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Reconsideration of Wind-Induced Vibration Mitigation of Long-Span Cable Supported : Effects of passive control and Strategy of active control

Lin Zhao, Yaojun Ge State Key Lab for Disaster Reduction in Civil Engineering, Tongji University, Shanghai, email: [email protected], [email protected]

ABSTRACT: Passive aerodynamic control methods with fixed shapes and installation positions have been widely involved in researches and applications in wind engineering. However, some shortcomings could not be ignored for the increasing demands of robustness in life cycle period for super long-span bridges in plan. Development of aerodynamic control methods, especially for main girder of bridge, for wind-induced vibration is briefly reviewed. Aiming at three aspects, including numerical calculation, wind tunnel test and on-spot measurement, some reasons about obvious difference among them are concluded as theoretical algorithm, structural size effects and complex incoming flow, etc. Finally, more concentration are focused on active aerodynamic control, the past more than 30 years development has been reviewed, some conclusion are also reached, then alternative method named self-adaptive active control plate with real time feedback mechanics is proposed, and some possible characteristics of new approach are also discussed.

KEY WORDS: long-span bridge; wind-induced performance; robustness; multiple scale comparison; active and passive control

1 INTRODUCTION For the over 70 years since Tacoma Narrow Bridge, WA, USA was destroyed by wind in 1940 until now, under joint efforts of structural engineers and aerodynamicists, various wind-induced vibrations have been basically explained scientifically and a modern bridge wind engineering system that integrates theoretical research, wind tunnel test, on-spot measurement and numerical simulation has been formed gradually on method level. At present, a theoretical analysis system on bridge aerodynamic force characterized by its linearity and stability effect has been well built and major wind-induced issues that endanger the stability and safety of flexible structures like bridge have been deeply understood and mastered as well. Located at Pacific Northwest, China is one of the few countries that are seriously influenced by windstorms around the world. Majority of the most serious tropical cyclones/violent typhoons in the world are generated at the Pacific Ocean and then move along northwest or to the west, and then they land and attack our coastal areas from the south to north frequently. At present, number of the super long-span bridges that are newly constructed and to be constructed in our economically developed coastal areas has obviously increased. Limit wind load caused by gale/typhoon is often the critical factor used to control bridge design and construction (Zhao Lin et al., 2009), which makes driving/people’s comfort level and structure durability during bridge operation also have obtained the same status with structural stability and safety and it is shown as the comprehensive requirements for wind resistance and robustness within structural life cycle. With the development of modern high-strength materials and construction technologies, bridge structure is developing towards long-span and flexible, which undoubtedly will increase the wind sensitivity of bridges continuously. Thus, wind-induced vibration of bridge structure has become one of the non-ignorable controlling factors in long-span bridge design. Continuous breakthroughs at limit span of cable supported bridges (Xiang Haifan and Ge Yaojun, 2005) rely on the improvements and applications of control methods for wind-induced vibrations of primary members like main girder. According to actual applications and exploratory researches on bridge engineering, such control methods could be divided into three categories: structure method, aerodynamic method and mechanical method. Structure method is used to realize vibration suppression relatively passively through adjusting bridge structure system; mechanical method is used to realize vibration compression through increasing structural damping with ingenious mechanical devices; aerodynamic method is used to remove incentives that cause wind-induced vibrations fundamentally through decreasing wind load. Compared with the other two methods, vibration compression thinking of aerodynamic method is more initiative, its control effect is more obvious and its controlling cost and price are even lower. Attaching small aerodynamic control methods to the surface of primary members of bridge is simple, easy to use and of a stable operating status. At present, during construction and operation of long-span bridges,

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except the long and large flexible cables adopt the wind-induced vibration control strategy with both aerodynamic method (surface pit, helix, etc) and mechanical method (various kinds of active and passive dampers), most of the other members use aerodynamic method alone to control wind-induced vibrations (Xiang Haifan, 2005). Aerodynamic methods that are used on main girder of cable supported bridges as additional members generally include: stabilizer, diversion plate, suppression plate, injection plate, apron board and flange. Besides, the aerodynamic methods attached to main girder surface (such as diversion plate to overhauling rail or suppression plate to accommodation rail, etc) also might exert decisive influences on aerodynamic performance of main girder cross-section (Chen Haixing, 2012). Most of the aerodynamic methods share similar appearance with straight/folded plates, belong to non-bearing structure method, their positions and forms are fixed after installation (please refer to Figure 1), and they belong to aerodynamic methods of fixed plate, which is called “fixed plate” for short. Besides, chamfering of main girder cross-section, central slotting and linear optimal design of tuyere are also selectable design combination. The above aerodynamic methods guide or interfere the air fluid flow distribution near bridge girder cross-section to improve aerodynamic performance of the overall structure. Potential wind- induced vibrations and their aerodynamic control methods of the cable supported bridges that have been completed at present, span of which rank the top all over the world shall refer to Table 1.

Figure 1. Summary on Aerodynamic Methods Commonly Used in Box Girder Cross-section

Table 1. Conditions on Adopting Aerodynamic Methods for Cable Supported Bridges with Maximum Span Bridge Main Form of main Wind-induced Aerodynamic/mechanical Country Bridge name type span girder vibration method Akashi-Kaikyo Japan 1991m Truss Flutter Central stabilizer Bridge Central slotting, chamfering Split steel Vortex-induced at the bottom of girder, China 1650m Xihoumen Bridge box vibration, flutter level flange, gear variable windshield (windbreak) Denmar Great Kelp East Vortex-induced 1624m Flat steel box Diversion plate Long- k Bridge vibration Runyang Yangtze span China 1490m Flat steel box Flutter Central stabilizer suspensi River Bridge on UK Humber Bridge 1410m Flat steel box Flutter Level flange Jiangyin Yangtze bridge China 1385m Flat steel box -- -- River Bridge Hong Kong China 1377m Flat steel box Flutter Slotting Tsingma Bridge USA Verrazano Bridge 1298m Truss -- -- San Francisco USA 1280m Truss -- -- Golden Gate Bridge Yangluo Yangtze China 1280m Flat steel box -- - River Bridge Suzhou-Nantong China Yangtze River 1088m Flat steel box Cable vibration Flute/damper Bridge Long- Hong Kong Split steel Flutter, cable Central slotting, span China 1018m Stonecutters Bridge box vibration flute/damper cable- Vortex-induced Rail of adjustment and stayed Edong Yangtze China 926m Flat steel box vibration, cable inspection vehicle, bridge River Bridge vibration helix/damper Japan Tatara Bridge 890m Flat steel box Cable vibration Flute/damper France Normandie Bridge 856m Flat steel box Cable vibration Helix/damper

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During safety evaluation of wind-induced effect of long-span bridge structure, wind tunnel test is the leading research method. Bridge section model test provides some necessary load parameters and primary wind-induced vibration information on bridge structure for numerical calculation of bridge structure; full bridge aeroelastic model test could better represent the features of wind-induced vibration of 3D bridge tower-main girder structure. On this basis, various wind load effect theoretical systems could be combined to develop finite element numerical calculation, which could represent or deduce deep structural behaviors from a certain extent and the safety of structural behaviors also could be verified from a certain extent based on this (Ge Yaojun, 2011). As the most important research method at present, wind tunnel test is entrusted with an important post in bridge wind- resistance design, but some deviations still have happened during actual applications in the design of bridge wind-resistance for many times. Vortex-induced vibration has been discovered while building the 193m-main girder of Great Kelp East Bridge and the actually measured frequency of vortex-induced vibration and wind speed of oscillation starting are inconsistent with the results of wind tunnel test; during wind tunnel test on Hong Kong Stonecutters Bridge, it has been found that amplitudes of vortex-induced vibrations at 1:20 and 1:80 are quite different. Later, the above issues have been boiled down to Reynolds Number Effect and then explained (Larsen et al., 1998). It could be concluded that in the face of super long-span bridge planned in the future, such factors that influence little on aerodynamic effect of small-span bridges will become major bottlenecks in researches and applications gradually. Since bridge wind-resistant technologies that need to be systematically checked properly developed until now, similar issues or hidden dangers also have happened. Internal mechanism and improving measures should be analyzed to take precautions on future bridge wind engineering and technological improvement.

2 PASSIVE CONTROL EFFECTS AND ISSUES Super long-span bridge structures are of the characteristics like large in size, multi-factor coupling, etc. Generally, single- factor small-scale (<1:200) model tests cannot reveal the real performance of full-scale bridge structures under multi-factor coupling. As it generally has many limitations on the methods and equipment for measurement and control in wind tunnel test and there are also many assumptions in theoretical analysis on wind-induced effects of bridge structure, researches on wind- induced vibrations of bridge structure have been made in ideal model framework with many assumptions all the time, so that deviations certainly exist in such researches, compared with real structures and real sites. Under existing theoretical and test conditions, wind-induced vibration performance of bridge structure obtained through theoretical analysis and wind tunnel test cannot completely simulate the performance of real structures under real site conditions. In general, it mainly has three shortcomings during safety evaluation of the wind-induced effects of long-span bridges: 1) Theoretical algorithm: existing researches on wind-induced effects of bridge structure are mainly based on strip theoretical assumptions, and as existing aerodynamic expression is linear (Simiu and Scanlan, 1996), its nonlinear effect is difficult to be considered. And some phenomena like soft flutter happened in lab under actual conditions are difficult to be explained with existing theories. 2) Scale effect of physical model test: existing research results have indicated that for the physical models with different reduced scales, test results might be quite different. Possible reasons include simulation of Reynolds Number Effect (Li Jiawu et al., 2003), difference of integral scale (Zhou Yufen et al., 2010), etc. 3) Complex conditions of inflow state: existing wind tunnels only could represent uniform inflow and good-state low- turbulence stochastic wind field, and the sites of real structures might face strong turbulence of gale/typhoon. Besides, some actions made by special wind effects on bridge structures are also difficult to be represented and simulated in lab.

Table 2. Comparison of Multi-scale Verification Results of Typical Bridges Main Form of Country Bridge name Bridge type Comparison of multi-scale verification results span main girder Physical model Full-scale verification verification result result Vortex-induced vibrations Zhoushan Suspension Split steel Vortex-induced resonance China 1650m have happened for many Xihoumen Bridge bridge box won’t happen in real times within the wind structures speed of 6-10m/s (Ge Yaojun et al., 2010) (Li et al., 2011) Large-scale model test Small-scale model test result result First-order vortex-induced Cable-stayed China Xiazhang Bridge 780m Flat steel box vibration locking range of First-order vortex-induced bridge 6-9m/s, amplitude of vibration locking range of 80mm 8-12m/s, amplitude 50mm (Zhao Lin et al., 2010) Steel box Theoretical analysis Full-scale verification China Lupu Bridge 550m (arch rib) result result

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Main Form of Country Bridge name Bridge type Comparison of multi-scale verification results span main girder Accumulated hours for vortex-induced vibration No vortex-induced since its completion until vibration has happened now is about 69h (Ge Yaojun et al., 2002) Normative analysis Full-scale verification result result Prestressed Yibin Minjiang China Arch bridge 160m concrete box Buffeting under Second Bridge Passed buffeting checking (arch rib) construction has caused under construction state structure collapse

In recent years, structure health monitoring systems with complete equipment have been installed on some super long-span bridges to collect real location, characteristics of wind field and wind-induced vibration information of the structure in real time, conduct real-time monitoring on the whole performance of bridge structure and make intelligent evaluation on structure performance and state. But obvious deviations exist between obtained monitoring results and existing numerical calculation, theoretical framework system of physical wind tunnel test. While bridge span is developing towards super long, some technical issues that have not been completely solved and some ideological issues that are difficult to predict have been generated, such as large amplitude vortex-induced vibration at main girder cross-section exceeded the anticipation of preliminary test and research, when Denmark Great Kelp East Bridge was completed in 1998 (Frandsen, 2001); large amplitude rain-wind induced vibration happened to the cable of Jintang Bridge of Zhoushan Island-land Project with span of 620m. Design and research of super long- span cable supported bridges have become the top priority in front of bridge workers. Construction of cable-stayed bridges that are longer than 1,500m and suspension bridges that are longer than 3,000m has been listed in the agenda. All of the issues about aerodynamic stability that include flutter stability, static wind stability and vortex-induced resonance with low wind speed are the key factors that restrict cable supported bridges to increase span (Xiang Haifan et al., 2011). Besides, special climate pattern, extremely severe typhoon and wind-rain multimedia coupling at engineering site (Zhao Lin et al., 2014) and bridge structure combining form as well as multiple nonlinear combining effects of material, geometry and wind load of components also will exert positive or negative influences on bridge span increase at safety and usability. Current deep development trend of super long-span bridges forces us to have to possess understanding leap on superior consciousness and scientific forecasting. In view of this, it needs to systematically investigate, survey and collect physical wind tunnel test results of many existing bridges, health monitoring results of wind-induced performance of real bridges, numerical simulation and analysis results of infinite elements to make comprehensive evaluation. And when it is allowed, the physical test standard model that considers comprehensive scale effect will be built, differences of several types of issues will be answered quantificationally and understanding will be made clear. In Form 2, it has listed the representative conditions that there are differences in multi-scale verifications of typical bridges, such as numerical calculation, wind tunnel test, engineering site, etc. From a certain extent, it indicates that part of the doubts and problems to be solved in details still exist in existing theoretical, test and research systems. For the conditions in Form 2 that wind-induced performances of many long-span bridges are inconsistent with anticipations, it indicates that anticipation abilities on aerodynamic structure, aerodynamic method control effect or wind tunnel test are insufficient. Take such problems together, the following reasons might exist upon analysis: Control ability is only for specific inflow conditions: inflow conditions of the bridge site are often more complex than the working conditions of bridge model in the flow field of wind tunnel test. And in wind tunnel tests, for the verifications on wind- induced stability of long-span bridges that center on flutter and vortex-induced vibration, most of them focus on the inflow angle that is within attack angle ±5°. In recent years, observation analyses and wind tunnel simulation researches on characteristics of strong wind in coastal areas and mountainous areas indicate that under the inflow wind field conditions that exist within the range of attack angle ±5°~±15° with large probability (Pang Jiabin, 2006; Sun Jianchao, 2006), and under large attack angle, further passivation of bridge girder cross-section is easier to lead to aerodynamic instability that goes against wind-induced stability. If considering specific wind effects represented by strong turbulent typhoon, thunderstorm wind characterized by strong updraft or downdraft and tornado, this issue will become more complex. Control performance is usually for specific wind-induced vibrations: aerodynamic control methods are mainly applied in the wind-induced self-excited unstable vibration (such as main girder flutter, self-support galloping of steel bridge tower, galloping of steel arch rib in construction, etc) that are closely related to the safety and stability of bridge structures at first. The aerodynamic methods (such as adopt slotting to separate double-box cross-section at main girder, add central stabilizer, etc) adopted in order to improve wind-resistant stability will influence streamline of bridge girder cross-section, so as to make peak energy of wake flow signature turbulence more prominent. While improving wind-induced stability of bridges under high wind speed, transfinite lateral displacement, large amplitude vortex-induced vibration and random buffeting response that go against structural performance are easy to be induced under the low wind speed that is commonly seen. Control effect is sensitive to structural dimension: it is concluded from wind tunnel tests and researches on many real bridge projects, wind-induced vibrations caused by passivation of main girder cross-section are extremely sensitive to the subtle change of aerodynamic dimension. For wind tunnel test on Xihoumen with main span of 1,650m, vortex-

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induced vibration with limited amplitude that exceeds the anticipation of wind tunnel test exists for a short time during construction process, for the difficulty of scale reduction simulation of detail structure of main girder central slotting and its similarity; for both cable-stayed bridge across the sea from Xiamen to Zhangzhou, Fujian and Jintang Bridge of Zhoushan Island-land Project, quite different vortex-induced vibrations (Cao et al., 2008) have been found during wind tunnel tests and researches for the small differences (≤5cm) between cross-sections of pedestrian guard rail on bridge deck and size of the block at the bottom of rail. Wind tunnel test and research on typical main girder cross-section indicate that change of the windward angle of tuyere and vertical dimension of central stabilizer could change critical wind speed of bridge wind-induced vibration for over 100% the most (Yang Yongxin, 2002; Zou Xiaojie, 2005). With engineering technicians’ deep understanding on the development of bridge technologies, wind-induced vibration of super long-span bridges tends to focus on comprehensive evaluation and optimization of wind-induced vibration effects based on both safety and usability gradually. It is very difficult to find out some general aerodynamic method that could satisfy wind-resistant comprehensive performance upon wind tunnel test and numerical calculation. From present experience of wind tunnel tests and researches, various traditional aerodynamic methods cannot control strong robustness and universality of complex inflow and wind-induced vibration. If an applicable motion with single-frequency, multi-frequency or even broadband could be found to change the location and form of aerodynamic methods specifically according to inflow conditions and structural motion morphology, or aerodynamic methods could intelligently generate countermeasure for the condition of flow field, the above shortcomings could be greatly improved. It means the fact that control effects of traditional aerodynamic methods are stable could be come into play. Then for aerodynamic methods are of simple structure and light, the shortcomings that design of the actuating device of active and passive mechanical control methods that only provide energy and damping effects are complex, action real-time response efficiency is difficult to guarantee and maintenance is tedious could be avoided. Such aerodynamic methods with active ability to change could be called as “self-adaptive active aerodynamic methods”. Based on commonly seen “fixed plate”, adding mechanical or intelligent motions or changes of form and location, it belongs to controllable plate aerodynamic methods, which is called “control plate” for short. This name is similar to specialized vocabulary in the field of mechanical manufacturing or artificial intelligence.

3 DEVELOPENT REVIEW Aerodynamic methods mainly refer to install ancillary facilities on main girder of cable supported bridge to improve aerodynamic performance of cross-section and to decrease aerodynamic force on main girder, so as to decrease or control harmful vibrations. According to if moving relatively to main girder, aerodynamic methods could be divided into fixed aerodynamic methods, movable aerodynamic methods and active aerodynamic methods. Fixed aerodynamic methods: mainly include stabilizer and tuyere (Yang Yongxin, 2002; Zou Xiaojie, 2005), corrugated plate (Gammal, et al., 2007), flange-type methods (Savage et al., 2003; Liu, 2006) (such as diversion plate, separating plate, suppression plate, etc). As such methods do not need additional energy supply while operating, are highly reliable and do not need large amount of daily maintenance, they are most widely used to control wind-induced vibrations of long-span cable bridges (please refer to Form 1). Movable aerodynamic methods: refer to the aerodynamic methods that are installed on main girder driven by specific actuating device through considering the motion of main girder or main cable, and their main characteristic is moving in fixed pattern. Generally, they do not need additional energy input to maintain operating status. Zhoushan Xihoumen Bridge has skillfully adopted the windbreak that could be adjusted to horizontal or vertical (please refer to the variable windshield shown in Figure 1). Besides improving driving wind environment on bridge deck, these methods also could be used in safety control on static stability of main girder while powerful typhoon is landing and other vortex-induced vibrations under low wind speed. Active aerodynamic methods: have active control plate of self-adaptive feedback mechanism. Strictly speaking, such aerodynamic methods are limited to experimental research. Therefore, how to truly realize “intelligent” self-adaptive feedback mechanism is the bottleneck in current researches, and current research reserve is not enough to support this technology to transfer to actual application of bridge engineering. Summaries will be made one by one on the research progress of some commonly seen movable/active aerodynamic methods: In 1987, Raggett has come up with the idea that installing ancillary control plate on main girder of suspension bridge to change the aerodynamic forces exerted on main girder and improve flutter performance of suspension bridge.

a) Active tuyere b) Active control plate Figure 2. Devices of the Active Aerodynamic Methods Used by Ostenfeld

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In 1992, Ostenfeld and Larsen from Denmark COWI have come up with two active aerodynamic methods used in flutter control: one is to actively change the shape of cross-section edge of stiffening girder to improve the flow field surrounding stiffening girder, which is called active tuyere; the other is to install streamlined control plate at the bottom of cross-section of stiffening girder, which is called active control plate. Detailed sketch map on the devices shall refer to Figure 2. In 1992, Kobayashi and Nagaoka from Ritsumeikan University in Kyoto, Japan have designed a wind tunnel test device for active control plate (Figure 3), in which upon wind tunnel test, after installing active control plate, flutter critical wind speed of the model has been raised twice as the original. It is a pity that feedback gain coefficient of active control plate is artificially selected and modern control theories have not been used to optimize it.

Figure 3. Test Devices of the Active Control Plate Used by Kabayashi and Nagaoka

In 1999, Del Arco and Aparicio have summarized and put forward four different installation positions of control plate shown in Figure 4. Hereinto, for the control plate in Case 3, control plate could slide to wake zone of box girder through setting a set of horizontal sliding device.

Figure 4. Devices of the Active Aerodynamic Method Used by Del Arco and Aparicio In 1999, for the aerodynamic methods of control penal, Fujino has come up with an actuating device and designed a set of movable control penal. The device is shown in Figure 5. As the plummet in stiffening girder drives control plate to move, adverse motion between stiffening girder and plummet drives anti-phase twist motion between control plate at windward side and main girder (phase difference is 180°) and drives in-phase twist motion between leeward control plate and main girder. Then, Fujino has made wind tunnel test and numerical calculation to verify vibration compression effect of control penal.

Figure 5. Device of the Active Aerodynamic Method Used by Fujino In 1999, inspired by some method of flutter control in aeronautical engineering, Kwon has come up with a new flutter control method that could be used in bridge girder cross-section. Sketch map on the device is shown in Figure 6. As aerodynamic

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turbulence is quite similar with stabilizer, it is called movable “control stabilizer”. For this device, it requires to open two slots at the bottom of box girder, and control stabilizer could shuttle up and down driven by TMD device in box girder. The research indicates that control stabilizer could improve flutter critical wind speed for over 50%.

Figure 6. Device of the Active Aerodynamic Method Used by Kwon In 2000, Omenzetter has put forward and developed a device used in flutter control, through which tyuere moves driven by relative motion between main cable and stiffening girder. For this system, a set of complete numerical calculation models have been built. Numerical calculation indicates that according to asymmetric tyuere in stiffening girder cross-section, critical wind speed of flutter has been greatly increased. When width of tyuere exceeds 3m, critical wind speed of flutter reaches 160m/s, which has increased as much as 202%, compared with the 53% without methods. In 2002, Omenzette’s further research results have been generalized in 3D models successfully, and various different layout lengths have been compared and selected to consider 3D effect of tyuere system.

Figure 7. Device of the Active Aerodynamic Method Used by Omenzetter Test devices of aerodynamic methods mainly include movable control plate, movable “similar stabilizer”, movable tyuere, etc. Movable aerodynamic methods are only under preliminary research at present, and they only could control specific wind- induced vibrations and cannot adopt targeted countermeasure for different forms of wind-induced vibrations.

Figure 8. Aerodynamic Model Attached to Active Control Plate Used by Huynh Analysis algorithm on the effects of movable aerodynamic method also has developed for a certain degree. In 2001, Huynh has set up the flutter numerical analysis on stiffening girder of active control plate. In order to simplify, during numerical calculation, active control plate and flow field of main girder have been supposed not to interfere each other, aerodynamic parameters of control plate and main girder have been recognized independently and the aerodynamic forces exerted on both control plate and main girder could be overlaid. Thus, flutter derivative could be expressed as a function between motion amplitude and phase of control plate, which could be used to make flutter analysis. In 2004, Nissen has pointed out irrationality of the assumption in numerical calculation that flow fields around main girder and control plate do not interfere each other. He

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also has suggested qualitative evaluation method of the interference effect between the two factors and completed numerical calculation theory of this system preliminarily. Aerodynamic force exerted on control plate could be directly obtained though measuring the pressure on the surface of control plate, compare this force with that exerted on control plate while supposing motions of main girder and control plate won’t interfere the flow fields surrounding each other, additional aerodynamic force generated for interference is obtained upon separation and dissolve it into the numerical calculation theoretical model put forward by Huynh with the form of aerodynamic derivative. In 2009, with 2D model cross-section and aerodynamic force load mode of active control plate, Kirch has proved that active control plate could improve aerodynamic stability of bridge cross- section through solving with characteristic value of state space within linear elastic range upon theoretical analysis, and he also has pointed out that active control plate is difficult to obviously improve random vibration of buffeting theoretically. In 2013, upon wind tunnel test and theoretical analysis, Guo Zengwei has developed active control research on flutter and vortex-induced vibration of active control plate and set up aerodynamic model of active control plate system and its flutter and vortex-induced vibration governing equation. Set a suspension bridge with main span of 3,000m as an example, deflation handing method of system state in active control of flutter and selection method of active control algorithm have been discussed. Try to make control researches on flutter and vortex-induced vibration of active control plate with open-loop manner, namely adopt steering engine (servo motor) controlled by MCU and input with digital signals and use self-developed software to realize the interactive transmission of control commands between computer and MCU, so as to realize accurate control on vibration frequency, phase and amplitude of control plate. Upon wind tunnel test, influences made by frequency and phase of torsional vibration of active control plate on control effect of flutter and vortex-induced vibration under open-loop control have been researched. In Figure 9, it shows the loading process and test process of active control plate based on open-loop plan.

a) Sketch Map on Mechanical Driving and Control of Control Plate

b) Drawing on Bridge Section Model and Control Plate c) Sketch Map on Control Plate to Bridge Section Model Figure 9. Loading Principle and Test Implementation of Active Control Plate Based on Open-loop Plan Besides the method of moving control plate to realize active aerodynamic control, there are still some atypical researches and applications of active aerodynamic control methods in recent years. In 1999, Kubo et al. have sprayed airflow at windward side of bridge box girder to control the separation of boundary layer at the surface of box girder, which has realized restraint on torsional flutter of bridge and indicated that boundary layer of flow field could be controlled through air injection or air suction.

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In 2002, Hiejima has made periodic air injection to control boundary layer of column to decrease its degree of passivation, so as to realize that passivated column tends to be streamlined. In 2004, adopting this principle, Kubo has put forward a new device to control vortex-induced vibration through actively controlling characteristics of boundary layer, which means scrollable columns could be installed at both sides of bridge girder cross-section to control the strength of airflow vortex effect. In 2011, Xin Dabo has adopted CFD technology to systematically analyze the flow state of bridge box girder under steady suction condition. This research has indicated that suction makes mainstream get closer to the wall, which has reduced the aerodynamic torque of the model under twist motion and improved stability of wind-induced flutter of long-span bridges.

4 CONCLUSIONS Wind load is much smaller than the weight of bridge, so that the bridge cannot be stimulated to generate large amplitude vibrations only with forced vibration. Thus, wind-induced vibration of bridges could be boiled down to resonance of low damping system from a large extent. Compared with other vibration suppression methods, aerodynamic methods could remove the incentive of wind-induced vibrations of bridge fundamentally though damaging “bridge-flow field coupling resonance system” skillfully. Compared with mechanical methods and structure methods, the vibration compression thinking of aerodynamic method is more initiative, its control effect is more obvious and its controlling cost and price are even lower. Active aerodynamic design is of good vibration suppression performance, it could better solve the problem that wind-resistant safety reserve of super long-span bridges is insufficient, it is of good universality to various kinds of wind-induced vibrations and it could better solve the problem that wind-resistant performance of super long-span bridges becomes worse as well. But structure of active aerodynamic methods might be complex and it might face the serious problem of reliability in engineering application as well. Normal operation of active aerodynamic methods relies on the reliable operation of sensing system, control system and actuation system, and careless mistake that happens to any link might invalidate the entire control system and even might aggravate structural vibrations. Therefore, ensure active aerodynamic methods be of good robustness is important to guarantee it operates reliably, and it is also the important prerequisite in future engineering applications. In general, aerodynamic methods of control plate applied in bridge structures is still under preliminary study and although the control effect that has been reported has obtained satisfactory research results, for flutter instability of main girder of single sinusoidal divergence form, they are not suitable for different types of wind-induced vibrations or adopting self-adaptive counter motions. And it also lacks of reasonable proof for control effect within life cycle of bridges to meet the requirements of comprehensive robustness. Detailed performances are shown as follows: 1) All the test effects are mainly trial, control effect mechanism is not completely clear and control objective of self-adaptive real-time feedback has not been realized; 2) Simplified analyses have been adopted too much in theoretical systems built during researches, so that it lacks of strong support to modern structural control theories; 3) Practical calculation model of main girder-control plate system cannot reasonably reflect the influence of motion phase and vibration amplitude of control plate on control effect, and the mutual interference between flow fields of main girder and control plate system generated for relative motion between main girder and control plate has been neglected at the same time; 4) Recognition and optimized analysis algorithms of the structure of main girder-control plate system and aerodynamic load parameters need to be built to further verify the effectiveness of active control plate to complex inflow requirements. Wind-induced performance of bridges is of two distinct characteristics, including crosswind vibration and relying on the interaction between wind and structure, which not only bring convenience to wind-vibration control, but also bring larger challenges. It is convenient as layout of bridges could be properly changed or some diversion devices could be added to reduce the interaction between wind and structure, so as to reduce amplitude of wind-induced vibrations; the larger challenge is for main girder, as vertical restraints of main girder are difficult to change and the space inside main girder that could be used is very limited, which cannot meet installation space for passive reactive mass damper and needless to say the travel space for mass block required by reactive mass damper to come into play. Thus, for wind-induced vibration control on main girder, traditional mechanical vibration suppression methods are helpless, more proper control methods should be found out for its specific characteristics of wind-induced vibrations. Applications of traditional aerodynamic methods have played a non-ignorable role in breaking through the span of long-span cable supported bridges, but all the different kinds of traditional aerodynamic methods cannot control the robustness and universality of complex inflow and wind-induced vibrations. For the demand for development of super long-span bridges, it is necessary to integrate modern active control theories to further discuss the applications of more effective self-adaptive active aerodynamic methods in structural wind engineering. In the future, wind-induced vibration control of bridges is better to focus on giving play to the characteristics of aerodynamic control methods as they are flexible, stable and easy to be used, to the advantages of mechanical methods as they are of strong coping capacity and good controllability and to the shortcoming in current research progress as active control plate system lacks of self-adaptive feedback mechanism. While exploring control effect mechanism of main girder-control plate and its practical recognition algorithm of engineering applied numerical model and aerodynamic load parameters, and integrating physical wind tunnel test to further optimize motion and structural parameters of control plate of active aerodynamic methods and verify its actual application effects. When it is allowed, we should try to transform to actual applications of bridge engineering.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of National Key Basic Research Program of China (i.e.973 Program) (2013CB036300), Ministry of transport application foundation research project (2013319822070) and the National Natural Science Foundation of China (91215302, 51222809 and 51178353), and the support of Program for New Century Excellent Talents in University.

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14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015