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RE: Rigid vs. Flexible Diaphragm

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I agree with Paul and Dennis.  This subject has been hashed over a number of
times over the several years I have been on this list.  There are other
issues.  Until I moved to the southwest I saw very few (if any) homes where
all the shear walls fit the nice sanitary tested models of "rectangular"
shear walls.  Unless I have missed something I haven't seen any literature
that shows how to compute the stiffness of a gable end wall or for that
matter a gable end wall with openings.  How about the stiffness of an end
wall with a mono pitch roof with openings?  Is the diaphragm on a gabled
roof rigid?    If so, is the deflection calculated in the same manner as a
nice "tested" diaphragm model.  Then include dormers, intersecting gable
roofs, skylights, etc. etc.  Then there is the question for deflections for
unblocked diaphragms.  How about all those interior walls not calculated as
shear walls?  What about all those pesky little panels that don't quite fit
the criteria for aspect ratio.  Their stiffness doesn't go to zero at a
3.5001:1.0000 ratio.  Seems that would also mess with the precise
calculations of a rigid diaphragm.

I add these questions to the topic and I haven't done the amount of research
as some one like Dennis, or Ron Homberger (sp?) when he was contributing.

Rigid diaphragm distribution just makes no sense for this type of
construction.

Joseph R. Grill, PE 

-----Original Message-----
From: Paul Crocker [mailto:pcrocker(--nospam--at)reidmidd.com] 
Sent: Tuesday, April 20, 2004 1:22 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Rigid vs. Flexible Diaphragm


"To the many engineers that claim that they have been doing wood design (or
any design) in a certain fashion for so many years û my question is ôso
what? ö.  People in the steel industry that had been designing
pre-Northridge moment connection for years before Northridge hit and were
forced to change."

But that's just the problem.  With the exception of certain rotational
mechanism/soft story issues that affected certain very specific layouts,
traditional wood frame has done just fine in recent earthquakes.  Why fix
what was not broken?  Doing a more time-consuming analysis does not
necessarily correlate to a better end product for the user.   

Paul Crocker, PE, SE



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