U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-05-083
Date: August 2007
Cable-stayed bridges have become the structural form of choice for medium- to long-span bridges over the past several decades. Increasingly widespread use has resulted in some cases of serviceability problems associated with stay cable large amplitude vibrations because of environmental conditions. A significant correlation had been observed between the occurrence of these large amplitude vibrations and occurrences of rain combined with wind, leading to the adoption of the term “rain/wind-induced vibrations.” However, a few instances of large amplitude vibrations without rain have also been reported in the literature.
In 1999, the Federal Highway Administration (FHWA) commissioned a study team to investigate wind-induced vibration of stay cables. The project team represented expertise in cable-stayed bridge design, academia, and wind engineering.
By this time, a substantial amount of research on the subject had already been conducted by researchers and cable suppliers in the United States and abroad. This work has firmly established water rivulet formation and its interaction with wind flow as the root cause of rain/wind-induced vibrations. With this understanding various surface modifications had been proposed and tested, the aim being the disruption of this water rivulet formation. Recently developed mitigation measures (such as “double-helix” surface modifications) as well as traditional measures (such as external dampers and cable crossties) have been applied to many of the newer bridges. However, the lack of a uniform criteria or a consensus in some of the other key areas, such as large amplitude galloping of dry cables, has made the practical and consistent application of the known mitigation methods difficult.
The objective of this FHWA-sponsored study was to develop a set of uniform design guidelines for vibration mitigation for stay cables on cable-stayed bridges. The project was subdivided into the following distinct tasks:
The initial phase of the study consisted of a collection of available literature on stay cable vibration. Because of the large volume of existing literature, the information was entered into two electronic databases. These databases were developed to be user friendly, have search capabilities, and facilitate the entering of new information as it becomes available. The databases have been turned over to FHWA for future maintenance. It is expected that these will be deployed on the Internet for use by the engineering community.
The project team conducted a thorough review of the existing literature to determine the state of knowledge and identify any gaps that must be filled to enable the formation of a consistent set of design recommendations. This review indicated that while the rain/wind problem is known in sufficient detail, galloping of dry inclined cables was the most critical wind-induced vibration mechanism in need of further experimental research. A series of wind tunnel tests was conducted at the University of Ottawa propulsion wind tunnel to study this mechanism. This tunnel had a test section 3 meters (m) (10 feet (ft)) wide, 6 m (20 ft) high, and 12 m (39 ft) long, and could reach a maximum wind speed of 39 m/s (87 mi/h). With a removable roof section, this tunnel was ideal for the high-speed galloping tests of inclined full-scale cable segments.
The results of the project team’s dry inclined cable testing have significant implications for the design criteria of cable-stayed bridges. The 2001 Post-Tensioning Institute (PTI) Recommendations for Stay Cable Design, Testing, and Installation indicates that the level of damping required for each cable is controlled by the inclined galloping provision, which is more stringent than the provision to suppress rain/wind-induced vibrations.(1) The testing suggests, however, that even if a low amount of structural damping is provided to the cable system, inclined cable galloping vibrations are not significant. This damping corresponds to a Scruton number of 3, which is less than the minimum of 10 established for the suppression of rain/windinduced vibrations. Therefore, if enough damping is provided to mitigate rain/wind-induced vibrations, then dry cable instability should also be suppressed.
The project team obtained matching funds from Canada’s Natural Sciences and Engineering Research Council for the testing at the University of Ottawa, effectively doubling FHWA funding for the wind tunnel testing task. The project team also supplemented the study by incorporating the work of its key team members on other ongoing, related projects at no cost to FHWA.
Analytical research covering a wide spectrum of related issues, such as the behavior of linear and nonlinear dampers and cable crossties, was performed. The research included brief studies on parametric excitation and establishing driver/pedestrian comfort criteria with respect to stay cable oscillation.
Based on the above, design guidelines for the mitigation of wind-induced vibrations of stay cables were developed. These are presented with two worked examples that illustrate their application. This is the first time such design guidelines have been proposed. They are meant to provide a level of satisfactory performance for stay cables with respect to recurring large amplitude stay oscillations due to common causes that have been identified to date, and are not intended to eliminate stay cable oscillations altogether (as this would be impractical).
It is expected that these guidelines can be refined suitably based on future observations of the actual performance of stay cables in bridges around the world as well as developments in stay cable technology. With the widespread recognition of mitigation of stay cable vibration as an important issue among long-span bridge designers, all new cable-stayed bridges are more likely than not to incorporate some form of mitigation discussed in this document. Such would provide ample future opportunities to measure the real-life performance of bridges against the design guidelines contained here.
As a precautionary note, the state of the art in mitigation of stay cable vibration is not an exact science. These new guidelines are only intended for use by professionals with experience in cable-stayed bridge design, analysis, and wind engineering, and should only be applied with engineering judgment and due consideration of special conditions surrounding each project.