Fatigue Damage of a Diagonal Member in a Steel Truss Bridge Due to Wind-Induced Vibration

Abstract

Ikitsuki bridge is a three span continuous steel truss bridge completed in July, 1991. Approximately 18 years after the completion, a crack was found in a diagonal member in the vicinity of an intermediate support. In order to identify its cause, fracture surface of the crack was observed and a long-term monitoring of wind and vibration of some diagonal members has been carried out. The study reveals that (1) the crack was initiated and propagated as fatigue crack; (2) vibration of the target members is induced by the wind of 6–8 m/s and more than 15 m/s in the direction approximately normal to the bridge longitudinal axis; (3) the members mainly vibrate in the plane of truss and the vibration frequencies coincide their 1st natural frequencies; and (4) the maximum stress range induced by the vibration due to 6–8 m/s wind is approximately 30–40 N/mm², while that due to wind more than 15 m/s can be 195 N/mm². These wind-induced vibrations are thought to be the main cause of the fatigue crack.