Stay Still

Nov. 8, 2004

Sometime around 2008, the longest cable-stayed bridge in the world will link the banks of the Yangtze River with a twin-tower, six-lane highway structure whose central section will reach an astonishing 1,088 meters.

Sometime around 2008, the longest cable-stayed bridge in the world will link the banks of the Yangtze River with a twin-tower, six-lane highway structure whose central section will reach an astonishing 1,088 meters.

The proposed Sutong Bridge, in China’s Jiangsu Province and part of a massive government-sponsored coastal highway construction project, will require some 20 miles of steel cable and rise about 62 meters to allow modern container ships to pass beneath in all kinds of weather. At 1,088 meters in length, the Sutong Bridge surpasses Japan’s Tatara Ohashi Bridge by 198 meters, as well as Hong Kong’s Angchuanzhou Bridge, now under construction, by 70 meters.

The bridge also will feature the world’s first semiactive cable damping system that will monitor and control cable vibration. Since the 1920s, more than 700 cable-stayed bridges have either been built or are under construction worldwide; China and Japan build the most, and in the past decade China has built more than half of the top 20 longest spans.

To an engineer, the design of a cable-stayed bridge is appealing for a number of reasons. Chief among them is the great flexibility for location it offers; the design doesn’t require the huge structural anchors conventional suspension bridges demand.

Cable-stayed bridges cost less to build and to maintain than other designs. And as bridge spans have tended to become longer in recent years, the cost advantage has become clearer. Cable-stayed bridges begin to have a pronounced cost advantage over other designs when the main span exceeds about 245 meters, and more clearly so when the main span exceeds about 300 meters. This may explain why the longer cable-stayed bridges tend to be newer and why cable-stayed bridge construction in recent years has been trending toward longer spans.

Whoa the gallop

Harder to quantify but no less important, cable-stayed bridges are beautiful. With soaring towers and radiating steel support cables, cable-stayed bridges represent one of the most aesthetically pleasing designs in civil engineering today.

Nevertheless, like most engineering feats, the cable-stayed bridge carries problems unique to its design. Its cables, central to its extraordinary appearance, tend to “gallop” or vibrate from wind and rain excitation.

Cables are efficient structural elements used in cable-stayed bridges, suspension bridges and other cable structures. Because steel cables are flexible and have low inherent damping, they are susceptible to significant vibration, particularly when used as a component of cable-stayed bridges. This uncontrolled movement can result in premature cable or connection failure and breakdown of the cable corrosion protection systems, reducing the life of the structure and eroding public confidence in the safety of cable structures.

Consequently, controlling cable vibration has become an increasingly important concern for engineers, since bridges continue to grow in size and length. And while engineers today are testing a number of cable damping systems, data are just beginning to be made available that compare the performance of leading systems.

Technically, preventing vibration is relatively simple. Conventional transverse tie cables and passive dampers can do the job, but they alter—and, some argue, harm—the structure’s aesthetics. To preserve the graceful lines, one or more passive dampers must be located near the bottom end of the cable, typically at a distance of no more than 1% or 2% of the cable’s overall length. The result is a damping force so weak as to be virtually useless. Place dampers that are too strong, on the other hand, and they lock down the cable, and again the result is a damping force that does essentially nothing.

Transverse passive viscous dampers, for example, have been applied to cables on the Brotonne Bridge in France, the Sunshine Skyway Bridge in Florida and the Aratsu Bridge in Japan. The damper location is typically close to the bridge deck for aesthetic and practical reasons. What recent tests have shown, generally, is that for short cables passive dampers have been found to provide sufficient damping. For longer cables, however, such as in the planned 1,100-meter main-span bridge in Hong Kong or the Normandie and Tatara bridges with cables more than 450 meters long, passive dampers may not provide enough damping without changing the aesthetics of the structure. Further, passive dampers are not capable of accommodating more than one mode of vibration, and longer cables are likely to experience a variety of vibration modes.

Modifying the surface shape of cables to be more aerodynamic has been explored, but it is impractical for retrofit applications and may increase the drag on the structure. And engineers are looking into the benefits of helical beads, which channel wind and water off the cables to prevent vibration.

Fluid non-movement

The proposed Sutong Bridge, however, will test the first semiactive damping system that uses a “smart” magnetorheological (MR) fluid technology capable of continually sensing vibration in individual cables and dissipating energy before it reaches destructive levels. The semiactive dampers will be used as part of a structural monitoring system for the bridge.

North Carolina-based Lord Corp., along with Dr. Y.Q. Ni at Hong Kong Polytechnic University, developed the MR damper system used in China.

In its normal state, MR fluid, a free-flowing suspension of micron-size iron particles, can have the viscosity of light motor oil. When exposed to a magnetic field, however, the fluid thickens within milliseconds to a semisolid state. Using sensors and control algorithms, the MR fluid dampers respond to a magnetic field practically immediately, allowing for real-time control.

The MR damping system, with specially formulated fluid incorporated into specially designed dampers, has been relatively inexpensive to develop. The system requires little maintenance and demands minimal amounts of power to operate.

Similar smart dampers, using the same MR fluid technology but tuned to a static state, have already been tested on another bridge in China at the Dong Ting Lake Bridge in Hunan Province. Recent research on the Dong Ting Lake bridge cable damper system has produced useful data on vibration control, durability and service life.

Test data from the Dong Ting Bridge represents the world’s first full implementation of MR-based smart damping technology to cable-stayed bridges. The experiment results show that MR dampers can effectively suppress cable vibration caused by wind and rain by up to 85% to 94%.

Recent data demonstrate that semiactive smart dampers may provide greater levels of damping than can be achieved with passive viscous dampers. An analytical study demonstrated also that smart, semiactive damping provides increased supplemental damping for cables with sag, a condition in longer cables that presents particular modes of vibration and movement. This study looked at the effects of cable sag, inclination and axial flexibility on the performance of smart cable damping. Although semiactive damper performance appeared to degrade somewhat for certain combinations of sag and damper location, the degradation was less so than with passive dampers. Overall, semiactive dampers were shown to perform better than passive dampers for a wide range of cable sag.

Researchers also have compiled a solid body of data measuring durability and service life of MR dampers in harsh environments. Using specially designed dust covers for bridge applications, MR dampers have passed rigorous durability testing at 2 million cycles with no leaks and less than 10% change in force. Based on this proven performance, the dampers used with an appropriate protection against harsh environments can be expected to provide up to two decades of operation.

The data have convinced Chinese engineer Gu Jinjun, general manager at Tsinghua Hechang Vibration Suppression Technique, to use MR dampers for all bridge retrofitting projects his company plans to undertake over the next several years. His company is currently working with Lord Corp. to design an optimum MR damper system for bridge retrofit. It is expected that two to three cable-stayed bridges a year will be retrofitted with semiactive MR dampers to control cable vibration.

About The Author: Carlson is the engineering fellow with the Materials Division at Lord Corp.

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