United States Department of Transportation
i
Wind Induced Vibration of Stay Cables: Techbrief [July 2007]
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2007-07-01
[PDF-627.77 KB]
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Abstract:Cable-stayed bridges have been firmly established as the most efficient and cost effective structural form in the 152-meter (m) to 472-m span range. With widespread popularity of cable-stayed bridges around the world, longer spans are being constructed employing increasingly longer stay cables. The stay cables are laterally flexible structural members with very low fundamental frequency and very little inherent damping. For this reason, the stay cables have been known to be susceptible to excitations, especially during construction, wind, and rain-wind conditions.
Recognition of this susceptibility of stay cables led to the use of some mitigation measures on several of the earlier structures. These included cable cross-ties that effectively reduce the free length of cables (increasing their frequency) and external dampers that increase cable damping. Perhaps due to the lack of widespread recognition of the stay cable issues by the engineering community and the supplier organizations, the application of these mitigation measures on early bridges appear to have been fairly sporadic. However, those bridges incorporating cable cross-ties or external dampers have generally performed well.
During the mid-1980s to mid-1990s a number of early cable-stayed bridges were observed exhibiting large stay oscillations under certain environmental conditions. From field observations it became evident that these vibrations were occurring under moderate rain combined with moderate wind conditions, and hence were referred to as rain-wind vibrations. The formation of a water rivulet along the upper side of the cable and its interaction with wind flow have been solidly established as the cause through many recent studies and wind tunnel tests. Exterior cable surface modifications that interfere with the formation of the water rivulets have been tried and proven to be very effective in the mitigation of the rain-wind vibrations.
At the time of the present investigation, it was evident that the rain-wind problem had been essentially solved at least for practical provisions for its mitigation. However, some further experimental and analytical work was needed to supplement the existing knowledge base on several other stay cable vibration issues in order to formulate adequate design guidelines.
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