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# Re: Differential Settlement

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Differential Settlement
• Date: Thu, 2 Jan 2003 12:50:36 -0800

```Larry,
The 1.7" differential settlement for 20' is way too much. It seems like it
has come from the common estimation of 0.75 of max. total settlement. I
would limit the differential building slope to 0.002 for steel or concrete
frame. This would give you a 0.48" for 20' span.

Reza Dashti P.Eng.
Vancouver, BC

----- Original Message -----
To: "SEAOC" <seaint(--nospam--at)seaint.org>
Sent: Thursday, January 02, 2003 8:24 AM
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?
>
>
>
>
> Larry Hauer, S.E.
>
>
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