Pile head cut

[Okiryu]

Hi,

In my area it is common to cut piles in order to bring they at a level where the pile caps can be constructed. The piles are precast presstressed concrete piles. My question is about if there is any issues affecting the steel reinforcement due to cutting these piles. Since the piles are presstressed, I was assuming that there may be some effect in the concrete presstressed material? On the other hand, it is difficult to determine exact pile lenghts so pile cutting is inevitable.

Thanks !!

 

[hokie66]

In trimming precast concrete piles, whether prestressed or conventionally reinforced, you don't cut the reinforcing. You saw around the pile, remove the balance of concrete mechanically, and use the remaining reinforcement to tie into the pile cap. The prestressing strands are bonded, so you can trim as required.

 

[Okiryu]

hokie66, Do you mean that you may have regular reinforcement plus presstressed strands? Our piles have spiral reinforcement and in the longitudinal direction all are presstressed strands. To connect to the pile cap, we hang rebars inside the hollow portion of the pile. We also hang a bucket inside the hollow portion, so concrete does not fill the entire length of the pile.

Based on your comment, we can trim our piles since all are presstressed strands. I am not familiar with presstressed concrete as I am involve more with the geotechnical side of the project. Can you give me a rough explanation of the bonding effect in presstress concrete, or if you can direct me a good reference, I can check by myself.

Thanks !!

 

[hokie66]

It sounds like you are using round spun precast piles. No, I don't believe they would have both types of longitudinal reinforcement. I have experience with using spun poles, usually tapered, and also spun building columns, usually straight, but have never used spun piles. Sounds like a good use of the product. The spinning process creates a very dense concrete. A dish of laitance is left, which should be removed where you are connecting.

Yes, you should be able to cut the entire precast section, then use the void to insert reinforcement to tie the pile to the cap. As the prestressing strands are fully bonded, you don't have to worry about them going anywhere.

 

[Okiryu]

Thanks hokie66. Yes, we are using spun precast piles with very dense concrete. The info you provided is very helpful... thanks again.

 

[Okiryu]

hi hokie66, I was talking with some engineers and some of them mentioned that when trimming the piles, embedment of the pile head into the pile cap becomes more critical. They said that after trimming, they assume that the top 50cm below the trimmed area as non reinforced concrete pile and therefore at least 50cm of the pile head should be embedded into the pile cap. Any comments?

 

[KootK]

When you cut a prestressed member, you change the state of stress near the cut. The pre-stressing needs to "re-develop" at the fee end so to speak. This leaves the surrounding concrete essentially un-prestressed for a short distance. That can be a problem if concrete pre-compression is part of the durability program and the decompressed concrete is left exposed.

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.

 

[Okiryu]

KootK, in case of a hollow circular member like spun piles, how can the un-presstressed zone/lenght can be calculated. As I was mentioning, some enginners told me to assume 50 cm below the trimmed portion. Thinking more carefully, based on my last post, a 50 cm embedment into the pile cap appears to be very large. Is any way to re-reinforce the weakened area at the pile head after trimmed? That may be close to the area where the max. moment acts for fixed head conditions.

 

[KootK]

KootK, in case of a hollow circular member like spun piles, how can the un-presstressed zone/lenght can be calculated

Any basic prestressed concrete text will have some examples to guide you. Like most things, it comes down to equilibrium and strain compatibility.

Is any way to re-reinforce the weakened area at the pile head after trimmed? That may be close to the area where the max. moment acts for fixed head conditions.

I can't think of anything practical. And I'm not sure that it's necessary. You may be able to just treat the 500 as you might rebar development in a non-prestressed member. Or perhaps the "bucket" can supply the reinforcement, either compositely with the pile or not. I'd recommend taking a step back and asking yourself "how am I getting bending moment out of the pile and into the cap?". Or "Should I be transferring moments into the cap?". With the basic load path in hand, you can then turn your attention to the granular details.

I should add that prestressed piling is not my area of expertise. If Hokie offers conflicting advice, use his.

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.