106. A Study in the Long-Term Performance of Specialized Low Friction Hermetically Sealed Fluid Viscous Dampers under Nearly Continuous Operation on a Pedestrian Bridge
This paper will outline the specifics in quantifying the continued damper performance through an intermediate inspection after seven years, followed by a successful comprehensive inspection after eleven years. This included the removal, dynamic testing, and re-installation of three selected dampers.
The well known collapse of Tacoma Narrows Bridge in 1940 clearly identified the importance of aeroelastic effects on long span bridge performance. Extensive research has been carried out since then to better understand the effects of wind on long span bridges, producing various analytical response prediction techniques. An example of the application of such techniques is presented. However, due to challenges related with full scale measurements, these prediction techniques have commonly been validated using only wind tunnel experiments. Recent research has revolved around the conduct of long term full scale measurements on a cable stayed bridge to compare actual bridge performance with those of analytical predictions. In order to ensure the reliability of predicted response, the input parameters, such as wind conditions at the site and modal properties of the bridge are also calibrated using corresponding measured quantities. This paper summarizes some of the preliminary results and outlines their implications.
Fluid Viscous Devices have been found to be a highly effective protection system for bridges. Introduced to China in 1999, the Taylor Devices damper systems have been successfully installed or will be installed in both large and super large bridges in China for protection from earthquake, wind, vehicle and other vibration. Seventeen different bridge projects include the Sutong Yangtze River Bridge, the longest cable stayed bridge in the world, the Nanjing 3rd Yangtze River Bridge, the fifth longest suspension bridge in the world, and the Xihoumen across Sea Bridge, the second longest suspension bridge in the world. The performance of the bridges and dampers have been reported as “very good” during the May 12, 2008 Wenchuan earthquake. All of the dampers produced have been subjected to rigorous static and dynamic testing, which show the dampers will perform well for the next 50 years and possibly much longer.
Fluid viscous dampers have found commercial applications on buildings and bridges subject to seismic and/or wind storm inputs. They are now being used as well on footbridges to suppress undesirable pedestrian induced vibrations. This paper provides a brief overview of fluid damping technology with specific case studies for pedestrian bridges now equipped with fluid viscous dampers. These viscous dampers are used to suppress the feedback between the pedestrians and the bridge and/or wind induced vibrations. On-site tests show that fluid viscous dampers provide significant gains in performance at relatively low cost.
The spring-damper isolators described in this paper were used on the world’s largest cable stayed bridge – the Sutong Bridge over China’s Yangtze River, completed in 2008. The Sutong Bridge is located north of Shanghai in China’s Jiangsu Province at a site where catastrophic earthquakes, typhoons, and ship impact are key design issues. The total length of the bridge is 4.7 miles, with a .67 mile long center span. The tall support towers of this bridge and the long support cables create long period motions along the primary axis of the bridge. The need to accommodate thermal expansion and contraction of the deck axially means that extensive motion can occur in this direction. The configuration of the bridge permits large axial motion of the suspended deck during earthquakes, typhoons, and synchronized truck/car braking loads such as would occur during a mass vehicular accident on the bridge. During dynamic earthquake loading, the long period of the suspended deck provides inherent isolation, albeit essentially undamped. Analysis indicated that added viscous damping would reduce deck motions substantially. During other events like typhoons and vehicle loading, analysis determined that the most cost-effective solution was to incorporate a snubbing type spring element that would only engage (become active) when the damper was approaching its end of travel in either extension or compression. The spring-dampers on this bridge have only damping forces for roughly 85% of the available displacement from the neutral (center of travel) position. Beyond this travel the spring element engage and a combined response of spring plus damper forces results. Essentially, the spring elements are “gapped” through all but approximately the last 15% of the damper stroke in either direction.
Modern pedestrian bridges tend to be long and slender, usually causing relatively low frequency primary modes of vibration. This type of structure can be excited to resonance by synchronized crowd footfall. Added damping is often required to prevent excessive structural motions and loadings. This paper describes the Modular Tuned Mass Dampers used to provide the required added damping for the three Spring Mountain footbridges in Las Vegas.
The Millennium Footbridge was opened to the public on June 10, 2000 – the first new bridge across the River Thames in historic London in more than a century. Nearly 100,000 people used the new bridge in its first day of operation. On June 12, 2000, the Millennium Bridge was ordered closed, due to hazardous deck motions. Seemingly random pedestrian footfalls were causing resonance of the bridge deck, with lateral accelerations measuring up to 0.25 g. The selected method of retrofit was to add fluid damping to the bridge. This paper describes how this was done, including testing of the bridge with groups of up to 2,000 people.
Fluid Lock-up Devices have recently become popular for passive control of large structures subjected to earthquake or wind storm effects. The Lock-up Device, a variation of the Fluid Viscous Damper, allows unrestricted motion at low translational speeds. When a transient event occurs the Lock-up Device activates and forms a rigid connection. After the transient event the Lock-up Device reverts to low force output, permitting structural sections to thermally expand or contract without added stress. The operation of the device is completely passive. It enables multi-mass structures to be dynamically braced without resorting to the cost and complexity of an active actuator system..
55. EXPERIMENTAL STUDY OF BRIDGE ELASTOMER AND OTHER ISOLATION AND ENERGY DISSIPATION SYSTEMS WITH EMPHASIS ON UPLIFT PREVENTION AND HIGH VELOCITY NEAR-SOURCE SEISMIC EXCITATION
A series of shake table tests on an isolated bridge model included low and high damping elastomeric isolation systems, and low damping elastomeric systems with added linear and nonlinear viscous dampers. Each of these configurations could withstand much stronger seismic excitations than the non-isolated configurations. A set of low intensity tests was conducted to form a basis for comparison with the non-isolated configurations and also to test the effectiveness of these systems under low intensity excitation. The results of these tests are presented, followed by a discussion of the effects of scragging, the benefits of seismic isolation, and the significance of damping, the importance of added damping in near source seismic excitation and on the benefits and drawbacks of using nonlinear viscous damping.
43. PRE-QUALIFICATION TESTING OF VISCOUS DAMPERS FOR THE GOLDEN GATE BRIDGE SEISMIC REHABILITATION PROJECT
This report presents the results of the testing of a viscous damping device provided to the Earthquake Engineering Research Center (EERC) of the University of California at Berkeley for pre-qualification testing as part of the seismic rehabilitation of the Golden Gate Bridge. In all, four different viscous dampers from four different manufacturers were tested in the prequalification program. This report presents the test results for the damper denoted Damper C. The test results for the other three dampers, Dampers A, B, and D, are presented in separate reports. Conclusions were that Damper C performed consistently and well throughout the entire testing/pre-qualification program. This report also includes a complete specification for production dampers for this project.