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Accident Report Detail

 
Collapse of a Suspended Span of Inter- state Route 95 Highway Bridge over the Mianus River

Executive Summary

‚ÄčAt 1:30 a.m., e.d.t., on June 28, 1983, a 100-foot-long suspended span between piers 20 and 21 of the eastbound traffic lanes of the Interstate Route 95 highway bridge over the Mianus River in Greenwich, Connecticut, collapsed and fell 70 feet into the river below. Two tractor-semitrailers and two automobiles plunged Into the void In the bridge and were destroyed by impact from the fall, Three vehicle occupants died, and three received serious injuries.

The suspended span which collapsed was attached to the bridge structure at, each of its four corners. To support the weight of the northeast and southeast corners of the suspended span, each corner was attached to the girders of the cantilever arm of an adjacent anchor span by a pin and hanger assembly. The pin and hanger assembly includes an upper pin attached through the 2 1/2-inch-thick web of the girder of the cantilever arm and a lower pin attached through the 2 1/2-inch-thick web of the girder of the suspended span. One and one half-inch-thick steel hangers connect the upper and lower pins-one on the inside and one on the outside of the web.

Sometime before the collapse of the suspended span, the Inside hanger in the southeast corner of the span came off of the inside end of the lower pin. This action shifted the entire weight of the southeast comer of the span onto the outside hanger. The outside hanger gradually worked Its way farther outward on the pin, and over a period of time, a fatigue crack developed in the top outside end of the upper pin. The shoulder of the pin fractured off, the pin and hanger assembly failed, and the span collapsed into the river.


Probable Cause

‚ÄčThe National Transportation Safety Board determines that the probable cause of the collapse of the Mianus River bridge span was the undetected lateral displacement of the hangers of the pin and hanger suspension assembly in the southeast corner of the span by corrosion-induced forces due to deficiencies in the State of Connecticut's bridge safety inspection and bridge maintenance program.


Accident Location: Greenwich , CT    
Accident Date: 6/28/1983
Accident ID: DCA83AH003

Date Adopted: 7/19/1984
NTSB Number: HAR-84-03
NTIS Number: PB84-916203