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RE: Rigid Wood Diaphragm?

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Bill,

I believe the diaphragm cantilever limit is 25 feet.

The drift will never work at the outside if this is more than 1 story.

How is the roof supported? Is it also a cantilevered diaphragm? If so,
did they extend the corridor shearwalls all the way to the top chords of
the trusses (assuming trusses & pitched roof)?

Perhaps it was designed using rigid diaphragm principles, but did they
consider torsional irregularity and accidental torsion in the design?

You being shocked is not shocking to me. There is a lot a really bad
stuff out there that gets by plan check no problem. Owners and
contractors like it too because they are getting it built cheap. When
the rest of us try to do work like that, the owners want to go back to
their garbage engineer.

The only thing preventing this is consistent/uniform plan checking from
each building department. This is a dream and many plan checkers (NOT
ALL) are just plain difficult to work with (i.e. "Seismic Design of a
Garden Trellis"). IMHO, I think 3rd party plan checking is a scam (along
with soils reports). I'm not sure what they charge, but these firms just
seem to fill up two pages minimum worth of comments, to justify their
fees. Most never open the calcs and never read the general notes. The
ICBO Certified plans examiner is not sufficient (although I have no idea
what it requires) to qualify someone to be a plan checker. In the bay
area, most of the cities do a good job in house and are fairly
consistent. County agencies are hit or miss. 3rd party is nearly always
a huge pain in the ass.

If your project is in Zone 3, perhaps they are even more lax in their
checking efforts (I don't know)... but I think any decent plan checker
would seriously object to the design as you described it.

Thank god I'm not an architect and have to deal with Planners at the
city on top of plan check. They throw around other people's money like
it's a game.

DISCLAIMER: There are good plan checkers out there and at least three or
four on this list. But MOST ARE NOT - I STAND BY THAT STATEMENT. SOME
PLAN CHECKING IS STILL BETTER THAN NOTHING.

-gerard
Lodi, CA


-----Original Message-----
From: Bill Allen [mailto:T.W.Allen(--nospam--at)cox.net] 
Sent: Friday, October 10, 2003 10:04 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Rigid Wood Diaphragm?

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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