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Site Class B versus C

Site Class B versus C

Site Class B versus C

(OP)
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.

RE: Site Class B versus C

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.

RE: Site Class B versus C

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.

RE: Site Class B versus C

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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