Section 3.4.3/Compression Design Value Adjustment
NDS 2018 Section 3.4 covers adjustment factors for compression parallel to grain including the column stability factor.
The adjusted compression design value parallel to grain (Fc') is the reference value (Fc) multiplied by applicable factors: duration of load (CD), wet service (CM), temperature (Ct), size factor (CF), incising (Ci), and the column stability factor (Cp). The column stability factor accounts for buckling and depends on the slenderness ratio (le/d), where le is the effective column length and d is the least dimension. Cp reduces the compression capacity as slenderness increases.
Why this section exists
Wood columns, like steel columns, can fail by buckling before reaching their material crushing strength. The column stability factor (Cp) reduces the allowable compression stress based on the slenderness of the member. A short, stocky post can develop its full compression strength. A tall, slender column buckles at a fraction of its material capacity. The Cp calculation ensures wood columns are sized for both material strength and stability.
What plan reviewers look for
Plan reviewers check that the column stability factor is included in wood column calculations. They verify the effective length (le) based on the end conditions and bracing. They check that the slenderness ratio (le/d) does not exceed the maximum of 50. They also verify the species, grade, and reference Fc value match the NDS Supplement tables.
Common violations
Related NDS requirements
Section 3.3.3 covers bending design value adjustments. Section 4.3.1covers the column stability factor calculation in detail. Section 3.7 covers combined bending and compression (beam-column interaction). NDS Supplement Table 4A provides reference Fc values.