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RE: diaphragm rigidity

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I've thought about this one and have come to the conclusion that the plan
reviewer knows as little about the methodology as your or anyone.
The only accounting system that will balance is that of either the flexible
OR rigid design - not both. In most cases the finished product is an
envelope solution which inflates the demand by taking the worst case load
distribution (rigid vs. flexible) and you simply can not work backwards to
balance the system.

As long as you considered the relative stiffness of the walls in each line
of shear AND the relationship of the relative stiffness of walls in adjacent
lines of shear (and in most cases you won't be able to adjust this because
of the restrictions in the architectural design). So, if you end up with a
long solid wall in one line and two very short piers in an adjacent line,
you might have a problem.

At least with an envelope solution (worst case of each) you will have had to
"beef" up the weak walls which is about the best you can do if the adjacent
line of shear is much more stiff.

Unfortunately, there are no good documents written about this. This is
because there is not general agreement about the fine points of the
methodology since nothing has been tested. Common sense is the only thing
you have to protect yourself on this one and you may need to turn the tables
on the plan checker and ask him (or her) to explain exactly what they want.
If you don't know how to accomplish it (and most of us probably don't) make
the plan checker show you how to do it and then you evaluate his work as to
whether or not it makes any real sense.

I wouldn't suggest the Blue Book commentary on this since it spends most of
it's rhetoric explaining in simple terms that "you can't get there from
here" and even more on why this is not the best method for wood design. If
anything - the commentary is a great defense for flexible diaphragm design.

Dennis S. Wish, PE

At 10:19 PM 02/10/2000 EST, you wrote:
>I recently received a city comment that I would like input on. I have an
>apartment building in seismic 4, within 2 km of an active fault. I am using
>plywood diaphragms and shear walls. Based on stiffness of the diaphragm vs.
>the walls, the diaphragm is considered rigid. Based on the wall rigidities,
>appropriate loading has been distributed to the walls and appropriate
>diaphragm nailing has been applied adjacent to each wall.
>I received a plan check comment that asked to account for differences in
>diaphragm rigidities in distributing the load to the shear walls.