I was recently trying to model a BRB using SAP2000 by defining axial plastic hinges at either ends of the brace. But the problem that I am facing while doing pushover analysis is that the brace is buckling in compression and hence achieving much lower axial strength than what it is having in tension. I have also referred to the paper : Karimi, M. R. B., Yaghin, M. A. L., Nezhad, R. M., Sadeghi, V., & Aghabalaie, M. (2015). Seismic Behavior of Steel Structure with Buckling-Restrained Braces. World Academy of Science, Engineering and Technology, International Journal of Civil, Environmental, Structural, Construction and Architectural Engineering, 9(4), 488-496. But could not solve the issue. Can anyone help?
If you are performing nonlinear analysis with sap2000, you should uncheck the option "P-delta" or "P-delta plus large displacements" in analysis settings. In this way, second order effects are not considered in the entire model, including braces.
Differently, if you want to include buckling of other members, such as columns, you can modify the inertia moduli of the section assigned to the brace. In this way, elastic buckling is avoided and in non linear branch it will follow the rule of the plastic hinge.
I would suggest you to use nonlinear two node link element in SAP2000 since link element does not account for geometry nonlinearity. The newest version i think has the BRB hysteresis model included already.
As for the main frame column i usually assign out of plane notional load to incorporate buckling by activating the P-Delta effect based on AISC-LRFD DAM method.
Thank you everyone for your help.
I think I have found a solution to my problem. First I would like to clarify that I have been working with axial plastic hinges placed at either end of the brace. Also for the load case I had unchecked the P-Delta option so that it does not perform buckling analysis. But my fault lay in the fact that I had not specified the yield force (or stress) in the dialog box for defining the hinge, rather I had kept the box beside it checked, relying on the software to determine the yield force appropriately based on the material properties defined. While this worked just fine for the brace under tension, it curiously did not work for the one which is under compression. This seems to be an anomaly in the software. I have attached a screenshot herewith. After having specified this value it is working fine both ways.
It would be of immense help if someone can explain what was wrong in keeping the "Yield stress" or "Yield force" box checked instead of defining the value itself.
That´s what it happened to me, a soon as I put the yield force and the yield displacement the software threw me good results. That´s because the software automatically puts a values of a standard section property if you checked the dialog box.
But I would like to know, if I´m modeling correctly the BRB´s because i´m trying to found the correct values about the displacement control parameters, acceptance criteria when I define the hinge( BRB´s).
@ Sushovan Dutta Thanks for your information. Would you please let me know how to calculate 'Stress SF' and 'Strain SF' values in Sap?
Karimi M, Yaghin M, Nezhad R, Sadeghi V, Aghabalaie M. (2015). Seismic Behavior of Steel Structure with Buckling- Restrained Braces. International Journal of Civil and Environmental Engineering. 2015. 9(4), 503 - 511.
Article Seismic Behavior of Steel Structure with Buckling- Restrained Braces
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