The natural frequency of axial-loaded concrete beam decreases with increasing applied compressive force, and the natural frequency of tensioned cable increases with increasing tensile force. However, the variation of natural frequency of prestressed concrete beam (PCB) consisting of concrete and cable has rarely been discussed based on both |the rigorous mathematic model and experimental results. In this study, a testing program was conducted and Rayleigh's method was used to derive an approximate equation for computing natural frequency. The results indicate that (1) the natural frequency decreases with increasing prestressing force for PCB with eccentric parabolic tendon, but it is no change for PCB with eccentric straight tendon, (2) the proposed equation was proved to be an adequate method for estimating first mode natural frequency in vertical direction, and (3) a modified computed method for effective moment of inertia of PCB was also proposed.

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