From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Fri, 12 May 2000 22:34:18 -0700
At 02:02 PM 05/12/2000 -0700, you wrote:
>The answer to your question is yes, however with some reservation. The exact
>wording in section 1617.6.2 is:
>"For light frame, flexible diaphragm buildings, of seismic use group I and
>two stories or less in height: resisting elements are permitted to be
>designed using the least value of R for the different structural systems
>found on each independent line of resistance........"
>The catchword again is "flexible" diaphragm. Pooh! I don't even want to go
>Ben Yousefi, SE
>San Jose, CA
I understand everyone's disgust with the "flexible" issue, but I wonder...
Whoever proposed the above exception's language, were they adhering strictly
to the definition of "flexible diaphragm" that has only been added to code
as a comprehensive, applies-to-everything definition for the 2000 IBC? Or
did they mean by "flexible" any wood diaphragm?
For many decades the term "flexible diaphragm" was the insider's standard
euphemism for "wood diaphragm", and also for untopped steel deck diaphragm.
"Flexible" became customary jargon for type of material, NOT for one of two
black/white pigeonhole categories that a certain relative rigidity
calculation divides diaphragms into.
That calculation was only concocted in 1986 by Ed Zacher and his SEAONC
associates, and only pursuant to shear distribution and eccentric rotation
concerns (two code subsections only, within which the definition appeared
and was limited to.) Offhand naming of ANY wood diaphragm as a "flexible
diaphragm" certainly has persisted in general use through the present.
Only for the 2000 IBC was that special-use definition (which defies
unambiguous interpretation, no matter how cleverly parsed) moved to a
position of vastly broader application. In fact, in the July 1998 Final
Draft and apparently as a result of floor debates at the April 1998 ICC
hearings, "Diaphragm, Flexible" newly appeared in the general definition
section 1602 of code, far removed from earthquake provision definitions in
1613.1, as well as far from its usual, specialized location.
The exception in 1617.6.2 that's now under discussion wasn't in that July 98
Final Draft, which is the most recent version in my possession. So there is
a time disconnect between codewriting attention to "flexible diaphragm"
definitions and attention to cantilever Column R factor relief exceptions.
How do we really know that the person who used the word "flexible" in the
1617.6.2 exception for Cantilever Columns was aware of the drastic change in
the meaning of "flexible" from traditional usage? How do we know that
approvers of the exception remembered? Could it be that this relief
exception has been ambushed and shot out of the saddle by a definition whose
new power the exception's enactors had forgotten?
Charles O. Greenlaw SE Sacramento CA