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Soil-Structure Interaction

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I believe that your first impression is right but I would guess that very
few engineers would include the mass of the slab in their calculations.

If the slab rests on a layer of sand and a membrane then the coefficient of
friction between the slab and the ground would be relatively small.  In a
large to medium earthquake the acceleration levels could be larger than the
coefficient of friction and then one should consider including the mass of
the slab when checking the piles.  Sililarly when the ground settles you
would have a similar concern.

If the passive resistance against the grade beams is large enough you might
be able to assume the slab moves with the ground.  But if this were the
case you would not have to transfer any lateral loads to the piles since
the passive pressure would also resist the lateral forces from the building
on the slab.

It is my belief that when calculating the inertial loads generated by the
slab on grade you should use the zero period acceleration from the spectrum
since the slab and the piles would not be able to vibrate at lower
frequencies.  Also unless the piles are designed to yield, the use of the R
factor in the Uniform Building Code is probably questionable.

Mark Gilligan

Message text written by INTERNET:seaint(--nospam--at)

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.

Gunnar Hafsteinn Isleifsson