Wood Beam (LRFD, NDS 2018)

How it calculates

Design method: LRFD per NDS 2018

The calculator applies Load and Resistance Factor Design as specified in NDS 2018. Every load combination entered (dead, live, roof live, snow, wind, seismic) is factored using ASCE 7 load factors. The factored demand at each critical section - maximum moment Mu, maximum shear Vu, and bearing stress at each support - is compared against the corresponding factored resistance: φMn, φVn, and φc⊥Fc⊥' respectively. The resistance factors are fixed per NDS: φb = 0.85 for bending, φv = 0.75 for shear, and φc = 0.90 for compression parallel to grain. This approach makes wind and seismic load combinations directly compatible without the ASD conversion step.

Reference design values and adjustment factors

For every sawn lumber, glulam, LVL, or I-joist selection, the calculator retrieves the tabulated reference design values (Fb, Fv, Fc⊥, E, Emin) from the NDS 2018 Supplement. It then applies all required adjustment factors per NDS 2018 Section 2:

• CD (load duration) - derived from the governing LRFD load combination
• CM (wet service) - applied when moisture content exceeds 19% for sawn lumber
• Ct (temperature) - for elevated temperature environments
• CF or Cv (size or volume) - size factor for sawn lumber, volume factor for glulam
• Cr (repetitive member) - for closely spaced members sharing load
• CL (beam stability) - calculated from the slenderness ratio RB based on the lateral bracing configuration entered

The adjusted design value F'b (LRFD) = Fb × KF × φb × λ × (all applicable C factors), where KF is the format conversion factor and λ is the time effect factor per NDS 2018 Appendix N.

Bending check

The critical moment Mu is computed by structural analysis across all spans and load combinations simultaneously. The factored bending resistance is φMn = φb × F'b × S, where S is the section modulus of the selected member. For multi-span continuous beams with unlimited supports, the calculator determines the governing span and load combination automatically. The utilization ratio Mu / φMn must be ≤ 1.0 to pass.

Shear check

Shear Vu is taken conservatively at the face of each support. The factored shear resistance is φVn = φv × F'v × (2/3 × A) for rectangular sections. For I-joists, the calculator uses the manufacturer-specified allowable shear capacities. Utilization Vu / φVn must be ≤ 1.0.

Bearing (compression perpendicular to grain)

At each support, the bearing stress is checked against the factored bearing resistance φc⊥ × Fc⊥', where the bearing length is entered per support. The bearing area factor Cb may increase the allowable bearing stress for short bearing lengths per NDS 2018 Section 3.10. Utilization must be ≤ 1.0 at every support.

Deflection checks

Three independent deflection limits are checked for each span:

• Short-term (live load only): Typically L/360 for floors, L/240 for roofs - configurable per span
• Long-term (total load with creep): Typically L/240, with long-term creep factor applied per NDS 2018 Section 3.5
• Hard absolute limit: A fixed maximum deflection (default 2 in / 50 mm) regardless of span length

Creep for sawn lumber and glulam uses the NDS duration of load and moisture adjustment. When shear deflections are significant for deep short-span members, the calculator adds a shear deflection component per an NDS-referenced approximation method.

Flitch plate and multi-ply beams

For flitch plate configurations, the calculator computes the composite transformed section properties based on the steel-to-wood modular ratio. Bending capacity accounts for the composite section, but shear and bearing are assigned entirely to the wood elements as a conservative simplification. The CL stability factor is calculated using the wood section only. Multi-ply sawn lumber beams in strong-axis bending are checked as a single composite section assuming full load sharing between plies.