Also, note that, for risk category I or II buildings, IBC Section 2309 permits the use of the WFCM and its load assumptions for buildings within the WFCM scoping limitations. The WFCM is a referenced alternative approach to the IRC based on IRC Section R301.1.1. Scoping provisions for the IRC, IBC 2308, and IBC 2309 relative to the roof and wind loads for light-frame wood construction. The International Building Code® (IBC), IRC, and WFCM have uplift connection load tables that can be used to size roof-to-wall uplift connections. Uplift connectors at rafter or truss bearings are based on main wind force resisting system (MWFRS) loads. Uplift connectors for gable endwall rake overhang outlookers to the endwall require engineering or can be sized based on Wood Frame Construction Manual (WFCM) prescriptive tables to account for increased C&C loads at roof edges. For similar reasons, in the International Residential Code® (IRC), overhang detailing includes limits on gable endwall WSP cantilevers. For other roof slopes, Table 1 shows a comparison of C&C loads for ASCE 7-16 versus ASCE 7-10.īetween the increase of C&C roof areas assigned to corner and edge regions and the increase in C&C roof pressures, nail schedules for wood structural panels (WSP) changed, with nail spacing cut in half in some cases. In the case of a flat roof on a low-rise building using the simplified method, pressures for corner, edge, and interior areas increased from 13 percent to 81 percent, with an average increase of over 40 percent. Courtesy, American Wood Council, Leesburg, VA. Comparison of C&C wind loads for ASCE 7-16 versus ASCE 7-10. 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). 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 (0.79 rad) from the horizontal. Purlins shall be sized not less than the required size of the rafters that they support. Installation of purlins to reduce the span of rafters is permitted as shown in Figure R802.5.1. (continued) TABLE R802.4(2)-continued CEILING JOIST SPANS FOR COMMON LUMBER SPECIES (Uninhabitable attics with limited storage, live load = 20 psf, L/Δ= 240) CEILING JOIST SPACING (inches) TABLE R802.4(2) CEILING JOIST SPANS FOR COMMON LUMBER SPECIES (Uninhabitable attics with limited storage, live load = 20 psf, L/Δ = 240) CEILING JOIST SPACING (inches) (continued) TABLE R802.4(1)-continued CEILING JOIST SPANS FOR COMMON LUMBER SPECIES (Uninhabitable attics without storage, live load = 10 psf, L/Δ = 240) CEILING JOIST SPACING (inches)Ĭheck sources for availability of lumber in lengths greater than 20 feet.įor SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 0.0479 kPa. TABLE R802.4(1) CEILING JOIST SPANS FOR COMMON LUMBER SPECIES (Uninhabitable attics without storage, live load = 10 psf, L/Δ = 240) CEILING JOIST SPACING (inches) For other grades and species and for other loading conditions, refer to the AWC STJR or utilize the American Wood Council ("AWC") Maximum Span Calculator for Wood Joists & Rafters found at. Spans for ceiling joists shall be in accordance with Tables R802.4(1) and R802.4(2). Collar ties shall be not less than 1 inch by 4 inches (25 mm by 102 mm) (nominal), spaced not more than 4 feet (1219 mm) on center. Collar ties or ridge straps to resist wind uplift shall be connected in the upper third of the atticspace in accordance with Table R602.3(1). Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice. Rafter ties shall be not less than 2 inches by 4 inches (51 mm by 102 mm) (nominal), installed in accordance with the connection requirements in Table R802.5.1(9), or connections of equivalent capacities shall be provided. Where ceiling joists are not parallel to rafters, rafter ties shall be installed. Where ceiling joists are not connected to the rafters at the top wall plate, joists connected higher in the atticshall be installed as rafter ties, or rafter ties shall be installed to provide a continuous tie. Ceiling joists shall be continuous or securely joined in accordance with Table R802.5.1(9) where they meet over interior partitions and are nailed to adjacent rafters to provide a continuous tie across the building where such joists are parallel to the rafters. Ceiling joists and rafters shall be nailed to each other in accordance with Table R802.5.1(9), and the rafter shall be nailed to the top wall plate in accordance with Table R602.3(1).
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