Tension Reinforcing in Auger Cast Piles

[dirtStruct]

We are using auger cast piles for a project and I have some very high tensile loads I need to resist. We have the loads per the geotechnical engineer that the piles can handle, but we need to design the pile itself to take the load and transfer the force to the soil. We plan on using 120ksi bar for the center bar to resist the tensile forces and will take this down for the full length of the pile. We will then use a rebar cage to resist the lateral forces and bending for the top 30 feet. My question is in the design of the tensile reinforcing, do I design the tension rebar like a column or like an anchor? For instance do I design it for T = As(0.9fy) or do I design it for T = 0.75(As(1.9fy,Fu or 125000 psi whichever is smallest)) and follow Appendix D? This is assuming that I am designing the load for strength design.

 

[KootK]

I vote for "like a column" based on the proportions involved. Sounds as though you've got miles available to accomplish your lap between the center bar and the rest of the pile concrete and rebar. I've always assumed that the 0.75 on strut and tie tie capacity was there because, with strut and tie in a deep members, everything is really a permutation of shear and, thus, a shear-ish material resistance factor makes sense for the ties.

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.

 

[dirtStruct]

Thanks KootK. So, really just checking the capacity for T=As(0.9fy) should suffice? Is there anything else I am missing that I should be considering for tension?

 

[KootK]

So, really just checking the capacity for T=As(0.9fy) should suffice?

Yes. So say I at least.

Is there anything else I am missing that I should be considering for tension?

This will surely fall into overthinking territory for most folks but I would consider the following:

1) Due to strain compatibility, I believe that your center bar will try to develop rapidly near the top of the pile and transfer much of its tension to the outer rebar cage. In case this is correct, I'd provide a concentrated group of rebar ties in this area to allow this non-contact lap splicing effect to occur without splitting the pile concrete.

2) Similar to #1, I think that your pile rebar cage will attempt to transfer tension back to the center bar near where the cage is discontinued. Again, I'd provide some cocentrated ties in that area.

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.

 

[JedClampett]

dirtStruct, I'm not sure what size center bar you're using, but it seems that development is worth checking. It's possible (but unlikely) that the bar doesn't develop full tension even in the whole pile.
I had a chance to watch some Auger Cast pile installation recently. I was particularly concerned about the installation of the reinforcing cage. I don't like "stabbing" reinforcing into concrete and didn't think it would work. Well, of course, the guys doing the installation were way smarter than me on this. Their main issue was keeping the cage from advancing too fast. They needed a crane to control the cage. Gravity did a great job vs. the grout. My point is, don't be afraid of using the cage to the bottom of the pile. If that gets away from the 120 ksi bars, it might be well worth it.

 

[PEinc]

What you are designing is very much like an uncased micropile. Check the FHWA Micropile design manual.
Look for: Micropile Design and Construction Reference Manual 2005 FHWA-NHI-05-039

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

There is a newer, 2005 manual that can be bought.

http://www.PeirceEngineering.com

 

[ATSE]

Similar to what other smart guys have said above, Appendix D is not appropriate unless you are trying to use a short embedment length, which you are not doing.
A few years ago I tension load tested some #10 75-ksi bar x 10ft embed to 90% yield in a low strength grout column (3-day breaks = 200psi), with zero bond slip. Even though ACI 318 Chapter 12 is not intended for low strength grout, it will provide conservative results for development.

As an aside, I would avoid using 120ksi bar. Most reinforcing (deformed or threaded) gets manhandled a bit during its life before placement. High strength bar is used all the time, I know. Not quite as ductile as you might think - you will specify it less after you see it fracture.

 

[dirtStruct]

Thanks all, this was very helpful! The pile is 75 feet long and the center bar we are specifying is 1 3/4" dia, but we have plenty of length to develop it.

 

[EZBuilding]

I have typically designed deep foundation elements such as auger cast piles in consideration of both Ultimate and Service level loads. Service level loads are compared against allowable stresses for materials used in deep foundation elements (Table 1810.3.2.6 FBC - I believe IBC uses the same table nomenclature). This table limits nonprestressed reinforcement in tension "other conditions" to 0.5fy or 24 ksi. The use of 125 ksi or 60 ksi reinforcement both yield the limit of 24 ksi.

In this condition I would check the pile for allowable compressive and tensile stresses based upon the aforementioned table and for lateral bending in both a compression and tension case based upon a strength level state. Another thing to note is that the pile will likely have zero concrete shear capacity in the tension case, all shear capacity would need to be provided by shear reinforcement.

Several members have responded to the OP and none have mentioned this code requirement. Am I missing something here?

 

[dirtStruct]

EZBuilding, I know what tables you are referencing for service level loads, but where does it say that I have to check both strength and service design for a pile? Especially if I have high lateral loads I would want to design the pile and reinforcing for strength level design, correct? Looking through the codes there does not seem to be a similar limit on the stress of the reinforcing if strength level design is used, but this would lead to a much higher acceptable load than if service level is used, which is a very interesting point. I have used strength design to check the pile reinforcing design, which seems to be perfectly acceptable per code, but if I used service level loads and previsions I would get a MUCH different answer based on the stress limits on reinforcing. In my case, the 120 ksi bar (ASTM 722) actually is prestressed, but is not used for prestressing, so perhaps

http://www.eng-tips.com/viewthread.cfm?qid=417446

this condition wouldn't apply anyways.

In terms of shear, I knew about the fact that the concrete would have zero capacity and designed the reinforcing of the rebar cage to take the full load, but thanks for pointing that out.

 

[irishengdave]

It's technically an anchor as a column is normally in compression.

Check the dev. Length as you're using a high yield bar. Are you using couplers in the pile for the central bar? I don't use them in cases where corrosion is a risk. If the threads on the coupler degenerate over time you will weaken the tensile capacity. I use centralizers to make sure the bar does not displace during concrete placement. I always specify a tremmie for CIP pile concrete placement. Segregation can occur otherwise. I've seen contractors just drop the concrete in from the top.

 

[EZBuilding]

dirtStruct,

I think the closest defining statement would be 1810.3.2.6 "Allowable Stresses :The allowable stresses for materials used in deep foundation elements shall not exceed those specified in Table 1810.3.2.6." I could not find anything else in the chapter stating that using strength design would allow one to ignore these values.

There is a huge change of capacity based upon the tables for allowable stress and traditional strength design. I interpret this to require a certain level of overdesign by the code as inspection, repair and or replacement of deep foundation elements are significantly more challenging to perform than for other structural elements.

I'm not sure if the use of prestressed reinforcement would change this provision.