Mianus River Bridge

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The Mianus River Bridge on Interstate 95 in the Cos Cob section of Greenwich, Connecticut is famous for the collapse of a 100-foot deck of its eastbound span on June 28, 1983. Three people were killed when their vehicles fell with the bridge into the Mianus River 70 feet below, and three were seriously injured.

The collapse was caused by the failure of two pin-and-hanger assemblies that held the deck in place on the outer side of the bridge. The hanger on the inside part of the expansion joint at the southeast corner was forced from the pin that was holding it, and the load was shifted to the only other pin in the joint. The problem was caused by rust formation within the bearing on the pin, exerting a tremendous force on the hanger. The extra load on the remaining pin started a fatigue crack at a sharp corner on the pin. It failed catastrophically and the deck was then supported at just three corners.

When two heavy trucks and a car entered the section, the remaining expansion joint failed and the deck crashed into the river below. The ensuing investigation cited corrosion from water buildup due to inadequate drainage as a cause. The highway drains had been deliberately blocked during roadmending some 10 years before, and water leaked down through the pin bearings, causing them to rust. The outer bearings were safety-critical and non-redundant, a design flaw of this particular type of structure. The bearings were difficult to inspect close-up, although traces of rust could be seen near the affected bearings.

The incident was also blamed on inadequate inspection resources in the state of Connecticut. At the time of the disaster, the state had just 12 engineers, working in pairs, assigned to inspect 3,425 bridges.

The interstate was not fully reopened for six months, and then only with a temporary truss. In total, final repairs cost over $20 million USD.

The replacement span is officially named the Michael J. Morano Bridge.