Why did Florida International University's new bridge crumble onto traffic on SW Eighth Street, killing six people last week? That's a question investigators will spend months trying to answer. But one enterprising and extremely entertaining YouTuber might have already solved the mystery.

In a 16-minute clip that's been viewed nearly a half-million times since he uploaded it Friday, a foulmouthed, strongly accented Canadian who posts under the handle "AvE" says that by combing through video, photos, and original schematics for the bridge, he's found the "smoking gun" that led to the disaster: steel tension rods that popped out of place after last-minute onsite changes to the engineering plans.

"This is what is happening around North America in technical firms... discussing about what the fuck went wrong and how we can prevent it," says the man, who dedicates his video to fellow "engin-nerds and brainiacs" around the world trying to solve the mystery from their laptops.

The key to his theory: Post-collapse photos that show a long, slender metal rod perfectly intact protruding from a metal tube just above the carnage of crumbled concrete and mangled cars.

Yes, the video is 16 minutes long, but if you're trying to wrap your head around how a new bridge could fail so quickly, it's a hell of a convincing argument. Plus, the host, whoever he is — a question engineering enthusiasts online have tried to answer before — has an excellent accent that includes pronouncing "video" as vi-jay-o.

Here's the gist: To help stabilize the bridge, engineers planned to have transporters connected by steel plates below each of the trusses. But FIU's engineers evidently had to change that plan onsite because of sloping ground next to SW Eighth Street and a traffic divider in the way. So one transport was shifted toward the center of the bridge.

AvE even dug up footage of an engineer from Munilla Construction Management, the main contractor, discussing those plans a few days before the collapse. (It's at the eight-minute mark in the video.)

"We can see here the difference between what was planned and what was enacted," AvE says, pointing to the original plans and videos from the site showing the actual locations of the transporters.

To help support the bridge, the engineers included "post-tensioning rods" in tubes in the trusses between the concrete layers. Each rod is meant to hold a certain amount of tension and can be tightened to meet the plan's specs.

But AvE's theory is that by changing where the transports were holding up the bridge, the rods' tension was also changed. When the engineers noticed this, he says, they likely tried to yank the tension tighter on one of the rods to compensate.

AvE then vividly demonstrates the dangers of this technique in his home lab using a hydraulic cylinder, which he uses to place increasing tension on a smaller rod. "This thing'll kill you right dead, quick fast in a hurry if'n you're not careful," he says as he loads up the experiment. "Really don't want to get killed by this thing."

Sure enough, as he films, the rod bears an increasing load of tension until, suddenly, it rockets out of the cylinder like a bullet.

AvE believes that's exactly what happened on FIU's bridge last Thursday. Investigators from the National Transportation Safety Board have already confirmed the tensioning rods were being tightened onsite before the collapse.

"Someone sees one of these PT rods doesn't have enough strain, so they start tightening it... but it's weakened," AvE says. "All the sudden it fails, this shatters, and this whole member collapses."

His proof: that original photo he highlighted. He says it shows a tension rod sticking straight out of its tube — the same way his test rod rocketed out of his cylinder.

"The rod is dangling midair with the cylinder still attached," he says. "If the concrete truss section had broken first, it would have mangled up that steel something fierce."

The crews knew there was a problem, he says; otherwise, they wouldn't have been trying to add more tension to a rod. And FDOT Friday said engineers had spotted cracking in the bridge that suggested a problem with how the weight was being distributed.

"There was a crane there at a time," he says. "Maybe it was there to lift up the span. They knew there was a problem and they were trying to remediate it — while the fucking traffic is still going, which is a terrible... just incredible... I can't even."

With time and in-depth analysis and more evidence, his theory might be disproven. But in light of what we know today, it's a compelling argument that's been echoed in other engineering-focused corners of the web, such as the engineering subreddit.

Once the technical culprit is nailed down, investigators will have to tackle even thornier questions: Who knew there was a problem before the collapse, and who allowed traffic to continue driving underneath?

