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Re: Residential Flexible/Rigid Diaphragm Analysis

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Dennis,

I think this issue is more of common sense rather than what the code is
forcing on us.

A lot of residential products have relatively short diaphragm spans which
will obviously perform more in line with the rigid diaphragm analysis.
Sure, this will add a few hours to our calculations and I have to admit that
I am not sure if the end result will yield a "better" design.  Bottom line
is that SEAOC is promoting this level of analysis and until we all have
several projects behind us, we should all take special care to our best job
possible.

As for the cantilever column.  The "exception" we have proposed to SEAOC is
also based on common sense.  The lateral force resisting elements at the
front of a garage have a stiffness and likewise the is true with the shear
walls at the back.  Fortunately, the shear wall at the back doesn't know the
make up of the front element ( only a relative deflection ).  In order to
get this exception through we (seismology) felt it was important to have a
certain level of over strength  within the column.  I am still to come
across an example where it will govern the design.


-----Original Message-----
From: Seaintonln(--nospam--at)aol.com <Seaintonln(--nospam--at)aol.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Sunday, March 21, 1999 10:25 AM
Subject: Residential Flexible/Rigid Diaphragm Analysis


>I happen to speak with a few engineers on different committees for both
SEAOSC
>and SEAOC. There appears to be (and I agree) a growing disagreement with
the
>prospects of designing custom (or conventional) residential structures
(single
>family) using rigid diaphragm techniques.
>There is also a growing concern or disagreement with penalizing the
structure
>when using embedded (Pendulum) steel columns for resisting lateral loads on
a
>small portion of the structure. The disagreement here is not that the loads
to
>the columns should not be amplified, but that the entire structure should
not
>be forced to comply to a much lower Rw when only a portion of the building
is
>strengthened using embedded columns.
>Personally, I would rather wait the results of the Cal-Tech/CUREE study
before
>subjecting clients to higher design and construction costs which may be
>indicative of construction quality rather than design deficiencies.
>With July approaching quickly, I am concerned that local municipalities
will
>not be aware of the number of engineers who question this approach and
would
>hope that local building officials will take this into consideration.
>
>It is further my understanding from discussing this with a knowledgeable
>SEAOSC committee member than the wood committee questions the need for this
>change, but has been overruled by the seismology committee. I am not sure
>whether this was at SEAOSC level or at the final State level (SEAOC).
>
>Perhaps some of you can shed some light on this topic and we can make at
least
>our opinions heard.
>
>Sincerely,
>Dennis S. Wish PE
>Editor SEAINT Online
>
>
>