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RE: Rigid plywood diaphragms

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I have a couple of comments (of course:>)

1. I intend to design using rigid Diaphragm analysis - reason: more
business, better fee's. I don't have to agree with the methodology (as I'll
discuss later) to be pleased to have sufficient reason to raise my fee's.
2. I received a call from a local Architect who historically engineered his
custom homes. They were as we might expect, proof that an architect should
stick to architecture and not cross over into engineering unless he shows
competency in this area. This Architect asked me to submit a current resume
to him - he has need to hire an engineer on projects coming up. I know he is
motivated by fear of the '97 design methodology (no one has asked for
compliance to rigid diaphragm analysis as many mentioned is required in the
'94 code).

Now the reality of the situation:
I agree totally with Lynn on this issue and I do not support the need for a
rigid diaphragm analysis. Surely a much simpler approach is warranted. I
reviewed the notes from the Wood Seminar held last February. The
presentation and design examples were done by Bill Nelson (SEAOSC Board
Member and Chair (I believe) of the Wood Committee). The design notes were
very clear as to the comparisons of Wind, Flexible and Rigid Diaphragm.
What struck me most compelling was that Bill compared each analysis and used
the worst reactions in each line of shear. I do this when analyzing a
flexible diaphragm comparing Wind to Seismic. However, a rigid diaphragm
analysis depends heavily upon distribution to shearwalls by wall rigidity
distribution. How is this supported when the load originating from the
diaphragm is not balanced and exceeds or other rigid reactions throughout
the diaphragm. This unbalanced condition suggests that one line of shear may
be deliberately stiffer than the next which will force the diaphragm

The design example clearly demonstrated that rigid diaphragm analysis
governs when the aspect ration of the walls in the line of shear are narrow
and tall (i.e., too flexible). Lines of shear where aspect ratio is nearer
to 1 are shown in the distribution analysis to be loaded more from a
flexible diaphragm design than a rigid design.

Now, assuming that the closer a wall aspect ratio gets to 3.5:1 the greater
the deflection (any surprises here). Therefore, it would be a much simpler
solution (albeit I might lose all this new business if this gets out) to
either restrict wall aspect ratios or treat the wall as a pendulum and
design the panels deflection for 3Rw/8 (or an agreeable substitute) to force
an analysis that can predict the true stiffness of the wall.

I invite others to review the design examples and compare results. I am not
saying that the shear is any less or more where the wall is longer and
shorter. Only that narrow panels absorb more shear in rigid analysis than
they do in flexible diaphragms.

Caution: I have not verified this with other examples (which I could not
find) and I have not done sufficient rigid analysis to stake my reputation
on this. However, if my conclusions drawn from this example are incorrect I
would like someone to bring it to my attention and explain why.

As far as liability - Some stated that this is a stated requirement in the
'94 code. I don't know one engineer who has designed in this manner. Are we
currently liable?
We are screwed any way we look at it. There is no protection from a
dissatisfied client and there are plenty of expert witnesses willing to bias
their opinions based upon what they can earn. No one is free from
litigation. Second, this section of the code is not supported by the entire
engineering community even if it was enacted into code. there is still
something called engineering judgment which allows an engineer to plead his
position. If successful, the building official has the right to accept a
deviation from the code based upon supportive engineering documentation.
Regardless of our liability, we need to be thinking with our intellect and
not our pocketbook or fear.

If the local building official enforces rigid diaphragm analysis, I may be
able make those of you like Lynn happy when I demand 2% of Construction

Dennis Wish PE

PS: Here is an example of dammed if you do and damned if you don't. It was
rumored that there are unresolved issues related to the new Simpson's
Strongwall and the belief is that the values need to be reduced by a safety
factor because the wall is expected to deflect more than code allowable. I
heard that these concerns were brought up at the last Existing Buildings
Committee meeting by John Cariotis. John, by the way, established the
testing procedure and wrote the ICBO submittal representing Simpson. So
what's it suppose to be?
He also mentioned concern about brittle failure in the Hardy Frame. Can
anyone offer any information on the Hardy Frame? I am installing them on two
custom homes and have, by nature of the design, placed the equivalent of two
or three conventional walls into one Hardy Frame. I have designed the
foundations accordingly and have added additional steel and depth. The
Frames are designed to less than the maximum allowable load as noted on the
Hardy Schedule and in accordance with L.A.City additional requirements. The
expected Hardy Frame deflection for a 4' x 8' section is less than a 1/4"
when loaded with over 5 kips. I don't know if Kariotis was aware of these
deflection statistics tested by Hardy Industries and their engineers. It's
not too late for me to work out a correction so if anyone can enlighten me
on the Hardy Frames I'd be very appreciative.

-----Original Message-----
From: Lynn [mailto:lhoward(--nospam--at)]
Sent: Wednesday, November 18, 1998 5:38 PM
To: seaint(--nospam--at)
Subject: Re: Rigid plywood diaphragms

"Since most residential buildings typically have deep, short span
diaphragms, it is virtually certain that these diaphragms can not be
considered flexible under [1994 Uniform Building] code definition and
force distribution to the shear walls must be made on the basis of wall
rigidities.  Engineers should adjust their design practices to conform
these 1994 and 1997 UBC provisions in order to avoid lawsuits at the

Hahahahahahahaha!!!!!  That is a good one!

Who is going to sue me and on what grounds.  Certainly it WON'T be
because the diaphragm did not perform properly in an earthquake.  If we
are sure about ANYTHING it is that our seismic design of plywood
diaphragms (vertical and horizontal) perform very well on residential
construction.  This will be even more true now that the height to width
ratios are more restrictive.
I have a REAL hard time with this one.  There are plenty of structural
issues that are of real concern that talented people could be spending
their time on.  Plywood diaphragms perform well for residential
buildings as currently designed, period.

Anybody can sue for any reason.  And some people do just that.  There is
no protection from those kinds of people.  What should happen is that
there should be an exemption in the Code for certain kinds of buildings
with a proven history of successful performance during strong ground

I have a question for the engineers that are going to design future
buildings to this new provision.  Are you going to contact all the
Owners of buildings you have previously designed and notify them that
the plywood diaphragm building they own may not provide adequate
life-safety protection in an earthquake??
I would guess that no one will do this, because we all know that it is
simply not true.

Sorry to vent like this, but some things just really get to me :)