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Wood Beam (ASD, NDS 2018)

Run the calc
Design and verify wood beams to NDS 2018 ASD, bending, shear, bearing, and three deflection limits, each shown with the governing NDS adjustment factors and code reference. Load reactions at each support link directly to connected column and footing calculations so changes propagate automatically through the full load path. Supports simple and continuous spans, multi-ply beams, flitch plate composite beams, and inclined configurations.
This calculator uses an earlier code edition (NDS 2018 (ASD)). It stays available for existing projects. For new designs, check the Calculator Library for the current edition.

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Worked example

See Worked Example: Wood Beam to NDS 2018 for a full worked example with this calculator.

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Method & scope

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Calculation method

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.

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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 Allowable Stress Design (ASD) per the 2018 National Design Specification for Wood Construction (NDS 2018). Reference design values are adjusted using all required NDS factors: load duration (CD), wet service (CM), temperature (Ct), size (CF), flat use (Cfu), incising (Ci), repetitive member (Cr), beam stability (CL), and volume factor (CV).
Key inputs include beam plan length, support locations and types with bearing lengths, section type (standard from the section library or fully custom), number of plies, and applied loads (distributed, line, point, and moment). You also specify service conditions, wet or dry, temperature range, and incising treatment, and lateral-torsional bracing configuration (top face, bottom face, both, or no continuous bracing).
The calculator returns six utilization checks: bending moment (M/M’), shear (V/V’), and bearing (R/R’), each expressed as a ratio where 1.0 or less is acceptable, plus three deflection checks: short-term (ST), long-term (LT), and a simplified DL+(LL or SL) envelope. Adjusted allowable capacities account for all applicable NDS adjustment factors.
Yes. The calculator supports multi-ply beams, flitch plate composite beams (steel plates sandwiched between timber plies), and inclined beams in Simple Slope or Hip/Corner Slope configurations. For flitch beams, composite bending stiffness is derived automatically from the combined timber and steel section properties.
The calculator lets you specify three independent deflection limits: a hard absolute limit (in inches), a live-load span ratio (e.g., L/360), and a total-load span ratio (e.g., L/240). NDS and IRC commonly use L/360 for floors under live load and L/240 for total load. For roof members with no finished ceiling, L/180 is often acceptable. Enter the limits appropriate for your occupancy and finish conditions, the calculator flags any exceedance automatically.
Yes. Beam support reactions (vertical loads at each bearing point) can be linked directly to column and footing calculators in the same Calcs.com project. When span, section, loading, or service conditions change in this beam, all linked calculations update automatically, no manual re-entry of reactions. This is particularly useful for member chains where a beam frames into a column that bears on a footing, as a single load change propagates through the full load path without risk of stale values downstream.

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Next steps

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