Abstract Summary
Tibetan footbridges are very slender, light and flexible structures, and are used in mountainous terrains, often to overcome deep valleys. Consequently, these structures are very sensitive to the dynamic wind action, not only in terms of stability and ultimate resistance but also serviceability limit states. A Tibetan bridge with a span of about 215 m has been designed in the North of Italy. The deck is suspended by four main cables running at a height of more than 110 m from the bottom of a V-shaped valley. The structure is stiffened by two stabilizing cables, running below the bridge and outside the planes of the main cables, and connected to the deck through a system of hangers. The deck consists of a system of steel profiles and bracings, supporting an about 1 m-wide porous steel grid working as walkway. For the ultimate limit states, the possibility that the grid is iced or covered by snow has also to be considered. The lateral protective nets are very porous, supported by a system of horizontal elements and vertical struts. The resulting structure is very light and presents many low-frequency modes. Moreover, several modes exhibit very similar shapes and close vibration frequencies. The bridge has recently been tested in the CRIACIV Wind Tunnel Laboratory in Prato, Italy. A sectional model of the deck and suspension cables was reproduced at the scale 1 : 7.5. A set of static tests allowed measuring through a pair of high-frequency force balances the aerostatic coefficients of the deck. Noteworthy is the significant value of the drag coefficient. Afterwards, the model of the bridge was elastically suspended in the wind tunnel by means of eight coil springs, using also several cables to restrain the unwanted degrees of freedom. Transversal and rotational (torsional) vibrations were allowed, and the aerodynamic derivatives were measured through a free-vibration procedure (Bartoli et al., 2009). The aerodynamic damping for the oscillations in the mean wind direction was estimated based on the classical quasi-steady theory. Mean wind angles of attack of ±5 deg were considered in addition to the baseline null incidence. Afterwards, the results of modal analysis and wind tunnel tests were employed to perform frequency-domain calculations of the bridge dynamic response under stationary turbulent wind. The effect of considering a simplified aerodynamic admittance function was also investigated. The structure exhibited a significant susceptibility to the wind excitation, including some concerns about flutter stability. Under moderate mean wind velocities, non-negligible lateral and torsional oscillations are expected, requiring the setting of a limit wind speed for the closure of the bridge. Finally, the considered Tibetan bridge resulted to be a very interesting case study to address the problem of wind-induced vibrations from both ultimate and serviceability points of view. References: Bartoli, G., Contri, S., Mannini, C., Righi, M. (2009): Toward an improvement in the identification of bridge deck flutter derivatives. Journal of Engineering Mechanics 135 (8), 771-785
