Resisting Cathedral Ceiling Rafter Thrust Forces
Resisting Cathedral Ceiling Rafter Thrust Forces
(OP)
I am working on a set of plans for a house where the Architect wants the 2nd story to be formed by rafters with collar tie as the ceiling line. Collar tie is about 1/3 from the top. Span is huge, about 30 ft wide. I think I'm OK with the "standard" version of this(there are CJs below), using 2x10 rafters with 2x10 tie, but at the dormers (I know great right?) I have a double 9.25" LVL rafter/tie frame and the thrust at the tails is about 1500 lbs.
Do you guys have any tricks for dumping this into the wall.? Of course its open in this area and all I have to work with down below is a balcony on one side.
Do you guys have any tricks for dumping this into the wall.? Of course its open in this area and all I have to work with down below is a balcony on one side.






RE: Resisting Cathedral Ceiling Rafter Thrust Forces
I have, only once and in a low wind region, used large stud wall (again 2x8, or at least the metric equivalent in NZ) and a 4-2x10 continuous top plate as a beam. That then brought the loads to the return walls. Worked great, but a bit of a pain to construct (the contractor has to be very careful about the height of the studs, and it is a non-standard length detail; always a high chance of screw up). In our case the two walls were of different heights when built, but thank God the shorter one was correct and the contractor just had to pull all the fasteners from the hangers we had turned 90 and cut down the taller studs before reassembling.
Once built, it worked great. I would recommend it in any case where your span would be of reasonable length. Could also work for you if you could put a reasonable size moment frame across the room, but 30' might preclude making with work without a bump out on the outside.
You can also get some modest gain from a double collar, but it is a pain to analyse and get the connections to work.
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
Without ceiling joists, a roof of this size this will not work without a structural ridge.
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
XR250-This is not a true 2-story house. It's one of those 1.5 story deals. The CJs are from the 1st story below. The collar ties form the 2nds story ceiling.
On each end (left and right sides) there are floors so my concerns about thrust are limited there. In the middle though its open with just a walkway/balcony at the front. Thought about using the walkway sort of as a deep beam and coupled with the bump out thinking maybe I can get it to work.
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
Dik
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
Moment is almost never an issue for long span wood members unless you are designing for minimum code on deflection.
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
Dik
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Resisting Cathedral Ceiling Rafter Thrust Forces
This is the upper story floor layout with the lower first floor walls shadowed.