The B1 factor is a moment amplifier to account for the P- δ effects (second order effects caused by displacement of the member between brace points, sometimes called small P-delta). If the Apply B1 factors option is selected, the B1 factor is calculated for each member and for each load combination according to Eq. (C2-1b) in Chapter C of AISC 360-05. If the option is not selected, the B1 factor is not applied to member forces. Note that the B1 factor is applied to moments from the gravity load cases (this is based on an assumption that all of the moments due to gravity loads are non-translational, and that all moments due to lateral loads are due to lateral translation).

The B2 factor is an amplifier to account for the P-Δ effects (second order effects caused by relative displacement of the member ends, sometimes called large P-delta). The B2 factor is calculated for each diaphragm and for each load combination. If the Apply B2 factors option is selected, B2 is calculated according to Eq. (C2-3) in Chapter C of AISC 360-05, based on the specified Rmx and Rmy values (1.0 or 0.85). If the option is not selected, B2 is not calculated and member forces are not modified with B2. Note that B2 factors are calculated for each of the global X- and Y-axes, and then they are resolved along the member local coordinate system. This factor is then applied to members' axial loads and moments obtained from lateral analysis.

It is advised that the user review the AISC 360 Direct Analysis Validation report in Steel - Standard mode.