| Summary: | 碩士 === 國立雲林科技大學 === 營建工程系 === 102 === The tension variations of either stay cables, external tendons, or suspenders of arch bridge usually play the most critical role in the structural health monitoring of the corresponding bridges. The ambient vibration method is commonly adopted to determine the cable force by first identifying the cable frequencies from the vibration signals. With given vibration length and flexural rigidity, an analytical or empirical formula is then used with these cable frequencies to calculate the cable force. It is, however, usually difficult to decide the two required parameters, especially the vibration length due to uncertain boundary constraints. To tackle this problem, a new concept of combining the modal frequencies and mode shape ratios was recently by this research group for developing an accurate method merely based on ambient vibration measurements. A simply supported beam model with an axial tension was adopted and the effective vibration length of cable was then independently determined based on the mode shape ratios identified from synchronized measurements. With the effective vibration length obtained and the identified modal frequencies, the cable force and flexural rigidity could then be solved using simple linear regression techniques. This method has so far been extended from its initial assumption of symmetric boundary constraints to a more general formulation for dealing with unsymmetrical boundary constraints. Excellent results have also been achieved for its practical applications in stay cables and external tendons. The current study is aimed to further generalize this method for its applications in more complicated situations, especially for the force estimation in the suspenders of arch bridges where the boundary constraints at both end are obviously different and multiple measurements are only convenient to be conducted near the deck end. In this case, the quality of identified mode shape ratios can not be guaranteed because the multiple synchronized measurements concentrate at one end. On the other hand, the provided starting point of the sinusoidal mode shape function can more faithfully reflect the virtual hinged boundary at the near end of the simply supported beam model with an axial tension. Based on this advantage of the near-end hinged boundary, this research first establishes a finite element model with a hinged boundary at the near end and another fixed boundary at the far end for corresponding to the actual conditions of the suspenders of an arch bridge. Initially guessed cable force and flexural rigidity are then input to the model for analyzing the effective vibration length of each mode, followed by solving a new round of cable force and flexural rigidity with the identified frequency of each mode. Finally, the iteration analysis is performed until the cable force is convergent under a specified tolerance error. The feasibility and accuracy of the proposed method is extensively verified with demonstrative numerical examples, single strand test in the laboratory, and actual applications to different arch bridges. Furthermore, several important issues in engineering practice such as the arrangement of sensors and the parameter selection in the optimization process are also thoroughly investigated.
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