Reverse engineer the mechanical failure just from grainy video

[FatPhil]

Source: https://www.youtube.com/watch?v=hlk_GcRI6LM


Let me fabricate the quasibunk "the bridge collapsed *because* they cut the ribbon".
Come on boffins - debunk me!

 

[Mendel]

Let me fabricate the quasibunk "the bridge collapsed *because* they cut the ribbon".

why else would it collapse? someone below dislodging a support?

I'd handwave this away with vibratory mode not anticipated by the builders. triggered by the sharp impulse from releasing the tension on the handrails—sort of like the amateur version of the Tacoma Narrows bridge failure.

 

[FatPhil]

why else would it collapse? someone below dislodging a support?

I'd handwave this away with vibratory mode not anticipated by the builders. triggered by the sharp impulse from releasing the tension on the handrails—sort of like the amateur version of the Tacoma Narrows bridge failure.

I think it's even simpler that that. I think that the rails were providing almost all of the stiffness against simple flexion of the surface of the bridge under the load of that many people. However, nothing was keeping the rails perpendicular to the bridge's surface apart from the ribbon, and without that restorative force, they buckled immediately and catastrophically.

I recently built a tower out of spaghetti and masking tape that could support a cup of water at the top as a challenge, and the rigidity of the whole thing ended up being dependent on just the tiniest of fixups on the bottom layer. One single snapped strand of spaghetto, and 200 - soon to be 400 - other bits of spaghetti and half a litre of water would have come crashing down. This bridge kinda reminded me of that tower (and which still stands in my lounge, I'm kinda proud of it).

 

[Mendel]

However, nothing was keeping the rails perpendicular to the bridge's surface apart from the ribbon

they're still perpendicular on the other side?

near side at 0:05

The deck body seems deep enough to resist bending (and the handrail clearly doesn't), my guess is that it consists of two sections that were inexpertly joined. Some stress then overloaded that joint, and the sudden jerk from the ribbon-cutting, amplified by a bridge loaded at one end but otherwise free to oscillate, may have done it.

 

[Mendel]

It's possible that the holes in the sides of the deck body that accomodate the handrail slots actually weakened the deck and caused the collapse.

 

[econ41]

Laser Beams from space???

 

[jarlrmai]

The ribbon is already broken before the cut/collapse, he is holding one end of it.

The loose end is being held up by the wind.

 

[Ann K]

How about the simplest explanation: it wasn't designed to hold the weight of that many people, but all the dignitaries crowded on to see the opening and/or get their pictures in the paper.

 

[Trailblazer]

View attachment 54546

The ribbon is already broken before the cut/collapse, he is holding one end of it.

The loose end is being held up by the wind.

Good spot. Maybe this was take two!