Question about Nonlinear Modeling of RC Frames (Lumped Plasticity)

[luckypants]

Dear STKO Team：

I try to build a column with plastic hinges (Lumped Plasticity).

I use elastic beam column and zerolength element to build my model, and apply Elastic Uniaxial Material on zerolength element.

But there is an error when I performed the analysis. The picture is the error message and the following is my model.

How could I solve the problem?

Any advices and helps that you provide are highly appreciated.

Best Regards

[STKO Team]

Dear user, that error means that your model is singular.
It happens because in the definition of the zeroLength material, you only assigned a uniaxial material in some of the 6 DOFs. This means that the other DOFs are released. If you want to create continuity in the other DOFs you need to put a stiff uniaxial material in those DOFs.

[luckypants]

Dear STKO Team：

Thank you very much for helping me.

I have solved the problem, but there are two questions I want to ask.

1. After I watched E-Learning Courses(Learning Opensees: Nonlinear Modeling of RC Frames (Lumped Plasticity)), I confused about why they set plastic hinges on direction 6 of zerolength element (Mz). Because Mz is torsion moment.

I think plastic hinges should be set on 5 or 6 direction(Mx or My), because there is bending moment on that direction.

Likewise, we set plastic hinges in Mx or My in ETABS and SAP2000. It is diffrent in OpenSees. Why they set in direction 6(Mz)?

2. We can set zerolength element in two ways.

Case 1: manual (2 points and 1 line)

Case 2: automatic with the HingedBeam model in STKO

We should use equaldof command in Case 1 to create continuity, but we should use a stiff uniaxial material in Case 2.

Is there any difference between these two ways? If their results will be different?

Best Regards

[Clarabella]

Dear Luckypants,

1. After I watched E-Learning Courses(Learning Opensees: Nonlinear Modeling of RC Frames (Lumped Plasticity)), I confused about why they set plastic hinges on direction 6 of zerolength element (Mz). Because Mz is torsion moment.

I think plastic hinges should be set on 5 or 6 direction(Mx or My), because there is bending moment on that direction.

Likewise, we set plastic hinges in Mx or My in ETABS and SAP2000. It is diffrent in OpenSees. Why they set in direction 6(Mz)?


First of all, OpenSees lumped plastic hinges are more general than those of common commercial programs. You can define a plastic behavior for each degrees of freedom including torsion.
Said that the behavior depends on the local axes defined. However, in STKO/OpenSees X local axis is always along the longitudinal deirection (therefore torsion Mx), and y and z directions depend on how you defined your local axes


2. We can set zerolength element in two ways.

Case 1: manual (2 points and 1 line)

Case 2: automatic with the HingedBeam model in STKO

We should use equaldof command in Case 1 to create continuity, but we should use a stiff uniaxial material in Case 2.

Is there any difference between these two ways? If their results will be different?

you are correct. In case 2 you have a larger number of degrees of freedom, however if you use a reasonable number to assign the stiffness of the rigid material, the results will be the same.

Regards

[luckypants]

Dear Clarabella：

Thank you very much .

Best Regards