After Thursday's Burlington Skyway crash, how do you determine the structural integrity of a bridge?

Sunday, August 3, 2014, 2:37 PM - According to the latest reports, the Burlington Skyway is closed for at least the long weekend after Thursday afternoon's accident caused part of the bridge support structure to collapse, with no word yet on when it will reopen. After all that, how do you test something that complex to be sure it's safe?

From the pictures posted to social media, by people on the scene, police and news agencies flying overhead, the damage to the Skyway is pretty serious.

Pic of Burlington #Skyway incident with Dump truck. That truck driver better have his license revoked. pic.twitter.com/PG9NBrpzwp

— Nate Vandermeer (@vandernate) July 31, 2014

Going away for the long weekend? Here are the details on the Burlington Skyway Bridge CLOSURE: https://t.co/CCq5Tso4fi pic.twitter.com/cHOy5Lmgez

— Leeanna Mclean (@leeannamclean) August 1, 2014

Even after they clear away the dump truck that caused the problem and the crushed tractor trailer that fell victim to the incident, engineers from the Ministry of Transportation have quite the job on their hands, especially when they're dealing with a bridge crossed by upwards of 80,000 vehicles every day. The engineers know their stuff, though, and political pressure - as tough as it may be - doesn't enter into it, according to McMaster University civil engineering professor Samir Chidiac.

"When it comes to life safety," he told CBC News, "we don’t mess around."

If there's one benefit to the timing of this accident, it's that the bridge had recently gone though a full inspection, and is currently undergoing maintenance work. With the results of that inspection in hand, it is much easier to go through the damage, to see what structural elements have been affected, and how that affects the overall integrity (not to mention knowing exactly what damage was caused by the accident, as opposed to what may have already been there from before).

RELATED: Dramatic photos of Skyway Bridge Crash

It's difficult to test the strength of materials sometimes, especially when you can't simply apply brute force to them to make sure they'll stand up to punishment. So, how does one go about inspecting a complex through-arch bridge will multiple interconnected support structures, to ensure that it's going to stand up to those tens of thousands of vehicles rumbling along it every day, without causing more damage to it and possibly even bringing it down around you?

NEXT PAGE: The science behind the assessment

According to the Ontario Ministry of Transportation (MTO), visual inspection plays a large part in these efforts, as engineers are trained what to look for - cracked concrete and stressed, fatigued or rusted metal. However, high-tech science comes into play as well.

Ultrasonic testing. Credit: MTO

Ultrasonic testing is very useful for this purpose. An emitter sends out or induces sound waves, which travel through the materials, bounce off of any surfaces they meet and return to the receiver. If the results return more surfaces than there should be, it very likely means that there's a crack inside that particular structural piece.

Magnetic particle inspection is another method that's used for materials like iron and iron alloys, which can even show you exactly where the damage is and how extensive. This is especially useful if the crack is inside a metal beam, where you can't see it. By applying an electric current through the material, so that it induces a magnetic field inside, or applying a magnetic field directly to the material, cracks in the structure can be found simply by exposing the area to small ferromagnetic particles. If everything is fine, the magnetic fields are fully contained inside the material and no particles will stick to the exterior. If there is a crack inside the material, this will deform the magnetic field lines, causing them to 'leak' outside of the material's surface. If the particles are applied to this area, they will stick, and they'll stick exactly where the crack is, revealing its exact location to the inspector.

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A new method that engineers are looking at recently to read a bridge's structural integrity uses something far more common - rain.

Researchers at Brigham Young University, in , have been investigating a common type of non-destructive test, called impact-echo testing, but using water droplets as their impactors.

"There is a difference between water hitting intact structures and water hitting flawed structures," Prof. Brian Mazzeo said in a University press release. "We can detect things you can’t see with a visual inspection; things happening within the bridge itself."

The key is listening to the acoustics of the bridge - the 'singing' or 'groaning' it does as the raindrops strike the surface. With so many drops hitting during even a short rain shower, it could give a very good periodic look at the structural integrity of a bridge. This would not only tell them where there are weaknesses in the structure, but also how big they are. This method is focused on a structural weakening known as delamination, where layers of concrete separate away from another, weakening the overall structure. However, if it is refined, it could be applied to look for other kinds of damage, and even to other tests beyond bridges.

"We would love to be able to drive over a bridge at 25 or 30 mph, spray it with water while we’re driving and be able to detect all the structural flaws on the bridge," Mazzeo said. "We think there is a huge opportunity, but we need to keep improving on the physics."

As for the Skyway, the latest word is that inspections are not yet complete, and the Toronto-bound side of the bridge will remain closed at least through the long weekend as they clean up the mess and continue assessing the damage.