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# rotational spring for a pile cap

## rotational spring for a pile cap

(OP)
I am looking for a method or reference for calculating the rotational spring value for a pile cap in sandy soil.  The soil about 4 feet below the bottom of the pile cap is liquefiable.

Is it conservative/sufficient to multiply the rotational spring for an individual pile by the number of piles in the pile group to arrive at a rotational spring value for the pile cap?

### RE: rotational spring for a pile cap

I'm not a geotechnical engineer, however, I've seen numerous reports that specifically present springs values for use with structural analysis.

Naturally, when a structural engineer is presented with a liquefaction as a possible soil failure mode, the response of the structure is assumed to be driven by the stiffness of the foundation including the contribution of the pile group.  This assumes, of course, that it is somewhat deeper than the liquifiable zone else there is assumed to be rigid body translation and or rotation.

Once the soil stiffness is no longer included in the analysis, the foundation stiffness may be computed similar to a structural matrix formulation.  Having said that I don't believe that all values of stiffness (three orthogonal translations and rotations) will sum.  The values for the translation will as you might expect.  For a circut analogy - resistors that are in parallel will sum but not those in series.

As for references, Many of the reports that I have read mention, amoung others, Gazeates and Lam and Martin.  As I come across others I will post those as well.

You should be able to find much information on the modelling aspects by perusing the following databases: EERI's Earthquake Spectra abstract search, EERC at Berkeley (also at www.eeri.org), and MCEER's quakeline database at University of New York at Buffalo.

### RE: rotational spring for a pile cap

Here is the publication I have at work which has been very useful - Seismic Design of Highway Bridge Foundations Vol I-III.  This was completed by the Earth Technology Corp. for the FHWA.  The report is dated Jan. 86.  As I mentioned earlier this is the work of Po Lam and Geoffery Martin et al.

Also, please note that the other name I mentioned should be Gazetas.

Good Luck.

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