Flutter and galloping of cable-supported bridges with porous wind barriers
Introduction
Strong cross-winds on bridges and viaducts cause dynamic instabilities of vehicles and trains. Due to these adverse wind effects, vehicles may overturn, collide with each other or with structural elements. Hence, during extreme wind events, viaducts and bridges are often closed to traffic.
To protect vehicles from cross winds, roadway wind barriers are commonly designed, e.g. Kozmar et al., 2009, Kozmar et al., 2012a, Chu et al., 2013, Chen et al., 2015, as vehicles are particularly vulnerable to cross-wind effects on viaducts and bridges, e.g. Argentini et al., 2011, Dorigatti et al., 2012, Kozmar et al., 2012b, Kozmar et al., 2015, Zhou and Chen, 2015.
The major properties of wind barriers that determine their sheltering efficiency for vehicles are porosity and height. Flow characteristics on bridges equipped with wind barriers are predominantly influenced by the bleed flow through the wind-barrier cavities, separated shear layer and the reversed flow downwind of the barrier, e.g. Telenta et al. (2014).
Chen et al. (2015) indicate that larger porosity of wind barriers is unfavorable for dynamic stability of vehicles on bridges, as the obtained velocity reduction may not be sufficient in case the wind-barrier cavities are too large. Sheltering efficiency of wind barriers is strongly affected by the wind-barrier height, Chu et al. (2013). An optimal wind-barrier design with respect to wind perpendicular to bridges is considered the one with 30% porosity and 5 m height, e.g. Kozmar et al. (2014).
While the protective effects of wind barriers for vehicles are fairly known, their influence on aerodynamic forces and dynamic stability of bridges is quite unknown. Only some recent studies consider aerodynamic forces for bridges with wind barriers, Guo et al. (2015). The effects of bird-protection barriers on aerodynamic and aeroelastic behavior of high-speed train bridges are reported in Ogueta-Gutierrez et al. (2014).
Apart from wind barriers, other structural elements of bridges and viaducts, e.g. railings, crash barriers, central slotting, prove to influence aerodynamic forces and moments of bridges as well, e.g. Raggett, 2007, Diana et al., 2013, Xu et al., 2014a.
Design of bridge-deck cross sections may influence their aeroelastic behavior as well, Xu et al. (2014b), while bluff cross sections are commonly more susceptible to flutter, e.g. Nikitas et al. (2011). Vehicles can significantly alter the dynamic stability of bridge decks, e.g., Han et al., 2014, Han et al., 2015, Pospíšil et al., 2016.
The 5 m high wind barrier with 30% porosity, suggested by Kozmar et al. (2014) with respect to the protection of vehicles on bridges from cross-winds, proved to deteriorate dynamic stability of bridge decks, Buljac et al. (2017). However, in practice, wind barriers are manufactured with various porosities and heights, depending on specific wind characteristics for a certain geographic location and respective terrain characteristics. At this moment, it is not completely known whether and to what extent the aerodynamic and aeroelastic characteristics of cable-supported bridges alter due to wind-barrier porosity and height.
The present study focuses on effects of the wind-barrier porosity and height on aerodynamic characteristics of three typical wide long-span cable-supported bridge decks and their sensitivity to self-excited vibrations. Wind-barrier models with different porosities and heights are placed at the windward (leading) edge of the bridge-deck section models, as strong cross winds that may destabilize or overturn vehicles on bridges predominantly blow from one direction only, and wind barriers are commonly placed at the windward bridge-deck edge with respect to the dominant wind direction. Aerodynamic drag and lift force, as well as the pitch moment coefficients, are determined in a boundary layer wind tunnel for various flow incidence angles, and the susceptibility of the studied bridge-deck sections to galloping and flutter is analyzed.
Section snippets
Aerodynamic loads and galloping instability
The aerodynamic coefficients are determined for flow incidence angles from −10° to +10° with an increment of 1° using the following equations:where FD and FL are aerodynamic drag and lift forces, respectively, M is aerodynamic pitch moment. CD, CL and CM are aerodynamic drag force, lift force and pitch moment coefficients, respectively. v∞ is average flow velocity in undisturbed freestream flow, α is flow incidence angle, ρ is air
Description of the wind tunnel and bridge-deck section models
Experiments are carried out in the climatic boundary-layer wind tunnel of the Institute of Theoretical and Applied Mechanics in Prague, Czech Republic. The aerodynamic section of this wind tunnel is 1.9 m wide and 1.8 m high rectangular cross-section. The flow is uniform along the wind-tunnel aerodynamic cross section and the turbulence intensity is less than 2%.
The studied bridge-deck sections are: (i) Great Belt Bridge (GBB) with a streamlined cross section, e.g. Bruno and Mancini (2002),
Aerodynamic force and moment coefficients and galloping instability
Coefficients of the aerodynamic drag and lift forces and the pitch moment are reported in Fig. 4, Fig. 5, Fig. 6, Fig. 7 for flow incidence angles between −10° and +10° with an increment of 1°.
The trends in aerodynamic coefficients obtained for the empty Great Belt Bridge without the wind barrier correspond relatively well with previous studies on similar bridge-deck sections, e.g. Reinhold et al. (1992). Some differences in the results are likely due to minor discrepancies in the Reinhold
Conclusions
The influence of wind-barrier porosity and height on aerodynamic and aeroelastic characteristics of wide long-span cable-supported bridges is studied experimentally in a boundary layer wind tunnel. The experiments are carried out on sectional models of the Golden Gate Bridge (USA), Kao-Pin Hsi Bridge (Taiwan), and Great Belt Bridge (Denmark). The wind-barrier models are placed at the windward (leading) edge of the studied bridge-deck section models only.
The obtained results indicate a strong
Acknowledgments
The support of the Croatian Science Foundation, GAČR No. 15-01035S, CET sustainability project LO12 (SaDeCET) is gratefully acknowledged.
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