AMBIENT VIBRATION TESTING AND MODAL ANALYSIS OF AN RCC ARCHED DAM Nouredine BOURAHLA 1 , Abdelghani SI-CHAIB 2 , Mustapha NOURI 1 , Takieddine MENAOUER 1 , Idriss ROUAZ 3 ABSTRACT Monitoring the behavior of strategic structures such as high-rise buildings, large bridges and dams is essential to determine their "state of health". Vibration analysis, for preventive maintenance, is a relevant tool that has been used for few decades in this field, relying on correlation between in-situ measurements and finite element model properties. The elastic dynamic properties and particularly the natural frequencies and mode shapes can be successfully estimated, especially in elastic range, using the well-known ambient vibration testing. The present paper presents the structural dynamic aspect needed for health monitoring of an RCC arched dam. Such approach permits to calibrate numerical model to predict the mechanical characteristics of the dam and then to identify qualitatively the possible damage pattern under strong earthquake. The Tabellout Roller Compacted Concrete arched dam was chosen for this study. It presents a good example for the application of vibration analysis, since it is located in close proximity to three regional potentially seismogenic faults. For this purpose, a correlation between experimental ambient vibration measures and finite element numerical modal properties of the dam is first presented. Once the modal properties are calibrated, series of nonlinear dynamic analyses under seismic loading have been carried out, to predict the behavior of the dam. On the basis of the obtained results an optimized sensor monitoring scheme is proposed to track the structural status of the dam and to detect the damage levels in real time. Keywords: Identification; Ambient vibration; Dam; Model calibration; Dynamic response 1. INTRODUCTION One of the most frequent uses of ambient vibration testing involves identification of natural frequencies, mode shapes of vibration and equivalent viscous damping ratio of various full scale structures (Si-Chaib et al. 2014). These results are used to validate or update FE models by matching the experimental and numerical modal properties and can also be used for damage detection and localization (El-Ouafi et al. 2009-2013), (Li et al. 2016). The structural health monitoring (SHM) is the process of implementing a damage identification strategy, involving the observation of a structure over time and space in order to determine the health state of the structure. The main objective in SHM is to keep track of the changes in the characteristics of the structural system in order both to detect and locate the damage, and to make a decision automatically whether the damage is in dangerous level for the structure or not. Several developments were made to address the difficulties in implementing structural health monitoring systems based on ambient vibration recording Elmasry and Johnson (2004), Oth and Picozzi (2012). In this context, this paper presents a methodology based on ambient vibration validation of a FE model of the Tabellout dam that has been used to identify qualitatively the progression of damage level under seismic loading using nonlinear dynamic analysis. This approach aims to provide a basis for locating potentially strategic points for an optimal instrumentation for health monitoring of the dam. 1 University Saad Dahlab, Algeria, nbourahla@univ-blida.dz 2 Tractebel-Engie, France, abdelghani.si-chaib@tractebel.engie.com 3 CNERIB, Algeria, rouaz.idriss@gmail.com