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Re: diaphragm rigidity

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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.

        It is not clear from this what the plan reviewer wants that you have
not already provided. You say the diaphragm has been determined to be
"rigid" (or "not flexible", as the UBC and SEAOC Blue Book since the 1988
edition calls it.) Then you say in effect that the distribution has been
carried out as per code mandate when the diaphragm is not flexible. 

        Are there multiple diaphragms or some complication not apparent in
your description of the building? What "differences" in what "diaphragm
rigidities" are contemplated?

>Does anyone have any good references for how to distribute loads to shear 
>walls based on relative diaphragm rigidities?


        The Commentary to the SEAOC Blue Book is a good reference in that it
comes from the committee that created these distribution requirements over
the years, but did not do so with any real focus on wood-sheathed
construction like of concern here. Also, the latest commentary reflects the
results through late 1999 of raging controversies during the last year on
just this matter. There are more than four large pages on this topic,
beginning with Sect C805.3 on page 259.

        A fair reading of this contrite commentary indicates that there is
no settled method of doing this distribution, in part because there is no
way the relative rigidities of the elements, even in the simplest layouts,
can be known with any precision. One method of coping with this uncertainty
in rigidities is to design the elements with extra capacity to cover an
estimated range or band of load distribution, rather than for one unique
calculated result. Perhaps this is what the plans reviewer is after.

        The Blue Book comes with membership in any of the regional sections
of SEAOC, and is sold by ICBO along with other SEAOC publications. 

Charles O. Greenlaw SE   Sacramento CA