A high-ranking official with the Ontario Ministry of Transportation says the bolts holding together a section of the new Nipigon Bridge snapped off, causing a portion of the bridge to rise about 60 centimetres.

Gerry Chaput, the ministry's assistant deputy for provincial highways, told CBC News the broken bolts will be subjected to special testing.

"We'll take the bolts that broke, we'll take them into a laboratory. We'll check the grade of the steel," he said.

"We'll try breaking a few of them ourselves to see what load they break under. We'll look at the structure from a computer-modelling perspective to understand what the loads were to determine what might have played a role in causing them to break."

Nipigon River Bridge fails in cold

A 'mechanical failure' of the Nipigon River Bridge forced the closure of the Trans-Canada Highway, until one lane was reopened on Monday. (Provided/Ashley Littlefield)

Chaput said similar bolts are holding together another section of the bridge — and all the bolts were fully inspected before the bridge was opened.

Workers are currently stationed beneath the bridge to watch for any movement, he noted.

Mark Green

Mark Green is professor of structural engineering at Queen's University. He says the way the bridge broke is very unusual. (Supplied by Mark Green)

'Unusual situation'

A professor of structural engineering at Queen's University in Kingston said the way the bridge broke is unusual.

If cold was a factor, Mark Green said, it's a reminder of the extra challenges that come with building structures for northern climates.

"We always do have to bear in mind — whenever constructing in such regions — to really make certain that we think doubly about what the effects of the cold climate are, and not to underestimate it."

Nipigon River Bridge repairs

An expert on bridge structures from Queen's University in Kingston, Ont., says, while it's unclear what caused the failure, cold certainly could have been a factor. (CBC)

Sometimes, at low temperature, bolts and steel can become more brittle. It's also possible for cables to shorten in colder temperatures.

That doesn't mean cable-stay bridges aren't suitable for the North, he said, adding the reason we don't have more cable-stay bridges in Ontario is because they are ideal for longer crossings, and the province doesn't have many of those longer crossings.

While one does have to be mindful of thermal effects on cable bridges, with proper design there should be no problem, Green noted. He said he was confident the problem would be fixed and the overall structure of the bridge would be fine.

Nevertheless, news of the bridge's failure was shocking, he said.

"It's a really unusual situation … I've never heard of this type of situation."

Currently, one lane is available to cars and regular-weight transport trucks, and engineers were still working to determine whether the Nipigon River Bridge can sustain the weight of oversized trucks.

Map of Nipigon River Bridge

The Nipigon River Bridge, about 100 kilometres northeast of Thunder Bay, is the only route through northern Ontario to the eastern and western parts of the country. (Google Maps)

Detour details being kept secret

Roughly 1,300 trucks cross over that stretch of the Trans-Canada Highway each day, carrying $100 million worth of food, mail, machinery and other goods, according to 2012 figures provided by the Canadian Trucking Alliance. 

"Every truck that drives across Canada, will go across that bridge," Nipigon Mayor Richard Harvey said Monday. "The country has been cut in half … and from that perspective, it's an issue of national importance."

There are rural logging roads in the area, about 100 kilometres northeast of Thunder Bay, Ont., but those don't have the infrastructure to support most commercial or passenger vehicles. There's no alternative to the bridge, other than taking a detour through the United States.

Nipigon River Bridge reopens1:28

Police initially shut the bridge to traffic Sunday evening after the crossing's west side began to pull away from its abutment, lifting the deck by about 60 centimetres.

The $106-million expansion of the Nipigon Bridge was slated to finish in 2017. It was also touted as making the country's shipping route less vulnerable, the community's mayor said, as it would boost the two-lane crossing to four lanes.

Harvey said the town has three different emergency management plans, depending on the season, to redirect passenger and commercial traffic in cases when the bridge is blocked or broken.

One of those possibilities includes reinforcing some of the logging roads so they could support traffic, but Harvey said the exact details are being kept secret.

"The last thing we want right now is people to start trying to go around that bridge."