System Identification and Response Simulation of Reinforced Concrete Buildings Seismically Retrofitted By Base Isolation

Abstract eng:
The dynamic identification of a four story reinforced concrete building retrofitted by base isolation is performed on the basis of push and release tests performed on the building. The identification is performed in two stages. In the first stage, the superstructure is considered rigid and the isolation system is identified by using a tri-linear model for the high damping rubber bearings and a constant Coulomb friction model for the low friction sliding bearings. In the second stage, the motion recorded just above the isolation system is considered as the input of a detailed free-body SAP2000 model of the superstructure. For identification purposes, a condensed model is derived from the detailed model. This is used in combination with the Covariance Matrix Adaptation-Evolution Strategy (CMA-ES) optimization algorithm for the identification of the structural parameters consisting of a multiplier of the stiffness matrix and a parameter specifying stiffness proportional damping. The identified model parameters are used for the simulation of the experimental recorded accelerations and the match is quite satisfactory. The use of the identified parameters for the isolation system and for the superstructure in the detailed SAP2000 model produces results that match the general trend of the isolation mode but fail to reproduce the high frequency response exhibited by the experimental results. The reaction history of the isolation system on the superstructure is evaluated by using both the rigid superstructure model and the one considering deformation of the superstructure. It is shown that while the former does not exhibit high frequency content, the latter shows considerable high frequency response. Application of the reaction containing high frequency components to the detailed SAP2000 model results in accelerations that match reasonably well the experimentally measured ones. Finally, the identified stiffness parameters point towards a significant reduction of the modulus of elasticity of concrete for the evaluation of the cross section rigidities when using gross section second order moments of inertia. The significant identified damping ratios appear to be considerably large consistent with the large released force and resulting initial acceleration and subsequent rapid attenuation.

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