Experimental Study On Structural Performance Of Prestressed Concrete Beams Under Alternating Loading

Prestressed concrete bridges are widespread in our country,the effective prestressing force and residual strength of main beams are directly related to the safety and reliability of the bridges.The main girders of prestressed concrete bridges not only resist the permanent actions but also the variable actions,such as vehicle load and wind load during its service life,which will lead to cumulative fatigue damage of materials and prestressing loss,and then causes the emergence of cracks and excessive deflection,eventually the structural residual capacity degrades and even results in collapse accident.In this paper,the 30m-span prestressed concrete T-type girder on the usual highway was studied and 4 scale model beams with the same reinforcing bars were fabricated on the basis of similarity theory.By the static and fatigue loading tests,the prestressing loss and residual capacity with cycle time increasing were studied and the crack mechanism of prestressed concrete structures under fatigue load was discussed.The main research works and conclusions are included as follow:(1)Based on the static test of model beams,the mechanical behaviors of component materials are similar and structural integrity is good.The model presents ductile failure,which shows the similar design method is reliable.(2)By comparing numerical simulation results with the static test ones,the failure loads and the development of cracks are similar.The results of static test are effective,which can be used to determine the minimum and maximum of cyclic loads.(3)Based on the fatigue test of model beams,the mid-span deflection and the strain of reinforcing bars,prestressed tendons and concrete have no noticeable change,which indicates fatigue load has a little influence on the prestressed concrete beams.(4)Based on the fatigue test of model beams,prestressing loss can be divided into three stages in the whole loading process.In the earlier and later period,the prestressing loss changes apparently,but during the middle stage the effective prestressing force almost unchanges.The ultimate bearing capacity of model girders after fatigue loading has a certain increase and sufficient safety factor comparing tocurrent specification.(5)Based on the formulas of fatigue strains given by the CEB-FIP 1990 Mode Code and Balaguru,the theoretical calculated values are less than the test ones.For the analysis of normal section fatigue crack resistance for prestressed concrete beams,we should adopt the experimental value.If there is no adequate test data,the design formula of CEB-FIP 1990 Mode Code can be employed by considering modified coefficient.(6)The flexural fatigue strength of concrete and prestressing loss must be considered in the design of prestressed concrete beams for normal section fatigue crack resistance under fatigue loading.Based on the calculation results and test phenomena,the checking computation method is proposed,which can provide the design gist for the fatigue crack resistance of prestressed concrete members.