Nondestructive evaluation of tuned-mass-damper-type vibration absorbers using vibration signatures

Nondestructive evaluation methods are often essential in order to objectively assess the condition of structural and ancillary systems for bridges, particularly in cases where visual methods provide only limited or subjective data and the function of the systems being evaluated may not be disturbed using destructive evaluation techniques. This paper describes a nondestructive evaluation procedure that was developed to objectively assess the current condition of a system of 920 aged vibration absorbers installed on a long span cantilever truss bridge. The vibration absorbers, which are essentially tuned mass dampers consisting of a steel weight attached to the end of a viscoelastic rubber stem, were installed soon after the bridge was opened to mitigate the effects of excessive wind-induced vibrations that had occurred in many of the truss members. A nondestructive evaluation procedure was developed because the vibration absorbers had to remain in service during the evaluation process, and there were only a limited number of spare absorbers that could be used to replace any that might be destructively tested. The complete evaluation program included nondestructive vibration testing in the laboratory using impact methods to determine the current dynamic properties of the absorbers, and field testing to evaluate the energy dissipation characteristics of the installed vibration absorbers as they were actually operating. In addition, a limited number of destructive material tests were conducted in order to corroborate the results of the nondestructive tests. This evaluation program revealed that the current performance characteristics of the aged vibration absorbers were within the limits of the original design specifications, and that the synthetic rubber material used for the vibration absorbers had experienced no appreciable deterioration after nearly 25 years of continuous service.