Aerodynamic Instability Investigations of a Novel, Flexible and Lightweight Triple-Box Girder Design for Long-Span Bridges

Abstract

The present paper investigates the possibilities for avoiding classical flutter and static divergence for very long-span suspension bridges with a novel, flexible, and lightweight triple-box girder. Previous studies have shown that the critical classical flutter wind speed tends to decline with the main span width. Other studies have shown that flutter can be avoided if the torsional frequency is lower than the vertical. The road to get there in practice, however, is complicated. The possibility for tuning the torsional natural frequency without affecting the vertical frequency is used in the present paper. The effect on aerodynamic stability is analyzed in detail for low torsional-to-vertical frequency ratios typical for very long-span bridges with lightweight and flexible girders. The present study includes nonlinear finite-element analysis and static, free vibration and forced motion wind tunnel tests. Aerodynamic stability has been obtained in a section model test with a lightweight setup corresponding to a bridge girder mass of only 6.38 t/m in full scale. Flutter was not observed for any torsional-to-vertical frequency ratios in the range from .97≤γω≤1.55. Stability was observed up to wind speeds of approximately 88 m/s in full scale for a 2,125.2-m span. The aerodynamic stability obtained in the configurations of the present section at γω≈1.2 shows that this might be an economically feasible solution for future long-span suspension bridges because aerodynamic stability is achieved even though the torsional stiffness and the mass of the deck are low.