nipigon river bridge (update)
January 11, 2016 § 1 Comment
Oh, here’s a doozy…The Trans-Canada highway has been closed this morning because of a bridge failure over the Nipigon River in Ontario. Not, as you can see, the most spectacular failure, but pretty tough to get the family sedan over that rather abrupt change in level.
Two SCI-TECH principles come immediately to mind.
First, it’s pretty clear what happened. Although no one’s saying anything yet, you can see that the failure happened at an expansion joint (note the ‘teeth’ drooping from the edge of the deck). The only thing authorities have mentioned so far is that they’re blaming “extreme cold conditions” for the failure. Concrete’s coefficient of thermal expansion is a pretty benign 14-ish 10-6/°C, about the same as steel, which is one of the reasons we use steel and not, say, aluminum for reinforcement. But it got down to -24°C in Thunder Bay last night, or a good 45°C colder than room temperature. The bridge is listed as spanning 252 meters. Assuming that’s measured at 21°C, at its coldest last night the bridge would have shrunk by a solid 15.8 centimeters from its design length (14 cm/cm x 10-6/°C x 252m x 45°C), or about 6 inches. Thermal expansion (or, in this case, contraction) is a powerful force, and it’s easy to imagine that amount of pull fracturing whatever pins were holding that joint together. You can see, too, the results of diagonal tension members in a cable-stayed bridge–the deck is not only pulled up, it’s also pulled back since the deck suddenly became a very skinny horizontal column. It’s buckling, but very gently.
Second point: Redundancy and resilience. The Trans-Canada is apparently the only automotive route between the eastern and western halves of the country. So today, if you want to drive from Toronto to Vancouver, you’re going to be stuck going around the southern edge of the Great Lakes. The Nipigon River Bridge is being replaced by a double-span, which would provide the kind of backup you’d expect in a system so reliant on a single node. But, of course, that backup would have been subject to the same forces of thermal contraction, and if both spans had been designed to the same standard, they could both have ended up looking like this.
Anyway, to any neighbors to the north enjoying a leisurely drive through the Midwest today, wave and smile, and enjoy the extended tour of the U.S., brought to you in this case by the laws of physics…
Update, 12 January 2016:
“A multimillion-dollar bridge on the Trans-Canada Highway, the sole east-west route across part of northern Ontario, has partially reopened after sustaining serious damage over the weekend, provincial officials said Monday.
“The Ontario Provincial Police and the Ministry of Transportation confirmed that one lane of the Nipigon River Bridge has reopened.
“A statement from Transportation Minister Steven Del Duca said the lane is available to cars and regular-weight transport trucks, but that engineers are still working to determine whether it can sustain the weight of oversized trucks.”