Pile to Pile Cap Reinforcement Detailing

[Cougar_15]

Hi All,

I currently have a 16" pipe pile coming in to a 6' deep pile cap with the bottom reinforcement placed on top of the imbedded pile (See attached for sketch). This pile will see mostly compression and tension loads. I am thinking about running the pile reinforcement as close as possible to the top layer of reinforcement in order to better engage the top mat for bending. I am hesitant to do this because I still want my compression load to be transferred on the outside of the bottom mat and not potentially transferred through the bars in compression to a deeper location in the mat. Ideally, the concrete filled pipe will take all of the vertical load through bearing. If the compression is transferred to a deeper location in the cap it would seem that my punching shear checks would need to have a closer look to ensure I am using an appropriate concrete thickness to resist the shear. Any thoughts or discussion is always appreciated !!!

 

[phamENG]

If you're worried the gravity load will follow a path through your hooks and down into the piles, do a quick check on the relative stiffness between 4 2' long steel bars and bearing on a concrete filled pipe. I think you'll see that, even if some of the load did go into the bars, bearing will take over really quickly.

 

[EZBuilding]

Cougar 15,

I would recommend getting your hands on CRSI' Design Guide for Pile Caps.

My primary experience with piles is with Auger-Cast piles. The 16 inch embedment you have shown for your pile into the pile cap would be excessive for that condition. For steel piles, CRSI recommends a minimum of 6 inches of embedment.

In the case of Auger-Cast piles, hooks are considered cost-prohibitive reinforcement detailing. It is common for reinforcement to be provided with development length into the pile caps.

I can't see your concern regarding the compression being taken up at a higher section of the pile cap due to the load coming into the reinforcement. That would not be one of my design considerations, but I will keep an eye on the thread to see if this is a concern shared by others. I expect that making sure your reinforcement is capable of transferring tension and shear from the pile cap and into the pile is the primary consideration.

 

[Brad805]

I agree with Pharm.

The hooks you show are not ideal from a practical point of view. If you cannot develop the pile rebar you may be interested in the HRC 670 ends. HRC 670

 

[Cougar_15]

I am able to develop the pile bars into the cap. It was more of a concern (or myself overthinking) that if I extend the pile bars to the top mat for tension/ flexural purposes that I will be impacting how the compression load is distributed into the cap. Ideally, the compression load from the pile would happen at the pile bearing interface regardless of where I place the tension reinforcement. Pharm had a great point about relative stiffness.

 

[dik]

and unless you have uplift, the hooked bars don't help a lot, best they are straight.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

@OP: You will be constrained, like us all, by what the industry deems to be practical. That said, from a theoretical perspective, I agree with every one of your concerns.

If the compression is transferred to a deeper location in the cap it would seem that my punching shear checks would need to have a closer look to ensure I am using an appropriate concrete thickness to resist the shear.

Yeah, just so. Frankly, I have this same concern with every concrete column on pad footing connection. The degree to which it is a valid concern is, of course, a function of how heavily reinforced your pile is. And it's exacerbated by the fact that concrete creep tends to see the compression in a pile or column migrate disproportionately away from the concrete and towards the steel. Is this a big deal in a column with 2% reinforcing? Probably not. Is it a big deal in NY where the "reinforcing" might be a 3" diameter, 105 ksi bar? Maybe.

I don't really have a great answer for you on this one with respect to why you shouldn't worry about this. I try not to worry about it because nobody else seems to worry about it.

This pile will see mostly compression and tension loads. I am thinking about running the pile reinforcement as close as possible to the top layer of reinforcement in order to better engage the top mat for bending.

If you're designing the connection per reinforced concrete principles then, technically, you have to run the bars across the top mat. In a pile, the only, somewhat practical way to do this, I think, is to splice starter bars (technically ender bars) onto the main pile reinforcement. Those bars might be smaller than the pile cage bars and might be headed rather than hooked.

I feel that a more practical solution is to treat the connection as an anchorage problem rather than as a reinforced concrete problem however. This would be to take a similar path to how one designs a post installed anchor group for tension.

 

[KootK]

If the compression is transferred to a deeper location in the cap it would seem that my punching shear checks would need to have a closer look to ensure I am using an appropriate concrete thickness to resist the shear.

This is the picture that I often have in my mind when I try to rationalize this. Consider the merits of this argument:

1) No matter what, the pile compression load needs to ultimately be dragged back to the lower face of the pile cap for RC concrete design.

2) Punching shear is how we justify the use of diagonal tension in the concrete to accomplish #1.

3) I would argue that punching shear, in this instance, is just one of several possible diagonal tension failure planes based on the tension field that develops.

4) If the delivery of the compression from the rebar into the pile cap is uniform, or better, are any of the other failure planes so much worse than the one that we assume for punching shear?

Yeah, I know: it's pretty loosey goosey as far as "proof" goes.

 

[KootK]

I should have indicated that the compression field lines emanate from the rebar as a byproduct of the bond stresses developed there.

 

[le99]

I suggest turning the pile and pile cap 180 degrees and treating it as a typical footing foundation. Then develop the reinforcement to meet the demand, whether tension or compression, according to the code provisions, and check the two-way shear capacity of the cap as the foundation pad subjects to the concentrated load delivered by the pile(s).

Also, you should develop the bars based on how the pile was designed. You don't want the connection too rigid if the pile was assumed pinned at the pile-cap interface. For significant tension/uplift load, it is not rare to provide a bar in the center of the pile and extend close to the top face. A plate washer is often placed on the top of the bar to ensure pull-out strength.

Unless required for bending, the pile embedment length is excessive. If necessary, you should provide shear reinforcement to protect the concrete core.