Hi All,
I wanted to get some opinions from the experts on an interesting project designed by a firm that is accustomed to contemporary styles and "contemporary" builders. Given the location of the structures, we would like to incorporate some nice local timbers and traditional joinery, and eliminate as much steel hardware as possible.


Structure as designed:
2 Canopy style open-air classrooms, identical
35' x 35'
Round posts (perimeter only, 12 total)
Low slope (roughly 2/12) pitch
Uninterrupted interior span
Standing seam roof
Coastal Carolina so high wind load/little snow load


The current design is simply round posts with double plates and double rafters bolted through the posts. Purlins 24" o.c. We want to replace these with good joinery and solid timbers. There are several challenges with this, but right now my issue is bracing. As you can see in the picture (hopefully!) there is cross bracing in each bay on the side elevations. These would be hidden behind wooden louvres. My question is about running bracing (hopefully timber but possibly steel rod) from post to rafter. I can see the ability to prevent uplift, but is that a suitable method of transferring lateral force to the ground? Previous experience tells me that rafters are not generally used to stiffen the whole frame, more for the roof structure. Any thoughts?

Are you going to move the rafters to larger members, and have them sit over the posts?

What about doing the same to the connectors along the front and the back between the posts?

Theoretically you'll want bracing in both axis - along the rafter line, and along the plate lines.

Any sheathing on the ceiling besides just the sheet metal?
That is a huge area that can help as it can become a large diaphragm for keeping the roof rigid.

What is your connection to the foundation? You may need a moment connection regardless of what you do up top.

The braces will have to be stout enough to not bend and buckle when they go into compression as the structure moves. The connections will have to have enough relish to work as the opposing pair moves into tension.

Then the rafter and the posts will need to be able to resist whatever load the braces bring to them. Its a bit of chasing ones tail.

I'd get a local TF engineer involved.

David Hourdequin is a TFG member and is on these forums occasionally.

http://www.dremy.com

The plan is to use larger single rafters instead of doubles, with each rafter sitting over the post. As to whether they would bear on a plate or directly on the post, I havent determined. I dont have any experience with making the connection of a sawn rafter to a round post, so thats something I'm looking into. As for the connecting beams between posts on the front and back, that will depend on the placement of the rafters.
The design is definitely lacking bracing in the plate axis. I think we can solve that with simple bracing running post to beam. The issue I'm running into is bracing along the rafter axis. I don't care for the bracing shown in the design. I think that was thrown in as an afterthought. The architect is pretty set on having the louvres installed to create shade, and I need to come up with a way to incorporate timber bracing that will fit behind these things.
We've touched on the connection to the foundation, but havent decided on anything yet. The architect would like to use his local engineer. I think he may be a little out of his comfort zone when it comes to timber, so its going to be a challenge to minimize steel connections...
Thanks for the thoughts Mike. Keep em coming

I can imagine a lot of tension between a spare modern design, the need for high level engineering and a desire for traditional timber frame detail. A core problem is that primary wood to wood joints are not rigid enough to resist racking forces hence the need of bracing. The downside of bracing is it can deflect the posts, even to the point of fracture, and break the primary joints. So in principle, I would look to trussed panels between the posts all around the perimeter, unfortunately I think this will conflict with the aesthetic of openness.

The second major concern is with the 35 ft. clear span interior rafters, that will require very deep glulam timber or a truss to replace a single member rafter. I believe rack bracing is important to these bents.

If you consider timber framing conventions, where one tries to build a quiet low tension frame, with joinery that works in compression, you will have to rely on a repetoire of steel connections, because a major storm will create havoc to the ordinary pegged joint. So honestly, in the limited way I understand, I think in order have a timber frame, a very different design needs to be developed.