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Re: SDC C or D ... ?

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I can't speak to southeast Alaska (wish I could), but I think the code "phenomena" you are encountering can best be summed up with referencing the experience of one particular earthquake: Mexico City 1986.
The deep soils of Sacto put many mid-rise buildings at great risk from distant earthquakes - such as ones that could originate on the Hayward and San Andreas faults.  It's a resonance thing.
So while the design forces may not be as high as in SF or LA, the cyclic demand may be even greater - thus the need for the most stringent of seismic detailing practices.
-Ben Maxwell

Drew Morris <dmorris(--nospam--at)> wrote:
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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Benjamin H. Maxwell, S.E.

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