Design of Purlins:

Purlins are members which directly carry the loads of the roof covering and are supported at the nodes (joints) of the adjacent roof trusses. Hence, they act like a beam with a span equal to the roof truss spacing. As the top members of the roof truss is sloping the purlins which are generally angles or channel sections (rarely light weight beam sections) are also inclined to the vertical. Hence, any vertical (live or dead load) causes biaxial bending in them, which may be referred to. However, wind loads acting normal to the roof truss rafter causes uniaxial bending of the purlins in the plane containing the y-axis.