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Register a new userDynamic Behaviour of the first instrumented building in France: The Grenoble Town Hall
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Authors
Clotaire Michel
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The French Accelerometric Network (RAP) launched in November 2004 a marked operation for the seismic behaviour assessment of a typical French building. The main goal of this project is to collect accelerometric data in the building and use them to calibrate models or alternative tools used for the seismic behaviour assessment. The final goal of this project is to help the vulnerability assessment of cities in moderate seismic hazard countries. The French Accelerometric Network (RAP) chose to install a permanent network of 6 accelerometers recording continuously the vibrations of the Grenoble City Hall at the basement and at the top. The 13-story building is a RC shear walls building, typical of the RC structures designed at the end of the 60s in France. All the data collected in the building are available on the online access database of the RAP. In addition to the permanent network, an ambient vibration experiment has been performed in 36 points of the whole building. Using the Frequency Domain Decomposition method, these data allowed estimating precisely the different modes of vibration of the structure for low amplitudes. Only the first bending modes in each direction (1.15 and 1.22 Hz) and the first torsion mode (1.44 Hz) are excited. We compared the frequencies obtained using ambient vibration to those for a moderate earthquake recorded by the permanent network. Thanks to the continuous recording, a statistical approach of the torsion mode pointed out the position of the centre of rotation of the building. A modal model extracted from ambient vibrations is proposed and validated thanks to the earthquake recordings collected in the building during the ML=4.6, September 8th 2005 Vallorcine (Haute-Savoie, France) earthquake.
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Seismic vulnerability analysis of existing buildings requires basic information on their structural behaviour. The ambient vibrations of buildings and the modal parameters (frequencies, damping ration and modal shapes) that can be extracted from them naturally include the geometry and quality of material in the linear elastic part of their behaviour. The aim of this work is to use this modal information to help the vulnerability assessment. A linear dynamic modal model based on experimental modal parameters is proposed and the fragility curve corresponding to the damage state ?Slight? is built using this model and a simple formula is proposed. This curve is particularly interesting in moderate seismic areas. This methodology is applied to the Grenoble City where ambient vibrations have been recorded in 61 buildings of various types and to the Pointe-`a-Pitre City with 7 study-buildings. The fragility curves are developed using the aforementioned methodology. The seismic risk of the study-buildings is discussed by performing seismic scenarios.
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