Philippe Gueguen

Dynamic parameters of structures extracted from ambient vibration measurements: an aid for the seismic vulnerability assessment of existing buildings in moderate seismic hazard regions

During the past two decades, the use of ambient vibrations for modal analysis of structures has increased as compared to the traditional techniques (forced vibrations). The frequency domain decomposition (FDD) method is nowadays widely used in modal analysis because of its accuracy and simplicity. In this paper, we first present the physical meaning of the FDD method to estimate the modal parameters. We discuss then the process used for the evaluation of the building stiffness deduced from the modal shapes. The models considered here are 1D lumped-mass beams and especially the shear beam. The analytical solution of the equations of motion makes it possible to simulate the motion due to a weak to moderate earthquake and then the inter-storey drift knowing only the modal parameters (modal model). This process is finally applied to a nine-storey reinforced concrete (RC) dwelling in Grenoble (France). We successfully compared the building motion for an artificial ground motion deduced from the model estimated using ambient vibrations and recorded in the building. The stiffness of each storey and the inter-storey drift were also calculated.

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

Abstract 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 during the lifetime of accelerometric networks, and the equations selected for the probabilistic studies are usually models established from foreign data. Although most GMPEs have been developed for magnitudes 5 and above, the minimum magnitude often used in probabilistic studies in low‐seismicity regions is smaller. Disaggregations have shown that, at return periods of engineering interest, magnitudes less than 5 may 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 300xa0km, 3.8≤ M w ≤4.5). The method is based on the log‐likelihood value proposed by Scherbaum etxa0al. (2009). The best‐fitting models (approximately 2.5≤LLH≤3.5) 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 the predominant factor 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 confirm that a dataset of ∼190 observations, the 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.

Using experimental data to reduce the single-building sigma of fragility curves: case study of the BRD tower in Bucharest, Romania

The lack of knowledge concerning modelling existing buildings leads to signifiant variability in fragility curves for single or grouped existing buildings. This study aims to investigate the uncertainties of fragility curves, with special consideration of the single-building sigma. Experimental data and simplified models are applied to the BRD tower in Bucharest, Romania, a RC building with permanent instrumentation. A three-step methodology is applied: (1) adjustment of a linear MDOF model for experimental modal analysis using a Timoshenko beam model and based on Anderson’s criteria, (2) computation of the structure’s response to a large set of accelerograms simulated by SIMQKE software, considering twelve ground motion parameters as intensity measurements (IM), and (3) construction of the fragility curves by comparing numerical interstory drift with the threshold criteria provided by the Hazus methodology for the slight damage state. By introducing experimental data into the model, uncertainty is reduced to 0.02 considering Sd (f1) as seismic intensity IM and uncertainty related to the model is assessed at 0.03. These values must be compared with the total uncertainty value of around 0.7 provided by the Hazus methodology.

Seismic vulnerability assessment of urban environments in moderate-to-low seismic hazard regions using association rule learning and support vector machine methods

AbstractnThe estimation of the seismic vulnerability of buildings at an urban scale, a crucial element in any risk assessment, is an expensive, time-consuming, and complicated task, especially in moderate-to-low seismic hazard regions, where the mobilization of resources for the seismic evaluation is reduced, even if the hazard is not negligible. In this paper, we propose a way to perform a quick estimation using convenient, reliable building data that are readily available regionally instead of the information usually required by traditional methods. Using a dataset of existing buildings in Grenoble (France) with an EMS98 vulnerability classification and by means of two different data mining techniques—association rule learning and support vector machine—we developed seismic vulnerability proxies. These were applied to the whole France using basic information from national databases (census information) and data derived from the processing of satellite images and aerial photographs to produce a nationwide vulnerability map. This macroscale method to assess vulnerability is easily applicable in case of a paucity of information regarding the structural characteristics and constructional details of the building stock. The approach was validated with data acquired for the city of Nice, by comparison with the RiskUE method. Finally, damage estimations were compared with historic earthquakes that caused moderate-to-strong damage in France. We show that due to the evolution of vulnerability in cities, the number of seriously damaged buildings can be expected to double or triple if these historic earthquakes were to occur today.n

Remote Modal Study of Reinforced Concrete Buildings Using a Multipath Lidar Vibrometer

AbstractOver recent years there has been growing interest in building frequency analysis using ambient vibrations in the fields of structural engineering with application to earthquakes, health monitoring, or operative testing. The direct applications are (1)xa0to define the modal characteristics of existing buildings for modeling their seismic response, (2)xa0to monitor the long-term variations in their structural health, and (3)xa0to detect and localize changes in the structure such as those produced by earthquake damage. Simultaneously, velocity measurements with laser remote sensing techniques have gained interest for several applications. For example, coherent lidar systems enable accurate measurement of the vibration velocity of remote targets. This allows operative modal analysis (OMA) of potentially damaged buildings, for their diagnosis from a safe distance after a seismic event. This paper compares the frequency analyses obtained using sensitive velocimeter sensors and coherent lidar sensors, applied ...

Classification of basic roof types based on VHR optical data and digital elevation model

In the frame of seismic vulnerability assessment in urban areas, it is very important to estimate the nature of the roof of every building and, in particular, to make the difference between flat roofs and gable ones. In order to perform this tedious task automatically on a large scale, remote sensing data provide a useful solution. In this study, we use simultaneously very high resolution panchromatic data, and an accurate digital elevation model. The fusion of these two modalities enables the extraction of two mixed features. Based on these features the classification between the two considered classes becomes a simple linearly separable problem.

Predicting Nonlinear Site Response Using Spectral Acceleration Vs PGV/Vs30: A Case History Using the Volvi-Test Site

In this study, we analyze the efficiency of the ratio between particle velocity and shear wave velocity as a strain proxy for evaluating the nonlinear seismic response of sediments. The in situ stress–strain relationships are derived from accelerometric vertical array recordings at the TST site in Volvi (Thessaloniki, Greece). First, the shear wave velocity between two successive sensors was computed by seismic interferometry and strain was computed as the velocity ratio or the relative displacement between sensors. The shear-wave velocity profile and in situ shear modulus degradation curve with strain were compared with previous studies performed at the TST site. Finally, the stress–strain relationships were derived from data recorded at the surface by extending the strain proxy and stress values to the ratio between peak ground velocity and the Vs30 parameter used for site classification, i.e. without requiring the accelerometric vertical array. Our model captures the in situ nonlinear response of the site, without consideration of azimuth or distance of the earthquakes. In conclusion, the acceleration (stress) values, based on the accelerometric response spectra instead of peak ground acceleration compared with the deformation (strain) proxy, provide an effective model of the in situ nonlinear response, providing information that can be integrated into ground motion prediction equations.

Correlation between ground motion and building response using Californian earthquake records

Using data from the California Strong Motion Instrumentation Program, we studied the relationship between building response and parameters describing the noxiousness of ground motion. According to vulnerability methods that use structural drift as damage criteria, we estimated the building response on the basis of the normalized relative roof displacement (NRRD), considered as damage criteria. The relationships between the NRRD and the intensity measures of the ground motion are developed using simulated annealing method. Grouping buildings by typology (defined according to their main construction material and height) reduces the variability of the building response. Furthermore, by combining IMs, the NRRD can be predicted more accurately by a building damage prediction equation. A functional form is thus proposed to estimate the NRRD for several building typologies, calibrated on the building responses recorded in California. This functional form can be used to obtain a fast and overall damage forecast after an earthquake.

Eurocode 8-compatible synthetic time-series as input to dynamic analysis

Nonlinear dynamic analysis of existing or planned structures often requires the use of accelerograms that match a target design spectrum. Here, our main concern is to generate a set of motions with a good level of fit to the Eurocode 8 design spectra for France. Synthetic time series are generated by means of a non-stationary stochastic method. To calibrate the input parameters in the stochastic approach, we select a reference set of accelerograms for a Eurocode 8 type B site category from the PEER Ground-Motion Database, which are then adjusted to the target spectrum through wavelet addition. Then, we compute nonlinear seismic responses of a soil column, including pore pressure effects, and brittle and ductile structures to the stochastic time-series, the natural accelerograms and time-series generated using stationary stochastic approaches. The results of these calculations reveal considerable variability in response despite the similarities in terms of spectral acceleration.

Interaction sismique sol-structure-sol en milieu urbain: Modélisation par éléments finis

ABSTRACT Under particular conditions, the radiated motion due to building vibration may considerably modify the surface free-field seismic motion. This paper presents the study of interactions between soil and building motion, performed by the finite element method. The model was excited by shear wave input motion (2D, plane strain). First, the numerical topmost soil response has been compared to the analytical formulation, considering several soil models. Then, the effects of the structural parameters variation (height and width) on the building response have been studied as well as the induced soil effects (β1 variable). The spatial decay of the radiated wavefield back into the soil from the building base has been considered, showing resonance effects between soil and structure.