According to Section 4.3.3.1 Partial Loading of Roofs of ASCE 7-22 [1], Where uniform roof live loads are reduced to less than 20 psf (0.96 kN/m2) in accordance with Section 4.8.2 and are applied to the design of structural members arranged so as to create continuity, the reduced roof live load shall be applied to adjacent spans or to alternate spans, whichever produces the greatest unfavorable load effect.

Reference: MBMA manual [2]


This means that, for the main frame and purlin design of a three-span PEMB, a total of Four Partial Live Loads  must be considered, as follows:




However, structural designers generally do not take the partial-loading cases into account, even though these cases may produce more critical effects than uniform loading. 


This significantly underestimate member demand, leading to substantially lighter members 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.

[2] Metal Building Systems Manual, 2024 Edition.