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

CHAPTER 8: CONCLUSIONS

A study on wind-induced vibration of stay cables equipped with different types of mitigation measures was conducted through numerical simulations and analyses. The use of transverse crossties, external dampers, and a combination of the two as mitigation measures were analyzed with respect to their relative merits and shortcomings. Explicit time-history analysis of the behavior of stay systems subjected to realistic wind events was performed as well as a sensitivity study to assess the influence of parameters involved in the design of these measures. The following summary highlights the findings and conclusions drawn from the current study:

• Crossties, in general, are effective in the mitigation of in-plane vibration of stay cables. However, the performance of a crosstied cable network is sensitive to the frequency contents of the input wind profile.
• Crossties are particularly effective in suppressing cable vibrations induced by wind events containing appreciable low-frequency components.
• The performance of a crosstied stay cable system is not necessarily proportional to the quantity (or the number of lines) of crossties. Excessive provision of crossties potentially makes the system susceptible to large-amplitude vibrations under highly turbulent
  wind events.
• Oversized (or large diameter) crossties make the system overly rigid, rendering the system vulnerable to high-frequency local vibration modes. Crosstie grounding increases the natural frequencies of the system's global modes of vibration.
• In-plane performance of a crosstied cable network is insensitive to the pre-tension level of crossties as long as this pre-tension level is high enough to prevent slackening of crossties under design wind events. Out-of-plane performance, however, is affected by the pre-tension level of crossties.
• External dampers, in general, are effective in suppressing stay cable vibrations via dissipation of vibration energies of the stay system. However, the efficiency of dampers depends on the spectral properties of the input wind profile.
• Viscous dampers are particularly effective in controlling stay cable vibrations induced by highly turbulent wind flows containing appreciable high-frequency components.
• The effect of a combined use of cable crossties and external dampers is not necessarily the sum of the effects of their independent uses, its efficacy depending on the layout of the crossties and dampers and on the spectral properties of input wind profiles.
• The optimal coefficients of dampers attached to stay cables networked with crossties cannot be determined from the dynamics of individual cable-damper systems. Further research may be needed to develop a practical method for estimating optimal damper coefficients for networked cables.
• External dampers installed at crosstie connections to the deck are found to be very efficient in dissipating the vibration energies of a crosstied stay system, constituting an excellent mitigation alternative.