Section H1.1/Combined Axial and Bending (Beam-Columns)
AISC 360-22 Section H1 covers the interaction equations for members subject to combined axial force and flexure.
Members subject to both axial compression and bending must satisfy the interaction equations in Section H1.1. When the ratio of required axial strength to available axial strength (Pr/Pc) is 0.2 or greater, the equation is Pr/Pc + 8/9(Mrx/Mcx + Mry/Mcy) must be less than or equal to 1.0. When Pr/Pc is less than 0.2, the equation is Pr/(2Pc) + (Mrx/Mcx + Mry/Mcy) must be less than or equal to 1.0. The required flexural strength Mr must include second-order effects (P-delta and P-Delta amplification).
Why this section exists
Most columns in real buildings carry both axial compression and bending moments from wind, seismic, or eccentric gravity loads. The interaction equations ensure that the combined effect of axial force and bending does not exceed the member's capacity. A column that is adequate for axial load alone and adequate for bending alone may fail under the combination because compression reduces the bending capacity and bending increases the effective eccentricity of the axial load.
What plan reviewers look for
Plan reviewers check that beam-column interaction is evaluated for every column subject to lateral loads (wind or seismic). They verify that second-order effects (P-delta) are included in the required moments. They check that both strong-axis and weak-axis bending are included where applicable. For moment frame columns, the interaction check is typically the controlling design criterion.
Common violations
Related AISC 360 requirements
Chapter E covers axial compression capacity (Pc). Chapter F covers flexural capacity (Mc). Appendix 8 covers the approximate second-order analysis (B1-B2 method). Chapter C covers the general stability requirements and analysis methods.