Aerodynamic response of slender suspension bridges

Abstract

This thesis studies the aerodynamic stability of suspension bridges with a streamlined single box girder or a twin box girder as cross section. The focus has been on studying how aerodynamic properties affects the response and the flutter stability limit of the bridge.

Response calculations is based on a modal approach in the frequency domain and has been carried out by use of MATLAB. Prediction of the stability limit has been based on a buffeting-response calculation instead of solving the impedance matrix in search for singularities.

For a streamlined box girder the Hardanger Bridge has been used as a case study. The results show that instability due to flutter is caused by motion induced loss of stiffness in torsion. Structural damping, motion induced stiffness coupling between the torsion and vertical displacements, and aerodynamic damping in torsion has no effect on the stability limit or the response of the system.

For a twin box girder, a proposed bridge over the Halsafjord with this cross section has been studied. If the distance between the box sections of the bridge is 20 meters or more, then the motion induced forces contributes to the stiffness in torsion. Since there's no loss of stiffness, no flutter stability limit was found. Structural damping and motion induced stiffness coupling between the vertical and torsion modes has no effect on the stability limit. Aerodynamic damping and stiffness in torsion does affects the response values of the system, but the stability limit remains unchanged. Based on the results, crossing the Halsafjord with a suspension bridge with a main span equal to 2050 meters will be possible.