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RE: Site classification - IBC

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Jason  - and Tom Hunt: 

You each say to average the top 100-ft - to which I agree if it can be done.
But I don't understand how you do that if you have standard penetration test
values for the soil in the top 20-ft and then have 80-ft of rock. There are
no standard penetration values for the rock - so what do you use to create
an "average"? 

William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


> -----Original Message-----
> From: Jason W. Kilgore [mailto:jkilgore(--nospam--at)leok.com] 
> Sent: Thursday, April 29, 2004 10:09 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: 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?
> 
> Whether you bear directly on the rock or on the 20 ft. of 
> soil shouldn't affect the soil site classification.  The 
> classification is based upon the AVERAGE of the top 100' of soil/rock.
> 
> Some might argue that since you're bearing on rock that you 
> should get to use the rock coefficients, even if it's 20' 
> down.  In that case, I'd say you need to include that 20' of 
> height, as well as the weight of the first floor slab and 
> grade beams, in your seismic analysis.  The piers and grade 
> beams would need to be designed as moment frames.
> 
> At any rate, any motion amplification that is caused by the 
> soft upper soil (the reason behind the higher Fa/Fv values 
> for soft soil) would be applied directly to the foundation.  
> It doesn't matter if the application is to spread footings or 
> to the sides of piers.
> 
> ---
> Jason W. Kilgore, PE, SE
> Leigh & O'Kane, LLC
> Kansas City, Missouri
> 
> 
> 
> 
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