Applied Moment on End of Pile 1

[timbercharlie]

I am working on a project where I have vertical loads, lateral loads (in X and Z directions) and an applied moment that needs to be resisted by piles. The contractor wants me to resist all of those loads on a single pile. Based on my limited experience with pile design, I am not certain that I can resolve all of these forces with a single pile. My approach is to use a pile group with a concrete cap that will distribute and resolve the moment into vertical components. Does anyone know of a design procedure where a single pile can resist all of these forces? Thank you for your valued input.

 

[BAretired]

Depends on the magnitude of the moment. A pile held laterally at the top is capable of resisting applied moment. It is treated as a beam on an elastic foundation.

BA

 

[spats]

Agree with BA, but don't let the contractor talk you into doing something you're uncomfortable with or can't prove by the numbers.

 

[timbercharlie]

Thanks guys... for lateral restraint at the top of the pile do you mean a pile cap or does it need to be more rigid (like a slab)? Please refer to the attached free body diagram... is that what you mean?

Also, my professional ethic doesn't allow me to do something that I can't support with numbers... don't know why you'd think otherwise.

Thanks for your help!!

 

[JAE]

timbercharlie - there are some pile charts that the Gaylord & Gaylord Structural handbook has that I've seen - these deal with the distribution of the moment down through the pile - depending on the soil parameters, pile diameter, etc.

See attached.

 

[azcats]

Is any deflection or rotation allowed at the top? Could you use the embedded pole formulas in the code?

 

[timbercharlie]

Thanks all. This gives me plenty to sink my teeth into.

 

[asixth]

The 12" pile is not going to have the greatest amount of flexural capacity. With the presence of shear as well, the cross-section with the highest moment may occur at 5-6 times the pile diameter below the surface.

If you have obtained the Mx and Mz reactions applied to the top of the pile by assuming a fixed rotational restraint in your analysis it would be worthwhile to model the pile as well or a spring rotational restraint which allows some rotation at the top of pile to eleviate some of the flexure induced.

 

[amec2004]

>>Does anyone know of a design procedure where a single pile can resist all of these forces?

That depends on how much lateral movement you allow the pile head to deflect. In industrial structures normally it's 1/4 inch or 6mm. It may relax to bigger value if the supported services can tolerate bigger lateral deflection. For example a mining equipment may allow up to 20mm lateral deflection while a downstream equipment may have less than 6mm tolerance. It's not correct to say a single pile can take or you have to go for a pile group. You have to check based on the actual deflection value under the applied loads and compare to allowable value.
For a single pile you can use LPile v6 and for a group of pile you can use Group v8. You can check the book Single piles and pile groups under lateral loading by Lymon C. Reese, W. F. Van Impe for the theories LPile and Group are based on. These two program use the non-linear spring which is called P-Y curve to simulate soil reaction. For a moderate allowable lateral deflection such as 10mm you can assume P-Y curve is linear to simplify the analysis and get a close result. Otherwise you have to go to many trial-and-error analysis and converge the analysis. STAAD Pro has non-linear spring and you can use STAAD Pro to do the same thing as LPile, but you have to create the non-linear P-Y curve manually for STAAD input.

>>for lateral restraint at the top of the pile do you mean a pile cap or does it need to be more rigid

For the pile head restraint there are two cases : free head or fixed head.

If in the lateral load applied direction there are two piles under a pilecap, you can assume the pile is fixed head in that direction, otherwise it’s free head. For a single pile it’s definitely free head in both directions. For a two-pile pilecap it is free head in one direction and fixed head in the other direction.

The fixed head boundary condition greatly increases the pile lateral resistance compared to free head, but it may lose the pile group efficiency if the two pile spacing is less than 5d, the leading pile may leave a loose soil in front of trail pile and cause the trail pile to be less efficient.

We may debate how the two-pile pilecap shall be oriented to maximize the pile group lateral resistance. If we put two piles along the vertical brace direction, the piles get fixed head but get group reduction factor. If we put two piles perpendicular to the vertical brace direction, the piles get free head but there is no group reduction. We end up get the same pile group resistance in both cases with the plus and minus effect.

There would be more funs if you go to analyze the batter pile in a group. Group v8 is a good software for the analysis.

For all the pile lateral analysis you shall also check the pile shaft flexure and shear capacity with factored moment and shear. For the concrete pile there is a cracked pile stiffness issue which also has non-linear value changing with the pile shaft moment. LPile v6 can do a good job to incorporate the non-linear P-Y curve and non-linear concrete pile stiffness.

For the pile and pile group lateral resistance analysis I can give two lunch and learn seminars to my colleagues but I haven’t got a chance to do so. Finally I find an opportunity to write some words here and I hope all of you have funs on reading above brief introduction.

 

[oldrunner]

Your problem is similar to a pole sitting on top of a pile of some diameter. To start with, no one has mentioned the soil and where the passive starts or the capacity of the passive pressure. The pole (or column)is anchored by anchor bolts at the top or is embedded some distance down into the pile, either way, something is cranking in a moment at the top. Someone else mentioned that the maximum moment on the pile will be somewhere below the surface of the ground and he was right. There are programs that can solve this for you - something like L-Pile or something more expensive.

 

[FixedEarth]

BA hit it on the nose- it depends on your shear and moment loads. If they are light (say shear < 3k, M<10k-ft +/-), you may get by with a single pipe pile. If they are moderate, you can have a composite pipe pile - concrete pier design. If it the loads are high, you will not get enough Sx and Ix from the pile, to make the design work.