Combined Bending and Compression

Design of a structural member for loading that induces both bending and axial compression should take into account not only the primary stresses due to the combined loading but also secondary effects. Commonly called P-delta effects, these result from two sources: (1) Incremental bending moments caused by buckling of the member that create eccentricity of the axial compression load with respect to the neutral axis, and (2) secondary moments produced in a member of a rigid frame due to sidesway of the frame that creates eccentricity of the axial compression load with respect to the neutral axis. Both the AISC LRFD and ASD specifications for structural steel buildings contain provisions for the influence of second- order effects; each specification, however, treats P-delta differently.

LRFD Strength in Bending and Compression

The LRFD specification presents two interaction equations for determining the strength of a member under combined bending and axial compression. The equation to use depends on the ratio of the required compressive strength Pu (kips) to resist the factored load to the nominal compressive strength Pn (kips) computed from Eq. (6.39), where   x  resistance factor for compression  0.85.

ASD for Bending and Compression

In ASD, the interaction of bending and axial compression is governed by Eqs. (6.68) and (6.69) or (6.70):