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  • Officially, this suspension bridge over the River Lee, in Cork,

  • in the south of Ireland, is called "Daly's Bridge",

  • after a businessman who helped fund it back in the 1920s.

  • But unofficially, it has a different name: the Shakey Bridge.

  • Because it shakes.

  • I was going to film this with my GoPro on a stick walking across,

  • but it's a really narrow bridge,

  • and I don't want to end up barging into the people who are actually using it.

  • I can confirm from a brief experiment, though...

  • Yep. It's bouncy!

  • - In the early 1700s, there was a ferry boat that actually brought people from here

  • across to the other side.

  • But unfortunately between 1906 and 1908, the Dooley family actually retired,

  • retired themselves from the ferry boat industry.

  • And it was through the philanthropy of a man called James Daly, who gave

  • nearly £700 towards the construction of a bridge project.

  • £700 in today's terms would be around €50,000.

  • And to be honest,

  • you would not be able to build a bridge in today's world for €50,000.

  • So the bridge was good value.

  • David Rowell and Company, in London, Westminster,

  • actually had a bridge catalogue.

  • And the Corporation of Cork bought a bridge off the catalogue,

  • and in early 1927 these huge, giant wooden boxes

  • arrived on the banks of the River Lee.

  • And the bridge actually was constructed.

  • So in the first couple of months the people of the area

  • discovered that the bridge actually shook when they walked over it.

  • - Now, all suspension bridges, actually, pretty much all bridges,

  • will move or shake or sway a little

  • as the dynamic loads on them from people and weather change.

  • And that's a good thing: because if they don't move a little...

  • ...they break.

  • The design of this bridge meant the shake was more obvious.

  • It bounces up and down.

  • And it probably wasn't intentional(!) But clearly it wasn't dangerous, as proven

  • by the fact that the bridge stood for 90 years,

  • taking all the jumping around and abuse

  • that local children, and adults, could throw at it.

  • But nothing lasts forever.

  • In 2017, an inspection looked at the corrosion and damage

  • that had accumulated over the years on the bridge.

  • And, well, it'd been built to last with good materials.

  • But we're near the ocean here, and 90 years of salty air and shaking

  • meant that the metal had corroded, and wires had broken and frayed.

  • The report said, in short: the bridge had about three years

  • before it had to be closed, and repairs were needed immediately.

  • So it was repaired.

  • Not rebuilt: the engineers reused or matched materials as far as possible.

  • They took the deck of the bridge apart, fixed it up off-site, and then put it back together,

  • taking care to keep the height and positioning the same.

  • They replaced the suspension cables with brand new ones,

  • and repaired the towers where they stood.

  • And they gave the bridge a fresh coat of paint.

  • But here's the problem.

  • This bridge is on the local Record of Protected Structures.

  • It must be maintained and repaired, but it also can't be changed.

  • - There was public consultation, and of course many people actually wrote in,

  • going "keep the shake, keep the shake, keep the shake, keep the shake".

  • And in fairness to the engineers who worked on the project,

  • I think they did achieve that.

  • People have had a lot of fun

  • walking across the bridge, admiring the river, jumping on it.

  • Like, this has been ongoing for 90 years.

  • - Before the work started, the engineers used accelerometers

  • to measure how the bridge responded to someone jumping right in the middle.

  • And they found that the bridge bounces up and down at about 2.3 cycles a second.

  • So as part of the design process, they used computer modelling

  • to work out how the new cables would behave.

  • And they made sure to keep the bridge's total mass as close as possible

  • to how it was before.

  • Bridges shake and twist in three dimensions, of course,

  • it's not just up-and-down.

  • But up-and-down is the direction

  • you feel the most when you're standing up there!

  • And all the data showed that, in all three dimensions, the repaired

  • the repaired Shakey Bridge was moving more or less as it did before.

  • There was a slight change.

  • But arguably that just took it

  • closer to how it shook when it was brand new, nearly a century earlier.

  • The bridge now goes up and down at 2.2 cycles a second.

  • It would have been entirely possible, with modern technology and materials,

  • to build an identical looking bridge that was much more stable.

  • Normally, that's what engineers do: they try to damp down vibrations.

  • But in this one case, their job was to keep things the same.

  • Fixing the shake would not have been preservation.

  • The goal here was to repair this bridge so that it can survive,

  • and shake the residents of Cork for another century to come.

Officially, this suspension bridge over the River Lee, in Cork,

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The bridge that must legally wobble

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    林宜悉 に公開 2022 年 03 月 10 日
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