Section

A range of building sections can be developed, depending on the functional, technical and aesthetic criteria that exist for the building.

Common roof forms require some means of shedding water. Although flat roofs are possible, the size of section required to prevent deflection and ponding is critical. Various degrees of pitch are possible, one of the commonest forms being the use of a steel truss. The choice of decking, weatherproofing and insulation also affect the choice of structure.

Whilst trusses give an overall deeper structure, they may be lighter than equivalent solid sections and allow the passage of service runs. Fabrication costs may be higher.

Diagrams of alternative columns

Diagrams of fabricated columns

Diagrams of trussed columns

The nature of the connections is an important consideration from both a structural and an economic viewpoint. The main connection methods in steel construction are bolting and welding.

Rigid joints are more difficult to achieve in steelwork and are therefore correspondingly more expensive. An exception to this is the portal frame, using a haunch or fabricated sections, and providing a very efficient structural form for single storey construction. Rigid joints may also be provided at column foundations, using a welded base plate on the bottom of the column. This will also require correspondingly larger foundation to resist overturning.

The ideal shape for an arch for resisting uniform gravity loads would be the inverted tensile catenary shape since under uniform vertical loading conditions no bending is induced in the arch.

However, horizontal wind loads and asymmetric loading, for example from drifted snow, will generate bending moments and the need for bending stiffness in the arch ribs. Inverted catenary forms are therefore rare, a circular shape being easier to fabricate.

Curved circular arch main steel members can be pre-bent, formed from fabricated sections, or made up as curved trusses.

Prebending is simplest but has increased time and cost for fabrication. Alternatively, by pre-bending small angle or tube sections and welding up trusses a curved lattice form can be constructed. Alternatively, an approximate curved section can be fabricated (somewhat expensively) by cutting and butt welding to shape. (Note that transportation is more difficult for very large sections and the truss may have to be delivered to site in sections to be bolted or welded before erection. This is also true of straight trusses generally over 18m in length.)

The easiest cross-sectional shape - and most traditional in terms of steeper tiled (30 - 45 degrees) or slated (18 - 24 degrees) roofs - is to have a sloping clad sheet roof and vertical columns. This can be achieved in various ways.

A traditional pitched roof form can be achieved in various ways:

• The traditional form consists of a lightweight truss of small angle sections (or angles and tie rods) bolted or riveted together. Knee braces were used to provide resistance to wind loading. The compression elements of these trusses were often made of double angles. The gap between could not be repainted and has often rusted badly in damp conditions; dust has also gathered. Smooth tubular elements are preferable in this respect. They are also more elegant and are increasingly common, although more expensive. These trusses had a slope of 18 degrees, a value dictated by then current wind loading design codes, resulting in neither suction nor pressure. A variation on this was a truss of tube or channel/angle sections with the top cranked upwards at midspan. This provided a greater central lever arm depth to resist the maximum applied load bending moment.
•  The portal frame, constructed using solid UB sections, is a very efficient form. The bending moment diagram gives peak values at the eaves joint. The strength at this point can be increased locally with the introduction of a cut section of UB welded to the rafter to provide a stiff haunch. The greater depth also facilitates a rigid joint between rafters and columns. These frames are usually designed using the "plastic" theory. A more recent development in has been the use of thin plates cut and continuously welded into I-section portal shapes. If masonry cladding is used for wall heights over about 4m then the outward deflection of the portal "knee" joint must be calculated carefully.

• A cranked truss and column might be more economical than a solid section portal but this depends on the relative costs of factory labour plus overheads and material (usually based on weight of steel).

Portal haunch

Cranked truss and column

The column dimension also requires consideration.

Tall columns may require addition thickness to resist buckling. Trussed columns may also be useful particularly in larger buildings and may provide depth for services

3D form: The overall arrangement of the frame must be stiff and stable, as well as being able to resist loads from not only gravity but also wind which can cause both lateral and uplift loads. Stability, and resistance to lateral loads, is normally achieved by rigid connections or cross bracing.

Lateral stability is most efficiently achieved using cross bracing, either using rods in tension or larger hollow, L or I sections in tension and compression. Bracing may also be required in the roof, particularly if lateral loads are to be transferred to braced gable ends. As discussed in relation to the section, some lateral resistance may be provided by the cross sectional shape eg. portal frames or rigid column/foundation connections, particularly if the gable ends are to be kept open. Design stages should identify places where cross bracing may be placed, eg. where large openings are not required. Lateral forces can be transferred to braced gables by trusses in the plane of the roof or where the construction of the roof allows it to act as a plate.

Forms of bracing

Forces being transmitted from side walls to wall plane to end walls