constraints, based on current wisdom, are not included and the engineer has the freedom to justify the

design in any way.

The other extreme is a highly prescriptive set of design rules providing 'recipes' for satisfactory solutions. Since these can incorporate the

results of previous experience gained over many years, supplemented by more recent research work they might appear to be more secure.

However, such an approach cannot be applied to the conceptual stages of design and there are many cases where actual circumstances faced



by the designer differ somewhat from those envisaged in the rules. There is also a psychological danger that such design rules assume an

'absolute' validity and a blind faith in the results of using the rules may be adopted.

Clearly there is a role for both the above approaches. Perhaps the best approach would be achieved by specifying satisfactory performance

criteria to minimise the possibility of collapse or any other type of 'failure'. Engineers should then be given the freedom to achieve the

criteria in a variety of ways, but also be provided with the benefit of available data to be used if appropriate. Perhaps the most important

aspect is the attitude of the engineer which should be based on simple 'common sense' and include a healthy element of scepticism of the

design rules themselves.

2. UNCERTAINTIES IN STRUCTURAL DESIGN

Simply quantifying the design process, using sophisticated analytical techniques and employing powerful computers does not eliminate the

uncertainties associated with structural design, although it may reduce some of them.

These uncertainties include the following:

loading.

constitutive laws of the material.

structural modelling.

structural imperfections.

Loading is discussed in more detail in Lecture 1B.3. Although it is possible to quantify loads on a structure, it is important to recognise that

in most cases these represent little more than an estimate of the likely maximum load intensity to which a structure will be exposed. Some

loads, such as the self weight of the structure, may appear to be more easily defined than others, such as wind loads or gravity waves on

offshore structures. However, there is a significant degree of uncertainty associated with all loads and this should always be recognised.

Constitutive laws are typically based on the results of tests carried out on small specimens. For convenience, the mathematical representation

of the behaviour, for instance in the form of a stress-strain curve, is considered in a simplified form for the purpose of structural design. In

the case of steel the normal representation is linear elastic behaviour up to the yield point with plastic behaviour at higher strains (Figure 2).

Although this representation provides a reasonable measure of the performance of the material, it is clearly not absolutely precise.

Furthermore, any material will show a natural variability - two different samples taken from the same batch will typically fail at different

stresses when tested. Compared with other materials, steel is remarkably consistent in this respect, but nevertheless variations exist and

represent a further source of uncertainty.

Methods of analysing structural behaviour have advanced significantly in recent years, particularly as a result of developments in computing.

Despite this, structural analysis is always based on some idealisation of the real behaviour. In some cases, such as isolated beams supported

on simple bearings, the idealisation may be quite accurate. In other circumstances, however, the difference between the model and the real



structure may be quite significant. One example of this is the truss which is typically assumed to have pinned joints, although the joints may

in fact be quite rigid and some members may be continuous. The assumption that loadings are applied only at joint positions may be

unrealistic. Whilst these simplifications may be adequate in modelling overall performance the implications, at least with regard to

secondary effects, must be recognised.

Yet another source of uncertainty results from structural imperfections which are of two types: geometrical, i.e. out of straightness or lack of

fit, and mechanical, i.e. residual stresses due to fabrication procedures or inhomogenities in the material properties. It is not possible to

manufacture steel sections to absolute dimensions - wear on machinery and inevitable variations in the manufacturing process will lead to

small variations which must be recognised. In the same way, although steel construction is carried out to much tighter tolerances than for

most other structural materials, some variations (for instance in the alignment of individual members) will occur (Figure 3).

In adopting a quantified approach to structural design, all these uncertainties must be recognised, and taken into account. They are allowed

for by the following means:

specifying load levels which, based on previous experience, represent the worst conditions which might relate to a particular structural

type.

specifying a sampling procedure, a test plan and limits on material properties.

specifying limits or tolerances for both manufacture and execution.

using appropriate methods of analysis, whilst recognising the difference between real and idealised behaviour.

These measures do not eliminate the uncertainties but simply help to control them within defined bounds.

3. DESIGNING TO AVOID COLLAPSE

3.1 Historical Background

Structural design is not something which is new. Ever since man started building - dwellings, places of worship, bridges - some design

philosophy has been followed, albeit often unconsciously. For many centuries the basis of design was simply to copy previous "designs".

Where "new developments" or modifications were introduced, trial and error techniques were all that was available. As a result many

structures were built, or partially built only to collapse or perform inadequately. Yet these failures did have a positive value in that they

contributed to the fund of knowledge about what is workable and what is not.

This unscientific approach persisted for many centuries. Indeed it still forms part of the design approach adopted today. Rules of thumb and

empirical design recommendations are frequently used, and these are largely based on previous experience. Nor is structural engineering

today totally free of failures, despite the apparent sophistication of design methods and the power of computers. The dramatic box girder

bridge collapses in the early 1970s were a grim reminder of what can happen if new developments are too far ahead of existing experience.



The emergence of new materials, notably cast and wrought iron, required a new approach and the development of more scientific methods.

The new approach included testing, both of samples of the material and proof testing of structural components and assemblies. New concepts

too were sometimes justified in this way, for instance in the case of the Forth Rail Bridge.

The first moves to rationalise structural design in a quantitative way came at the beginning of the 19th century with the development of

elastic analysis. This type of analysis allowed engineers to determine the effect (on individual structural components) of forces applied to a

complete structure.

Testing of materials provided information concerning strength and, in the case of iron and steel, other characteristics such as the elastic limit.

Of course there were often great variations in the values measured, as indeed there are even today with some materials. In order to ensure a

safe design, a lower bound on the test results - a value below which experimental data did not fall - was normally adopted as the 'strength'.

Recognising some of the uncertainties associated with design methods based on calculation, stresses under maximum working load

conditions were limited to a value equal to the elastic limit divided by a factor of safety. This factor of safety was specified in an apparently

arbitrary fashion with values of 4 or 5 being quite typical.

This approach provided the basis of almost all structural design calculations until quite recently, and for some applications is still used today.

As understanding of material behaviour has increased and safety factors have become more rationalised, so design strengths have changed.

Changes in construction practice, and the development of new, higher strength materials, have necessitated detailed changes in design rules,

particularly with regard to buckling behaviour. However the basic approach remained unchanged until quite recently when certain

limitations in classical allowable-stress design became apparent. The limitations can be summarised as follows:

i. there is no recognition of the different levels of uncertainty associated with different types of load.

ii. different types of structure may have significantly different factors of safety in terms of collapse, and these differences do not appear in

any quantifiable form.

iii. there is no recognition of the ductility and post-yield reserve of strength characteristic of structural steelwork.

The last of these limitations was overcome by the work of Baker [1] and his colleagues in the 1930s when plastic design was developed.

This method was based upon ensuring a global factor of safety against collapse, allowing localised 'failure' with a redistribution of bending

stresses. A comparison of elastic and plastic design is given by Beal [2].

In recognition of the disadvantages of the allowable stress design method, an alternative approach, known as limit state design has been

adopted. Limit state design procedures have now become well established for most structural types and materials. The approach recognises

the inevitable variability and uncertainty in quantifying structural performance, including the uncertainties of material characteristics and

loading levels. Ideally, each uncertainty is typically treated in a similar manner using statistical techniques to identify typical or

characteristic values and the degree of variation to be expected from this norm [3]. It is then possible to derive partial safety factors, one for

each aspect of design uncertainty, which are consistent. Thus different load types, for instance, have different factors applied to them. The

structure is then examined for a variety of limit states. In that case the structure is designed to fail under factored loading conditions, giving a

clearer picture of the margins of safety than was previously the case with allowable stress design.

3.2 Stability

Inadequate strength is not the only cause of collapse. In particular the designer must ensure adequate stability, both of the complete structure

(a function of the overall structural form) and of each part of it (dependent on individual member proportions and materials). The latter is

generally dealt with by modifying the material strength to account for individual conditions. Overall stability is very much more difficult to

quantify and must be carefully considered at the earliest stage of structural design. In this sense structural stability can be defined by the

conditions that a structure will neither collapse (completely or partially) due to minor changes, for instance in its form, condition or normal

loading, nor be unduly sensitive to accidental actions. Some examples are shown in Figure 4.



In designing for stability the positioning of the main load-bearing elements should provide a clearly defined path for transmitting loads,

including wind and seismic actions to the foundations. In considering wind loads on buildings it is important to provide bracing in two

orthogonal vertical planes, distributed in such a way as to avoid undue torsional effects, and to recognise the role of the floor structure in

transmitting wind loads to these braced areas (Figure 5). The bracing can be provided in a variety of ways, for instance by cross-bracing

elements or rigid frame action.



Consideration of accidental actions, such as explosions or impact, is more difficult, but the principle is to limit the extent of any damage

caused. Limitation of damage can be achieved by designing for very high loads (not generally appropriate) or providing multiple load paths.

Design requires consideration of local damage rendering individual elements of the structure ineffective, and ensuring the remaining

structure is able to carry the new distribution of loads, albeit at a lower factor of safety. Alternative strategies are to provide for dissipation of

accidental actions, for instance by venting explosions, and to protect the structure, for instance by installing bollards to prevent vehicle

impact on columns (Figure 6).



Structural stability must of course be ensured when alterations are to be carried out to existing structures. In all cases stability during

execution must be very carefully considered.

3.3 Robustness

In many ways robustness is associated with stability. Construction forms which fulfil the primary function of accommodating normal

loading conditions - which are highly idealised for design purposes - may not perform a secondary function when the structure is subject to

real loading conditions. For instance the floor of a building is normally expected to transmit wind loads in the horizontal plane to the braced

positions. Transmission of wind loads can only be achieved if there is adequate connection between the floor and other parts of the structure

and building fabric, and the floor itself is of a suitable form of construction.

4. OTHER DESIGN OBJECTIVES

Although design against collapse is a principal consideration for the structural engineer, there are many other aspects of performance which

must be considered. None of these aspects can be quantified and only certain ones will normally apply. However, for a successful solution,

the designer must decide which considerations can be ignored, what the most important criteria are in developing the design, and which can

be checked simply to ensure satisfactory performance.

4.1 Deformation

The deflection characteristics of a structure are concerned with stiffness rather than strength. Excessive deflections may cause a number of

undesirable effects. They include damage to finishes, (particularly where brittle materials such as glass or plaster are used), ponding of water

on flat roofs (which can lead to leaks and even collapse in extreme cases), visual alarm to users and, in extreme cases, changes in the

structural behaviour which are sufficient to cause collapse. Perhaps the most common example of deflection effects occurs in columns,

which are designed for largely compressive loads but may become subject to significant bending effects when the column deforms in a

horizontal plane - the so called P-delta effect.

The normal approach in design is to check that calculated deflections do not exceed allowable levels, which are dependent upon structural

type and finishes used. For instance, deflection limits for roof structures are not normally as severe as those for floor structures. In

performing these checks it is important to recognise that the total deflection max consists of various components, as shown in Figure 7,

namely:



max = 1 + 2 - 0

where 1 is the deflection due to permanent loads

2 is the deflection due to variable loads

0 is the precamber (if any) of the beam in the unloaded state.

In controlling deflections it is often necessary to consider both max and 2, with more severe limits applying in the latter case.

Although the calculated deflections do not necessarily provide an accurate prediction of likely values, they do give a measure of the stiffness

of the structure. They are therefore a reasonable guide to structural performance in this respect. With the trend towards longer spans and

higher strength materials, design for deflection has become more important in recent years. In many cases this consideration dictates the size

of structural elements rather than their resistance. In the case of certain structures, deflection control is of paramount importance. Examples

include structures supporting overhead cranes and those housing sensitive equipment. Design for deflection is likely to be the critical

condition in such cases.

4.2 Vibration

The vibration characteristics of a structure are, like deflection behaviour, dependent upon stiffness rather than strength. The design principle

is to adopt a solution for which the natural frequency of vibration is sufficiently different from any source of excitation, such as machines, to

avoid resonance. Longer spans, lighter structures and a reduction in the mass and stiffness of partitions and cladding have all contributed to a

general lowering of the natural frequencies for building structures. Cases of human discomfort have been recorded and Eurocode 3 [4] now

requires a minimum natural frequency of 3 cycles per second for floors in normal use and 5 cycles per second for dance floors.

Wind excited oscillations may also need to be considered for unusually flexible structures such as very slender, tall buildings, long-span

bridges, large roofs, and unusually flexible elements such as light tie rods. These flexible structures should be investigated under dynamic

wind loads for vibrations both in-plane and normal to the wind direction, and be examined for gust and vortex induced vibrations. The

dynamic characteristics of the structure may be the principal design criterion in such cases.

4.3 Fire Resistance

The provision for safety in the event of fire is dealt with in Group 4B. It is a common requirement that structural integrity is maintained for a

specified period to allow building occupants to escape and fire-fighting to be carried out without the danger of structural collapse. For steel

structures alternative design strategies can be adopted to achieve this requirement. The traditional approach has been to complete the

structural design 'cold' and to provide some form of insulation to the steelwork. This approach can give an expensive solution and alternative

methods have now been developed, allowing reductions, and in some cases complete elimination, of fire protection. In order to implement

these alternatives in an effective manner, it is important that, at an early stage in the design process, the structural design considers how the

fire resistance of the steelwork is to be achieved. Adopting a design solution which may be relatively inefficient in terms of the weight of

steel for normal conditions may be more than offset by savings in fire protection (Figure 8).



Buildings close to a site boundary may require special consideration to prevent an outbreak of fire spreading to adjacent sites due to

structural collapse. Again quantitative design procedures have been developed for such circumstances [5].

4.4 Fatigue

Where structures, or individual structural elements, are subject to significant fluctuations in stress, fatigue failure can occur after a number of

loading cycles at stress levels well below the normal static resistance. The principal factors affecting fatigue behaviour are the range of

stresses experienced, the number of cycles of loading and the environment. Structures which need particular consideration in this respect are

crane gantry girders, road and rail bridges, and structures subject to repeated cycles from vibrating machinery or wind-induced oscillations.

Design guidance is included in Eurocode 3 [4].

4.5 Execution

One of the principal advantages of steelwork is the speed with which execution can proceed. In order to maximise this advantage it may be

necessary to adopt a structurally less efficient solution, for instance by using the same profile for all members in a floor construction, even

though some floor beams are less highly loaded than others (Figure 9). Temporary propping should be avoided as must late changes in detail

which might affect fabrication.

It is important that the structure is not considered in isolation, but rather treated as one part of the complete construction, along with services,

cladding and finishes. By adopting a co-ordinated approach to the design, integrating the parts and eliminating or reducing wet trades, speed

of execution of the project as a whole can be maximised. A good example of this is the two-way continuous grillage system used for the

BMW Headquarters at Bracknell and other projects [6].

The installation of services can have significant implications for speed, cost and detail of construction. In buildings with major service

requirements, the cost of the services can be considerably greater than the cost of the structure. In such circumstances it may well be better to

sacrifice structural efficiency for ease of accommodating the services. The design of the total floor zone including finishes, structure, fire

protection and services also has implications for other aspects of the building construction. The greater the depth of floor construction, the

greater the overall height of the building and hence the quantity of external cladding required. In many commercial developments very

sophisticated and expensive cladding systems are used. Savings in cladding systems may more than offset the use of shallower, but less

efficient, floor construction. Where there is strict planning control of overall building height, it may even be possible to accommodate

additional storeys in this way.



4.6 Maintenance

All structures should be inspected and maintained on a regular basis, although some conditions are likely to be more demanding in this

respect. For instance, steelwork within a dry, heated interior environment should not suffer from corrosion, whilst a bridge structure in a

coastal area will need rigorous maintenance schedules. Some structural forms are easier to maintain than others, and where exposure

conditions are severe, ease of inspection and maintenance should be an important criterion. Principal objectives in this context are the

avoidance of inaccessible parts, dirt and moisture traps, and the use of rolled or tubular individual sections in preference to truss-like

assemblies composed of smaller sections.

5. DESIGN RESPONSIBILITIES

One engineer should be responsible for ensuring that the design and details of all components are compatible and comply with the overall

design requirements. This responsibility is most important when different designers or organisations are responsible for individual parts of

the structure, such as foundations, superstructure and cladding. It should include an appraisal of the working drawings and other documents

to establish, inter alia, that requirements for stability have been incorporated in all elements, and that they can be met during the execution

stage.

Effective communication both within the design team and between the designer and constructor before and during execution is essential.

Good communication will help to avoid potential design conflicts, for instance when services have to penetrate the structure, and also to

promote safe completion of the structure in accordance with the drawings and specification. The constructor may also require information

concerning results of site surveys and soil investigations, design loadings, load resistance of members, limits on positions of construction

joints, and lifting positions on members to be erected as single pieces. A statement accompanied by sketches detailing any special

requirements should be prepared when necessary, e.g. for any unusual design or for any particularly sensitive aspects of the structure or

construction. This statement should be made available to the contractor for appropriate action regarding temporary works and execution

procedures.

The designer should be made aware of the proposed construction methods, erection procedures, use of plant, and temporary works. The

execution programme and sequence of erection should be agreed between the designer and constructor.

Full and effective communication between all parties involved will help not only to promote safe and efficient execution but may also

improve design concepts and details. Design should not be seen as an end in itself, but rather as an important part of any construction

project.


There are very many uncertainties associated with structural design. However powerful the tools available, the engineer should always

recognise that the design model is no more than an idealisation and simplification of the real condition.

A quantified approach to structural design can take different forms with a view to providing a framework for satisfactory solutions.

The application of design rules should be tempered with common sense and understanding.

Structural design must consider many aspects of both performance and cost. The most efficient structural solution may not result in the

most efficient solution overall if other interdependent aspects of the construction are not considered in a co-ordinated fashion.

7. REFERENCES

[1] Baker, J.F., and Heyman, J. "Plastic Design of Frames 1: Fundamentals", Cambridge University Press, 1969.

[2] Beal, A.N. "What's wrong with load factor design?", Proc. ICE, Vol. 66, 1979.

[3] Armer, G.S.T., and Mayne, J.R. "Modern Structural Design Codes - The case for a more rational format", CIB Journal Building Research

and Practice, Vol. 14, No. 4, pp. 212-217, 1986.

[4] Eurocode 3 "Design of Steel Structures" ENV1992-1-1: Part 1: General Rules and Rules for Buildings, CEN, 1992.

[5] Newman, G.J. "The behaviour of portal frames in boundary conditions", Steel Construction Institute.

[6] Brett, P.R. 'An alternative approach to industrial building", The Structural Engineer, Nov. 1982.





ESDEP WG 1B

STEEL CONSTRUCTION:



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.

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.

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.



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.

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.

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.

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?



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.



of 19ESDEP LECTURE NOTE [WG1B]


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.

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.

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 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.

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.

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 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.

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

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.


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.

[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'.

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?





ESDEP WG 1B

STEEL CONSTRUCTION:


Lecture 1B.2.2: Limit State Design

Philosophy and Partial Safety Factors

OBJECTIVE/SCOPE

To explain the philosophy of limit state design in the context of Eurocode 3: Design of Steel Structures. To provide information

on partial safety factors for loads and resistance and to consider how the particular values can be justified.

RELATED LECTURES



Lecture 1B.8: Learning from Failures

Lecture 2.4: Steel Grades and Qualities

Lecture 2.5: Selection of Steel Quality

SUMMARY

The need for structural idealisations is explained in the context of developing quantitative analysis and design procedures.

Alternative ways of introducing safety margins are discussed and the role of design regulations is introduced. The philosophy of

limit state design is explained and appropriate values for partial safety factors for loads and strength are discussed. A glossary of

terms is included.

1. INTRODUCTION

The fundamental objectives of structural design are to provide a structure which is safe and serviceable to use, economical to

build and maintain, and which satisfactorily performs its intended function. All design rules, whatever the philosophy, aim to

assist the designer to fulfil these basic requirements. Early design was highly empirical. It was initially based largely upon

previous experience, and inevitably involved a considerable number of failures. Physical testing approaches were subsequently

developed as a means of proving innovative designs. The first approaches to design based upon calculation methods used elastic

theory. They have been used almost exclusively as the basis for quantitative structural design until quite recently. Limit state

design is now superseding the previous elastic permissible stress approaches and forms the basis for Eurocode 3 [1] which is

concerned with the design of steel structures. In the following sections the principles of limit state design are explained and their

implementation within design codes, in particular Eurocode 3, is described.

2. PRINCIPLES OF LIMIT STATE DESIGN

The procedures of limit state design encourage the engineer to examine conditions which may be considered as failure - referred

to as limit states. These conditions are classified into ultimate and serviceability limit states. Within each of these classifications,

various aspects of the behaviour of the steel structure may need to be checked.

Ultimate limit states concern safety, such as load-carrying resistance and equilibrium, when the structure reaches the point

where it is substantially unsafe for its intended purpose. The designer checks to ensure that the maximum resistance of a

structure (or element of a structure) is adequate to sustain the maximum actions (loads or deformations) that will be imposed



upon it with a reasonable margin of safety. For steelwork design the aspects which must be checked are notably resistance

(including yielding, buckling, and transformation into a mechanism) and stability against overturning (Figure 1). In some cases

it will also be necessary to consider other possible failure modes such as fracture due to material fatigue and brittle fracture.

Serviceability limit states concern those states at which the structure, although standing, starts to behave in an unsatisfactory

fashion due to, say, excessive deformations or vibration (Figure 2). Thus the designer would check to ensure that the structure

will fulfil its function satisfactorily when subject to its service, or working, loads.



These aspects of behaviour may need to be checked under different conditions. Eurocode 3 for instance defines three design

situations, corresponding to normal use of the structure, transient situations, for example during construction or repair, and

accidental situations. Different actions, i.e. various load combinations and other effects such as temperature or settlement, may

also need to be considered (Figure 3).



Despite the apparently large number of cases which should be considered, in many cases it will be sufficient to design on the

basis of resistance and stability and then to check that the deflection limit will not be exceeded. Other limit states will clearly not

apply or may be shown not to govern the design by means of quite simple calculation.

At its most basic level limit state design simply provides a framework within which explicit and separate consideration is given

to a number of distinct performance requirements. It need not necessarily imply the automatic use of statistical and probabilistic

concepts, partial safety factors, etc., nor of plastic design, ultimate load design, etc. Rather it is a formal procedure which

recognises the inherent variability of loads, materials, construction practices, approximations made in design, etc., and attempts

to take these into account in such a way that the probability of the structure becoming unfit for use is suitably small. The

concept of variability is important because the steelwork designer must accept that, in performing his design calculations, he is



using quantities which are not absolutely fixed or deterministic. Examples include values for loadings and the yield stress of

steel which, although much less variable than the properties of some other structural materials, is known to exhibit a certain

scatter (Figure 4). Account must be taken of these variations in order to ensure that the effects of loading do not exceed the

resistance of the structure to collapse. This approach is represented schematically in Figure 5 which shows hypothetical

frequency distribution curves for the effect of loads on a structural element and its strength or resistance. Where the two curves

overlap, shown by the shaded area, the effect of the loads is greater than the resistance of the element, and the element will fail.



Proper consideration of each of the limits eliminates the inconsistencies of attempting to control deflection by limiting stresses

or of avoiding yield at working load by modifying the design basis (formula, mathematical model, etc.) for an ultimate

resistance determination.

The procedure of limit state design can therefore be summarised as follows:

define relevant limit states at which the structural behaviour is to be checked.

for each limit state determine appropriate actions to be considered.

using appropriate structural models for design, and taking account of the inevitable variability of parameters such as

material properties and geometrical data, verify that none of the relevant limit states is exceeded.

3. ACTIONS

An action on a structure may be a force or an imposed deformation, such as that due to temperature or settlement. Actions are

referred to as direct and indirect actions respectively in Eurocode 3.

Actions may be permanent, e.g. self-weight of the structure and permanent fixtures and finishes, variable, e.g. imposed, wind

and snow loads, or accidental, e.g. explosions and impact (Figure 6). For earthquake actions, see Lectures 17 and Eurocode 8

[2]. Eurocode 1 [3] represents these by the symbols G, Q and A respectively, together with a subscript - k or d to denote

characteristic or design load values respectively. An action may also be classified as fixed or free depending upon whether or

not it acts in a fixed position relative to the structure.



3.1 Characteristic Values of Actions (Gk, Qk and Ak)

The actual loadings applied to a structure can seldom be defined with precision; liquid retaining structures may provide

exceptions. To design a structure for the maximum combination of loads which could conceivably be applied would in many

instances be unreasonable. A more realistic approach is to design the structure for 'characteristic loads', i.e. those which are

deemed to have just acceptable probability of not being exceeded during the lifetime of the structure. The term 'characteristic

load' normally refers to a load of such magnitude that statistically only a small probability, referred to as the fractile, exists of it

being exceeded.



Imposed loadings are open to considerable variability and idealisation, typically being related to the type of occupancy and

represented as a uniform load intensity (Figure 7). Dead loads are less variable although there is evidence that variations arising

in execution and errors can be substantial, particularly in the case of in-situ concrete and finishes such as tarmac surfacing on

road bridges.

Loadings due to snow, wind, etc. are highly variable. Considerable statistical data on their incidence

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