THE EFFECT OF BOLTED CONNECTION TIGHTNESS ON NATURAL FREQUENCY AND STIFFNESS OF TRUSS BRIDGES

Abstract

Monitoring and inspection of bridge health are compulsory in maintaining the reliability of the bridge structure. One parameter commonly measured is the natural frequency, which can be determined using a vibration test. The natural frequencies can be post-processed to obtain bridge stiffness and the corresponding deflection. For truss bridges, these parameters depend on joint tightness and load conditions at the time of measurement, which is the aim of this paper. An experimental test on a steel truss model was conducted to investigate the interaction between bolt clamping forces and load conditions on the truss bridge's natural frequency and stiffness. The results showed that the natural frequencies of truss bridges typically decreased as clamping forces were reduced, even in the absence of an external load. However, the added external weight to the truss bridge increased the natural frequency, as in a chain mechanism. The measurement of natural frequencies for truss bridges should account for the bridge's existing load condition. Moreover, natural frequency predictions for bridges with loose bolt connections are recommended to be applied conservatively by reducing theoretical natural frequencies by 30% (based on the typical assumption of a tight bolt connection).