Pile cap included in seismic weight

[swooop]

We have a 9x9 grid of 14" dia. concrete piles with a 2'-6" thick pile cap supporting a steel structure. Top of pile cap is 1' below grade. What is the correct procedure of including or not including the pile cap weight when checking lateral seismic forces? Some say that it moves with the earth, so ignore weight. Some say just be conservative and apply a lateral force due to the weight.

FYI: The lateral force, if used, due to the pile cap self weight is 20 kips. The base shear of the steel structure, not including the pile cap, is only 6 kips. This issue obviously has a great impact on our piles.

What do you all typically consider? Any input would be helpful!

Thanks!

 

[WillisV]

Foundation systems are not generally considered in the seismic weight of the structure as their inertia does not impart any additional load on the lateral force resisting system of the building above.

Similiar idea holds true for say a tilt up building. For determining seismic forces, the weight of the walls parallel to the direction of loading can be ignored for seismic weight calculations in that direction, because they carry their own load as shearwalls and do not impart any additional force on the remainder of the building;s lateral system.

 

[swooop]

"inertia does not impart any additional load on the lateral force resisting system of the building above. "

I completely agree with this. I should have clarified. This question is in regards to the design of the piles, not the structure above. Same answer?

 

[blackmaddog]

I haven't done pile design before, but my logic would be to ask what is resisting the seismic load of the pile cap? Is it the surrounding soil or the pile? I would think that if the pile caps are backfilled with compacted structural fill, it would be rigid enough to resist the load. If they are backfilled with softer materials, the piles will take the seismic load.

Maybe some of the geotechs in here can shed some light?