Member configuration and shape
Trusses can be designed to suit a wide range of requirements.
Trusses are suitable for short, medium and long span conditions. Members can be arranged to satisfy external cladding slopes and internal requirements, including the accommodation of services. Whilst the top chord follows the line of the roof slope, the lower chord can typically be horizontal or parallel. A horizontal chord provides a deeper service zone, but bracing members are of varying length, and is more restrictive on headroom. A parallel bottom chord results in constant bracing lengths (and therefore simpler fabrication), smaller overall dimensions facilitating transport to site, and clearer internal headroom.
Truss configurations may be designed to limit the unrestrained lengths along the top chords to minimise buckling.
Some typical forms of standard trusses are shown below. These are basically for trusses in simple bending. For normal loading conditions acting vertically downwards the top boom members would be subject to compression. Individual lengths between joints in compression chords are deliberately kept shorter than for the bottom boom members which are normally in tension. This is because the tendency for compression members to buckle is largely dependent on the length between restraints while tension members are not affected by length. Consequently the longer diagonal members are also positioned to be in tension.
The shape of the truss can be varied to reflect the bending moment diagram.
The system offers considerable flexibility in deciding on the configuration and detail of the structure. For instance, it is logical to deduce that further advantage is available if the distance between flanges varies with the applied moment, and this leads to the adoption of curved booms. Strictly speaking this meant that the members were no longer under simple axial loads but also had a bending component. This was usually small enough to be ignored but nowadays curved truss girders are less popular.
Lighting requirements may influence the truss shape.
Lighting requirements may necessitate the use of monitor or saw tooth (north light) trusses which allow high levels of natural lighting without excessive solar gain.
Individual truss members are often of small section, and tubes are particularly attractive.
The choice of cross-sectional shape for individual members of trusses is not restricted. Whereas beam members are predominantly I-shaped to accommodate the bending effects, truss members are generally subject to small direct forces. Consequently angles, tees, or any other standard structural section may be used. For lightly loaded structures the members are usually angles or tees while for more heavily loaded trusses universal beam sections may be more appropriate. Open sections make the connections simpler but when aesthetic considerations are important circular and rectangular hollow sections are commonly used along with welded joints providing neat details.
A variety of section types can be used for the individual truss elements.
Trusses may be fabricated from a variety of steel sections including circular, square and rectangular hollow sections, angles, flats, rods and cold formed profiles. Circular hollow sections are often used for exposed trusses where appearance is important.
Trusses may be plane, triangular or rectangular.
Trusses may be plane - ie they have flat cross-sections with the top chord directly above the bottom chord - triangular, or (less commonly) rectangular forming a lattice structure of box section. The triangular and box forms can provide increased strength without using deeper forms or using heavier elements. They can also be used to accommodate walkways or services.
