Need a book? Engineering books recommendations...
Re: FW: Ceiling joists/rafter ties IRC R802.3.1[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: FW: Ceiling joists/rafter ties IRC R802.3.1
- From: Jim Wilson <wilsonengineers(--nospam--at)yahoo.com>
- Date: Wed, 5 Oct 2005 10:23:29 -0700 (PDT)
Designing hip rafters as structural beams has a lot of practical limitations when applied in a conventional home: -Deep hips cause detailing issues with tray ceilings (which can be bad structural details anyway) -They inevitably require that the horizontal cut of the beam hang off the inside edge of the top wall plate. Local inspectors accept the hip beam without questioning its capacity, but they require an engineer to say the "notch" is acceptable. -It is very difficult to provide a conventionally framed connection to support the top end of the hip rafters at the roof. It is common to see a built-up hip beam lean on a ridge board. That's no good... I also see 2x4 T-section posts kicked at 30 to 45 degrees to the top of an interior wall. That's not much better. -Taking all of this additional roof load down through the house via posting poses other major problems. I won't bring up house designer's plans that don't work, but even providing an appropriate structural post in the attic space is a complication with high roofs. I.M.O., its a waste of wood and effort and design time to provide these massive LVL hip beams when these other issues are not being taken into account. The code could stand to put additional limitations on hip beams, such as Buddy states about 12'x12' hip areas. Anything beyond that requires an engineered design. And not just one from the lumber yard that says a hip beam has to be such and such. My 2 cents... Now I have to go deal with a house with singe 12" LVL hip "beams" spanning over 20ft, intercepting a tray ceiling, with discontinuous rafters ties, single 2x4 roof posts toe nailed into the bottom of the hip rafters, etc., etc.... And the inspector only wants a letter saying the "notched" hip is acceptable. Apparently the rest is okay. Jim Wilson --- AWC Info <AWCInfo(--nospam--at)afandpa.org> wrote: > Dennis, > > 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. > > HTH > > Buddy > > John "Buddy" Showalter, P.E. > Director, Technical Media > AF&PA/American Wood Council > 1111 19th Street, NW, Suite 800 > Washington, DC 20036 > P: 202-463-2769 > F: 202-463-2791 > http://www.awc.org > > 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? > > Best Regards, > Dennis S. Wish ,PE > California professional Engineer > Structural Engineering Consultant > ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
- FW: Ceiling joists/rafter ties IRC R802.3.1
- From: AWC Info
- FW: Ceiling joists/rafter ties IRC R802.3.1
- Prev by Subject: FW: Ceiling joists/rafter ties IRC R802.3.1
- Next by Subject: Cement Pricing for January, 2006?
- Previous by thread: FW: Ceiling joists/rafter ties IRC R802.3.1
- Next by thread: Modeling Techniques (deck)