1,721,156 research outputs found
Introductory notes for the Acta IMEKO Special Issue on the XXIX Italian National Congress on Mechanical and Thermal Measurements
No full textAn analytical perspective about structural damage identification based on transmissibility function
No full textTransmissibility functions have been widely applied into the field of structural health monitoring, especially for damage detection. However, due to the existence of few analytical research works to conceive the significance of damage onset, the inherent mechanism of damage identification based on transmissibility function has not yet been clearly and deeply evaluated. In this article, an analytical approach has been investigated to demonstrate how to localize damage components caused by mass and stiffness change in terms of multiple-degree-of-freedom mass–spring–damper system, based on the inherent analysis between the transmissibility functions for any two consecutive masses. This study also points out the importance of the system input: various input location may lead to different recognized damage regions. Related simulation case studies and laboratory activities are carried out, proving the effectiveness and accuracy of the proposed approach
Scour at river bridge piers: real-time vulnerability assessment through the continuous monitoring of a bridge over the river Po, Italy
No full textBridge failures are not uncommon during floods. Therefore, the assessment of bridge vulnerability is crucial to help a bridge's administration take the best decisions during emergencies (i.e. closing the bridge). The first step to achieve this is to implement a monitoring system providing needed information to develop scenarios based on administration response time. The proposed system seeks a real-time evaluation of the loads acting on the bridge. Wind and water contributions, debris accumulation upstream of piers and river bed level are evaluated by specific devices. The most difficult measurement is the identification of the riverbed position in close proximity to piers. Scour model predictions are still far from robust and accurate and no standard devices are available to measure it. Unfortunately, the main reason for bridge failure is scour around bridge piers, called local scour. A monitoring system was developed for this purpose and installed on a critical bridge over the river Po in Italy, as a result of the cooperation between the Politecnico di Milano and the Province of Mantova. The aim was to measure the main parameters for a proper evaluation of the bridge's vulnerability. The paper's focus is twofold: to show long-term data and all the phases needed to assess bridge safety. The first aspect, not so common in the literature, is useful to understand the temporal evolution of the environment around the bridge. The second one provides a roadmap for risk assessment and management decisions relying on the widely available database. With regard to this second topic, another paper describing the overall strategy has already been published. The main focus here is on potential implementation, with an emphasis on the available data
Assessment of a Vision-Based Technique to Estimate the Synchronization of Jumping Crowds in Civil Structures
No full textVibration serviceability criteria for civil structures follow a three-step framework, namely the excitation source, the path and the receiver. The first step, which is also the focus of this study, deals with the characterization of human-induced loads. However, the design models reported in the current guidance and codes are very often overly conservative and cannot adequately represent the real nature of crowd excitation. In this work, we present a computer vision technique, based on the use of Digital Image Correlation (DIC), as a solution to this problem. In addition to a cheap and an easy to install set up, the system can provide a comprehensive assessment of the coordinate motion induced by occupying crowds of various sizes. To demonstrate the efficacy of the proposed method, the measured DIC data are compared to those coming from the accelerometers installed on multiple subjects while performing jumping activities on a real grandstand. Then, the vision-based approach is used to study and to quantify the level of synchronization among the individuals for a range of songs and metronome beats. Results demonstrate that the DIC technique achieves similar performance as the inertial sensors but overcomes some practical limitations related to these traditional systems
Progettazione e realizzazione di un sistema di isolamento sismico per la Vittoria Alata
No full textLa classica filosofia della progettazione strutturale alle azioni sismiche, basata sulle forze
(Force-Based Design) o sugli spostamenti (Displacement-Based Design) prevede di costruire
strutture sismo-resistenti, basandosi sui concetti di capacity design. Tale progettazione, quindi,
è indirizzata alla realizzazione di strutture in grado di resistere agli eventi sismici attraverso
lo sfruttamento della loro duttilità, locale e globale.
Tale filosofia progettuale, se la progettazione è correttamente eseguita e in caso di eventi
sismici paragonabili a quelli di progetto, garantisce un adeguato grado di sicurezza per il
collasso della struttura ma comporta di fatto un globale danneggiamento irreversibile della
stessa, con le relative conseguenze negative di tipo economico e sociale, nel caso ad esempio
di edifici sensibili che debbano interrompere le loro attività in seguito al sisma (ospedali,
aziende)
Detecting Real Damage in Operating Tie-Rods Under Uncontrolled Environmental and Operational Conditions
No full textGiven the current availability of increasingly advanced sensors and processing capabilities, structural health monitoring (SHM) is the most suitable approach to the maintenance of operating structures. The recent trends in literature indicate that unsupervised data-driven damage detection algorithms may allow a transition from human inspections to continuous and automatic monitoring. However, there are still few examples that prove their effectiveness in real scenarios, i.e., in presence of uncontrolled environmental and operational conditions. This paper tries to bridge this gap, by presenting an application where vibration-based unsupervised damage detection is used to spot the existence of an ongoing corrosion process on operating tie-rods. These structural elements are metallic slender beams subject to axial load, used to balance lateral forces of arches and vaults, in both modern and ancient civil structures. Like all slender structures, they undergo significant vibration levels which make the use of modal identification algorithms particularly effective. In a recent study, the authors demonstrated how modal parameters can be used to define a multivariate damage feature which allows a separation between the environmental effects and damage. This paper investigates the potential of using a Gaussian mixture model for detecting damage in tie-rods through unsupervised data clustering. The potential of the proposed approach is demonstrated considering real data, acquired under uncontrolled environmental and operational conditions, and in presence of real damage
Vision-Based Method to Measure the Synchronization Level of Jumping Crowds
Full text linkThe prediction of the dynamic loads produced by groups of people is a crucial aspect of the design of stadiums or entertainment venues. This is because the coordinated motion of lively crowds may induce severe vibration levels in the structure, which can become critical for both human comfort and structural safety. However, the available information on this topic is very limited. Human loads often rely on deterministic models that do not consider the interaction and the coordination achieved by the participants or try to account for them through empirical assumptions. Therefore, they could find very little correspondence in realistic scenarios. This article aims to close this gap by introducing a vision-based technique able to directly measure crowd loading and quantify the synchronization level between individuals. Starting from a sequence of images of a jumping crowd, digital image correlation (DIC) is used to extract the vertical velocity of different regions occupied by the participants; then, the vertical force time record is estimated. Finally, the comparison between the actual force signals and their envelopes allows for estimating the crowd synchronization over time. The method has been successfully validated with two field tests on the grandstands of the Giuseppe Meazza stadium in Milan, demonstrating its ability to reliably estimate the synchronization level reached by the participants
A cointegration-based approach for automatic anomalies detection in large-scale structures
Full text linkIn recent years, the development of structural health monitoring (SHM) solutions for the automatic evaluation of the health state of engineering structures is continuously growing. However, when considering real-world applications, structures are highly influenced by meteorological variations or human activities (like temperature, wind and traffic loading) which can overwhelm the changes induced by a damage. Thanks to its ability to remove the long-term trends from a set of variables of the same process, cointegration, a technique born in the field of econometrics, has been introduced about ten years ago in SHM applications as a valid method to project out the confounding influences, such as environmental and operational variations. Because of the few examples of implementation currently available, this paper provides an in-depth review of all the relevant aspects to consider when cointegration is used as damage detection strategy and data are acquired from real-world structures of large dimensions. The methodology is applied for the first time on a complex structure of a singular nature, i.e. the steel roof of the G. Meazza stadium in Milan, which consists of multiple modular elements referred to as rafts. The time series which measures the rotations of the rafts are used as input data for the development of the cointegration-based method. Then, Johansen procedure is adopted to create a unique feature from the multivariate dataset, namely cointegrating residual, in which the effects of environmental and operational variables are suppressed, while the effects due to damage remain evident. The obtained residual is therefore used for novelty detection by means of a control chart, demonstrating its effectiveness into identifying the presence of anomalies or modifications in the structure in a clear and timely manner
Wind actions on the London Eye cables. Part II: damping device design and field measurements
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