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ANALISI DELLE VIBRAZIONI INDOTTE DAI PEDONI SU PASSERELLE PEDONALI BASATE SU TEST SPERIMENTALI E SIMULAZIONI NUMERICHE
No full textNegli ultimi decenni, le passerelle pedonali sono diventate strutture sempre più snelle e leggere, grazie allo sviluppo di materiali da costruzione ad alte prestazioni e nuove esigenze estetiche. Tuttavia, queste strutture possono essere sensibili a sollecitazioni dinamiche essendo solitamente caratterizzate da frequenze naturali prossime a quelle tipiche dell’azione dinamica dei pedoni. Pertanto, si rendono spesso necessarie misure atte a evitare che la funzionalità di queste strutture sia compromessa da eccessive vibrazioni, siano esse modifiche strutturali o previsioni delle vibrazioni indotte.
Il presente studio caratterizza le principali fasi della procedura di valutazione delle vibrazioni indotte dai pedoni su passerelle pedonali, valutando e confrontando le prestazioni di modelli di simulazione dell’azione dei pedoni proposti in letteratura. Tale procedura include campagne sperimentali atte a caratterizzare i parametri modali della struttura e valutare le sollecitazioni indotte dai pedoni, la calibrazione di modelli numerici strutturali e l’adozione di appropriati modelli matematici per simulare l’azione dei pedoni. Le prestazioni dei modelli di simulazione sono infine valutate attraverso un confronto tra le accelerazioni sperimentali e numeriche.
Test dinamici su scala reale sono effettuati su due passerelle pedonali al fine di caratterizzare il loro comportamento dinamico e valutare la risposta a diverse condizioni di carico dei pedoni. Poiché l’affidabilità della risposta simulata dipende sia dall’adozione di adeguati modelli di carico del pedone sia dalla capacità di rappresentare il reale comportamento strutturale, la caratterizzazione del comportamento dinamico è una fase chiave nella procedura di previsione della risposta. Nel caso in cui il esso sia rappresentato attraverso modelli a elementi finiti, l’impego di tecniche di calibrazione numerica si rende necessaria per ottenere una descrizione accurata del reale comportamento dinamico.
Infine, si testano e confrontano le prestazioni dei diversi modelli matematici per la simulazione del carico dei pedoni, ponendo l’attenzione sulla componente verticale. Il primo modello analizzato è il classico modello di carico periodico, basato sull’ipotesi che la forza prodotta da ciascun piede sia la stessa e che sia periodica. In questo approccio, il modello FE della struttura è utilizzato per effettuare analisi dinamiche simulando diverse condizioni di carico. In seguito, per tenere in conto che i principali parametri che definiscono le forze indotte dai pedoni sono caratterizzati da un certo livello di variabilità, si considera un modello di simulazione probabilistico. La caratterizzazione statistica della risposta dinamica è basata sulla simulazione di numerose condizioni di traffico pedonale e perciò può essere molto onerosa in termini di tempi di calcolo. Un’interessante alternativa riguarda la modellazione dell’azione dei pedoni come un processo stazionario, basata su una definizione analitica della densità di potenza spettrale delle forze indotte. Questo modello può rappresentare il carico indotto dai pedoni anche in condizioni di intenso traffico pedonale, grazie alla definizione di una funzione che descrive la correlazione tra i pedoni. Inoltre, allo scopo di fornire una più accurata predizione della risposta strutturale in condizioni di traffico intenso, nel presente studio si tiene in conto dell’interazione meccanica tra la folla e la struttura. Negli ultimi due modelli, il comportamento strutturale è caratterizzato attraverso i parametri modali sperimentali, senza la necessità di sviluppare modelli FE; ciò implica sia un’approssimazione nella descrizione del comportamento strutturale ma anche l’eliminazione delle incertezze legate alla modellazione.Over the last decades, footbridges have become more and more lively and slender structures, due to the increasing strength of new structural materials accompanied with aesthetic requirements. However, these kinds of structures are typically characterized by natural frequencies within the range of the pedestrian dynamic actions and thus they can be very sensitive to dynamic excitation. For this reason, measures to prevent the functionality of the structure to be compromised by excessive vibrations are often required, whether they are structural modifications or vibration serviceability assessment.
This work characterizes the main stages of the procedure for the evaluation of human-induced vibrations of footbridges, assessing and comparing the reliability of pedestrian load models proposed in literature. The procedure includes experimental investigations to identify the footbridge dynamic behavior and characterize human-induced actions, calibration of structural numerical models and adoption of suitable mathematical models to simulate pedestrian loads. Finally, the reliability of the different pedestrian load models is assessed comparing numerical and experimental accelerations and the influence of the different load model hypothesis is evaluated.
Full-scale field-tests are performed on two real footbridges to experimentally identify their dynamic behavior as well as evaluate the structural response due to pedestrian actions, namely different-sized groups of pedestrians crossing the footbridge at different velocities. Since the reliability of the predicted response depends on both the adoption of adequate models to simulate pedestrian loads and the capability of representing the actual footbridge behavior, the characterization of the structural dynamic behavior is an important stage in the prediction of the structural response. When finite element models are adopted to represent the structural behavior, model-updating techniques are applied to calibrate those models in order to achieve a reliable description of the dynamic behavior.
Finally, the performances of different mathematical models to simulate human-induced loads are tested and compared, with a specific focus on the vertical components. At first, the classic periodic load model is considered, which is based on the assumption that both human feet produce the same force and that the force is periodic. In this approach, updated FE model is adopted to perform dynamic analyses simulating different pedestrian loading conditions. Then, to take into account that main parameters of the walking force are characterized by a certain level of randomness, a probabilistic simulation model is considered. As the statistical characterization of the dynamic structural response relies on the numerical simulations of many samples of pedestrian traffic conditions, it can be time consuming. An interesting alternative is the modeling of pedestrian loading as a stationary random process through the analytical definition of an equivalent spectral model of pedestrian-induced forces. The model can represent the pedestrian-induced loading even in crowded conditions thanks to the definition of a physically based function describing the correlation among pedestrians. To allow for a more accurate prediction of the maximum structural response in crowded conditions, the present study in addition accounts for the vertical mechanical interaction between pedestrians and the supporting structure. In the last two models, the structural behavior is defined by the experimentally identified dynamic parameters, with no need to develop FE models, which implies, on one hand, an approximation in the description of the footbridge behavior and, on the other, avoiding uncertainties associated with the modeling
Dynamic Identification and Model Updating of a Masonry Chimney
No full textThe paper presents the results of tests performed on a historical masonry chimney and its damage evaluation. The studied masonry chimney exhibits a clear and well visible crack pattern. To evaluate the safety condition and to design rehabilitation interventions, an extensive non-destructive test campaign is performed. The paper describes the dynamic tests, the identification of the structural modal properties, the calibration of a structural Finite Element (FE) model based on the experimental results and the evaluation of the effect of cracks on its dynamic properties. Modal identification is performed using the so called covariance-driven Stochastic Subspace Identification method (SSI-COV) to estimate natural frequencies, mode shapes and modal damping ratios. Then, the model updating is performed to localize the damage starting from the identified modal properties. Instead of adjusting the stiffness properties for all the elements, a stiffness distribution is determined by means of damage patterns. The results of two damage patterns are compared with those of the undamaged model and with the visual inspection carried out on the structure
Dynamic Assessment of Masonry Towers Based on Terrestrial Radar Interferometer and Accelerometers
Full text linkThis paper discusses the performance of a terrestrial radar interferometer for the structural monitoring of ancient masonry towers. High-speed radar interferometry is an innovative and powerful remote sensing technique for the dynamic monitoring of large structures since it is contactless, non-destructive, and able to measure fast displacements on the order of tenths of millimeters. This methodology was tested on a masonry tower of great historical interest, the Saint Prospero bell tower (Northern Italy). To evaluate the quality of the results, data collected from the interferometer were compared and validated with those provided by two types of accelerometer-based measuring systems directly installed on the tower. Dynamic tests were conducted in operational conditions as well as during a bell concert. The first aimed at characterizing the dynamic behavior of the tower, while the second allowed to evaluate the bell swinging effects. Results showed a good agreement among the different measuring systems and demonstrated the potential of the radar interferometry for the dynamic monitoring of structures, with special focus on the need for an accurate design of the geometric aspects of the surveys
A computationally efficient procedure for calibrating model parameters of multiple specimens
Full text linkModel calibration can be a very intensive and time-consuming task, especially when dealing with non-linear
and large finite element models. The computational effort further increases when multiple specimens have to
be calibrated. This is typical of laboratory experiments where several specimens made with the same and/or
different constituent materials are tested. This paper proposes a calibration procedure aimed at reducing
the computational effort of multiple specimen model calibration. The calibration procedure combines the
robustness of a surrogate-assisted evolutionary algorithm with the exploitation of a database collecting the
results of the previously calibrated specimens. In this research, the proposed procedure is applied to the
calibration of the parameters of a cohesive crack model for fiber-reinforced concrete specimens. The benefit
of the proposed procedure is shown by comparing the results with those obtained from the same calibration
method but without accounting for the previous results in the calibration of a new specimen
Dynamic monitoring of the Pasternak footbridge using MEMS-based sensing system
No full textThe aim of this paper is to investigate the dynamic behaviour of a steel curved cable-stayed footbridge using an advanced MEMS-based Structural Health Monitoring system. Experimental campaigns were carried out in July and December to characterize the dynamic behaviour of the footbridge subjected to ambient vibrations and human-induced loading actions and to evaluate the effects of temperature shifts on structural modal properties. The monitoring system is composed of a controller and storage unit and several intelligent bus-connected sensing units that can record both the accelerations along two orthogonal axes and the temperature. The main features of this system are the transmission of data in digital form and its high signal-to-noise ratio in the low and medium-low frequency range. The structural dynamic properties are identified through the classic Enhanced Frequency Domain Decomposition (EFDD) method that is based on the diagonalization of the spectral density matrix. A preliminary FE model of the footbridge is built and the numerical results are compared with the experimental ones
Dynamic behaviour of the San Felice sul Panaro Fortress: Experimental tests and model updating
Full text linkThis paper describes the experimental tests and numerical analyses performed to characterize the dynamic behaviour of the principal tower of the San Felice sul Panaro Fortress (Modena, Italy). After the Emilia earthquake that occurred in 2012, the Fortress reported serious damage, such as severe cracks on the walls and collapses of several towers and the roof. As a part of a research that aims at evaluating the vulnerability of the Fortress and designing retrofitting interventions, full-scale ambient vibration tests were performed to evaluate the dynamic properties of the principal tower. Afterwards, a Finite Element (FE) model is calibrated to obtain a good match between the numerical and experimental modal properties. The optimization process is carried out through an improved surrogate-assisted evolutionary strategy. Due to the serious damage of the Fortress, the effective stiffness of the cracked masonry and the efficiency of connection at the interface between the principal tower and the rest of the Fortress are considered the main uncertain quantities to be calibrated. A multi-objective optimization is performed, considering the frequency and mode shape residuals. These are defined as the difference between experimental and numerical modal properties. The multi-objective optimization is reduced to a series of a single-objective optimization adopting the weighted sum method. The set of optimal solutions that form the Pareto front is obtained performing the optimization for different values of the weighting factors. Then, two criteria are used and compared in order to find the preferred solution among the Pareto front solutions. Finally, a comparison of the identified structural parameters obtained varying the weighting factors for natural frequencies and mode shapes in the optimization process is presented, highlighting the importance of a proper choice of the weighting factors
Contribution of anthropogenic consolidation processes to subsidence phenomena from multi-temporal DInSAR: a GIS-based approach
Full text linkThe paper introduces an approach based on the combination of multi-temporal Differential Interferometric Synthetic Aperture Radar and geographical information systems analysis to investigate and separate several contributions to subsidence phenomena over the municipality of Ravenna (Emilia Romagna, Italy). In particular, the relationship between displacements detected over built environment and consolidation processes after construction was assessed and filtered out from the subsidence map to quantify the local overestimation of subsidence phenomena due to the mentioned processes. It requires descriptive attributes related to the age of construction and intended uses. The outcomes of the present study highlight ground consolidation processes that seem to be active over areas settled in the last 30 years with a component contributing to vertical rates up to 3 mm/yr. Such contribution represents the 20% of the cumulative displacements reported for coastal villages where different sources of subsidence increase the vulnerability to coastal erosion. We discuss the contribution of consolidation processes over a couple of recently settled areas to separate among contributions and avoid the misinterpretation of effects due to other anthropogenic sources of subsidence
Ambient vibration-based finite element model updating of an earthquake-damaged masonry tower
Full text linkThis paper presents a vibration-based model updating procedure for historical masonry structures which have suffered severe damage due to seismic events. This allows gathering in-depth insights on the current condition of damaged buildings, which can be beneficial for the knowledge of their actual structural behaviour and, consequently, for the design of repairing and strengthening interventions. The methodology, based on the experimentally identified modal parameters, is tested on the San Felice sul Panaro medieval fortress, which was heavily damaged by the 2012 Emilia earthquake. The finite element mesh of the structure in its post-quake condition is generated by means of a nonstandard semi-automatic mesh generation procedure based on a laser scanner points cloud. Ambient vibration testing is performed on the main tower of the fortress. Mechanical properties of the tower and the level of connections with the rest of the fortress in its current damaged state are investigated. To fully characterize the actual behaviour of the tower in operational condition, mesh elements corresponding to the damaged masonry are identified and different material properties are assigned to them. This allows to account for the effect of damage and cracks, which appeared essential in the calibration process. The updating procedure is carried out by means of an advanced surrogate-assisted evolutionary algorithm designed for reducing the computational effort
Human-structure interaction effects on the maximum dynamic response based on an equivalent spectral model for pedestrian-induced loading
No full textThe paper investigates the effects of the human-structure interaction (HSI) on the dynamic response based on a spectral model for vertical pedestrian-induced forces. The spectral load model proposed in literature can be applied for the vibration serviceability analysis of footbridges subjected to unrestricted pedestrian traffic as well as in crowded conditions, however, in absence of HSI phenomena. To allow for a more accurate prediction of the maximum structural response, the present study in addition accounts for the vertical mechanical interaction between pedestrians, represented by simple lumped parameter models, and the supporting structure. By applying the classic methods of linear random dynamics, the maximum dynamic response is evaluated based on the analytical expression of the spectral model of the loading and the frequency response function (FRF) of the coupled system. The most significant HSI-effect is in the increase of the effective damping ratio of the coupled system that leads to a reduction of the structural response. However, in some cases the effect of the change in the frequency of the coupled system is more significant, whereby this results into a higher structural response when the HSI-effects are accounted for
Dynamic identification of an ancient masonry bell tower using a MEMS-based acquisition system
No full textIn this paper results of dynamic tests performed on a bell tower located in Ficarolo (Italy) are reported. After the Emilia earthquake that occurred in 2012, the bell tower reported a serious damage pattern and, as a consequence, retrofitting interventions were carried out. Dynamic tests before and after the strengthening were performed to investigate the modal properties of the bell tower and to evaluate possible changes in dynamic behavior due to the intervention. Accelerations during ambient vibrations were recorded by means of an advanced MEMS-based system, whose main features are the transmission of the data in digital form and the possibility of performing some system analyses directly on-board of the sensors. Accelerations were acquired using 11 biaxial MEMS units. First 8 modes are clearly identified, with natural frequencies in the range 0.5-9.0 Hz. Finally, a comparison between the performances of the installed MEMS-based system and a traditional analog (piezoelectric) system is carried out and results are critically compared
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