Lateral loading analysis in piles

[Okiryu]

Hi, when you analyze piles under lateral loading, do you consider the axial force applied to the pile? I understand that there are axial-moment interaction curves, but most of my analysis have been done assuming axial force is zero. I am just curious to know what is your approach for lateral loading analysis. As always, thanks for your input !!

 

[jayrod12]

Are you checking the pile, or the soil? If the pile directly, then yes you can account for both, however in general for concrete piles, axial load improves bending capacity.

 

[Okiryu]

Thanks jayrod12. Well, for lateral loading analysis you need the soil reaction, so I would say I am analyzing both.

In my case, commonly, geotechnical investigations are done at earlier phases of the design, so I do not have lateral loading info from the structural, so I try to back-calculate the allowable lateral force for a specific lateral displacement while I am not exceeding the crack moment of the pile (pre-stressed concrete piles). As you mentioned, I noted that for pre-stressed concrete piles, the balanced point gives larger moments than if I assume axial load equals zero, then I started to think that in reality the pile is subjected to axial and lateral forces simultaneously, so considering the axial load as zero for lateral loading analysis maybe too conservative.

 

[jayrod12]

I don't think given the constraints you've given you're being too conservative.

At the time of your analysis you do not have any more information to go off of. What some of the geotechs around here do is give the conservative value based off of similar assumptions to yours, and then add a clause that if more capacity is required that the geotech should be contacted and they will examine it on a case by case basis.

 

[Okiryu]

jayrod12, thanks again. That's good to know.

 

[bdbd]

If retaining system, yes, 0 axial force assumption.
If foundation pile, no.

 

[MotorCity]

Is the load due to the axial force also causing the lateral force? If so then yes, you should account for both.

 

[Okiryu]

bdbd, thanks for your reply. Most of the analysis I have seen so far, assumes axial load to be zero. Precast piles info for crack moments also are provided by pile manufacturers assuming axial force as zero (at least in my area). This assumption may be conservative, but I am just wondering why this is the typical assumption...

 

[bdbd]

First time I did that, I didn't do it because it is conservative. I have looked at how to calculate reinforcement. I needed M and N. I didn't load it vertically, but anchors did. However, for example, for upper parts axial load is 0, when you go deeper, axial load increases. So, checked out, which is the most conservative.

 

[Okiryu]

MotorCity, the axial force is not causing the lateral force/displacement, but if there is an axial eccentric load, it will cause a moment at the pile head which I consider in the laterally loaded pile analysis.

 

[Okiryu]

bdbd, you mentioned that "for upper parts axial load is 0, when you go deeper, axial load increases". Load-transfer diagrams for combined friction and end-bearing piles show the opposite...