Wandering of the modal parameters in existing building: application to structural health monitoring

International audienceAmbient vibrations in building is of increasing interest for applications in mechanical engineering, civil engineering and earthquake engineering. With advances in data acquisition systems (number of measurement points, continuous recording, low-noise instrument) and advances in signal processing algorithms, further and better studies can be conducted on civil engineering structures for evaluating their modal parameters and their physical properties. This study is focused on long- and short-term variations of frequency and damping in buildings. After a brief overview of the physical meanings and the practical interest for earthquake engineering, some examples are shown. They concerns the transient variations of modal parameters related with non-linear behaviour of the system, the permanent decrease of frequency and damping after extrem event and the natural wandering of modal parameters, often related to atmospheric conditions. These changes, however, confirms the great stability and confidence measurements in buildings using modal analysis. This information helps to consider the relevancy of analysis of existing monitoring (damage, ageing, and so on) and can better calibrate the mechanical models used for the analysis of seismic vulnerability of existing structures, and thereby help reduce variability of their estimates

Correlation between ground motion and building response using Californian earthquake records

International audienceUsing 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

Distributed archive and single access system for accelerometric event data : a NERIES initiative

We developed a common access facility to homogeneously formatted accelerometric event data and to the corresponding sheet of ground motion parameters. This paper is focused on the description of the technical development of the accelerometric data server and the link with the accelerometric data explorer. The server is the third node of the 3-tier architecture of the distributed archive system for accelerometric data. The server is the link between the data users and the accelero- metric data portal. The server follows three main steps: (1) Reading and analysis of the end-user request; (2) Processing and converting data; and (3) Archiving and updating the accelerometric data explorer. This paper presents the description of the data server and the data explorer for accessing data

A Natural Seismic Isolating System: The Buried Mangrove Effects

The Belleplaine test site, located in the island of Guadeloupe (French Lesser Antilles) includes a three-accelerometer vertical array, designed for liquefac- tion studies. The seismic response of the soil column at the test site is computed using three methods: the spectral ratio method using the vertical array data, a numerical method using the geotechnical properties of the soil column, and an operative fre- quency domain decomposition (FDD) modal analysis method. The Belleplaine test site is characterized by a mangrove layer overlaid by a stiff sandy deposit. This con- figuration is widely found at the border coast of the Caribbean region, which is exposed to high seismic hazard. We show that the buried mangrove layer plays the role of an isolation system equivalent to those usually employed in earthquake engineering aimed at reducing the seismic shear forces by reducing the internal stress within the structure. In our case, the flexibility of the mangrove layer reduces the distortion and the stress in the sandy upper layer, and consequently reduces the potential of liquefaction of the site

Damage Detection and Localisation Using Mode-Based Method and Perturbation Theory

International audienceIn this paper, the detection and the localization of a local perturbation are assessed by analysing the frequency changes only (fundamental mode and overtones). After describing the method used herein applied to the bending beam and based on the perturbation theory, experimental application to a 1D plexiglas beam is shown using frequency and modal analysis technique. The damage is considered as a local perturbation of Young's modulus. Finally, the localisation of damage is done using classical modal-based methods and perturbation theory. The frequency values are caught by the Random Decrement Technique applied to the time history vibrations for one sensor at the free extremity of the beam. Detection and localization are successful, even for small and transientchanges of the structure properties

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

Contribution of ambient vibration recordings (Free-field and buildings) for post-seismic analysis: the case of the Mw 7.3 MARTINIQUE (French lesser ANTILLES) earthquake, november 29, 2007

Following the Mw 7.3 Martinique earthquake, November 29th, 2007, a post-seismic survey was conducted by the Bureau Central Sismologique Français (BCSF) for macroseismic intensities assessment. In addition to the inventories, ambient vibration recordings were performed close to the particularly damaged zones in the free-field and the buildings. The objective of the paper is to show the relevancy of performing ambient vibration recordings for post-earthquake surveys. The analyses of the recordings aim at explaining the variability of the damages through site effects, structure vulnerability or resonance phenomena and to help the characterization of the post-seismic building integrity. In three sites prone to site effects, we suspect damage to be related to a concordance between soil fundamental frequency and building resonance frequency. Besides, the recordings of ambient vibrations at La Trinité hospital before and after the earthquake allow us to quantify the damage due to earthquake in terms of stiffness loss

Macro-scale vulnerability assessment of cities using Association Rule Learning

International audienceIn this paper, a datamining method based on Association Rule Learning (ARL) is applied to define a vulnerability proxy between the elementary characteristics of buildings and the vulnerability classes of the European Macroseismic Scale EMS98 (Grunthal, 1998). The method was applied to the Grenoble city test-bed described in the first part of this paper. The ARL method is then presented and a vulnerability proxy was derived for a Grenoble city-like environment. The vulnerability proxy is tested in Nice in the third part, a city that has been the subject of a vulnerability study (Spence and Lebrun, 2006). Finally, the damage produced by historic earthquakes was computed, considering the (equivalent) earthquake-era and the present-day urbanization for simulating seismic damage

Frequency and Damping Wandering in Existing Buildings Using the Random Decrement Technique

International audienceWith the increasing use of permanent, continuous and real-time networks, ambient vibrations can provide a simple tool for the identification of dynamic building parameters. This study is focused on the long-term variation of frequency and damping in several buildings, using the Random Decrement Technique (RDT). RDT provides a fast, robust and accurate long-term analysis and improves the reliability of frequency and damping measurements for structural health monitoring. This reveals particularly useful information in finding out precisely how far changes in modal parameters can be related to changes in physical properties. This paper highlights the reversible changes of the structureÕs dynamic parameters, correlated with external forces, such as temperature and exposure to the sun. Contrasting behaviours are observed, including correlation and anti-correlation with temperature variations

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 significant variability in fragility curves for single or grouped existing buildings. This paper 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 3-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 inter-storey 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