TWI was involved as technical experts for the defence in a lawsuit following the failure in 1984 of the Valleyfield lift bridge on the Saint Lawrence Seaway in Canada. United Dominion Industries, TWI's client, and Canron Inc. who designed and built the failed lifting mechanism respectively, were sued by the Saint Lawrence Seaway Authority (SLSA) in a case that concluded in 1993. If the Saint Lawrence Seaway is a large navigable channel between the Saint Lawrence river and the Great Lakes. It was completed in 1959 and the Valleyfield bridge numbers one of many complex structures along its length. The bridge carries road and rail over the Seaway and can be raised vertically to allow shipping to pass.
The lift bridge operates on a counterweight basis. Ropes pass from each of the four corners of the 1250 ton lifting span over larger pulleys (sheaves) of about 5m diameter to counterweights, each weighing 1/2 of the weight of the span. The sheaves have 500mm diameter forged axles (trunnions) which rotate on greased bearings, the grease being distributed by means of grooves cut both in the bearings and on the trunnions. Grooves in the latter are not normally recommended but were provided at the specific request of SLSA.
Valleyfield lift bridge, Canada.
In 1962 (three years after the bridge opened) problems occurred with the bearings when grease fell in abundance on to the road and railway below. To solve this, SLSA decided to block the grooves on the trunnions with silver solder (babbitt metal). They realised that this would not adhere to the surface of the grooves so to provide a key, " diameter holes were drilled at 45◦ every 3 inches along the grooves.
In November 1984 the bridge was in mid course when one of the counterweight sheave trunnions broke free and fell down onto the platform. The road, rail and seaway were rendered unusable and, consequently, ships which were waiting to pass through the seaway became stranded in the ice for several months.
Composite photograph showing SE and NE towers. The broken shaft and sheave have not yet been taken down.
Investigation revealed that the trunnions had failed by fatigue, cracks having initiated from the severe stress raisers caused by the three" diameter anchor holes nearest to the shoulder of the trunnion and thus subjected to the highest bending stress which alternated with every revolution of the sheave. Furthermore, beach marks on the fracture surfaces could be related to the shipping seasons (the seaway is closed every winter during the freeze up). These showed that cracks had been growing ever since the modifications were made in 1962.
The judge concluded that the defendants were not responsible for any of the modifications which led to the failure, these having been carried out under the supervision of the SLSA. Better maintenance procedures could have averted the disaster as the cracks could easily have been detected during in-service inspections.
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