Hello. I'm new to timberframing and have some questions concerning lateral forces on a common rafter timberframe with a structural ridge. Mainly im interested to know what, if any, amount of lateral force is absorbed by the common rafters in a high wind situation. For this, I'm assuming the cross section of the building with post/girt/rafter is forming a triangle and thus is able to withstand a "racking" force. If anyone can help with this and understands what i'm trying to get at here, let me know. thanks, Steve.

I wouldn't rely on the rafters themselves. With any externally applied lateral force (wind, earthquake, etc), the force needs to be transfered down to the ground / foundation, so bracing upward just means you will have to brace down somewhere else.

If the lateral force is applied to one of the eaves, and the force tried going into the rafters, in order for it to be doing anything, it has to go back out the opposing rafter into the other eave wall, so you've not gained anything. And that is assuming there is a collar tie or something like that.

If the roof is adequately sheathed, then, you may get some diaphragm action where the load applied to the eaves travels in to the roof sheathing, which is attached to the gable walls, and then is transfered down that way. But, this requires a fairly shallow pitch to be effective.

Hope this helps,

Joe

@ Stevenk,

I agree with Joe. Your frame would only work if there is a tie beam or at least rafter ties.
What is the size of your structure?

Hey, thanks for the input.
I was mainly interested in what the general school of thought is on this. I have seen some photos ( for example: Tedd Benson's The Timber Frame Home pg. 56) of similar frames with plenty of kneebraces in the plane perpendicular to the common rafters but none going the other way (i.e. from eave post to tie beam). I assume these frames must be relying on structural panels or sheathing to resist lateral force, but was curious about the rafters acting as a brace in some fashion. My frame has a 28' x 36' footprint (ridge goes long ways) and will have 3 10" x 10" pine posts supporting the 10" x 20" structural ridge. I think I will have to place larger rafters over gables and center post (principal rafters?) and brace these back to posts to help with shear. How does this sound? any input is helpful. thanks again, Steve.

Stevenk,

I thought that this is the kind of frame you have in mind. Only thing I would add, at least in the gable walls where they are not in the way, are braces on the post underneath the gird and the ridge beam down onto the tie beams. Otherwise you do rely on the SIPs somewhat. There is a picture on the previous page in the book mentioned where it is done that way. Also like the note underneath the picture: "Best frames are often ........"
In your case the loads from the rafters, which will have to be sized so they can span and carry the loads, will be transfered the gird and ridge beam. From there it will get transfered down through the posts onto the foundation.
If you put the rafters too far apart they will actually have to be very heavy. I prefer this kind of roof construction myself. Besides the aesthetic aspect it is also easier and faster to cut then the typical bend and purlin approach. It is also easier to add roof overhangs at eave and gabel with out having to rely on SIPs. It makes the house more attrative and you do not have to box them in like the SIPs.
And as for lateral forces, since they are no braces in direction of the rafters. Since the rafters are fastened to the beams you end up with a rigid triangle formed by rafters and girds (which are held together by the floor joists) that can and will resist any force it has to withstand.
So this is a sound frame by itself that does not rely on SIPs for strength.
But just like any other frame that is covered with SIPs they will add additional strength thus stiffen the frame even more.
I'd say go ahead with your idea and post some pics as you go along.