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Re: Soil-Structure Interaction

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Of course you have to include the slab weight in your calcs. Your structure
is founded on piles precisely because of the low bearing capacity of the
underlaying soil, otherwise it would be a mat foundation, and the piles
would be useless. Not only that, the slab itself has to be designed as if it
was an elevated slab (not a slab on grade).

Javier Encinas
ASDIP Structural Software
 ----- Original Message -----
From: Gunnar Hafsteinn Isleifsson <gunnarhi(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Wednesday, January 03, 2001 11:16 PM
Subject: Soil-Structure Interaction

> Hi
> I have a sort of a geotechnical question:
> A steel structure stands on a thick concrete slab, which in turn stands on
pile footings. When evaluating lateral seismic forces acting on the piles,
do I include the weight of the slab, or does it just "move with the ground"
as an integral part of it.
> My first impression is that one has to include the weight of the slab,
when determining the base shear at the top of the piles, because that
although the slab will also be ground supported (cast on ground), any soil
settlement will cause the slab to rest solely on the piles, and then one
can't rely on any friction (sliding resistance) between the slab and the
> Our client wants to know if anything can be done to exclude the slab
weight, since it has great effect on the pile foundation costs. Geotechnical
investigations have not been undertaken yet, apart from some rough
estimates, so I don't know if any liquefaction potential exists on the site.
> Any thoughts on the subject are appreciated.
> Regards
> Gunnar Hafsteinn Isleifsson
> Denmark