Study On Bearing Capacity Degradation Of Prestressed Concrete Beams Under Fatigue Loading

Due to the unique advantages,prestressed concrete bridges are widely applied in Chinese bridge construction.During long-term service of prestressed concrete bridges,they must not only resist the dead load but also the cyclic loading such as the vehicle load.As the number of cyclic loading increases,cumulative fatigue damage and prestressing loss occur inside the structure.Under such condition,cracks and excessive deflection occur during the service phase,and eventually the structural bearing capacity degrades,affecting structural safety,durability and service life.Thus,it is necessary to study the bearing capacity of prestressed concrete beams under fatigue loading.In this paper,the standard 30m-span simply supported prestressed concrete Tbeam is selected and 6 identical model test beams are designed and made with the scale ratio of 1:5.By the static and fatigue loading tests,the effective prestress and flexural stiffness of prestressed concrete beams under fatigue loading are analyzed.And the bearing capacity degradation mechanism of prestressed concrete beams under fatigue loading is studied.The main research contents of this paper are summarized as follows:(1)Tests are carried out for prestressed concrete test beams under static load.By static test,the failure characteristics of the test beam and the mechanical properties of the constituent materials are analyzed.The rationality of the test beam's cross-section dimension and reinforcement design is verified.According to the static test results,the upper and lower load limit are determined for fatigue loading test.(2)Fatigue loading tests of uncracked test beams are carried out.During the fatigue loading process,the variation law of prestressed tendons' effective prestress is studied and the relationship between prestressing loss and residual deflection of the test beams is analyzed.Taking into account that the irreversible fatigue creep of concrete in compression zone is the main factor causing prestressing loss,the calculation formula of concrete fatigue creep is proposed based on the test results and the main influential factors of fatigue creep.Based on the static crack resistance calculation method in the specification,and considering the fatigue strength reduction of concrete in the tension zone and prestressing loss of the prestressed tendons under fatigue loading,the fatigue crack resistance formula of the prestressed concrete beam is given.(3)Fatigue loading tests of pre-cracked test beams are carried out.According to the fatigue loading test results,the deflection and flexural stiffness variation laws of the test beams under different number of cyclic loading are analyzed.It is found that the flexural stiffness of the prestressed concrete test beams with fatigue failure shows a three-stage variation law in the whole fatigue loading process,which specifically means sharp decline-stable slow decay-accelerated decline rule.And there are still 60% residual flexural stiffness after beam's failure.On the basis of the above law and the initial deflection expansion coefficient method,the calculation formula for the first two-stage flexural stiffness of the beam is proposed.In the third stage,the flexural stiffness of the beam shows a linear accelerated attenuation trend and there are still 60% residual flexural stiffness after beam's fatigue failure.Thus,the calculation formula for the third-stage flexural stiffness of the beam during the fatigue loading process is proposed.(4)Static tests of the test beams after a certain number of fatigue loading are carried out,and the mechanical behavior of the test beam after fatigue loading is analyzed.Compared with the bearing capacity of test beams for static tests,the effect of fatigue loading on the bearing capacity of test beams is investigated.It is found that the residual bearing capacity of prestressed concrete beams after fatigue loading is related to the state of ordinary tensile steel bars,and the fatigue life of prestressed concrete beams is closely related to the stress amplitude of ordinary steel bars.Based on the above findings,the calculated values by current codes and experimental data of the stress amplitude of ordinary steel bars are compared and analyzed.The fatigue life of the four test beams is verified by the S-N curve equation of the deformed steel bar proposed by domestic and foreign research institutes or scholars.Finally,the ultimate load test values of test beams for static or fatigue loading tests are compared with the calculated value by current codes.