Seismic - Site and Soil Coeff with piles

[PhDE]

Both IBC and BOCA do not address the question how does foundation type effect the soil/site class. Suppose I have building on very soft soil - one with battered piles to rock and another building with spread footings. Do I really have to design those buildings for the same lateral forces? I would assume the building with piles to rock can give an equivalent site class of rock rather than soft soil. In turn, my lateral forces would not be amplified. The "jiggling" of the soil would translate to axial forces in the battered piles down to rock. Does this make sence to assume both buildings have the same "amplifcation" of lateral forces independent of the foundation system? Help!

 

[rlflower]

For clarification, I believe you are refering to the Soil Profile Types designated in Table 16-J of the 1997 UBC. These do affect the seismic coefficients in Tables 16-Q and 16-R. I am not familiar with the IBC, for that code has yet to be adopted in my jurisdiction.

I would think you should have a soils engineer/ geologist involved in this project, and their report would cover such concerns. You can ask for an addendum to their report if this subject is not adequately covered.

Shooting from the hip, I would say that you cannot mix a soft soil bearing with a bearing on rock. If these buildings are in any way attached, the long-term effect would be a foundation system severed in half. Either design a foundation system "floating" on the soft soil, or full bearing on bedrock.

 

[JAE]

PhDE:
I would suggest that the piling does not directly affect the seismic response of your structure as much as you surmise, even if they are battered. The accelleration of the building is dependent upon the soil accelleration around the vicinity of the building and this enters the building at the soil/foundation interface which is usually the perimeter gradewalls / foundation walls / basement walls etc.

The battered piling would perhaps help in that they direct the resulting response forces into the rock in a fairly rigid manner, but I don't believe that there is any direct research on this subject.

I do believe that the IBC and UBC do not have any language that would permit you to ignore the upper strata of soil due to a battered pile system.