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Re: Site classification - IBC

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I will shoot from the hip a little on this since I have not encountered this situation myself nor have I used the IBC.  The fact that your piles bear on the rock probably has negligible effect on the horizontal response of the soil structure interface.  Most piles are assumed (hopefully this is in your soils report) equivalently fixed at about 10 to 15 feet below grade; well within your 20 feet of softer soils.  Therefore, horizontally, your building is reacting to the soils and foundation in the upper portion.  I would use the IBC method as shown for averaging multiple layers of different soils.  Theoretically you could probably make a case that for vertical seismic response your building and foundation will react directly to the rock layer.

Thomas Hunt, S.E.
ABS Consulting

"John C. Jones" <john(--nospam--at)>

04/28/2004 04:40 AM

Please respond to

Site classification - IBC

I have a situation where rock is about 20' down and is good for 40 ksf.  The top 20' of soil isn't too good so the building will be on drilled piers.  IBC 1615.1.5 gives a procedure for determining the site class based on different strata of soil.  I don't see it clearly spelled out when you have solid rock that you're bearing on and then soft soil above.  This is making the difference in lateral load of 350k versus 200k.  Anyone run into this?

John C. Jones, PE
Barnett Associates
Pell City, AL
205-884-0099 (fax)

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