As a first order approach, the weight of the slab may be ignored as long
as top of slab is used as the embedment surface. Remember that the slab
density is not much more than the soil density. What the slab may do,
particularly if it is continuous, is "slave" the piles at the surface to
move in phase, and at the same limear displacements.
That said, however, a large and thick slab will alter the seismic motions
at the site. For critical applications (specified performance, vibration
isolation, base isolation, etc.), additional analyses may be in order.
For less critical applications, simplified analyses may suffice.
Include in your geotechnical budget some time for a design consultation
with the GE (following the full site investigation). A few hours
consultation may save you hundreds of hours of unnecessary numerical
modeling and soil-structure analyses.
Russ Nester, SE, GE
On Thu, 4 Jan 2001 04:16:22 +0100 "Gunnar Hafsteinn Isleifsson"
> 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 ground.
> 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