Glulam Volume Factor Cv (NDS 5.3.6)
The volume factor that reduces a glued-laminated beam's bending strength -- the glulam-specific penalty that the sawn-lumber CL check never applies. Cv = KL x (21/L)^(1/x) x (12/d)^(1/x) x (5.125/b)^(1/x), capped at 1.0, with L the span (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 and source
Cv = min( KL x (21/L)^(1/x) x (12/d)^(1/x) x (5.125/b)^(1/x), 1.0 ); allowable bending uses the lesser of Cv and CL.
NDS 2018 (National Design Specification for Wood Construction) ยง5.3.6, the glulam volume factor Cv, by name; the reference bending value and factors come from the NDS Supplement.
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