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All across the world people need to get from one place to another. And sometimes to get to that other place we need bridges. Here are eight bridges that are extraordinary in their own way, from standing the test of time to handling hundreds of millions of people crossing every year.

Hosted by: Michael Aranda
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Images: https://www.flickr.com/photos/dickdotcom/2724142781
https://www.flickr.com/photos/nordique/4346766797
Michael: So you want to get from point A to point B, but there’s a bunch of water in the way. What do you do? Well, you could always just build a bridge.   Humans have been building bridges for thousands of years, and we’ve come up with a lot of different designs for them, depending on what we’re trying to cross. Building a bridge across a low-lying lake is a little different than stringing one between two mountaintops. Someone has to design and build these bridges, and it can take a lot of clever engineering to get from point A to point B. Whether it’s the cost, the extreme height, or the sheer distance that needs to be crossed, here are 8 bridges that people had to get pretty creative to build.   [SciShow intro plays]   The highest bridge in the world, as in, the one with the longest drop to the ground below, is the Sidu River Bridge in China. It’s a suspension bridge that connects two mountain peaks, and between them is an absolutely stupefying drop of almost 500 meters.   Even though the bridge is pretty remote, it does save an awful lot of time compared to having to climb two mountains and cross a canyon. The location turned out to be a challenge: the mountains are super steep and full of thick forests. Plus, it’s windy! So you can’t even use helicopters! So… they decided to build a bridge with rockets, instead.   See, suspension bridges are usually built with towers for support. They also have cables that run all the way across the bridge, connecting the ends to the towers, and the towers to each other. And once you’ve built the towers, you have to string the cables between them somehow. Starting with the pilot lines.   But how do you hang a cable between two towers five hundred meters up in the air? The engineers turned to the army for help, and the army gave them some rockets. They launched the rockets, carrying the pilot cables, from one mountain to the other. The rockets crossed the 1100 meter gap and landed within 15 meters of their targets, and the engineers had the beginnings of a bridge. As incredibly drastic solutions go, that’s not bad.   The Millau Viaduct in France is the tallest bridge in the world from the top to the bottom. Its tallest mast checks in at 343 meters. That’s taller than the Eiffel tower. It crosses a deep gorge carved by the River Tarn, and instead of hanging from the ends of the gorge as a suspension bridge, it was designed as a cable-stayed bridge. That’s where the bridge has support pillars that go all the way down, with cables running between the pillars and the deck of the bridge.   The architect, Norman Foster, tried to make the bridge actually look nice. But it also had to cross a distance of 2500 meters, and relieve the heavy traffic choking the area. The result is strangely delicate-looking. It’s held up by cables suspended from seven slender masts that look almost too graceful to hold the thing up.   The design of the bridge was meant to minimize the amount of material they needed, and therefore the cost. But less material meant the bridge needed to be designed very carefully. The columns are just about as narrow as they can possibly be while still being strong enough to hold up the bridge, which weighs about 36,000 metric tons, by the way. Meanwhile, each supporting column splits in two to handle any sliding around due to earthquakes or temperature changes. And it works!   The bridge across Louisiana’s Lake Pontchartrain, adjacent to New Orleans, wasn’t designed with aesthetics in mind. It’s a trestle bridge, meaning that it has short spans with supports underneath. Basically, it’s a bunch of concrete pilings. But it’s also very, very long. Nearly 38 kilometers.   In the mid-1950s, when it was opened, it was a pretty impressive engineering accomplishment. It was precast, meaning it was manufactured in modular, basically-identical pieces, which, for the time, was pretty unusual. The bridge even had its own personal manufacturing plant. Individual segments of the bridge were also pre-stressed. Pre-stressed concrete contains stretched-out steel beams or cables, which makes the concrete more resistant to bending or buckling. Stronger concrete means you can use less of it to cover more distance, which is pretty convenient for building bridges.   The precast concrete parts were moved out over the lake, and all the construction workers had to do was slot them together. The parts were easy to fit together without spending too much time in the lake. So all that concrete may have been kinda unglamorous, but it was useful.   Originally, the bridge was only two lanes across that entire distance. But it turned out that a lot of people needed to cross Lake Pontchartrain. So a second, nearly identical span opened in 1969. It split up the north and southbound directions so there was no longer any chance of head-on collisions. It also meant people could finally pass each other. Imagine being stuck behind someone going 10 kilometers below the speed limit for 38 kilometers.   Some bridges were built to last. The oldest bridge in the world that people still use is the Caravan Bridge in Turkey. It’s not all that impressive, compared to some of the other bridges we’ve talked about so far: it’s a stone slab about 13 meters long, supported by a single arch across the river Meles. But that simplicity is probably the reason it’s been around so long.   The Caravan Bridge dates back to about 850 BCE, making it nearly three thousand years old. Arches are pretty cool. They work by distributing the weight pressing down in the center out to the supports at the ends, called abutments. That can be really convenient when you’re trying to build a bridge over a deep river where you can’t put down any supports.   The arch just sends all the weight of the bridge to the sides. But there is an upper limit to what arches made of stone can do. The bigger the semicircle, the more tension it will experience, and if it’s too big it’ll collapse. Arches made of other materials can be stronger, but 3000 years ago, they didn’t have too many options. And in the case of the Caravan Bridge, the stone arch made a perfect support.   It didn’t seem like it was going to be all that hard, at first. All they had to do was build a second bridge alongside the existing one to ease congestion, easy peasy. But the Benicia Bridge in the Bay Area of California came in years late and more than four times over budget. They thought it would cost $300 million, and ended up spending nearly $1.3 billion. How could one bridge be so expensive?   Well, they forgot about the fish. They were using pile drivers to dig underwater for the bridge’s supports, and the noise was killing fish. So the engineers had to invent a completely new technology to muffle the sound under water. Their solution? A curtain of air bubbles that smothered the noise and saved the fish.   Air and water have very different densities, which makes it hard for sound to travel from water to air to water again. So they set up some perforated rings, and pumped air through them, which surrounded the column and blocked the noise.   Now, this is going to come as an absolute shock to any of you who live in New Jersey or New York City, but... The George Washington Bridge, which connects Fort Lee, New Jersey with Manhattan, is the busiest bridge in the world. The bridge has a total of fourteen lanes, a record for a suspension bridge. Except they’re not all side by side. The GWB is a double-decker, with eight lanes above and six below. It’s had to be renovated and expanded multiple times to deal with the ridiculous demand.   The bridge opened in 1931 with six lanes and one deck, and was the longest bridge in the world for a few years, until the Golden Gate Bridge opened in 1937. In 1946, they added two more lanes. Then the lower level was added in 1962. And this 85 year old bridge sees more than one hundred million vehicles every year. That’s pretty impressive for a bridge that’s over 1000 meters long. All that traffic creates a lot of wear and tear. The bridge isn’t in any danger of collapsing from all the traffic, but it sure does take maintenance.   The longest bridge in the world that you’re allowed to drive a car on is the Bang Na Expressway in Thailand. It’s not a water crossing, exactly, although some water does happen to go under it. It’s basically a freeway built up in the air, because why go around stuff when you can go over it?   And this bridge goes over a lot of stuff: it soars through the air for 55 kilometers. Like with the Lake Pontchartrain causeway, engineers decided precast and pre-stressed was the way to go here. The whole thing is six lanes, but widens to twelve at the toll plazas. You didn’t think you were going to cross this thing for free, did you? Someone’s got to pay for those 1.8 million cubic meters of concrete.   But there is a bridge that’s longer than the Bang Na. It’s just not for cars. It’s for trains. Specifically, it’s the Danyang-Kunshan Grand Bridge, which is part of China’s high speed rail network. And the world’s second longest bridge is on the very same line. At three times the length of the Bang Na, the Grand Bridge stretches for 165 kilometers, crossing numerous bodies of water.   Building such an enormous bridge was going to take a massive effort, so the builders went with a brute force approach: Ten thousand people built the whole thing in four years. Like the other long bridges we’ve talked about, it uses pre-stressed concrete, which makes sense, because if you’re going to build the longest bridge in the world, you’re probably going to want to use as little building material as possible.   So it takes a lot to build a bridge, a lot of people, a lot of money, and often a lot of concrete. But with new building techniques and some creative thinking, record-breaking bridges are being built all the time.   Thanks for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show, just go to Patreon.com/SciShow. And don’t forget to go to YouTube.com/SciShow and subscribe!