Wood Beam (LRFD)

How it calculates

The Wood Beam (LRFD, NDS 2024) calculator checks sawn lumber, glulam, LVL, I-joist, and flitch plate beams against the Load and Resistance Factor Design provisions of the 2024 National Design Specification for Wood Construction. For each ASCE 7 load combination, the calculator resolves factored demands and compares them against factored resistances adjusted through the NDS 2024 LRFD framework.

LRFD adjustment framework

NDS LRFD converts reference design values - tabulated on an ASD basis in the NDS Supplement - into the LRFD resistance space using the format conversion factor K_F. Each adjusted resistance is then multiplied by the applicable resistance factor φ and the time effect factor λ, which accounts for the duration of the governing load:

• λ = 0.8 for occupancy live loads
• λ = 1.0 for snow (short duration) and impact
• λ = 1.25 for wind and seismic

The full adjusted moment resistance is: φM_n = φ_b · K_{F,b} · λ_b · C_M · C_t · C_F · C_fu · C_i · C_L · C_r · F_b · S

Adjustment factors applied per the 2024 NDS Supplement include: wet service (C_M), temperature (C_t), size (C_F), flat use (C_fu), incising (C_i), repeating member (C_r), and beam stability (C_L) for lateral-torsional buckling.

Bending check (NDS 2024, Cl. 2.3)

utilization = M_u / φM_n ≤ 1.0

The calculator evaluates all ASCE 7 load combinations, identifies the one producing the maximum factored moment M_u, and compares it against the factored moment resistance φM_n. For beams without continuous bracing, beam stability factor C_L is computed from the effective slenderness ratio R_B and governs when lateral-torsional buckling is critical.

Shear check

utilization = V_u / φV_n ≤ 1.0

Factored shear V_u is taken conservatively at the face of support rather than at distance d as permitted by code. The factored shear resistance φV_n applies the shear resistance factor and the full suite of adjustment factors relevant to the shear reference design value F_v.

Bearing check

utilization = R_u / φR_n ≤ 1.0

Governing bearing demand R_u (maximum factored vertical reaction at any support) is compared against the factored bearing resistance φR_n. Minimum required bearing lengths at end and interior supports are output separately. Bearing at point loads is not checked per the calculator assumptions.

Deflection checks

Three independent deflection limits are verified using unfactored (service-level) loads:

• Live/short-term deflection: δ_ST ≤ Δ_ST,max (default span/360, user-adjustable)
• Long-term deflection: δ_LT ≤ Δ_LT,max (default span/240, accounts for creep under sustained loads)
• Simplified DL+(LL or SL) deflection: δ_D+(L|S) ≤ Δ_D+(L|S),max

Deflection limits can be doubled for cantilevers. When explicitly enabled, shear deflection is added to flexural deflection using the approximation method described in the NDS supplement.

Reactions and load linking

At each support, the calculator outputs maximum factored vertical and horizontal reactions (R_u,vertical and R_u,horizontal). These reactions can be linked directly to connected wood column or footing calculations in the same project, so downstream members update automatically when beam loading or geometry changes.