Silver Bridge — One Corroded Eyebar Dropped a Bridge into the Ohio River

At approximately 17:00 on 15 December 1967, in heavy Christmas rush-hour traffic, the Silver Bridge carrying US Route 35 across the Ohio River between Point Pleasant, West Virginia, and Kanauga, Ohio, collapsed in seconds and dropped 37 vehicles toward the water; 46 people died and 9 were injured. The cause was not flood, not overload, not a barge strike. It was a single 2.5 mm (0.1 in) crack in the head of one eyebar — link 330 at the north joint C13N — that had grown silently for 39 years by stress-corrosion cracking and corrosion fatigue until the bar fractured in a brittle cleavage, unzipping a chain that had no second load path to catch it.

The Silver Bridge was an eyebar-chain suspension bridge, in which the main cables were replaced by chains of flat, pin-connected steel links called eyebars. Each link in the Silver Bridge chain was made of only two eyebars side by side, joined to the next by a large pin through the forged eye at each end. The 1928 design — by the J. E. Greiner Company, built by the American Bridge Company — used a high-strength heat-treated steel run at unusually high working stresses to save weight and money, carrying a 700 ft main span and 2,235 ft of total deck on chains barely thicker than they had to be. The economy that made the bridge cheap also made it fragile: a two-bar link has no redundancy, and heat-treated steel run at high stress has little fracture toughness in reserve.

The fatal crack began at the rim of the pin hole in the head of eyebar 330, where fretting against the 11 in pin and water pooling in the joint set up a corrosive environment under sustained tension. Over four decades a crack roughly 2.5 mm deep grew by the joint action of stress-corrosion cracking and corrosion fatigue. On a near-freezing December evening, with the steel at its most brittle and the deck loaded with stalled holiday traffic, the crack reached critical size and the eyebar head cleaved. Its partner bar could not carry the doubled load; the link parted, the chain went slack, and the span fell. The National Transportation Safety Board — in its first major highway bridge investigation — fixed the cause unambiguously, and the failure rewrote how the United States inspects its bridges. No routine inspection of the era could have seen a 2.5 mm crack sealed at a pin-hole rim 100 ft above the river; the bridge was built so that finding it was the only thing that could have saved it.