Glulam Volume Factor Cv (NDS 5.3.6)
The volume factor that reduces a glued-laminated beam's bending strength.
Example
You enter
- Length between points of zero moment L (ft; the span of a simple beam) 20
- Beam depth d (in) 18
- Beam width b (in) 5.125
- Species exponent x 10
- Legacy NDS 1997 loading factor KL (1.0 = current NDS) 1
You get
- Volume factor Cv 0.965
Checked against published worked examples from: Jonathan Ochshorn.
Details, formula, and sources
ScopeThe glulam-specific penalty that the sawn-lumber CL check never applies. Cv = (21/L)^(1/x) x (12/d)^(1/x) x (5.125/b)^(1/x), capped at 1.0, with L the length between points of zero moment (ft), d and b the depth and width (in), x = 10 for softwoods (20 for Southern Pine), because a larger stressed volume is more likely to contain a strength-limiting defect. A 5-1/8 x 18 in glulam over 20 ft gives Cv = 0.965 (a 3.5% cut); a 6-3/4 x 24 in girder over 32 ft gives 0.870 (13%). The allowable bending uses the lesser of Cv and the beam-stability factor CL. A design aid, not a substitute for the engineer of record.
Formula or tableCv = min( (21/L)^(1/x) x (12/d)^(1/x) x (5.125/b)^(1/x), 1.0 ), L between points of zero moment; allowable bending uses the lesser of Cv and CL. The optional KL multiplier (default 1.0) is the NDS 1997 loading factor, absent from the current NDS.
Source and editionNDS 2018 (National Design Specification for Wood Construction) §5.3.6, the glulam volume factor Cv, by name; the reference bending value and factors come from the NDS Supplement.
Free accessThe Cv equation is stated in NDS 5.3.6, published free read-only by AWC at awc.org; the (21/L)^(1/x) arithmetic is public.
What governsEstimate. AHJ and licensed professional govern.
API field namesspan_ft, depth_in, width_in, x, kl, cv
- Volume factor Cv = (21/L)^(1/x) x (12/d)^(1/x) x (5.125/b)^(1/x), capped at 1.0; L between points of zero momentNDS 2018 §5.3.6 (Eq. 5.3-1)
- Species exponent x = 10 for all species except Southern Pine, where x = 20NDS 2018 §5.3.6
- Lesser of Cv or CL the allowable bending uses the lesser of Cv and the stability factor CL, not their productNDS 2018 §5.3.6 / §3.3.3