Energetic considerations on the effects of inelastic stiffness on nonlinear seismic response

In this paper, the effects of post-yielding stiffness on the response of non-linear structural systems subject to seismic actions are investigated. For this purpose, two hysteretic models are analyzed and then compared. The first is a bilinear plastic model, representing structures that exhibit strain hardening characteristics. The second is the elastic-perfectly-plastic model, that, even if does not simulate post-elastic stiffness effects, is widely used in design codes to predict conservative structural response. For these models, a parametrical study is carried out to assess the influence of post-yielding stiffness ratio on some non-linear response quantities, such as the response modification factors, the ductility demand and the damage index. The analysis is developed on a SDOF system subject to El Centro earthquake confirm the presence of a postyielding stiffness sensitive region of vibration period, and demonstrates that the elastic-plastic constitutive law is excessively conservative to evaluate structural deformations. The study suggests an understanding of the results also from an energetic point of view, considering the energy dissipated by systems under investigation. Key-Words: Post-yield stiffness ratio, Inelastic Displacement Ratio, Strength Reduction Factor, Dissipated energy, Damage Functional