Performance Evaluation And Causal Analysis Of Cracking And Excessive Deflection Of In-service PC Girder Bridges With Variable Section

PC girder bridges with variable section are widely used in highways,railways and urban roads and other important artery traffic.However,cracking and deflection often occur after long-term operation,which are very important and difficult problems in engineeringBased on the investigation and statistics of real bridge distress causes,this paper systematically analyzes the main causes of cracking and excessive deflection.Two methods for performance evaluation of in-service bridges are proposed,one is to identify the state of the bridge and the other is to identify the development law of long-term prestress loss.The main research work and innovation points are as follows:(1)Taking the typical PC girder bridges with Variable Section in jiangsu province as an example,the main causes of the web crack and bending crack were gained through the parametric sensitivity analysis and statistical investigation of the disease characteristics,and the regions prone to the occurrence of bending crack and web oblique crack were obtained.The analysis shows that the overload of vehicle is one of the main causes of bending crack of box girder.(2)A performance evaluation method based on the state identification of the bridge with characteristic parameters of cracks is proposed.The effective prestress of longitudinal prestressed tendon and the bending stiffness of structure are identified combined with the height of bending crack characteristic parameter.The effective shear stiffness of the structure and the effective stress of the vertical prestressed tendon are identified by the grade of oblique cracks.Finally,T he feasibility of this method is verified by the crack distress causes of an in service bridge in jiangsu province.(3)Based on the ABAQUS three-dimensional entity element analysis function,the concrete shrinkage and creep subroutine was developed to analyze the time-varying effect of PC girder bridges with Variable Section.The analysis shows that the bridge whose prestressed bars are bent down and anchored to the web can effectively suppress the tensile stress in the web compared with whose prestressed tendon bundles are anchored to the rooft;The effects of long-team prestress loss and the mid-span deflection of the structure under ACI209,ceb-fip(1990)and B4 shrinkage creep models were compared and analyzed;it is also found that shrinkage creep and vertical prestress loss have a great influence on the main tensile stress of the web in the 1/4~1/2 region of the middle span,which is one of the main reasons for the occurrence of oblique cracks in the web in this region.(4)The influence of critical section stress and creep coefficient variation on the long-term deflection of the bridge is studied.The results show: t When the structure satisfies the reasonable stress bridge state,it is beneficial to restrain the long-term deflection of the structure.Creep coefficient variation has a great influence on the longterm deflection and a small influence on the growth rate of long-term prestress loss.(5)The influence of ambient temperature and stress inhomogeneity of prestressed tendons on the relaxation loss of prestressed tendons was studied.The loss of relaxation of prestress doubles under the condition of over normal temperature.When the non-uniform dispersion coefficient of prestressed tendons is 0.4,the actual relaxation loss of prestressed tendons is 2.3 times of that calculated according to the code.(6)A performance evaluation method for long term prestress loss identification based on measured deflection data is proposed.The method considers the stiffness degradation of the structure,is simple to operate and practical,and the feasibility of the method is verified by the measured deflection data of the auxiliary passageway of humen bridge...