Comprehensive investigation on the cause of a critical crack found in a diagonal member of a steel truss bridge

Ikitsuki Bridge is a three span continuous truss bridge with a length of 800 m, which was completed in July 1991. In December 2009, a crack which seriously damages the safety of the bridge was found in a diagonal member near an intermediate pier during an inspection for determining points to be annually inspected. In order to identify its cause, some tests on the material used for the cracked member were conducted, and a long-term monitoring of wind and vibration of some diagonal members with similar structural characteristics to the cracked member had been carried out since December 2011. In this article, the outline of the crack is presented and the main causes of the crack are discussed based on the results of the tests and monitoring. The conclusions can be summarized as follows: (i) The crack initiated and propagated to become approximately 200 mm long as a fatigue crack. The quality of material and weld satisfies the requirement for them. (ii) The vibration of the diagonal members is induced by the wind with the velocity of 6-8 m/s and higher than 15 m/s in the direction approximately normal to the bridge longitudinal axis. The maximum stress range induced by the vibration due to 6-8 m/s wind is approximately 30-40 MPa, while that due to the wind blowing at the velocity of over 15 m/s can be 195 MPa. (iii) These wind-induced vibrations are thought to be the main cause of the fatigue crack.