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# RE: Rigid vs. Flexible Diaphragm

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Rigid vs. Flexible Diaphragm
• From: "Bill Allen" <T.W.Allen(--nospam--at)cox.net>
• Date: Wed, 5 May 2004 11:00:47 -0700

```Guatam -

You've obviously done this a lot more than I have, but is this true ("the
validity could be questioned")?

IOW, have you done a design based on RDA using the capacities as the
stiffness and then gone back and checked the structure using the actual
stiffness to see if the load distribution changes enough to change a shear
wall specification? If there's no change, that would be great news to save
on the iterations. IMO, the time would be better served doing better
detailing than performing multiple iterations. I believe WoodWorks does the
distribution based on capacity also.

After you've done a comparison, who could question this assumption?

The one downside I can see with using the capacity as the stiffness of
course is in a mixed system where you have a steel frame (braced or moment)
and/or a cantilevered flag pole column. This is also a problem with
WoodWorks, but for other reasons.

T. William (Bill) Allen, S.E. (CA #2607)
ALLEN DESIGNS (http://www.AllenDesigns.com)
San Juan Capistrano, CA

<SNIP>
:I used the envelope
:method.  In my spreadsheet, my wall stiffness is based on the capacity
:rather than demand.  This helps establish the wall rigidity rather than
:reiterating for every small change in force redistribution; also it is in
:line with the fact that the rigidity of the strong wall and hardy frame is
:based on the capacity rather than demand.  So, in terms of analysis, it
:takes one iteration only.  Of course, the validity of this assumption could
:be questioned - but I had to have a starting point.
:
:Gautam

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```