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RE: Site classification - IBC

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C makes a pretty good difference.  Maybe that's what I'll wind up with.  "D" is how it's already designed, but if I could back off of that some it would be nice.

John C. Jones, PE
Barnett Associates
Pell City, AL
205-884-5334
205-884-0099 (fax)


-----Original Message-----
From: Jason W. Kilgore [mailto:jkilgore(--nospam--at)leok.com]
Sent: Thursday, April 29, 2004 3:05 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Site classification - IBC


This doesn't mean you have to skip all the way from "A" to "D" - like I
said, it's an average.  You could probably at least use "C", but I'd let
your geotech make that call.

---
Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

> -----Original Message-----
> From: John C. Jones [mailto:john(--nospam--at)struct-engr.com]
> Sent: Thursday, April 29, 2004 2:28 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Site classification - IBC
> 
> That's what I was afraid of.  That's sort of how I pictured it, but hoped
> there was an out I didn't know about.
> 
> Thanks,
> 
> John C. Jones, PE
> Barnett Associates
> Pell City, AL
> 205-884-5334
> 205-884-0099 (fax)
> 
> 
> -----Original Message-----
> From: Jason W. Kilgore [mailto:jkilgore(--nospam--at)leok.com]
> Sent: Thursday, April 29, 2004 11:09 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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