Wood Bending Member (NDS Beam Stability Factor CL and Adjusted Fb')
Whether a wood beam bends without buckling its unbraced compression edge sideways, the 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 and source
RB = 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.
NDS 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.
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