Inelastic bar buckling models implemented in OpenSees

Modeling of bars’ buckling is fundamental in the simulation of the flexural response up to collapse of RC members under seismic loading. When bar buckling occurs, indeed, strength and stiffness degradation starts to significantly increase, limiting the deformation capacity of the member. For typical members’ dimensions and layout of the reinforcement, buckling occurs at or after yielding. For these reasons, particular attention must, therefore, be given to the stress-strain model used to represent the behavior of reinforcing steel bars. The present work focuses on the models developed by Gomes-Appleton and Dhakal-Maekawa for inelastic buckling, which are among the most widely used ones and are those implemented in OpenSees. Assumptions adopted by the two models on initiation of buckling, reloading stiffness from post buckling compression state, stiffness of lateral reinforcement and interaction between the bar and concrete are discussed. Modifications introduced in the OpenSees implementations of the models are also reported. To evaluate the models, it is necessary to compare the analysis results with experimental responses. The experimental dataset is expanded with the high-fidelity numerical tests to consider non-standard loading protocols.