Study On Residual Mechanical Properties Of Steel-concrete Composite Beam Bridges Based On Cumulative Fatigue Damnification

There are mainly two reasons for the degradation of the mechanical properties of steel-concrete composite bridges.One is the deterioration of the materials and performance degradation of the shear connectors due to the cumulative fatigue damage under vehicle loading.The other is the corrosion of the shear connectors caused by the water seepage and carbonation due to the concrete cracking,poor performance of the combination surface and other factors or the weakening of the combination action caused by the initial corrosion of the shear connectors.In view of this,this dissertation combines with the project of the National Natural Science Foundation of China "Research on residual mechanical properties of steel-concrete composite beam bridges based on cumulative fatigue damage effect"(51278119).Taking the steel-concrete composite beam as the research object,the method of the combination of experimental research and theoretical analysis is used and two aspects of the component level and the structural level are analyzed in this dissertation.The static and fatigue tests of 20 push-out test specimens and 10 test beams and the corrosion fatigue tests of the 6 test beams were carried out.Based on these tests,dynamic changes of the initial static performance of the structures under the fatigue loading were further explored.The change rules of including the bond-slip performance at the junction surface,flexural capacity,stiffness,long-term deformation and other mechanical properties under the single or coupling actions of corrosion and fatigue loads were revealed.The main research contents and conclusions are summarized as follows:(1)Residual mechanical properties of shear connectors under fatigue loadingStud connectors and PBL shear connectors which are most commonly used in engineering are selected as the research object.The variation in the mechanical indexes such as shear capacity and shear stiffness after fatigue loads was analyzed through the standard push-out tests.The results show that residual mechanical properties of shear connectors and PBL connectors under fatigue loading show nonlinear degradation;compared to the PBL connector,the degradation effects of the bearing capacity and the shear stiffness for the stud connectors are more obvious;the ductility coefficient of PBL connector is far greater than the stud connectors,which indicates better ductility of PBL connectors.(2)The calculation model of the residual bearing capacity and the load-slip constitutive model of shear connectorsBased on the statistics analysis of the stud and PBL connectors push test results and considering the two element fatigue failure criterion and residual material strength theory,an exponential degradation model of shear capacity for the stud connectors after fatigue loading was proposed;the stress state of PBL connectors under cyclic loading was analyzed using the method of elastic foundation beam.A calculation model of the residual capacity of PBL connectors after fatigue loads was derived considering the cumulative fatigue damage effect on the bearing capacity reduction.At the same time,taking the stud connectors for example,the degradation rules of the key indexes and related parameters of stud connectors were obtained based on numerical fitting method.Combined with the simplified three-stage load-slip model function,a time-varying load-slip constitutive model of stud connectors considering the mechanical property degradation was established.(3)Study on residual mechanical properties of steel-concrete composite beams under fatigue loadingBased on the beam scale test,a series of mechanical indexes such as residual capacity,flexural stiffness and bond-slip behavior of composite beams with different shear connectors were studied.The results show that the failure modes of the test beams of stud connectors after fatigue loading change from the concrete crush at the mid span to the stud failure.Due to the shear strength degradation,the stud connectors fail before the composite beam.It should be paid attention to in the design of composite beam bridge in the future.After the fatigue loading,the PBL test beams exhibit less bearing capacity degradation,less residual deflection accumulation,less relative-slip growth and smaller lift values.(4)Study on calculation method of residual stiffness and residual bearing capacity of steel-concrete composite beams under fatigue loadingBased on the beam test results,the structural fatigue stiffness based on the effective moment of inertia theory was defined.With the introduction of fatigue stiffness degradation function,which was fitted and verified by the measured stiffness,a degradation calculation model of the deflection and the residual flexural stiffness of stud beams after cyclic loading was established.Considering degradation of concrete,steel girder and shear connectors during the whole process of the residual strength degradation of the materials,the nonlinear damage model was introduced into the calculation of bearing capacity of steel-concrete composite beam bridge.And then through the beam test data comparison and quantitative analysis,a residual bearing capacity model of composite beam under fatigue loading was established and the corresponding computation analysis software was put forward.In addition,an example of a real bridge is calculated and analyzed by this method.(5)Study on mechanical properties tests and degradation behaviors of steel-concrete composite beams with corroded shear connectors under fatigue loadingThe corrosion problems of the studs and PBL connectors on the composite beams were simulated through the electrochemical accelerated corrosion tests.And then fatigue tests of the composite beams with different corrosion degrees were performed.Degradation rules of mechanical properties of composite beams under conditions of coupling corrosion and fatigue loads were studied.The results show that a series of mechanical parameters including the residual bearing capacity,residual deflection,and relative-slip of the beams with two types of shear connectors under corrosion and fatigue loading show different degrees of degradation with the increase of the corrosion rate.Whether the corrosion object is stud or PBL connector,reduction on the overall performance of the composite beams are obvious.It is suggested that some corrosion prevention measures should be taken in the design of composite beams and shear connectors at the same time,and the number of shear connectors should be improved so as to ensure the normal service performance of the bridge.