字幕表 動画を再生する 英語字幕をプリント 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.