Pile Cap - rationalising force path to neglect lateral load in tension piles.

[JSD1986]

I often have to design the piles in say a 4 pile cap. The Cap is subject to a lateral load, a bending moment and compression load. Pile are conventional bored pier/CFA pile.

Due to the moment applied to the cap and the compression force we end up with some piles in compression and some piles in tension. Due to the lateral load being applied we end up with an induced bending moment somewhere down the pile. At this stage I end up with piles that are subject to tension and bending moment, and as you might well know requires a substantial amount of reinforcement because the concrete is basicially doing nothing.

Here's what i'd somehow like to rationalise and hoping to get some constructive comments.

1) For the case above, if the piles in compression could take ALL the lateral load could we justify that the piles in tension are taking none of the lateral load and hence none of induced moment.

Example.

MY HYPOTHESIS

Pile cap with 4 x 600 dia piles. Shear Load on cap 400kN,(say 100kN per pile). say we get 3000kN/pile compression and 1500kN tension/pile.

Compression piles - If we do it my way we get 200kN shear per pile, (lets say moment arm of 1.2mtrs (2D). M = 240kn.m. Compression load is 3000kn. say we are working in ultimate I get something like 5N20 bars, 40mpa, 600 dia pile

Tension Piles - Going off my assumption we have no lateral load so therefore no induced moment. Tension load 1500kn tension - Reo something like 9N24.

So designing it like this we'd use 9N24 for all piles, 1.4% reo.

OTHER WAY

Tension piles subject to 1500kn tension and 100kn lateral >>> 100 x 1.2(moment arrm) = 120kn.m. . Based on this I get something like 8N32 bars needed, about 2% reo. (wont bother checking compression as I know this case will govern.)



The example above probably doesn't really show the extremes of what results we can get, but as a piling contractor the costs can be huge especially if there are multiple pile caps, large diameter and bigger loads.

Any thoughts?

 

[WARose]

There is a lot in the literature on what pile sees what in the group. There is one school of thought that perimeter/front piles may carry much more lateral load than the interior/back piles (especially if the spacing is tight enough; most books O have recommend a pile spacing of about 5-8 diameters so that each piles is mobilized equally).

In any case, especially if the the piles are fixed to the cap (and the spacing is right) I don't know that you can justify any reduction in the lateral load to a pile. It's going along for the ride when the pile moves anyway. And a note on that: I've seen a lot of guys ignore P-Delta in such a case. Don't forget to throw that in as moment your pile will see.

 

[KootK]

I find your proposal pretty convincing. And, realistically, I would expect the compression pile to attract more shear/moment than the tension pile. The only concern that I can think of would be for cyclic, reversible loading. Redistribution of the shear and moment from the tension pile to the compression pile may well involve yielding the tension pile rebar and opening up significant cracks in the tension pile. Those cracks would need to close, and the rebar re-yield when the the tension piles go into compression. I'd think that would tend to open things up some.

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.

 

[JSD1986]

I know for a fact that this type of rationalization occurs in practice and as a reasonably inexperienced engineer about to get my CPENG I'd love to know if there are any legs to stand on should this need to be officially debated.

 

[Teguci]

In engineering, any load path that resolves the loads while keeping the elements and their connections in the elastic range, is a valid solution. Corollary, glass structures don't have many solutions.

Thoughts
- The shadowing of subsequent piles from lateral loads is appropriate for piles spaced less than 6 diameters apart. (Ref FHWA-NHI-10-016 pg 14-17)
- Per KootK's point, make sure to model the moment/deflection on the tension pile to verify any plastic deformation
- For shear loaded pile caps with overturning moments, I have always been a fan of battering the piles in order to get more shear into the axial component of the piles.
- With any shear loading (ie all pile caps) A (partial) moment connection between the pile and pile cap is always required to resolve forces.