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RE: Rigid Wood Diaphragm?

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David -

To be clear, I am not advocating the omission of shear walls at the
exterior. I'm just trying to ensure I'm not being overly conservative.

To play devil's advocate, the (cantilevered) diaphragm depth is on the
order of magnitude of 80-110 ft. while the cantilevered span is about 30
ft. Still an issue?

IIRC, isn't there a limitation on cantilevered diaphragms in the Code
(something like 2:1, one story, yadda, yadda)? Yeah, I know, there are
plenty of transverse shear walls which will resist rotation. I'm
extremely comfortable with this concept with a traditional "rigid"
diaphragm (i.e., concrete or metal deck with concrete fill), but not
with wood other than a garage or one story appendage.

I can definitely see bumping up the loads on the corridor walls to
account for stiffness, but I am uncomfortable ignoring the shear
resisting elements at the outside walls. Even in the concept of
"enveloping", loads due to FDA would be applied to the outside walls.

Thanks,

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

:-----Original Message-----
:From: David Merrick [mailto:mrkgp(--nospam--at)winfirst.com]
:Sent: Friday, October 10, 2003 9:31 AM
:To: SEAINT
:Subject: RE: Rigid Wood Diaphragm?
:
:A non-generic design that will increase problematic deflections is not
:good.
:
:Concrete topping may act rigid but at higher shear values, the concrete
may
:delaminate. Rigid and flexible analysis must be considered. Gluing down
the
:sheathing may have the same argument. For a ductile-load-criteria,
maybe
:one
:should greatly increase the shear to the relied-on rigid/brittle
diaphragm.
:Maybe use the OMEGA factor.
:
:Identifying a wood diaphragm as rigid, is only for the purposes of
:distributing the shear to the walls, not to justify deformation
:compatibility and not to exceed allowed cantilevered diaphragm
dimensions.
:
:All systems will deflect. Compatibility of deformation in non-shear
walls
:is
:required. Unusual engineering approaches are difficult to defend.
(Drift,
:Glass, and life-safety). Drift is rarely checked in wood construction
but
:the designs are usually accepted because they follow generic shapes
with
:generic expectations. I have heard arguments against checking for wood
:Deformation Compatibility. Such as, structures are rarely over three
:stories
:high and windows are usually over planted areas or non-public areas.
:
:Is observing no movement a true test? The 500 year (or more) event has
:probably yet to occur.
:
:A generic wood frame design allows 1/2" (or more) story movement. I  do
not
:like to wake up, before my coffee, to see 1/2" wall cracks and broken
:windows. You want to make it worse?
:
:David Merrick, SE
:
:
:
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