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Publication Number:  FHWA-HRT-14-049    Date:  August 2014
Publication Number: FHWA-HRT-14-049
Date: August 2014


Mitigation of Wind-Induced Vibration of Stay Cables: Numerical Simulations and Evaluations


Cable-stayed bridges have been recognized as the most efficient and cost effective structural form for medium-to-long-span bridges over the past several decades. With their widespread use, cases of serviceability problems associated with large amplitude vibration of stay cables have been reported. Stay cables are laterally flexible structural members with very low inherent damping and thus are highly susceptible to environmental conditions such as wind and rain/wind combination.

Recognition of these problems has led to the incorporation of different types of mitigation measures on many cable-stayed bridges around the world. These measures include surface modifications, cable crossties, and external dampers. Modifications to cable surfaces have been widely accepted as a means to mitigate rain/wind vibrations. Recent studies have firmly established the formation of a water rivulet along the upper side of the stay and its interaction with wind flow as the main cause of rain/wind vibrations. Appropriate modifications to the exterior cable surface effectively disrupt the formation of a water rivulet.

External dampers and cable crossties have gained increasing popularity among bridge designers as measures for controlling wind-induced stay vibrations. External dampers dissipate the mechanical energy of vibrating cables and increase cable damping. Crossties transform individual stay cables into a cable network and increase the in-plane stiffness of a stay cable system. The increased system stiffness is translated into increased vibration frequencies of the system, especially in their fundamental modes. These increases in fundamental vibration frequencies due to the addition of cable crossties have been viewed as a merit to lower the potential of aerodynamic instabilities of the cable system subject to wind flow.

However, the effectiveness of crossties as a means of counteracting undesirable stay cable oscillations has not been unequivocally established, and the potential benefits of increased fundamental frequencies of crosstied cable networks under realistic wind flow has not been substantiated by explicit analysis. The problem of potentially undesirable behavior of local vibration modes of crosstied cable networks has been pointed out by other researchers. Local modes of vibration are characterized by a set of intermediate segments of specific cables involved in the oscillation of a cable network.

External dampers provide mitigation effects through dissipating the mechanical energies of vibrating cables. However, the mitigation effectiveness of these dampers depends on the geometrical and mechanical properties of the cable-damper assemblies and the characteristics of wind flow. Also, there would be synergistic effects from a combined use of cable crossties and external dampers. No detailed studies have been reported in the literature that address the combined use of cable crossties and external dampers.

The objective of this study is to supplement the existing knowledge base on some of the outstanding issues of stay cable vibrations and to develop technical recommendations that may be incorporated into design guidelines. Specifically, this project focuses on the effectiveness of cable crossties, external dampers, and the combined use of crossties and dampers. Finite element simulations are carried out on the stay cable systems of constructed stay cable bridges under realistic wind forces in order to address these issues. Explicit time-history analysis has enabled the performance of stay cable systems with different mitigation strategies to be assessed and compared for their relative advantages and disadvantages.

This current study indicates that the effectiveness of cable crossties as a mitigation measure depends on the configuration of stay cables and the condition of wind flow. The optimal provision of crossties for a given stay system depends on the nature of the design wind event to be used. For example, stay cable networks with overly equipped crossties are not very effective to mitigate highly turbulent wind events. Stay networks with large crosstie quantities have increased fundamental frequencies and tend to pose greater potential for resonance with highly turbulent wind excitations. A medium-to-low level of crosstie provision helps to combat high-frequency dominant wind events more effectively.

Conversely, analysis indicates that external viscous dampers are very effective in controlling vibrations of stay cables subjected to wind events containing appreciable high-frequency components. It was also found that combined use of cable crossties and external dampers is effective in combating a wide range of wind events containing both low- and high-frequency components. In particular, external dampers attached at crosstie anchorages to the bridge deck are found to be much more efficient than dampers attached to individual stays. Dampers attached to individual cables are very limited in their influence on cable damping due to the close proximity of the dampers to the anchorages of the cables.