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RE: Diaphragm Chord Wood Roof

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Scott,

Thanks for your response.

I'm not sure we are thinking of the same thing when we discuss chord design.
My question is the diaphragm chord element.  This is the member that is
supposed to take the compression\tension couple force of the diaphragm deep
beam analogy.  The force in the chord is perpendicular to the wind
direction.  I think your discussion is the end forces transferring the shear
to the top plate and shear wall.  That is the case where the force is
parallel to the wind.

I think I will do something similar to what you discussed below as far as
blocking between trusses and connecting the roof sheathing to the blocking
on the inside face of parapet.

Rich


-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu] 
Sent: Saturday, July 12, 2008 2:14 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Diaphragm Chord Wood Roof

For wind parallel to the truss span, I would think you could just use the
top chord of the truss as your diaphragm chord member.  It would appear per
the drawing that the top chord is continuous.  This assumes that the "last"
truss is RIGHT at the wall.  The real challenge would be to get the
diaphragm force from the top of the truss down to the bottom chord of the
truss.  I could see either designing the truss for this force (i.e. a linear
shear force on the top chord...and then design the truss members and
connection to handle that shear) or by placing sheathing on the truss (which
would likely happen any way) and use the sheathing to transfer the force.
If the "last" truss is held back from the edge of the wall, then you just
run the wall up to the diaphragm level and do traditional connections.

For wind perpendicular to the truss span, I could see doing blocking between
each truss and then tie the blocking together with straps to connect all the
blocking to "make" your chord member.  I would think you then would use
sheathing on the outside face to transfer the force from the top chord level
down to the wall (I would likely look to see if I could overlap the
sheathing from the truss space to the first part of the wall below...with
blocking at the joint, of course.  The tough part would be were to put the
blocking relative to the parapet (which I think you hinted at).  You could
either extend the roof sheathing INTO the paraphet "space" (which would
require notching it around the paraphet posts) and thus put the blocking at
the outside face of the building...which would make it rather easy to also
attach the wall sheathing to the blocking.  Or you could hold the sheathing
at the inside face of the paraphet (no notching required)...but then if you
use vertical blocking, you would have to figure out how to get the diaphragm
force from the blocking (chord) to the wall sheathing since the blocking
would be held back from the outside face.  Now, you could look at putting
the blocking in horizontally, giving you more nailing surface for the roof
sheathing such that you might still be able to hold the sheathing back but
still have the blocking extend out to the wall sheathing.  Thus, roof
sheathing nails down to the horizontal blocking which then gets nailed from
the side into the narrow face for the wall sheathing.

Don't know if that helps or not.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Rich Lewis [mailto:seaint04(--nospam--at)lewisengineering.com] 
Sent: Saturday, July 12, 2008 1:39 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Diaphragm Chord Wood Roof


I appreciate the replies, especially on a Friday night.  I wasn't expecting
answers until Monday.

I have tried to attach a JPG image of the truss to this file but the Server
kicked it out.  I have setup a web page link to a JPG image you can view to
see a schematic of the truss.

http://www.lewisengineering.com/RF_truss.jpg
 
As you can see from the image the truss is parallel chord with bottom chord
bearing.  There is also a tall parapet that kind of cantilevers up.  This is
for a chain restaurant.  It came from a Prototype design.  I assume this has
been built in several locations already but I don't see from the prototype
drawing how they accounted for the shear transfer from the diaphragm to the
shear walls and for the diaphragm chord force transfer.  There is no
discernable load path from the roof sheathing to the shear wall.  

Several have mentioned using coil strap anchors and I'm not sure I picture
what you are describing.  If I understand correctly I would put solid
blocking between the trusses at the top chord.  The sheathing would be
nailed to the blocking.  I would add a strip of coil strapping on the top of
the sheathing that would act as the tension chord member.  For the
compression chord I would consider the line of blocking between trusses as
adequate.  Due to the parapet wind and the long narrow building my chord
force is over 8,000 pounds.  I don't have a Simpson book with me now, but I
imagine this will be a hefty strap. I have not thought to use coil strapping
before, but then again I typically design top chord bearing or gable style
trusses and use the double top plate of the stud wall.

If I didn't have this parapet I could see this as being feasible.  The
additional issue I see is getting the shear from the diaphragm into the
shear wall.  Ideally the roof sheathing would stop at the inside face of
parapet wall so the sheets don't have to be cut.  To get shear transfer I
think I need to require the sheets to be notched around the vertical parapet
stud and run my top chord blocking out at the exterior face of wall.  For a
prototype, this seems to have been ignored by others.

Hopefully this JPG image will help provide additional insight and response
from the List.

Thanks again for your help.

Rich


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