Suspension bridges represent a valid solution for covering very long space gaps thanks to their deck’s low weight. Composed by a stiffening girder, designed to support active loads, and a cable system sustaining the deck through hangers, suspension bridges are characterized by a high interaction between these two elements of the structure. Many models have been studied and have improved during the last two centuries. The first model, called deflection theory, considered inextensible hangers and, therefore, only the deflection. This model has been improved by introducing the possibility of rotation of the deck and the elasticity of the hangers. The vibrational response is governed by structural parameters related to the deflection, the torsion and the stiffening behavior of the cables. Their influence is studied by means of parametric analysis in order to understand their effects on frequencies and modal shapes. The presence of internal instability is well known in non linear systems where, under sufficient initial energy, the motion changes from one degree of freedom to another. In the case of suspension bridges, this instability corresponds to the transfer of energy from flexural to torsional motion. This work provides a description of the models for suspension bridges and the derivation of their equations of motions. Then, it proposes a parametric analysis for the different models, showing the effects of structural parameters in terms of natural frequencies and modal shapes. At last, the activation of the flexural-torsional instability is approached by numerical integration of the equations of motions. The aim of this analysis is to show the effects of the parameters on the system response.
I ponti sospesi rappresentano un’ottima soluzione per coprire lunghe distanze graziealla leggerezza dell’impalcato. Sono composti da un impalcato di irrigidimento, atto a sostenere i carichi variabili, e da un sistema di cavi che sostiene l’impalcato tramite una serie di pendini. L’influenza tra le due componenti del ponte è molto elevata. Nei due secoli scorsi, sono stati studiati e ampliati numerosi modelli. Il primo, conosciuto come teoria della deflessione, considera pendini inestensibili e, quindi, solamente la flessione verticale dell’impalcato. Questo modello è stato migliorato con l’aggiunta della torsione e la rimozione dell’ipotesi di pendini inestensibili. La risposta vibrazionale è governata da parametri strutturali legati alla flessione, alla torsione e alla rigidezza incrementale dei cavi. La loro influenza sulle frequenze naturali e sulle forme modali è stata studiata attraverso analisi parametriche. È ben noto che la non linearità nei sistemi genera un’instabilità interna. Data una sufficiente energia iniziale, il moto passa da un grado di libertà ad un altro. Nel caso dei ponti sospesi, questo fenomeno avviene tramite il trasferimento di energia dal moto flessionale a quello torsionale. Questo lavoro offre una descrizione dei modelli e la derivazione delle equazioni del moto dei ponti sospesi. Successivamente, è stata svolta un’analisi parametrica per i diversi modelli che mostra gli effetti dei parametri strutturali sulle frequenze naturali e sulle forme modali. Infine, l’instabilità flesso-torsionale è studiata attraverso l’integrazione numerica delle equazioni del moto nel tempo. Lo scopo di questa analisi è di mostrare come i diversi parametri influenzano la risposta del sistema.
Some features on the dynamic response of suspension bridges
Aspesi, Francesco
2020/2021
Abstract
Suspension bridges represent a valid solution for covering very long space gaps thanks to their deck’s low weight. Composed by a stiffening girder, designed to support active loads, and a cable system sustaining the deck through hangers, suspension bridges are characterized by a high interaction between these two elements of the structure. Many models have been studied and have improved during the last two centuries. The first model, called deflection theory, considered inextensible hangers and, therefore, only the deflection. This model has been improved by introducing the possibility of rotation of the deck and the elasticity of the hangers. The vibrational response is governed by structural parameters related to the deflection, the torsion and the stiffening behavior of the cables. Their influence is studied by means of parametric analysis in order to understand their effects on frequencies and modal shapes. The presence of internal instability is well known in non linear systems where, under sufficient initial energy, the motion changes from one degree of freedom to another. In the case of suspension bridges, this instability corresponds to the transfer of energy from flexural to torsional motion. This work provides a description of the models for suspension bridges and the derivation of their equations of motions. Then, it proposes a parametric analysis for the different models, showing the effects of structural parameters in terms of natural frequencies and modal shapes. At last, the activation of the flexural-torsional instability is approached by numerical integration of the equations of motions. The aim of this analysis is to show the effects of the parameters on the system response.File | Dimensione | Formato | |
---|---|---|---|
Tesi_CIV.pdf
accessibile in internet per tutti
Dimensione
2.6 MB
Formato
Adobe PDF
|
2.6 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/177945