How much does vertical load affect the lateral load capacity of piles? 1

[abusementpark]

In general, how much does vertical load (compression or tension) affect the lateral load capacity of piles?

It seems like whenever I get the geotechnical engineer to run an L-PILE analysis for me, the vertical load doesn't seem to be that consequential.

 

[FixedEarth]

None. We assume the vertical loads are in the centroid so the only lateral loading is from shear and overturning moment. Why don't you try LATERAL FOUNDATION software and run your own analysis. By the way, you get less deflection bu increasing width/diameter of the pile/pier than by increasing embedment depth.

 

[Ron]

FixedEarth...I tend to agree if you are considering geotechnical issues only; however, in many cases piles follow column theory in a structural sense. In that case, the vertical load (axial compression in particular) is a critical load case for buckling, particularly if the pile is end bearing and has reduced lateral support from the soil at one or more locations along its length.

 

[FixedEarth]

Ron- The equations from Hetenyi can be applied to a laterally loaded pile or pier and we can adjust the horizontal subgrade modulus with depth as most soils are layered. So if there is a squeezing layer, we assign a smaller kh value. A weak soil layer will only affects the lateral load capacity if it is within the upper 6B or so. Correct, the weak layer will have more effect on axial capacity, even if it is at greater depth than 6B but the imposed vertical loads do not play any part in the lateral capacity of the pier or pile.

 

[hokie66]

FixedEarth,
I think you are still talking about the geotechnical capacity of the pile, while Ron was just noting that the structural capacity of the pile shaft itself can in some instances control. The OP asked for an answer "in general", but the answer is "it depends".

 

[Ron]

hokie66...thank you...that is correct.

FixedEarth...check a little deeper in Hetenyi....His equations apply to a beam on an elastic foundation, and he does consider axial compression in one series of equations (which you will notice, significantly changes his influence line); however, his equations do not touch on column theory at all and a pile acts as a column in many instances.

 

[FixedEarth]

I did that checking early last year. With combined footings and mats, you can only have one k value (vertical subgrade modulus). You then load a beam with point or line load and moments at the ends. However, the ends are not fixed.

For laterally loaded piles/piers, If you look at Bowles '74 (Analytical & Computer Methods in Foundation Engineering) and R.F. Scott's '81 (Foundation Analysis), Reese & Van Impe 2011 (Piles and Pile Groups under Lateral loading), they all took the Beam on Elastic Foundation equations and say we can apply layering with depth. I wouldn't argue with Bowles, Scott and Reese.

Ron- I think we answered the original question that a vertical load does not have any influence on the lateral capacity of the pile. Hokie66, there is no "it depends", the answer is no relation between vertical load and lateral capacity.

 

[abusementpark]

None. We assume the vertical loads are in the centroid so the only lateral loading is from shear and overturning moment.

Ok thanks. That is what I thought. However, sometimes the geotechnical engineer requests the corresponding vertical load on the pile or shaft. So, it got me thinking.

By the way, you get less deflection bu increasing width/diameter of the pile/pier than by increasing embedment depth.

I will keep that in mind. Thanks!

 

[FixedEarth]

Anytime abusementpark. Try to get Foundation Analysis by R.F. Scott for more insight.

A soils guy may ask for vertical load for one of three reasons:
1- To check that individual pier/pile settlement is less than 1/3 inch for example and then see if differential settlement over two adjacent piers/piles are acceptable (L/480).
2- To check the required embedment depth for a vertical and uplift load or
3- To check that required embedment for a combination of lateral/moment is not under that for axial loads.

For example, a 15 ft embedment may be needed for a 35 kip axial load but only 11 ft and 18" pier diameter may be needed for lateral deflection of 0.3 inch. Here is a current guide to drilled shafts:

http://www.fhwa.dot.gov/engineering/geotech/foundations/100521.cfm

 

[dik]

A compression load on any flexural member reduces it stiffness if linearly elastic. This loss of stiffness is marginally reduced further by the Ec reduced value at higher Fc.

Dik

 

[Ron]

dik...exactly...put some practicality back into this thread. Further, with any axially loaded member, the axial load and the lateral load must pass a unity check. One or the other might have to be reduced to achieve this.

 

[hokie66]

Yep. Piles have a geotechnical capacity and a structural capacity. The two aren't necessarily equivalent.

 

[dik]

Hokie... I would suggest that they converge at failure

Dik

 

[apsix]

The faint glimmer of hope in this thread is that the OP is a structural engineer and should be expected to realise that there is more than one failure mode possible.

 

[abusementpark]

The faint glimmer of hope in this thread is that the OP is a structural engineer and should be expected to realise that there is more than one failure mode possible.

Haha, yes. I'm definitely aware of all the structural failure modes. I should have clarified that I was really asking about the effect of the geotechnical capacity.