Research On Natural Frequency Of Unbonded Prestressed Concrete Simply Supported Beams

The value of prestress is one of the most important factors which influence the structural safety of prestressed concrete bridge and the loss of the prestress will speed up the development of the bridge crack leading to the decline of the load carrying capacity of bridge, therefore, how to correctly evaluate the effective prestress in the bridge not only can accurately evaluate the performance of the prestressed bridge but also can provide scientific advice for the maintenance and reinforcement of the prestressed bridge. But under the premise of maintaining the integrity of the structure, unless the prestress detecting device such as pressure sensors is embedded in advance in the structure, there is no effective means to detect the existing prestressed in the structure now. Relevant research results show that there is a certain sensitivity between the natural frequency of the prestressed beam and the prestress, so it is of great significance in engineering practice to identify the effective prestress according to the natural frequency of the prestressed beam.This paper is based on the traffic science and technology project of Jilin province “natural frequency calculation method and prestress identification algorithm of unbonded prestressed simply supported concrete beams”, the following work has been carried out:1. Prestressed simply supported beam is regarded as a combination of two sub structures: compressive concrete beam and prestressed tendons. Vibration differential equation of unbonded prestressed concrete beam is established by the Hamilton principle, the deformation coordination relation and the boundary condition. Then considering the influence of the initial deflection of beam on the free vibration characteristics of prestressed concrete beam under the condition of eccentric prestress, based on the Darren Bell principle, the vibration differential equation of simple supported beams under the condition of eccentric prestress is derived and a nonlinear differential equation is obtained. Finally, the differential equation is solved by improved shooting algorithm.2. In order to further verify the relationship among natural frequency, the value of the prestress and layout of prestressing tendons, 5 unbonded prestressed concrete beams with different forms of tendons layout are made and the corresponding test plan is determined. And the relationship between the value of prestress and natural frequency of unbonded prestressed concrete beams is got through tests.3. In order to identify the value of effective prestress under the condition that the natural frequency of prestressed concrete beams is known, the BP neural network was constructed and the deficiency of this network was discussed. A particle swarm optimization algorithm was used to optimize the threshold value and the initial value of the BP neural network and the values were passed to the BP neural network. Then the BP neural network was trained and predicted. The test data were classified into training samples for training the network and prediction samples for verify the predicted results.