![]() ![]() The braces shall be spaced not more than 4 feet (1219 mm) on center and the unbraced length of braces shall not exceed 8 feet (2438 mm). ![]() Multiply the span given by the multiplier for the wind zone (low and medium 1.3) or (high and very high 1.1) to calculate the extended span for the wind zone chosen. Overview Resources provide a simplified system to determine allowable joist and rafter spans in one- and two-family dwellings, and the companion supplements for tabulating allowable bending and modulus of elasticity design values for visually graded and mechanically graded dimension lumber. Installation of purlins to reduce the span of rafters is permitted as shown in Figure R802.5.1. The span of each rafter shall be measured along the horizontal projection of the rafter. Purlins shall be continuous and shall be supported by 2-inch by 4-inch (51 mm by 102 mm) braces installed to bearing walls at a slope not less than 45 degrees (0.785 rad) from the horizontal. Choose the rafter size and the spacing intended and read off the permitted span. For other grades and species and for other loading conditions, refer to the AF&PA Span Tables for Joists and Rafters. When calculating the maximum span of an rafter. The second span table is aforementioned alike such the first, apart items assumes a base snow load of 50 Psf. Purlins shall be sized no less than the required size of the rafters that they support. The first span table belongs for roofs where the ceiling is none attached to the rafts (with no snow load), a live load off 20 Psf, a dead load of 20 Psf, and a deflection limit of L/180. CEILING JOIST SPANS FOR COMMON LUMBER SPECIES (Uninhabitable attics without storage, live load = 10 psf, L/Δ = 240) CEILING JOIST SPACING (inches)ĬEILING JOIST SPANS FOR COMMON LUMBER SPECIES (Uninhabitable attics with limited storage, live load = 20 psf, L/Δ = 240) CEILING JOIST SPACING (inches)
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