Experimental and numerical studies on flutter stability of a closed box girder accounting for ground effects

Abstract

The aerodynamic loads that the bridge deck experiences as a result of interactions with the ground are not well studied when bridges are constructed at low heights from the ground. Flutter stability is the primary goal in the modern bridge wind-resistant design. Therefore, it is necessary to investigate the ground effects on the flutter stability and its influence mechanism. The present study investigated the ground effects on the flutter stability of a closed box girder based on wind tunnel tests and numerical simulations method. First, we presented a vibration measurement system for wind-tunnel test to simulate static ground effects on the bridge girder. Second, we investigated the system torsional damping ratios, the flutter derivatives, and the flutter critical wind speeds and their variation laws correlated with the heights from the ground in two flow fields (two different ground conditions) through a series of wind tunnel tests. The outcomes showed the following: that there is an approximate linear relationship between the height from the ground and the flutter critical wind speed in the uniform flow field, but this does not appear in the turbulent flow field; that the flutter critical wind speed dramatically decreases when the height from the ground is reduced in the uniform flow field; that the ground effects increase the absolute values of H*3 and A*1, thus resulting in faster growth in negative aerodynamic damping induced by the coupling effects of the torsional and heaving motions; and that the flutter derivative A*2 shows a tendency from negative to positive and ground effects make this tendency occur in advance at +3°wind attack angle. Finally, we applied the numerical simulations method to study the ground effect mechanism on the flutter stability. The calculated flutter critical wind speeds are in agreement with test results. The numerical results showed that existence of ground boundary layer generally reduces the flow speed in the region between the deck and the ground and increases both the positive pressure on the stagnation point of the lower surface and the local negative pressure on the upper surface. As a result, the positive and negative pressure increase as the height from the ground decreases, leading to large aerodynamic forces on the section, the flutter critical wind speed consequently decreases.