On this page:

The 3D interaction (yield) surface of P-M2-M3 hinges may be defined explicitly, or automatically through AISC-LRFD eqn. H1-1a and H1-1b (Φ=1) or FEMA-356 eqn. 5-4 for steel, or ACI 318-02 (Φ=1) for concrete. Post-yield behavior is interpolated from one or more user-defined P-θ curves, in which θ represents the relationship between M2 and M3. During analysis, an energy-equivalent moment-rotation curve is generated relative to the input P-θ curve(s) and the interaction-surface yield point.

...

• Design forces (Pu, M2, M3) must be within the interaction surface for the section to pass design. This can be checked visually by plotting the M2-M3 interaction diagram for a constant axial load P (design force), then by making sure that the design point (M2, M3) falls within this domain.

See Also

• Fiber hinge article