Wood Beam-Column Interaction (NDS 3.9.2)
A stud carrying wind on a bearing wall is a beam and a column at once: NDS 3.9.2 combines them as (fc/Fc')^2 + fb/[Fb'(1 - fc/FcE)] <= 1.0, where FcE = 0.822 Emin'/(le/d)^2 and the 1 - fc/FcE denominator is the P-delta magnifier. A 4x4 stud at 3,000 lb + 3,000 in-lb passes at 0.55 with the bending term already amplified 1.6x; double the axial load and the amplifier nearly quadruples it to a 1.55 fail - the effect a designer must not drop. Enter Fc' with Cp and Fb' with CL from the companion tiles. A design aid, not the engineer of record's stamped design.
Formula and source
fc = P/A; fb = M/S; FcE = 0.822 Emin'/(le/d)^2; interaction = (fc/Fc')^2 + fb/[Fb'(1 - fc/FcE)] <= 1.0.
The NDS 2018 3.9.2 combined bending and axial compression interaction with the Euler stress FcE = 0.822 Emin'/(le/d)^2 and the 1 - fc/FcE P-delta moment magnifier, by name.
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