Dynamic Properties of Stay Cables on The Penobscot Narrows Bridge

CHAPTER 1. INTRODUCTION

Of particular importance is the dynamic behavior of the deck's supporting cables for cable-stayed bridges. These structurally critical components are often excited into several vibration modes by the ambient wind conditions. Study of the damped behavior of cables is fundamental to ensure a safe and structurally sound system.

Stay cable vibration up to amplitudes of 6.5 ft (2 m) under conditions of moderate wind, sometimes in conjunction with light rain, has been observed with increasing frequency in recent years.⁽¹⁾ This problem is not new and has been studied extensively over a period of several decades; however, gaps remain in our understanding of the problem. With a growing inventory of cable-stayed bridges, we have experienced a significant increase in reports of large amplitude cable vibrations. Some structures have been retrofitted to mitigate these vibrations. Cable-stayed bridges under design and/or construction are currently incorporating dampers, cross-ties, and/or aerodynamic surface treatments into the cable system. While retrofits have been deployed to fix existing problems and mitigation details have been evolving for new structural designs, few full-scale investigations have been conducted to either establish the detailed site/structure conditions or evaluate the effectiveness of mitigation measures. To fill the information gap, the Federal Highway Administration (FHWA) is performing long term monitoring of cables on existing cable-stayed bridges and is conducting vibration tests on cables during various stages of construction on new bridges.

The Penobscot Narrows Bridge is a cable-stay bridge connecting the town of Prospect and Verona Island in Maine, which opened to the public on December 30, 2006. During construction, the Maine Department of Transportation (MaineDOT) and the FHWA agreed to test the longer cable-stays to determine their mode frequencies and damping ratio values, both before and after the installation of dampers. Figure 1 below shows the bridge next to the suspension bridge it replaces, while the inset shows a close up view of the cable-stays threading the pylon.

Figure 1. Photo. The Penobscot Narrows Bridge.

Identical dampers were installed on each cable-stay, with a damping value chosen based on specifications determined by the Figg Engineering Group, the bridge designers. These short-term tests would serve to establish and benchmark cable properties such as vibration frequencies and inherent damping of each stay cable as well as the additional damping provided by viscous dampers installed on each cable. Information obtained in this study can be used not only to assess if design objectives have been met, but also to catalog representative cable properties and for comparison with future measurements to determine if performance has changed perhaps requiring inspection and/or repairs.