Time-dependent Performance Monitoring And Condition Assessment Of Long-span PSC Box-girder Bridge

The unfavorable factors,such as environmental erosion,material aging and overloading vehicles,etc.,make the bearing capacity and reliability of bridge structure decrease continuously.In particular,long-span PSC box-girder bridges generally suffer from excessive deflection at midspan and cracks in the web owing to the combined action of shrinkage and creep,stress relaxation and corrosion of reinforcing steel,which seriously affects the serviceability and durability of structure,and even induces safety accidents.Therefore,it is very important to carry out long-term health monitoring and accurately evaluate and predict the existing condition and future trend of long-span PSC box-girder bridges,so as to ensure its safety and normal operation.Based on the disease characteristics and maintenance requirements of long-span PSC box-girder bridges,this dissertation focuses on the time-dependent performance monitoring and reliability evaluation of Jinghang Canal Bridge(one continuous box-girder bridge with main span of 150m).The main research contents are as follows:(1)Firstly,the health monitoring strategies,methods and applications of existing PSC bridges are summarizd and discussed.In order to well capture the behavior of the bridge during its whole life and to maximize the use of sensors,a life-cycle health monitoring system,consisting of construction monitoring,completion test and operation monitoring,has been established on the Jinghang Canal Bridge.The monitored structural responses are analyzed and compared with the calculated results,and a good agreement is observed between these.Therefore,the effectiveness of the multi-output support vector machine in updating FE model of bridge structure is validated based on the test results.(2)A deflection control strategy is proposed on the basis of long-term monitoring and probabilistic finite element analysis,given the existence of excessive deflections at mid span in the operation of long-span PSC box-girder bridges.The mechanical behaviors of thin-walled box girder are described using composite degenerated shell elements,and the effects of shear lag and stress relaxation on long-term deformation are also included.Considering the uncertainty of shrinkage and creep model and the randomness of influence parameters,the uncertainty and sensitivity of long-term deformation are analyzed based on the LHS method.The confidence limit with 95% guarantee rate for the deflection at the mid-span is obtained,which is used for the quantitative design of backup tendons.The results of sensitivity analysis show that the creep and shrinkage model uncertainty factors,relative humidity and effective prestress of tendons have great influence on the long-term deformation.(3)In the laboratory environment,the long-term prestress losses of eight post-tensioned concrete beams are monitored for more than one year by using the load cells,elasto-magnetic(EM)sensors and vibrating wire strain gauges(VWSGs).Based on the monitored data,the influence of tendon layout,reinforcement ratio,duct grouting,stress level and jacking sequence,etc.,on long-term prestress losses are analyzed.And a proposed model for predicting the time-dependent prestress losses is thus presented,in which non-prestressed steel and the interaction among the shrinkage,creep of concrete and steel stress relaxation are taken into account.Compared with existing methods in standards,the accuracy of overall prediction of the proposed method is higher,and the maximum difference between test and predicted results is within 10%.Finally,the uncertainty analysis of long-term loss is carried out based on the proposed method.(4)In this paper,a method is proposed to analyse the random field-based time-dependent reliability of PSC bridges.The random field model is used to describe the variability of spatial distribution of structural parameters,such as the concrete elastic modulus and the distributed load,etc.,which are discretized by using the local average method.The importance sampling is used to estimate the failure probability of structure,in which the influence of shrinkage and creep,stress relaxation and shear lag on the time-varying performance of box-girder is also taken into account.Discussion is made about the effects of random field parameters on the time-dependent reliability.The analysis results show that the reliability of box-girder bridge is significantly affected by the shrinkage and creep of concrete,and the deflection reliability indices may fall below the target value prior to the expected service life.Compared with the discrete element numbers and correlation models,the correlation length of random fields has great effect on the time-dependent reliability index of structure.