Structural change monitoring of a cable-stayed bridge by time-series modeling of the global thermal deformation acquired by GPS Abstract In this study, the use of ARIMA model coefficients extracted from the response of global thermal deformation, which can be acquired by the GPS monitoring, was proposed for the structural change monitoring of the long-span bridge. The daily periodic air temperature change causes the characteristic global thermal deformation, which is suitable to be acquired by the GPS in the long-span cable-stayed bridge. The pattern of this global thermal deformation was then expected to have sensitivities to changes on global structural properties. The procedures of feature extraction based on the ARIMA model estimation and the Mahalanobis distance comparison were presented, and their applicabilities were verified both by the numerical study and by the application to actual GPS monitoring data. In the numerical study, a FE model of a cable-stayed bridge was constructed, and the time-series displacements under the periodic temperature load were obtained with some cases of structural conditions by varying the boundary conditions and the cable tensions. The procedures of feature extraction and comparison were then applied to the obtained displacement time-series. In the results, the Mahalanobis distance of feature vector, which was configured by estimated AR and MA coefficients, showed significant changes both in the two cases of boundary condition and stayed-cable tension changes. The procedure was then applied to year-round GPS data acquired in the actual cable-stayed bridge. It was shown that the Mahalanobis distance comparison could provide proper assessment to the structural changes that was consistent with the actual structural condition.

Dynamic response measurement of steel plate structure utilising video camera method Abstract Vibration measurement is a common method used to assess a structure’s condition. The basic premise for the detection of damage in a structure is based on vibration measurement, whereby changes of stiffness, vibration mode, and energy dissipation in the system can cause changes in the dynamic response. The vibration measurement is conducted by obtaining the fundamental period to judge the health condition of the monitored structure. This research used the high-speed camera (HSC) to capture the structure motion during the vibration of steel-plated structure. Laboratory experiments were performed on a thin steel plate (600 × 600 × 2.3 mm) and orthotropic steel deck. The HSC was used to measure the resonance frequency of the structure monitored.

Strategies of structural health monitoring for bridges based on cloud computing Abstract The collapse of the Polcevera bridge in Italy represents a serious event which seems to be a direct result of cumulated local damages due to the aggressive environment of the construction site. Recently, evidence of corrosion of both ordinary and post-tension steel reinforcements were detected, in addition to concrete carbonation. Such phenomena generally lead to an increase in the deformation of all the elements of the bridge structure, which start to increase in time, leading to a progressive deterioration of the overall system. As a consequence, a proper structural monitoring layout would provide an extremely useful tool, for a correct plan of maintenance for all the elements of the considered infrastructure. In this work, strategies for the definition of structural health monitoring systems for bridges are discussed, from both software and hardware points of view. More specifically, a Cloud computing interface is considered, to make recorded data available for further analyses and post-processing procedures. The presented definition of the monitoring architecture could lead to the proper maintenance of all the structural elements, preventing the unexpected collapse of the structure.

Responses of the odd couple Carquinez, CA, suspension bridge during the M w 6.0 south Napa earthquake of August 24, 2014 Abstract The behavior of the suspension bridge in Carquinez, CA, during the Mw6.0 24 August 2014 South Napa, CA earthquake is studied. Utilizing data from an extensive array of accelerometers that recorded the earthquake-excited motions, dynamic characteristics such as modes, corresponding frequencies and damping are identified and compared with previous studies that used ambient data of the deck only plus mathematical models. Data are systematically analyzed for vertical, transverse and torsional motions of the deck, and transverse, longitudinal and torsional motions of the towers. The transverse and vertical fundamental mode frequencies of the deck are the same (0.17 Hz) due to coupling. Higher frequencies for transverse and vertical coupled modes are also the same at 0.46 Hz and 0.98 Hz. Tower translational frequencies are 0.39 Hz in the transverse direction and 0.46 Hz in the longitudinal direction, and are also coupled with those of the deck. Coupling of torsional modes of the tower and deck is also identified. A beating effect is observed, particularly for torsional motions.

Structural health status assessment of a cable-stayed bridge by means of experimental vibration analysis Abstract The results of an experimental dynamic analysis and structural modelling of the Indiano Cable-Stayed Bridge in Florence, Italy, are presented in this paper. Ambient and traffic-induced vibration tests were first carried out. These allowed extracting the dynamic characteristics of the structure in terms of resonance frequencies, modal shapes and damping. The experimental results were used to set up a finite element model. The geometrical characteristics and the mechanical properties of the materials used in the structural design of the bridge were assumed. The model was then used to evaluate the effects of the static and seismic loads according to the present Italian Technical Code. The results pointed out the good performance of the bridge, even though it had been designed without accounting for the seismic actions.

Continuous monitoring of the Milan Cathedral: dynamic characteristics and vibration-based SHM Abstract The traditional collaboration between Politecnico di Milano and Veneranda Fabbrica del Duomo di Milano—the historic institution established by Gian Galeazzo Visconti in 1387 and having in charge all operational aspects related to the Milan Cathedral since more than 600 years—recently focused on the design and installation of a structural monitoring system, with the objective of assisting the condition-based structural maintenance of the historic church through the continuous interrogation of sensors installed in the structure and the extraction from measured data of features which are representative of the current state of structural health. The new monitoring system of the Milan Cathedral includes different types of measurements and sensors: quasi-static acquisition of strain in selected tie-rods and biaxial tilt of selected piers and the main spire, monitoring of inner and outer environmental parameters and dynamic measurement of the velocity response at the top of 14 piers and at 3 levels of the main spire. After a concise description of the historic church and of the monitoring system, the paper focuses on the dynamic characteristics of the Milan Cathedral, their evolution during the first months of monitoring (since October 16th, 2018) and the lessons learned in view of the structural health monitoring of the monument. The presented results from the vibration monitoring highlight that: (a) 8 global modes of vibration are automatically detected in the frequency range 1.0–5.0 Hz; (b) the resonant frequencies exhibit a distinctive trend of variation, which is mainly driven by temperature; (c) the mode shapes of the cathedral do not show appreciable fluctuations associated with the environmental effects.

Ambient and free-vibration tests to improve the quantification and estimation of modal parameters in existing bridges Abstract The correct estimation of modal parameters has an important role in ensuring the structural reliability of existing bridges. Operational modal analysis provides rather accurate extraction of natural frequency and mode shapes, but the corresponding damping estimates are subjected to higher uncertainties. This paper reports the results of ambient and free-vibration tests performed on five different typologies of road and railway bridges: steel trusses, steel box, multi-girder reinforced concrete, masonry and reinforced concrete arch bridges. In one case, data acquired by a continuous structural health monitoring system over a period of 1 year are available. In another case, the passage of trains is exploited to perform free decays. The processing of ambient vibrations is performed through several frequency and time-domain identification techniques, whereas the measured free decays are analyzed by the logarithmic decrement method applied to the autocorrelation functions of the signals. Outcomes are compared, evaluating the quality of modal damping estimates and the accuracy of results. The influence on modal parameters extraction of structural typologies, length of acquired time histories and ambient noise, loading and environmental effects are studied. At the end, it is demonstrated that the estimation of modal damping is more reliable for flexible structures when SHM and free-vibration data are available.

Vibration-based structural health monitoring using symbiotic organism search based on an improved objective function Abstract Vibration-based structural health monitoring (VSHM) relying on model updating methods has been developed greatly and nowadays, not only serves as a major subset of SHM, but also shapes a special class of optimization problems. The historical course of evolution of this field via different research groups and with different goals and objectives, has resulted in the emergence of multiple objective functions, each appropriate only for certain damage scenarios and incapable of reasonably addressing others. The natural frequency residual (NFR) is an objective function sensitive to the damage intensity, which in the meantime, fails to predict the damage location in the symmetric structures. The total modal assurance criterion (TMAC) is another objective function which is sensitive to the damage position, but fails to estimate of the damage severity in the uniformly damaged structures. The present work successfully provides an innovative solution to unify both aforementioned objectives in a holistic objective function (HOF), through a particular combination of NFR and TMAC. Additionally, the competency of the above HOF for solving the VSHM problems has been investigated and demonstrated using a symbiotic organism search (SOS) optimization algorithm. The robustness and efficiency of the proposed VSHM method are verified through assessment of two un-damped benchmark structures. The obtained results indicate that in all damage scenarios, the HOF predicts with a high precision the damage location and succeeds in the high accuracy estimation of the damage extent. Consequently, the combination of the proposed HOF criterion and the SOS optimization algorithm is recommended as a reliable technique for VSHM.

Elementary concepts of structural robustness of bridges and viaducts Abstract This paper will explain the elementary aspects related to structural robustness. The concept of structural integrity is firstly introduced and next it is considered how it varies in time. The different kind of progressive collapses are introduced with reference to real cases. Design strategies are then explained with reference of bridges and viaducts, considering examples. Finally, considerations about a more general view of structural robustness are enlightened. The role of design clima and conceptual design is stressed also in connection with structural health monitoring. In the whole paper, a practical point of view is used, as needed by designer.

Measured properties of structural damping in railway bridges Abstract Dissipative properties of a structural system are difficult to be characterized in real structure. Nevertheless, damping features may be dominant in several operating conditions of railway bridges influencing fatigue life or passenger comfort during train passage. Observations treating real data acquired in operational condition on steel and concrete railway bridges belonging to the Italian network permits to highlight dissipative sources and features. Consequently, linearized modal damping ratios are evaluated through a recursive process on the acceleration signals acquired before, during and after train passages and/or in environmental conditions. Stochastic Subspace Identification has been used to identify state-space dynamical models able to reproduce the vibrations. Through these models, characterized by an increasing number of state-space variables, it is possible to extract modal damping ratios. A mechanical interpretation of damping characteristics is pursued through the evaluation of the differences with respect to a classical Rayleigh proportional damping matrix of the viscous matrix belonging to the identified state-space models determined through the system spectral features. A non-proportional damping index is presented as a basis to determine the influence of different sources of non-proportionality in the damping matrix (as the ballast layer under the track) and to justify the high value of damping observed in specific experimental campaigns.