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Wood Column (LRFD, NDS 2018)

Run the calc
Column loads link from beam reactions above to footing calculations below, change a load once and all linked calculations update automatically. Design wood columns and studs to NDS 2018 LRFD with checks for axial compression, column stability factor Cp, combined axial and biaxial bending interaction per §3.9, and bearing.
This calculator uses an earlier code edition (NDS 2018 (LRFD)). It stays available for existing projects. For new designs, check the Calculator Library for the current edition.

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Method & scope

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Calculation method

Design method
This calculator implements Load and Resistance Factor Design (LRFD) per the 2018 National Design Specification for Wood Construction (NDS 2018). In LRFD, tabulated reference design values are first multiplied by a format conversion factor K_F to express them in LRFD terms, then adjusted by the time-effect factor λ and all applicable LRFD adjustment factors. The factored demand must not exceed the adjusted factored resistance at every limit state: φ × K_F × F_ref × (adjustment factors) ≥ factored demand For axial compression, φ_c = 0.90. For bending, φ_b = 0.85. For shear, φ_v = 0.75. All utilization ratios must be ≤ 1.0.
Adjustment factors
The calculator applies the following adjustment factors to each reference design value:
  • K_F (format conversion factor), converts ASD reference design values to LRFD reference resistance; K_F = 2.40/φ for compression and bending
  • λ (time-effect factor), parallels the ASD load duration factor; λ = 0.8 for occupancy live load, 1.0 for wind and seismic, 0.6 for permanent dead load
  • C_M (wet service factor), reduces design values when in-service moisture content exceeds 19% for sawn lumber or 16% for glulam and SCL
  • C_t (temperature factor), reduces values for sustained temperatures above 100°F
  • C_F (size factor), adjusts F_c and F_b for sawn lumber based on cross-section dimensions
  • C_i (incising factor), applied when preservative treatment requires incising, reducing most reference values by a fixed fraction
  • C_P (column stability factor), the primary buckling reduction factor for compression, computed per NDS 2018 Eq. 3.7-1
Column stability factor and buckling
The column stability factor C_P is the central calculation for compression capacity. It is derived from the ratio of the Euler critical buckling stress F_cE to the adjusted reference compression stress F*_c: F_cE = 0.822 × E’_min / (L_e / d)² where L_e is the effective unbraced length on the axis being checked and d is the cross-section dimension resisting buckling on that axis. E’_min is the adjusted LRFD modulus of elasticity for stability, incorporating φ_s = 0.85 and the applicable wet service and temperature factors. The stability factor C_P is then computed from NDS 2018 Eq. 3.7-1: C_P = [1 + (F_cE / F_c)] / (2c), √{[(1 + (F_cE / F_c)) / (2c)]², (F_cE / F*_c) / c} where c = 0.8 for sawn lumber and 0.9 for glulam and structural composite lumber (SCL). A separate C_P is computed for each principal axis using the corresponding effective slenderness ratio. The governing axis, the one yielding the lower C_P, controls the factored compression resistance.
Compression checks
Two independent factored compression checks are performed, one for each principal axis: φ_c P’_n,x = φ_c × K_F,c × F_c × λ × C_M × C_t × C_F × C_i × C_P,x × A Utilization (X-axis): P_u / φ_c P’_n,x ≤ 1.0 Utilization (Y-axis): P_u / φ_c P’_n,y ≤ 1.0
Combined axial compression and biaxial bending
For columns subject to simultaneous axial compression and bending about one or both axes, the calculator evaluates the NDS 2018 LRFD combined interaction equation per §3.9. The denominator amplification terms account for the P-delta effect: axial load increases effective bending demand as the column deflects laterally. P_cE,x and P_cE,y are the Euler buckling loads on each axis. The interaction utilization must be ≤ 1.0 across both combined equations.
Bending and shear
Adjusted factored bending resistance for each axis uses the beam stability factor C_L per NDS 2018 Appendix E. For columns fully braced on both faces, C_L = 1.0. The adjusted factored shear resistance is computed with φ_v = 0.75 and the applicable wet service and temperature factors. Utilization Vu / φVn must be ≤ 1.0.
Bearing
Bearing is checked at the top and base of the column per compression perpendicular to grain. The bearing area factor C_b may exceed 1.0 for bearing lengths under 6 inches located at least 3 inches from the member end. The utilization R_u / φ_c P’_n,⊥ must be ≤ 1.0.
Built-up columns
For nailed or bolted built-up (multi-ply) columns, the effective column stability factor C_P is reduced relative to an equivalent solid section of the same cross-section area. The calculator applies the built-up column reduction to C_P to account for reduced composite action between plies, consistent with NDS 2018 provisions for multi-ply assemblies such as doubled or tripled studs.

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How to use it

1

Open the calculator

Open it from Run calc in the About this calculator panel above. To start from a typical setup, choose one of the presets listed there.
2

Enter your inputs

Work through the input sections from top to bottom. Click any input label to see its reference explanation, clause, conditions and assumptions. See Checks, References, Conditions and Assumptions.
3

Review the results and export

Check the utilization of each governing check in the summary, then export a PDF report. See Views and Export.

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Common questions

This calculator uses Load and Resistance Factor Design (LRFD) per the 2018 National Design Specification for Wood Construction (NDS 2018). Factored compressive demand Pu is checked against factored resistance φcPn, where the resistance factor φc = 0.90. All reference design values are converted to LRFD format values using the format conversion factor KF, then multiplied by the applicable LRFD adjustment factors (λ, CM, Ct, CF, Ci, CP) before each limit state is evaluated.
Key inputs include column cross-section (species, grade, and dimensions selected from the built-in US wood section library or entered manually), unbraced length on each principal axis, effective length factors (Ke,x and Ke,y) for each end condition, factored axial load from ASCE 7 load combinations, and biaxial bending moments. Service conditions, moisture content, temperature, incising treatment, and number of plies for built-up columns are additional inputs that govern which adjustment factors apply.
The calculator performs factored compression checks on both principal axes (Pu / φcPn,x ≤ 1.0 and Pu / φcPn,y ≤ 1.0), incorporating the column stability factor Cp computed per NDS 2018 §3.7. Combined axial compression and biaxial bending is evaluated using the NDS 2018 LRFD interaction equation per §3.9. Bearing at the column base and top are also checked. Outputs include utilization ratios for every limit state, the governing slenderness ratio (Le/d) on each axis, and the calculated Cp value.
Yes. The combined load interaction equation per NDS 2018 §3.9, adapted for LRFD, evaluates axial compression together with bending about both principal axes simultaneously. The denominator terms account for P-delta amplification, where axial load increases effective bending demand as the column deflects laterally. This handles columns subject to eccentric loads, lateral forces from wind or seismic, or out-of-plane forces from rafter thrust, all within a single utilization check.
Use LRFD when your project’s governing load combinations include significant wind, seismic, or other short-duration loads expressed as ASCE 7 factored combinations. The LRFD time-effect factor λ directly parallels the ASD load duration factor CD, but LRFD factored combinations and resistance factors are better suited to projects requiring consistent LRFD framing across wood, steel, and concrete elements. For routine gravity-only residential and light commercial columns, ASD remains the more common choice.
Yes. This is a load-path calculator designed specifically for linked project workflows. Axial load from a beam or joist above links directly into this column calculation, the column’s factored demand Pu updates automatically whenever the upstream beam span, loading, or section changes. The column’s factored reaction in turn links downstream to a footing or foundation calculation, so a single load change propagates through the complete beam-column-footing chain without re-entry at any step.

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Next steps

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Wood Column to NDS 2018 - Validation Examples

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