Following the near collapse of an Interstate bridge in the City of St. Louis, Missouri, a procedure using ultrasound has been developed to check the integrity of the pin in a pin and hanger strap connection. Ultrasound is transmitted into the ends of pins and reflections from defects are displayed on the scope of the ultrasound instrument indicating the presence and location of cracks or wear. Using this procedure, 90 bridges containing 675 pin and hanger strap connections have been checked. Four percent of the pins have been found to be defective. The pins from the Interstate bridge in St. Louis were not removed intact. However, the predictions obtained with the ultrasound procedure correlated closely with the information obtained from the pins and pin pieces after they had been removed by burning. A pin pusher has since been developed, built, and used to remove pins intact. Eight pins were removed intact from one bridge and good correlation between predictions and actual conditions was found. Of the eight pins, it was predicted that one would be completely severed, one would be cracked, and six would be sound. After removal of the pins, the predictions were verified as accurate by visual examination. A limitation of the procedure is the inability to precisely predict the defect size. However, the threshold depth where a crack can be identified has been determined to be approximately 1/8 in.
Abstract