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Re: SDC C or D ... ?

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Dave Adams wrote:
Hello all,

I've been finding a fairly consistent and frustrating discrepancy in
seismic design category determination between SDS and SD1 and am
wondering if perhaps the TARGET values shown in Table 1613.5.6(2) are
incorrectly low.  I find many of the regions here in the Central Valley
(California) defined as SDC C by Table 1613.5.6(1) by a comfortable
margin, but then wind up falling into SDC D by a larger margin when
reviewing SD1.  There are a number of detailing, inspection, and
reporting provisions required for SDC D that are not required for SDC C
and it seems out of whack for old "Seismic Zone 3" cities to now require
the same triggers that San Francisco or Los Angeles require (old
"Seismic Zone 4" cities).  I am aware of Section 1613.5.6.1, but the
requirements for the exception are fairly restrictive (40' between shear
wall lines).

Is anyone aware of discussions or research that has brought this up?  Is
this a more refined consideration of design values -- where do the
TRIGGER values for determining SDC in these two table come from?

This is not unique to California. I have noticed a similar behavior when looking at sites in southeast Alaska. Using the short period response puts you in SDS C, but looking at the longer period you end up in SDS D.



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