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RE: Differential Settlement

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Before you get into this too deep, you might ask the geotech if the
recommendations are based on any sort of soil-structure interaction
considerations. I think the geotechnical engineer usually does these
evaluations based on what the soil would do if the foundation stiffness were
not part of the equation, which may overestimate differential settlement. If
you have the software to do it, I'd encourage you to do an elastic
foundation analysis if the geotechnical engineer feels comfortable giving
you some subgrade modulus values to work with.

-----Original Message-----
From: lrhauer(--nospam--at)earthlink.net [mailto:lrhauer(--nospam--at)earthlink.net]
Sent: Thursday, January 02, 2003 8:25 AM
To: SEAOC
Subject: Differential Settlement


I am engineering a 3 story building which requires a mat foundation due to
the
soil conditions, (the bearing value is okay- 1500 psf, but there is a layer
of
dry sands btwn 15 to 40 ft below grade which are susceptible to seismic
induced settlement). The soils report is requiring designing the mat
foundation for 1.7 inches of differential settlement for a span of 20 feet.

Using the equation M=(6EI x defl.) / L x L, I get very large moments for the
potential differential settlement, particular if the mat is considered
uncracked, (24" thick). And this is not even considering the moments due to
gravity loads. It really doesn't seem practical, and maybe the solution is
to
get through the layer of sand with end bearing caissons to eliminate the
differential settlement.

Anyone have any experience or thoughts on this problem?


Thanks in advance.


Larry Hauer, S.E.


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