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Thanks a lot fmk. Indeed, I don't need to work on the local stiffness matrix but only global one. Specifically for Bouc-Wen model, I guess what I need to do is like this: write down the global stiffness matrix first, then modify it by multiplying *alpha_1* (Bouc-Wen model parameter corresponding wit...

[quote="AURELIO"]Help! I need to model a structural frame element, instead of SDOF or shear beam, using Bouc-Wen model. It means I need to couple material nonlinearity (Bouc-Wen model) into local stiffness element of all the frame members and assemble them. I have problem to write the local stiffnes...