Batter Pile Foundations Example - Resolving lateral demands into pile reactions 1

[EZBuilding]

I am working on a building which must utilize batter piles to resist lateral loading as the upper soil strata are too weak to provide meaningful lateral resistance. I have found limited information regarding battered piles, especially worked out examples from a structural engineering perspective. Accordingly, the below and attached are my attempts at resolving lateral loads into a battered pile foundation system.

I believe there are two "methods" in which batter piles resist lateral loads, either through passive or active resistance (self described nomenclature here). Through passive resistance the batter piles resist lateral loads based on the horizontal component of the axial load in the batter pile. Through active resistance, the lateral load increases the axial load in the batter pile.

The attached worked out example attempts to work through the reactions on each of these methods.

Assumptions:

Reinforced concrete shear wall supported on pile cap with piles.
1 to 6 horizontal to vertical batter
0.6 D + 0.6W is the controlling load combination and the only one considered.
No seismic loading
No lateral resistance provide by pile bending or lateral earth pressure
Pile loads based on axial and moment demands are provided based on rigid pile group analysis

Generally, I am looking for feedback on my understanding of how to resolve lateral loads in a battered pile foundation. Specifically in the "active resistance case" as I appear unable to close the loop on the pile reactions. If anyone believes I am missing something in my worked out examples, please do share and I will aim incorporate the comments into this understanding.

 

[KootK]

It would be useful to know if your piles are skin friction, end bearing, or both.

 

[EZBuilding]

These piles are skin friction

 

[KootK]

One approach that I've seen in the past is this:

1) Assume that the battered piles take all of the lateral shear. Equal distribution if no end bearing.

2) Assume that the non-battered piles take all of the gravity and overturning without the help of the battered piles.

This approach risks overloading the battered piles since they may see the lateral shear AND a share of the gravity & overturn. In many cases such as yours, the governing pile failure mode is settlement. In this context, it doesn't really matter if the battered piles have some extra settlement since #1 and #2 above already have that covered. If one wanted to be a bit more conservative, I suppose that run a second gravity and overturning analysis assuming that the battered piles participate and then add that demand to #1.

Deep foundation work is rough stuff so there's often little to be gained by trying to be very accurate. Of course, the scale of your shear demand here may neuter this approach if every pile needs to be battered.

 

[EZBuilding]

I have seen this approach used in the past - and I may very well have to rely upon it here.

Nonetheless, this approach ignores the "push-pull" effect at adjacent piles that I describe in the "active pile behavior". While your comments regarding the settlement behavior of the skin friction piles are well taken the batter piles typically need to occur on the outer layer of piles - locations to which they would be picking up the largest component of the overturning loads.

Also, even if there are shortcuts available that provide reliable results - I still feel the need to try and push to understand the behavior.

 

[sticksandtriangles]

I have struggled with battered pile design before as well (very little documentation as you mentioned).

I can't help much in regards to the battered pile geotech theory, but I will throw in some quick computer modelling I did.

Image one is the all piles battered option, I went into etabs and made a link that had a battering angle of 10 degrees and a U1 (axial direction) stiffness of 1000 kip/in:

Image two is the last piles being battered:

S&T -

http://www.re-tug.com

 

[KootK]

I was also going to suggest simply modelling a rigid block on top of some spring representations of your piles. Because your shear resistance is coupled with your flexural resistance, I don't believe that a closed form solution is possible without introducing one or more simplifying assumptions.

Another possibility if you don't like that:

1) Faux represent each battered pile as a combination of a vertical pile and a horizontal pile. This, obviously, is one of those dreaded simplifications.

2) Call the load at the outermost faux vertical pile "P".

3) Express the axial force in every pile, including the faux horizonal ones as %P, based on the pile's distance from the center of rotation.

4) Solve the relatively simple set of equations that results from enforcing shear, moment, and axial equilibrium.

Also, for a symmetrical setup, it may be possible to strip out the wall axial force initially and just add it back in afterwards via superposition.

 

[sticksandtriangles]

Updated with images with the 0.6D + 0.6W, before was 1.0DL + 1.0WL.

All Battered:

Ends Battered:

S&T -

http://www.re-tug.com

 

[EZBuilding]

Sticks and triangle,

I did a similar trial model within Etabs - both with springs and fixed supports at the end of the piles. Similar to you - I also observed the "push pull" behavior in the "active pile" case.

Applied Loads:

Pile Axial Force:

Pile Cap Moment Diagram:

Pile Cap Shear Diagram:

This trial model only looks at the lateral load component on this exercise. Traditionally I expect that the push pull behavior in a condition such as this does not have as much impact on the "pull" piles as this force would be counteracting the pile demands due to overturning and axial. The shear diagram does identify additional pile cap shear which would need to be accounted for in a traditional design. I wonder if both of these conditions would hold true in a situation where a shear wall pile cap is experiencing bi-axial bending.

 

[EZBuilding]

KootK,

I'll take a stab at working through the load distribution as you described it - although I believe it will help in distributing the loads but will not capture some of the concerns about how the battered piles interact with the pile cap and adjacent piles.

In reality, my use case for battered piles is substantially more complex than the example described. Multiple shear walls and columns on a single pile cap with a higher magnitude of load. In addition to the number of wind directions and load combinations that we need to consider makes it so that I need to be able to identify a path forward that utilizes traditional foundation analysis software (SAFE in my case). Similar to your load distribution example, I expect that I will end up landing in a situation that utilizes vertical and lateral springs to represent the different piles and I consider some of these other effects as a superposition to the traditional behavior that is identified.

 

[EZBuilding]

Brad805,

I have found few batter pile examples, and none that address the concerns that I brought up. One of those positions where you don't know if you're making a mountain out of a mole-hill or if you should keep digging and try and get a better more fundamental understanding. If you have any examples handy that I could review, I would be grateful if you would share.

I will be discussing this with my project's geotechnical engineer, however, the feedback I have received from geotech's in the past has been in line with not considering the effects I described. I do feel that the distribution of the loads to the piles is more of a Structural Engineering effort versus a Geotechnical one. Although Kootk's point about skin friction behavior and settlement does have some merit here.

 

[sticksandtriangles]

Be careful with the X and Y springs, I went down that route originally, but the results often don't match our axial load only assumption. I think the best way to approach this in ETABs or Safe is to make a link and rotate the local axis to the battering angle of the pile.

S&T -

http://www.re-tug.com

 

[KootK]

In reality, my use case for battered piles is substantially more complex than the example described. Multiple shear walls and columns on a single pile cap with a higher magnitude of load. In addition to the number of wind directions and load combinations that we need to consider makes it so that I need to be able to identify a path forward that utilizes traditional foundation analysis software (SAFE in my case).

Yeah, in my experience these things get so complex, so quickly, that the analysis really does have to be quite rough in order for it to be tractable at all. As an added wrinkle that is rarely dealt with explicitly, many of these setups will actually show a demand for plan torsion on the pile cap. Have. Fun. With that.

 

[EZBuilding]

Stickandtriangles,

Thanks - I will review accordingly.

Can you post a moment and shear diagram from your trial model?

 

[Brad805]

I don't think you are making a mountain out of a mole hill given the loads described. I think the fundamental problem is finding a linear solution to a non-linear (soil) problem. Deciding on the spring constants in S&T's model is difficult. I have compared our pile models using the soil spring analogy to the analysis by our geotech's that include the soil strata and was not thrilled at the differences. If you do not mind posting a snip of one of the soil logs I will mock up a model later to compare with others.

 

[sticksandtriangles]

Shear - All Battered

Moment - All Battered

Axial - All Battered

Note that with a stiff wall above this element, I would be there would be a lot less shear and moment in the pile cap itself.

S&T -

http://www.re-tug.com

 

[EZBuilding]

Kootk,

My configuration will have some in plan torsion as well. Makes for quite the stew.

The first time I was taught about designing a shear wall pile cap - I was advised to pin the pile cap for lateral loads and then just divide the sum of the direction by the piles shear capacities. No regards for any of those intricacies there...

 

[EZBuilding]

Brad805,

I do not yet have a soil log on the property, but I know that the first dozen or so feet have completely inadequate soil conditions. My load demands are actually a fair bit larger than my example - I just wanted to try and put something together that gave the team a common set of guidelines.

In order of magnitude I have a initial trial run for axial flexure which requires roughly 40 piles. Based on pile shear/bending capacity only I would need 130 piles. Ignoring my concerns noted above and at a high level - I would need roughly 15 batter piles to resist the same lateral demand. That is a massive delta.

 

[EZBuilding]

Sticksandtriangles,

I think the critical condition here is the "active" case where I think there is going to be a spike in shear that is unaccounted for traditionally. I agree with your assessment that the wall can help stiffen things up substantially, but it is common to see the outer row of piles be outside the confines of the shear wall. Those are typically the most commonly battered piles.

 

[KootK]

I believe that it's that very spike in pile cap shear that is the reason for battered piles being frowned upon in high seismic applications (which yours is not).