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Re: Differential Settlement

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Larry,
Having just designed a 900mm thick mat for a heavy reinforced concrete building with a modulus of subgrade reaction of 10pci/i I would ask the following questions:

1. Is the differential settlement given as to limit the minimum stiffness of the mat supporting the building. 2. What is the estimated total settlement? It may be much greater than 1.7 inches. Keep this in mind when connecting utilities and inform the civil.

The differential settlement between bays or within a given distance is most likely a recomendation to prevent potential cracking in the systems above the mat. I limited the differential differential settlement of the mat to around 20mm for 8 meters. The geotech estimated a total settlement of around 150mm that would occur mostly during construction.

Talk to your geotech and ask yourself again can you really have 1.7 inches of differential in the building systems above in only 20 feet.

Utilized SAFE and would recommend it for mat design. You can obtain differential settlements for sub grade modulus, bearing pressures, and reinforcement.

James

From: <lrhauer(--nospam--at)earthlink.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: SEAOC  <seaint(--nospam--at)seaint.org>
Subject: Differential Settlement
Date: Thu, 02 Jan 2003 08:24:48 -0800

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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