Towards Better Understanding of Bridge Aerodynamics - Turbulence Effects

1 Towards better understanding of bridge aerodynamics - turbulence effects Shuyang Cao State Key Lab for Disaster Reduction in Civil Engineering, Tongji University, Shanghai, China email: [email protected] ABSTRACT: The importance of aeroelastic performance of bridges is becoming increasingly significant for wind-resistant design, in the trend of variation of cross sections used for long span bridges from truss-stiffened to quasi-streamlined, and then to multiple-box cross section geometries. This paper reviews phenomena of major concern in bridge aerodynamics: VIV, galloping, flutter and buffeting, with a particular attention to turbulence effects. The analytical, wind tunnel and CFD approaches to generate turbulence that are necessary for studying the turbulence effects are discussed also. This paper shows that either qualitative or quantitative understanding of the turbulence effect, particularly the mechanism behind, is surprisingly insufficient. Although turbulence might help the stabilization of long-span bridges and thus it is not a conclusive parameter for wind-resistant design, the turbulence effects on the aerodynamic and aeroelastic behavior of a bridge must be better understood because the interaction between the bridge and the turbulence always exists. KEY WORDS: Active Wind Tunnel; Buffeting; CFD; Flutter; Galloping; Turbulence Effects; VIV. 1 INTRODUCTION It is really very exciting, however a little bit uneasy, to notice the mass realization of long-span bridges and rapid increase of bridge span in the world in the past several decades. The maximum span of cable-stayed bridge increased from 200-300m in 1950s to more than 1000m recently, represented by the 1088m long Sutong Bridge completed in 2008 in China and 1104m long Russky Bridge completed in 2012 in Russia. Meanwhile, the mid-span of suspension bridge has reached to 1991m in 1998, with the opening of Akashi Kaikyo Bridge in Japan, and the researchers have been planning or proposing to challenge super long bridges with longest span of about 3000-5000m to cross straits, possibly in order to the meet the requirements of globalization. Meanwhile, since the beginning of the years 2000, China has taken a leadership in the realization of an impressive series of long span bridges (Ge and Xiang, 2008). As of 2015, China occupied 6 and 5 seats respectively in the lists of top-ten cable-stayed and top-ten suspension bridges, and these Chinese bridges were completed in this century. Furthermore, the most notable is that the top-ten longest cable-stayed bridges on construction that will be opened in the coming 3-4 years are all in China, and six of the top-ten longest suspension bridges on construction are also in China. It goes without saying that innovations in structure system, design method and construction technique have played important roles in the realization of long span bridges. Every time of growth of bridge span in the past is related to the scientific and technical innovations, among which the contributions from the wind engineering field becomes more and more significant when the bridges becomes more wind-sensitive with increase in bridge span. The collapse of Tacoma Narrows Bridge in 1940 boosted research in the field of bridge aerodynamics - aeroelastics, the study of which had influenced the designs of all the world's great long-span bridges built since 1940. After the failure of Tacoma Narrows Bridge, the wind effects on bridges were enthusiastically investigated by the researchers and engineers involving in bridge aerodynamics, among which the outstanding series work (i.e. Davenport, 1962; Scanlan and Tomko, 1971; Scanlan and Jones, 1990) successfully laid foundations of wind- resistant design of long-span bridges. The Honshu-Shikoku project, which was initiated in 1960s and comprised several long span bridges including the famous Akashi Kaikyo Bridge for which the design against wind load and aeroelastic phenomena was one of the main concerns, accelerated the research of bridge aerodynamics. The Messina Crossing project, the research on which can date back to the 1970s, also stimulated many innovative researches on nonlinear analysis methods of the complex interaction phenomena encountered by long span bridges in turbulent wind (i.e., Argentini et al., 2010; Diana et al., 2013). All the efforts made in the community of wind-resistant design of long-span bridges led to current sophisticated theory and comprehensive and integrated method for wind-resistant design of long-span bridges, which guaranteed the safety and severability of the long span bridges constructed around the world. The significant contribution of aerodynamic research to the realization of long-span bridges can be realized easily from the revolution of bridge deck shape. It is well known that, Humber bridge with a span of 1410m, completed in 1981 in UK, plays an important role in the historical development of suspension bridges because it is characterized by an aerodynamic section, although the first example of long-span bridge with this type of section was the bridge over the River Severn, UK, characterized 14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015 2 by a main span of 988m, which had been completed in 1966 (Borri et al. 2013). For the design of the Humber Bridge, the depth of the deck was increased by 50% from that of the Severn Bridge, in order to increase the torsional stiffness and then avoid flutter at the design wind speed. The same design concept was followed for the Great Belt East Bridge with a main span of 1624m, built in Denmark in 1998. A streamlined section obtained by optimizing the wind performances by introducing wedge- shaped edge fairings was chosen for the Great Belt East Bridge. As many as 16 different trapezoidal box sections were tested in order to appreciate how modifications to the geometry could influence the aerodynamic stability (Larsen and Gimsing, 1992). A series studies on the aerodynamic performance of twin box girder has been carried out in China in order to meet the continuous need for creating long-span bridges and challenging the span limitation (Yang et al., 2015). A twin box girder solution was also chosen for the design of the 1545m long suspension Kwangyang Bridge in Korea, whose cross sectional shape was optimized by section model tests at three different scales to maximize the aerodynamic stability and minimize the drag force (Kwon et al., 2008). Furthermore, a tri-cellular cross section stiffened by several transverse beams was proposed for the Messina Strait Bridge because the exceptional span of the bridge (main span 3300m long) requires very high aerodynamic and aeroelastic performances. The improved stability brought by this innovative solution was checked with a vast wind tunnel test campaign. Meanwhile, instead of a box solution, a classic truss-stiffened deck configuration with reduced aerodynamic performance was selected for the Akashi Kaikyo Bridge in Japan in order to reach the required performance level for flutter stability (critical wind speed higher than 78m/s) (Miyata et al., 1988), because the mid-span could not exceed 1700m if a closed-box deck solution was chosen, in order to guarantee the required safety level for flutter instability. The options of perforated decks or laterally separated decks were not appropriate either because of the too low torsional stiffness. At the same time, the deck cross sections of almost all long cable-stayed bridges were optimized from an aerodynamic point of view. The cross section of the Sutong Bridge was chosen after various wind tunnel tests associated with aerodynamic instability (Chen et al., 2005). For the design of Stonecutter Bridge, a twin box girder deck with a wide clear separation of 14.3m was adopted, with which stability against flutter both during construction and in-service stages could be anticipated (critical 1-min wind speed higher than 95m/s) (Larose et al., 2003). As shown above, without the improved knowledge of bridge aerodynamics or wind-resistant design methods considering wind-structure interaction mechanism, it is impossible to realize long span bridges. However, it is a little difficult for the author to feel smartness in the process of wind-resistant design of a long span bridge. The modern analytical framework to calculate or predict the wind-induced response of a long-span bridge borrowed the aerodynamic knowledge of aerofoil with a streamline body. However, the shape of many bridge decks is not streamline. Furthermore, with the bridge being more wind sensitive, several assumptions for analyzing bridge vibration such as assumption of small amplitude of vibration do not stand and the nonlinear features of bridge vibration are not ignorable anymore. Changes in stiffness, mass and damping of bridges lead to new requirements in dealing with wind effects. The considerations for wind- resistant design of long bridges have to be adjusted continuously, in the trend of a bridge becoming longer and flexible, by adding more terms or adjusting the values of kinds of parameters in order to describe the complicated and delicate interaction between air and bridge. Currently, the wind-resistant performance of a long-span bridge is investigated mainly by wind tunnel experiments. From well-designed sectional or aeroelastic model tests to identify the static and aerodynamic parameters of bridge sections, safety against winds can be satisfactorily guaranteed. However, we have to admit that the stabilization of vortex induced vibration or flutter of a bridge is sometimes built on trial and error. Also, there are a lot of unknown and uncertain issues when people intend to refine the current methods for gust response analysis. In addition, the mechanisms of cable vibration and vibration control lack sufficient physical understanding. It is believed that more fundamental researches on the wind effects on bridges, in particular bridge aerodynamics, are necessary in order to facilitate the physical understanding of the interaction between wind and bridge, and then make the wind-resistant design more rational and reliable.

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