Fluid-dynamics and Flutter Analysis of the George Washington Suspension Bridge

The aim of this thesis is to analyze the behavior of suspension bridges under wind loads, in particular the George Washington bridge has been chosen as a case study, in both original and latest stiffened versions respectively of 1931 and 1962. The goal is to determine the critic wind velocity that leads the bridge to the well-known phenomenon of flutter instability adopting a commercial finite element software and to find a simplified approach for the estimation of the motion-related wind loads. The research is also focused on the study of the computational fluid dynamic models aimed at determining the aerodynamic parameters on which the approach mentioned before is based. The work is divided in two part, a first part containing the theoretical and bibliographical background in which are based the calculations carried out in the second part. The first part begins with considerations regarding the description of wind effects on suspension bridges, some analytical approaches and descriptions of the phenomenon of flutter instability are presented, with an in-depth reference to the method used by the software adopted. A chapter was subsequently dedicated to the description of the load acting on an oscillating body in a uniform airflow by different authors. The strong duality between time-domain and frequency-domain representations is focused with the aim of reporting a relationship between flutter derivatives, Theodorsen functions and aerodynamic static coefficients. The final chapter of the first part is dedicated to a brief introduction to the computational fluid dynamic followed by some example of application on bridge decks. The second part starts with an application of the formulas developed in the first part for the calculation of flutter derivatives, the Great Belt and the Akashi Kaykio bridges were chosen as case studies. In the next chapter several CFD analysis with ANSYS FLUENT have been carried out with the aim of obtaining the aerodynamic static coefficients of the bridges mentioned before. The results were compared with the data found in the literature and then the same parameters of George Washington Bridge have been calculated with the same approach. Thereafter considerations were made about numerical modelling of suspension bridges in ANSYS APDL, some bridges have been modeled in order to make a comparison between the results obtained and the literature data. In the end a flutter analysis of the two versions of George Washington bridge have been carried out and it was possible to draw conclusions on the influence of the lower deck on the aerodynamic performance of the bridge.