Study On Bearing Damage Assessment And Replacement Technology Of High-Speed Railway Concrete Box Girder Bridges

High-speed railway(HSR)networks offer a fast and robust travel option that enhances the quality of life,supports economic growth and is widely built worldwide.The harsh smoothness and stability of HSR determine that it has the characteristics of a large proportion of bridges.By the end of 2019,about 35,000 km of HSR,including 38,200 bridges with 19,600 km(account for 56%),have been built in China.As an essential part of a bridge,bearings easily suffer damage when exposed to various loading and environmental changes during their lifetime.However,the maintenance of HSR bridge bearings does not attract much attention from engineering personnel and bridge management departments.Because on the one hand,HSRs have only been in service for less than 20 years.On the other hand,the initial cost of bearings accounts for a small proportion of the expenses in HSR bridge engineering,not more than 0.5%～1%.These situations result in many defective bearings still in service in bridges,which brings serious safety risks to the performance of the operation and bearing capacity of bridges.Compared with small-and medium-span road bridges,bearing damage assessment and replacement for bridges under operating HSRs is more challenging due to the higher safety requirements for the operation of trains and the complex bearing structures.Based on the background,a set of rapid damage assessment methods and efficient repair schemes for bearings on HSR bridges considering structural performance and operation safety are proposed and established through on-site investigation,theoretical analysis,numerical simulation,laboratory experiment,and engineering demonstration.The contents and achievements are as follows:(1)Relationship between the abnormal grade of bearings based on apparent defects and potential inducement.Through the field investigation of bearing diseases of 5 HSR and 19 road bridges in East China,the similarities and differences between bearing diseases and potential incentives are analyzed.Through statistical models of survey data,the relationship between the abnormal grade of bearings and potential incentives is established,which suggests the direction for the maintenance of different types of bridge bearings.The degradation mechanism of mechanical properties is revealed based on the summarized types of bearing diseases.The main conclusions drawn from the survey and statistics are as follows:the abnormal inducement“environmental factors" will lead to serious bearing diseases for the plate rubber bearings.In contrast,the potential abnormal inducement "installation defects" often cause serious bearing diseases for pot or spherical bearings commonly used in HSR bridges.(2)Damage assessment for bridge support boundary based on impact factor.Taking a Bernoulli-Euler beam with elastic supports at both ends and a two-degree of freedom(DOF)moving spring-mass model as the research objects,the vehicle-bridge interaction model is derived and established based on the balanced equation.Then,the dynamic response of the bridge is obtained by taking road irregularity as input and solving the coupling equation through the Newmark-β method.Finally,the influence of support boundary damage on the dynamic responses of vehicle and bridge is evaluated based on impact factor,which aims to provide reliable suggestions for bridge design and maintenance.Conclusions drawn from the numerical simulation are that the low-order mode is more sensitive to the damage of the support boundary than the highorder mode.Even if the road irregularity level is good,impact factor may exceed 1.0 while the bearing is seriously damaged.(3)Bearing damage evolution based on train-track-bridge coupling vibration.First,a finite element(FE)model of bridge bearing is constructed and integrated into a train-track-bridge coupling vibration model which is established based on the ANSYS platform.Then,the impact of the degree,and location of bearing damage on the vibration response of the train-track-bridge coupling system are analyzed.The reaction transfer mechanisms of the diseased bearing are revealed.Finally,numerical simulation concludes that bearing damage has a more significant impact on the bridge vibration response than the train’s;The displacement response of the bridge is far more sensitive to the deterioration of the bearings than the acceleration response.In addition,the combined damage condition’s influence on the train-track-bridge system’s vibration response is more significant than the superposition of the system vibration response caused by the single damage in the combined damage.(4)Adjustable height temporary support for bearing replacement of large tonnage HSR bridges.First,a modular wedge-shaped temporary support suitable for bearing replacement of large tonnage HSR bridge with continuous height adjustment is designed based on material mechanics and self-locking theory.Then,through full-scale and scale experiments,the bearing capacity and stability of the designed temporary support are verified under working and extreme conditions,respectively.Finally,the safety performance and working efficiency of the designed temporary support are demonstrated by applying it to the demonstration project to replace a continuous prestressed concrete box girder bridge where the spans combination with(60+100+60)m.It is concluded from the experiment and application that the designed temporary bearing capacity can reach 30,000 kN and has the advantages of high efficiency,labor-saving,and stability,which can be popularized and applied in similar projects.(5)Safety monitoring and evaluation system for replacing HSR concrete bridges.Taking two HSR continuous bridges with the span of(60+100+60)m and(40+56+40)m as research object,a set of pot or spherical bearing replacement and repair schemes,monitoring measures,and new safety evaluation methods for HSR bridges are proposed based on field practice,which is applied and implemented under the conditions of operating HSRs.Specifically,the internal damage to structure formed in construction process is explored through acoustic emission(AE)monitoring,and the structural damage state is evaluated by cointegration analysis based on long gauge FBG strain monitoring data.The application shows that AE monitoring based on parameter analysis can accurately capture the start time and evolution law of substantial damage in bearing replacement process,and strain cointegration analysis can effectively determine the safety state of the structure before and after construction.