Problem $5(30$ points$)$

Determine the LRFD available strength of an A$572$ Gr $50,20ft$ long, $2$L$6$x$4$x$5/8$ double$-$angle compression member in a planar truss. The angles are long legs back$-$to$-$back with a $\frac{3}{8^{\text{'}\text{'}}}$ gap. The angles are welded at each end to a gusset plate and along the length at two intermediate points with a spacing of a$=80$ in$.$ Compare the results to that found in AISC Manual Table $4\mathrm{.}9.$