I really need help with Pinching4

[omalvarez]

Hi all, Nowadays I am working at my thesis (damage index of bridge piers jacketing with reinforced concrete). I want to use Pinching4 material to model reinforced concrete elements' hysteresis loop, but I have a problem I do not know the material parameters. I know that there are a lot of papers which they are explained, but I need a more detailed explanation. For example How are calculated the unload-reload path parameters? On bridge structures What are the ideal range of the unload-reload path parameters or the damage rules range? How can I get a backbone response curve? Can backbone curve be a relation moment-rotation? I have been more than one month trying, but my material model is wrong. Really I am desperate because I have not carried on my work. If somebody has a model that can send me I would be very grateful (I already check the manual example) or somebody else may orient about other material or hysteretic model that incorporate strength and stiffness deterioration for reinforced concrete elements.
I hope some one can help me. Thank you

[omalvarez]

Ok. In order to understand the behavior of the pinching4 material I built a plain frame model whose columns are beamWithHinges elements and the beam is an elasticBeamColumn. First I run a gravity analysis and eigen analysis and I think my model is correct. After I run a dynamic analysis (section aggregator [like example 2 at OpenSEES wiki] at columns hinges) and I obtained a good hysteretic loop and displacement history. After I return to run a dynamic analysis but I assign pinching4 material at columns hinges. I got the backbone curve (moment-rotation) from a calibrated pushover analysis at SAP2000 and I got the others parameters from OpenSEES wiki example http://opensees.berkeley.edu/wiki/index ... ialExample, but both the displacement history and hysteretic loop do not have form. I really need help because I have read some papers but I need a more detailed information.
Some questions: the procedure to obtain the backbone curve is ok? How can I calculate the unload-reload path and damage rules parameters? On bridge structures (bridges columns or circular columns) what are the ideal range of the unload-reload path parameters and the damage rules range? Have I to use a BeamColumnJoint element and assign it pinching4 material or can I use beamWithHinges element like I did it? Have I to model bar-slip response, joint shear response and interface-shear response?
Please help me, I have not could carry on my thesis work

[brag006]

If you want to understand a material model, build a very small example with 4 nodes connected by zerolength elements. Fix the nodes in all directions except the loading direction. Then apply cyclic displacement and plot the results. Then change the different parameters and see what happens to the hysteresis loop. You will quickly figure out the purpose of each parameter.

Calculating the appropriate input parameters requires you to understand the theory and I don't think you will find much help here on that. You just need to read papers and any other theses on the topic.