Design of Angle Iron Purlins:

For normal inclined roof trusses (slopes not greater than 30^o) and minimum basic imposed load of 0.75 kN/m² the following guidelines are recommended for angle iron purlins (conforming to steel grade Fe 415)

(a)        A depth of the purlins should not be less than 1/45 of its span.

(b)       A width of the purlin should not be less than 1/60 of its span.

(c)        A maximum bending moment in the purlin may be taken as

M_max     = WL/10                               

where,   W = Total uniformly distributed load (including wind load) on the purlin, and

L = Span of the purlin (distance centre to centre between roof trusses).

Here the bending of the purlin about their minor axis is neglected.

(d)       The maximum bending stresses in compression or tension may be increased by 33% (1.33 times) as the effect of wind loads is also included.

(e)        The purlins might sometimes be designed as continuous beams supported over the roof trusses.