Rafter Ties on Hip Roof?

[quasi904]

I'm planning a pavilion structure, 28 x 24 with a hip roof supported by doubled 2x10 wall plates. The plates are bolted in to rabbet atop 6x6 posts set in concrete. No walls, no ceiling. Will I need to add some type of rafter tie and if so, how?
Thanx,
Jim

 

[BAretired]

One way to avoid rafter ties is to use a post at each end of the ridge beam but that clutters up the internal space and may not be an acceptable solution.

A hipped roof can be designed without rafter ties or internal columns by using the wall plates as tension members and the roof deck for stability. Without roof deck, the four hip members together with a four foot long ridge beam form an unstable structure when hinged at all six nodes.

If there was no ridge beam, the four hip beams would meet at a point and would form a stable structure if hinged at all five nodes.

BA

 

[XR250]

BA's 2nd option can be a detailing nightmare however due to the trying to resolve the hip thrust into the plates.

 

[quasi904]

I guess my confusion is in how to tie the rafters together along the longer dimension. The only place where rafters are directly opposed and tied to the ridge beam is the 8 common rafters in the middle of the structure. With a roof pitch of 3.5:12 I think there will be quite a bit of lateral stress. The columns are planted in concrete at 4' deep and are either 8' or 12' apart. The connection between the posts and the rafters is via a doubled 2x10 rim with a 1.2" plywood core glued and screwed. This is bolted to the post and is in a mortise or rabbet cut into the top of the post.

Thanx,

Jim

 

[BAretired]

A pitch of 3.5:12 was not mentioned in the original post. Why can't you use a greater slope?

Another option is to use a truss at each end of the ridge beam designed to support a concentrated load at midspan from the hip beams and ridge beam. Then you do not need rafter ties.

BA

 

[quasi904]

The slope was an aesthetic choice and it allowed for smaller hip joists since dropping them on that pitch meant that the birds mouth at the rim wasn't too deep to significantly weaken it. I could go with a slightly higher pitch and chamfer the hip joists if necessary. With a clear span between support columns, I'm not sure how (where) to place a truss. Are you thinking of a single truss incorporating the ridge beam?
Jim

 

[BAretired]

I was thinking of two trusses spanning 24', one at each end of the ridge beam and normal to it.

BA

 

[msquared48]

The two truss bottom chords then become the rafter ties you seek.

Mike McCann
MMC Engineering

 

[XR250]

If you want to get cute and have the budget, you can get an I-beam cut and welded at a 3 1/2:12 pitch and span it 24' - one at each hip.
Then you have not rafter ties.

 

[quasi904]

BAretired. I've attached a drawing of the roof plan below. I can see where the trusses in the short dimension could help but what about the longer dimension. I'm about a heartbeat away from a timberframe truss and a gable roof. . .
Jim

 

[BAretired]

My Autocad is not operational right now. Can you provide a PDF?

BA

 

[BAretired]

I guess I don't really need to see the drawing. Each truss supports the interior end of two hip beams and one end of the ridge beam. Rafters span from wall to hip beam or wall to ridge beam. All rafters have a vertical reaction at each end (no horizontal) so no rafter ties are needed.



BA

 

[4thorns]

Jim. Your dwg includes an external reference (jpg) that is on your computer. We can't see it. You would be better off to print it to a pdf and upload that.

 

[4thorns]

I understand your truss concept BA but with the ridge only being just short of 5' the trusses would be quite close together. How about making the building a little narrower? This would move separate the trusses some. A little more symmetrical maybe. I like the steel idea too. It would result in a completely open ceiling. How about this. Add 2 feet to the width and subtract 2 from the length. Make it square. Then incorporate the timber truss idea in the center, heading in one direction or the other and designed to carry the point loads from all 4 hips. Eliminate the thrust from the hips. I think that without doing something a little off the wall, someone is going to have to dig a little deeper in his pockets!

 

[BAretired]

Nowadays, the usual method of framing a hip roof is to use trusses spaced at 2' centers with a girder truss 8' from each end.

Personally, if I wanted to stick frame a hip roof, I would use the wall plate as a tension tie and the roof deck for structural stability as mentioned in my first post. I might use some temporary rafter ties or interior posts during construction but would remove them upon completion.

That seems to me to be the most practical and satisfactory solution but it does require special attention to the connection of the hip beam to wall plate at the four corners of the building. Also, the wall plates would need to be spliced adequately to carry the calculated tension.


BA