Seismic cracking mechanism and control for pre-stressed concrete box girders of continuous rigid-frame bridges: Miaoziping bridge in Wenchuan earthquake as an example
Abstract: The box girder of the Miaoziping Bridge, a three-span prestressed concrete continuous rigid-frame bridge, suffered a serious crack in its box section’s web near the 1/6 to 1/2 length of the side span and the middle-span length of 1/4 to 3/4, as a result of the 2008 Wenchuan earthquake, which also caused large lateral residual displacements at both ends of the side span. In this study, eight strong-motion records near the bridge site and two other records (El Centro and Taft) are selected as inputs for time-history analysis of the bridge. The cantilever construction process and initial stress of the box girder are considered in a bridge model for seismic numerical simulation. Further, the simulation results are compared with the actual earthquake damage. The cracking mechanism, influencing factors and control of the girder crack damage are discussed. The high-stress zones of the box girder agree with the seismic damage observed, even various seismic inputs are considered. The findings reveal that the maximum (principal) tensile stress of the girder exceeds the tensile strength of the concrete under the seismic excitations, and cracks occur. Under various input directions of ground motions, the proportion of the main girder stresses induced by the earthquake shows differences. After the failure of the shear keys in the transverse direction of the bridge, the stresses of the girder decrease in the mid-span. However, the beams at both ends of the side spans revealed large lateral displacements. Considering that the uplift of the beam ends, stress and axial torque of the girder’s side span are greatly reduced. Setting bi-directional friction pendulum bearings on the transition pier is an effective damping measure to control web cracking of the mid-span and lateral drifts of the beam ends.
Keywords: Pre-stressed concrete continuous rigid-frame bridge / Seismic damage / Box girder cracking damage / Beam end uplift / Shear keys / Friction pendulum bearing
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