Hip Roof Pavilion Framing

[TRAK.Structural]

I've been asked about an open pavilion type structure with a hip roof (see attached inspiration image). I'm trying to sus out how these are typically framed. It's high end residential construction, likely 20 feet x 20 feet in plan, and stick framed with conventional sawn lumber and/or engineered wood products as needed. It will have a flat ceiling so my thought is that this works similar to a typical rafter/ceiling joist system however there are some differences that I haven't quite wrapped my head around yet:

1. Ceiling joists that have the bottom flush with the ceiling can really only run in one direction; to resolve the outward rafter thrust in the other direction should another layer of ceiling joists be run right on top of the aforementioned joists?
2. If the structure is a square in plan and the roof framing meets at a common peak, do the hip members need to be designed as beams spanning from the corners to the peak?
3. Does anyone have any guidance on detailing at the central peak? Getting (4) hips cut so they all meet just right seems like it may be tough, but I'm not sure that some sort of compression ring for the hips to frame in to would be any better.

 

[bones206]

You could have a king post truss (blue) pick up the hip girders (red) on each side of the ridge (magenta). I out sketched one half of the framing:

 

[phamENG]

1) Essentially, yes. They don't need to be full ceiling joists, though. It's often accomplished with 2x4s laid flat perpendicular to the ceiling joists. You can calculate the thrust and design an efficient tension tie. Steel straps over blocking can also be used.

2) Not if you have adequately tied everything together.

3) I wouldn't worry about it. This is pretty small and the details of cutting the lumber at that connection should be doable by any competent framer. After all, they're not really beams.

Traditionally framed hip roofs look like they shouldn't work, but there's a load path there.

 

[TRAK.Structural]

Bones - That's one idea I've considered. I think it offers a more easily understood (for my mind anyway) load path and force resolution.

Pham - 1. See sketch below. Where the opposing rafters do not meet due to intersecting with the hips is the compression between these opposing rafters resisted by the diaphragm? Does a couple rows of blocking in this direction at each side make sense to help stiffen this?

2. I assume the stiffness of providing a rafter/tie system is what prevents the hips from acting like beams and resulting in large hip thrusts at the corner posts that needs to be resolved in a "tension ring"?

 

[phamENG]

Correct - the diaphragm ends up resisting that compression. Note that where the compression will be highest (longest rafters), the length of the compression element is shortest. Some blocking will help if you are concerned about it. I usually require blocking in rafters anyway to avoid problems with uplift.

And yes, if you have adequate ties, there won't be any outward thrust at the hips.

Now, if your client wanted no ceiling, things would be very, very different...

 

[TRAK.Structural]

Pham - Makes sense. I'll probably add some collar ties each direction as well.

Detailing of wood connections is always a pain, IMO. I've done the beam/col joints different ways in the past, wondering what your opinion/preference is for this situation. Beams on top of cols with some sort of post cap connector, or notch the top of cols and set top of beam equal to top of col with through bolts?

 

[DoubleStud]

I don't understand. Why not just get prefab trusses? That would be the cheapest and quickest.

 

[TRAK.Structural]

DoubleStud - Agreed that trusses would be easiest. Cost is debatable, and I'm working for the contractor who wants to stick frame it.

 

[bones206]

Detailing of wood connections is always a pain, IMO. I've done the beam/col joints different ways in the past, wondering what your opinion/preference is for this situation.

There are so many ways to join wood members. If you already have a carpenter on board, maybe see what they want to do and check that the numbers work.