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RE: Federal Project - Seismic Question

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The soil type (i.e. Site Class) can have BIG impact.  In the IBC world, "bad" soils can bump you from a low seismic (such as SDC B) to a moderate seismic risk (SDC C or even D for an important building like a hospital) even in area of low seismicity.  Seismic evaluation in things like FEMA 310, the NEHRP provisions, the IBC and ASCE 7 are NOT just based upon seismicity of the region...but also importance of the building and site classification.  Based upon the general soil description (i.e. mainly sandy soil on limestone), it kind sounds like a site class C to me...maybe at worst D.
 
The fact that Florida excludes seismic design in the Florida Building Code is meaningless.  It is purely an administrative decision that seismic is not enough of concern to make people design for it.  Does not mean that strictly speaking all areas of Florida have the same seismic risk OR that seismic risk is not possible at all.  Just means that they decided that it was not a significant enough of a risk to warrant dealing with.  The feds don't tend to take that belief no matter WHERE their buildings are....but then they are putting in place methods and procedures for use in ALL areas of the country whether there is a perceived seismic risk or not.
 
Regards,
 
Scott
Adrian, MI
-----Original Message-----
From: Andrew Kester, P.E. [mailto:akester(--nospam--at)cfl.rr.com]
Sent: Friday, June 20, 2008 12:19 PM
To: seaint
Cc: Scott Maxwell
Subject: Federal Project - Seismic Question

I have a potential client asking for a Life Safety Performance Level building certification, it looks to be a Tier 1 according to ICSSC RP 6. This is detailed in FEMA 310. The project is assumed to be near Orlando, FL.
 
I have followed through the FEMA 310 formulas, they are rather simple, and have come up with a Moderate Category according to Table 2-1 (Sds & Sd1), assuming Site Class D or E. This is hard for me to believe that this is the case, Seismic Design is excluded from the Florida Building Code, and according to a VA Seismicity Map, based on FEMA 310, they have all of Florida as "LOW" Seismicity. I should be getting the same results as the VA Map if I used FEMA 310 and that is what the map is based on, I am assuming?
 
This area is generally very sandy, with some organics and clayey soils, and then you may have limestone down 100+ feet... We usually get allowable soil bearing capacities in the 2-3ksf range. Using ASCE Table 9.4.1.2 it would seem we would be D or E, though if soil brg capacity is anything like su (avg undrained shear strength in top 100ft) then maybe we could have a Class C.
 
The VA Map I have does not show what site class they assume, so that has to be the difference, unless it is spectral response that is the issue. I was using ASCE 7 to determine both the Ss and S1 values, as well as the Site Class. FEMA 310 says to use one of their seismic maps, though I have not been able to locate it. It is called Seismic Map Package. I would be surpised if these were not the same or close to ASCE, but this is not may area of expertise.
 
Anyone have any input? Please CC me directly. Or a link or PDF or FEMA Seismic Map Package.
 
I am just trying to determine my scope for a proposal, and would like to be in the ballpark with my time required... TIA!
 
 
Andrew Kester, P.E.
Principal/Project Manager
ADK Structural Engineering, PLLC
1510 E. Colonial Drive, Suite 301
Orlando, FL 32803