Q: I have defined a new section and used the “-section” option to define the section properties. Why do the AISC LRFD results reported not match my hand calculations?
REV 7.05

A: The problem is due to your model. The option

       -SECTION

was used to change the area and inertias of the section (in particular, the area was changed by increased by a factor of 85). The options -P_N and -M_P were however, not used to change these factors. According to the documentation, if a value is not overridden with an option, the computed value will be used. For this case, the Pn value used is probably 1/85 smaller than what you would like.

A good rule is that you should model as close to what you have as possible and override as few items as possible. I can think of no reason at all to override the area of a "prismatic" beam. My guess is that is it not prismatic at all but a shape that would be better modeled with a tube, box, or W.