Wood Column (ASD, NDS 2018)

Theory: wood column (ASD, NDS 2018)

Design method

This calculator uses Allowable Stress Design (ASD) per the AWC National Design Specification for Wood Construction, 2018 edition (NDS 2018). All reference design values (F_c, F_b, F_t, F_v, F_c⊥) are multiplied by applicable adjustment factors to obtain allowable values before each limit state is evaluated. The governing utilization ratio across all checks must not exceed 1.0.

Adjustment factors

The key adjustment factors applied by this calculator are:

• C_D (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
• C_M (wet service factor) - reduces allowable values when in-service moisture content exceeds 19% for sawn lumber or 16% for glulam and SCL
• C_t (temperature factor) - reduces values for members subject to sustained elevated temperatures above 100°F
• C_F (size factor) - modifies F_b, F_t, and F_c for sawn lumber based on cross-section depth and width
• C_i (incising factor) - applied when preservative treatment requires incising, reducing most reference values
• C_r (repetitive member factor) - increases F_b by 1.15 when three or more parallel members at 24 in. or less spacing share loads through adequate sheathing
• K_f (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 = F_c × C_D × C_M × C_t × C_F × C_i × C_P,x

F'_c,y = F_c × C_D × C_M × C_t × C_F × C_i × C_P,y

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

Utilization (X-axis): f_c / F'_c,x ≤ 1.0

Utilization (Y-axis): f_c / F'_c,y ≤ 1.0

Bending and shear checks

Biaxial bending is evaluated independently on each principal axis:

F'_b = F_b × C_D × C_M × C_t × C_F × C_i × C_L × C_r

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

Shear is checked on each axis:

F'_v = F_v × C_D × C_M × C_t × C_i

Utilization (shear): f_v / F'_v ≤ 1.0

Combined loading

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

(f_c / F'_c)² + f_b,x / (F'_b,x × [1 - f_c / F_cE,x]) + f_b,y / (F'_b,y × [1 - f_c / F_cE,y - (f_b,x / F_bE)²]) ≤ 1.0

The denominator terms [1 - f_c / F_cE] 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 Eq. 3.9-1 and 3.9-2 are evaluated:

f_t / F'_t + f_b / 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⊥ = F_c⊥ × C_M × C_t × C_i × C_b): applies where load transfers across the grain, such as at a sill plate or beam seat

The bearing area factor C_b 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.

Deflections

Short-term and long-term lateral deflections are computed for both principal axes. Long-term deflections apply a creep multiplier based on service condition. User-specified limits (e.g., L/360 for live load, L/240 for total load) are checked against computed deflections and reported as a separate utilization ratio for each axis.