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- To: "SEAINT Listserver (seaint(--nospam--at)seaint.org)" <seaint(--nospam--at)seaint.org>
- Subject: FW: Ceiling joists/rafter ties IRC R802.3.1
- From: AWC Info <AWCInfo(--nospam--at)afandpa.org>
- Date: Wed, 5 Oct 2005 09:15:30 -0400
Title: FW: Ceiling joists/rafter ties IRC R802.3.1
There is no clear-cut answer to this...yet. Here's what I gleaned from our database.
The prescriptive "conventional construction" provision for hip roof systems have existed in the building code for many years. In general, the practice of framing jack rafters into a common nailing piece (i.e. into a 2x hip rafter) has had acceptable performance on small roof systems (i.e. building widths <=24') where offsetting hip thrust loads and sheathing have been able to provide adequate, though unengineered, resistance.
As the home building industry moves to larger buildings with more complicated roof lines and new framing systems, some of these "conventional construction" provisions which had inherent limitations (i.e. a 20' - 2x12 hip rafter on a 4:12 roof slope with a 2' roof overhang would limit the hip area to about 12'x12') are being exceeded.
In 1992, the wood industry began work on the first Wood Frame Construction Manual for One and Two-Family Dwellings (WFCM) in an attempt to provide engineering support and/or determine acceptable engineering based limits on residential systems used in high wind areas. The 2001 WFCM has been expanded to cover most dead, live, snow, wind and seismic load cases. One of the issues that was examined in the WFCM development process was the hip rafter roof system (hip-rafter system)... However, it was quickly realized that the performance of a 3D hip-rafter system is beyond the scope of the WFCM and simple engineering calculations, especially given the numerous and un-defined load paths that exist to resist the thrust and gravity loads. For this reason, the hip design provisions in the 2001 WFCM are based on designing the hip as a beam (hip-beam system) supported at both ends.
We have been getting requests to re-re-review (review this issue for at least the 3rd time) and see if we can come up with provisions for the hip/valley rafters to be designed as "connection" boards, like ridgeboards. To date, the problem is resisting the significant three-dimensional thrust (and associated ridge settlement, if the thrust isn't resisted adequately). That's where some of the points you raised will have to be addressed.
In fact, a typical hip/valley rafter system probably works somewhere between these two models... depending on the size of the roof and the support of the hip/valley at the ridgeline, there will be some settlement which may or may not be acceptable.
Given that the hip-rafter system is permitted prescriptively in the building code with no limits, how does one determine the adequacy of a prescriptive design...? When acceptance of unengineered construction, such as a hip-rafter system, is primarily based on satisfactory historical performance and not engineering calculations, one needs to review what has historically worked. Generally speaking, homes built 30-50 years ago with unengineered hip-rafter systems utilized relatively small hip areas. Hip areas under 12'x12' (hip rafter span of about 17') have generally proven acceptable in low snow load areas. For a 12'x12' hip area (24' wide building), the ridge displacements are generally observed to be minimal. It should be noted that structural requirements in the 2001 WFCM for engineered HIP-BEAMs are still somewhat reasonable for these smaller hip roof areas. For larger hip areas, the structural requirements become large, very quickly. Again, the structural requirements in the 2001 WFCM for HIP-BEAMs become very large as well.
We are aware that some companies have tested stick-built hip systems. We are working with them to see if they would be willing to share that data.
That's the best information I can provide at this point.
John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036
The American Wood Council (AWC) is the wood products division of the American Forest & Paper Association (AF&PA). AWC develops internationally recognized standards for wood design and construction. Its efforts with building codes and standards, engineering and research, and technology transfer ensure proper application for engineered and traditional wood products.
The guidance provided herein is not a formal interpretation of any AF&PA standard. Interpretations of AF&PA standards are only available through a formal process outlined in AF&PA's standards development procedures.
What is the best way to stick frame a hip end roof?
Dennis S. Wish ,PE
California professional Engineer
Structural Engineering Consultant
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