Unique roof rafter configuration and the analysis thereof...
Unique roof rafter configuration and the analysis thereof...
(OP)
Stop me if you've heard this before (but I'll need a picture to believe you). I'm reviewing the roof framing for a client with a 48' deep center hall colonial. The 10/12 pitch is formed with 11.88" LVL rafters at 16" on center, and they're tied at the top of the outside wall with continuous, 48' 11.88" LVL ceiling joists running parallel to the rafters. Halfway up into this cavernous space (10') there is an attic ceiling joist framed with (you guessed it) 11.88" LVLs. All connections are made with 4ea 1/2" bolts, except for the rafter tops, which are strapped to a non-structural 16" LVL ridge board (and each other).
Question: has anyone had any luck convincing a code official that an attic itself cannot have an attic? He wants me to apply significant live loading to the top of the attic ceiling joists because (let's face it) there's 10' of headroom up there.
Since the system appears to work without the ceiling joists (members, connections good), should I have the attic ceiling LVLs removed? Because the way I see it, those forces could (on paper) exceed the moment capacity of the rafters.
It seems a bit counterintuitive. Of course, as always, I may have missed something in my hastily drawn FBDs.
Question: has anyone had any luck convincing a code official that an attic itself cannot have an attic? He wants me to apply significant live loading to the top of the attic ceiling joists because (let's face it) there's 10' of headroom up there.
Since the system appears to work without the ceiling joists (members, connections good), should I have the attic ceiling LVLs removed? Because the way I see it, those forces could (on paper) exceed the moment capacity of the rafters.
It seems a bit counterintuitive. Of course, as always, I may have missed something in my hastily drawn FBDs.






RE: Unique roof rafter configuration and the analysis thereof...
If the lower floor joists are adequately connected to serve as collar ties, then the upper set probably has little effect on the rafters as the moment arm is so much greater than the upper ones.
You could remove the upper ones, but why? Just label the upper area as attic space, providing a small limited access to it, and the lower space - well apply the standard residential 40 psf figure.
I would be surprised if that did not work. Then again...
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Unique roof rafter configuration and the analysis thereof...
But check in the Chap 16
RE: Unique roof rafter configuration and the analysis thereof...
The roof was so huge it was like we were framing an entire house upon a house.
I'll see if I can find photos.
P.S. the roof configuration was installed because trusses that were originally installed by another contractor, collapsed in a wind storm.
RE: Unique roof rafter configuration and the analysis thereof...
RE: Unique roof rafter configuration and the analysis thereof...
Any ideas? And it must add to the wall thrust, too.
RE: Unique roof rafter configuration and the analysis thereof...
Bolted trusses have been use for well over 100 years - probably 200!!
This is not rocket science. The first guy did a lousy job!!
RE: Unique roof rafter configuration and the analysis thereof...
RE: Unique roof rafter configuration and the analysis thereof...
Well what kind of loads do they want to support. A plain attic for "lightweight" storage is 20 psf -- for "sleeping rooms" it is 30 psf.
Need a bit more info.
Slightly inclined to allow the bottom chord to carry any "attic" and/or ceiling load while the top chord carries snow and live loads.
But without dimensions and specifics - that is just a guess.!! Have done dozens of "room-in-attic" applications where there is a loft/bedroom/office built into the truss. Worked great!!