With reference to the bridges, it is mandatory to perform a static load test (law n.1086 / 71), the directives for which are reported in Section 9 of the New Technical Standards for Construction (NTC 2018), while the dynamic tests they are mandatory only for road and rail bridges of significant importance. In particular, the fundamental period determined experimentally must be comparable with that foreseen in the design phase. The dynamic tests, whose purpose is to identify the dynamic parameters of a structure, are taking place in civil works, especially with reference to bridges. When dynamic tests have to be performed on a bridge, Operative Modal Analysis (OMA) is a fast, cheap and efficient solution because the dynamic characterization is based on the response to environmental vibrations to which a structure is always subject. In order to give consistency to the results and guarantee the validity of the model, these dynamic tests are carried out more and more frequently in the case of the static test and the relative load test. The tests on the loaded bridge generally offer the possibility of evaluating the dynamics of the structure subject to a different (known) mass distribution compared to that of the permanent loads. The loads are usually made up of heavy trucks distributed in one or more rows both longitudinally and transversely to the bridge span. Their weight can be up to 50 tons and can significantly change the dynamic behaviour of the bridge. In the numerical model, trucks used for static tests are generally modelled only in terms of added masses on the bridge. This method does not consider that the dynamic system consisting of the truck, the shock absorbers and the tires, interacting with the dynamic system constituted by the bridge, can also significantly change its modal response. In other words, the fact that the mass of the trucks acts through a flexible system is neglected. In particular, when the frequencies of the bridge and those of the trucks are similar, the latter behave as a tuned mass damper. Not only they reduce the amplitude of mechanical vibrations on the bridge, but they also change its frequencies. Not taking into account the truck-bridge interaction, and the related effects on vibration frequencies, could lead to an incorrect interpretation of the dynamic test result, and consequently the test result. As part of this thesis, the effective importance of the truck-bridge interaction phenomena was first investigated, through an adequate modelling of the structure and trucks, and subsequently the problem was studied in parametric terms, focusing on the effects of the aforementioned interaction on first bending frequencies of the deck. At the beginning the study was limited to simple analytical models, in which the bridge and the truck were both reduced to a 1DOF system. Driven by the observations made on these simplified models, a finite element modelling of a bridge on which the trucks are present was performed. To have a measure of the effect of the bridge-truck interaction, the variations of the frequencies and the participating mass factors of the first two modes of vibrating of the bridge were monitored. In order for the results to be of practical interest, the study was conducted on realistic case studies obtained taking into account four parameters. These are the ratio between the mass of the trucks and the mass of the loaded portion of the bridge; the ratio between the loaded length and the length of the bridge; the number of trucks present in the longitudinal direction; the number of trucks present in the cross section.
Con riferimento ai ponti, è obbligatorio eseguire una prova di carico statica (legge n.1086 / 71), le cui direttive per l'esecuzione sono riportate nella Sezione 9 delle Nuove Norme Tecniche per le Costruzioni (NTC 2018), mentre le prove dinamiche sono obbligatorie solo per ponti stradali e ferroviari di significativa importanza. In particolare, il periodo fondamentale determinato sperimentalmente deve essere confrontabile con quello previsto in fase di progettazione. I test dinamici, il cui scopo è quello di identificare i parametri dinamici di una struttura, stanno prendendo campo nelle opere civili, specialmente con riferimento ai ponti. Quando i test dinamici devono essere eseguiti su un ponte, l'Operative Modal Analysis (OMA) è una soluzione veloce, economica ed efficiente perché la caratterizzazione dinamica è basata sulla risposta alle vibrazioni ambientali, a cui una struttura è sempre soggetta. Per dare coerenza ai risultati e garantire la validità del modello, sempre più frequentemente queste prove dinamiche sono eseguite in occorrenza del collaudo statico e della relativa prova di carico. Le prove sul ponte carico in genere offrono la possibilità di valutare la dinamica della struttura soggetta ad una differente distribuzione (nota) di massa rispetta a quella che compete ai carichi permanenti. I carichi sono solitamente costituiti da autocarri pesanti distribuiti su una o più file sia longitudinalmente che trasversalmente rispetto alla campata del ponte. Il loro peso può arrivare fino a 50 tonnellate e può modificare in modo significativo il comportamento dinamico del ponte. Nel modello numerico, i camion utilizzati per le prove statiche sono generalmente modellati soltanto in termini di masse aggiunte sul ponte. Questo metodo non considera che il sistema dinamico costituito dal camion, dagli ammortizzatori e gli pneumatici, interagendo con il sistema dinamico costituito dal ponte, ne possa cambiare anche significativamente la risposta modale, in altre parole si trascura il fatto che la massa dei camion agisce per il tramite di un sistema cedevole. In particolare, quando le frequenze del ponte e quelle dei camion sono simili, questi ultimi si comportano come un tuned mass damper e non solo riducono l'ampiezza delle vibrazioni meccaniche sul ponte, ma ne modificano anche le frequenze. Non tenere conto dell’interazione camion-ponte, e dei relativi effetti sulle frequenze di vibrazione, potrebbe portare ad un'errata interpretazione dell’esito del test dinamico, e conseguentemente dell’esito del collaudo. Nell'ambito di questa tesi, si è dapprima indagata l’effettiva importanza dei fenomeni di interazione camion-ponte, attraverso una adeguata modellazione della struttura e degli autocarri, e successivamente si è studiato il problema in termini parametrici concentrandosi sugli effetti della suddetta interazione sulla prime frequenze flessionali dell’impalcato. All'inizio lo studio si è limitato a modelli analitici semplici, in cui il ponte e il camion erano ridotti entrambi ad un sistema 1DOF. Spinti dalle osservazioni fatte su questi modelli semplificati, è stata eseguita una modellazione agli elementi finiti di un ponte su cui sono presenti gli autocarri. Per avere una misura dell'effetto dell'interazione ponte-camion, si sono monitorate le variazioni delle frequenze e dei fattori di massa partecipante dei primi due modi di vibrare del ponte. Affinché i risultati fossero di interesse pratico, lo studio è stato condotto su casi studio realistici, ottenuti tenendo conto di quattro parametri: il rapporto tra la massa dei camion e la massa della porzione caricata del ponte; il rapporto tra la lunghezza caricata e la lunghezza del ponte; il numero di autocarri presenti in senso longitudinale; il numero di camion presenti nella sezione trasversale.
L’interpretazione di prove di caratterizzazione dinamica eseguite nelle fasi di collaudo dei ponti
ORSELLI, STEFANO
2019/2020
Abstract
With reference to the bridges, it is mandatory to perform a static load test (law n.1086 / 71), the directives for which are reported in Section 9 of the New Technical Standards for Construction (NTC 2018), while the dynamic tests they are mandatory only for road and rail bridges of significant importance. In particular, the fundamental period determined experimentally must be comparable with that foreseen in the design phase. The dynamic tests, whose purpose is to identify the dynamic parameters of a structure, are taking place in civil works, especially with reference to bridges. When dynamic tests have to be performed on a bridge, Operative Modal Analysis (OMA) is a fast, cheap and efficient solution because the dynamic characterization is based on the response to environmental vibrations to which a structure is always subject. In order to give consistency to the results and guarantee the validity of the model, these dynamic tests are carried out more and more frequently in the case of the static test and the relative load test. The tests on the loaded bridge generally offer the possibility of evaluating the dynamics of the structure subject to a different (known) mass distribution compared to that of the permanent loads. The loads are usually made up of heavy trucks distributed in one or more rows both longitudinally and transversely to the bridge span. Their weight can be up to 50 tons and can significantly change the dynamic behaviour of the bridge. In the numerical model, trucks used for static tests are generally modelled only in terms of added masses on the bridge. This method does not consider that the dynamic system consisting of the truck, the shock absorbers and the tires, interacting with the dynamic system constituted by the bridge, can also significantly change its modal response. In other words, the fact that the mass of the trucks acts through a flexible system is neglected. In particular, when the frequencies of the bridge and those of the trucks are similar, the latter behave as a tuned mass damper. Not only they reduce the amplitude of mechanical vibrations on the bridge, but they also change its frequencies. Not taking into account the truck-bridge interaction, and the related effects on vibration frequencies, could lead to an incorrect interpretation of the dynamic test result, and consequently the test result. As part of this thesis, the effective importance of the truck-bridge interaction phenomena was first investigated, through an adequate modelling of the structure and trucks, and subsequently the problem was studied in parametric terms, focusing on the effects of the aforementioned interaction on first bending frequencies of the deck. At the beginning the study was limited to simple analytical models, in which the bridge and the truck were both reduced to a 1DOF system. Driven by the observations made on these simplified models, a finite element modelling of a bridge on which the trucks are present was performed. To have a measure of the effect of the bridge-truck interaction, the variations of the frequencies and the participating mass factors of the first two modes of vibrating of the bridge were monitored. In order for the results to be of practical interest, the study was conducted on realistic case studies obtained taking into account four parameters. These are the ratio between the mass of the trucks and the mass of the loaded portion of the bridge; the ratio between the loaded length and the length of the bridge; the number of trucks present in the longitudinal direction; the number of trucks present in the cross section.| File | Dimensione | Formato | |
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Tesi Magistrale Orselli Stefano Definitivo.pdf
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https://hdl.handle.net/20.500.12075/3768