Partial depth pile reinforcement

[canwesteng]

Recently, a project at the office only reinforced the top 20' of some cast in place friction piles. This helped a lot with installation, it was a small project so schedule was tight. My thinking is the piles can be designed such that all flexure and upflift is taken by the upper portion of the pile, leaving the unreinforced portion to act only in bearing. I can't really find any sources to back me up, foundation books don't mention reinforcing and concrete books don't really mention piles. Any thoughts?

 

[KootK]

My understanding of this is the same as yours. Under tension, the pile would crack through where the rebar ends. It's a very common system in my area and local geotechnical reports often mention the partial length pile reinforcing explicitly. I found the partial length reinforcing pretty shocking the when I first encountered it though. Now, my only real concern is durability. I imagine a crack forming 1" above the rebar ends an water and air somehow getting down in there.

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.

 

[jayrod12]

If you are concerned. Check how much depth it takes until the tensile stress in the pile is less than fr. Then provide reinforcing that extends beyond that depth. If I ever have extremely significant uplift then I'm providing full length reinforcing anyway.

My bet (I may have ran this number a few times before ;D) is that once you start accounting for pile dead load, over and above the dead load on the pile already, at about 20' you'll find that concrete tensile resistance is enough.

 

[canwesteng]

My preference is wait until there is no more tension in the pile at all - that way compression can keep the cracks sealed.

 

[jayrod12]

I agree there. It's likely that point not much further than the point at which the concrete itself can take it.

 

[KootK]

Check how much depth it takes until the tensile stress in the pile is less than fr. Then provide reinforcing that extends beyond that depth.

My preference is wait until there is no more tension in the pile at all - that way compression can keep the cracks sealed.

I don't believe that either of these approaches would prevent tensile cracking of a pile in uplift. Developing shaft skin friction in uplift requires some upwards displacement of the pile. For that upwards pile displacement to not crack the pile, the un-reinforced length of pile and any soil engaged by it would have to come along for the ride. In reality, I don't think that there is a zero tension point in an uncracked, uplifted pile.

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.

 

[wannabeSE]

Some reinforcing requirements are in the IBC chapter 18. The minimum length of reinforcement depends on seismic design category. For the 2012 IBC, see

http://publicecodes.cyberregs.com/icod/ibc/2012/icod_ibc_2012_18_sec010.htm

. The following is a quote for Seismic Design Category D thru F

1810.3.9.4.2 Seismic reinforcement in Seismic Design Categories D through F.
For structures assigned to Seismic Design Category D, E or F, cast-in-place deep foundation elements shall be reinforced as specified in this section. Reinforcement shall be provided where required by analysis.

A minimum of four longitudinal bars, with a minimum longitudinal reinforcement ratio of 0.005, shall be provided throughout the minimum reinforced length of the element as defined below starting at the top of the element. The minimum reinforced length of the element shall be taken as the greatest of the following:
1. One-half of the element length;
2. A distance of 10 feet (3048 mm);
3. Three times the least element dimension; and
4. The distance from the top of the element to the point where the design cracking moment determined in accordance with Section 1810.3.9.1 exceeds the required moment strength determined using the load combinations of Section 1605.2.

 

[FixedEarth]

In addition to wannabeSE comments, if the pier length to diameter ratio is 10 or less, we recommend full length reinforcement. For piers with L/D > 10, we suggest you perform lateral foundation analysis and determine point of zero moment and that is a reasonable place to stop the drilled pier reinforcement.

http://www.soilstructure.com/

 

[canwesteng]

I'm thinking the point of zero tension would occur based on using the weight of the pile to resist uplift, not skin friction. I agree if skin friction is resisting uplift likely the whole pile would be in tension to some degree

 

[Brad805]

In our region many of the geotechs have been recommending full length reinforcement for piles; however, the frost depth can be in the realm of 10'-0" in some cases. Frost pullout forces tend to be our most significant problem for our most common CIP piles.

 

[Lion06]

We have not had geotechnical engineers using skin friction with the soil for uplift resistance on drilled piers supporting lateral elements. Every drilled pier project we've done has had rock sockets considering only the skin friction with the rock. This requires full length bars.

 

[jayrod12]

Where I am, there could be anywhere from 20-100 feet of sticky clay. We have to rely on either skin friction or belling the piles for uplift.

The issue with belling for uplift is there's always some reason the contractor can't do it, i.e. sloughing of the bell, boulders, high water etc.