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# RE: Rigid vs. Flexible Diaphragm

• To: "'Rand Holtham, P.E.'" <rand(--nospam--at)sigmaengineers.com>
• Subject: RE: Rigid vs. Flexible Diaphragm
• From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
• Date: Wed, 28 Apr 2004 15:46:35 -0700
• Cc: <seaint(--nospam--at)seaint.org>

```Rand,
No, I am not trying to balance stiffness between parallel adjacent
walls. This is a concept with rotational design (rigid diaphragm
analysis). I do believe that we need to be considerate of the rigidity
of a potentially soft story, but whether you use steel frames,
cantilevered columns or proprietary shear walls you need to consider the
demand in the line of shear and use engineering judgment as to the
amount of drift or deflection you get in the shearwalls you have
resisting the diaphragms demand.

Much simpler is the idea that the next adjacent shearwall parallel to
this one is in the center of a home and picking up shear from diaphragms
that are stacked (at different elevations). This wall essentially must
resist 50% of the entire lateral force for the defined block or area of
the diaphragms. I said essentially because the home may be irregular in
shape with attached blocks that will contribute more load - unbalanced -
such as the legs of a "U" shaped structure.

I do look at the deflection of the panels and with Hardy's they are
generally less than 1/4-inch at full load. I like this because they are
not simply calculated lateral displacement but empirically tested and
reduced down to a working stress load. Therefore they are stiffer than
any plywood wall so if I have an adjacent long plywood wall, the
relative difference in the wall stiffness will be small. Unlike
Rotational Analysis, this is an intuitive opinion based on historic
performance of walls with embedded columns or moment frames. There is
also a cost differential to consider as a moment frame will be a very
expensive alternative to a proprietary shearwall. Furthermore, the
Hardy's are cold-form steel and unlike their competition (ShearMax and
Strongwall) they are much more difficult to "bastardize" in the field by
contractors who decide to add an opening - and this happens all the
time.

Proprietary shear elements are the new technology to offset the
constraints of the new codes. H/b ratios don't need to be complied with
if the wall is properly tested for a rated deflection at a given height.
The downside of this for now is that most engineers ignore the
foundation design when using a highly loaded panel that is narrow (such
as a 3-kip lateral load applied to a 24" wide by 12'-0" tall panel).
This is the serious side of foundation design that has not been done in
the past by engineers using plywood (or older stucco and gypsum) walls.

Hope this clears up the question. One more thing - these panels do not
extend to the roof and I do consider the area above the header that is
designed to transfer diaphragm shear to the proprietary frames, but
there is two feet of wall that needs to be converted into a drag strut
by plywood sheathing, blocking and nailing and possibly strapping to
drag the diaphragm down. This also needs to be considered by the
designers when using proprietary panels that do not make a connection to
the underside of the double top plates.

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: Rand Holtham, P.E. [mailto:rand(--nospam--at)sigmaengineers.com]
Sent: Wednesday, April 28, 2004 3:09 PM
To: dennis.wish(--nospam--at)verizon.net
Subject: Fw: Rigid vs. Flexible Diaphragm

Dennis,

For some reason this did not post on the list but it's directed to you
if

TIA,
Rand
----- Original Message -----
From: "Rand Holtham, P.E." <rand(--nospam--at)sigmaengineers.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, April 27, 2004 2:26 PM
Subject: Re: Rigid vs. Flexible Diaphragm

> Dennis,
>
> I may be in this too late, are you doing this (the hardie frame that
is
> ((I'll have to look this up, I've not seen this before))) to provide
> equivalent shear strength on each side of the diaphragm? Is this your
> to the building with only three walls? How about a steel frame?  I've
been
> hesitant about the proprietary wall assemblies since you can go to
another
> material and get much more strength at the same or less cost than the
> strongwall (and others).
>
> Rand
>
>
>
> ----- Original Message -----
> From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Monday, April 26, 2004 7:17 PM
> Subject: RE: Rigid vs. Flexible Diaphragm
>
>
> 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
> 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
> (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:
>
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>
> >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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