Dead load on the roof trusses in single storey industrial buildings consists
of dead load of claddings and dead load of purlins, self weight of the trusses in
addition to the weight of bracings etc. Further, additional special dead loads
such as truss supported hoist dead loads; special ducting and ventilator weight
etc. could contribute to roof truss dead loads. As the clear span length (column
free span length) increases, the self weight of the moment resisting gable frames
(Fig. 2.2b) increases drastically. In such cases roof trusses are more economical.
Dead loads of floor slabs can be considerably reduced by adopting composite
slabs with profiled steel sheets as described later in this chapter.
Live load
The live load on roof trusses consist of the gravitational load due to
erection and servicing as well as dust load etc. and the intensity is taken as per
IS:875-1975. Additional special live loads such as snow loads in very cold
climates, crane live loads in trusses supporting monorails may have to be
considered.
Wind load
Wind load on the roof trusses, unless the roof slope is too high, would be
usually uplift force perpendicular to the roof, due to suction effect of the wind
blowing over the roof. Hence the wind load on roof truss usually acts opposite to
the gravity load, and its magnitude can be larger than gravity loads, causing
reversal of forces in truss members. The calculation of wind load and its effect on
roof trusses is explained later in this chapter.
Earthquake load
Since earthquake load on a building depends on the mass of the building,
earthquake loads usually do not govern the design of light industrial steel
buildings. Wind loads usually govern. However, in the case of industrial buildings
with a large mass located at the roof or upper floors, the earthquake load may
govern the design. These loads are calculated as per IS: 1893-2002. The
calculation of earthquake load and its effect on roof trusses is explained later in
this chapter.
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