Wouldn't the frame itself provide the necessary wind shear plane?

Just blasphemous!

Don Wagstaff

If you designed for it.

Hello,

I am just blown away by the implications of that statement.

Don Wagstaff

Don, do I detect a bit of sarcasm there?

This same thought kept coming to me, why can't the frame just be made to handle all that? But I felt as if I must surely be missing something since no one else had mentioned what seemed to me to be the simplest solution. Maybe I am still missing something?

But brave Mr. Pfield-Steader seems to have created a user name solely to share this groundbreaking bit of wisdom with us (I am not at all sarcastic there, just to be clear). I guess he is braver than me.

Or have we fallen into the trap of letting SIPs or whatever enclosure method we may use serve a structural function, and not designing our frames to be able to handle themselves any more? Just a hop, skip, and a jump away from balloon framing I'm afraid! (OK, so that's maybe a bit of a stretch!)

But in all seriousness, why can't the frame be made to handle the wind shear associated with this system? There is no reason immediately obvious to me

DL,

Timber frames are not inherently stiff structures. They flex. Yes, even frames with a lot of knee braces flex when loaded by wind. They are strong but not stiff.

Modern building regulations require stiff buildings that do not flex. It is incredibly difficult (you can substitute "expensive" for difficult) to build an all wood-joined timber frame that meets code requirements for lateral stability. So to build code compliant timber frames buildings, most often we rely on the envelope to provide lateral stability.

This is typically not a problem when the frame is enclosed in any of the typical manners -- sips or stick framed walls. Those systems can be made to meet the requirements quite easily. It becomes an issue with alternate enclosure systems and especially with open air structures.

But there are forms of bracing other than knee bracing that do provide lateral stiffness. This would be lateral bracing.

Lateral bracing is a technique employed in much of Europe, it employs long slanted braces interfacing with horizontal and vertical timbers. For this to work, you must have either a studded timber frame, such as in the English or French styles, or a relatively close posted timber frame with horizontal ties, such as in the German style. I don't know much about the English (I assume it would achieve the same properties) but the Germans say that by having long braces pass over the horizontal members, they achieve a tremendous amount of lateral stiffness. Having the verticality of a cavity interrupted by one or more horizontal ties does a tremendous job of stiffening the structure against wind loading (that in combination with the Windrispen, very long roof braces that cross many rafters and tie them together) The Germans do all this, because they teach that you should rely on the enclosure -be it it boarding or infill- for absolutely nothing other than keeping out the cold. They have also striven to make frames which are very stiff and rigid.

I imagine that having long braces that pass from one bent to the next, with a few studs in between, would do the same thing.

Whenever you have vertical members that can brace against other vertical members, it improves the stiffness of the whole assembly. Vertical members braced against horizontal members is a flexible assembly. The same is true of Horizontal members braced against other horizontal members.

Alternately, you could build the frame as normal, and as part of a modified Larsen-Truss or similar enclosure method include very long 2x4 braces nailed diagonally across the frame, crossing over many of the studs thus tying them together.

The American timber frame is a flexible design, likely designed to be flexible. Keep in mind though that other cultures designed their frames to be as stiff as they could get them.


By enclosure the frame in typical manners you are essential doing the same thing I am suggesting here, tying many vertical members to each other to achieve stiffness. Sheathing can do a decent job of achieving lateral stiffness, I won't say much for relying on it for keeping things square...

As I said, if you design for it, you can do it. It may mean extra long tenons and braces, some tension steel, rigid connections to the foundation, or all of the above. It also depends on location, use, and live, dead, and wind loading.

I've worked on projects where the frame did all the work. On the coast in a hurricane zone it added plenty of steel and rigid connections to the foundation. In the mountains it required stiff posts with strong braces and equally strong tenons or tension joinery. In California it required lots and lots of embedded steel.

And don't forget about seismic. That adds a whole nother level of requirements.

And remember that every action has a reaction - and this is where simply adding braces or increasing their size doesn't work in all situations. That long brace can become an equally strong crowbar trying to pull apart your joinery.

The crowbar effect, that is why you will notice the Germans don't have their braces pushing against the posts, or if they are against the posts they are directly opposed with another brace on that same post. These posts typically extend all the way from sill to plate, or they may start at the sill just inside the corner post and brace against the next post, with another brace directly opposite in the next cavity there to ensure the post doesn't go anywhere.

Use wall ties too, 1 or 2 horizontal timbers that span the cavities between posts. This will have the effect of causing the posts to press against each other. If one post wants to flex, it would have to flex all of the others with it. These don't have to be very big at all, they aren't bearing any roof loads or other structural loads. They just need to be stiff enough to be able to push. They would work with the same dimensions as brace stock.