Study On Stress Increment Of External Tendons Of Strengthening Concrete Beams With External Prestressed Tendons

External prestressing is an important branch of post-tensioning system, which has widely adopted in bridge engineering and building structure as well as structure strengthening.The up-to-date state of stress increment of external tendon was given importance to introduced firstly in this paper and the method of calculating secondary moment of statically indeterminate structure by using equivalent load method is mainly presented.By using fibre integration method and nonlinear numerical analysis method, the externally prestressed simply-supported beams, from initial load to ultimate state, were analyzed. A compute program based on incremental loading is compiled. The analytical results of calculate programs agree well with the test results.Stress increment and second-order effect are the two critical factors in design of concrete beams with external prestressing.The study on concrete beams with external prestressing domestic and overseas,most deal with simple-supported beams and rarely deal with statically indeterminate beams. so it is necessary to do an in-depth study on statically indeterminate concrete beam with external prestressing. specimens of 1 4 external prestressing concrete beams, including 6 simply-supported beams ,4 two-span continuous beams and 4 single span frame beams were designed elaborately relying on the natural science fund project of Guangxi area"test study on concrete frame beams strengthening with external prestressed tendons",and the static load tests were finished. Several critical dates as deformations,strains of the beams and the stress increments of external tendons were measured, the occurrence and development of concrete crack under each grade loading were recorded elaborately, and the test results were analyzed.The test results show that there is a linear relationship between loads and increments of external tendons in the initial loading period, when the period passed, such a relationship changed. The increment speed of stress increment is obviously faster than that of the load. The ultimate stress increment of each tendon is obviously different, the biggest one is 762 MPa and the smallest one is 325 MPa. An approximately linear relationship between the tendon stress increments and deflection of the concrete beams is found when plotted them in a graph.By analyzing the inherent law between stress increment and deflection variation,the analytic model of the stress increment of external tendon related to concrete beam deflection is established,and the simplified calculation equation of stress increment varying with deflection variation is obtained. Result calculated by this formula is almost as accurate as the result of the test. However,calculation by this formula needs less computational effort and is more beneficial to engineering project.The test result of this paper may be as reference for future modification to present unbonded-prestressing concrete code equation for tendon ultimate stress f_ps.