Wood Beam (ASD, NDS 2024)

How it calculates

The Wood Beam (ASD) calculator checks sawn lumber, glulam, LVL, PSL, LSL and I-joist beams to NDS 2024 Allowable Stress Design. Beams can be single or multi-ply, with optional steel flitch plates. Each check divides demand by adjusted capacity, and the beam passes when every ratio is 1.0 or less.

Six checks

You get six utilization ratios, each from its governing load combination:

• Bending: M / M'
• Shear: V / V'
• Bearing: R / R' at the most critical support
• Short-term deflection: under L + Lr, S or 0.42W
• Long-term deflection: under kD + L + Lr, where k adds creep to the dead load (0.5 dry, 1.0 wet)
• Dead plus live or snow: D + (L or S), which you can switch off

Each deflection limit is a span ratio (L/n) capped by an absolute limit you set. Cantilever limits can be doubled.

Adjusted bending stress

F'b is the reference value Fb multiplied by the factors that apply to the product you pick:

• Sawn lumber: F'b = Fb × CD × CM × Ct × CL × CF × Cfu × Ci × Cr
• Glulam, strong axis: F'b = Fb × CD × CM × Ct × Cfu × the lesser of CL and CV
• Glulam, weak axis: F'b = Fb × CD × CM × Ct × Cfu
• LVL, PSL and LSL: Fb × CD × CM × Ct × Cr, with beam stability (CL) and volume (CV) factors
• I-joists: the manufacturer's moment capacity, adjusted by CD, CM, Ct, CL and Cr

Moment capacity is M' = F'b × S, where S is the section modulus of all plies. Reference values come from the built-in species and grade database. LVL, PSL, LSL and I-joists use manufacturer data.

Load duration and service conditions

• CD is set per load combination by its shortest-duration load, from 0.9 for dead load alone to 1.6 for wind and seismic.
• CM applies in wet service, above 19% moisture content (16% for glulam).
• Ct reduces capacity above 100°F, in two bands up to 150°F.
• Ci applies to incised dimension lumber.
• Cr adds 15% for repetitive sawn lumber members and 4% for LVL, PSL and LSL.

Beam stability

CL comes from the critical buckling moment of each unbraced segment, using the AWC TR14 method. The moment factor is taken conservatively as 1.72, or 1.0 for cantilevers. You choose continuous bracing on the top face, the bottom face, both or neither. Bracing both faces removes lateral-torsional buckling, so CL = 1.0.

Shear and bearing

Shear capacity is V' = (2/3) × F'v × A. Demand is taken at the highest point along the span, not at distance d from the support, which is conservative.

Bearing is checked at every support from the bearing length you enter. Short bearings get the bearing area factor Cb. Bearing under point loads is not checked.

Flitch plate and multi-ply beams

• Bending uses the transformed timber and steel section.
• Deflection uses the combined stiffness of timber and steel.
• Shear and bearing are assigned to the timber.
• Multi-ply beams bending about the weak axis act non-compositely.

Assumptions

The calculator states its assumptions on every sheet. The ones that most often matter:

• Axial load is negligible. Check this yourself for inclined beams.
• Members are straight, prismatic and not notched.
• Glulam has at least four laminations.
• I-joists are fully braced, with all holes within manufacturer limits.