Dionysius Siringoringo

Performance of a suspension bridge under wind, seismic and other live loads depends upon its structural properties such as mass, stiffness and damping and their distribution. Although these properties can be modeled using sophisticated analytical models, the real behaviors of the bridge remain to be verified from a full-scale vibration test. The fullscale vibration test would facilitate identification of dynamic characteristics (e.g. natural frequency, damping ratio and mode shape), whose quantities serve as the basis for validating and/or updating analytical models of the structure, as well as providing the actual structural properties and boundary conditions. Furthermore, frequent measurements and analysis of these characteristics will facilitate the evaluation of structural safety and health monitoring.

There are two most common techniques for vibration test of a bridge, namely, the measured-input test and the ambient vibration test. In the measured-input tests, the structure is excited by artificial means using large inertial shakers or drop weights. Measured input excitation is usually applied at a single location where the force input to the structure can be monitored. The tests with measured inputs are usually conducted on small- or moderate-span bridges. The results are generally sufficient for modal identification since the inputs can be well defined and the excitations can be optimized to the response of vibration modes of interest. However, the test features that require extensive instrumentations and disruption of traffic have made frequent tests less favorable. Furthermore, in the case of large and flexible bridges (such as cable-stayed and suspension bridges), where the natural frequencies of the predominant modes are closely spaced within the frequency range 0–1 Hz, the controlled use of specific exciters to obtain significant levels of response is often difficult and costly. In such cases, ambient vibration becomes the only practical means of exciting the structure. This type of test makes use of ambient environment effects such as wind, traffic load, and environmental load as excitation force.

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