From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Fri, 26 May 2000 10:21:19 -0400
Charles Greenlaw wrote:
>> A federally-funded 1978- vintage NEHRP precursor, ATC 3-06,
"suggested" this way, and the 1982-88 SEAOC Seismology Committee followed
suit, for the 1988 UBC seismic sections. Reportedly there was code
adoption-endangering "political" resistance in Zone 2 territory in the US
Midwest to the O.2 factor, and as a sop to them, a member of Seismology
Committee in the fall of 1986 offhandedly suggested dividing Zone 2 into A
and B portions, with the reduced value in the eastern portion (2A) that you
have noted. This gambit was embraced eagerly, and it worked.
There was no known technical research whatsoever as backing for the Zone 2
split or the .15 factor. This I know, because I was the one who suggested
the idea in committee.
It's hard to tell what in seismic code isn't fictitious or hastily
concocted, and then sold as solid, accurate science and time-proven wisdom
to eager believers.
At least this Zone 2A stunt didn't add needless workload to code users, or
add interpretation quandaries. To my amusement, Zone 2A has sprouted up in
Arizona and Hawaii since originally appearing east of the Rockies.<<
When the ill-advised and inappropriate 1985 UBC seismic zone map placed most
of Arizona in Seismic Zone 1, Tucson and Southern Arizona, by local
amendment, adopted Seismic Zone 2A as this would keep the base shear forces
at the same level that they were previously, 7.5%g. Going to Seismic Zone 2B
would result in base shear in the neighborhood of 9%g, which probably would
not have succeeded. It was then to either go with Seismic Zone 2A and to
keep the seismic forces the same as they had been, or to go with the
inadequate Seismic Zone 1 as shown in the UBC. Since Seismic Zone 2A would
not reduce the level of seismic design forces, it was a natural compromise.
While you were the author of Zone 2A, I was the principal proponent of
keeping Arizona in, at least, Seismic Zone 2, although only the Southern
Arizona structural engineers were in general agreement. It was not until
about 1994 that the UBC map was changed to reflect what was actually the
requirements in Tucson and Southern Arizona.
A. Roger Turk, P.E.(Structural)