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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, 14 Apr 2004 11:31:51 -0700

```Bruce -

With regards to your first question, IMO, I would think you would/could
apply that fading skill of "engineering judgment". But, I wouldn't design
the "soft" line for anything less than the load induced by the flexible
diaphragm analysis (FDA). Otherwise, you will be faced with similar problems
of some "tuck under" structures. IOW, if I'm visualizing your strip mall
problem correctly, I wouldn't use RDA to reduce the load at the front
storefront, but use it to increase the load at the rear wall. Sorry.

With regards to your last question, one of the documents Bucky Showalter
provided to us recently (thanks, Bucky), 2001 Edition Supplement, Special
Design Provisions for Wind and Seismic, provides pretty clear guidance. On
page 4, there is a comment in italics which reads:

"For the first iteration, an arbitrary load is applied to each line of shear
walls to determine the relative stiffness of the lines of walls. Once the
relative stiffnesses (sic) of the wall lines have been determined, the
applied lateral load is distributed proportionally. The shear walls are
redesigned and the lateral stiffness is recalculated and the applied load is
re-apportioned. This is continued until convergence."

Personally, the "arbitrary load" I would apply would be from the FDA. With
regards to iterations, make sure you have your options in Excel set up so
that it will handle circular cell references (Tools, Options, Calculations,
Iteration box checked). Your problem should converge pretty quickly.

Of course, that's just my take on it.

HTH,

T. William (Bill) Allen, S.E. (CA #2607)
ALLEN DESIGNS (http://www.AllenDesigns.com)
San Juan Capistrano, CA
:-----Original Message-----
:From: Bruce Holcomb [mailto:bholcomb(--nospam--at)brpae.com]
:Sent: Wednesday, April 14, 2004 10:43 AM
:To: seaint(--nospam--at)seaint.org
:Subject: Rigid vs. Flexible Diaphragm
:
:The IBC code gives a way to check if a diaphragm is "rigid" or
:"flexible" based on the deflection of the diaphragm relative to the
:sidesway of the lateral bracing system below the level of the diaphragm.
:Does this check for "rigid" or "flexible" need to be made in both
:orthogonal directions?  Depending on the building configuration, it may
:be possible to classify a diaphragm as flexible for loading from one
:direction, but rigid for the other.  This may be the case for a building
:that is very long, but not very wide.
:
:IMO, this gets messy where you have a long, narrow building with one of
:the long sides nearly free of shear walls... such as for a strip mall
:type building.  If I check it with wind or seismic force against the
:long dimension, the diaphragm could be flexible.  For load from the
:other direction, I need the diaphragm to be rigid, since one side of the
:building is free of shear walls.
:
:Also, how do I really apply the check for "flexible" vs. "rigid"?  Do I
:assume either "flexible" or "rigid" and design the lateral force
:resisting system, then check my assumption?
:
:
:Bruce D. Holcomb, PE
:Butler, Rosenbury & Partners
:300 S. Jefferson, Suite 505
:Springfield, MO 65806
:ph. 417-865-6100
:fax 417-865-6102
:www.brpae.com
:Architecture, Engineering, Interior Design, Planning & Development
:
:
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