Footage captures what happened to bridge seconds before collapse

When you think of a bridge collapsing you might imagine that it would be something that would happen relatively quickly.

In the case of the Tacoma Narrows Bridge in Washington state, the process took longer than you might expect.

The bridge - which cost $6 million to build, around $1 billion in today's money - lasted for a grand total of four months and seven days.

But the really interesting thing about Tacoma Narrows Bridge is the weird reason for its collapse.

Footage from before the bridge's collapse in 1940 shows it oscillating wildly.

The entire bridge, including steel beams and tarmac, looks like someone furling out a bedsheet.

You might think that it was simply that the wind blew it down, this is not actually correct.

While the wind - which reached 40mph - was a crucial factor, it wasn't the sheer force of the wind which caused the bridge to collapse, but something far more subtle.

In fact, it's not something that you would even imagine you'd have to take into account in building a bridge.

YouTube / Debbink Physics

The bridge collapsed due to a phenomenon called ‘aeroelastic flutter’. Yep, we haven’t a clue either.

In simpler terms, while most bridges have trusses which allow the window to flow through them, the Tacoma Narrows Bridge didn’t.

This ended up causing ‘vortices’, which led to either side of the bridge rocking in a perpendicular direction to the wind.

After one of the steel suspension cables snapped, the bridge started rocking in earnest.

Because the bridge had stiffening plate girders rather than web trusses, there was nothing to absorb and dissipate this rocking motion and it just got worse, eventually destroying the bridge.

YouTube / Debbink Physics

So how might you prevent this from happening in newer bridges?

Well one answer is to use girders which are not solid but instead supporting by a criss-cross pattern. This means the wind passes through gaps rather than the girders taking the full force.

Despite collapsing, the story was far from over, as the reason for the bridge's collapse led to numerous studies in science and engineering.

The bridge ended up being reconstructed in 1950, with a parallel eastbound bridge opened in 2007 to deal with growing congestion.

Those bridges - you'll be glad to hear - are currently still standing.

However, not all engineers learned their lesson from the Tacoma Narrows Bridge, with London's famously wobbly Millennium Bridge also having problems with 'flutter'. Oh dear.