![]() Updates to the prescriptive fastener tables in the 2021 IBC and 2021 IRC now provide consistency with the building codes and their referenced standards. Reduced nail spacing is based on wind loads from ASCE 7-16 and is consistent with roof sheathing nailing requirements in the 2018 WFCM.Ī previous article (STRUCTURE, Changes to the 2018 WFCM, June 2018) provided background information on increases to component and cladding wind loads in ASCE 7-16 which led to these changes ( Table 1). In the 2021 IBC and 2021 IRC, nailing patterns for wood structural panel roof systems have been updated. Table 2 shows scoping provisions for the IRC and IBC Sections 23 relative to the roof and wind loads for light-frame wood construction. 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. I would not be interested in 1/2, but I might be in the area between 1/3 and 1/2.Table 1. using the factor.īut, If I wanted to go above 1/3rd, could I use Fibonacci calculations to further reduce the span and be in compliance with the code? If I wanted to place the rafter tie 1/3rd the way up, I would have to find a appropriate span, species, spacing, thickness. We know that using the span adjustment factor reduces the allowable span because you are multiplying the table spans by the decimal factor. ![]() But could you? The 0.17 to 0.33 range is a Fibonacci Sequence (3,5,8.) that can be extended to: There are no restrictions that say you cannot place the rafter tie above 1/3. The upper limit shown is 1/3 the way up, above the wall plate. Table 802.5.1 specifies a Rafter Span Adjustment Factor in Note a.
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