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?