# Discussion Forum

## Need help setting up a model

 Topics: geotechnical, 4.2a, Structural Mechanics
RSS feed   |   Turn on email notifications   |   3 Replies   Last post: October 10, 2012 7:54pm UTC

Byron

October 8, 2012 4:36pm UTC

Need help setting up a model

I'm having a hard time figuring out how I would do this: I need to model a static load being applied to a deep foundation (steel pile, concrete pile, etc.) in 3D, but I'm having some problems. For simplicity, I am just using two cylinders with the inside cylinder being the shaft and the surrounding cylinder representing the ground. The problem arises when I run the first step of the model, because the soil and shaft stiffnesses are not the same, so the ground settles (deforms) and it imposes stress on the shaft. What should happen, just like in real life, is that the ground would have settled, and then the hole would be excavated and the shaft installed. What I need to do is have a step where the ground settle from self-weight, and then a second step where the shaft is "installed". If I could switch materials during model steps, that would work because I could assume the shaft is made of soil in step 1, then switch it to concrete/steel during step 2, but I don't think this is possible. Another option would be to change the height of the shaft in the 2nd step by the amount of settlement so that the strain, and therefore the stress, in the shaft is zero in the second step, but I don't know if this is possible either. Does anyone know a straightforward way of accomplishing what I'm trying to do here? I can't seem to get it to work!

thanks in advance

Nagi Elabbasi
Certified Consultant
Veryst Engineering

October 8, 2012 6:57pm UTC in response to Byron

Re: Need help setting up a model

Hi Byron,

Trying adding initial stresses to the model of the form “Stress_z=Acc_Gravity*density*z” where z is the direction of gravity. The deformation resulting from that initial stress will balance the deformation due to gravity. For a uniform column with a linearly increasing stress field due to gravity this initial stress can get your displacement back down to zero. Having an initial condition like that makes sense (and is not just a numerical trick) because you are trying to model the foundation in its deformed state rather than its unknown stress-free state.

Nagi Elabbasi
Veryst Engineering

Byron

October 9, 2012 4:20pm UTC in response to Nagi Elabbasi

Re: Need help setting up a model

Hi Nagi,

Thanks for the reply. I tried your suggestion, but it just zeros out the vertical stress, assuming I'm doing it correctly. I need the vertical stress to be present because it affects the horizontal stress, which I need to be present in the model.

Here is another thought I had... Can't I just initialize the stress in the soil domain instead of using a body load on the soil domain to come up with the vertical and horizontal stresses throughout the depth? I tried this by initializing the stress as a function of depth as solid.rho*g_const*Z for the vertical stress (solid.sz) and as 0.5*solid.rho*g_const for the horizontal stress (solid.sx and solid.sy) but when I check the stress in the model it says the stresses are ~ zero. Shouldn't it be possible to initialize the in-situ stresses this way? If I can do this it solves my problem! Thanks!

Byron

October 10, 2012 7:54pm UTC in response to Byron

Re: Need help setting up a model

Nagi,

Nevermind my last post.... I reread your post and realized I didn't do quite what you suggested. I fixed the model and now it works! Thanks very much for your suggestion.

Byron

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