Pile Reinforcement

[slickdeals]

Folks,
I wanted some clarification on the longitudinal reinforcing requirements for piles. The IBC seems to be pretty clear on what the reinforcing requirements are for SDC C and lower and SDC D and higher.

However, my question stems from a design standpoint.

The geotechnical engineer usually gives the flexural length of the pile (or by request), the allowable compression/tension values and shear capacities. We as structural engineers are required to design the reinforcing for the piles.

1. Assume I have a cluster of 40 piles for a shear wall pile cap subject to shear, axial and bending. The T/C loads in a pile can be easily calculated. The shear loads can be assumed to distributed equally to all piles.
LONGITUDINAL STEEL FOR MOMENT:
How is the longitudinal reinforcing in the pile calculated? Is it calculated based on (shear load on pile * flexural length)? If so, then should the pile be treated as a cantilevered beam/column with axial load and moment with an unbraced length = flexural length? We can either use a pile with a fixed head or pinned head which will change the moment values.

I have commonly seen piles with cage of 6#6 bars with #3 hoops extending 1/3 the length of pile + a full length bar. I don't know if that's just common practice that works for almost all conditions.

LONGITUDINAL STEEL FOR TENSION:
The tension capacity of a pile comes from skin friction. The shaft sheds tension into the surrounding soil based on depth of penetration. The tension in the pile is zero at its tip. Can the quantity of reinforcing be dropped based on how much tension exists in the pile?

SHEAR CAPACITY OF PILE CAP:
I assume the shear capacity of a pile group and pile cap is a function of the shear capacity of pile + passive resistance of pile cap + sliding resistance of cap.

Is it common to use the summation of all three to calculate the shear capacity? Or do you only use shear capacity of pile + passive of pile cap?

Sorry for the long winded post. I really wanted to summarize all my questions in one post.

 

[ishvaaag]

I can't say what is the "common" situation since not in a position to see lots of these works. However, my opinion is that you design the foundation according to your engineering model. Your model must give you reasonable figures at stable limit states about flexure, tension and shear distribution in the piles; even you might extoll some compression pile cap-soil (the more dubious component of your exposition) as long you in some reasonable way model the existing interaction.

Rewording what above, and aiming reinforcement, your quoted list above is more than anything the list of results of your analysis, that must be as complete as to reasonable portrait the particularities of required design for your structure.

 

[dirtydude]

You can reference CRSI and ACI. CRSI tables for concrete columns can be used to determine the amount of reinforcing needed. ACI has sections that dictate what the shear reinforcing is required. I forget the section off hand, but it indicates a formula for the minimum volume of reinforcing needed in shear.

 

[ron9876]

I assume that you are talking about augercast piles.

Bascially like any other member. If good soil it is a short column fixed on both ends with a moment based on shear per pile and the point of fixity provided by the geotech. Then PCA Column with cover per the code.

Shear resistance is combo of the items that you indicated with the limit for the pile based on the recommendations of the geotech. Then check shear stress in pile without reinforcement. Don't think I ever had a condition when the shear stress was very high since the limitations of the soil are relatively low.

How far the cage extends depends on the required tension capacity and moment capacity. I extend the cage at least a few feet past the point of fixity. On a high capacity tension pile the amount of reinforcement required will likely require that the cage extends full length to a rock socket. I guess that you could drop a few bars as you go down if it is a friction pile but I haven't ever done that. The geotechs are estimating the relative friction/bearing resisteance and they won't know for sure until they do the load tests.

In a compression pile something like you indicated seems about right unless you are counting on the compression steel to contribute to the capacity of the section. If it is a high capacity pile I would use more steel. In a condition where the cage stops I always add one bar full length to be sure the hole didn't collapse.

 

[rscassar]

They are all good questions.

RE Flexural design:

Interesting that you say the geotech performs the analysis. I always ask the geotech for soil properties and then run through my own analysis and design. If the pile is behaving like a column then it must be designed as a column.

RE Termination

If reinforcement is no longer required then it is acceptable to terminate the reinforcement.

 

[slickdeals]

ACI designs for concrete are based on ultimate strength design. So obviously my piles will also have to be designed per strength design requirements.

The allowable shear loads and axial loads are given by the Geotech. How do you folks address the following issues?

1. Do you design the piles for the ultimate shear (Allowable * 2)? Or do you design it for the shear per pile based on ASCE/IBC load combinations (~1.6 factor)?

2. Similarly, for P-M interaction, do you design it for ultimate axial load and moment due to ultimate shear * flexural length? Or do you use values based on actual load combinations?

3. For tensile loads, do you use:
As,reqd = (ultimate tension in pile) / (0.9*60 ksi)
[or]
As,reqd = (geotech allowable tension capacity) / (24 ksi)

 

[rscassar]

Request from the geotechnical engineer that they provide working loads for each individual load case (this may be covered in a performance specification). That way, you can combine load cases as per the code requirements (ASCE/IBC load combinations).

 

[ash060]

One thing is that for non-seismic categories, ACI 318 specifically states that the code does not apply for the design of piles. You have to look in ACI 543 and they have recommendations for the design of reinforcing in the pile. Alot of the info is based on allowable stresses.

I ask the geotech for the l-pile (or similar program) output. They can give you the shear and moment diagram for the pile with whatever lateral load you want on it. Then you can just design for those moments and shears. As for axial I am pretty sure (not positive) that the ACI 543 gives you allowables for the axial load.

 

[Lion06]

Our typical process for the design of drilled piers is as follows:
we provide maximum bases forces (shear, axial compression, axial tension, and moment if applicable) to the geotech. He does a run in L-pile to and gives us shear and moment diagrams for the caissons. You have to be careful here, though. If you have grade beams the geotech will try to say the top of the caisson os restrained to keep deflections under control. This puts a very large moment at the caisson/grade beam joinT and can cause a lot of congestion. It's best to leave the caissons unrestrained, if you can.

We take the shear and moment diagrams and design them just like a column - always as a short column.

 

[Brad805]

You can also model the piles using soil springs if the geotech defines the subgrade modulus. I have compared the results using this method to lat-pile and the results are resonably close. Waiting for the geotech is not always feasible for us.

Unless you have large lateral forces that benefit from passive soil pressures, I typically find that the pile stresses are relatively small since the design is governed by the soil characteristics.

Brad

 

[slickdeals]

How do you calculate tension reinforcing needed for a pile based on the tension capacity of the pile?

1. Do you use a overload multiplier such as 1.6 and the calculate using Pu / 0.9Fy?

2. Do you use the provided tension capacity of the pile and use a stress of 24 ksi or some other number i.e. P / 0.6Fy