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RE: Rigid vs. Flexible Diaphragm

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I agree with Bill on all but Buddy Showalter's name (Buddy not Bucky).
The problem is such that there is a greater likelihood that commercial
buildings will calculate as having a flexible diaphragm in at least the
longitudinal direction as the intent of most commercial buildings is to
maintain an open interior so as to redefine leasable space and every
square foot results in money earned or wasted.
The argument for Rigid Diaphragm analysis (RDA) is misleading and just
for the reasons that Bill brings up - it allows you to design (based on
your engineering judgment) a building with an open front. We first
discussed this a couple of years ago when an engineer in Wisconsin
wanted to design a restaurant that faces the lake (Michigan) and wanted
the view side completely glass. The way the code is stated, an engineer,
after proving the diaphragm is rigid, could design rotational shear -
something that those of us in California (Southern) learned was a
dangerous thing to do after the 1987 Whittier Narrows Earthquake when
garages with living units above collapsed as did buildings with parking
at alley level which we termed "tuck-under" parking. Rotational analysis
alone in light-framing will not work in areas of high risk to cyclic
loading and I believe in areas subject to moderate and high winds. In
other words - most of the US with the exception of low-risk seismic
design areas where brick is used on the exterior and tied to the
light-framing.

RDA, IMO, seeks the additional shear that is transferred through the
horizontal diaphragm to resisting walls based on differences in relative
stiffness. However, the problem is much easier to solve if we simply
apply some judgment that we failed to apply before the 1994 UBC. Most
wood framed buildings had shear walls designed to aspect ratios rather
than to wall deflection or relative stiffness. The current code still
does not allow relative stiffness analysis for light-framed walls in the
same line of shear and this may be a mistake in my opinion.

Mike Cochran, Dave Merrick and I debated the problems when I was trying
to write the 1997 Multi-Lat spreadsheet (flexible diaphragm portion) and
I think we all agreed that the solution to the problem was not more
difficult code rhetoric or higher shears, but more attention by the
design engineer to consider relative stiffness of shear resisting
elements in parallel and adjacent lines of resisting shear. With
proprietary shear elements, it is possible to install a narrower and
taller product that has the equivalent stiffness of four or more plywood
shearwalls. Furthermore, it is important that we not try and highly load
a plywood shearwall because when we do look at deflection, the drift
will most likely exceed the allowable story drift.

To safely contain yourself within the intent of the code but without
consideration of RDA, you might want to consider boosting the base shear
according to the guidelines of the Simplified Static Design - however,
if wind controls, you may wish to increase the exposure or add 20% to
the demand so as to design a greater capacity in plywood walls and
Holddowns. If you review the Seismic Design Manual Vol. II you will see
that the majority of walls (where the aspect ratios don't come close to
4:1) had the reserve capacity to handle the increased demand due to the
rotational shear in the horizontal diaphragm. Most of all - detail the
hell out of your projects and add a pre-construction meeting to make
sure that the builder understands what you are showing in your details.
Don't skimp and reduce the amount of generic details that you may use
from job to job if the conditions require something specific.

Enough - back to work.

Dennis


Dennis S. Wish, PE


California Professional Engineer

Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net

http://www.structuralist.net

 


-----Original Message-----
From: Bill Allen [mailto:T.W.Allen(--nospam--at)cox.net] 
Sent: Wednesday, April 14, 2004 11:32 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Rigid vs. Flexible Diaphragm

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
:Your Vision.  Our Focus.
:
:
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