Engineering and Construction

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The solution of this type of beam consists, in the first instance, of the evaluation of the fixing or negative moments at the supports.

The most general method is the use of Clapeyron’s Theorem of Three Moments.

The theorem applies only to any two adjacent spans in a continuous beam and in its simplest form deals with a beam which has all the supports at the same level, and has a constant section throughout its length.

The proof of the theorem results in the following expression:

where MA,MB and MC are the numerical values of the hogging moments at the sup- ports A, B and C respectively, and the remaining terms are illustrated in Fig. 10.7.

In a continuous beam the conditions at the end supports are usually known, and these conditions provide starting points for the solution.

The types of end conditions are three in number:

(1) simply supported
(2) partially fixed, e.g. a cantilever
(3) completely fixed, i.e. the end of the beam is horizontal as in the case of a fixed beam.

The SF at the end of any span is calculated after the support moments have been evaluated, in the same manner as for a fixed beam, each span being treated  separately.

It is essential to note the difference between SF and reaction at any support, e.g. with reference to Fig. 10.7 the SF at support B due to span AB added to the SF at B due to span BC is equal to the total reaction at the support.

If the section of the beam is not constant over its whole length, but remains  constant for each span, the expression for the moments is rewritten as follows:


in which I1 is the second moment of area for span L1 and I2 is the second moment of area for span L2.

Fig. 10.7 Clapeyron’s Theorem of Three Moments

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Propped cantilevers
Beams which are built-in at one end and simply-supported at the other are known as propped cantilevers. Normally, the ends of the beams are on the same level, in which case bending moments and reactions may be derived in two ways: by employing the Theorem of Three Moments or by deflection formulae.Appropriate formulae for propped cantilevers under various types of loading are  presented in the Appendix Bending moment, shear and deflection tables for propped cantilevers.

Sinking of supports
When the supports for a loaded propped cantilever do not maintain the same relative levels as in the unloaded condition, the BM and SF may be obtained by using the deflection method (Fig. 10.2).When the prop, B, sinks the load which it takes is reduced, while the fixing moment at the other end is increased. Two special cases arise: the first when the prop sinks so much that no load is taken by the prop, and the second when the built-in end sinks so much that the fixing moment is reduced to zero, i.e. the cantilever resembles a simple support beam. The two special cases are shown in Fig. 10.2.

Fig. 10.2 Bending moment diagram for propped cantilever with sinking support

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The calculations required to obtain the shear forces (SF) and bending moments (BM) in simply-supported beams form the basis of many other calculations required for the analysis of built-in beams, continuous beams and other indeterminate  structures.

Appropriate formulae for simple beams and cantilevers under various types of loads are presented in the Appendix Bending moment, shear and deflection tables for cantilevers and simply-supported beams.

In the case of simple beams it is necessary to calculate the support reactions before the bending moments can be evaluated; the procedure is reversed for built- in or continuous beams. The following rules relate to the SF and BM diagrams for beams:

(1) the shear force at any section is the algebraic sum of normal forces acting to one side of the section
(2) shear is considered positive when the shear force calculated as above is upwards to the left of the section
(3) the BM at any section is the algebraic sum of the moments about that section of all forces to one side of the section
(4) moments are considered positive when the middle of a beam sags with respect to its ends or when tension occurs in the lower fibres of the beam
(5) for point loads only the SF diagram will consist of a series of horizontal and vertical lines,while the BM diagram will consist of sloping straight lines, changes of slope occurring only at the loads
(6) for uniformly distributed loads (UDL) the SF diagram will consist of sloping straight lines, while the BM diagram will consist of second-degree parabolas
(7) the maximum BM occurs at the point of zero shear, where such exists, or at the point where the shear force curve crosses the base line.

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The finite element idealization should represent the real structure as closely as possible with regard to geometrical shape, loading and boundary conditions. The geometrical form of the structure is the major factor to be considered when deciding the shape of elements to be used. In two-dimensional analyses the most frequently used elements are triangular or rectangular shapes. The triangular element has the advantage of simplicity in use and the ability to fit into irregular boundaries. Figure 9.29 shows an example using triangular elements and Fig. 9.30 shows a combination of triangular and rectangular elements. These figures also demonstrate the need to use relatively small elements in areas where high stress gradients occur. In many such cases the geometrical shape of the structure is such that  a  fine mesh of  elements is required in order to match this shape.

Fig. 9.29 Circular hole in uniform stress field

Fig. 9.30 Perforated tension strip (plane stress) – a quarter of a plate is analysed

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The evaluation of the stress resultants in members of skeletal frames involves the solution of a number of simultaneous equations.When a structure is in equilibrium, every element or constituent part of it is also in equilibrium. This property is made use of in developing the concept of the free body diagram for elements of a structure.

The portal frame sketched in Fig. 9.16 will now be considered for illustrating the concept. Assuming that there is an imaginary cut at E on the beam BC, the part ABE continues to be in equilibrium if the two forces and moment which existed at section E of the uncut frame are applied externally.The internal forces which existed at E are given by (1) an axial force F, (2) a shear force V and (3) a bending moment M. These are known as stress resultants. The external forces on ABE, together with the forces F, V and M, keep the part ABE in equilibrium; Fig. 9.16(b) is called the free body diagram. On a rigid jointed plane frame there are three stress resultants at each imaginary cut. The part ECD must also remain in equilibrium. This  consideration leads to a similar set of forces F, V and M shown in Fig. 9.16(c). It  will be noted that the forces acting on the cut face E are equal and opposite. If the two free body diagrams are moved towards each other, it is obvious the internal forces F, V and M cancel out and the structure is restored to its original state of equilibrium.As previously stated, equilibrium implies ΣPx = 0; ΣPy = 0; ΣM = 0 for a planar structure. These equations can be validly applied by considering the structure as a whole, or by considering the free body diagram of a part of a structure.

In a similar manner, it can be seen that a three-dimensional rigid-jointed  frame has six stress resultants across each section. These are the axial force, two shears in two mutually perpendicular directions and three moments, as shown in Fig. 9.17.

Fig. 9.16 Free body diagram

Fig. 9.17 Force and moments in x, y and z directions

With pin-jointed frames, be they two- or three-dimensional, there is only one stress resultant per member, viz. its axial load.When forces act on an elastic structure, it undergoes deformations, causing displacements at every point within the structure.

The solution of forces in the frames is accomplished by relating the stress result- ants to the displacements. The number of equations needed is governed by the degrees of freedom, i.e. the number of possible component displacements. At one end of the member of a pin-jointed plane frame, the member displacement has translational components in the x and y directions only, and no rotational displacement. The number of degrees of freedom is two. By similar reasoning it will be apparent that the number of degrees of freedom for a rigid-jointed plane frame member is three. For a member of a three-dimensional pin-jointed frame it is also three, and for a similar rigid-jointed frame it is six.

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Commercial buildings use energy primarily for space and water heating, lighting, cooling, ventilation and small power. In order to reduce operational energy, buildings should be designed to provide maximum occupant comfort throughout the year with minimum energy requirements. The charts in Fig. 8.2 show typical energy requirements and carbon dioxide emissions for two types of commercial building.

Fig. 8.2 Comparison of operational energy requirements and resulting carbon emissions for a typical air conditioned office and a good practice naturally ventilated open plan office (source: Econ 19, Energy use in offices, Energy Efficiency Best Practice Programme)

Although heating accounts for a significant part of the annual operational energy use, it produces a relatively small proportion of the overall carbon emissions since it is generally provided by burning natural gas. In the UK, gas produces only 41% of the carbon dioxide emissions per kilowatt hour that are produced by electricity.

Therefore the greatest scope for reducing carbon emissions from commercial buildings is in reducing the energy required for cooling, mechanical ventilation and artificial lighting.

Energy use is heavily dependent upon building design, including such factors as layout, orientation, thermal capacity, glazing arrangements, solar  shading, cooling and ventilation.The choice of structural system (steel or concrete frame4 ) has been found to have very little effect on operational energy requirements, so the follow- ing notes apply in general to all commercial buildings.

Structural steel gives the design team the flexibility it needs  to exploit these factors to the full.

One of the major ways of providing the necessary comfort levels is to make use of fabric energy storage.

This involves using the floor plate to absorb heat during the period when the building is occupied during the day and then purging it at night as shown in Fig. 8.3.

Initially there was a perception that thick concrete floors were needed for the system to be effective. However, it has been shown that, with a 24-hour cycle of heating and cooling, the necessary volume and disposition of concrete can be provided by composite construction.

Perforated permeable ceilings can be used to permit the necessary contact between the warm air and the floor. Enhancements can be achieved by ducting air immediately above or below the slab. Examples are illustrated in the SCI publication Environmental Floor Systems.

Fig. 8.3 Diurnal cycle for fabric energy storage

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Residential buildings account for about 30% of the total energy used in the UK. This relatively high percentage of energy use, together with increasing demand for new housing, indicates the importance of achieving improved energy efficiency in this sector.

Light steel frame residential buildings have proved to have good standards of environmental performance, particularly in terms of energy conservation. Many have significantly exceeded the requirements of UK Building Regulations in terms of thermal performance, as shown in Table 8.1.

Table 8.1 Comparison of thermal performance of light steel frame residential buildings with UK Building Regulations

The following measures are recommended to reduce operational energy  requirements:

• Minimization of heat loss through the building envelope
• Provision of an airtight building envelope with controlled ventilation
• Provision of efficient and controlled heating
• Use of appropriate passive heating, cooling and ventilation methods.

These measures can often be achieved at little or no extra overall cost, particularly where savings can be made in the capital cost of heating systems as a result of additional insulation and airtight construction. To accompany the regulations, a document is being developed showing robust details of plans which will, if properly constructed, give compliance with the UK Building Regulations. To achieve compliance with the new regulations will be easier with framed construction than  with traditional methods.

Light steel frame housing has several generic advantages but as with all forms of construction attention should be paid to the detailing in order to maximize the benefits.

Placing the insulation on the outside of the frame is known as warm frame construction; this minimizes heat loss since there is minimal thermal bridging. Risk of condensation is eliminated since all the members are contained within the warm internal environment. High levels of insulation are possible however without  significantly increasing wall thickness by the addition of insulation between the frame members. Heat loss resulting from air infiltration can also be conveniently minimized by the inclusion of an air tight membrane in the walls.

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The embodied energy burdens of commercial buildings vary considerably depending upon their specification, as illustrated in Fig. 8.1. For example, prestige office buildings can have twice the embodied energy content of lower specification offices.

This is due to the way they are clad and fitted out rather than  the effects of the structural frame.

Embodied energy is typically small in comparison to operational energy over the life of a commercial building (generally an order of magnitude less).The balance is however changing as improved designs reduce operational energy and the rate of change of use increases so requiring more refurbishment.

Fig. 8.1 Comparison of embedded energy and operating energy for a basic  grade and prestige office over a 60-year life

The embodied energy of the structural components of a typical office building is only about 25–30% of the total embodied energy and tends to vary relatively little irrespective of the type of structure.A life cycle assessment (LCA) study of a typical four storey office building compared five alternative steel and concrete structures.

No significant difference was identified between the embodied energy burdens of the different options.Although steel has a high embodied energy content per kilo- gram, its high strength to weight ratio means that a much lower volume of steel is required to construct a steel frame than the volume of concrete needed to construct an equivalent frame. The cross-sectional area and mass of steel structural members is very much smaller than that of equivalent concrete members.

The comparative lightness of modern steel frame structures has been important in reducing their embodied energy burdens. In addition to savings associated with the frame materials, designers can often adopt lighter foundation systems than would be required for other forms of construction. Alternatively the number of foundations may be reduced, since the spanning capabilities of steel can allow larger structural bays, and consequently fewer column bases requiring support.The larger bays also allow for more flexible use of space; steel frames are  easily adaptable, thereby prolonging the building life and reducing yet further the impact of embodied energy over the life cycle.

In composite construction, where beams act in tandem with the floor slab, the additional strength and stiffness that are achieved allow the use of relatively shallow beams. These have lower embodied energy and the volume of concrete in the floor is barely altered.

Modern portal frame design has played a significant role in reducing the em- bodied energy burden of single storey industrial and commercial buildings. Portal frames are capable of spanning considerable distances using relatively light sections.

Roof beam depths are generally of the order of span divided by 60, as opposed to span divided by 25 for a conventional beam and column roof. The combination of steel frames with light gauge purlins and rails supporting insulated cladding systems provides a very efficient form of construction resulting in the very high (95%) major share. Systems are available to meet the requirements of the new Part L to the UK Building Regulationsin terms of both insulation and air tightness.

A key element in achieving low embodied energy is the pursuit of structural  efficiency. By reducing the amount of structural steel required per unit area  of commercial building, designers can not only achieve elegant and economic  structural solutions, but also reduce embodied energy burdens.

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The UK steelwork sector has worked and continues to work hard to provide the most economic solutions and work methods. It has become world-class through its investments in IT and associated technologies, based on the use of 3-D modelling for detailing with direct links to CNC machines and suppliers. This is proving to be a strong foundation for the release of further benefits which stems from integration with design activities through the adoption of data transfer protocols such as the CIMSteel Integration Standards.

Since long before the needs of sustainability were fully recognized, there has been a history of continuous improvement in structural design methods which have resulted in economies to clients over a period of years.

This has been encouraged and has been complemented by the development of systems and components,many involving the use of light gauge steel in addition to the more traditional hot rolled sections.

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Introduction
The UK Government, along with many others, has a very strong policy to encour- age sustainable development. In May 1999, it published A better quality of life – a strategy for sustainable development for the United Kingdom. The principles in this document were further developed for the construction industry in Building a better quality of life – a strategy for more sustainable construction, which was published in April 2000.The underlying principles of sustainability lie in the appropriate balance of economic, social and environmental impacts, the so-called triple bottom line.The strong message is that for a viable long term future any enterprise must pay due concern to the following issues:

• Maintenance of high and stable levels of economic growth and employment
• Social progress which recognizes the needs of everyone
• Prudent use of natural resources
• Effective protection of the environment.

Clearly, to be successful all these issues need to be balanced, as over-commitment to any one of them at the expense of the others will lead to failure in the long term.

There are many examples of how the steel construction sector has progressed to be itself more sustainable and contribute to sustainable construction. Some of these are set out below.

Steel has a significant role in construction and has established a strong position.

In the UK structural frames now have approximately 70% of the multi-storey frame and 95% of the single storey industrial buildings market.

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The design requirements for steel structures in which brittle fracture is a consideration are given in most structural codes.There are several key factors which need to be considered when determining the risk of brittle fracture in a structure. These are:

(1) minimum operating temperature
(2) loading – in particular, rate of loading
(3) metallurgical features such as parent plate, weld metal or HAZ
(4) thickness of material to be used.

Each of these factors influences the likelihood of brittle fracture occurring.

From experimental data and parametric studies examining the maximum tolerable defect sizes in steels under various operating conditions, codes such as BS 5400-3 and BS 5950-1 provide tables giving the maximum permissible thickness of steel at given operating temperatures.Where a design does not fit into these broad categories attempts have been made to use fracture mechanics to provide a criterion for material selection in terms of Charpy test energy absorption as this information is usually provided by the steel makers. When true fracture mechanics toughness values are available, methods such as those described in References 8 and 9 should be employed to assess the acceptability of flaws in fusion welded structures.

As discussed earlier, at normal operating temperatures and slow rates of loading valid KIc values are not usually obtained for structural steels, and in the offshore and nuclear industries critical CTOD and J tests are widely used. Under these conditions, valid KIc values from low temperature tests will be conservative. If the structure is acceptable when assessed using such conservative data then there is often no great need to pursue the problem, particularly as all forms of fracture mechanics testing are expensive compared with routine quality control tests such as Charpy testing.

In general, the fracture toughness of structural steel increases with increasing temperature and decreasing loading rates. The effect of temperature on fracture toughness is well known. The effect of loading rate may be equally important, not only in designing new structures, but also in understanding the behaviour of existing ones which may have been built from material with low toughness at their service temperature.The shift in the ductile–brittle transition temperature for structural steels can be considerable when comparing loading rates used in slow bend tests with those in Charpy tests. Results from experimental work have shown that the transition temperature for a BS EN 10025 S355 J steel can change from around 0°C to -60°C with decreasing loading rate.

Materials standards set limits for the transition temperatures  of various steel grades based on Charpy tests.When selecting an appropriate steel for a given structure it must be remembered that the Charpy values noted in the standards apply to parent plate. Material toughness varies in the weld and heat-affected zones of welded joints, and these should be checked for adequate toughness. Furthermore, since larger defects may be present in the weld area than the parent plate, appropriate procedures should be adopted to ensure that a welded structure will perform as designed. These could involve non-destructive testing including visual examination. In situations where defects are found, fracture mechanics procedures such as those in References 8 and 9 can be used to assess their significance.

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The structural implications of the above can be summarized as follows:

• Structural fire resistance: The required fire resistance for elements of structure is equivalent to that of the building without the atrium.
• Facade: The structure needs to ensure that the facade can achieve its  required performance.This may equate to ensuring that the deflections in a fire condition are limited or, more simply, by specifying a fire-resisting system for the facade.

Although BS 5588: Part 7 defines a performance criterion for ‘smoke retarding’ construction, consideration should be given to specifying fire-resisting construction instead; this is because there is no standard specification for smoke-retarding construction, and there will be necessity to develop ad-hoc material specification, construction methods and QA procedures to ensure that the system meets the required performance and to demonstrate this to the regulators.

• Atrium roof: Whilst structural elements solely supporting the roof neither  formally nor usually require fire resistance, it is prudent to consider the effect of collapse in development of the fire strategy. Thus potentially there may be a requirement for fire resistance. Also, as normal, where the roof structural elements support elements requiring a fire-resistance (such as the atrium facade), then these structural elements must achieve the required fire performance.

The fire strategy for a building with an atrium is complex, particularly where there is a need to incorporate local government Acts or property protection requirements, or where there are non-standard client, architectural, or engineering objectives.

A fire-engineered approach will enable the optimum strategy and best value to be achieved. Structural fire engineering can be used with an atrium building in the same way as for a building without an atrium, taking account of  the inherent  performance of the structure to reduce or omit the need for fire protection. Fire engineering enables best integration of the fire strategy within the design, to ensure that all systems are tailored to meet the precise performance and reliability requirements (for fire and all other requirements).

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Background
The fundamental principle of atrium fire safety design is to ensure that the design standard is not compromised by the presence of the atrium; the standard of the building with the atrium should not be less than the standard of an equivalent building without the atrium. This section describes the considerations and processes involved in the development of the atrium fire strategy, concluded by summarizing the implications for the structural design.

The primary objective of the fire strategy is to achieve an acceptable standard of life safety. BS 5588: Part 7 is the relevant ‘life-safety’ guidance for the incorporation of atria into buildings, and is applicable in all but limited circumstances.A package of fire safety provisions is developed to address the following requirements:

• Means of escape
• Compartmentation
• Structural fire resistance
• External spread of fire
• Facilities for the fire service. Smoke clearance is relevant for this situation.

This package depends on the size and use of the building.As examples:

(1) In a small office building, the design is for simultaneous evacuation and there is no need for compartmentation between storeys. Thus in this situation the atrium can be open, as the consequences of a fire anywhere in the building with an atrium are no different (in terms of life safety) than those in the same building without an atrium.
(2) In tall office buildings, storey-to-storey compartmentation is defined to enable phased evacuation; this vertical compartmentation needs to be maintained in the building with an atrium.
(3) In a hospital, as well as vertical compartmentation there is a need for horizontal compartmentation.This enables progressive horizontal evacuation of the fire compartment, with all other compartments remaining in place.
(4) With an atrium it is necessary to consider thermal radiation across the atrium, to ensure that this does not create untenable conditions in a non-fire compartment on the same storey adjacent to the atrium.

In situations where occupants remain in the building, the psychological effects of smoke in the atrium must also be considered, even though there may be no other risk posed by the fire.

The property protection and business continuity implications must be carefully considered. Even in situations where there is an acceptable standard of life safety, the atrium provides the opportunity for substantial spread of smoke, especially  with an unenclosed atrium. These additional requirements are usually specified by the client, operator, or insurer and not regulated under legislation. However, it should be noted that there are Local Government Acts (such as Section 20 in London) and national legislation outside England which have property protection implications.

The design procedure of the atrium can be generalized by consideration of:

• Use of the accommodation adjacent to the atrium
• Use of the atrium base
• Performance of the atrium façade
• Control of smoke within the atrium.

Effective smoke control is usually the key to the success of the atrium fire strategy.

This can be achieved either by preventing smoke entering the atrium (e.g. an atrium façade with fire-resisting performance and a fire-sterile atrium  base) or, alternatively, by managing any smoke within the atrium. The atrium can be divided into three zones:

(1) Atrium base
(2) The smoke reservoir at the top of the atrium, where any hot smoke will collect and can be extracted
(3) Remainder of the atrium.

Different design considerations will apply for these different zones.

The smoke management will normally take advantage of a smoke extract system.

The design of this system depends on the quantity of smoke within the atrium and the extract capacity. The quantity of smoke is dependent on the fire size and width of spill, where the smoke spills into the atrium. Complex spill geometries create large quantities of smoke. Smoke extract can be by either mechanical extract or natural systems. In either case it is necessary to arrange for low-level make-up for replacement air.Wind-effects on natural systems must be addressed.The extract can serve either to limit the depth of the smoke reservoir, or to limit the smoke reser- voir temperature, or both. In either case the fire size must be controlled.

The fire size is normally controlled either by use of sprinklers or by management control. Management control defines islands of fire load, the size  of the island dependent on the amount of combustibles, separated to ensure that the fire does not spread between islands. Typical rules are 10m2 islands separated by 4m.

Sprinklers are usually assumed in smoke control calculations to limit the fire size to a maximum area equal to the sprinkler-grid area. In practice, sprinklers may extinguish the fire or control it to a substantially smaller area.As an alternative to sprinklers or management control, a fire-engineered approach can  be used to account for the likelihood of a particular fire size occurring, given the use of the accommodation (‘natural fire’ based design).

It is important also to account for cool smoke, which cannot be assumed to rise (and may even drop).Where the smoke cannot be assumed to be hot enough to rise to the reservoir by buoyancy, it will be carried by the airflows that exist within the atrium. CFD analysis may be required to design the cold smoke system. Careful coordination with the building services engineer will enable the best value solution, particularly with natural ventilation systems, as the cold smoke is primarily carried by the environmental flows. An upward flow of air is beneficial for both the fire strategy and the environmental strategy.

Use of the atrium base can be enabled, provided that the rules for smoke and fire spread are respected. It is difficult to use sprinklers for controlling the fire size in the atrium base, as there is no ceiling above it (the atrium roof is usually too high to enable effective operation of sprinklers), and thus fire load control is either by management control or by an engineered approach.

Where the atrium facade is required to have a fire performance, this is defined either to prevent the fire breaking into the atrium, or to prevent the smoke breaking back in.Where an enclosure is required to have a fire-resisting performance, this can either be achieved by a formal fire-resisting system, or alternatively a smoke-retarding construction may be used in combination with an extract system to limit smoke temperatures. There is currently no standard for smoke-retarding construction, and thus it is often preferable to specify fire-resisting construction to avoid complicated specifications and enhanced site control.

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There are several factors that affect the cost of steel structures of this type:

• Type of section used
• Complexity of fabrication
• Complexity of installation
• Type of finish
• Quality of workmanship
• Procurement arrangements

It is common to budget steelwork on the basis of rates applied  to each tonne of material.While this is a useful indication of cost, it is important to recognize that the appropriate rates for these types of structure may be two to four times greater than those for standard structural steelwork.

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Ongoing and long term cleaning and maintenance will be required and may impact on the design. Ongoing work will require a simple safe means of access. It will be necessary to account for the loading and connection of gantries, walkways, harness points, etc., in the design of the primary structure.

Long term maintenance could be facilitated by independent access systems such as scaffolding. The main impact on the design of the structure will be the quality of the specification. The life to first maintenance of the finishes and components will increase with initial cost.This needs to be balanced by the costs of the maintenance activity.The client should be consulted on the options available and choose the most appropriate option. This will depend on his/her long term interest in the building fabric.

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An atrium structure will have several complex interfaces:

• The cladding
• Cleaning equipment
• Supporting structure

The management of these interfaces is critical to the successful implementation of the design. If the structure, cladding, etc are procured through separate contracts it is even more critical to define the technical requirements and responsibilities at interfaces. The following issues should be addressed:

• Forces and movements to be accommodated
• Compatibility requirements of materials
• Design responsibility for components and fixings
• Definition of overall theoretical and as-built geometry.

It can be convenient to incorporate several packages of work into a single contract.

This can reduce risk associated with the management of interfaces. This route may increase cost due to the increased responsibilities carried by the contractor.

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The engineer should always consider the erection method for a structure. In the case of atria this may be of particular concern for the following reasons;

• The site is constrained by surrounding buildings
• The application of load to existing structures to be carefully controlled
• The need to precamber the geometry
• Restrictions on the location, reach and capacity of craneage
• Limits on the size of components
• The formation of neat site connections
• Safe access for operatives for the erection of the primary structure, cladding and other items.

The erection methodology may preclude the use of some structural forms. It is, therefore, essential to consider this aspect at concept stage.

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The detailing of individual members and their connections is crucial to achieving a satisfactory architectural treatment. It is commonly the fabricator’s responsibility to detail connections, and often this expertise can inform the design process.However, it is important to ensure that aesthetic considerations are properly defined and controlled. The following should be considered:

• Nature of the connection between components of the primary structure
• Connection of the cladding system
• Connection of sundry components such as access equipment, lighting, etc.

Connections can be expressed or ‘invisible’.The strategy for connecting components should take into account the practicality of achieving the desired outcome. Site welding can offer ‘invisible connections’ within certain limitations. The following should be considered:

• The type of welding technique and position required to form a connection
• Butt welding can be ground flush whereas fillet welds cannot
• The type of weld testing required to verify connection adequacy
• The effects of heat distortion on the structural strength and geometry
• The effect of welding on shop-applied finishes.

Where connections are aesthetically critical, sample workmanship should always be called for as a basis for selecting a contractor. Samples should be kept as a record against which to judge finished workmanship.

Steelwork will require a paint finish for reasons of aesthetics and durability. Paint technology provides an ever increasing number of options.However, the choice can be guided by consideration of the following criteria:

• The steelwork fabricator who delivers and erects the steel will inevitably damage any paint applied at the works no matter how carefully the protection measures are prepared.
• The contractor who has a very tight programme may like to see primer, under- coat and finish coat on the steelwork applied at the works since it may be very difficult to apply the finish in high-level steelwork without extensive temporary works.
• Very hard paint finishes are more difficult to damage although when the inevitable occurs they are usually more difficult to repair to a good standard.
• If the finish-coat is applied on site, some parts of the steelwork may be difficult to reach and the standard of finish will not be as good as that achieved in the paint shop.
• Unless touching up to damaged areas is very minor, the result is unlikely to  be satisfactory and therefore a full site applied decorative finish may prove  necessary.
• The choice of system will also depend on the fabricator’s paint shop and its relationship with the paint supplier.

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Atria generally require transparent or translucent forms of cladding. Glazing is a traditional solution but is relatively heavy. If the space beneath is heated the thermal performance may dictate double-glazing, which is both heavy and  costly. Other forms of cladding such as ETFE foil cushions can offer a lightweight, thermally  efficient and cost-effective alternative to glazing.

Glazing systems may be framed or unframed.The allowable spans of the systems vary.The relationship between the layout of the primary structure and the cladding system will determine whether there is a need for a secondary system of support.

It is important to consider this relationship at concept stage to achieve the minimum  of elegantly arranged members. The following dimensions provide guidance:

• Planar glazing – spans in either direction around 2m
• Framed systems – spans 4–6m by 2m
• ETFE foil cushions – spans 6–8m in either direction.

Manufacturers should be consulted for more precise information.

Framing systems are generally made from aluminium extrusions, and therefore have thermal expansion and contraction characteristics which differ from steelwork.

This needs to be considered in the detailing of the connectivity between the two elements.

Tolerance between cladding and supporting steel framework also needs careful consideration. The accuracy with which steelwork can be fabricated and erected is generally lower than that of the cladding system. This incompatibility should also be accounted for in the detailing of connections.

Deflections of the atrium structure under loads can have significant bearing on the cladding system. Some glazing systems require a minimum slope to ensure their water tightness. The deflected shape of the supporting structure must satisfy these requirements under all relevant loading conditions. This may require the structure to incorporate a precamber from the theoretical final geometry.The cladding system must also be capable of accommodating any differential movements of the sup- porting structure.

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For a new building there will be significant choice in the types of structural systems that can be adopted, as the perimeter supports can be designed accordingly.Where an enclosure is being added to an existing building, available support will depend on the capabilities of the existing structure and may determine the appropriate options.

The structures in Figs 5.36 to 5.40 illustrate a series of solutions in which there is a generally decreasing reliance on the perimeter structure.

Fig. 5.36 Arch

Fig. 5.37 Beam or truss

Fig. 5.38 Half portal

As a group the arch, the beam and the dome are potentially the most structurally efficient solutions but rely totally on the support provided by the perimeter structure. In the case of an existing building, the following should be considered:

• The strength and integrity of the existing structure if this is to be used for support.
• The percentage increase in vertical load imposed on the existing structure by the new.This will be smaller where new loads are introduced towards the lower levels of existing walls and columns than at the top.
• The horizontal forces imposed at the supports. Horizontal thrusts introduced  at lower levels will have smaller resultant overturning effect  than at a higher level.















Failure to investigate the implications of the choice of structural form could result in a costly and clumsy solution.

As town centres and streets are redeveloped to compete with out-of-town shopping, the need for enclosure is a common solution, often considered. In these cases, the adjacent buildings are typically of a wide variety of constructions of various heights and condition.The solutions defined in Fig. 5.39 may be appropriate to avoid the complexities of resolving the various boundary conditions.The perimeter structure is required to support a small proportion of the vertical  load and relatively small horizontal loads from wind and stabilizing forces for the new columns, which does not generally pose a problem since there is usually some redundant horizontal capacity due to the wind shielding effect of the new enclosure. The portal style solution (Fig. 5.40) is substantially self-supporting.

Temperature effects are often a governing design consideration  for an atrium structure due to exposure to heating from sunlight. Temperatures in steelwork painted black can be 30° higher compared with white where directly exposed to the sun’s radiation. It is important to allow for movement of the structure, to prevent locked-in stresses from building up.Releases of this sort are often incompatible with providing horizontal stability. The choice of structural form needs to reflect these opposing requirements. For example, a statically determinate structure such as a three pinned arch can accommodate temperature movements and differential  settlements without generating secondary internal forces (see Fig. 5.41).

Fig. 5.39 Beam and column

Fig. 5.40 Portal frame

(b) Fig. 5.41 Movements caused by temperature changes

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An atrium is an enclosed courtyard space often rising through several storeys of a building. Its function is usually to provide daylighting within the plan of a building.

Often it provides covered circulation space between floor levels which readily orientates the building occupants. In common with all aspects of building design, there are a number of design considerations which come into play in the creation of an atrium space. These broad design requirements generate specific technical requirements for the design of the structure. These need to be taken into consideration at the developmental stages to ensure that an elegant design, consistent with all of the technical criteria, can be achieved.

The following design issues are common to the design of an atrium space:

• Structural form
• Type of cladding
• Tolerance and deflection
• Finishes and detailing
• Erection of structure, cladding and fit out
• Procurement
• Maintenance of the cladding, structure and fit out
• Internal and external cleaning
• Acoustic performance of the space
• Cost
• Fire safety of the atrium and surrounding spaces
• Provision of heating and cooling to achieve required environmental conditions.

The following sections look at some of the technical implications for the structural design and specification imposed by these design issues.

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The galvanized strip steel from which the light steel framing is formed is usually designated as either grade S280GD or grade S350GD to BS EN 1014710 (formerly Fe E 280 G or Fe E 350 G). These designations indicate the yield strength (280 or 350N/mm2 ) and the fact that the material is galvanized with a minimum G275 coating. Cold formed steel sections are usually rolled from galvanized sheet steel that is typically 0.9–3.2mm thick. The normal thickness of zinc coating (275 g/m2 ) has excellent durability for internal applications. Heavier coatings are available for more aggressive external environments.

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‘Stick-build’ or panel components may be assembled in either ‘platform’ or ‘balloon’ construction, as illustrated in Fig. 5.35. In platform construction, walls and floors are built sequentially one level at a time, so the walls are not structurally continuous.

In some forms of construction, loads from the walls above are transferred through the floor joists to the wall below.

In ‘balloon’ construction, the wall panels are often much larger and are continuous over more than one storey. Such panels are more difficult to erect than  single storey height panels and have to be temporarily braced whilst the floors are installed. The main advantage of this approach is that loads from the walls above are transferred directly to those below.

In both forms of construction, the external cladding or finishes  are generally installed and attached to the frames on-site.

Fig. 5.35 ‘Platform’ and ‘balloon’ forms of panel construction

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In modular construction, units are completely prefabricated in the factory and may be delivered to site with all internal finishes, fixtures and fittings in place, as illustrated in Fig. 5.34. Units may be stacked side by side, or one above the other, to form the stable finished structure.

Modular construction is most cost-effective where large production runs are possible for the same basic configuration of modular unit. This is because prototyping and set-up costs, which are essentially independent of scale, can be shared across many units. Details of the use of modular construction in residential construction are given later.

Fig. 5.34 Modular construction using light steel framing

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Wall panels, floor cassettes and roof trusses may be prefabricated in a factory and later assembled on site, as in Fig. 5.33. For accuracy, panels are manufactured in purpose-made jigs. Some of the finishing materials may be applied in the factory, to speed on-site construction. Panels can comprise the steel elements alone or the facing materials and insulation can be applied in the factory. The panels are connected on site using conventional techniques (bolts or self drilling screws).

The main advantages of panel or sub-frame construction are:

• speed of erection of the panels or sub-frames
• quality control in production
• reduced site labour costs
• scope for automation in factory production.

The geometrical accuracy and reliability of the panels and other components is better than with stick-build construction because panels are prefabricated in a factory environment. The accurate setting out and installation of foundations is a key factor to achieve rapid assembly of the panels and to obtain the maximum  efficiency of the construction process.

Fig. 5.33 Site assembly of light steel wall panels

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In this method of construction (illustrated in Fig. 5.32), discrete members are assembled on site to form columns, walls, rafters, beams and bracing to which cladding, internal lining and other elements are attached. The elements are generally delivered cut to length, with pre-punched holes, but connections are made on site using self-drilling self-tapping screws, bolts, or other appropriate site techniques.

The main advantages of ‘stick-build’ construction are:

• construction tolerances and modifications can be accommodated on site
• connection techniques are relatively simple
• manufacturers do not require the workshop facilities associated with panel or modular construction
• large quantities of light steel members can be densely packed and transported in single loads
• components can be easily handled on site.

‘Stick-build’ construction is generally labour intensive on site compared with the other methods, but can be useful in complex construction, where prefabrication  is not feasible. This form of construction is widely used in North America and  Australia, where there is an infrastructure of contractors skilled in the technique.

This stems from a craft tradition of timber frame construction that now uses many power tools. In these countries, traditional timber contractors have changed to light steel framing with little difficulty.

Fig. 5.32 ‘Stick-build’ site construction using wall studs and floor joists

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The basic building elements of light steel framing are cold formed sections which can be prefabricated into panels or modules, or assembled on site using various methods of connection.The different forms of construction are reviewed in the fol- lowing sections and are illustrated in Figs 5.29 to 5.31.

Fig. 5.30 Light steel framing using prefabricated panels

Fig. 5.31 Modular construction using light steel framing

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Light steel framing uses galvanized cold-formed steel sections as the main structural components.These sections are widely used in the building industry and are part of a proven technology. Light steel framing extends the range of steel framed options into residential construction, which has traditionally been in timber and masonry.

The Egan Task Force report called for improved quality, increased use of off-site manufacture, and reduced waste in construction, and the Egan principles have  been adopted by The Housing Corporation and other major clients in the residential sector. Light steel framing satisfies these Egan principles and it combines the benefits of a reliable quality controlled product with speed of construction on site and the ability to create existing structural solutions.

Light steel framing is generally based on the use of standard C or Z shaped  steel sections produced by cold rolling from strip steel. Cold formed sections are generically different from hot rolled steel sections, such as Universal Beams, which are used in fabricated steelwork.The steel used in cold formed sections is relatively thin, typically 0.9–3.2mm, and is galvanized for corrosion protection.

Cold formed steel sections are widely used in many sectors of construction, including mezzanine floors, industrial buildings, commercial buildings and hotels, and are gaining greater acceptance in the residential sector. Light steel framing is already well established in residential construction in North America,Australia and Japan.

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Wire rope cables are spun from high tensile wire. For structural work the cables  are multi-strand, typically 6 x 19 or 6 x37 with independent wire rope core and  galvanized to Class A. For increased corrosion-resistance, the largest diameter  wire should be used, and cables can be filled with zinc powder in a slow-setting polyurethane varnish during the spinning process. For even greater corrosion-resistance, filled strand or locked-coil strand can be used to which a shrunk-on polyurethane or polypropylene sleeve can be fitted. Stainless steel, although appar- ently highly corrosion-resistant, is affected by some aggressive atmospheres if air is excluded; the resulting corrosion can be more severe than with mild steel.

The simplest and cheapest type of termination is a swaged Talurit Eye made round a thimble (Fig. 5.28(a)) and connected into a clevis type connection or on to the pin of a shackle.The neatest and most streamlined fitting is a swaged eye or jaw end termination (Fig. 5.28(b)). Hot-poured zinc terminations have to be used for very heavy cables of greater diameter than 50mm (Fig. 5.28(c)).

Epoxy resin with steel balls can be used as a filler in place of zinc, offering an improvement in fatigue life at the termination.

On-site connections can be made with bulldog clips but they are ugly and damage the rope. For cable net construction the standard detail is a three-part forged steel clamp, of which the two outer parts are identical (see Fig. 5.28(d)). Forging is expen-

sive for small numbers and so for smaller structures machined aluminium components may be preferred. Double cables can have a swaged aluminium extrusion  prefixed to each pair of cables, which can then be connected with a single bolt. For the attachment of net cables to edge cables, forged steel clamps are generally used (Fig. 5.28(e)).Lower cost alternatives are bent plate or machined aluminium clamps.

Cable life is reduced by corrosion and fatigue. Galvanized cables under cover suffer very little corrosion; external cables properly protected should have a life of 50 years. Plastic sheathing has the great disadvantage of making inspection of the cable impossible.

Fatigue investigations have shown that it is wise to limit the maximum tension in a cable to 40% of its ultimate strength for long-life structures. For structures with  a design life of up to ten years a limit of 50% is acceptable. Flexing of the cables at clamps or end termination will cause rapid fatigue damage.

Fig. 5.28 Cable fittings: (a) swaged eye, (b) swaged terminator, (c) hot-poured white metal eye, (d) cross clamp, (e) net and boundary connection

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The analysis of space frames results in the production of large sets of linear simultaneous equations which must inevitably require the use of a computer for their solution. For these equations to be formulated it is necessary to input into the  computer significant amounts of information relating to the topology of the structure and properties of the individual members. This operation can be very time- consuming unless modern pre-processing techniques which allow rapid data  generation are adopted.

The members forming double-layer space frames are principally stressed in  either axial tension or axial compression. Consequently it is usual in the analysis  of double-layer space trusses to assume that the members in the structure are  pin-jointed, irrespective of the actual joint rigidity.This assumption may lead to the overestimation of node deflections, but because only three degrees of freedom are permitted at each node this approach minimizes computer storage requirements and processing time.

Single-layer grid and barrel vault structures carry the imposed load by flexure of the members so it is important to include in the analysis the flexural, torsional and shear rigidity of the members. Linear analysis only is required  for the majority of skeletal space structures but to ensure stability of shallow domes it is essential to undertake a non-linear analysis of these structures.Large-span space structures may benefit from a full collapse analysis where the ultimate load-carrying capacity and collapse behaviour of the structure are determined.

The behaviour of both single-layer and double-layer space trusses is influenced to a great extent by the support positions of the structure. The effects of joint and overall frame rigidity also have a commanding influence and affect the buckling behaviour of the compression members within the structure. Great care must be taken in assessing the effective lengths of compression members and it is unlikely that internal and edge compression members will exhibit similar critical buckling loads.

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The inherent characteristics of steel skeletal space frames facilitate their ease of  fabrication, transportation and erection on site. There are two main groups into which the majority of space frames may be classified for assembly purposes: the  particular structure may be assembled from a number of individual members  connected together by purpose-made nodes or alternatively may be constructed by joining together modular units which have been accurately fabricated in a factory before transportation to site.

There are numerous examples of ‘chord and joint’ space frame systems available for immediate construction. These systems offer full flexibility of member lengths and intersecting angles required in the construction of skeletal dome structures.

Many jointing systems are available; Figs 5.15 and 5.16 show a typical spherical node used in the MERO system and a cast steel node used in the NODUS system. 

Fig. 5.15 MERO node connector

Fig. 5.16 NODUS node connector

The ‘modular’ systems are usually based on pyramidal units which are prefabricated from channel, angle, circular hollow section or solid bars. The individual  units are designed to nest together to facilitate storage and transportation by  road or sea. Most manufacturers of modular systems hold standard units in stock, which greatly enhances the speed of erection. Figure 5.17 shows  typical details of the prefabricated steel modular inverted pyramidal units used in the Space Deck System.

Fig. 5.17 Exploded view of 2.4m square section of Space Deck

Steel space frames are generally erected rapidly without the use of falsework.

Double-layer grids of substantial span can be constructed entirely at ground level including services and cladding and subsequently lifted or jacked up into the final position.Dome structures can be assembled from the top downwards using a central climbing column or tower. A novel approach adopted for the erection of a dome with a major axis of 110m and a minor axis of 70m involved fabrication of the dome on the ground in five sections, which were temporarily pinned to each other.

The central section was then lifted and the remaining segments of the dome locked into position as shown in Fig. 5.18.

Fig. 5.18 Novel method of dome erection

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Space frames are classified as single-, double- or multi-layered  structures, which  may be flat, resulting in grid structures, or may be curved in one or two directions, forming barrel vaults and dome structures. Grid structures can be further categorized into lattice and space grids in which the members may run  in two, three or four principal directions. In double-layer lattice grids the top and bottom grids are identical, with the top layer positioned directly over the bottom layer.Double-layer space grids are usually formed from pyramidal units with triangular or square bases resulting in either identical parallel top and bottom grids offset horizontally to  each other, or parallel top and bottom grids each with a different configuration  interconnected at the node points by inclined web members to form a regular stable structure.

Single-layer grids are primarily subject to flexural moments,whereas the members in double- and triple-layer grids are almost entirely subject to axial tensile or com- pressive forces. These characteristics of single-, double- and triple-layer grids  determine to a very large extent their structural performance. Single-layer grids, developing high flexural stresses, are suitable for clear spans up to 15m while double-layer grids have proved to be economical for clear spans in excess of 100m.

The main types of double-layer grids in common use are shown in Fig. 5.12.

Fig. 5.12 Lattice and space grids

Skeletal space frames curved in one direction forming single- or double-layer barrel vaults also provide elegant structures capable of covering large clear spans.

Single-layer vaults are suitable for column-free spans of up to 40m, which may be substantially increased by incorporating selected areas of double-layer structure forming stiffening rings. Double-layer barrel vaults are normally capable of clear spans in excess of 120m. Figure 5.13 shows the main types of bracing used for single- layer barrel vaults.

Fig. 5.13 Bracing of single-layer barrel vaults

Dome structures present a particularly efficient and graceful way of providing cover to large areas. Single-layer steel domes have been constructed from tubular members with spans in excess of 50m while double-layer dome structures have been constructed with clear spans slightly greater than 200m.

Skeletal dome structures can be classified into several categories depending on the orientation and position of the principal members. The four most popular types usually constructed in steel are ribbed domes, Schwedler domes, three-way grid domes and parallel lamella domes.

Ribbed domes, as the name suggests, are formed from a number of identical rib members, which follow the meridian line of the dome and span from the foundations up to the top of the structure. The individual rib members may be of tubular lattice construction and are usually interconnected at the crown of the dome using a small diameter ring beam.

The Schwedler dome is also formed from a series of meridional ribs but, unlike the ribbed dome, these members are interconnected along their length by a series of horizontal rings. In order to resist unsymmetric loads the structure is braced by diagonal members positioned on the surface of the dome bisecting each trapezium formed by the meridional ribs and horizontal rings.

Three-way grid domes are formed from three principal sets of members arranged to form a triangular space lattice. This member topology is ideally suited to both single-layer and double-layer domes, and numerous beautiful large-span steel three- way domes have been constructed throughout the world.

The steel lamella dome is formed from a number of ‘lozenge’-shaped lamella units which are interconnected together to form a diamond or rhombus arrangement.The spectacular Houston Astrodome is an excellent example of this type of construction. This impressive steel double-layer dome was constructed from lamella  units 1.52m deep and has an outside diameter of 217m with an overall height of 63.4m. Figure 5.14 shows the four main dome configurations now in prominent use worldwide.

Fig. 5.14 Dome configurations

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Steel skeletal space frames are three-dimensional structures capable of very large column-free spans.These structures, constructed from either individual elements or prefabricated modules, process a high strength-to-weight ratio and stiffness. Steel space frames may be used efficiently to form roofs,walls and floors for projects such as shopping arcades, but their real supremacy is in providing roof cover for sports stadia, exhibition halls, aircraft hangars and similar major structures.

Fig. 5.11 Outrigger stability: (a) Vierendeel, (b) plan bracing, (c) diagonal bracing

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Consideration of a masted solution arises from the need to provide a greater flexibility in the plan or layout of the building coupled with its aesthetic value to the project as a whole. At the same time it offers the opportunity to utilize structural materials in their most economic and effective tensile condition.The towers or masts can also provide high-level access for maintenance and plant support for services.

The plan form resulting from a mast structure eliminates the need for either internal support or a deeper structure to accommodate the clear span.By providing span assistance via suspension systems the overall structural depth is minimized giving a reduction in the clad area of the building perimeter.The concentration of structural loads to the mast or towers can also benefit substructure particularly in poor ground conditions where it is cost effective to limit the extent of substructures (Figs 5.6 and 5.7). However, differential settlement can have a significant effect on the structure by relaxing ties on suspension systems. The consequent load redistribution must be considered.

Fig. 5.6 Traditional long-span structure

Fig. 5.7 Suspension structure

Most tension structure building forms consist of either central support or perimeter support, or a mixture of the two.Any other solutions are invariably a variation on a theme. Plan form tends to be either linear or a series of repetitive squares.


The forces and loads experienced by towers and masts are illustrated in Figs 5.8 and 5.9. In all cases it is advantageous but not essential that  forces are balanced about the mast. Out-of-balance loads will obviously generate variable horizontal and vertical forces, which require resolution in the assessment of suitable structural sections.

Fig. 5.8 Types of perimeter support.

Fig. 5.9 Central support

Suspension ties must be designed to resist not only tension but also the effects of vibration, ice build-up and catenary sag. Ties induce additional compressive forces in the members they assist. These forces require careful consideration, often necessitating additional restraints in the roof plane in either the  open sections or top chord of any truss.


Longitudinal stability is created either by twinning the masts and creating a vertical truss or by cross bracing preferably to ground (Fig. 5.10).

Fig. 5.10 Longitudinal stability