engleski

Aerodynamic and aeroelastic characteristics of cable-supported bridges with roadway wind barriers

Strong cross-winds on bridges and viaducts may cause dynamic instabilities for passing vehicles. To protect vehicles from those adverse cross-wind effects, wind barriers are commonly placed on bridges. While these barriers proved to be successful in sheltering vehicles from cross- winds, their influence on bridge aerodynamic and aeroelastic characteristics is still fairly unknown. This is particularly important for long- span cable-supported bridges that are susceptible to dynamic instabilities due to wind effects. Hence, the present thesis focuses on the effects of wind barriers on aerodynamic characteristics of three typical long-span cable-supported bridge decks and their sensitivity to self-excited vibrations. Experiments were carried out in the climatic boundary-layer wind tunnel of the Institute of Theoretical and Applied Mechanics in Prague, Czech Republic. Experiments were performed on sectional models of the Golden Gate Bridge (USA), Kao-Pin Hsi Bridge (Taiwan), and Great Belt Bridge (Denmark). Wind- barrier models of different porosities and heights were placed at the bridge-deck section models in various arrangements (windward, leeward and both windward and leeward). Flow characteristics around bridge-deck section models and their average aerodynamic loads (drag force, lift force and pitch moment) were determined for various flow incidence angles. Galloping instability was analyzed using the quasi-steady approach. Flutter sensitivity was studied via dynamic free-vibration tests and eigenvalue analysis of a two-degree-of-freedom system. The obtained results generally indicate a substantial influence of wind barriers on aerodynamic loads and stability of studied bridge decks. The drag force coefficient increases as the porosity of the wind barrier decreases, and as the height of the wind barrier increases. Wind barriers change the trends and absolute values of the lift force coefficient of bridge decks, which is more exhibited for more solid and higher wind barriers. The pitch moment of bridge decks decreases when the wind barriers are in place, while the influence of the porosity is more dominant than the wind-barrier height. The effects of wind barriers on galloping vibration of bridge-deck sections are rather negligible ; however, bridge decks become quite prone to flutter when wind barriers are in place. For the windward wind barrier only, flutter susceptibility of bridge decks is more exhibited for less-porous wind barriers. The effects of increasing wind-barrier height are not unambiguous, as they are simultaneously influenced by the aerodynamic shape of bridge deck sections. The wind-barrier arrangement has a major influence as well. For the configurations with the windward wind barrier only as well as both windward and leeward wind barriers, the flutter sensitivity of bridge decks increases substantially, as the critical flow velocity for bridge-deck flutter in those experiments decreased significantly in comparison with the respective empty bridge-deck sections. For the leeward wind barrier only, the flutter susceptibility of bridge-deck sections did not change and remained the same as it was for the empty bridge-deck sections.

Cable-supported bridges ; roadway wind barriers ; aerodynamic forces and moments ; flutter ; galloping ; wind-tunnel experiments

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