Combined Compressive Axial Load and Bending

Members under combined compression axial load and bending are generally referred to as beam-columns. Bending in such members may be caused by eccentric loading, lateral loads, or end moments, and the compression load can amplify the bending. These members must satisfy the interaction equations given by the AISI Specification to prevent both buckling and yielding. Separate equations are given for ASD and LRFD but symbols have common definitions except as noted.

ASD Method

where P = required compressive axial strength
Mx , My = required flexural strengths with respect to the centroidal axes of the effective section determined for the required compressive axial strength alone. For angle sections, My should be taken either as the required flexural strength or the required flexural strength plus PL/1000, whichever results in a lower permissible value of P
Pn = nominal axial strength determined in accordance with Art. 10.14
Pno = nominal axis strength determined in accordance with Art. 10.14, with Fn  Fy
Mnx , Mny = nominal flexural strengths about the centroidal axes determined in accordance with Art. 10.12

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