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Base Shear for Pile Foundation

Base Shear for Pile Foundation

(OP)
Good day everyone! A coworker of mine is working on a project in San Francisco, CA. One aspect of the project is a multi-story concrete shear wall building. The building is part of the wastewater treatment plant and is only 4 stories tall but with heavy dead loads due to the equipment. A number of structural engineers from multiple firms are collaborating on the project and there is some disagreement on how to design the pile foundations for the seismic load. Some people believe that the piles should be designed for the base shear of the building calculated with the redundancy factor and importance factor, R and I. Other people believe the piles should be designed for the maximum load the piles can 'see' which would basically be the design acceleration, Sds, without the R and I factors times the seismic weight. I can understand both sides of the argument so I'm interested in seeing what other people's opinions are on this.

RE: Base Shear for Pile Foundation

I'm not familiar with the 'R and I' approach you describe, though by the sounds of it it seems quite arbitrary and subjective. The latter approach seems more grounded in actual engineering.

RE: Base Shear for Pile Foundation

You're probably hearing the same arguments that I'm familiar with:

- it's tough to inspect or repair foundations so we should be very conservative.
- designing for the elastic level earthquake load is very expensive and, perhaps, impractical.

From what I hear, in CA, it's best to contact AHJ to figure out their local policy. The owner should also be consulted since this is a public utility and a need to be operational post EQ may justify the additional conservatism.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Base Shear for Pile Foundation

There are two factors that need to be considered in the pile design. The pile deformation caused by the soil movement (without the structure) and the deformation induced by the structure's seismic forces. The first is based on the actual ground movement and is not reduced due to the structures response (R, T etcetera). For the structure's contribution, I think it is more appropriate to consider the structures response and Importance Factor.

To help ensure continued operation after an earthquake, it might be worth basing the design on the "A" chapters in the CBC ("A" chapters are for hospitals and K-12 public schools). Section 1616A1.16 requires foundations be designed for the lesser of:
  • The strength of the super structure elements.
  • The maximum force that would occur in the fully yielded structure.
  • Forces from load combinations with the overstrength factor.


  • Seismic requirements for piles are spread through several places in the codes with some overlap (ASCE7, IBC/CBC chapters 16 and 18, ACI 318 chapter 21). CBC chapter 18(A) has some specific requirements for piles that should not be overlooked.

    For non-Californians interested in the CBC, it can be viewed online at http://codes.iccsafe.org/California.html#2013. The structural chapters (chapter 15 to end) are in volume 2

    RE: Base Shear for Pile Foundation

    Everybody's comments are spot on, as usual. However, I'd like to add my two cents to the discussion, as this is a question I've encountered frequently and would like to check what are other colleague's typical approaches.

    I think that the importance factor should be used regardless as it is a way to account for a larger seismic demand associated with a larger return period.

    Regarding the redundancy factor R, I'm not entirely familiar with the American standards but if it is similar to the behavior factor q in Eurocode 8, it is basically a measure of structural ductility. If you can't allow any plastic hinges in the piles, you should design for the elastic earthquake load (considering an overresistance factor of 1.5, the minimum in Eurocode 8).

    However, if plastic hinges are detailed properly in the piles to allow for sufficient ductility (usually limiting axial compression and providing sufficient ties), this could potentialy be explored and smaller earthquake load levels may be used in design. Resistance hierarchy should also be maintained, ensuring that the overall foundation resistance is larger than that of the shear walls base section. It basically depends on where you need energy dissipation in your structure.

    I've done both approaches, depending on structure, client and fees.

    RE: Base Shear for Pile Foundation

    (OP)
    Thanks for all the tips. In my opinion the piles should be designed to act elastically with the maximum seismic load, so not including the redundancy factor (which appears to be similar to the q factor in the Eurocode 8 based on a structures ductility) but including the importance factor which bumps the design acceleration back up toward the MCE acceleration. Like KootK mentioned, I fall in the camp of people who believe piles are difficult to repair after an earthquake so being conservative is the best thing to do within reason. I don't have the specifics of the project as I'm not involved with it but from what I gather the building base shear is already extremely high and any increase would push the project toward the impractical stage. They're trying to find the fine line between being conservative and practical. Whatever solution they come up with will go for approval by the City.

    WannabeSF - Thanks for pointing me to that section. I rarely ever look at the 'A' chapters but I'll forward it to my coworker and see if it can help in any way.

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