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RE: shear diaphragms and trusses

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Andrew,
I haven't read your entire post - figuring I got the gist in the first
paragraph and I do agree as long as the shear from the diaphragm can
pass down to the chord. An example will be a truss of a flat (or small
slope) roof truss. Bearing may be on the plate and the diaphragm
somewhat higher. The shear transfer is in the truss-blocking where the
connection of the diaphragm is transferred down by a panel to the double
plate. The double plate acts as the chord AS LONG AS IT IS CONTINOUS OR
TIED IN SUCH A WAY TO INSURE THAT CHORD DISCONTINUITY IS REPAIRED. This
can be seen in a "U" shaped building where the roof is continuous across
the exterior courtyard or patio between the legs of the "U".

Dennis

-----Original Message-----
From: Andrew Kester [mailto:akester(--nospam--at)bbma.com] 
Sent: Friday, November 14, 2003 10:19 AM
To: seaint(--nospam--at)seaint.org
Subject: shear diaphragms and trusses

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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