This paper presents the research on mechanism of overturning failure on box girder under overload and partial load for the single-column pier bridges. It includes (1) the definition of two key states before the box girder overturas(i.e., the disengaging state leading to the unbalanced load bearing and the ultimate state of overturning stability),(2) the depiction of three stages along overturning (stable stage, transitional stage, and overturning stage, (3) the development of an ABAQUS finite element model considering contact nonlinear and geometry nonlinear for analyzing Chunhui bridge collapse, and (4)the undertaking on identifying the relationships between overturning ultimate load and elastic modulus of box girder, load position, bridge length and friction coefficient of bearing and box girder. The results indicate that the overturning load calculated by finite element simulation conforms to the measurements collected at the collapse site of Chunhui bridge, reflecting the validity of the proposed study. The ultimate state of the overturning stability can be used to judge whether the bridge is overturning. It is more accurate than using the disengaging state of one side end bearing to judge, and the overturning ultimate load is 1.5-2 times than the rotating ultimate load.
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