Aerodynamic stability of slender suspension bridges

Abstract

This thesis studies the aerodynamic stability of a proposed suspension bridge crossing the Sognefjord. Such a bridge will require a main span of approximately 3700 m, almost double the span length of the current record holder. For such a long span and slender suspension bridge its aerodynamic properties are highly important.A comparison of the aerodynamic stability limits using three different configurations of a so-called vented or dual box girder has been done. The only difference in each configuration is the center-to-center distance between the individual box girders. The design of the individual girders and the cable system is kept constant.A modal approach to solving the equation of motion has been employed where the eigenfrequencies and eigenmodes have been computed using a computer program called ALVSAT. The numerical calculations of stability limits as well as dynamic response have been done using self-written MATLAB scripts based on well-established theory of bridge aerodynamics. Aerodynamic derivatives from wind tunnel tests have been obtained from a report on the similar cross section of the proposed Brusymfonien bridge. The results obtained show an increasing critical mean wind velocity with increasing the center-to-center widths between the box sections. For the widest dual box girder, no stability limits were found. The response results confirm the stability limits and indicate a stabilization effect of the dual box girders at wind velocities below the point of modal coupling.