According to ASCE 07-22 [1], The minimum uniformly distributed roof live loads, Lo, in Table 4.3-1, are permitted to be reduced in accordance with the requirements of Sections 4.8.2. Table 1.3.4 of MBSM [2] summarize this section as follows:



Section Roof Live Load Example of MBSM [2] present an example demonstrating calculations for a typical roof live load for a low slope multi-span rigid frame building, as follows:




As shown in the solution, two different roof live load values are calculated: one for application to the purlins and the other for application to the frames. This difference arises from the difference in the tributary areas associated with the frames and the purlins.






As presented above, the tributary load area is 100 ft² for the purlins and 500 ft² for the frames. Therefore, according to the table below, the roof live load for the purlins shall be taken as 20 psf, since AT<200A_T < 200AT​<200 and F<4F < 4F<4. For the frames, since 200<AT<600200 < A_T < 600200<AT​<600 and F<4F < 4F<4, the roof live load shall be calculated using the applicable formula: 20(1.2−0.001AT).




Accordingly, MkaPEB calculates the tributary loaded areas for purlins and frames separately, and designs each based on the corresponding roof live load.



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.