Wood Beam (ASD, NDS 2018)

How it calculates

Design methodology

The calculator implements Allowable Stress Design (ASD) as defined in the 2018 National Design Specification for Wood Construction (NDS 2018). In ASD, each load effect is compared directly to an adjusted allowable capacity derived from the tabulated reference design value multiplied by a chain of adjustment factors. The beam passes a given limit state when the demand-to-capacity ratio does not exceed 1.0:

utilization = demand / adjusted allowable ≤ 1.0

All six limit states - bending, shear, bearing, short-term deflection, long-term deflection, and DL+(LL or SL) deflection - must satisfy this inequality simultaneously.

Adjusted allowable bending stress

The adjusted allowable bending stress F'b is the product of the reference bending design value Fb and every applicable adjustment factor:

F'b = Fb × CD × CM × Ct × CF × Cfu × Ci × Cr × min(CL, CV)

• CD (load duration): amplifies allowable stress for short-duration loads - 1.6 for wind/seismic, 1.25 for construction, 1.15 for snow, 1.0 for occupancy live load, 0.9 for permanent dead load. The governing value is the highest CD among all loads acting simultaneously.
• CM (wet service): reduces capacity when moisture content exceeds 19% in service.
• Ct (temperature): reduces capacity for sustained temperatures above 100°F.
• CF (size factor): adjusts for member depth; applied to sawn lumber sections.
• Cfu (flat use): applies when the member is loaded on its wide face.
• Ci (incising): reduces design values for preservative-treated incised members.
• Cr (repetitive member): allows a 15% increase when three or more members are spaced 24 in. or less and share a load-distributing element.
• CL (beam stability) / CV (volume factor): NDS 2018 §3.3.3 requires using the lesser of CL and CV, not both simultaneously.

The bending utilization check is:

M / M' ≤ 1.0, where M' = F'b × S × (number of plies)

Beam stability and lateral-torsional buckling

CL is computed from the slenderness ratio RB = √(d × Le / b²), where d is beam depth, b is breadth, and Le is the effective unbraced length. The calculator accepts four LTB bracing conditions: no continuous bracing, top-face only, bottom-face only, and top-and-bottom. Full bracing at both faces suppresses lateral-torsional buckling and sets CL = 1.0.

Shear check

The adjusted allowable shear stress F'v is:

F'v = Fv × CD × CM × Ct × Ci

Shear demand is taken at a distance equal to the beam depth d from each support face per NDS 2018 §3.4.3. The utilization check is:

V / V' ≤ 1.0, where V' = F'v × (2/3) × A × (number of plies)

Bearing (compression perpendicular to grain)

At each support, the bearing stress is compared to the adjusted allowable perpendicular-to-grain compression:

F'c⊥ = Fc⊥ × CM × Ct × Ci × Cb

where Cb is the bearing area factor. The utilization check is:

R / R' ≤ 1.0, where R' = F'c⊥ × bearing area

Bearing length is entered for each support in the Supports table.

Deflection checks

Three deflection limit states are checked independently:

• Short-term (ST): instantaneous elastic deflection under the specified live-load or short-duration combination.
• Long-term (LT): total deflection including creep, computed as the ST deflection amplified by the applicable creep factor.
• DL+(LL or SL): a simplified envelope combining dead load and the governing transient load.

Each limit is checked against user-specified thresholds: a hard absolute limit (inches) and span-ratio limits (L/n for ST and LT). Deflections are computed from the composite stiffness EI of the full cross-section, including any steel flitch plates.

Flitch plate and multi-ply beams

For flitch plate beams, stiffness and moment capacity are distributed between timber plies and steel plates in proportion to their EI contributions. Timber capacity checks (bending, shear, bearing) apply only to the timber portion of the composite section, allowing accurate sizing of combination timber-steel headers and girders without conservative hand-simplifications.