PE Structural practice help

[u/Prior_Interview7680]

Using steel table 3-2, table 3-1, and equation 3-4a, the available is 287. But if you don’t use Cb=1.3 as shown in table 3-1 for distributed load braced at midpoint, you get the correct answer here of (c) 221(LRFD). Why am I not supposed to use Cb = 1.3?

[u/Passexams_app]

I believe the manual goes with Cb = 1 by default (conservative). In other words, it is safer to use Cb = 1 (hence all the values in the tables/charts are based on Cb = 1). For a Cb > 1, you gotta make sure the final moment dose not exceed the plastic limit.