Dynamic Test And Theorotic Analysis Of Externally Prestressed Concrete Simple-supported Beams

Externally prestressed concrete structure refers to a certain prestressed structure with pre-stressing tendons which arearranged out of the concrete beam body. It has the advantages of rapid construction speed and the convenience for the maintenance and replacement of the external tendons. Nowadays it has been widely used in newly-built structures and in the reinforcement of existing structures. Meanwhile it has been the hotspot in the structural engineering recent years. Since few studies have been done on the dynamic property of externally prestressed concrete structure, this paper applies simply-supported beam as the research object. The main work is as follows:Five prestressed concrete simply-supported beams were designed, and three of them were simply-supported beams with typical arrangement. The corresponding static and dynamic parameters were measured, and sensitivity analysis of modal parameters was made. The results indicate that during the stage of pre-stress application, the natural frequencies of the internally prestressed concrete beams increase with the increasing pre-stressing force, which is the same case for the externally prestressed concrete beams, and the testing results are in contradiction with the theoretical ones of classical dynamics. In addition, the sensitivity analysis shows that natural frequency is sensitive to pre-stressing force changes.The results from the existing formula for natural frequency show a contradictive tendency compared with the testing results which is caused by the change of the stiffness. According to the testing results, the stiffness of externally prestressed concrete beam is modified in this paper and the corresponding modification of the formula is given. The theoretical results calculated by the modified formula are in agreement with the testing ones at the stage of prestress application. Meanwhile the model of test beams is built by the finite element analysis software (ANSYS), to which the formula of stiffness is applied. The theoretical results calculated by the ANSYS are in agreement with the test results.It's not avoidable for the concrete material to produce micro-fractures and micro-holes during the shaping process. In light of this feature, the author defines a negative damage variable based on the concrete elastic modulus, and introduces the negative damage variable to the damage constitutive equation. The dynamic behaviors of the externally prestressed concrete structures are analyzed utilizing the damage mechanics and the loaded finite-element method. The results indicate: the damage theory with the negative damage variation based on the concrete elastic modulus can simulate the variation of the concrete elastic modulus well, especially when applying pre-stress; the natural frequency determined by the elastic modulus is in accord with the test results, and the error is acceptable.The paper derives the vibration equations of external cables both with and without the flexural rigidities of the external tendons. Then it analyzes the factors impacting on the external tendons' vibration, and obtains the practical relation between the externally prestressed concrete structure's external tendons force and the free frequency. ANSYS is used to simulate and analyze the external tendons' vibration, and the results are of much difference with the test ones. In the end of the article, the artificial neural networks are used to forecast the force of the external tendons. The outcomes are fit well with the test results.