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Diaphragm flexibility: code history

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At 12:15 AM 8/19/99 EDT, Ben Yousefi, SE, wrote, under "Re: Taking Questions
for Seismology Submission":

>1- Is there any documentation (SEAOC Seismology archives) that proves the 
>addition of provisions for diaphragm flexibility determination in the 1988 
>UBC was intended for wood diaphragms? I remember clearly the Saturday seminar 
>I attended in San Diego when the 88 UBC was adopted. Allen Poroush and Nabih 
>Yousseff were among the speakers. However, I don't have any recollection of 
>hearing anything significant about this issue and whether the new provision 
>was intended for wood diaphragms. 
>A couple of weeks ago I called ICBO and spoke with Susan Dowty in regard to 
>their archives. 
>Normally there is a paragraph or two given as "Reason" for any code change 
>proposal. However, for that particular year (1987), SEAOC had presented a 
>wholesale rehaul (20 pages) of the seismic provisions and according to Susan, 
>one brief sentence was given for reason to the effect that "the intent to is 
>to update seismic provisions to keep up with the new technology, 
>etc........." (nothing specific) She mentioned that Ken Luttrel was the SEAOC 
>seismology chair at that time and all questions regarding the code change 
>proposal were to have been addressed to him. (I would be very curious to hear 
>from Ken in this regard)
>I do recall that prior to the 88 UBC we (plan check engineers) always had a 
>rough time with metal decks without topping and whether they should be 
>assumed as rigid or flexible. So I personally welcomed the new addition to 
>the code (the metal deck industry since then developed flexibility 
>coefficients for their diaphragms) little did I know that it would someday 
>come back to haunt us.

C.G., offering some items on history and intent as to whether the shear
horizontal distribution provisions and flexible diaphragm definition were
aimed at wood specifically, or only in an incidental, peripheral way, or not
at all:

Ben, Ken Luttrell is a good record keeper and was indeed the seismology
chair in 1986, and Allan Porush was chair the next year. Ken, Bob Thacker,
and myself were Central section's delegates to SEAOC Seismology in 1986.
Central was in charge of the wood section during the whole rewrite that
produced the 1988 Blue Book and UBC. I have in front of me the agenda
package page which, at the 4th meeting in 1986, introduced the Flexible
Diaphragm definition to the committee for consideration. 

What this Flexible definition was aimed at occupies much of a long posting I
made on this list on May 13, 1999, under the subject, "Seismic Upgrade...
Appeal to those who created the code."  The thrust of deliberations is
discussed, as is significance of the ensuing Commentary entries on this
Flexible definition, which I believe Ted Zsutty had charge of. 

Mark Gilligan on the same day posted a corroboration that woodframe was of
scant interest in this diaphragm matter, and "Cain, William" suggested the
next day that Ed Zacher, the definition originator, should be consulted.

These postings can be read in the archives on the Seaint website. (It may be
best to look under author first, as the "Re:" goofs up the subject titles.)

On May 24, 1999 I posted some more on this matter, under the subject, "For
Urgent Review and Comment..."  Some commentary to the 1973 Blue Book was
excerpted in that missive; it sheds more light on whether actual analysis of
diaphragms to separate them into into named categories is necessary to
satisfactorily distribute horizontal shears among resisting elements. That
1973 Commentary reveals that Seismology Committee contemplated requiring
tributary span-based distribution of diaphragm shear for flexible "type"
diaphragms, but rejected such a requirement in favor of the less arbitrary
"consideration" of relative rigidities.  

Very recently Frank Lew, SE, now retired, informed me as follows:

>During the many 
>years I participated on the SEAONC Seismology Committe, I've 
>sensed a long-standing attitude that residential wood frame design 
>was not worthy of engineering attention, but also that whenever the 
>committee deigned to venture into this area, it expected its position 
>to be accepted, however uncertainly arrived at.

>A posting from Mark Swingle caught my eye:
>> It is not just the 1997 code that has
>> this requirement, and it didn't start in 1988
>> either.  The 1988 UBC simply TRIED to define what
>> a flexible diaphragm is, which had not been done
>> in the UBC before then.
>>  ....With no offense to the original author, the
>> definition of "flexible" diaphragm (that first
>> appeared in the 1988 UBC) is not well-defined (Ed
>> Zacher already knows this).
>Mark has it correct.  I was chairman of the ICBO Seismology 
>Committee in 1986 when the omnibus SEAOC seismic package to 
>the '88 UBC was processed, and recall this subject was discussed 
>at the committee meetings, with inconclusive results.  Ed Zacher, 
>representing SEAOC, acknowledged the concerns and questions 
>related to wood frame that were raised at the meeting, but either 
>could not or was not authorized to offer any changes.  It was raised 
>again on the floor at the annual code development meeting by an 
>individual (engineer?), but since there were no input or or positions 
>offered by industry representatives, a proposed amendment to the 
>SEAOC language was not successful. That wasn't surprising - on 
>seismic provisions in the UBC, SEAOC was the 800 pound gorilla.  
>There were a number of 600 pound  industry gorillas who 
>represented steel, concrete, masonry, etc. who often challenged 
>SEAOC in order to protect their vested interests.  When they don't 
>do so on an item, the presumed wisdom of SEAOC on all matters 
>seismic prevailed by default.
>During the committee debate, I had doubts that the rigidity issue as 
>applied to wood frame construction was ready for codification, but 
>with limited design experience in wood frame, I lacked the 
>confidence to argue the point.  In retrospect, I should have.
>Frank Lew
Hope that helps unravel some of the history, and gives leads to those who
could answer from their own participation at the time. Thanks for your
interest in it. I believe the views you have expressed on the lack of need
to make (or attempt to make) precise stiffness determinations, pursuant to
doing woodframe structure shear distribution, will find support in the
record, when taken in context and the whole of the record fairly considered.

Charles O. Greenlaw  SE   Sacramento CA