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Re: Plywood rigid diaphragms

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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 by
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.  I
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 deflection
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
rigidity."

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 able
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 should
be somewhere between the 100% flexible diaphragm and the 100% rigid diaphragm.
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 total
length or sum of the lengths of the individual walls the way we normaly do it.
Then do another simplified analysis using distribution by relative rigidity of
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 values
obtain from either the uniform wall distribution or the relative wall rigidity
distribution. We don't have to do another set of calculatons and iterate since
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 from
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 our
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 to
do something about it. I would like to know if anybody else out there have
other ideas crazier than mine.

Ernie Natividad