Nowadays one of the most challenging problems to be solved in Civil Engineering is the crossing of waterways of relevant depth. As a matter of fact, conventional solutions, such as cable stayed bridges and suspension bridges, can be effectively employed to cover long distances but only in shallow waters, example being the 1915 Çanakkale Bridge opened in March 2022 in the Dardanelles strait, and the Akashi Kaikyo Bridge, respectively the first and the second world longest suspension bridges. As the depth increases, alternative solutions are needed and a promising one is offered by Submerged Floating Tunnels. A Submerged Floating Tunnel (SFT), also known as Archimedes Bridge, consists in a hollow tubular structure floating in water thanks to the upthrust exerted by the means and kept in place by anchoring systems. The shape of the cross section, as well as the type of anchoring systems may differ depending on the environmental conditions and also on the maritime traffic that interests the site. The concept origin dates back to the 1860 and several studies have been undertaken in the recent past, especially in Norway, China and Italy since multiple would be the advantages that its realization would bring. In fact, such a structure would allow to preserve beautiful natural landscape surrounding fjiords, lakes and shores, being it completely out of human sight. In addiction, travel times would be reduced, there would be no risks associated to extreme weather conditions, improving the overall users experience, pollutant agents would be disposed of. Being it a modular structure, it is reasonable that construction costs would be linearly proportional to the tunnel lenght, and so they would also be reduced with respect to those associated to other solutions, such as a suspension bridge, of the same magnitude, making the SFT a more cost-effective solution. Nevertheless, such a structure has never seen the light, even though its feasibility has been proven with the Høgsfjord project, which has been discharged for political reasons. Safety is the main concern and, on this page, many are still the aspects to be investigated, connected to all the various possible loading conditions and extreme events that could interest such tunnel. Not only, there is a psychological aspect to be accounted for that obstacles its realization, which is relative to the general belief that travelling in a floating underwater tunnel could somehow be unsafe, and that disreguards completely the fact that this type of technology is already in use, even if in a different form, as the case would be of underwater tunnels on the seabed. This being said, starting from a semi-analytical continuous model recently proposed in literature, the aim of this research work is to develop a continuous model for the static and dynamic analysis of SFTs under different loading conditions, able to significantly reduce the computational cost associated with the evaluation of displacements and internal actions with respect to a numerical analysis performed by means of FEM models. Focus has been laid on the reconstruction of internal actions of the tunnel when subjected to seismic excitation and hydrodynamic load. For what concerns the seismic excitation, a response spectrum analysis has been performed and the overall structural response has been determined by means of modal superposition, exploiting the CQC modal combination rule. Then the results have been compared with the ones coming from a finite element model, showing excellent agreement. As for the hydrodynamic load, this was modelled by means of Morison Equation, which assumes the force as a linear combination of an inertial and a drag term. Then, numerical applications have been developed with reference to the inertial dominated regime only, evaluating the statistics of the response over a certain number of time histories. However, the time domain framework through which the analyses have been carried out is suitable for further expansion to the drag case. Also the case of seabed depth variability has been implemented, with the purpose of investigating its effect on the overall structural behaviour if compared to the case in which, instead, this variability is disregarded.
Oggigiorno, una delle sfide più grandi per l'Ingegneria Civile rimane quella associata alla ricerca di soluzioni per l'attraversamento di corsi d'acqua caratterizzati da notevoli profondità. Difatti, ponti strallati e sospesi possono essere efficacemente impiegati per coprire lunghe distanze ma solo in acque poco profonde, ne sono un esempio il Ponte dei Dardanelli, inaugurato nel marzo 2022 e il famosissimo Akashi Kaikyo Bridge, rispettivamente il primo e il secondo ponte sospeso più lungo al mondo. All'aumentare della profondità del fondale, tali strutture, per le quali abbiamo a disposizione conoscenze e tecniche realizzative ben consolidate, diventano di difficile realizzazione. Promettente appare allora l'alternativa offerta dal cosiddetto Tunnel Flottante, noto anche come Ponte di Archimede. Si tratta di una struttura tubolare sommersa, galleggiante grazie alla spinta esercitata dal fluido e tenuta in posizione da sistemi di ancoraggio, la cui sezione può avere forma variabile, dipendente dalle condizioni ambientali e di traffico del sito. Pensato per la prima volta nel 1860, è stato ed è tutt'ora oggetto di studio in diversi paesi, tra cui Norvegia, Cina e Italia. Il grande interesse nei suoi confronti sorge dal fatto che molteplici sarebbero i vantaggi connessi ad una sua realizzazione, come ad esempio una riduzione dei tempi di percorrenza e la preservazione del paesaggio naturale circostante fiordi, laghi e rive. Trattandosi di una struttura modulare, è ragionevole pensare che i costi connessi ad una sua realizzazione sarebbero linearmente proporzionali alla lunghezza del tunnel, quindi anche di molto inferiori se confrontati a quelli che sono affrontati per la realizzazione ad esempio di ponti sospesi. A dispetto dei vari aspetti positivi che lo caratterizzano, il Tunnel Flottante non ha ancora trovato realizzazione in nessuna parte del mondo, nonostante la sua fattibilità sia stata dimostrata nell'ambito del progetto Høgsfjord, che è stato respinto per motivi politici. Tra le varie ragioni, la principale è da ricercarsi nella necessità di individuare soluzioni e tecniche costruttive in grado di far fronte a condizioni di carico estreme che potrebbero interessare il tunnel e che vanno dunque adeguatamente studiate, al fine di garantire un adeguato livello di sicurezza strutturale. Un altro elemento ostativo è poi associato alla convinzione generale che viaggiare in un tunnel sottomarino galleggiante possa in qualche modo essere pericoloso, e che prescinde completamente dal fatto che questo tipo di tecnologia è già in utilizzo, anche se in forma diversa, nel caso di tunnel sottomarini adagiati sul fondale. Detto questo, partendo da un modello semi-analitico recentemente proposto in letteratura, lo scopo di questo lavoro di ricerca è quello di sviluppare uno strumento per l'analisi statica e dinamica dei ponti flottanti soggetti a diverse condizioni di carico, che sia in grado di ridurre significativamente il costo computazionale associato alla valutazione degli spostamenti e delle azioni interne rispetto ad un'analisi numerica eseguita mediante modelli FEM. Particolare attenzione è stata posta nella ricostruzione delle azioni interne del tunnel soggetto a carico sismico e idrodinamico. Per quanto riguarda l'eccitazione sismica, è stata eseguita un'analisi con spettro di risposta e la risposta strutturale complessiva è stata determinata mediante sovrapposizione modale, sfruttando la regola di combinazione modale CQC. Quindi i risultati sono stati confrontati con quelli provenienti da un modello ad elementi finiti, esibendo un ottimo accordo. Per quanto riguarda il carico idrodinamico, questo è stato modellato mediante l'Equazione di Morison, che assume la forza agente sul tunnel come combinazione lineare di un termine inerziale e uno di drag. Applicazioni numeriche sono poi state sviluppate solo per il caso di regime inerziale, valutando la statistica della risposta su un certo numero di storie temporali. Tuttavia, l’aver svolto le analisi nel dominio del tempo apre le porte per un'ulteriore espansione allo studio della risposta strutturale quando il caso di drag viene considerato. Attenzione è stata posta anche al caso di variabilità della profondità del fondale, con lo scopo di indagare il suo effetto sul comportamento strutturale complessivo rispetto al caso in cui, invece, tale variabilità è trascurata.
A semi-analytical model for the evaluation of the structural response of submerged floating tunnels subjected to seismic and hydrodynamic excitations
Morleo, Eleonora
2021/2022
Abstract
Nowadays one of the most challenging problems to be solved in Civil Engineering is the crossing of waterways of relevant depth. As a matter of fact, conventional solutions, such as cable stayed bridges and suspension bridges, can be effectively employed to cover long distances but only in shallow waters, example being the 1915 Çanakkale Bridge opened in March 2022 in the Dardanelles strait, and the Akashi Kaikyo Bridge, respectively the first and the second world longest suspension bridges. As the depth increases, alternative solutions are needed and a promising one is offered by Submerged Floating Tunnels. A Submerged Floating Tunnel (SFT), also known as Archimedes Bridge, consists in a hollow tubular structure floating in water thanks to the upthrust exerted by the means and kept in place by anchoring systems. The shape of the cross section, as well as the type of anchoring systems may differ depending on the environmental conditions and also on the maritime traffic that interests the site. The concept origin dates back to the 1860 and several studies have been undertaken in the recent past, especially in Norway, China and Italy since multiple would be the advantages that its realization would bring. In fact, such a structure would allow to preserve beautiful natural landscape surrounding fjiords, lakes and shores, being it completely out of human sight. In addiction, travel times would be reduced, there would be no risks associated to extreme weather conditions, improving the overall users experience, pollutant agents would be disposed of. Being it a modular structure, it is reasonable that construction costs would be linearly proportional to the tunnel lenght, and so they would also be reduced with respect to those associated to other solutions, such as a suspension bridge, of the same magnitude, making the SFT a more cost-effective solution. Nevertheless, such a structure has never seen the light, even though its feasibility has been proven with the Høgsfjord project, which has been discharged for political reasons. Safety is the main concern and, on this page, many are still the aspects to be investigated, connected to all the various possible loading conditions and extreme events that could interest such tunnel. Not only, there is a psychological aspect to be accounted for that obstacles its realization, which is relative to the general belief that travelling in a floating underwater tunnel could somehow be unsafe, and that disreguards completely the fact that this type of technology is already in use, even if in a different form, as the case would be of underwater tunnels on the seabed. This being said, starting from a semi-analytical continuous model recently proposed in literature, the aim of this research work is to develop a continuous model for the static and dynamic analysis of SFTs under different loading conditions, able to significantly reduce the computational cost associated with the evaluation of displacements and internal actions with respect to a numerical analysis performed by means of FEM models. Focus has been laid on the reconstruction of internal actions of the tunnel when subjected to seismic excitation and hydrodynamic load. For what concerns the seismic excitation, a response spectrum analysis has been performed and the overall structural response has been determined by means of modal superposition, exploiting the CQC modal combination rule. Then the results have been compared with the ones coming from a finite element model, showing excellent agreement. As for the hydrodynamic load, this was modelled by means of Morison Equation, which assumes the force as a linear combination of an inertial and a drag term. Then, numerical applications have been developed with reference to the inertial dominated regime only, evaluating the statistics of the response over a certain number of time histories. However, the time domain framework through which the analyses have been carried out is suitable for further expansion to the drag case. Also the case of seabed depth variability has been implemented, with the purpose of investigating its effect on the overall structural behaviour if compared to the case in which, instead, this variability is disregarded.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/198856