Spring element to use for piled foundation?

[davejryan]

Hi guys, hoping I could get a bit of advise on something I'm stuck on at work.I've recently started using Ansys and have been thrown in the deep end!
The structure I’m modelling in Ansys is on a piled foundation, with each pile descending about 20m into the ground. I want to model these piles using spring elements with a stiffness K. I am unsure, what is the right element spring type to use as I wish to model the support provided by a combination of both the soil and the piles.
I have been using COMBIN14 and COMBIN40 elements. With the COMBIN14 element, I have modelled the support conditions at each pile location by creating, from a node at ground level, a vertical element 1m below the surface and 2 elements in the horizontal directions. These elements represent all 3 orthogonal directions I wish the stiffnesses to be in.
This seems to work fine, but I want to compare this COMBIN14 element to other element types and maybe use an element that’s more robust. Hence I’ve been trying to use the COMBIN40 element! .When I use this element type in a similar fashion as the COMBIN14 and analyze (modal – subspace - extracting 10 modes), it says my nodes i and j for element X are not coincident.
Does this mean I must create 3 nodes at the same location and then create the 3 elements at this same location, between these nodes, specifying the direction/ dof I wish each element to act?
I’m basically unsure how to use the COMBIN40 element type, what are the benefits of one over the other?
I would appreciate any help/advise you could give me and I hope I havn’t completely confused you!

 

[Drej]

I wouldn't worry about the warning regarding coincident nodes, just check the keyopts you've stated for the element.

I'm not sure that your pile can be better described by the combin40, hence for what you've described I think combin14 is fine. What would concern me is the shear resistance given by the pile. Is this important? How is this modelled currently? Is it captured by a horizontal combin element? I'd be interested to know how you obtained the values of stiffness for the horizontal/vertical springs. To be honest, ANSYS isn't really built to cope with a soil/pile interaction in the classical sense, although it does offer some soil models (Mohr-Coulomb for example). I don't think it will model the interaction between the pile/soil either (only a fully bonded condition), usually carried out by interface elements. However, I think you might be able to use the Mohr-Coulomb model with some embedded line elements to model at least this fully bonded condition.


------------
See faq569-1083 for details on how to make best use of Eng-Tips.com

 

[davejryan]

Drej,
Thanks very much for your help, it was greatly appreciated.I managed to get both my Combin14 and Combin40 models to work. With my Combin40 model, I had entered the wrong keyopt settings as you said, which solved my coincident nodes problem, but had also fully fixed the end node on each support rather than the spring elements, which gave some strange deflected shapes. At the moment we are modelling a very simple structure to iron out some of the problems we may encounter with our actual,larger model.You are right to be concerned with shear resistance. This will be important in our structure and is currently modelled using a horizontal Combin14/40 elements. K values in our simple model are from previous work done in the past.The actual values used will be determined from on site tests of concrete piles. These results will give us(we hope) a value for K which is a combination of pile and ground stiffness and will be the K value for our springs.From what I've been reading Ansys doesn't seem to model soil/pile stiffness very well, but I will look into using the Mohr-Coulomb model anyway and see what happens!
Thanks again

 

[Drej]

Thanks for the feedback, Dave. Best of luck.


------------
See faq569-1083 for details on how to make best use of Eng-Tips.com

 

[gwolf]

Davejryan,

What you are trying to model is very complex. I am not an expert myself but I have seen papers given on this sort of thing. From this I remember that the shear interaction betwen soil and pile is very strong. If anything, the pile (as long as it isn't earthed to bedrock) acts as a rigid post on a soil shear spring.

I think that you will have to look up specialist literature on this - I have no references.

Good luck.