Pile Cap STM

[briancpotter]

I'm currently working on the design for the cast in place bulkhead on top of a steel sheet pile. To resist the rather significant lateral loads, battered precast piles are being placed every 5 ft. I'm trying to check the design with a STM, essentially treating it as a sort of vertical concrete corbel. But I'm having trouble resolving the forces in a way that makes any sense - right now I have sort of a lonely tie force that I can't seem to pair up, and other force breakdowns I've played with give me similar problems. What exactly am I missing here?

 

[SlideRuleEra]

I believe you have got it:

1. The horizontal soil pressure is resisted by the horizontal component of the concrete pile compression.

2. The vertical component of concrete pile compression is resisted by vertical tension in the steel sheet pile. In other words, if the steel pile can't resist extraction, they will move upward, out of the ground. As the sheet pile wall collapses towards the left, it will tend to rotate the concrete pile counterclockwise.

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[briancpotter]

Figured out why I'm having such a tough time getting this to work. The sheet pile is embedded partway into the bulkhead. But to get the global forces to balance, you need to assume the lateral load comes in at the intersection of the pile centerlines, well above where the sheet pile stops (see attached).

I've discussed it with another engineer, who's of the opinion that typically for battered piles that so long as your centerline intersection occurs within the pile cap, the cap is "ok" and you just need to check the possible failure surfaces/planes. Which makes a certain amount of sense. But I'd still feel better if I could cash it out in terms of struts and ties.

 

[KootK]

Your structural spidey senses are obviously tingling here. And I think that you should listen to them as, in my estimation, you're on to something. Even if your T&C forces intersect within the cap, I don't believe that your cap can be in equilibrium without:

1) A moment at the top of the precast piles.
2) A moment at the top of the sheet piles.
3) Both #1 and #2 in some measure.

I've told one version of this story in STM below. It would seem to have rather important implications for:

1) The extension of the sheet pile into the cap.
2) The design of the precast piles.
3) The design of the cap connection to the precast piles.

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.