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RE: perforated shear walls

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I am one engineer who can not buy into the idea of perforated shear
walls unless the structure is designed in a low-risk region of the
country or has the mass to resist lateral forces. I have read the
AF&PA's version and attended the first discussions of Perforated Shear
Walls, but I have to say that my "intuition" does not agree with the
imperical testing when it comes to high risk regions for seismic design.
I also admit that I don't have the gut intuition to make this judgment
in Hurricane or Twister regions of the country.

One of the things that I discovered in my nearly twenty years of
practice is that engineers tend to believe that controlled models
subjected to shake table testing can become models for methodologies
leading to the design of light-framed structures. However, few, if any,
light-framed structures are constructed to the same tolerances that the
models used for shake-table testing are subjected to. In fact, we have
no means to determine the off-setting values that are caused by the lack
of construction quality when considering the total system.

Specifically with regard to perforated walls, I can not accept the lack
of need for holddowns at the sides of piers adjacent to large openings.
I can believe that small windows can be strapped (above and below) to
take out most of the negative effect associated to individual
shearwalls, by the use of drag struts created above and below openings.

When I started lateral design studies - it was related to Unreinforced
Masonry buildings. Here there was a rationality that was proven by
impirical methods and published in the original ABK method. Here it was
easy to understand that in existing buildings, the capacity of the
diaprhagm generally controlled and we designed to the weakest link. 

What was important here was that we were able to understand that there
was a difference between shear piers allowed to rock until equilibrium
was reached, but the piers were considered independet as were
conventional shearwall design until recently with perforated wall
methods. Piers either failed, rocked or were stable without rocking
(such as a wall with sufficient dead load to resist uplift).

I am still unconvinced that perforated shearwall design (as I am the
180-plus year old Conventional Construction) methods provide sufficient
value as to provide protection of the homeowner against excessive
damage. It might be of value if life-safety is the only consideration -
but then again, even Conventional Construction will statisfy life-safety
issues and still not meet the minimum level of compliance for
full-compliance code methods (engineered).

I would like to see something more definitive in the style before I
decide to mix and match various full-compliance methods of design. In my
opinion, if we simply designed to the old methods with some intelligent
consideration for wall stiffness and deflection, we would not need to
consider more complicated methodologies.

Sincerely,
Dennis S. Wish, PE
California Professional Engineer
Structural Engineering Consultant
http://www.structuralist.net
dennis.wish(--nospam--at)verizon.net



-----Original Message-----
From: Casano, Karen [mailto:Karen.Casano(--nospam--at)dgs.ca.gov] 
Sent: Tuesday, May 06, 2003 4:58 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: perforated shear walls


Chris:

reference:  SEAOC Seismic Design Manual Volume II, pg 69. You may have
to struggle a bit to understand the statics behind it, but it's very
"rational". It's also very tedious to do by hand.

The spandrals over the openings often have the highest shears using this
method.  It just doesn't make sense to try to use a perforated panel
analysis when your openings are very large in comparison to the size of
the solid panel that is left.

APA also has a publication on perforated shearwall analysis which I
believe you can download from their website.  I have not reviewed it.

Karen Casano, SE
San Diego, CA
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Re:  Timber Design.



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