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# RE: Rigid vs. Flexible Diaphragm

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Rigid vs. Flexible Diaphragm
• From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
• Date: Mon, 26 Apr 2004 17:17:52 -0700

```Gautam
The key words are that every E.O.R. must decide for himself which is the
best method to use. We are very much in agreement.
Remember too that our discussions provide both sides of the argument to
those who are attempting to make the same decisions that we have. The
depth of the discussion is important because each engineer will go
through the same process of education and balance that you and I have.
It doesn't matter which process or method we use in the end as long as
the intent of the code is preserved - to protect lives and to prevent or
minimize major structural damage.

elements and how they add to the complexity of the design process. I had
one hit me today that challenged what I thought was appropriate for the
design:

The problem was a wall. There are two openings. The plate height is
14-feet. The opening on the left is 12'-0" to the header. The opening to
the right of the shear panel (the only panel) is only 5'-6" to the
bottom of the header (these are glass blocks in a wall above the kitchen
counter. The shearwall is 7'-1" long; all other walls do not meet the
H/b ratio.

A plywood shearwall would need to be sheathed both sides and the height
just makes the 2:1 ratio. I don't like the amount of drift on the wall -
it's too flexible. I wanted to use a Hardy Frame, but the 12'-0" tall
frame by 64-inch wide will not meet the demand of 5,700 pounds.

I called Hardy and they recommended that I use two of their 24" x 12'-0"
tall panels with 1-1/8" diameter anchor bolt and design the foundation
for the moment created between the two frames (one column goes into
compression while the other a foot or more away is in tension).

The representative from Hardy recommended that I speak to another
engineer who has designed a tract development that required the same use
of installation of the frame. Understand that the area above the header
(and usually the entire wall) is redundantly sheathed in 15/32" 5-ply
Struct I plywood and nailed to drag the sear from the double top plate
into the header that is extended over the Hardy Panels.

The point is that I believed the design would work and was the best
choice of two or three that I thought of. I had the opportunity to
discuss it with another engineer who has had the same experience I have.
This helped me feel comfortable with the decision I had to make as the
E.O.R.

The discussion might have included the possibility that the other
engineer would not agree with me. It meant that I would have to make a
decision that tested my professional knowledge and practicality of
design.

The bottom line is that there is no expert - but there are enough
knowledgeable people on line to build an expert from. We all have
something to offer and neither you nor I can take credit for what we
contribute to others who must make the same decisions. Anyone and
everyone who contributes becomes part of this "genetic" pool of
professional engineers with opinions and the final choice rests on the
shoulders of the Engineer in Responsible Charge. I personally owe a
great deal to the engineer who spent the time to run over the options I
had and to share his experiences with me.

Keep sending your opinions - I very much appreciate what you have to say
and enjoy the debate.

Best Regards,
Dennis

Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

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

http://www.structuralist.net

-----Original Message-----
From: G M [mailto:newabhaju(--nospam--at)hotmail.com]
Sent: Monday, April 26, 2004 1:19 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Rigid vs. Flexible Diaphragm

Dennis:

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>
>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
>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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>
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