Why does the Coventry Basin Bridge bounce?

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Why does the Coventry Basin Bridge bounce?

Postby dutchman » Sun Jul 16, 2017 7:14 pm

We try to explain the science behind the swagger

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It’s the Coventry landmark long known for its small - but unnerving - bounce.

Walking across the Canal Basin footbridge over the ring road has been likened to "being drunk when you're sober".

Here we try to to explain the science behind that infamous sway.

We think bridges should be solid, rigid, structures that shouldn’t move.

But bridges are designed to move, which helps prevent snapping and breaking, much like trees swaying in the wind.

All bridges are made of beams, supported by bearings, which bounce and move.

Engineers design them this way because it's safer for bridges to flex and bend rather than fracture and shatter.

Although bridges and buildings appear to be solid, they all have a natural frequency, meaning the number of times they move back and forth per second.

A force applied at the same frequency, such as footsteps across a bridge, traffic moving underneath or wind blowing across it, will amplify the vibration, in a process called mechanical resonance.

It was this process that caused the newly-opened Millennium Bridge in London to famously wobble in 2000.

And if the frequency is amplified enough, in extremely rare occasions, it can cause the bridge to collapse, such as the Tacoma Narrows Bridge in 1940 and the Broughton Suspension Bridge in 1831.

What do pedestrians think?

Not one to worry about the odd bounce is a 49-year-old, going by the nickname Mac, who said: “It’s got to give way somewhere.

“It bounces all the time since I’ve been using it, which is about seven years.

“It’s especially when you’ve got a good few on it, it’s really fun then. It’s like being drunk when you’re sober.”

Meanwhile Melanie, aged 46, from Fillongley, has childhood memories of the bridge bouncing and compared the structure to a fairground ride.

She said: “I don’t like the sensation, but I don’t think it’s going to fall down, I know it’s safe.

“It’s a bit like being on a ride, so you lose the sensation between where your stomach is and where your feet are.”

A 23-year-old student, from Radford, said: “This is literally the only bridge I’ve been on that bounces. It’s been like this since I’ve known this bridge. It’s bouncing now!

“I don’t know why it bounces, I think it’s unstable. What if something happens? What if it collapses?

“I don’t get frightened, I do have at the back of my head: imagine one day I’m walking and it just drops.”

Another woman said: “The bridge is not firm, that’s why it’s bouncing.”

But the last word goes to this 60-year-old passer-by who said: “It bounces because of expansion. If it was rigid it would crack.

“The bounce is normal, the flexibility’s built into it.

“I’d laugh if it just collapsed now.”

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I don't know any other bridges in Coventry which do it? :roll:
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