• Special Issues
  • PREFACE

    Structural health monitoring (SHM) received in the last twenty years, a growing interest by both research- ers and professional, as pointed out by the number of monitoring systems installed today in various countries of the world. The main reasons for this development are on one side the limitations related to the use of traditional methods based on visual inspections an on the other side the great potential offered by automatic systems of damage detection allowing a preventive as- sessment of the seismic vulnerability and a sensible reduction of maintenance costs.
    The aim of the Special Issue is to report recent advances in this field and successful applications. It includes eight papers from several research groups that have devoted considerable effort to the identification of techniques and tools for the application of SHM to structures in seismic areas.
    For seismic-prone structures, health monitoring can sometimes take advantage of real responses recorded during strong earthquakes. These occurrences have a strategic importance both for the advancement of knowledge on the behavior of structures under strong seismic actions and for the calibration of realistic and reliable numerical models, able to reproduce the structural behavior and to formulate a diagnosis about possible sources of damage. Furthermore, the possibility to assess the seismic vulnerability based on data recorded on the monitored structure, opens new avenues in maintenance policies, shifting from a traditional ‘scheduled maintenance’, to a ‘condition-based maintenance’, carried out ‘on demand’, basing on the current structural condition.
    The papers by Saisi et al. and Ranieri et al. tackle this subject for the specific case of historical masonry towers which are important landmarks in the Italian heritage, severely hit by the Emilia earthquake of 2012. The two papers present techniques for assessing the seismic vulnerability by means of preventive non destructive testing based on ambient vibrations taking into account environmental changes as well. The preventive assessment of the seismic vulnerability is also the subject of the paper by Ditommaso et al. focusing on irregular buildings, for which heavy structural damage is often due to torsional effects. In the paper is proposed a simplified experimental approach to identify the existence of torsional modes, based on a limited number of sensors thus limiting the cost of the monitoring system.
    In recent years, thanks to the innovations in sensors and techniques for data transmission, a challenging goal for permanent seismic monitoring networks is developing: the possibility to quickly detect damage in a structure affected by a seismic event. This can be of crucial importance both for the management and coordination of immediate safety interventions or evacuation operation, and also for planning of the first operations of repair of damaged structures. In the aftermath of a strong seismic event this assessment is carried out using traditional methods based primarily on visual inspection that require quite a long time to cover the entire area affected by the earthquake, and also lead to estimations which are typically conservative, certainly on the safety side, but with a correspondent increase of the costs related to the downtime of the structure. Furthermore they may fail if damage is not visibly evident. Networks of sensors permanently installed on the structure supported by efficient damage identification algorithms constitute a promising alternative, able to provide real time information and a quick assessment of the damage state of the building after a seismic event. The application of algorithms for damage detection and localization to structures in seismic prone areas is the subject of the papers by Guéguen et al., Ponzo et al, Limongelli et al. and Benzoni et. al. that present numerical and experimental applications of different damage localization algorithms based on modal or non modal features. Sharing the same challenging goal the paper by Hernandez-Garcia et al. implements a robust data-driven methodology for detecting, locating and quantifying changes not only in linear structural parameters but also in nonlinear characteristics of chain-like systems.
    The Guest Editor wishes to thank Dr. Gianmario Benzoni, Editor of the Journal, for his support, supervision and assistance in producing this special issue and also all the Authors and the Reviewers for their work and contribution.

  • Special Issues
  • ON THE ESTIMATION OF THE FUNDAMENTAL MODAL PROPERTIES OF ITALIAN HISTORICAL MASONRY TOWERS

    The preservation of historical structures is a very challenging problem that requires strong interdisciplinary efforts, in particular in those countries where seismic risk is relevant and the integration between conservation and protection can be very difficult. Recent Italian earthquakes hit many valuable masonry structures marking the urban and rural landscape. Among those, masonry towers are often impressive signs of past cultures and folks. Non-destructive testing of such structures has been widely investigated and discussed in the technical literature. In the present paper, some aspects related to output-only modal identification of tower-like masonry structures for their non-destructive characterization in view of seismic performance assessment are discussed. Specific attention is paid to the role of Operational Modal Analysis in the seismic analysis of masonry towers and the development of empirical predictive correlations.

     

     

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  • Special Issues
  • DAMAGE LOCALIZATION IN A HIGH-RISE BUILDING USING SEISMIC EXCITATION

    The size of high-rise buildings and the significant expenses associated to using repeatedly ad hoc excitation sources make appealing the adoption of natural phenomena as sources of the excitation even if their randomness makes quite challenging the assessment of the accuracy of the damage identification algorithms implemented to check the structural health. in this paper the sensitivity of the interpolation damage detection method (IDDM) to the randomness of the base excitation is studied with reference to the case of an high-rise wall-frame building lately designed to be built in Salerno (Italy). a detailed numerical model of the building was used to simulate several damage scenarios and a recently proposed numerical procedure was applied to calculate a large set of realistic base inputs, corresponding to low intensity earthquakes complying with the Italian code for the location of the building. structural responses to the entire set of input calculated by the numerical model were used to check the reliability of the damage identification algorithms. results show that the interpolation damage detection method allows to take into account the effect of the variability of the input and provides a reliable detection of a damaged location both in the case of single and of multiple damage scenarios in case of medium to severe damages. The investigation on multiple damaged locations pointed out also a relationship between the severity of damage, the global damage pattern and the values of the damage index that can influence the reliability of the method in case of multiple light damages.

     

     

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  • Special Issues
  • UN APPROCCIO SEMPLIFICATO PER L’IDENTIFICAZIONE SPERIMENTALE DEI MODI ROTAZIONALI DELLE STRUTTURE INTELAIATE

    Evidenze dei passati forti terremoti mostrano che elevati danni strutturali sugli edifici sono spesso causati da significativi effetti torsionali. Questi effetti sono stati ampiamente studiati negli ultimi anni e per gli edifici nuovi molti delle normative antisismiche più recenti prevedono apposite procedure progettuali. L’approccio tipico per la valutazione sperimentale degli effetti torsionali prevede l’installazione di un sistema accelerometrico multicanale, sia per il monitoraggio su edifici in continuo, anche in occasione di sequenze sismiche reali, sia per indagare il comportamento degli edifici in presenza di vibrazioni forzate. Tale approccio è senz’altro molto efficace su singoli edifici ed in grado di restituire ottimi risultati pur se a prezzo di significativi investimenti in termini di risorse e tempo; per tale regione non sembra essere la soluzione migliore da utilizzare in studi su larga scala con scopi differenti quali, ad esempio, per la proposizione e/o validazione di previsioni normative o per valutazioni speditive finalizzate allo studio e mitigazione del rischio sismico. Al proposito, nel presente articolo sono presentati i risultati sperimentali ottenuti con un approccio semplificato, basato su vibrazioni ambientali, ampiamente e facilmente utilizzabile per la caratterizzazione dinamica di strutture. Al fine di testare l’approccio proposto sono state eseguite differenti prove sperimentali, con differenti configurazioni di strumenti. Ovviamente l’utilizzo di due soli strumenti installati in copertura dell’edificio rendono possibile identificare le frequenze fondamentali dello stesso e i modi torsionali. L’installazione di uno strumento per ciascun piano rende invece possibile individuare anche le forme modali. Per validare la procedura proposta, è stato effettuato un confronto tra i risultati forniti dalla stessa (utilizzando velocimetri), e quelli ottenuti da un’altra ben più ampia campagna sperimentale (utilizzando accelerometri) condotta su di uno stesso telaio campione, in acciaio, presso l’Università di Basilicata.

     

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  • Special Issues
  • DYNAMIC AND SEISMIC HEALTH MONITORING OF A HISTORIC MASONRY TOWER

    The paper presents some results of the continuous dynamic/seismic monitoring program carried out on the tallest historic tower in Mantua, Italy. This project follows an extensive diagnostic investigation aimed at assessing the structural condition of the tower after the Italian earthquakes of May 2012.
    A simple dynamic monitoring system was installed in the tower to evaluate the dynamic response especially to the expected sequence of far-field earthquakes and to check the possible evolution of the natural frequencies; the response to ambient excitation has been continuously collected in 1-hour records since late December 2012.
    The paper summarizes the results of the continuous dynamic monitoring for a period of 8 months, highlighting the effect of temperature on automatically identified natural frequencies, the dynamic response to few seismic events and the key role of permanent dynamic monitoring in the diagnosis of the investigated historic building.

     

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  • Special Issues
  • NUMERICAL AND EXPERIMENTAL ASSESSMENT OF THE PERFORMANCE OF FOUR NONDESTRUCTIVE DAMAGE EVALUATION METHODS IN SITUATIONS COMPARABLE TO POST-EARTHQUAKE DAMAGE ANALYSIS

    This study evaluates several methods for the detection and location of a perturbation along a bending beam. Unlike previous studies, the situation chosen is comparable to that encountered in-situ after an earthquake. Indeed, assessment of earthquake damage to buildings is a crucial factor in crisis management. Buildings can be tested appropriately using operational modal analysis methods based on ambient vibration data. In this study, we tested four modal-based methods to locate the perturbation. The mode shapes and eigenfrequencies of a beam were evaluated in the laboratory using the frequency domain decomposition method applied to ambient vibrations simulated along a one-dimensional bending beam, using the finite element method and recorded on a poly(methyl methacrylate) (Plexiglas) beam-like free-clamped structure. The data show that the uniform load surface curvature method is the most efficient, and it allows for accurate estimation of the location of the perturbation, whatever its nature (e.g., multiple, simple, different positions). The experimental data also show that frequency domain decomposition resolution of the mode shapes must be as accurate as possible, otherwise it is impossible to locate transient perturbations over short periods.

     

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  • Special Issues
  • AN EVALUATION OF A METHODOLOGY FOR DETECTION, LOCALIZATION, AND QUANTIFICATION OF CHANGES IN NONLINEAR SYSTEMS BASED ON EXPERIMENTAL MEASUREMENTS

    Experimental data from a test-bed structure tested is used to evaluate and validate a methodology for detecting, localizing, and quantifying structural changes in nonlinear structures using chain-like reduced-order models estimated from measurements. This study showed that variations in the mathematical representation (i.e., two-dimensional polynomial expansion) of the restoring forces in the estimated chain- like reduced-order models could be employed to confidently detect the presence of physical structural changes introduced into the test-bed structure, accurately locate the structural section where the changes occurred, and provide an estimate of the severity or magnitude of the structural changes.

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  • Special Issues
  • LOCALIZATION OF DAMAGE OCCURRED ON FRAMED STRUCTURES: ANALYSIS OF THE GEOMETRIC CHARACTERISTICS OF THE FUNDAMENTAL MODE SHAPE

    Damage detection techniques based on data acquired using permanent and/or temporary monitoring systems directly installed on structures, and/or infrastructures, have received a significant attention in the recent scientific literature. The recourse to experimental methods it is necessary also with the aim to characterize the seismic linear and nonlinear behaviour of real structures excited by earthquakes. Structural Health Monitoring (SHM) systems provide also the possibility to better understand the effects of the dynamic soil-structure interaction, together with the role played by the non-structural components on both linear and nonlinear behaviour of the monitored structure. A new methodology for damage detection and localization on framed structures, based on the maximum modal curvature variation related to the fundamental mode of the monitored structure, is proposed in this paper. Particularly, the main outcomes retrieved from several numerical nonlinear dynamic models, and from several shaking table tests, performed at the University of Basilicata using a scaled framed model, have been discussed.

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  • Special Issues
  • NUMERICAL AND EXPERIMENTAL VALIDATION OF A STRUCTURAL HEALTH MONITORING TECHNIQUE FOR CRITICAL INFRASTRUCTURE

    Damage Identification method developed for infrastructure equipped with seismic response modification devices is hereafter summarized and validated through numerical and experimental case studies. The output-only method was tested via Finite Element analyses of two bridge structures, the Vincent Thomas Bridge and the Benicia Martinez Bridge, equipped with viscous dampers and friction pendulum bearings, respectively. The application of the method to real ambient vibration data from the Vincent Thomas Bridge proved successful in identifying early stages of degradation of seismic response modification devices. The Level III damage detection method was also applied to a three-span cable-stayed bridge, the Yokohama Bay Bridge, based on accelerometric records from the 2011 East Japan Earthquake. The integration of the method in an innovative monitoring systems aimed at the real-time remote assessment of the structural adequacy of aging critical infrastructure is under development.

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