Pile length in analysis model

[TewitC]

Hello Guys. I have a quick question here.

I have a question about pile lengths in analysis model in particular. I'm doing the substructure part of a bridge design. For the piles, I'm responsible for determining the reinforcement, but the pile length, anchoring, etc. are handled by a geotechnical engineer.

The procedure for our design is that the geotech first gives as soil spring, we do the model of the bridge and do the sizing of pier, pilecap and pile. And then we send the loads at top of piles to the geotech. Geotech does his calculation and comes up with pile length calculation. We put that into the report and that's it.

However, I have realized that the pile lengths calculated by the geotech are shorter than in what we put into the model. When we did the model we did not know the actual pile length so we put it long enough that the spring at the toe wouldn't active. But when I tried putting the "actual" pile length to the model, the results in piles changed considerably, up to 20-30% in the worst element.

I don't understand how this is happening as the only differences between the two models are the piles that are so deep that they wouldn't move at all.

 

[structSU10]

Do you input the pile as a element? Are they end bearing or side friction?

Modeling the pile as an element makes it a spring based on its axial stiffness and given length. End bearing piles could use their full length, skin friction may be their half length or something in a model. When using a spring changing its stiffness will result in different stresses / force distribution in many cases so what you describe is not surprising. This should be an iterative process and it may make sense to use a few spring constants that represent the pile during design to capture this effect and discuss with the geotech.

 

[TewitC]

Piles will be socketed into a rock layer. I agree with what you said and realize it is the vertical stiffness that is changing the result.


Previously when we modeled the piles with longer length, we put the tip support condition as pin. My understanding is that eventhough the tip is pinned, the axial deformation of the excessive part of the piles caused the pile behavior to be similar to that of pile sitting on spring. Now that the length is decreased, this behavior is gone and the pile elements become much more stiff.

Now my question is, in the new model with actual length, which the length decreased considerably from the previous model, does it make sense to put vertical spring instead of pin support?

 

[EZBuilding]

I agree with structSU10, it appears that you have modeled the pile as a frame element and then placed a spring at the end of the frame element - which becomes two springs in a row and changing the "net spring stiffness" of the pile. I think the soil spring provided by the geotechnical engineer should account for all aspects of the stiffness of the pile - I would model the springs as supports at the base of your pile caps.

I am also assuming that you had your pile cap laterally restrained - if not this is likely throwing off some of your results if you are using a lateral spring constant at the base of your pile frame element.

 

[TewitC]

EZBuilding

Thanks for the answer. I'm not sure if I understand the last part. Can you explain a bit more? And no I am not putting any external restraints at the pile cap, as I assumed soil at the level is fill. I do put soil spring along the pile elements.

 

[EZBuilding]

My background is in buildings - so please read my responses through that lens.

I am assuming that your piles are resisting laterals loads from your bridge. Accordingly if your lateral restraint is provided at the base of your pile frame element - there is an additional eccentricity being introduced into your analysis model. Within the pile arrangements if you have maintained similar relative stiffness, I find it odd that your analysis is getting such different results. This additional eccentricity could be a reason for that.

In my experience - soil springs are applied at the bottom of the pile caps not a the bottom of the pile. Axial and lateral spring stiffness's are provided based on this assumption.

It may be helpful if you provide a few sketches or snapshots of your analysis model.

 

[TewitC]

I don't have access to the model at the moment but I have drawn some sketch. Hope this helps.

Please note that the model is in 3D.

 

[EZBuilding]

I think it's important for you to have a conversation with your Geotechnical Engineer to make sure that your methods for applying the soil springs are consistent with their intention.

In the example of your sketch - if your base support is a pin and not a roller the lateral rigidity of it will significantly impact your lateral load distribution which could be impacting the axial loads in some of the piles.

For the consideration of the vertical support - the pile stiffness in your model would be a function of the axial stiffness of your frame element and the soil stiffness. The axial stiffness of the pile should be much stiffer than the soil stiffness unless this is a very very long pile. Based on your model I would expect that using a vertical spring that is equal to the soil stiffness would be the more appropriate solution.

 

[TewitC]

sorry for the confusion, the support at the base is vertical only, which makes it a roller.

Also, the piles are embedded into rock (sandstone), in this case, would the soil stiff still less than the piles'?

 

[centondollar]

If you reduce the length of the pile, it will obviously become stiffer: k = EA / L . If the geotech has told you that the pile is end-bearing, embedded into rock (which prevents movement in all directions) and minimal settlement (or none at all) will occur in the axial direction, then the pile should be supported on a pin. If the pile is driven several meters into hard soil (e.g., rock), it may also provide fixity against rotation and thus rotational stiffness, but evaluating that is difficult.

If the geotech has specified a vertical end-bearing spring resistance, it is uncommon, since such a parameter is not readily available, and vertical piling design for geotechnical end-bearing capacity is done based on rough estimates and verification by driving to refusal (maximum settlement for a given amount of blows in a row). If the pile utilizes friction, it too will be roughly estimated (using nomograms and hand-calculations, usually) and the end-bearing resistance will be a major part of the capacity in that case also.