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shear diaphragms and trusses

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Roof Diaphragms:

Jason brought up a good point about diaphragm chords. But, does the chord need to be at the roof level? I have always heard/read and assumed that in a CMU wall the bond beam is the chord and that can never be at the roof (the plywood level). So would it matter that in a wood wall if the top plate is not at the roof level? If this were true, then all trussed roofs would need a chord at the roof level due to the heel height. I think the deep beam analogy of a diaphragm works as long as your have a chord at the end of the diaphram and the shear force is passed into that chord properly.

Simpson Catalogs give normal to the truss shear values for most of their roof connections. The arrow is shown at the base of the connection, not at the top of the roof (how do they know how high your heel is?). I would say anything but maybe a 4" heel height connected with a HETAL, LTA1, or HM9, would not take the shear from the diaphragm into the wall. To do so it would have to resist the OT moment, and the resultant additional tension in addition to uplift. The options of solid blocking and in extreme cases mini shear walls are very good, but be aware that diagonal braces and shear walls have uplifts at their ends (or resultants in the Y), and this has to be added to your uplift of the connection. This was a major issue on a job I had and I ended up with mini-shear walls in-between every other truss, the shear wall jambs nailed off to the truss web, and an epoxy bolt hold down to take care of the truss uplift and the shear wall uplift. DETAILS, DETAILS.....  But isn't it the weakest part of the chain that always fails?

Don't forget to check your connection for shear parallel to the truss, which is a resultant from the top of the wall passing the horizontal force into the roof diaphragm. This is an issue on tall walls in high wind zones. I also check using combined loading interaction for shear in and out of plane with uplift. This is per Simpson and to be honest I used to not consider this. If you are pushing a connection to the uplift limit, then you go and add combined shear, it won't work.

Is this the way others are doing it? Anyone disagree with anything?

I feel like I am considered nit-picky here in the office, but I guess because I am young and inexperienced, and until I have umpteen years under my belt I prefer to do most everything by the book.

Andrew Kester, EI
Longwood, FL 

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