Study On Safety Of Simply Supported Box Girder Bridge Structure Under The Action Of Flood

The flood impact caused by the heavy rainfalls, producing a serious threat to the safety of the bridge across the gully or the channel, which has attracted worldwide attention. Especially in the mountainous city, for the common simply supported box girder bridge, when flood attacks, water damage phenomenon is particularly serious. But now the flood prevention study for highway bridge construction is less, has been unable to meet the requirement of the flood prevention application for the practical engineering. Therefore, it is necessary to study the safety of the simply supported box girder bridge structure under the action of flood.This paper calculated the horizontal and vertical force of bridge at flood state according to the horizontal and vertical velocity distribution in open channel, and analyzed the damage characteristics and failure mechanisms of the simply supported box girder bridge in the flood impact. The typical failure modes were presented, including: the bearing sliding mode, the bearing shearing mode, the overturning of the superstructure mode, and set up the the function expressions of the main failure modes for the bridge. Taking a two spans simply supported box girder bridge as an example, considering the uncertainly of flood force and the structure resistance, and studied the flow velocity and its variability、the bridge water levels effects on bridge reliability. Finally, the correlation between different failure modes and bridge system reliability were analyzed. Analysis results can provide the theoretical basis for the design of the similar bridge structure.In this paper, with the main conclusions:①The reliability index of the three kinds of failure modes for the simply supported box girder bridge decreased with the increasing of the flow velocity, but the downward trend of each reliability index was different. Three failure modes, the reliability index of the overturning of the superstructure mode was relatively large, but when the bridge completely submerged, the reliability index of the bearing sliding mode was minimum, and in 75%、50%、20%, three kinds of bridge submerged water levels, the reliability index of the bearing shearing mode was minimum.②With different coefficient of variation of velocity and different inundation height of flood, the reliability index of the failure modes of the bridge structure decreased with the increase of flow velocity. The coefficient of variation of velocity and flood inundation height has a great influence on the reliability of bridge structure, at the same velocity, the larger of the coefficient of variation of velocity was, the higher of the inundation height was, the smaller of the reliability index of the failure modes, more disadvantageous to the safety of the bridge.③The flood velocity, the coefficient of variation of velocity and the bridge submerged water level are the key factors to affect the system reliability or the safety performance the simply supported box girder bridge. The system failure probability of bridge structure increased with the increase of the flood water level、flow velocity and the coefficient of variation of velocity. Namely the more part of the bridge submerged, the larger of the flow velocity, the larger of the flow fluctuation, the bridge structure was more prone to damage, which was consistent with a number of failure characteristics of bridge damaged by flood.