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Re: Diaphragm and wall deflections

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At 07:43 PM 3/2/98 EST, you wrote:
>Does anyone know of a code limit of flexible diaphragm
>deflections or of flexible diaphragm lengths?

        Which code edition is of interest?  Present, Pretty Soon, or
Portending? These days it makes a difference like never before.

        The 94 UBC in Sec 2314.1 specifies among other things that
"Permissible deflection shall be that deflection up to which the diaphragm
and any attached distributing or resisting element will maintain its
structural integrity under assumed load conditions, i.e., continue to
support assumed loads without danger to occupants of the structure."  (This
style of code provision leaves a measure of control over the outcome to
engineers other than the code-writers, and has fallen from favor as of late.) 

        The same section later says that "Size and shape of diaphragms shall
be limited as set forth in Table 23-I-I". Presumably those limits reflect
deflection concerns. (Conservative shape ratios have become paramount in
post-94 editions for controlling vertical wood diaphragms -or shear walls-
and no relief by calculating deflections against load is permitted.)   

        Story drift limits for seismic are covered in Sec 1628.8 of the 94
UBC; three forms of criteria are given. 0.005 times the story height is a
traditional limit. During SEAOC Seismology Committee deliberations leading
up to the 88 UBC version, masonry guru Jim Amrhein told me he felt 0.005 way
too stringent for drift of masonry walls. I recall hearing 0.020 spoken of
approvingly.  


>Since the UBC Standards includes a method for calculating
>diaphragm deflections, why is it that the deflection calculation
>is not required by code for large diaphragms?

        This is a guess, but those ratio limits are probably presumed to
cover deflection concerns in routine situations, and the qualitative limit
first quoted would tip off a professional that a calculated deflection to
compare to "structural integrity" would be in order for unusual situations.
The topic wasn't revisited in the 1980-86 Blue Book rewrite, anyway, and
seemed well-settled long ago.

        The opposite to what you ask about was, rather amazingly, proposed
in SEAOC Code Committee last July for inclusion in the 2000 IBC. Diaphragm
deflection calculations would be routinely required, but the deflection
formulas would be eliminated from the code and standards. I was there as a
proxy from Central Section of SEAOC, and pointed out that this was like
having speed limits enforced on the highway, but prohibiting speedometers in
cars. (An unwelcome analogy, I found out.) I think that proposal fizzled
out, but may not have. Pet agendas in committee have a way of coming out of
remission as soon as the skeptics aren't looking. And as others have pointed
out, it is pre-emptively tedious and difficult to keep track of what's
brewing in the 2000 IBC.

        Now there has been a vigorous debate on holding SEAOC committee
meetings by cyber conferencing as a replacement for face-to-face meetings.
Based on experience, I think the best way to have SEAOC code-writing
committees meet is to prohibit face-to-face meetings, but not to use
electronic means either. Instead, drafts and agendas should be sent back and
forth by carrier pigeon. Or preferrably, in the interests of speed and
brevity, by carrier hummingbird.  

        Charles O. Greenlaw, SE    Sacramento CA