Wrong Axial diagram

Hello! In this section we'll talk about the post processing module and the data visualization.
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GianmarcoManzo
Posts: 19
Joined: Tue Sep 17, 2019 2:38 pm

Wrong Axial diagram

Post by GianmarcoManzo » Tue Nov 12, 2019 7:43 pm

Hi,
I have a problem with an output. I have modelled the structure in photo but I don't understand why the axial force is wrong. It is as if the element was divided into 4 sub-elements (mesh edge by division = 4) and each of them was loaded with a different force. And a non-constant butterfly axial diagram comes out. Can you help me?
1.PNG
1.PNG (117.52 KiB) Viewed 928 times

MassimoPetracca
Posts: 37
Joined: Wed Jul 24, 2019 11:10 am

Re: Wrong Axial diagram

Post by MassimoPetracca » Wed Nov 13, 2019 9:29 am

Hi Gianmarco,
The results are actually right. You see, in each element, the same axial force but with opposite sign at the 2 end nodes. This is because you are visualizing the "force" result. "force" contains the element forces at nodes in global coordinates.

What you want to visualize is either 1) "basicForce" or 2) "section.force".
Be aware however that in OpenSEES "basicForce" is an output given by almost all beam elements in 2D, but in 3D only by the ElasticBeam!
While "section.force" works with all beam elements that have sections, both in 2D and 3D. This means that only the ElasticBeam does not support it.

If you still have problems send us your .SCD file and we will correct it for you

Massimo

GianmarcoManzo
Posts: 19
Joined: Tue Sep 17, 2019 2:38 pm

Re: Wrong Axial diagram

Post by GianmarcoManzo » Wed Nov 13, 2019 5:40 pm

Ok perfect. In 3D can I use also GlobalForces for ForcedBeamElement?
Another question: I have modelled Ipe550 section with STKO triangular fiber and I got a different result compared to the case in which the same section has modelled with quad patch model (writing manually section.tcl for opensees). Why? Is It correct?

Can I send different input file to you by mail? Thanks

MassimoPetracca
Posts: 37
Joined: Wed Jul 24, 2019 11:10 am

Re: Wrong Axial diagram

Post by MassimoPetracca » Wed Nov 13, 2019 6:35 pm

Yes of course, reply with your files attached

MassimoPetracca
Posts: 37
Joined: Wed Jul 24, 2019 11:10 am

Re: Wrong Axial diagram

Post by MassimoPetracca » Thu Nov 14, 2019 11:54 am

Dear Gianmarco,

After debugging the attached files, I've seen 2 errors that lead to the mismatch between the analysis with QUAD fibers and the analysis with TRIA fibers (generated by STKO).

The difference is not due to the different fiber discretization, but to the following inconsistencies:
  • The file with QUAD fibers has the cross section oriented in the opposite direction, just rotate them by 90°
  • The file with TRIA fibers has a mistake in the dimension of the "ipe300" section, while the correct dimensions are given for the QUAD file.
After correcting these two errors you will see a perfect match:
mismatch fibers.png
mismatch fibers.png (49.18 KiB) Viewed 908 times

GianmarcoManzo
Posts: 19
Joined: Tue Sep 17, 2019 2:38 pm

Re: Wrong Axial diagram

Post by GianmarcoManzo » Fri Nov 15, 2019 10:15 am

Hi Massimo,
I have rotated QUAD section and I have fixed TRIA section in STKO. Moreover I have modelled the same structure in SeismoStruct for compare results. I have also changed the trasformation from Linear to P-DELTA. But the results don't perfect match:
Cattura.PNG
Cattura.PNG (60.11 KiB) Viewed 899 times
I have sent again the attachment in PM. Thanks.

MassimoPetracca
Posts: 37
Joined: Wed Jul 24, 2019 11:10 am

Re: Wrong Axial diagram

Post by MassimoPetracca » Tue Nov 19, 2019 12:35 pm

Hi Gianmarco,

I found the problem. It is purely numerical. Your mismatch with Seismostruct happens in the elastic range, and this was very strange to me... But after some checks, I found that the use of the norm of displacement increment test with a large time step is causing the acceptance of a solution which is quite far from the equilibrium path in the elastic range. You were using a large tolerance (1.0e-4) but I even tried 1.0e-9 and nothing changed. This probably means your models has a stiff system.

Do the following:
  • Use NormUnbalance test (tol = 1.0e-5, iter = 100)
  • Change from Penalty to Transformation (Penalty does not work well with NormUnbalance)
  • With Newton or NewtonWithLineSearch it takes about 20 iteration each step, try KrylovNewton. It is the one that gives the best convergence in this problem (about 5 iterations)
Note 1: In this case use can safely use Transformation instead of Penalty as constraint handler because there are no strange or circular dependencies in the constrained nodes. I understand that in some complex models the relation between constrained nodes can be quite cumbersome, and so the transformation may not work.
In those cases you are forced to use Penalty, and thus a displacement test (the unbalance cannot be used with penalty). When using a displacement test with this kind of problem, use smaller time steps.

Note 2: With this new configuration, Linear or PDelta transformation give almost the same result (displacement are not so large), with PDelta showing just a lower strength in the nonlinear range. However I noticed something interesting. When I tried Corotational transformation (which is the most general and complete kind of nonlinear kinematics in OpenSEES for beams), the structure was twisting about the Z-axis. This happend because of the triangular discretization we use in the fiber section, because with those few fibers, fiber where distributed in a not-so-perfectly symmetric way, thus causing the twisting of the structure when corotational method was used. This did not happen with the quad mesh, which is instead symmetric. So we are currently implementing the quad fiber discretization in STKO, at least for geometries that allows it.

Hope this helps

Massimo

GianmarcoManzo
Posts: 19
Joined: Tue Sep 17, 2019 2:38 pm

Re: Wrong Axial diagram

Post by GianmarcoManzo » Mon Nov 25, 2019 10:07 am

The results between triangular fiber and quad fiber modeling are now the same. But unfortunately they are not coincident in the non-linear phase with seismostruct. I attach a photo
Cattura.PNG
Cattura.PNG (73.28 KiB) Viewed 797 times

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