Seismic Damage Analysis Of The Main Bridge Of Miaoziping Bridge In Wenchuan Earthquake

A large number of continuous rigid-frame bridges with long-spans and high-piers have been built in China,especially in the western regions with high seismic risks.Combing continuous rigid-frame bridges with long-spans and high-piers undergoing earthquake at home and abroad,the girder of main bridge of the Miaoziping Bridge suffered a severe cracking damage in Wenchuan earthquake.Aso Choyoo Bridge had a similar cracking damage of girder in Kumamoto earthquake.Instead of the bearings are used between them like the simply supported bridge or the continuous bridge,the main girder and the pier are rigid together in the frame bridge,The girder will certainly resistant the seismic force together with the pier in strong earthquake.At present,there is no seismic code for the main girder of continuous rigid frame bridge with high piers and long spans at home and abroad.The response behavior had been little researched under strong earthquake.In this paper,the seismic damage of main bridge in Miaoziping Bridge in Wenchuan earthquake is studied in detail by numerical simulation.The main study is as follows:(1)The finite element model for seismic response analysis of the main bridge joint construction of Miaoziping Bridge is established.In order to acquire the initial internal force and stress of the main girder exactly,the cantilever construction stage is simulated in the finite element method.The results of stress simulation are consistent with the monitoring results in the construction site.In addition,the accuracy of the modeling is verified by combining the actual test results of the natural vibration characteristics of main bridge.The records of eight strong motion near the bridge site in Wenchuan earthquake,El Centro and Taft are selected as the inputs for time history analysis.(2)The time history analysis results of the main bridge of Miaoziping Bridge show that the high stress zones of the girder are consistent with the seismic damage when various seismic inputs are considered.The results are less affected by the assumption of boundary conditions at the transition pier,which indicate that the cracking damage of the box girder is not an individual phenomenon.In strong earthquake,it is prone to reach high stress including the top plate near the main pier and the closure sections of the side span and the middle span,and the web near 1/6 to 1/2 side span and the middle span of 1/4 to 3/4,and the bottom plate near the closure section and the adjacent 2–3 sections of the side span and the middle span.The stress of the longitudinal prestressed tendons is estimated to increase nearly 100 MPa.(3)In consideration of the failure of lateral shear keys at the transition pier of the main bridge,the tensile stress and compression stress decrease in the middle span.While the tensile stress and compression stress increase in consolidation of pier and girder,and the maximum torque of pier reduces about 30%.The maximum tensile stress of the side span in the top plate is reduced by about 5 MPa,and the compressive stress of the side span in the bottom plate and web is reduced by about 6 MPa.The torque of the side span of girder is reduced by 75%,and the longitudinal bending moment of the bottom of the pier is reduced by about 50%.It seems to indicate that this damage behavior is beneficial to the seismic resistance of the main structure of the bridge.Under various input directions of ground motion,the response of the main beam stress is greatly different.In the seismic analysis of continuous rigid-frame bridges with long-spans and high-piers,which involves the damage of the main beam,the ground motion must be input in three directions.The bi-directional friction pendulum bearings are set at transition pier,which can reduce partly the principle tensile and compression stress of the web in the middle span.At the same time,the large displacement response of girder of the side span at transition pier can be controlled,which is an effective damping measure to control seismic damage of continuous rigid-frame bridges with high-piers and large-spans.(4)Another two bridges are introduced to discuss the seismic failure patterns of continuous rigid-frame bridges with long-spans and high-piers.Assuming two bridges under the same seismic action as Miaoziping Bridge,they are possible to crack in the girder.The height of piers has major influence on the stress of the main girders,and the span is second.The main piers and spans of the bridge are larger,the(principle)tensile and(principle)compressive stress are greater.As the main girders cracks,the bridges bearings can be destroyed simultaneously,and the piers may also crack even yield at the bottom.