16 th World Conference on Earthquake Engineering, 16WCEE 2017 Santiago Chile, January 9th to 13th 2017 Paper N° 4368 (Abstract ID) Registration Code: S- B1462534797 SYSTEM IDENTIFICATION AND RESPONSE SIMULATION OF REINFORCED CONCRETE BUILDINGS SEISMICALLY RETROFITTED BY BASE ISOLATION G. Oliveto (1) , A. Athanasiou (2) , A. Markou (3) , G. Marino (4) , N. D. Oliveto (5) (1) Professor, University of Catania, goliveto@dica.unict.it (2) Postdoctoral researcher, University of Catania, athanasiou@dica.unict.it (3) PhD, Civil Engineer, athanasiosmarkou@gmail.com (4) Civil Engineer, giovannim@alice.it (5) PhD, Civil Engineer, e-mail: noliveto@buffalo.edu Abstract 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. Keywords: base isolation; dynamic testing; structural identification; numerical simulation; non-linear behavior 1. Introduction Around the turn of the century, two four-story reinforced concrete buildings in the town of Solarino in Eastern Sicily, Fig. 1, were retrofitted against seismic excitation using base isolation. Upon completion of the retrofitting works in July 2004, one of the buildings (civic number 25, to the right in Fig.1) was subjected to free vibration tests by application and sudden release of the design displacement [1, 2]. The building was instrumented so as to record the accelerations of the six rigid body modes plus those associated with the deformation modes, Fig. 2. Transducer 07 at station S2 was oriented in the transverse direction (Y) in tests 1 to 5, and in the longitudinal direction (X) in tests 6 to 8. A simple model was then used to identify the properties of the base isolation system [3]. Subsequently, an analytical solution was provided for the simulation of the dynamic response of the base isolation system under earthquake excitation [4]. Finally, a numerical constrained optimization procedure was provided as an alternative to the analytical solution in view of applications to 2D ground motion excitation [5, 6]. All previous studies aiming at model parameter identification and response simulation of the seismic isolation system were based on considering the superstructure as a rigid body. The aim of the present contribution is to