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

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

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

Seismic vulnerability assessment to slight damage based on experimental modal parameters

International audienceThe aim of this paper is to adjust behaviour models for each class of structure for vulnerability assessment by using ambient vibration. A simple model based on frequencies, mode shapes and damping, taken from ambient vibrations, allows computation of the response of the structures and comparison of inter-storey drifts with the limits found in the literature for the slight damage grade, considered here as the limit of elastic behaviour. Two complete methodologies for building fragility curves are proposed: (1) using a multi-degree of freedom system including higher modes and full seismic ground motion, (2) using a single-degree of freedom model considering the fundamental mode f0 of the structure and ground motion displacement response spectra SD(f0). These two methods were applied to the city of Grenoble, where 60 buildings were studied. Fragility curves for slight damage were derived for the various masonry and reinforced concrete classes of buildings. A site-specific earthquake scenario, taking into account local site conditions, was considered, corresponding to an ML=5.5 earthquake at a distance of 15km. The results show the benefits of using experimental models to reduce variability of the slight damage fragility curve. Moreover, by introducing the experimental modal model of the buildings, it is possible to improve seismic risk assessment at an overall scale (the city) or a local scale (the building) for the first damage grade (slight damage). This level of damage, of great interest for moderate seismic prone regions, may contribute to the seismic loss assessment

Analysis of 1-Hz GPS data for the estimation of long-period surface motion of Tohoku-Oki Mw9.0 2011 earthquake

The GPS displacement time series are today mostly used for the estimation of the transient (period <2sec) and coseismic (static) displacement of an earthquake, while the estimation of the velocity and acceleration is based only seismic networks. In this study, we examine whether the GPS records can be used to capture the spectral characteristics of the long-period signal and their consistency with the corresponding seismic motion sensors, for the displacement, velocity and acceleration. For this purpose, we use the 1-Hz GPS network records of the Tohoku-Oki 2011 earthquake processed in Precise Point Positioning (PPP). The derived GPS and strong-motion waveforms were analysed resulting into displacement, velocity and acceleration for periods ranging from 3 to 100s. The derived GPS and strong-motion time series were compared and it was found that these are of similar pattern and amplitude. However, there is a non-constant phase shift between the corresponding GPS and strong-motion time series, resulting in significant difference between the time series in the time domain. On the contrary in the frequency domain, the GPS and strong-motion time series are consistent for periods larger than 3-4s. Finally the GPS and the strong-motion records were compared in the time-frequency domain based on wavelet analysis, revealing that both GPS and seismic records express consistently the variation of the long-period of the seismic signal. Thus, it is proved that the GPS records can be used for the estimation of the long-period ground motion and contribute to the reliable estimation of the corresponding characteristics (displacement, velocity, acceleration)

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

Analysis of 1-Hz GPS data for the estimation of long-period surface motion of Tohoku-Oki Mw9.0 2011 earthquake

The GPS displacement time series are today mostly used for the estimation of the transient (period <2sec) and coseismic (static) displacement of an earthquake, while the estimation of the velocity and acceleration is based only seismic networks. In this study, we examine whether the GPS records can be used to capture the spectral characteristics of the long-period signal and their consistency with the corresponding seismic motion sensors, for the displacement, velocity and acceleration. For this purpose, we use the 1-Hz GPS network records of the Tohoku-Oki 2011 earthquake processed in Precise Point Positioning (PPP). The derived GPS and strong-motion waveforms were analysed resulting into displacement, velocity and acceleration for periods ranging from 3 to 100s. The derived GPS and strong-motion time series were compared and it was found that these are of similar pattern and amplitude. However, there is a non-constant phase shift between the corresponding GPS and strong-motion time series, resulting in significant difference between the time series in the time domain. On the contrary in the frequency domain, the GPS and strong-motion time series are consistent for periods larger than 3-4s. Finally the GPS and the strong-motion records were compared in the time-frequency domain based on wavelet analysis, revealing that both GPS and seismic records express consistently the variation of the long-period of the seismic signal. Thus, it is proved that the GPS records can be used for the estimation of the long-period ground motion and contribute to the reliable estimation of the corresponding characteristics (displacement, velocity, acceleration)

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