How to consider the rotational mass in linear elastic static analysis?

[luckypants]

Dear STKO Team：

After I discussed with my advisor, I changed the method of comparing ETABS to STKO.

I tried to build the model by the same way in ETABS and STKO.

The following was the way I built the model in ETABS.

First, I made all the mass of material to be zero. (Figure 1) Then, I only assinged the translational masses in each node. (Figure 2) So I could confirm I built the model by the same way in STKO.

Then I input the same ground motion in X and Y direction. Finally, the result was only different in X direction.

Therefore, I guessed the rotational may influence the result and the rotational effect was not obvious in STKO.

If I input the wrong data, I thought the result would not be same in Y direction.

What do you think about it?

Any advices and helps that you provide are highly appreciated.

Best Regards

[STKO Team]

After I discussed with my advisor, I changed the method of comparing ETABS to STKO.
I tried to build the model by the same way in ETABS and STKO.
The following was the way I built the model in ETABS.

Of course they should be built in the exact same way, otherwise the comparison won't be fair.

Therefore, I guessed the rotational may influence the result and the rotational effect was not obvious in STKO.

I'm not sure about this. I you refer to the rotational partecipating mass you see in the output of the modal analysis, remember that those values are only coming from a post-processing, they do not influence at all the eigenvalue problem.

I'm pretty sure there is some difference somewhere else. Could you share your *SCD file here (zipped) ? We do not use ETABS so we cannot run your EBATS model. If you can convert it to SAP2000 send also that one, otherwise send us a report of all the details of your modeling in ETABS

[luckypants]

Dear STKO Team：

Thank you for your replay.

I put my *SCD file and user report of ETABS here.

Best regards

[STKO Team]

OK. We wil be busy in the next three days but we will have a look as soon as possible

[luckypants]

Thank you very much.

[AcetoDivino]

Hi luckypants,

I'm trying to compare your two models. I'll tell you a few observations that came to my mind:

- The two models have quite different fundamental periods. The masses are the same, so the problem should be on the stiffness side.

- I saw from the ETABS report that you assigned a "Shell-Thin" while in STKO a "ShellMITC4". Since the shells are in the Y direction, this could be the problem (out of plane). I suggest you repeat the analysis making sure they have the same formulation (you discretized the shell on STKO with a single element, typically they split more) or brutally removing the shells in both models to make sure that is the problem.

- I have never used ETABS, but in Sap2000 the default one-dimensional elements are "frames", so with shear deformability. To relate it back to a "beam" in SAP you impose = 0 shear area multipliers in the frames section.

I suggest you use this procedure https://wiki.csiamerica.com/pages/viewp ... d=15074050 to export from ETABS to SAP2000.

I hope I was able to help you.

[luckypants]

Dear AcetoDivino：

Thank you for your advices.

I can't convert it to SAP2000, because I don't have SAP2000. (The user must have both ETABS and SAP2000 installed on the same system)

To simplify the comparison, I deleted the shear wall. Then, I did the analysis again and compare ETABS to STKO.

There were some results I found.

1. The two models had almost same frequency (1.07 Hz), so the models was almost same.

2. The results of two models still be different with roof displacement.

Here is my STKO model. What do you think about it?

Any advices and helps that you provide are highly appreciated.

Best Regards

[AcetoDivino]

Hi luckypants,

having removed the walls, the only thing that differentiates the 2 models now is the floor diaphragms, try removing the membranes as well in ETABS.

Can you send me the ETABS output of the time vs displacement? That way I can do some testing with STKO.

Regard

[prasannahannur]

How can we define zero-length elements in SAP2000?

[Clarabella]

Please check the manual of SAP2000