Wood Column (ASD)

How it calculates

The Wood Column (ASD, NDS 2024) calculator designs sawn lumber, glulam, and structural composite lumber (SCL) columns and studs using Allowable Stress Design per the 2024 National Design Specification for Wood Construction. Reference design values are drawn from the NDS 2024 Supplement (Tables 4A, 4B, 4C, 4D, 4E, 4F) and adjusted by all applicable factors before each limit state is evaluated.

Design method

All reference design values (Fc, Fb, Ft, Fv, Fc⊥) are multiplied by applicable adjustment factors to obtain allowable values. The governing utilization ratio across all checks must not exceed 1.0. The primary design basis is NDS 2024 Chapters 3 and 4, with column stability governed by NDS 2024 Section 3.7.

Adjustment factors

The key adjustment factors applied by this calculator are:

• CD (load duration factor) - amplifies or reduces allowable stress based on the duration of the controlling load combination; short-duration loads (wind, seismic) allow higher stress than sustained dead loads
• CM (wet service factor) - reduces allowable values when in-service moisture content exceeds 19% for sawn lumber or 16% for glulam and SCL
• Ct (temperature factor) - reduces values for members subject to sustained elevated temperatures above 100°F
• CF (size factor) - modifies Fb, Ft, and Fc for sawn lumber based on cross-section depth and width per NDS 2024 Supplement tables
• Ci (incising factor) - applied when preservative treatment requires incising, reducing most reference values per NDS 2024 Table 4.3.8
• Cr (repetitive member factor) - increases Fb by 1.15 when three or more parallel members at 24 in. or less spacing share loads through adequate sheathing
• Kf (built-up column factor) - reduces the effective column stability factor for nailed or bolted multi-ply columns relative to an equivalent solid or glulam section

Compression checks

Two independent compression capacity checks are performed, one for each principal axis, to account for different effective lengths and bracing conditions:

F'c,x = Fc × CD × CM × Ct × CF × Ci × Cp,x

F'c,y = Fc × CD × CM × Ct × CF × Ci × Cp,y

The column stability factor Cp is computed per NDS 2024 Eq. 3.7-1, which is a function of the Euler critical buckling stress FcE = 0.822 E'min / (Le/d)², the adjusted reference compressive stress F*c (all factors except Cp), and the parameter c (0.8 for sawn lumber, 0.9 for glulam and SCL). A separate Cp is calculated for each axis using the corresponding effective slenderness ratio.

Utilization (X-axis): fc / F'c,x ≤ 1.0

Utilization (Y-axis): fc / F'c,y ≤ 1.0

Bending and shear checks

Biaxial bending is evaluated independently on each principal axis:

F'b = Fb × CD × CM × Ct × CF × Ci × CL × Cr

For glulam and SCL members, the volume factor CV replaces CL for strong-axis bending (the lesser of the two governs). The beam stability factor CL accounts for lateral-torsional buckling per NDS 2024 Appendix E; for columns with adequate bracing on both axes, CL = 1.0.

Shear is checked on each axis:

F'v = Fv × CD × CM × Ct × Ci

Utilization (shear): fv / F'v ≤ 1.0

Combined loading

For members subject to simultaneous axial compression and biaxial bending, the calculator evaluates NDS 2024 Eq. 3.9-3 and 3.9-4:

(fc / F'c)² + fb,x / (F'b,x × [1 - fc / FcE,x]) + fb,y / (F'b,y × [1 - fc / FcE,y - (fb,x / FbE)²]) ≤ 1.0

The denominator terms [1 - fc / FcE] amplify the effective bending demand to account for the P-delta effect of axial load on lateral deflection. The summary utilization is the maximum across both interaction equations.

For members subject to tension and bending, NDS 2024 Eq. 3.9-1 and 3.9-2 are evaluated:

ft / F't + fb / F'b ≤ 1.0

Bearing

Bearing is checked at both member ends:

• Parallel to grain (F'c,||): applies where the column end bears on a plate or sill aligned with the grain direction
• Perpendicular to grain (F'c⊥ = Fc⊥ × CM × Ct × Ci × Cb): applies where load transfers across the grain, such as at a sill plate or beam seat

The bearing area factor Cb may be greater than 1.0 when bearing length is less than 6 inches and the bearing location is at least 3 inches from the member end, per NDS 2024 Section 3.10.4.

NDS 2024 reference design values

Reference design values are sourced from the NDS 2024 Supplement tables for the selected species, grade, and size classification. Where NDS 2024 values differ from NDS 2018 for specific species-grade combinations, the 2024 values govern. If your project requires NDS 2018 compliance, use the timberColumnASD calculator instead.