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RE: Rigid vs. Flexible Diaphragm

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Dennis:

I have always enjoyed reading your postings. I have enjoyed reading the different viewpoints from you and many others in this list. I have learnt a lot form the many experts that contribute to this list. Regardless of what we read, each E.O.R has to satisfy himself that the design/analysis is correct before he stamps the drawings. Many are happy with the FDA method; I am in the minority that feels RDA is the way to go. The RDA method I use has many shortcomings ? especially at the roof diaphragm level (the many valleys that go every which way, gable end shear walls, and the shear wall deflection contribution of Simpson straps.) In term of time required to do a FDA versus a RDA, computers make this a non-issue. I have developed a set of spreadsheets that have FDA, FDA/RDA envelope, and the FDA method with the 20% increase options - which analysis one chooses is a mouse click away.

Like other contributors sated, I think I will agree to disagree with you on FDA/RDA issue and move on to other engineering topics.

Gautam Manandhar, SE

From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: Rigid vs. Flexible Diaphragm
Date: Wed, 21 Apr 2004 01:14:42 -0700

Joe, Paul and Gautam,

I thank you for your comments, but let me assure you that I am not an
expert - I am simply a practitioner who has questions and relies on the
information I receive from those of you on the list who are the real
experts working in the field. However, I weigh the information I receive
to decide if I am doing the job I was hired for and if the work I
produce is practical - in other words, is it safe, will it perform well
and most of all, is it capable of the dynamics of change that may occur
in the future.

The fact remains that reality is not to be found EXCLUSIVELY in text
books or in theory - it must fit into society and be culpable to future
owners of the buildings that results from our efforts. The owner has the
right to expect that his lifetime investment can support the growth of
his or her family and the needs of each occupant.

I am sorry that Gautam takes exception to my opinions or believes that I
am "branding" my peers who choose to design by RDA. They have the same
choice as I to design to the best of their ability, but I won't let the
letter of the code impair my professional judgment even if the rhetoric
makes me culpable for my choices. The code is not perfect and we have
proven this time and time again. What matters is not the theory we
subscribe to, but that we design to a standard that is no less than full
compliance to the code.

This argument can go on forever, but the bottom line is that we are not
designing to satisfy our own ego - we are attempting to design for the
possibility of expansion and the future marketability of the home. If
you can't sell the home because of the potential risk of destroying the
structural integrity then you will have a home that serves no purpose.

Gautam, you are arguing methodology while I am arguing practicality. You
are absolutely wrong if you believe that it is just as much work to
recreate an RDA model without knowing where the walls are than it is to
distribute forces by proportionality as you would on a simply supported
beam.

FWIW, I've yet to see any engineer from inside my area or from outside
that has submitted plans for review and who has used a rigid diaphragm
analysis even as part of an envelope solution. I did this with one house
and I hope the family is happy as they paid for a bomb shelter. RDA
alone would have created a soft-story failure, FDA was good but I wanted
to live up to the letter of the code. The next step was to rebalance the
RDA since I introduced forces into walls that would not normally have
been there in the RDA. I had to upgrade all of the walls and strengthen
every one of them.

It was overkill. When I went back to look at a simpler model like the
examples in the Seismic Design Manual Volume II - I discovered that all
but one or two walls had sufficient reserve capacity to resist the
additional forces induced by horizontal diaphragm rotational shear.

That's my argument. As one of you great thinkers' states: "If it ain't
broke, why fix it?" Focus on the conventional framed homes and attack
the politics between Engineers and the National Association of Home
Builders who would rather have engineers out of the housing industry and
whose designs represent the greatest amount of damaged homes in the
recent earthquakes and hurricanes of the last 15 years.

Dennis

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