Design Under Combined Effect
Strength of secondary beam is defined in consideration with combined forces. Design of Members for Combined Forces Steel Beam Design (AISC 360-16)
When purlins are included in the analysis system, they are designed by taking into account the axial force and biaxial flexural moment. If the purlins are not included in the analysis system, the axial force is 0. So, it is designed with only flexural moment.
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