3 Group Pile Cap Design 1

[JohnnySm]

Hi guys,

I am a 1 year grad structural engineer. Getting my head around the differences of beam and truss analogy in Pile Cap design.


my question is regarding the difference in rebar arrangement for 3 group pile caps designed under beam theory and strut and tie theory. Traditionally as far as i know, rebar is set in perpendicular directions across the bottom and then mirrored on the top to create the cage, with side bars etc. However with S and T theory surely the longitudinal reinforcement should span in three different directions across the effective widths from pile cap to pile cap making a triangular arrangement? But this seems complicated? how is that reinforcement continued to create the cage?

does anyone know of any books/ reading material that can explain this thoroughly, my general understanding of pile cap design isn't fantastic anyway. Iv referenced to "Reinforced Concrete Design to Eurocode 2" by Bill Mosley but the example in pile cap design is limited and based on a 4 pile group with truss theory. does anyone know of anything a bit more in depth?

Regards

R

 

[jayrod12]

If using S&T, technically there's no "cage". Our standard 3-pile cap only has bottom bars that run between piles to create the tension tie. The key is to ensure these bars are anchored appropriately to act as the tension tie for the 3-dimensional truss.

Kootk, if he has some time will probably chime in with some nifty sort of sketches illustrating the proper way. He's one of the S&T heavy hitters around here.

 

[hokie66]

Depends on your tradition, I suppose. I have always reinforced 3 pile caps in three directions, even before what is now called strut and tie theory.

 

[KootK]

I kinda dig this business of being formally introduced/summoned before I chime in. Makes me feel like the Pat Sajak of structural engineering. I really don't have the answer that the OP is looking for but I do know some relevant stuff.

Here's a few semi-relevant papers on STM for pile caps:

Link
Link
Link

Having been trained primarily in the US, most of what I think I know has been informed by the work of the Concrete Reinforcing Steel Institute (CRSI) I consider their design guide to be the state of the art in north america and highly recommend it: [link ftp://ftp.dfi.org/SuperPile2015Presentations/02-Mays%20final.pdf]Link[/url]. That link is just a slide presentation about the design guide, not the design guide itself.

CRSI's design guide doesn't approach pile cap design from an STM perspective so you may not want to run out and buy it. They take sort of a modified sectional design approach where the shear and detailing provisions are informed by strut and tie although STM is not used explicitly.

There's a blurb in the front matter where they state that STM is actually un-conservative for some pile cap designs. Frankly, that statement terrifies me and I'm hoping that it's a misstatement on the part of the author (haven't looked into it yet). The most powerful aspect of STM is that we consider it a valid, lower bound, ultimate limit state procedure that is universally applicable. If STM is only valid "sometimes" or even "usually" then, truly, I would consider the approach effectively neutered.

Traditionally, as far as i know, rebar is set in perpendicular directions across the bottom and then mirrored on the top to create the cage, with side bars etc.

I have every version of the CRSI manual ever published. Go me. I didn't review all of them before responding to your post but I reviewed a few. For some time, if not in perpetuity, CRSI has been specifying a discrete, three way reinforcing scheme just as you and Jayrod have suggested. And I certainly agree. A two way layout on a three pile cap seems, mechanically, like a terrible idea.

Our standard 3-pile cap only has bottom bars that run between piles to create the tension tie.

This is what I do as well. There is something about it that bothers me though. It would seem to not satisfy STM requirements for crack control reinforcing. Furthermore, it would be difficult to provide crack control rebar as the struts will not be parallel to your tie reinforcing. So far, I've been ignoring this hoping that the extra concrete around the struts is helping to confine things.

But this seems complicated?

It is, a bit, with respect to bar layering. What I've been doing is shown below. If one were willing to lift some bar ends to weave some of the bars beneath previously place bars, then an arrangement could be arrived at where each bar group has a high and a low end. That's not worth the trouble in my opinion. Pile caps are usually deep enough that small variations in bar layering don't matter much with respect to flexural capacity.

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.

 

[calvinandhobbes10]

I second the CRSI suggestion and have used it myself to learn about pile cap design. Bottom mat only unless you have uplift.

 

[JohnnySm]

Thank you all for your comments. especially KootK.

I will review the attached material and try my best to get hold of some of the CRSI material if feasible. Though working in imperial measures kills me...

Cheers guys!