"Pile zone" Reinforcing for Strut-and-Tie Designed Pile Caps

[edled]

Hello all,

We're working on a project using 240k capacity piles, in groups of 2 through 6. Pile diam = 18", spaced 27" O.C. typically. I've done my design using 3D strut-and-tie models, using tie forces to size the bottom reinforcing bars, and using normal one-way (never governs due to all piles being within critical section) and two-way shear methods to double check the depth I've chosen. It's my understanding, especially after reading "Design of Deep Pile Caps by Strut-and-Tie Models" (Adebar, Zhou, ACI Journal July-August 1996), as well as some AASHTO/DOT articles, that the reinforcing in my ties should be concentrated over my piles, thus creating true "ties" that are geometrically where I assumed they were in my STM. I've documented this by calling out separate "pile zone" reinforcing requirements.

Now naturally the contractor has come back to us and asked why we are doing this, and that the reinforcing needs to be distributed along the bottom of the pile cap. My colleague who has done a lot of concrete design in Europe scoffs at this, and insists that what we've shown is definitely doable. (For reference, the max we show is 5 bars in layer one and 3 bars in layer two over the pile "zone", which is a 2' wide zone centered on the pile). The main issue in the contractor's eyes, I assume, is the fact that there are the rebar cages coming up out of the piles.

Do any of you have experience trying to get contractors to group the reinforcing like this? If they really push back, the only alternative that I see is to just design the PC's based on the CRSI method. I was really enjoying figuring out how to do the 3D STM method though, and I feel that it is the most "correct".

Any and all feedback would be appreciated,

Ed

 

[Teguci]

A reasonable question that you should be prepared to answer. I enjoy getting myself nose deep into complicated problems that take pages of excel calculations to resolve into a refined answer. But, before implementing that beautiful answer, take a step back and try to simplify it for construction. Do you need 2-layers of reinforcement? you have a 2'-0" wide zone over each line of piles that are spaced 2'-3" apart. If you do need 2 layers, why not 2 layers of 4 bars each to minimize honeycombing? How many bars are you saving by using the strut and tie model over the CRSI method? How does the rebar cage and tension reinforcement mesh?

Some construction practices in Europe are not appropriate for us in the States for the very simple reason that we are not familiar with them (like the German concrete corbel design).

 

[KootK]

My thoughts:

1) I'm sure that your solution is doable as it sounds as though you've given due consideration to congestion.

2) I agree that the STM layout seems mechanically superior.

3) As long as there's a commonly accepted method that allows for distributed bottom reinforcement, you're likely to keep getting push back from contractors. I'd spare yourself the angst and rework and just use the CRSI method.

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.

 

[edled]

Thank you both for your quick responses. Teguci, the suggestion to fully utilize the two layers to limit the amount of reinforcement in one is appreciated, and I'll take it under consideration, and KootK thanks for your input. I feel that I will end up going with the CRSI method, since as KootK pointed out it's a commonly accepted method, and one which the contractor will appreciate.

One thing I neglected to mention is the fact that for a typical 4 pile cap, when designed using the CRSI method, minimum bottom reinforcing ends up controlling (14 #9 bars, each way). Based on STM, we need 7 #9's for each tie, so 14 each way, but then we also need to add a bar on either side and in the middle to meet the max spacing of 18" requirement of ACI. Ends up being more steel using STM, which was a little counter intuitive.