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Pseudo static acceleration and model size

Pseudo static acceleration and model size

Pseudo static acceleration and model size

When using numerical modeling software such as Plaxis 2D to model slope stability or a tall retaining wall with seismic loads, a quick method is to use the pseudo static acceleration (as opposed to several time histories and full dynamic analysis).
However, the stability and/or back of wall force results are dependent on the model size (horizontal X axis dimension).
If the model is very large dimensionally, the lateral inertia forces tend to accumulate artificially high at the wall face. I couldn't find any guidance from Plaxis on the Knowledge Base.
Is there a rational way of sizing the X and Y dimensions of the model so the inertia loads are in proportion to what a limit equilibrium model would show? Or is there a way to apply the acceleration to a portion of the soil clusters but not other clusters?
Yes, I realize I can simply run a Slide analysis, but limit equilibrium provides no real displacement or soil state information.

RE: Pseudo static acceleration and model size


I don't think there is a guidance, but do you think that force on wall incrase as you incrase X? Linearly? It seems impossible, if you want I can work on a parametric analysis for an embedded wall. Would not take too much time. Increasing X and checking the moment and shear on the wall.

RE: Pseudo static acceleration and model size

Yes, increasing the horizontal dimension X behind the wall increases the lateral force on a wall when using a pseudo-static acceleration. That is, applying a horizontal acceleration at the base boundary.
Not linearly, as you will see from a parametric study. There is a certain range of sensitivity. The insensitive range (very long) is not the correct range.
My experience and preference is Plaxis 2D, but I am certainly interested in the results of any nonlinear FEA program.
Not the case for time history.
Think about a pan full of not-so-rigid Jello, 1 inch thick. Then think about a pan that is 1/3 full of Jello, also 1 inch thick.
Now take both pans, and rotate them 90 degrees about the horizontal axis, so the Jello is now vertical, being sheared by gravity. More Jello in the pan = more force parallel with gravity. So it is with soil.[If you mother made very stiff Jello, this visual does not work so well. My mom made semi-fluid Jello that would drip in this experiment.]
I am not inferring this is reality. It is certainly not, of course. Pseudo-static is a crude approximate for structural analysis, and very crude for soil-structure interaction, but we use it because it is quick and relatively easy and somewhat intuitive. Just trying to bound this error inherent in the uniform, perfectly coherent acceleration simplification.

RE: Pseudo static acceleration and model size

Oh. If you are right, we are in big trouble.

As you say, pseudo-static analysis is really easy. I use it mostly on excavations and slopes. However, if this is the case, we have really a problem.

I get your jello example. However, I would expect dissipation of lateral force through soil into the boundaries. I will perform a simple parametric analysis to see for myself, in 2-3 days.

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