**On-line Span Calculator HELP**

## SPAN CALCULATIONS

Lumber design values used to
calculate maximum horizontal spans include modulus of elasticity (*E*),
bending strength (*F*_{b}), and shear strength (*F*_{v}). 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. 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}). The 2012 *National Design Specification®
for Wood Construction (NDS®)* specifies appropriate magnitudes for
lumber design values and adjustment factors.

Maximum
horizontal joist and rafter spans are taken as the smallest span (*L*)
calculated from the following three formulas:

_{}
based on bending strength (*F*_{b})

where *s* =
spacing between joists or rafters

*S*_{x} = section
modulus for strong-axis bending of joist or rafter

*w*_{T} = total
distributed load (D + L, or D + L_{r}, or D + S)

supported by joist or rafter, in terms of load per unit area

_{}
based on deflection limit and modulus of elasticity (*E*)

where *I*_{x}
= strong axis moment of inertia for joist or rafter

*w*_{L} = distributed
live load (L or L_{r}) or distributed snow

load (S) supported by joist or rafter,

in terms of load per unit area

deflection
constant = constant term in denominator of

deflection limit (e.g., *L*/**360**)

_{}
based on shear
strength (*F*_{v})

where *A* =
cross-sectional area of joist or rafter

## BEARING LENGTH

The minimum
required bearing length (*l*_{b}) at each end of a joist or rafter
is determined from the following formula:

_{}

where *t* =
thickness of joist or rafter

## LOADING ASSUMPTIONS

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. 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). 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.

## ACKNOWLEDGEMENTS

The Span Calculator was designed and created
by Cameron Knudson and Dr. David Pollock at Washington State University.
Support for development of the Span
Calculator was provided by
American Wood Council.