Research on Residual Bearing Capacity Calculation Method and Changing Law of Prestressed Concrete Box Girder with Damage

Due to the simple design and convenient construction of prestressed concrete box girder bridges, it is widely used in bridge construction in China. At present, the load test of bridge is the most direct and effective method to evaluate the actual bearing capacity of the structure. However, this method has barriers including high cost, long time, and unfriendly to the damaged structure. Besides, the calculation method of the ultimate bearing capacity of damaged structures is relatively subjective and uncertain. Therefore, this study combines the full-size model test results of a newly built 30 m prestressed concrete box girder, and the mechanical properties of the test beam under three different working states (elastic, elastic-plastic, and plastic stage) are obtained. The stiffness degradation and bearing capacity change law of precast prestressed concrete box girder during failure are investigated. Two different stiffness damage reduction coefficients are defined. Combined with the structural characteristics of this kind of precast prestressed concrete box girder and the calculation formula of flexural rigidity of cracked members given by the code, the calculation formula of the actual residual bearing capacity of members is proposed based on the stiffness damage reduction coefficient. The results show that the reduction coefficients of flexural stiffness defined by the two methods have the same variation trend. The stiffness reduction effect of the box girder after damage is evident. The stiffness reduction of the box girder from the cracking damage to the ultimate bearing capacity state is about 40%, and the stiffness reduction of the two adjacent sections can be approximately linear distribution. The deviation between the residual bearing capacity calculated based on the stiffness damage reduction coefficient and the test value is less than 5%. Combined with the distribution law of the stiffness reduction coefficient along the longitudinal section of the box girder, the distribution law of the actual residual bearing capacity along the longitudinal section of the box girder after the damage occurred in the midspan section can be calculated. This calculation method changes the previous assessment method of structural technical condition from qualitative assessment to quantitative assessment, which can be the reference for evaluating the technical condition of damaged bridges and the calculation of residual bearing capacity in engineering.