Return to index: [Subject] [Thread] [Date] [Author]

RE: Roof to top plate connection?

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Paul,
I agree with Bill, and would add that I (almost) never rely on hurricane straps to transfer diaphragm shear into a wall. They are designed for uplift and a small lateral load, which is what the strap would resist at the top of the wall, not at the top of the framing member or truss. I have talked to other engineers who rely on this, sometimes with trusses that have one foot + tall heels, and I will always disagree with that. You have to provide blocking or diagonal bracing to get the shear from the top of the roof framing member/truss to prevent overturning. Luckily you had 2x members, but like Bill said they are 1.5" short so not they have cross grain bending on that little cantilever. Luckily it is wood so you can scab on and fix the problem pretty easily.
 
The "almost" never case is when the truss has a tiny heel, like the top chord and bottom chord come together in a miter cut and are basically 4" or so tall, and you can use a beefy HETAL or something with some real lateral capacity to resist that overturning/lateral force. But I never rely on a plain ole wimpy twist strap for that.
 
One question on Bill's comments was shear transfer at the boundary. Now the way I have always understood it is in an unblocked diaphragm, you are not relying on blocking at ANY panel joints whether in the field or at the boundaries of the diaphragm. So if an unblocked diaphragm works for your design shear, then you do not need the blocking between the trusses/framing members for shear transfer out of the diaphragm and into the wall, though it will help.... They are there to transfer the lateral load in each framing member to the wall. But I guess that is accomplishing the same thing. But you could also use diagonal straps at every framing member/truss. Right?
 
Hope that made some sense to someone...
 
Andrew Kester, PE
Structural Engineering Consultant
Lake Mary, FL