Silver Bridge Collapsed
West Virginia's Silver Bridge was the first suspension bridge to hang from a linked chain of steel bars instead of standard wire cables. It was nearly impossible for inspectors to reach the steel chain, however. Even if they'd been able to, they didn't have the means to detect the crack that formed within one of the links. On December 15, 1967, that link—and the chain it was an integral part of—snapped, plunging the bridge into the Ohio River and killing 46 commuters. Today, engineers are developing sonar sensors that may be able to detect such cracks long before they cause trouble. With such devices, inspectors can send sound waves into steel and analyze the returning waves for changes in pitch, which can signal hidden cracks and other defects.