How to calculate steel structure normal stress

When the roof can prevent the purlins from breaking and twisting, the following strength formula can be used to check the cross section
When the roof can not prevent the purlin lateral instability and reverse should be calculated according to the stability formula cross section.
Deformation calculation
The real purlins shall be checked for deflection in the direction perpendicular to the roof. On both sides of the curled groove shaped section of the simple purlin.
Purlin steel structure requirements
When the purlins span more than 5m should be in the purlin between the middle of the set bar. When the purlins span 7m should be in the purlin span three points to set a bar.
The effect of the pull is to prevent the purlin from being laterally deformed and twisted and providing an intermediate fulcrum in the x-axis direction. The force of the intermediate fulcrum needs to be transmitted to a larger member. For this purpose a stay and a stiff stay are required at the roof or cornice.