Hip Roof with Vaulted Ceiling Below

[kmart30]

I have a 2.5/12 hip roof (see attached plan) with a vaulted ceiling below. Normally I treat the hips as beams and have a support where they meet at the ridge coming down on a joist that transfers load to walls. The ceiling doesn't allow us to do that unless it bears on a tie that would be connected back to the roof rafter. I believe trusses might be the best option here but they would rather not use trusses because of timeline. Any better ideas?

 

[Shotzie]

Could you use a cranked steel frame to pick up the hips/ridge?

 

[KootK]

Not a ton of options here that I can see. You might have to built your own truss to serve as that transfer joist that you mentioned. What's the interior ceiling pitch? A rule of thumb that I used to see as a truss designer was [max ceiling pitch = 0.5 x roof pitch]. That wouldn't leave you with much here.

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.

 

[XR250]

I typically use a dogleg steel beam (or cranked beam as Shotzie calls it).

 

[kmart30]

Archy has it to match the same pitch as the roof which doesn't leave room for much of anything. Just for the hips I have a 11.25 LVL right now with about a 1.5k reaction at the top. Double that and add the load from the ridge we are probably looking at 6kips coming down on to a tie or something. Can through bolt tie to rafter but now the rafter needs to take the additional point load which would require an LVL at least. Its a mess... Hopefully I can convince them to get to a 4/12 roof with a 2/12 interior pitch on the ceiling so we can go with trusses and be done with it...

 

[kmart30]

Never used a cranked beam or maybe I have called it something different in the past....anybody have a good reference or somewhere to get some more info on it?

 

[Shotzie]

I'll mention a flitch beam or a knife plate at the peak as well in case more wood is preferred, although I personally haven't used either for a situation like this.

 

[XR250]

Never used a cranked beam or maybe I have called it something different in the past....anybody have a good reference or somewhere to get some more info on it?

It is just a an I-beam or flitch beam cut and welded into the "A" shape of the roof. Add seats etc. to support the hips and ridge.
It typically runs in the rafter space at the intersection of the hips and ridge.

 

[Shotzie]

The context I was using "cranked beam" in was to refer to a steel beam that follows the pitch of the roof and is spliced at the peak to allow for two different slopes. The peak connection is usually welded. The beam would then frame down to steel columns. It's effectively a moment frame with a kink in it. The image search on google for "cranked steel beam" yields some results.

 

[Shotzie]

Also, you had mentioned that you may have collar ties in the initial post. This could alleviate some of the connection requirements at the columns as you can treat it like a tied frame, using a steel member installed horizontally between the bottom flanges (part way up the steel beams) to tie the two sloped steel beams together.