Plastic Rots higher than Chord Rots of Beams after Gravity

[hyperion]

Hello Everybody,

Title states my problem; at the end of gravity analysis, and at the onset of dynamic analysis, my beams loaded with uniform distributed loads register way too high plastic rotations than chord rotations which is physically impossible.

It is a 3-D model of an 8Story RC structure. All elements are defined as forceBeamColumn with HingeRadau option. Fiber sections at columns and no problems. As i employ rigid diaphragms I am forced to use Moment - Curvature relationships at the hinge ends of beams. The only nonlinearity in beams is very simple bilinear (hysteretic material) Mz-Curvature relationships assigned at end sections of the HingeRadau Model.

When I remove gravity loading or do not perform a priori gravity analysis, everything is fine. Chord and Plastic rotation histories during dynamic analysis are absolutely sensible. But slightest uniform load on beams result in this situation. Funnily enough, if I assign fully elastic sections to hinge regions and intermediate regions of all forcebeamcolumn elements (basically an elastic model) I still get plastic rotations! Yet they are still higher than beam chord rotations.

From my understanding by digging into source code, plastic rotations of forcebeamcolumn element are calculated by using the last committed element stiffness matrix and force matrix and by subtracting the multiplication (flexibility matrix, i mean) of these from the total chord rotations. I have previously read posts by developers that state this method is a simplistic way of calculating the plastic deformations. Is this the cause of my strange problem? Can multiplication of the increasing element end forces and the virtually unchanged flexibility matrix and subtraction of those from the actual rotations yield this behaviour? If this is the case, is there a reliable way of calculating plastic deformations in forceBeamColumn elements?

Or am i missing a very fundamental thing here? I am completely baffled about the situation. Any help would be greatly appreciated.

Developers and users of OpenSees, thank you very much in advance.

[mhscott]

Can you run the same simple analyses for a 2D model and report back here if the same behaviors are observed?

[hyperion]

Thank you Dr. Scott for your reply and sorry for the long delay,

I have finally been able to model individual frames from my full 3D Structure in a 2D analysis environment. I have conducted both gravity and pushover analysis and not encountered such a problem that I described. In the planar model that utilizes 3 DOF, plastic rotations are normal, i.e. they are negligible at the end of gravity analysis and completely sensible (less than total rotations) at the end of the pushover analysis.

Seeing this outcome, I suspected from the rigid diaphragms in my 3D model as the cause of the problem. I removed the diagrams and conducted a gravity analysis but the issue of registering way higher plastic rotations than chord rotations persisted in that case. Therefore I think rigid diaphragm is not the cause of this phenomenon.

I also inspected whether I assign correct force deformation relationships and section properties to the elements and I am fairly sure that the analytically modeled structure is in accordance with the designed one in terms of section properties, etc.

Now I could isolate the problem to the 3D model rather than the planar one. But that does not resolve the issue since I have to work on the 3D model.

Thank you very much for your help, I would be grateful for further suggestions/solutions.

[aminamini]

Hi everybody!

I have the same problem in my project.
can anybody help us?
thank you in advance!