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The study was conducted to evaluate the structural integrity and earthquake resistance using commonly employed traditional method in bridge engineering and dynamic plunking test for the selected existing bridges in central Taiwan. The plunking test, which has been developed for several decades in structural dynamics, may provide additional information to reveal the existing structural conditions in terms of structural fatigue and material decay within the structural elements. The results of the tests include the dynamic parameters such as stiffness k and damping ration D of the tested bridge girders to provide useful reference and guideline for the future repair/maintenance programs related to the structural capacity and durability of the bridges. The results of the study revealed that the most significant factors affecting the natural frequency and stiffness is the bridge girder support conditions (boundary conditions: fixed, roller, hinge, free). Other significant factors include the cross-sectional dimension and span length of the bridge. The results of the study indicated that the damping ratio D of the tested Lin-wei bridge is close or equal to 1 which may imply the bridge exhibits critical or over damped behavior during strong vibration. In this case, the amplitude of the bridge during earthquake or vibration resonance condition may be significantly reduced and may provide more smooth traffic driving patters. The results of the study also revealed that the stiffness k of the Chin-yuan bridge from tests is 15-30% less than the theoretical value. It is estimated that the significant reduction of the stiffness may be contributed to the quality decay of the bridge material and/or the long term lack of proper maintenance of the Chin-yuan bridge which was built about 80 years prior to the time of the bridge evaluation.
Keywords: bridge inspection, damping ratio, stiffness, plunking test
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