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RE: Plywood rigid diaphragms

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With all due respect to each of you who have contributed to this thread, I
believe that you can theorize this one into the ground. The conditions you
indicate are rather pristine. What I mean is that once you create
discontinuities between lines of shear you have a diaphragm with different
types of behavior.
I would believe that to consider a roof rigid it would need to be virtually
flat and the model would need to have an aspect ratio encroaching upon 1.0.
I'm not sure how creative your architects tend to get, but here in the
desert it is not uncommon to have a 6,000 square foot residence with a
combination or hip, shed, and gabled roofs - each originating at different
plate heights.
If you have a gable or hip roof, for example, how can you consider that
there is rigidity normal to the ridgeline? You may be able to assme this if
you have rafter ties and a flat ceiling, but consider the use of a vaulted

I would not disagree with your discussions if you were transfer shear from a
floor diaphragm down, but a roof tends to be at least twice or more flexible
than a horizontal floor diaphragm.

I read the SEAOC Plan review that came today and it appears that the
Seismology committee is not turning a blind eye on the damage caused to
single family homes after Northridge. They point to poor design,
construction defects and inappropriate field observation as the main causes
of concern.

I believe this to be accurate since most of what I have seen that suffered
damage was not due to an inadequate design proceedure but rather one that
was not properly executed and enforced.

I think that both Michael and Ernie can appreciate the amount of design work
involved in a complicated custom home. IMHO, much more work than any
commercial structure I've been involved with. We don't need to make the
design methodology more complicated, but rather to insure that the minimum
design proceedure we currently have is followed.
Furthermore, we need to revise the prescriptive measures to be equivalent to
or more conservative than the engineered design.

This leads to my last question:
Has anyone created a series of diaphragms in the lab that can be subjected
to various static and cyclic loads? If so where is the data and if not,

I have seen very few engineered homes with excessive damage resulting from
Northridge. I did have a Condo complex in Simi Valley where the garage piers
failed and which were engineered. However, the engineer used a 6x8 column
between garage doors and secured them to a pad foundation with a Simpson
Holddown at each side in plane with the Garage doors. This was destined to
fail from anyones point of view. This engineer obviously did not know that
you can not create a bolted moment connection in wood and try to create
fixity in this manner. Failure, of course, occured parallel to grain.

I'm not trying to end this debate. I would like to see some real world
conditions considered and then an opinion as to whether a difference in even
a 1/2" of deflection is predictable across the entire diaphragm. Personally,
I think the tributary method we are using today is as close as we are
realistically going to get before we make residential construction
unaffordable to the middle and lower incomes.

Dennis Wish PE

-----Original Message-----
From: ErnieNSE(--nospam--at) [mailto:ErnieNSE(--nospam--at)]
Sent: Monday, August 31, 1998 10:46 AM
To: seaint(--nospam--at)
Subject: Re: Plywood rigid diaphragms

Michael Cochran said:

"I agree that the design load to wall line should not be less than what you
get by tributary area, and that you can not reduce the load because of rigid
diaphragm redistribution.  I don't think that the use of the term rigid is
completely appropriate.  Lets say you have 5 parallel wall lines separated
30 feet between each wall line, as you go from left to right, with initial
calculated deflections of  0.25", 0.3", 0.35", 0.2" and 0.4" respectively.
don't imagine the deflection of the far right wall line with 0.4" defleciton
is going to transfer any (very little) load to the left wall which initially
had a deflection of 0.25".  The wall with 0.2" will see more load, and its
deflection will increase as the far right wall with 0.4"calculated
initially moves.  The stiffer wall lines will pick up load as the adjacent
wall lines deflect, but not like a rigid diaphragm which has a center of

I agree with you on this but I think it is too complicated to get into this
exact analysis at this time to be practical. Like Bill said, we got to be
to charge extra for any additional time we spend on the job over what we
normally do. I just used the 100% rigid diaphragm analysis as a compromise
since it can be done without a very complicated formula that will require a
three dimensional structural analysis software. Like I said, the value
be somewhere between the 100% flexible diaphragm and the 100% rigid
Let each engineer use his judgement and those who are not too experienced on
this can be conservative and use values at extreme cases.

Regarding the distribution of the lateral loads to the different walls along
the same wall line(assuming you have decided how to distribute the loads to
each wall line), we can distribute the lateral loads uniformly over the
length or sum of the lengths of the individual walls the way we normaly do
Then do another simplified analysis using distribution by relative rigidity
each individual wall. If somebody can develop a table of plywood shear wall
rigidity based on H and D dimensions, (whether simpified or complicated
considering holdowns, crushing of wood, slips, etc), similar to the table we
have for masonry wall in Armhrein's book, then this will make it a little
easier to do. Again, after the two calcuations, use the greater of the
obtain from either the uniform wall distribution or the relative wall
distribution. We don't have to do another set of calculatons and iterate
we are just approximating anyway.

Use this simplified method when the approximate relative rigidity difference
between wall lines is a certain value(say 50% or whatever). Or let each
engineer use his/her judgement for now until we can get better guidelines
test and experimental results.

I feel this is better than not addressing this issue at all. Especially if
your have a three story apartment unit with the first floor front using only
very narrow plywood shear walls between wide garage doors and the rear is a
full-length plywood shear wall. We can start including this extra work in
fee schedule for such projects which we feel is worth these analysis.

These are just my ideas that I think is worth considering if we all decide
do something about it. I would like to know if anybody else out there have
other ideas crazier than mine.

Ernie Natividad