Where t = thickness of joist or rafter Loading Assumptions The minimum required bearing length ( l b) at each end of a joist or rafter is determined from the following formula: Where A = cross-sectional area of joist or rafter Bearing Length W L = distributed live load (L or L r) or distributed snowĭeflection constant = constant term in denominator of
Where I x = strong axis moment of inertia for joist or rafter Supported by joist or rafter, in terms of load per unit areaīased on deflection limit and modulus of elasticity ( E) W T = total distributed load (D + L, or D + L r, or D + S) S x = section modulus for strong-axis bending of joist or rafter Where s = spacing between joists or rafters Maximum horizontal joist and rafter spans are taken as the smallest span ( L) calculated from the following three formulas:
The 2012 National Design Specification® for Wood Construction (NDS®) specifies appropriate magnitudes for lumber design values and adjustment factors. Calculated spans incorporate design value adjustments appropriate for repetitive-member use ( C r = 1.15), duration of load ( C D), lumber size ( C F), wet service conditions ( C M), and incised lumber ( C i). Bearing strength in compression perpendicular to grain ( F cp) is used to determine the minimum required bearing length at each end of joists and rafters. Lumber design values used to calculate maximum horizontal spans include modulus of elasticity ( E), bending strength ( F b), and shear strength ( F v).
