Various types of steel roof trusses are shown in Fig. 24.7(a–e) and 24.8(a–d). Steel roof trusses are designed in such a way that the members are either in compression or in tension only. Members of a truss which form the roof base depends upon the span, roof slope, covering materials, centre to centre of trusses, etc.
Tee-section is the most suitable section for principal rafter. For struts either angle iron or channel section may be used. Round or flat sections are used for tension members. Built-up sections are also used in certain cases. Members of a truss are connected by bolts, rivets and thin plates called gussets.
Trusses for Small Spans
Small trusses (span up to 15 m) are rested on bed plates at the ends. The bed plates may be of stone or concrete. The ends are bolted down with rag bolts which hold the truss down.
Small trusses generally consist of angles connected with gusset plates. In order to seat the foot of the truss on the bed plate short angles are fitted. For spans up the 7 m, 15 mm diameter rivets are used (Fig. 24.7(a–e)).
Figure 24.7 Various types of steel trusses for spans up to 15 m
Figure 24.8 Various types of steel trusses for span more than 15 m
2. Trusses for Large Spans
In large-span trusses the members may consist of more than one section. Two angles or channels or flats may be connected in the gusset with other members at the joints. As the truss is hung, handling may be difficult. Thus the trusses are riveted into two portions at the fabrication centre and gusset plates at the connected ends are riveted to them. The two halves are erected at the site and riveted. Bigger bearing plates are used for these trusses. Figure 24.8(a–d) show different types of trusses grouped under large span trusses.
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