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Site Class B versus C

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agnoyes

Structural
Jan 31, 2007
5
I have a project in Chattanooga TN that is classified as a Site Class C by shear wave velocity test to 2300 fps. However, by putting in a basement at -12'-0" below grade and updating the shear wave velocity test to discount the top 12' the shear wave velocity increases to 2600 fps and is then a Site Class B. The design team would like to utilize the site class B to eliminate various requirements necessary if it is a site class C. However they do not want a basement.

Therefore, if I design my structure starting from 12' below grade (drilled piers to bedrock) is it reasonable to use the lower base shear afforded by a higher site class B? I also would eliminate seismic ties as well.
 
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This sounds sketchy as heck. You're still going to get shear wave transfer through that top zone of soil even if your foundation is below that point.

I don't have code commentary in front of me, but I'm pretty sure site classification is based on the top 30m of soil, not the 30m below the foundation level.
 
Protect the public is first priority. Even if it's a private house where the owners tell you they don't care, future owners and guests expect that.

With that said, if you can reasonably make a case, then go for it. Otherwise, I'd follow the code. If the owners don't understand or care, you don't really want to be working for them anyway. I know that's easy to say from a distance, but I think it's true.
 
Even if you did put the basement in I do not think you can discount the top 12' as this material will apply horizontal loads to the basement walls during an earthquake.
 
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