Section 4.3.1/Compression Design Values
NDS 2018 Section 4.3 covers wood column design including the column stability factor Cp and slenderness ratio limits.
Wood compression members (columns and studs) must be designed using the adjusted compression design value Fc' which includes the column stability factor Cp. The Cp factor accounts for the slenderness of the column: the ratio of effective length to the least dimension (le/d). For le/d ratios up to 50, the column is permitted. Cp reduces from 1.0 for very short columns to near zero for very slender columns. Members subject to combined compression and bending must satisfy the interaction equation in Section 3.9.2. The effective length factor Ke depends on the end conditions (fixed, pinned, or free).
Why this section exists
A short wood post fails by crushing (material strength governs). A tall, slender wood column fails by buckling (stability governs) at a load far below the material crushing strength. The column stability factor Cp bridges these two failure modes with a single design equation. Without accounting for slenderness, a designer could specify a column that is strong enough to resist crushing but too slender to resist buckling, leading to a sudden and catastrophic collapse.
What plan reviewers look for
Plan reviewers check the column design calculations for the slenderness ratio le/d and the column stability factor Cp. They verify the effective length factor Ke matches the actual end conditions (bracing, connections). For wall studs, they check that the sheathing provides adequate lateral bracing to reduce the effective slenderness about the weak axis. For columns with bending (eccentric loads or lateral loads), they check the interaction equation.