Seismic vulnerability analysis of moderate seismicity areas using in situ experimental techniques: from the building to the city scale – Application to Grenoble and Pointe-à-Pitre (France)

International audienceSeismic 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-à-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

Dynamic Behaviour of the first instrumented building in France: The Grenoble Town Hall

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 60's 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

Time-frequency analysis of Transitory/Permanent frequency decrease in civil engineering structures during earthquakes

The analysis of strong motion recordings in structures is crucial to understand the damaging process during earthquakes. A very precise time-frequency representation, the reassigned smoothed pseudo-Wigner-Ville method, allowed us to follow the variation of the Millikan Library (California) and the Grenoble City Hall building (France) resonance frequencies during earthquakes. Under strong motions, a quick frequency drop, attributed to damage of the soil-structure system, followed by a slower increase is found. However, in the case of weak earthquakes, we show that frequency variations come from the ground motion spectrum and cannot be interpreted in terms of change of the soil-structure system

Seismic vulnerability analysis of moderate seismicity areas using in situ experimental techniques: from the building to the city scale – Application to Grenoble and Pointe-à-Pitre (France)

International audienceSeismic 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-à-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

Comparaison entre calculs de vulnérabilité sismique et propriétés dynamiques mesurées

National audienceLarge-scale seismic vulnerability assessment methods use simplified formulas and curves, often without providing uncertainties. They are seldom compared to experimental data. Therefore, we recorded ambient vibrations and estimated modal parameters (resonance frequencies, modal shapes and damping) of 60 buildings in Grenoble (France) of various types (masonry and reinforced concrete). The knowledge of resonance frequencies in the linear domain is essential in the seismic design. Hence, we compared resonance frequency formulas given in the design code with this experimental data. The variability is underestimated and only two parameters (type and height of the building) seem to be statistically significant. Moreover, we compared the linear part of capacity curves used in European Risk-UE method to the measured frequencies. The variability is still very large and these curve are often not relevant for the French buildings. As a result, ambient vibration recordings may become an interesting tool in order to calibrate the linear part of capacity curves

On the Testing of Ground--Motion Prediction Equations against Small--Magnitude Data

Ground-motion prediction equations (GMPE) are essential in probabilistic seismic hazard studies for estimating the ground motions generated by the seismic sources. In low seismicity regions, only weak motions are available in the lifetime of accelerometric networks, and the equations selected for the probabilistic studies are usually models established from foreign data. Although most ground-motion prediction equations have been developed for magnitudes 5 and above, the minimum magnitude often used in probabilistic studies in low seismicity regions is smaller. Desaggregations have shown that, at return periods of engineering interest, magnitudes lower than 5 can be contributing to the hazard. This paper presents the testing of several GMPEs selected in current international and national probabilistic projects against weak motions recorded in France (191 recordings with source-site distances up to 300km, 3.8\leqMw\leq4.5). The method is based on the loglikelihood value proposed by Scherbaum et al. (2009). The best fitting models (approximately 2.5\leqLLH\leq3.5) over the whole frequency range are the Cauzzi and Faccioli (2008), Akkar and Bommer (2010) and Abrahamson and Silva (2008) models. No significant regional variation of ground motions is highlighted, and the magnitude scaling could be predominant in the control of ground-motion amplitudes. Furthermore, we take advantage of a rich Japanese dataset to run tests on randomly selected low-magnitude subsets, and check that a dataset of ~190 observations, same size as the French dataset, is large enough to obtain stable LLH estimates. Additionally we perform the tests against larger magnitudes (5-7) from the Japanese dataset. The ranking of models is partially modified, indicating a magnitude scaling effect for some of the models, and showing that extrapolating testing results obtained from low magnitude ranges to higher magnitude ranges is not straightforward

In Situ Experiment and Modelling of RC-Structure using Ambient vibration and Timoshenko Beam

International audienceRecently, several experiments were reported using ambient vibration surveys in buildings to estimate the modal parameters of buildings. Their modal properties are full of relevant information concerning its dynamic behaviour in its elastic domain. The main scope of this paper is to determine relevant, though simple, beam modelling whose validity could be easily checked with experimental data. In this study, we recorded ambient vibrations in 3 buildings in Grenoble selected because of their vertical structural homogeneity. First, a set of recordings was done using a 18 channels digital acquisition system (CityShark) connected to six 3C Lennartz 5s sensors. We used the Frequency Domain Decomposition (FDD) technique to extract the modal parameters of these buildings. Second, it is shown in the following that the experimental quasi-elastic behaviour of such structure can be reduced to the behaviour of a vertical continuous Timoshenko beam. A parametric study of this beam shows that a bijective relation exists between the beam parameters and its eigenfrequencies distribution. Consequently, the Timoshenko beam parameters can be estimated from the experimental sequence of eigenfrequencies. Having the beam parameters calibrated by the in situ data, the reliability of the modelling is checked by complementary comparisons. For this purpose, the mode shapes and eigenfrequencies of higher modes are calculated and compared to the experimental data. A good agreement is also obtained. In addition, the beam model integrates in a very synthetic way the essential parameters of the dynamic behaviour

Analysis of Reinforced Concrete Buildings Using Multipath Lidar

International audienceThis paper compares the modal analysis of reinforced-concrete buildings obtained using sensitive velocimeters and coherent LIDAR. Ambient vibrations are recorded by these two systems and processing using operative modal analysis method for getting building frequency and mode shapes. Real-scale trials applied to five buildings located at Grenoble (France) are presented. The efficiency and reliability of the Lidar is discussed and the modal parameters measured by Lidar at a range of 200m and by in-situ velocimeters are compared. The results are in good agreement and allow us to conclude on the ability of the coherent Lidar to assess modal parameters of existing buildings at long range and without any retroreflectors placed on the structures. The results open new perspectives for remotely testing buildings, without getting inside, facilitating dynamic analysis of buildings for earthquake engineering applications

Parameter estimation of short-time multi-component signals using damped-amplitude & polynomial-frequency model

International audienceThis paper concerns the parameter estimation of multicomponent damped oscillations having non-linear frequency. In this paper, the instantaneous frequency is approximated by polynomials while the amplitude is characterized by damped exponentials to connect directly to its physical interpretations. A maximum likelihood procedure is developed via an adaptive simulated annealing technique which helps to speed up the convergence. Results on simulated signals show that the proposed algorithm is more efficient than the algorithm based on polynomial amplitude models, and allows the estimation of damping coefficients over a very short time duration. Finally, the proposed algorithm is applied for characterizing the ambient vibrations of a building