Seismosoft | Earthquake Engineering Solutions

Hi!

I'm trying to model a simple 2D wall using beam-truss model. I modelize the columns at both ends of the wall by infrmDB elements while the horizontal beams, vertical columns and diagonals inside the wall are made of truss elements. The model should be available on the picture below.

[img][/https://ibb.co/YbhRzFc]

The problem is that when I apply an incremental force perpendicular to the wall, only the nodes of the infrmDB elements are moving while the nodes in the middle of the wall (surrounded by truss elements) don't move at all. When looking at the "Action Effects Diagrams", there is actually no axial force in any truss element, even if there are axial loads applied on the top nodes and a big displacement of the infrmDB elements.

NB: It may sound weird to apply an out-of-plane incremental load on a 2D wall, but it's just a basic model to understand the phenomenon as I got the same problems with U-shaped wall for its part submitted to out-of-plane displacement

Can someone help me to know what's going on and how to deal with it ? It would really help me a lot!!
Thank you,
Christophe

You can check the elongation dl of the truss elements in the post-processor. If there are indeed deformation, then the problem is with the stiffness (or the maximum capacity) of the truss element. If there are no deformations, then it has to do with the kinematics of your system and/or the connections of the elements.
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Thank your for your quick reply and helping me!

I checked, there is deformation in the truss elements. The linear relation F = EA/L * dl works until the yielding of the steel.

I also get the "zero diagonal term" error appearing. Is it the problem ? I suspect it's because in my case the truss elements don't bring flexural rigidity in the out-of-plane direction while I guess it's needed here.

Do you know if the "Equal DOF" constraint is the way to add flexural rigidity in the out-of-plane direction ?

Thanks again,
Christophe

The "zero diagonal term" means that you have a zero diagonal term in the stiffness matrix. It means that one or more DOFs are completely free and they are not connected to the structure. This usually (but not always) causes numerical instabilities to the model. Probably it means that some parts of your model and not correctly connected to the rest of the structure.

We cannot say if the "Equal DOF" constraint can provide the desired effect in your model. I suggest that you run an eigenvalue with the model; by checking the eigensolutions, you will get a better perspective of your model and the possible problems in it. In any case, your problem is a FE connectivity problem and this forum is about the use of our programs. It is probably something that you need to discuss with your supervisor.
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