The WFCM also has tables for the number of jack studs required, so it covers that as well.
For an example from the 2018 WFCM, Table 3.22A1 tells you that for a laterally unsupported (dropped) header in an exterior load bearing wall supporting a roof and ceiling of 20 psf dead, with an allowable deflection of L/240, and a 20 psf live load with a 36' building width (so a header in an eave wall, not a gable wall) a 5.125" x 13.75" glulam can span 17' 10". The footnotes tell you that it assumes #2 DF-L, H-F, SP, or S-P-F, and to see Table 3.22F for the number of jack studs.
Table 3.22F tells you that for the 36' building width, 20psf live load, and 5" deep header, supporting just a roof and ceiling, for an 18' span you need 3 jack studs at each end, so the header would have 4.5" x 5" of bearing at each end. Presumably the cross grain compression capacity of the bottom plate is reflected in that prescription.
Then Table 3.23A tells you that for Exposure Category B, that 5.125" x 13.75" glulam dropped header is fine for the 17'10" gravity span for 3 second wind gusts up to 160 mph, but if the 3 second wind gust is 170 mph, that consideration limits the span to 17' 3".
Cheers, Wayne
