Section 3.9.2/Combined Bending and Axial Compression
NDS 2018 Section 3.9.2 covers the interaction equation for wood members with combined bending and compression including P-delta effects.
Wood members subject to both bending and axial compression must satisfy the interaction equation: (fc/Fc')^2 + fb1/[Fb1'(1 - fc/FcE1)] + fb2/[Fb2'(1 - fc/FcE2 - (fb1/FbE)^2)] must be less than or equal to 1.0. This equation accounts for the P-delta amplification effect: axial compression increases the bending moment through the (1 - fc/FcE) magnification term. Fc' includes the column stability factor Cp from Section 4.3. The equation must be checked for both strong-axis and weak-axis bending. Wall studs subject to wind load plus gravity load are the most common application of this equation.
Why this section exists
Most wood columns and studs carry both axial compression (gravity loads from the structure above) and bending (wind pressure, eccentric loads). The interaction equation ensures the combined effect does not exceed the member's capacity. The P-delta amplification term captures the second-order effect where axial compression increases the bending stress on a deflected member. Without this amplification, the interaction check would be unconservative for slender members.
What plan reviewers look for
Plan reviewers check stud wall and column calculations for the combined loading interaction check. They verify the axial load includes all tributary gravity loads and the bending includes wind or seismic forces. They check that both axes are evaluated. They verify the column stability factor Cp is included in the axial capacity. For tall stud walls, the P-delta amplification is significant and must not be omitted.