Need a book? Engineering books recommendations...
[SEAOC] Re: Basic Roof Rafter Analysis Question[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)power.net
- Subject: [SEAOC] Re: Basic Roof Rafter Analysis Question
- From: Steve Privett <eqretrodr(--nospam--at)earthlink.net>
- Date: Sat, 10 Aug 1996 09:25:50 -0700
Dennis, When wood shingles/shake were "outlawed" a few years ago, I was doing a lot of this work for various roofers. I found a lot of different conditions and very poor connections in the roof structure. But to your point... I checked the 2x purlins as rafter supports, which resulted in multiple span design of the rafters and found many times, the rafters would work, but not the purlins. The span of the purlins is typically reduced with struts that may go parallel or perpendicular to the purlin (and sometimes angles in between). Many times, these struts are supported by interior walls with no footing or double joist.... (another possible problem) For struts parallel to the purlin, I just analysed the purlin as a pinned frame. This quite often required the addition of a 2x member at the center span to handle bending or deflection problems. Again the connections at the pins need investigated. Many of these were just angle cut struts with one nail. I would use plywood gussets or nailed metal plates or scab a 2x on the side of the strut that would overlap the purlin. For struts perpendicular to the purlins, the problem is a bit more difficult as quite often there are not oposing members. Therefore I'd take the closest rafter to the strut and use that as a component of a "truss". The purlin would impose a vertical load that is resolved by the strut, and the opposing horizontal thrust caused by the slope of the strut is resisted by the closest rafter. This requires investigation of the purlin in shear as the the rafter may be 12 inches away from the strut. This also requires investigation of the connections. Many times the connection of purlin to the strut and the rafter needs improved not just to resist the forces, but also to address the cross grain bending as the strut is attached to the bottom of the purling and the rafter to the top. In addition, the rafter ties typically need checked. If they were face nailed to the rafters, quite often the nailing in inadequate to resist the horizontal thrust of the rafter. Sometime the addition of ties just above the purlins work and are easier to install. Additional loads to headers are not usually a problem when the change is from shake to lt wt, but to full mission tile it could be. The installation of the mission tile makes a big difference. Just the tile may not impose much but if installed with a lot of cement for "asthetic" reasons... it could kill you.. (Another thing to remember with any tile, is the addition of plywood shtg -- 5/8" in LA city I believe). Another thing to consider is the additional load to the lateral bracing system... While I believe the "Blue Book" used to have something in it about increases less than 3 percent could be overlooked, I use the change in roofing to "look" at the rest of the structure for inherent weaknesses and point these out to the owners. I believe UBC actually says ANY increase in the loads or change in the capacity requires the upgrade to current code. Most of the time I can convince the owner of the practicality of "upgrading" the system. Not only does the address the walls, but many times, the shear transfer from roof diaphragm to the walls is inadeqate. Drag struts, collectors and ties at re-entrant corners can then also be addressed. On complex roofs, the "engineered look" of an existing systems quite often reveals problems. Especially when the building was "just built" per general construction provisions... But that's a completely different thread <G> My apologies to any who think I get long winded and wordy.... Steve Privett CE eqretrodr(--nospam--at)earthlink.net ...
- Prev by Subject: [SEAOC] RE: ANSI
- Next by Subject: [SEAOC] Re: Bridges
- Previous by thread: [SEAOC] Basic Roof Rafter Analysis Question
- Next by thread: [SEAOC] Reactive aggregate