Dead Loads:
The dead load on a roof truss includes the weight of roof coverings, purlins, bracings, self weight of roof truss etc. As some of these weights can only be exactly known after their final design/selection, a rough estimate is made for preliminary design as follows:
Weight of Roof Covering
Weight of roof covering is given for some of the common material in Table as the weight per sq. meter of plan area.
Table : Weight of Roof Covering
Weight of Purlins
This can be assumed as given in Table , which are given for each sq. metre of roof plan area.
Table : Weight of Purlins
Weight of Bracings
The dead load of bracings might be estimated as 15 N /m2 of plan area.
Weight of Roof Trusses
The self weight of the truss depends upon the type of roof covering material and its weight, the span and rise of the truss and the truss spacing. It may vary from 90 to 150 kN/m2 of plan area. However, the following empirical formula is sometimes suggested to approximately estimate the truss weight.
W = 10 (l/3 +5) N/m2
where l is the span of the truss in metres.
Table may also be used to estimate the approximate weight of the truss when the design load q in N/m2 is known.
Table : Self Weight of Roof Truss
If the plan area A (= Span of truss × Spacing of truss) in sq. meter is taken as the variables, the subsequent expressions give the estimated weight of roof truss per sq. metre of plan area.
Therefore, after finally designing the members of the roof truss, a total weight of the roof truss must be compared with the estimated W, assumed in design calculations, and adjustments may be made if a wide discrepancy is observed.