Wood Bending Member (NDS Beam Stability Factor CL and Adjusted Fb')
Whether a wood beam bends without buckling its unbraced compression edge sideways.
Example
You enter
- Fb* (reference bending x factors except CL) (psi) 1350
- Emin' (psi) 620000
- Breadth b (in) 3.5
- Depth d (in) 11.25
- Effective unbraced length le (in) 144
You get
- Slenderness RB 11.50
- Critical FbE 5,626 psi
- Stability factor CL 0.985
- Adjusted Fb' 1,329 psi
- S (in³) 73.8281
- M prime (ft-lb) 8179.12
Checked against published worked examples from: Jonathan Ochshorn (Cornell University).
Details, formula, and sources
ScopeThe flexural twin of the wood column check. Form the beam slenderness RB = sqrt(le x d / b^2), the critical buckling value FbE = 1.20 Emin' / RB^2, and the NDS 3.3.3 beam stability factor CL, then the adjusted bending value Fb' = Fb* x CL and the allowable moment M' = Fb' x S. A stocky, nearly-braced 4x12 keeps CL near 1.0; leaving the same beam tall and unbraced drops it off a cliff. A design aid, not a substitute for the engineer of record.
Formula or tableRB = sqrt(le x d / b^2); FbE = 1.20 x Emin' / RB^2; r = FbE/Fb*; CL = (1 + r)/1.9 - sqrt(((1 + r)/1.9)^2 - r/0.95) (clamped to 1.0); Fb' = Fb* x CL; S = b d^2 / 6; M' = Fb' x S.
Source and editionNDS 3.3.3 beam stability factor CL and the slenderness / critical-buckling relations, by name; the reference bending value and its adjustment factors come from the NDS Supplement.
Free accessThe CL, RB, and FbE equations are stated in NDS 3.3.3, published free read-only by AWC at awc.org; the section-modulus and moment arithmetic is public statics.
What governsEstimate. AHJ and licensed professional govern.
API field namesfb_star_psi, emin_psi, b_in, d_in, le_in, rb, fbe_psi, cl, fb_prime_psi, s_in3, m_prime_ftlb
- Beam slenderness RB = sqrt(le d / b^2), capped at 50; geometry more slender than that is not a bending memberNDS 3.3.3
- Fb* user-supplied Fb* is the reference bending value times every applicable factor except CL; the reference and CD/CM/Ct/CF/Cfu/Ci/Cr come from the NDS SupplementNDS Supplement
- Effective length le is the effective unbraced length of the compression edge from NDS Table 3.3.3, not the clear spanNDS Table 3.3.3