![]() ![]() Support for development of the Span Calculator was provided by American Wood Council. David Pollock at Washington State University. The Span Calculator was designed and created by Cameron Knudson and Dr. The assumption regarding dead load distribution is based on historical and current criteria for Joist and Rafter Span Tables adopted in the model building codes. In addition, the dead load (D) is assumed uniformly distributed over the horizontal projected span (rather than being distributed over the true length of sloped members). Live load (L), snow load (S), and roof live load (L r) are assumed uniformly distributed over the horizontal projected span for sloped members, as specified in the model building codes. 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). All Inch inputs and dimensions are actual physical finished sizes (unless otherwise noted) ? Select output Fraction Precision, Decimal Inch or Metric mm. 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. Building Width Ridge Thick Actual: Rafter calculator ridge thickness adjustment diagram: Facebook LinkedIn Twitter WhatsApp Telegram eMail. Lumber design values used to calculate maximum horizontal spans include modulus of elasticity ( E), bending strength ( F b), and shear strength ( F v). ![]()
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