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Report Reveals Factors in Bridge ‘Split’

Wednesday, September 28, 2016

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Nearly 10 months after a new cable stay bridge in Ontario, Canada, appeared to split during a spell of cold weather, reports on the cause of the failure have been released.

The investigation found that three main factors led to the malfunction on a tie-down connection of the Nipigon River Bridge, the Ontario Ministry of Transportation announced Thursday (Sept. 22): the design of the shoe plate and its flexibility; a lack of rotation in the bearing that was constructed; and improperly tightened bolts attaching the girder to the shoe plate.

Although a cold snap and wind gusts were originally thought to have contributed to the failure, the office stated: “When combined, these three factors produced the malfunction. Neither cold temperatures nor wind contributed to the closure.”

Broken Bolts

As reported earlier, the Nipigon River Bridge appeared to come apart Jan. 10 as the west side of the bridge pulled away from the abutment, reports said at the time. The separation caused the deck to lift about 60 centimeters (approximately two feet), making it impassable to traffic and essentially cutting off access to southwestern Ontario.

The bridge, a significant link connecting the east and west along the Trans-Canada Highway, had only opened 42 days earlier, according to the Toronto Sun.

Two days after the incident, the Ministry of Transportation shared that bolts holding together a section of the new bridge had snapped off. Similar bolts were said to be in place in another section of the bridge.

However, with the release of the testing and investigation reports, Minister of Transportation Steven Del Duca and Michael Gravelle, Minister of Northern Development and Mines and Member of Provincial Parliament for Thunder Bay-Superior North, near the bridge site, emphasized, “The testing confirmed that the bolts broke due to overloading and not due to any flaw in the bolts themselves.”

Bolt Analysis

Following the closure, two firms were contracted to test the bolts from the bridge: Surface Science Western of London, Ontario, and Ottawa’s National Research Council Canada. Working independently, each lab received 14 broken bolts and 10 intact bolts to test, according to a Ministry of Transportation Fact Sheet.

Both labs concluded that the bolts met all requirements of the required standard (ASTM A490), exhibited good performance under cold temperatures, and broke due to overloading.

Nipigon River Bridge components
Images: Ontario Ministry of Transportation

The engineering reviews found that there three main factors led to the malfunction on the Nipigon River Bridge: shoe plate flexibility, a lack of rotation in the bearing, and improper tightening of the bolts, the Ontario Ministry of Transportation stated.

The reports determined that the damage “occurred progressively over several weeks based on the appearance of corrosion on the broken surfaces of some of the bolts.”

Contributing Factors

At the same time, ministry bridge engineers and Associated Engineering (Ont.), an independent engineering consultant with expertise on cable stay bridges, performed an engineering analysis to determine why the tie-down connection malfunctioned in the northwest corner of the bridge.

These investigations were conducted independently and reached the same conclusion, the related Fact Sheet noted.

Specifically, the analysis called out three items:

  • The shoe plate was too flexible, creating a “prying action” which increased the forces on the two outer rows of bolts. This additional force resulted in the bolt heads/nuts bending, stretching and eventually breaking. 
  • The bearing did not rotate, and the lack of rotation increased the forces being placed on the bolts, causing them to break.
  • In regard to the bolts, proper tightening keeps the forces in the bolts more consistent when the load on the bridge changes.

The bearing design did not measure up to the contract specifications, the Sun reported. Additionally, the shoe plate, bolted connection between shoe plate and girder, bolted connection between shoe plate and bearing, and bearing design did not meet the requirements of the Canadian Highway Bridge Design Code, the paper added.

Investigators noted that other factors were also identified but their impact was likely minimal. Cold temperatures and wind were eliminated as contributing factors.

Moving Forward

A temporary fix was put in place in February so the bridge could reopen to traffic. This consisted of a hold-down system anchored to the girder and bridge foundation.

Now, a permanent retrofit design has been developed to address the issues identified by the engineering reviews and to ensure the bridge “functions safely for road users throughout its intended useful life,” according to the Ministry of Transportation.

Nipigon River Bridge permanent retrofit

The permanent retrofit solution involves a linkage anchoring the girder to the bridge foundation, as well as vertical bars placed at each girder end with pins at top and bottom of the bars, allowing longitudinal movement and rotations but preventing uplift. 

The retrofit, which reportedly has been used on other on cable-stayed bridges, including spans in Quebec and Kentucky, consists of a linkage anchoring the girder to the bridge foundation. The design concept includes vertical bars placed at each girder end with pins at top and bottom of the bars, allowing longitudinal movement and rotations but preventing uplift, according to the ministry.  

This design has been reviewed by Associated Engineering (Ont.) and ministry bridge engineers and will also be applied to the south portion of the bridge, the office added.

Preliminary estimates for the initial repair work performed in February and the final repair to be implemented are reportedly between $8 and $12 million.

According to the Sun, taxpayers may not have to bear the financial burden of the repairs. Del Duca indicated that although the reports spell out why the bridge failed, they don’t assign responsibility. He added that the government has had a preliminary conversation with the contractor.

“I think it’s fair to say that there’s a lot of responsibility to go around for this one,” Del Duca said. “Now we need to go through a process to make sure that the responsibility or the liability for what’s taken place is apportioned in a manner that’s appropriate.”



Tagged categories: Asia Pacific; Bridges; Corrosion; EMEA (Europe, Middle East and Africa); Government; Infrastructure; Latin America; North America; Quality Control; Roads/Highways; Steel

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