Study Of Vibration Frequency Of Prestressed Beam Based On Damage Theory

At present the prestessed concrete is widely used in the civil engineering.It is known that the loss of the prestress force in tendons occurs due to elastic shortening, creep and shrinkage of concrete, steel relaxation, etc. in the condition of service of prestressed concrete. When the loss of prestress force oversized, the building destruction will be non-premonitory and tremendous,so the detection of prestress loss is the important part and difficult problem of the health monitoring and detection of prestressed concrete structure.The paper discusses the relation of frequencies of prestressed concrete beam and prestressed force based on the dynamic performance of the beam and the knowledge of the damage theory.In the foundation of prestressed concrete beam's dynamic characteristic research,two unbonded prestressed concrete beams were made. Prestress force was applied to these beams in grades, then their first third steps frequencies in every grade prestress force were measured. And relation of change between prestress force and frequencies which are used to analog the prestess loss is analysed in this paper. The test results indicate that the frequencies of these beams are increasing gradually with the increasing of the prestress force in the certain range, then the frequencies decreased slightly when the prestress force out of the range.The article has established finite element model of prestressed concrete beam with ANSYS, then the frequencies of test beams were calculated by it.The results of calculation indicate that the calculated frequencies of these beams are decreassing gradually with the increasing of the prestress force.This conclusion is opposite to the result of test.Experimental and the finite element analysis result indicated that, dynamic performance model is not appropriate if we regard the prestressed concrete as isotropic material.Therefore, this article embarks from the structure dynamics basic principle and the damage theory,using the elasticity coefficient change, the fomula of the prestressed beam's natural frequency has obtained. The formula results indicated that, frequency changing tendency compared with test result is consistent, the result precision of the first frequency is also higher.