Resisting Cathedral Ceiling Rafter Thrust Forces

[grogannc]

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.

 

[CELinOttawa]

This really depends on your building code, but with the fire stops that are available these days I have used baloon framing a couple of times. Would only work for you with reasonably large members (say 2x8) and a PWF, so the studs were continuous from the founding level of the basement.

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.

 

[XR250]

I don't understand your situation. You say you have collar collar ties as the ceiling line but then you say there are CJ's below.
Without ceiling joists, a roof of this size this will not work without a structural ridge.

 

[grogannc]

In fact one side is a 2x8 wall though it has a large punched door opening through it at the floor below. The other side does have a bump out which is slightly larger/wider than the width of the dormer so I am having to pick up the A-frame dormer loads with a large beam to get it into the bump out walls.

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.

 

[grogannc]

see if this comes out

 

[dik]

Can you use a ridge beam? or factory fabricated trusses?

Dik

 

[grogannc]

Already tried to go the truss route. I can possibly do a structural ridge but its about a 30ft. span in the shortest configuration. It would have to be pretty stout as much deflection in it will make the thrust show back up.

 

[grogannc]

Looks like it would be a fairly large glulam. Not the end of the world I guess but its about a 50 kip-ft moment using 35psf

 

[XR250]

I just designed a 40' Ridge for a house. W21x50. 30 ft. should not be a problem with multiple 24" LVL's if you ain't in a high snow area.
Moment is almost never an issue for long span wood members unless you are designing for minimum code on deflection.

 

[dik]

Glulam or paralam for 30' shouldn't be a problem... or even a small steel beam.

Dik

 

[KootK]

I'm having a tough time visualizing this. Can you share the lower floor plan as well?

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.

 

[grogannc]

I'm going with a triple 24 lvl ridge over the open portion in the middle with the balcony and use dimensional lumber for the floor system to tie the rafter tails to at the left and right portions of the plan. Should work fine I think unless I am missing something.

This is the upper story floor layout with the lower first floor walls shadowed.