Measurement of Human-Induced Vibrations on a Lively Footbridge By Microwave Remote Sensing

Abstract eng:
Microwave remote sensing is probably the most recent experimental technique suitable to the noncontact measurement of deflections on large structures, in static or dynamic conditions. The main ideas of the microwave-based measurement of deflections are: (1) to employ a radar to take consecutive images of the investigated structure; (2) to evaluate the displacement of each target detected in the images from the phase of the back-scattered electromagnetic waves collected at different times (microwave interferometry). The paper addresses the application of microwave remote sensing in the serviceability assessment of a lively pedestrian bridge. The investigated footbridge follows the modern trend to construct light and slender footbridges, accompanied with aesthetic requirements for greater slenderness. The adopted structural solution basically includes: (a) the deck, very slender and about 64.0 m long; (b) one main spatial system of suspension cables; (c) two steel frames supporting the suspension cables and the backstays; (d) one system of ropes with opposing curvature, aimed at laterally stabilising the deck. Firstly, operational modal testing was performed using conventional accelerometers and 12 vibration modes were successfully identified in the frequency range 0–7 Hz: the frequency of the fundamental mode was 1.03 Hz and 5 modes turned out to fall in the range 2–3 Hz. Subsequently, groups of volunteers (up to 32 adults) simulated normal walking and running at different step rates along the deck and the human-induced vibrations were measured by radar technique as well. The deflection responses recorded by the microwave interferometer clearly and easily suggested the exceeding of comfort criteria threshold and this result was confirmed by the acceleration levels derived from displacement data. In addition, an excellent comparison was found between velocity and acceleration time series obtained from data simultaneously acquired by accelerometers and radar. Hence, microwave remote sensing turned out to provide accurate information on the human-induced vibrations, in terms of displacement, velocity and acceleration, although these information were limited to few reflecting points of the deck.

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