Design of Steel Structure
Generally the purlins are placed at the panel points so as to avoid

Axial force in rafter
Shear force in rafter
Deflection of rafter
Bending moment in rafter

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Design of Steel Structure
When the axis of load lies in the plane of rivet group, then the most heavily loaded rivet will be the one which (where, Fa is the load shared by each rivet due to axial load and Fm is the shearing load due to moment in any rivet.)

gives the maximum angle between the two forces Fa and Fm
gives the minimum angle between the two forces Fa and Fm
is at the maximum distance from CG of the rivet group
is at the minimum distance from CG of the rivet group

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