Shallow foundation in 3D

[Humam]

Hey There
I intended to model a shallow 3D foundation in STKO. I have question about how to create and assign the Q-Z springs to tie the foundation with the soil? I have attached a picture for a shallow foundation in 2D just because I don't have one in 3D. do I have to create all of those springs individually or is there a way to create all of them by one or two commands?

[STKO Team]

dear User,
in order to create Q-z spring we suggest to define a zero lenght element.
Firs of all, geometry for fondation has to be define. It can be both a beam or a shell element. Then it is necessary to create another geometry, equal to the first one and in the same position, that will define the nodes of the soil. Do not assign any physical or element property to this second geometry, since it will be used only for generating nodes (not elements). We suggest to use the same global seed on both the geometries.
It is necessary to apply external boundary on the second geometry (the one without properties applied).
Second of all you need to create an interaction between these 2 geometries. Click on the button "New" in the interaction tab and select Node to Node Link on the interaction type list.
Then create a Qz uniaxial material and a zeroLength material with the Qz uniaxial material in the proper direction (typically Z if you are in 3D). In the Qz uniaxial material, insert material parameter per unit length.
Finally, create a zeroLength element, using the "distributed" option. In this way, the material parameters of the Qz material (per unit length) will be lumped automatically by STKO based on the mesh size.

We hope to be helpfull, but if you had any problems, please share your file with us.

Have a nice day.

[STKO Team]

Dear user, attached you can find a modified version of your file.

shallow footing.zip

(112.73 KiB) Downloaded 49 times

The main changes are:

• I created a copy of the foundation geometry, without any property or element assignment, just to generate base nodes for fix. For the sake of clarity I moved it at a finite distance from the foundation geometry. This will generate a warning because the zeroLength expects a distance = 0 between the 2 nodes. However this does not affect the results.
• then I merged the column and foundation geometries into a single one.

  

  copy_and_merge.png (134.68 KiB) Viewed 1125 times

• The I created an interaction between foundation and base edges (only EDGES!!)
• The zeroLength element property in this case should have the "distributed" option checked. This is a STKO feature. In this way, the material property defined per unit length, will be automatically lumped by STKO based on the mesh-size.

  

  qzinter.png (308.14 KiB) Viewed 1125 times

• I also made the beams corotational, so that you can see both p-delta and finite rotation effects.
• The constraint handler is now Penalty. In this model you will have many kinematic constraints, and the transformation method can fail.
• Vertical and horizontal load were defined into a single load condition. I split them in 2 coditions, then I applied the vertical load for the first gravity analysis, and the horizontal load after, for the second displacement control horizontal analsyis.
• I also changed the material parameters for the stiff elastic beam section you used for the foundation. They were 1.0e10, but it gives convergence problems, since this value is too much compared to the real stiffness of your model, and it makes the system ill-conditioned. I used 1.0e6. Keep in mind that 2-3 order of magniture more than the others materials is fine.

[Humam]

Thank you for explanation it was very helpful. one more question please. can explain how to use a shell element instead of these grid of elastic beam column elements?
Best,

[STKO Team]

The concept is exactly the same.

• instead of this beam-grid, create a surface for the foundation.
• then create a copy for the base fixity
• only the first one should have a physical property (ElasticMembranePlateSection), and an Element property (ShellMITC4). The copy is only used to generate base nodes to fix.
• create an interaction as I did for the beam-grid, but this time selecting faces.
• now the material parameters in the QZ uniaxial model should be per unit-area
• the rest is the same

shallow footing shell.zip

(112.34 KiB) Downloaded 48 times

[Humam]

Dear STKO team
I tried to imitate what you did in shell model for isolated foundation, but it didn't work. Can you please check my attached file.
Best regards.

[STKO Team]

Dear User,
the model seems just fine. Can you explain with more detail and/or screenshot what problem it has?
We will happy to help you.

Have a nice day.

[Humam]

Dear STKO team
I attached a screen shot of what I got when I run the program.
Best regards

[STKO Team]

Dear User,
the model runs correctly.
To see the results, you can open the database (.mpco) in the Post Processor.
Have a nice day