Bearing Stress between Pile and Pile Cap

[scairns]

Greetings!

I'm currently working on a job where we will be driving 9 5/8" OD x 0.472" pipe piles (open-ended) to refusal on bedrock at a depth varying from 3m to 23m. We have been given a safe design working load of 900kN/pile from our geotechnical consultant. As the piles are going to be driven open-ended, i was not going to concrete fill after installation. The piles will not be augered.

In a book by Joseph E. Bowles (Foundation Analysis and Design - 3rd Ed.), the author mentions that if the top of the pile is adequately embedded into the pile cap (say 150mm), a transfer plate is not necessary.

When i calculate the stress of the concrete on the area of the pile, i get a bearing stress of approximately 100MPa at the peak design load of the pile (!!!). Is there something i'm missing? Without concrete filling the pile, can you eliminate the transfer plate?

Thanks in advance for your time.

Regards,
Steve

==================================
Steve Cairns, M.A.Sc, P.Eng.
Halsall Associates
Sudbury, Ontario, CANADA

 

[Focht3]

Are you looking at the pressure along the exposed edge of the pipe? (I got 97.7 kPa using the edge area, 19.2 kPa using the gross area of the pipe end.)

I suspect you should re-think the problem, and look at partially filling the top of the pipe with concrete such that it develops a good concrete 'plug.' That way you can use the gross area of the pipe face, not just the steel area, to transfer the pile load into the cap's concrete.

I don't like Bowles' book for many reasons - this is a good example; the proper embedment of pipe piles into a pile cap isn't a 'cookbook' problem. Other references are probably available to guide you; in their absence, I'd look at the problem using St. Venant's principle - as well as treating it as a concrete punch-through problem.

While I haven't done this kind of design before, it seems to me that embedding rebar into the top of each pipe, along with the inclusion of a mat of steel that is tied to the 'pipe rebar', should be a fairly straightforward way to address this issue.

And if the pile group will be subjected to significant lateral loads, you will need to be sure that the pipe piles have adequate embedment into the pile cap with respect to pile flexure. Most references I have seen indicate the minimum assumed lateral force is 10% of the axial force.

And in my job site visits during the construction of pile caps, I have typically seen two layers (top and bottom) of steel -



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[PEinc]

I don't see how you can not fill the pipe pile and not use a closure, transfer plate. Even though the pipe is being driven open-ended, it probably will not fill completely with soil. If the upper portion of the pipe pile is empty inside, you can't easily pour the pile cap without concrete filling the pile.

I recommend filling the pipe piles with concrete and adding a rebar dowel if uplift is a consideration. On highway projects, bearing piles for abutments and retaining walls usually are embedded into the pile cap a minimum of 12 inches or 300 mm.

 

[scairns]

Thanks for the responses.

I've seen on a number of jobs the addition of a concrete plug, but i'm not entirely sure how it works. If the concrete is poured in the pipe, and it shrinks slightly (as concrete does) how is the friction developed between the smooth surface of the pile and the concrete? I can see this working if the concrete is put into a closed-ended pipe where we can ensure there is no debris in the pipe.

As for the bearing stress on the concrete... i'm not concerned about punch shear through the pile cap, it's reinforced adequately for that. But the local stress at the interface between the steel and the concrete is significantly higher than 98kPa. 900000 N of force. The area of the pipe is approximately 8700 mm2.

Thanks again.

==================================
Steve Cairns, M.A.Sc, P.Eng.
Halsall Associates
Sudbury, Ontario, CANADA

 

[PEinc]

You also have bond of the concrete to the exposed surface area of steel pile.

 

[dik]

Just recently did a project... needed a couple of 20M bars to develop the pile capacity and welded them to the top of the pile. Matter of filling the top of the pipe with newspaper or something to keep concrete from filling it up. Also bearing was based on allowable increase due to confinement...

 

[scairns]

But can you consider the concrete confined if it is pushing against newspaper or debris in the pile? I can see the concrete being well confined if the pipe is clean and fully filled with concrete.

Thanks again.

==================================
Steve Cairns, M.A.Sc, P.Eng.
Halsall Associates
Sudbury, Ontario, CANADA

 

[VoyageofDiscovery]

Hi scairns,

The old '73 AASHTO used to allow 10psi for bond between concrete and steel piles. We have to use this especially when a pile steel cap is near impossible such as submerged tremie footings in cofferdams.

Regards

VOD

 

[clayR]

Steve.

I would not even consider load transfer based on embedment length of steel pipe, although you might consider using nelson studs. This is cheap and fast! but you still got to keep the conc. out the pipe.

As PEinc. points out, the pile will need to be capped with something. If you use grout to fill the pipe the value of 10 psi bond stress grout/steel that VOD cited is way low for what you can expect to see. This has been documented by pulling on a grout plug within a pipe using a threadbar and hydraulic jack. This is with grout, not concrete.

Myself, I would feel most comfortable with a plate. You could look at using plates with gussets prefabricated ahead of time, really cuts down on the field fab. This method will also keep concrete out of the pipe.

Wouldnt it be much easier to just fill the entire length of pipe with concrete and be done with it?

 

[dik]

scairns:

The top couple of inches are filled with concrete... and with the relative stiffness of steel vs. concrete, I would consider the concrete restrained... A lesser condition exists if the steel pipe were bearing on a 'flat' chunk of concrete... and you could design the bearing as constrained in this latter case...

Yup!, the concrete is constrained...