Steel Beam-Column Interaction (AISC H1)
Perform the AISC 360-22 Chapter H interaction check for steel members under combined axial compression and biaxial bending. Determines whether Equation H1-1a (Pu/φPn ≥ 0.2) or H1-1b (Pu/φPn < 0.2) governs and computes the interaction ratio. Takes axial capacity from Chapter E and flexural capacity from Chapter F as inputs.
What this calculates
The Pu/φPn ratio (from Chapter E axial capacity), the Mux/φMnx and Muy/φMny ratios (from Chapter F flexural capacity), the governing interaction equation (H1-1a or H1-1b), and the resulting interaction ratio. A ratio ≤ 1.0 indicates the member is adequate.
Inputs
Required axial compressive strength (with second-order effects)
Required flexural strength about major axis (with second-order effects)
Required flexural strength about minor axis (with second-order effects)
Available axial compressive strength per Chapter E
Available flexural strength about major axis per Chapter F
Available flexural strength about minor axis per Chapter F
Outputs
Axial demand-to-capacity ratio
Major axis flexural demand-to-capacity ratio
Minor axis flexural demand-to-capacity ratio
H1-1a (if Pu/φPn ≥ 0.2) or H1-1b (if Pu/φPn < 0.2)
Combined interaction ratio — must be ≤ 1.0
Methodology
Per AISC 360-22 Section H1.1: If Pu/φcPn ≥ 0.2, use Equation H1-1a: Pu/(φcPn) + 8/9·[Mux/(φbMnx) + Muy/(φbMny)] ≤ 1.0. If Pu/φcPn < 0.2, use Equation H1-1b: Pu/(2φcPn) + [Mux/(φbMnx) + Muy/(φbMny)] ≤ 1.0. Second-order effects (P-δ and P-Δ) should be included in the analysis per Chapter C before entering demands into the interaction equations.
Code References
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