Nonlinear damping and nonlinear responses of recycled aggregate concrete frames under earthquake loading

The recorded data of a shaking table test on a recycled aggregate concrete (RAC) frame model conducted by the authors is used to analyze the nonlinear behaviors of the RAC structure under earthquake loadings and white noise excitations. Firstly, the nonlinear damping behavior of the RAC model with damage accumulation is studied. A damping model that combines a viscous damping and a quadratic damping mechanism is extended to consider ambient vibrations and to describe the nonlinearity of the damping properties as well as to detect the damage of the RAC frame model under white noise excitations. The quadratic damping factor 8 introduced is a reliable damage indicator with a promising damage-sensitive feature in the health monitoring for RAC structures. The percentage of the energy dissipated by the quadratic damping mechanism to total energy dissipation increases from 5.7% to 98.4% with damage accumulation, indicating that the energy dissipation mechanism changes from a viscous damping mechanism to a nonlinear damping mechanism. Secondly, a data-driven parameter identification method based on Hilbert-Huang transform (HHT) is proposed and conducted to analyze the nonlinear behaviors of the RAC model under strong ground excitations from the point view of the distribution of nonlinear story forces, natural frequency, and damping ratio. The equivalent damping ratio of the RAC frame increases up to 45.06% with the damage accumulation and demonstrating a significant percentage of the nonlinear energy dissipation. Finally, a quantitative damage classification method for RAC frames is proposed by using the quadratic damping factor delta as a damage indicator.