According to ASCE 7-22 [1], balanced and unbalanced snow loads shall be evaluated separately. For purlin design, the unbalanced snow load cases are similar to those used for main frame design; however, the applied load for purlin design shall not be taken as less than the minimum snow load, Pm​.

In practice, however, designers often use the unbalanced load cases developed for main frame design for both the main frame and the purlins.

For example: when Pi= 6.6 psf and Pm= 30 psf , the value of 6.6 psf may be appropriate for main frame design; however, 30 psf shall be used for purlin design. Using Pi= 6.6 psf instead of Pm= 30 psf for purlin design can significantly underestimate purlin demand, leading to substantially lighter purlins that are neither safe nor code-compliant.



References:

[1] American Society of Civil Engineers. "Minimum design loads and associated criteria for buildings and other structures." American Society of Civil Engineers, 2022.