Gabriele Comanducci

Vibration-based structural health monitoring of a historic bell-tower using output-only measurements and multivariate statistical analysis:

This article presents the development and the results of 1 year of implementation of a simple vibration-based structural health monitoring system for preventive conservation and condition-based maintenance of an Italian monumental masonry bell-tower. The system is based on the data recorded by a small number of high-sensitivity accelerometers, on remote automated frequency tracking and on a multivariate statistical analysis criterion for damage detection, combining data regression, principal component analysis, and novelty analysis. The analysis of monitoring data highlights the main characteristics of the response of the tower to wind, swinging bells, and low-return period earthquakes. Despite the low levels of vibration in operational conditions, the system is seen able to track the time evolution of five natural frequencies of the structure and successfully use such information for detecting anomalous deviations from normal conditions. More in general, the presented results show a promise toward a more widespread use of low-cost vibration-based monitoring systems for cultural heritage preservation.

On vibration-based damage detection by multivariate statistical techniques: Application to a long-span arch bridge

Structural health monitoring allows the automated condition assessment of civil infrastructure, leading to a cost-effective management of maintenance activities. However, there is still a debate in the literature about the effectiveness of available signal processing strategies to timely assess the health state of a structure. This paper is a contribution to this debate, by presenting the application of different vibration-based damage detection methods using up-to-date multivariate statistical analysis techniques applied to data acquired from a permanently monitored long-span arch bridge. Techniques based on dynamic regression models, linear and local principal component analysis, as well as on their combinations, including, in particular, the newly proposed method based on the combination of dynamic multiple linear regressions and local principal component analysis, and, finally, a method based on the recently proposed approach of cointegration, are considered. A first effort is made to formulate these methods within a unique mathematical framework, highlighting, in particular, the relevant parameters affecting their results and proposing objective criteria for their appropriate tuning and for choosing the length of the training period. Then, the considered damage detection methods are implemented and applied to field data, seeking for damage-sensitive features in the presence of variable environmental and operational conditions. The considered techniques are applied to time histories of identified modal frequencies of the bridge and their capability to reveal structural damage of varying severity is assessed using control charts. The case of an artificially imposed non-linear correlation between the features is also considered. The results provide, for the first time in the literature, an estimation of the minimum level of damage that can be realistically detected in the bridge using dynamic signatures and up-to-date signal processing algorithms, thus contributing to a more aware use of monitoring data and reliance over related health state assessment information.

The Stretching Method for Vibration-Based Structural Health Monitoring of Civil Structures

This article newly proposes the application of the stretching method, that is used in geophysics for detecting variations in the velocity with which waves propagate in the earths crust from seismic noise recordings, in the context of vibration-based Structural Health Monitoring SHM of civil structures. The result is a computationally efficient long-term vibration-based SHM tool, that follows the current trend of using a very limited number of sensors permanently installed on site to measure operational structural responses for the purpose of damage detection. In the SHM setting, the proposed method aims at a direct identification of small permanent shifts in the natural frequencies of the structure in a changing environment, which is achieved by maximizing the correlation coefficient between a reference waveform, computed in a training reference period in which the structure is assumed to be undamaged, and a stretched version of the same waveform evaluated at the current time. The comparison is performed in the frequency domain and the waveform of interest is obtained from cross-correlations of the ambient vibration measurements. More specifically, in the case of multiple sensors, the waveform can be either the cross-power spectral density of the signals recorded by a pair of sensors, or the largest singular value of the spectral matrix of the measurements. It follows that the method can be regarded as an extension of the classic Frequency Domain Decomposition FDD. A key feature of the proposed stretching method is mitigating the effects of environmental fluctuations by time domain averaging of cross-correlations over a proper period of time, before taking their Fourier transform to estimate the spectral densities. Such a time domain averaging is carried out in a relatively long period of time for estimating the reference waveform, whereas it is carried out in a shorter time for estimating the current waveform. The main features of the proposed methodology are its very low sensitivity to environmental fluctuations, resulting in a quite short training period length, and its low computational cost, which could be compatible with a direct integration within smart sensors with embedded electronics. The performance of the method is illustrated in the case study of an Italian historical monumental bell tower that has been monitored by the authors for more than 1 year.

A parametric study on reliability-based tuned-mass damper design against bridge flutter

We present a probabilistic methodology for designing tuned mass dampers for flutter suppression in long-span bridges. The procedure is computationally efficient and computes the probability of flutter occurrence based on a modified first-order method of reliability analysis, a reduced-order representation of the structure and a time domain formulation of aeroelastic loads. Results of a parametric investigation show that the proposed methodology is preferable to a deterministic design procedure, which relies on nominal values of mechanical and aerodynamic parameters and does not guarantee the maximum safety. Furthermore, the reliability-based approach can be effectively used in the design of multiple tuned mass damper configurations by enhancing robustness against frequency mistuning and by reducing costs associated with supplemental damping for a given safety performance level.

Earthquake-Induced Damage Detection in a Monumental Masonry Bell-Tower Using Long-Term Dynamic Monitoring Data

ABSTRACT This work investigates the use of an advanced long-term vibration-based structural health monitoring tool to automatically detect earthquake-induced damages in heritage structures. Damage produced in a monumental bell-tower at increasing values of the Peak Ground Acceleration (PGA) of the seismic input is predicted by incremental nonlinear dynamic analysis, using a Finite Element model calibrated on the basis of experimentally identified natural modes. Then, predicted damage effects are artificially introduced in the monitoring data to check for their detectability. The results demonstrate that a very small damage, associated to a low intensity and low return period earthquake, is clearly detected by the monitoring system.

DYNAMIC TESTING AND MONITORING OF HISTORIC TOWERS FOR SEISMIC DAMAGE DETECTION

Structural health monitoring (SHM) systems represent an acknowledged low-invasiveness and low-cost solution enabling real time condition assessment of civil structures, providing useful information for preventive conservation and maintenance activities. The various documented successful applications of SHM systems to bridges and large infrastructural systems has recently led many researchers to investigate their effectiveness in the case of monumental and historical buildings, often vulnerable to both anthropogenic a natural (e.g. seismic) hazards. The countries in the Mediterranean Basin are especially committed to this kind of research, being, at the same time, rich of built cultural heritage and highly exposed to seismic and other natural hazards. Among the various types of historic buildings, Italian Cities are especially characterized by an extensive presence of bell and civic masonry towers, which also deserve a special attention because of their slenderness, making them suitably monitored via vibration-based systems, and because of their seismic vulnerability. Framing in the afore-depicted scenario, the authors have started in the last years a research activity on vibration-based permanent SHM of slender historic towers, finalized at the implementation of new approaches for the optimal management of the limited available budgets for conservation and restoration activities. In the present paper the results concerning this activity are presented with specific reference to two study cases in the city of Perugia, Italy: the bell tower of the Basilica of San Pietro and the civic tower called “Torre degli Sciri”. The two considered monuments present completely different characteristics and complexities from both structural and architectural points of view, making them worth interesting investigating as typological benchmarks. The work discusses both ambient vibration testing and permanent mon-itoring applications, including considerations regarding the adopted sensing hardware. The feasibility of a rapid post-earthquake damage detection via SHM is finally explored with reference to the study case of the San Pietro bell tower, with particular attention to the recent seismic events. 2564 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 2564-2577