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Overloaded bolts blamed for bridge bearing failure

Nipigon River Bridge Aerial View

Overloaded bolts were ultimately to blame for the bearing failure on a stunning new Canadian bridge just weeks after it opened, a report has found.

The Nipigon River Bridge is a key east-west transport link, but its deck sections became disjointed in January this year, less than two months after it was opened to traffic.

The bridge was in the process of a £54M reconstruction by a Ferrovial Agroman/Bot Construction joint venture, with the former Hatch Mott MacDonald – which has now been separated into two independent businesses – providing construction contract administration. The new bridge was designed by MMM Group in Canada.

The failure involved the complete fracture of all 40 bolts connecting the tie-down bearing to the main girder bottom flange on the north-west bearing of the bridge.

The technical report by Associated Engineering into the failure said: “Immediately upon the failure of the bolts, the unbalanced weight of the spans, acting through the backstay cables, pulled the north-west end of the bridge deck upwards, coming to rest at approximately 600mm above the road level. The center-west bearing did not fail, which limited further damage to the bridge.”

Ontario minister of transport Steven Del Duca and Thunder Bay-Superior North minister of northern development and mines Michael Gravelle have now made a statement following the report into the reasons for the failure.

“The testing confirmed that the bolts broke due to overloading and not due to any flaw in the bolts themselves,” said the statement.

“The engineering reviews found that three main factors led to the malfunction: first was the design of the shoe plate and its flexibility; second was a lack of rotation in the bearing that was constructed; and third was improperly tightened bolts attaching the girder to the shoe plate. When combined, these three factors produced the malfunction. Neither cold temperatures nor wind contributed to the closure.”

Nipigon tech diagram

Nipigon technical diagram

Source: Ontario Ministry of Transportation

The technical report said that the shoe plate was too flexible, which created a prying action which increased the forces on the two outer rows of bolts. The extra force saw the bolt heads and nuts bending and stretching until they broke.

In addition, the bearing did not rotate, and, because there was not proper tightening, the forces on the bolts were not as consistent as they could have been with the load on the bridge. No problem was found with the actual bolts.

A permanent retrofit design has now been developed, which will cost up to £7M. An emergency detour route is also being developed, should there be any future need.

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