United States Department of Transportation
i
Dynamic Properties of Stay Cables on the Leonard P. Zakim Bunker Hill Bridge
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2021-05-01
[PDF-11.95 MB]
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Edition:Laboratory Report; December 2003–December 2008
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Abstract: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 low inherent damping and thus are highly susceptible to environmental conditions such as wind and rain–wind combinations. Recognition of these problems led to incorporating different types of mitigation measures on many cable-stayed bridges around the world. These measures included surface modifications, cable crossties, and external (or internal) dampers. Modification of cable surfaces has 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 modification of exterior cable surface effectively disrupts the formation of a water rivulet. The objective of this study was to supplement the existing knowledgebase as to some of the outstanding issues of stay-cable vibrations and develop technical recommendations that may be incorporated into design guidelines. Specifically, this project focused on identifying in-situ cable dynamic properties and assessing the performance of dampers and crossties on the Leonard P. Zakim Bunker Hill Bridge. Forced vibration tests were conducted on the stay cables during the latter stages of construction, just prior to and following installation of dampers as well as before and after installation of a single line of crossties. Cable properties, such as vibration frequencies and damping levels, were established and compared with design targets. The measured levels of inherent damping in the cables were low, as expected, and the resulting low Scruton numbers confirmed the need to install cable crossties.
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