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RE: Rigid Wood Diaphragm?

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	I will own up to having done this on a project.  Not quite as bad as your
description, we didn't have air conditions in the remaining panels.  And I
won't say it was my first choice.  I designed the roof to go to the outside
walls and used the corridor walls to carry the floors.  Now I know as well
as you do that the force is not zero at the outside walls.  But I also feel
that if there is a defined load path, the loads will generally find it.  The
corridor walls can carry direct shear and the transverse walls will deal
with any torsion.  The topping will make the floor much more rigid in
reality than calculated by code.  I know that is not the perfect code
answer, but I believe we satisfied the intent.  And with short outside
flexible walls and long rigid interior walls, my analysis may actually be
closer to reality.
	We detailed the outside walls to carry the roof load and some reserve.  But
also detailed the corridor walls to carry all of the load.  Those walls are
much longer and more rigid than the exterior walls.  The building will need
to move a long ways before the outside walls will carry much.  Another way
to look at it, we had a diaphragm that "cantilevered" about 30 feet each
side and was close to 300 feet long.  Call that engineering judgment, but I
don't see it moving much.  Bottom line, I think it can make sense if
detailed thoughtfully.

Jake Watson, P.E.
Salt Lake City, UT

P.S. I don't think all the dinosaurs are extinct, I suspect you are in good
company. :)

-----Original Message-----
From: Bill Allen [mailto:T.W.Allen(--nospam--at)cox.net]
Sent: Thursday, October 09, 2003 4:41 PM
To: seaint(--nospam--at)seaint.org
Subject: Rigid Wood Diaphragm?


Yesterday, I saw something that really shocked me.

Either I saw a serious design problem, or I'm a dinosaur and I'm losing
touch with reality.

I design a few projects that are wood framed hotels. They are three and
four story boxes, basically. Visualize a rectangular building with a
room, corridor and room forming the transverse section. In the
longitudinal direction, there are substantial shear walls in the
corridor and just a few between the windows on the exterior walls. The
floor diaphragms are wood sheathing with 1-1/2" lightweight concrete.
The roof diaphragm is wood sheathing. My challenge has always been shear
walls on the exterior walls, but specifically the hold downs. I can't
use a perforated shear wall (or a wall frame) because there is an AC
unit under the window and the top of the window is very near the top of
framing (16" or so). By the time the uplift forces accumulate all the
way down, you can imagine what kind of uplift forces I have at the
foundation for a 3 or 4 story structure in seismic zone 3 or 4,
especially with near source effects.

When doing a preliminary review of an upcoming project with the owner's
rep (the contractor - with whom I have a very good relationship), he
pulls out a set of drawings for another job (in seismic zone 3) that
made my jaw drop. The only shear walls (and thus hold downs) shown on
the drawings in the longitudinal direction are on the corridor walls.
There are no shear walls on the exterior and, in fact, the ac units have
been moved out from below the windows and right in the middle of where
the shear walls would be (in between the windows).

The only thing I can think of that the design engineer did was consider
the diaphragm rigid and cantilevering off the corridor walls. Now, I
know the corridor walls will pick up more than just the tributary load
from a flexible diaphragm analysis, but I can't believe that, in a wood
framed structure, NO load would go to the exterior walls.

Does anyone else do this?

Should I check my meds?

TIA,

T. William (Bill) Allen, S.E. (CA #2607)
V/F (949) 248-8588
San Juan Capistrano, CA
http://members.cox.net/ballense/





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