Drilled Pier Reinforcing

[mrengineer]

This is a survey: What is your practice in establishing amount (if any) of longitudinal reinforcing in drilled piers that do do not have any moment other than that due to piers placed off-center?

For starters:

1) ACI would let you use plain concrete if that works.

2) The FHWA publication on drilled shafts recognizes the ACI approach, but goes on to say that FHWA prefers to use ACI requirements for reinforced columns.

3) CRSI 2002 Design Handbook recommends at least 1/2 percent longitudinal reinforcement, extending 3 shaft diameters (10' minimum) into the top of the pier, assuming adequate lateral support by the surrounding soil.

 

[prsconsultant]

We usually get a recommendation from the geo-technical engineer. In cases where the geo-tech does not make any recommendations we use 1/2% longitudinal reinforcement extending a minimum of 1/3rd length of pile or 10 ft minimum.

 

[JAE]

Here are a few threads from the past on this topic (sorta).

thread507-186921

thread256-6852

thread256-166950

Most of the drilled piers I've designed were in expansive clay regions and the vertical bars were based on two criteria:

1. An uplift force provided by the geotechnical engineer. This usually was some function of the clay and the pier diameter.

2. A minimum amount of 0.0075 x Ag
where Ag = gross area of the pier.

Item 2 would provide these results for various shaft diameters:

18" - 6 #5
24" - 6 #7
30" - 8 #8 (or 10 #7)
36" - 12 #8

 

[BlastResistant]

I don't feel comfortable have unreinforced sections in drilled shafts. The firm I work for has a policy of carrying a minimum of 0.008 x Ag throughout the full length of the shaft. This also seems to be a nice compromise between 0.5% and 1%.

 

[archeng59]

I use 1/2% to 1% steel, no vertical bars smaller than a #6 and #4 ties. Not so much for code reasons but because the reinforcing cage needs to be stiff when it's being lifted and placed into the hole. It's my experience that contractors lift the cage with hooks attached to the ties. The #4's do not bend as easily as #3 ties. The #6 bars and larger help keep the cage straighter than #5. I live in an area where we have a lot of expansive soils. Usually, if it's 30 ft or less, I'll reinforce the entire length. If it's a longer pier, I'll look at reinforcing less than the full length because standard bar lengths are typically 30 ft. Longer reinforcing cages require couplers at splices. I just had a senior moment and can't recall the technical name of these (I'm at home and can't look it up), but I use the precast pier reinforcing guides for deep piers. The guides are half-moon shaped and tie to the longitudinal bars to help keep the cage centered in the shaft.

 

[Lion06]

We typically provide 1%.

Archeng- Lenton is one proprietary name for the couplers. I think NMB splice is another.

 

[miecz]

We are not in a region of high seismic risk, and we use 1/2% minimum. We reinforce the full length of the shaft.

 

[archeng59]

StructuralEIT, I wasn't referring to the couplers although I can see how you interpreted it that way. I could not recall the name of the cage alignment devices manufactured by Pieresearch, which are called Centraligner Pier Sleds. I use them often on pier shafts over 10 feet. Also, I use Pieresearch's Hijacker pier bolsters on the bottom of the cage to help keep the rebar off of the bottom of the hole.

 

[hokie66]

Working in Australia, if the bored piers are not required to resist uplift and the soil provides adequate lateral support, normal practice is to use .5% longitudinal reinforcement. Normally use a spiral as the tie reinforcement. I usually specify the length as the full length expected, but do not add on extra length if a pier is lengthened due to bearing requirements.

 

[haynewp]

The OP mentions "no moment".

I hope there is a foundation or earthquake person to correct me if I am wrong; for piles that are in a seismic area but are not part of the lateral resistance of the building, I think some bending should still be considered. There is horizontal movement in the layers of soil beneath the building that is not uniform. In other words, there are layers of soil shifting at different rates and causing bending in deep foundations that are only supposed to be part of the gravity carrying members of the building. Do any codes require a minimum amount of reinforcing in all piles on a site of a certain seismic category?

 

[JLNJ]

Placing 1% steel in a pier where it is not warranted will get us accused of playing with house money.