Pile cap detailing for eccentric loads

Does CRSI has any detail/table for pile cap with moment?

In many situations, the practical solution will be to resist the moment created by the load eccentricity by designing the cap and piles for a distribution of axial pile load that mirrors that eccentricity as shown below. In that case, there is no explicit moment at the top of the piles nor in the pile to cap connections. I don't believe that the CRSI figure that you reference is intended to produce a moment connection at the tops of the piles per se.

fne said:

CRSI Pile Cap Design Guide, Figure 4.3 shows minimum pile embedment and a distance to the bottom reinforcing in the pile cap. The distance to the reinforcing places it above the top of the pile. [highlight orange]If the loading on the cap is eccentric to the pile or pile group, there will be a moment in the cap. Looking at the pile cap as fixed at the pile and an applied moment at the top of the pile, the moment will be constant throughout the depth of the pile. [/highlight] This would mean at a point near the top of the pile(s) the concrete section is unreinforced. Would you design the cap as unreinforced, or extend the pile further up into the cap - be sure the minimum embedment of the pile is above any reinforcing so it can be developed?

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Fig. 4.3 shows a group of 8 piles. I can't reproduce it here as it is copyright material.

For cast-in-place piles, there is no reason why the pile reinforcement can't be extended into the pile cap. For driven piles, continuity may be achieved in other ways.

A large eccentricity on a group of eight piles could put some piles in tension and if so, extended reinforcement would be a requirement. A small eccentricity would create differing loads on individual piles, but they would all be in compression; in that case, reinforcement would not be needed between pile and cap, although it is still a good idea.

If you are dealing with a single pile, the moment is not constant throughout the depth of pile because soil provides lateral resistance to movement, but continuity between pile and cap is desirable and, in fact, necessary if some of the bars are stressed in tension.

BA

Thank you for the responses. It is a two pile group and the axis of the pile group is the axis of the moment. It is a new column in an addition up against an existing building.

One thing I did not take into consideration when posting the original thread is the compressive force in the cap due to the axial load of the column. In very simple terms the allowable tension due to bending in the cap may not be reached since it first must overcome the compression in the cap due to the axial force. Something we will have to check.

Any other comments are welcomed.

IMO, two piles arrangement is not a good/stable choice. Use at least 3 piles with closer spacing, if it supports column.

Yeah, I'm with retired13 100% on that being an undesirable system. It's inefficient, difficult detailing, and a recipe for unpredictable behavior since your moment winds up being resisted by lateral soil stresses.

If it's two piles it should be two piles parallel with your eccentricity. The rational end game is the cap acting as a propped cantilever with a back span of some kind.

If the column is up against an existing building, there may not be much choice. A third pile, if added, may increase the eccentricity but might be a good idea anyway.

BA