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RE: Site classification - IBC

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I have never had to actually do the IBC averaging technique but here are a few observations.  The first is that using SPT blow counts to determine site class is a very indirect and I would add probably very crude method, so do not get hung up on trying to get precise numbers.  The second is that when conducting SPT tests (ASTM 1586) you typically stop at 100 blow counts if you get into very hard soils.  I would probably use 100 for your rock.  Note that using shear wave velocity's is probably a lot more applicable in your situation than SPT blow counts however you generally do not get this in a soils report unless you specifically asked (and pay) for it.

I would also assume there must be a geotechnical report you are looking at to get the data you are reporting.  The site class is typically determined by the geotechnical engineer so I would try to get them involved.

Thomas Hunt, S.E.
ABS Consulting



"Sherman, William" <ShermanWC(--nospam--at)cdm.com>

04/29/2004 02:20 PM

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RE: Site classification - IBC





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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