Brakes – Inertia of damping mass dissipates motion
Reexamination Certificate
2000-12-18
2002-08-20
Lavinder, Jack (Department: 2683)
Brakes
Inertia of damping mass dissipates motion
C188S267100, C188S268000, C188S381000, C267S136000, C174S042000
Reexamination Certificate
active
06435323
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed generally to the aerodynamic and mechanical damping of cable stays. More specifically, the present invention is directed to the aerodynamic and mechanical damping of cable stays by utilizing active devices. Most particularly, the present invention is directed to the structure and to the use and control of active aerodynamic damper bands applied to cable stays for the purpose of mitigation of wind/rain induced cable stay vibrations.
Current approaches to controlling large amplitude cable stay vibrations are passive. No active sensing or control mechanisms are utilized. The implementation of an active, smart cable vibration damping system is presented in the subject invention. The system of the present invention employs distributed aerodynamic rings along with small, embedded mechanical dampers, such as shiftable media, pendulums, and/or spring type inertial masses that may be energized using active smart control when the cable vibration reaches a threshold limit. Due to the extreme heights at which cables are mounted, efforts in active smart control are focused on low-maintenance damping techniques and low-cost cable modifications.
DESCRIPTION OF THE PRIOR ART
In recent years, large-amplitude cable stay vibrations have been observed on a number of bridges in the U.S. and abroad during relatively low wind speeds in the range of 7 to 14 m/s (15-30 mph), with and without the presence of rain. Rain and wind-induced cable stay vibration is an aerodynamic phenomenon that was relatively unknown and did not receive adequate attention from bridge designers thus resulting in the need for mitigation devices. Excessive vibrations are detrimental to the fatigue life of the cable stays and cause distractions to the passing motorists.
The vibration of cable stays is most prevalent during low wind speeds, below 14 m/s (30 mph), and accompanying moderate to heavy rain. In addition, vibrations may also occur at high wind speeds, above 22 m/s (50 mph), without rain. The cause of the vibration problem at low wind speeds with rain is believed to be the change in cross-sectional shape of the cable or cable stay that occurs when rain forms a rivulet along the cable. This modification of the cross section of the cable stay affects the aerodynamics of the cable stay, resulting in large vibrations at wind speeds well below known vortex shedding speeds for cylindrical cable stays in a specific vibration mode. Cable stay vibrations can be severe and have led to concerns that they are contributing to significant fatigue loads on the cables. At risk is the material that makes up the cable stay itself; as well as the anchorage devices.
Investigations at the Fred Hartman Bridge located at Baytown, Tex., and at the Veteran's Memorial Bridge located at Port Arthur, Tex. have shown the existence of a large number of rain/wind induced cable stay vibrations. Over 5000 five-minute “triggered” events of cable stay accelerations have been recorded in just over two years. “Triggered” events are recorded when a predetermined acceleration and/or wind speed threshold is exceeded. It has been noted that each individual cable seems to vibrate at a particular lower-mode shape, but typically not the first mode. For example, a long Fred Hartman stay cable, 183 m (601 ft) in length with a fundamental frequency of 0.65 Hz, vibrates predominately in the
3
rd
mode and not in the first two. Similarly, a mid-size Fred Hartman stay cable, 87 m (286 ft) in length with a fundamental frequency of 1.2 Hz, was found to vibrate predominately in the 2
nd
mode and not in the first.
Higher modes of vibration in the cables were also found on both the Fred Hartman and the Veterans's Memorial bridges. It is generally accepted, though unproven, that cables vibrating in lower modes cause more damage than cables vibrating in higher modes, since lower-mode vibrations generally cause larger displacements. However, it is entirely possible that higher mode vibrations occur often enough to produce significant fatigue loadings on the stay cables due to cycles of reversed stressing.
Considering the physics of the rivulet formation, it is difficult to conceive that the rivulet is consistently located at the most critical location along the full cable length; there is lack of full-scale information. It is possible that the rivulet that primarily causes the vibration at the lower wind speeds forms at the critical location only over a partial cable length. This could explain why there is a preference for certain lower-modes to vibrate.
Currently, cable stay oscillations caused by wind/rain induced aerodynamic forces are controlled by one, or by a combination, of the following methods: 1) single-point mechanical dampers, typically at the base of each cable, 2) restraining cable devices connecting adjacent cables at various locations along the length of the cable, resulting in a reduced effective length for each cable and/or 3) aerodynamic damping approaches such as grooves, protuberances or circular rings. The former two methods are considered concentrated damping mechanisms, while the latter is considered distributive.
For a distributed mitigation device, such as the aerodynamic rings, it is possible to completely solve the vibration problem by installing the rings only on a partial length of the cable-and not along the full cable length. A distributed aerodynamic ring system will be effective in eliminating significant vibrations in all vibration modes, unlike a linear mechanical damper (hydraulic) that is optimized to be effective for a single mode.
Mechanical dampers generally are linear viscous mechanisms, somewhat similar to an automobile shock absorber. However, they also can be non-linear, computer-controlled mechanisms. Mechanical dampers are a proven technology and are relatively easy to install. However, they generally are: 1) expensive systems-and can be expensive to install, 2) may need periodic maintenance, and 3) typically require substantial cable stay displacements to occur before the damping mechanism becomes functional.
Restrainers are employed to tie adjacent cable stays together at discrete points along the cable. Restrainers generally are effective solutions, as one cable adjacent to another oscillating cable generally will not be oscillating. For cases when adjacent cables do oscillate together, many times they will vibrate out of phase or in different modes from each other. In these cases, restrainers are able to utilize the stiffness of adjacent cables to prevent a particular cable from oscillating. If the restrainer is unable to prevent oscillations, it continues to be considered beneficial in that it causes the cable stay to vibrate at higher modes as it “fixes” intermediate nodal points. Again, though a higher mode vibration is visually less dramatic, significant fatigue loadings can occur. Restrainers also are a proven technology. However, they are fairly difficult to install—particularly at cable stay heights generally required. Also, restrainers have had problems due to failure through loosening of attachments to the cable stays.
Although mechanical dampers are more popular, aerodynamic devices have certain advantages. They can be very effective over a wide range of wind speeds, and perform even better at high wind speeds, if properly designed. These aerodynamic devices are generally cost-effective and demand little maintenance efforts, thus they can function reliably. They can also be designed to be aesthetically pleasing; and reduce the effect of the aerodynamic forces before the cable begins to vibrate, where mechanical devices must dissipate energy of the cables that are already vibrating.
Various forms of aerodynamic solutions to the vibrations of smooth-surfaced, circular cable have been sought. While some can be adopted only at the design stage, others are feasible for retrofitting as well. Aerodynamic countermeasures usually modify the surface of the cable cross section to improve its aerodynamic performance in terms of reducing the excitation from th
Gardner Thomas B.
Mehta Kishor C.
Phelan R. Scott
Sarkar Partha P.
Zhao Zhongshan
Jones Tullar & Cooper PC
Lavinder Jack
Sy Mariano
Texas Tech University
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