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Re: Lateral Resistance for Structured Pile Supported Foundation

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We encounter the same situation all the time. We always use batter piles in our bridge design projects. I developed pile footing design program for local Water Tank Company and they don?t like to use batter piles. I always do the following steps to take care the lateral force.
Neglecting Passive Earth:
  1. Calculates pile design loads (P & M) at top of pile, at point of contraflexure and at point of fixity.
  2. Check pile strength for the applied P & M.
  3. Pile head is considered fixed if it has at least 12 inches embedment into the pile cap and the rebars are placed in-between piles.
  4. Piles are normally fixed (no movement) 10 to 15 ft below pile cap. I always use 12 ft.
  5. If the pile head is fixed (allow to move but no rotation), the point of contraflexure is always at mid-point (= 6 ft below pile cap)
  6. Each pile?s design loads (P & M) can easily be calculated based on pile group geometry and applied loads.
Considering Passive Earth:
  1. Use AASHTO (LRFD) Table C3.11.1-1 ?The Movement requirements for minimum Active and maximum Passive earth pressure (Example: Dense Sand: 0.001H for active and 0.01H for passive)
  2. Apply earth pressure behind pile cap and pile based on the actual pile movement.
  3. Apply the unbalanced lateral force to the pile group.
T.J. Fu, P.E.
Louisville, Kentucky


"Stuart, Matthew" <mstuart(--nospam--at)schoordepalma.com> wrote:
I have conducted a peer review of a structured (one-way slab and grade beam) pile supported foundation design. The building is considered an emergency shelter under the IBC and is subject to a considerable seismic base shear. The total lateral load required per pile is 15 kips. The geotechnical engineer has indicated that the vertical piles are good for about 1 to 2 kips of lateral capacity. We have recommended that (since the foundation is already constructed) structural fill be placed next to the grade beams to provide enough passive resistance against the seismic base shear. The design engineer is of the opinion that as long as the piles remain adequately braced by the underlying soils during an earthquake that there is no need to provide lateral resistance for the base shear. I am not aware of any Code or design provisions that allow one to ignore the load path mechanism and not provide some form of lateral resistance (either via battered piles, passive soil resistance or vertical pile lateral resistance) against applied forces. Anyone interested in providing some input concerning this issue?
 
D. Matthew Stuart, P.E., S.E., F.ASCE, SECB
Senior Project Manager
Structural Department
Associate
Schoor DePalma Engineers and Consultants
200 State Highway Nine
Manalapan, NJ 07726
732-577-9000 (Ext. 1283)
908-309-8657 (Cell)
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mstuart(--nospam--at)schoordepalma.com
 


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