SAFE foundation design - Soil set to compression only, still gives uplift 4

[missstructures]

I have the soil properties set to "Compression Only" and all cases as linear/static - why am I still getting tension in the soil pressures?

 

[BAretired]

Because either your program doesn't understand your command or you don't understand your program's output.

BA

 

[r13]

Double check your input for foundation type, make sure the "compression only" statement is there.

 

[JLNJ]

There is an error in your modeling.

 

[jimstructures]

missstructures,

Do You have any uplift on your structure?

Do You have any horizontal load above the top of the pad applied to your structure?

Either of these condition may apply an eccentric load to your foundation.

Where is the centroid of the reaction relative to the centroid of the load?

Jim

 

[missstructures]

I know there is eccentricity and expected the initial tension - I am asking why SAFE calculates it if it's supposed to be set to "compression only" - why doesn't it just say ZERO? It's a program question more than a design question.

 

[PROYECTOR]

missstructures,

You must perform a nonlinear analysis. "Compression Only" is a contact type nonlinearity, so it does not work with linear analysis.

 

[StructEngBrah]

PROYECTOR is correct, you need to run a nonlinear analysis. I haven't used SAFE, but other CSi products will default to using the compression only stiffness as a tension/compression stiffness in linear analyses.

 

[HTURKAK]

If you want to run linear analysis, delete the tension springs and run again.. after running a few times, eventually you will obtain all springs compression (provided that the foundation is stable ).

This thread reminded to me an old saying regarding the computer

 

[r13]

You must perform a nonlinear analysis. "Compression Only" is a contact type nonlinearity, so it does not work with linear analysis.

Is that the SAFE only feature or generic throughout the analytical programs? Then what I did (using other programs) was wrong then.

 

[PROYECTOR]

Is that the SAFE only feature or generic throughout the analytical programs? Then what I did (using other programs) was wrong then.

That's the way SAFE (and most CSI software's) works. But regardless of the software, whenever you want to incorporate nonlinearities into a structural analysis model (eg, material plasticity, tension only braces, second-order effects, cables, etc.), the type of analysis performed should be able to capture that nonlinear behavior. This cannot be done with a linear analysis. Some software's, however, perform nonlinear analysis by default when there is any nonlinearity defined in the model. For example, STAAD.PRO does this when you define compression only springs and use the "Repeat Load" command.

 

[r13]

PROYECTOR,

Thanks for the clarification, I am feeling better now. However, I don't recall that STAAFD addressed that issue, at least in the early releases.

 

[missstructures]

Thank you - SAFE does not perform the nonlinear iteration by default - you have to find the combinations causing the uplift, then convert them into non-linear load cases. Then it iterates and converges. It has to be a combined load case. Not the most straight forward but I think I figured it out.

 

[radiocontrolhead]

I have the soil properties set to "Compression Only" and all cases as linear/static - why am I still getting tension in the soil pressures?

your issue above in bold

change it to non linear static,

be sure the model is stable in the plane direction by either providing restraints or checking the analysis box to limit degrees of freedom.