<title>Bridge stay cable condition assessment using vibration measurement techniques</title>

Abstract

In this paper, results of a research project sponsored by the Federal Highway Administration (FHWA) on a non-destructive method for measurement of stay cable forces in cable-stayed bridges are presented. This project included development and verification of specific analytical and experimental procedures for measurement of stay cable forces. In one set of procedures, a single laser vibrometer is used to measure low- level cable vibrations due to ambient (wind and traffic) excitation. The laser device allows rapid measurement of cable vibrations at distances of up to several hundred feet. Procedures are also developed for utilization of accelerometers attached to cables. Contact sensors are more appropriate when long-term remote monitoring is desired. Measured natural frequencies of vibration are related to cable tension through a mathematical formulation developed during the course of this study. This formulation includes the effects of cable sag-extensibility, bending stiffness, various boundary conditions, intermediate springs or dampers, etc. This method can also be used during construction in lieu of the 'lift off' method. The accuracy and effectiveness of this methodology was tested in the laboratory on a scaled model of a cable, and on two cable-stayed bridges. This ability to rapidly measure stay cable forces provides an opportunity for global condition assessment of these major structures.