Building a bridge is no easy task. There are structural issues to contend with and, especially in Canada, weather variability that engineers have to take into consideration.
And weather may have been a factor when part of the newly opened Nipigon Bridge separated from the structure by 60 cm on Sunday night.
The bridge, which cost an estimated $106 million, is part of the TransCanada Highway and links eastern and western Canada.
While details as to exactly what caused the bridge to suffer such a major mechanical failure are yet to emerge, some are blaming the cold weather.
After days of above-normal daytime highs, Ontario experienced a deep freeze. In Nipigon, the temperature went from -6.3 C on Saturday to -15.7 on Sunday. As well, the area experienced high wind gusts as the system bringing the cold weather rolled in.
But bridges — especially in Canada — are built to withstand wind and cold temperatures.
“In this case, it would have been designed to withstand wind,” Mark Green, professor and associate head in the Department of Civil Engineering at Queen’s University, said.
Bridges collapsing due to wind isn’t anything new. Perhaps the most well-known failure occurred in 1940, when the Tacoma Narrows Bridge began to sway wildly in 56 km/h wind, finally breaking apart and collapsing into the Tacoma Narrows strait of Puget Sound.
WATCH: Tacoma Narrows Bridge collapses in wind
“Wind can get underneath the structure and provide uplifting forces, as happened in Tacoma,” Green said. “It causes problems with dynamics, etc.”
When it comes to the cold weather, bridges — typically comprised of steel cable and concrete — react just like anything else. The steel and concrete expand in warm weather and contract in cold.
The cables of a cable-stayed bridge like the one in Nipigon are fasted from a main tower to the ends with bolts. In the cold, the steel in the cables would contract causing the bridge to want to uplift. But that’s what the bolts at the ends are for: to counter that.
Though it’s unknown what happened, Green said that it could have been a combination of factors that made the perfect storm: cold weather could have compromised the bolts and the wind could have caused uplift. If the cables had iced, that could have compromised the bridge as well.
But all these factors are always taken into consideration by engineers when designing a bridge.
“I suspect something like this was a confluence of a couple of things that weren’t expected to happen.”
“We have the engineering knowledge to design them for those circumstances,” Green said.
“It shouldn’t really, the only thing that I can think that could be a problem, is if the cables iced,” he said. “But it’s generally something that is known. Something out of the ordinary must have happened.”
“Our engineers are on-site and continue to investigate,” Ontario’s Ministry of Transportation spokeperson Bob Nichols said. “We’re still working on it.”
Green said that once the investigation determines the cause, the bridge will be safe, if not safer.
“The engineers will be able to go back and say this is what went wrong; this is what was missed, and then they go back to rectify the problem.
“There’s a lot of expertise there.”
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