Bridge-safety evaluation using ultrasonic stress measurement

Fracture mechanics can be used to evaluate the consequence of having a crack in a bridge structure. To do so requires that the stress state near the crack be known including the contribution of residual and fabrication stresses. In general these must be measured. Stress causes a small but measurable change in the speed of sound in many materials. Hence measurement of velocity in a bridge provides a means to determine all the components of stress. This concept has been demonstrated in laboratory situations by various researchers. Here we report results from field tests on actual bridges. The stress in flange and web regions of two bridges was measured with ultrasonics. In the first bridge we determined the residual stress in the girders. The second bridge was an integral backwall bridge with no expansion joints. It had been instrumented at time of construction. Strain gage readings indicated compressive stresses near yield. Ultrasonic measurements showed the bridge to be safe. Subsequent replacement of suspect electronics in the monitoring instrumentation verified the ultrasonic results to be safe.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.