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Re: allowable foundation and lateral pressure

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allowable foundation and lateral pressureThis is to account for increased
bearing capacity caused by increased confining pressure from the surrounding
soil.  If you didn't have this, the soil would fail under its own weight
after 15-20 ft.

A common practice is to increase the surface bearing pressure by the density
of the surrounding soil times (the depth minus some value).  The "some
value" might be the top 12", the local frost depth, or the maximum possible
scour depth if you're near water.

So, if you're allowable a net bearing pressure of 1500 psf and the bottom of
footing is 3' below grade you could justify a (3'-0" - 1'-0")*(110pcf)=220
psf increase in bearing pressure.  You gross is now 1720 psf.

That being said, I haven't found a direct provision in the 2000 IBC that
addresses this.  If a plan checker flags it you can try justifying it by
pointing at the UBC, but that might not fly.

Also, if you're using the table I'm assuming that you're NOT have a soil
boring done by a geotech.  To cover you're rear, you need to personally
visit the site and verify the local conditions.  Take a shovel and a metal
rod with you.  The shovel is to get past the topsoil and determine exactly
what is there.  The rod is to push in the ground to see how soft the soil

Jason Kilgore
Leigh & O'Kane, L.L.C.
816-444-9655 (FAX)
----- Original Message ----- 
From: Schwan, Martin K.
To: seaint(--nospam--at)
Sent: Tuesday, October 21, 2003 7:12 PM
Subject: allowable foundation and lateral pressure

I am wondering if someone could tell me if the IBC has a similar provision
as found in the 1997 UBC table 18-I-A allowable foundation and lateral
pressure, footnote 2.   Basically, for footings with a minimum 12" width and
minimum depth of 12" and placed on natural grade, you could increase the
allowable foundation pressure by 20% for each additional foot of width or
depth up to a maximum value of 3X's the designated value.
Thank you in advance for any information or direction.Martin in the land of
increasing darkness.

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