Steel members subjected to axial forces are generally more efficient than members in flexure since the cross section is nearly uniformly stressed. Trusses, consisting of essentially axially loaded members, thus are very efficient in resisting external loads. They are extensively used, especially to span large gaps. Since truss systems consume relatively less material and more labour to fabricate, compared to other systems, they are particularly suited in the Indian context.
Trusses are used in roofs of single storey industrial buildings, long span floors and roofs of multistory buildings, to resist gravity loads Trusses are also used in multi-storey buildings and walls and horizontal planes of industrial buildings to resist lateral loads and give lateral stability . Trusses are used in long span bridges to carry gravity loads and lateral loads
Trusses often serve the action of the girder in transferring the gravity load over larger span, and are referred to also as lattice girders. Such lattice girders are usually deeper and much lighter than regular girders and hence are economical, particularly when repetitive fabrication is taken advantage of. These are used as flooring support systems in multi-storey buildings, within which depth all the ducts can be easily accommodated without increasing the depth of the ceiling.
Steel trusses can also be efficiently used along with concrete slabs in buildings and bridges by mobilising composite action between structural steel and concrete. In this chapter, initially, the details of structural steel trusses are discussed. Subsequently, the behaviour and design of structural steel – concrete composite trusses are discussed.
ECONOMY OF TRUSSES:
As already discussed trusses consume a lot less material compared to beams to span the same length and transfer moderate to heavy loads. However, the labour requirement for fabrication and erection of trusses is higher and hence the relative economy is dictated by different factors. In India these considerations are likely to favour the trusses even more because of the lower labour cost. In order to fully utilize the economy of the trusses the designers should ascertain the following:
- Method of fabrication and erection to be followed, facility for shop fabrication available, transportation restrictions, field assembly facilities.
- Preferred practices and past experience.
- Availability of materials and sections to be used in fabrication.
- Erection technique to be followed and erection stresses.
- Method of connection preferred by the contractor and client (bolting, welding or riveting).
- Choice of as rolled or fabricated sections.
- Simple design with maximum repetition and minimum inventory of material