Dynamic Response and Fatigue of Steel Tied-Arch Bridge

Abstract

The Toutle River Bridge is a steel tied-arch bridge, one that vibrates extensively and has sustained significant fatigue cracking. An experimental study into the cause of this behavior is described. Computer analyses of the bridge behavior are used to estimate the expected response and to establish appropriate locations for instrumentation. The instruments were installed and field tests were performed. Controlled tests were performed with trucks of known axle weight and spacing. Some controlled tests were performed with trucks traveling at known speed and in a specific driving lane with no other traffic on the bridge. Controlled tests were used to calibrate the instrumentation and establish the basic bridge behavior. The results showed that composite action had been lost in the heavily loaded stringers, and little amplification of dynamic response was noted. The measured periods of vibration generally compared well with computer predictions. Uncontrolled truck traffic was then measured for approximately one month. This data was used to establish load spectra and to estimate the fatigue life of critical components. Fatigue, which is caused by calculated stress ranges, should not be important on this bridge for another 20 to 30 years. Existing fatigue damage is driven by distortional fatigue caused by the large bridge deformations. Several options for dealing with the problem are presented.