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RE: 94 UBC and Rigid Plywood Diaphragms??????

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Bill Cain posted without comment a relevant code section:

>The pertinent section of the 1997 UBC reads:
>"Section 1930.6 Horizontal Distribution of Shear. The design story shear 
>...SHALL be distributed to the various elements of the vertical
>lateral-force-resisting system in proportion to their rigidities,
>CONSIDERING the rigidity of the diaphragm. 
>Diaphragms SHALL be considered flexible for the purposes of distribution of
>story shear and torsional moment when the maximum lateral deformation is
>more than two times the average story drift of the associated story. This
>MAY be determined by comparing the MIDPOINT in-plane deflection of the
>diaphragm itself under lateral load with the story drift of adjoining
>vertical-resisting elements under equivalent tributary lateral load."
        [emphasis added, for comments below]

So what parts of this are rigid orders, and what parts allow discretion to
the designer?

"Shall" is mandatory; "May" is discretionary at the option of the actor.
Thus, one doesn't have to determine if the diaphragm is "flexible",
so-called, by the "midpoint" deflection method given. 

And what if the diaphragm IS deemed flexible, by whatever method of
determination? I don't see where that imposes a particular array of design
limitations on the designer. It doesn't impose a strict tributary diaphragm
span method on distribution to those vertical resisting elements. 

In fact, the word "considering" remains operative. You are still required to
"CONSIDER the rigidity of the diaphragm" in distributing the design story
shear. How you consider it remains discretionary. If the code writers meant
that a "flexible" diaphragm lacked all rigidity and could not be considered
to function to distribute story shear past any vertical resisting element,
they could have removed your option by a change of code language. But they
did not. Thus your considering of distribution remains open to your judgment.

Note that the "equivalent tributary lateral load" language is only part of
an optional deflection comparison that you need not use; it is not part of
mandatory consequences to be used in lateral load distribution. 

And please, what does "midpoint" mean? Midpoint of what length or span? I
can't discern any clear meaning from either the words, the context, or use
of my imagination. It must mean whatever the hostile litigant's hired expert
says it means. 

Charles O. Greenlaw, SE   Sacramento CA