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The Rio-Antirrio Bridge: The Most Challenging Bridge Ever Built

Before we get started, let’s play a little game. Imagine that you, the viewer, are an engineer, and your job is to build a bridge over some water. Now, imagine a few ways to make building that bridge more difficult. What if the water you want to build over is incredibly deep? Done. How about having no solid ground at the bottom? Okay. What if the site is an active earthquake zone, and plate tectonics is actively making the huge gap you want to bridge even wider, every year? Sure. What if the site acts as a natural wind tunnel, with winds reaching speeds of up to 70mph? At this point, why not.

As you can guess, this place is not in your imagination – it is very much real, as is the bridge that had to overcome all of those challenges, in what can only be described as an engineering masterpiece. This is the story of the Rio-Antirrio Bridge, the most challenging bridge ever built.

Over Troubled Waters

Rio-Antirio bridge with over troubled waters
Rio-Antirio bridge by Eusebius is licensed under CC-BY-SA

Our story takes us to Greece, specifically the Gulf of Corinth, historically a pain in the neck for any Greeks who wanted to get from one side of it to the other. Anyone wanting to get from, say, the town of Rio, on the Peloponnese peninsula, to the town of Antirrio, on the Greek mainland, you had to either take a boat or go 280 miles (~450 kilometers) out of your way around the Gulf of Corinth to get there. But the Gulf is only 3 kilometers long at its narrowest point; one Greek prime minister, Charilaos Trikoupis, allegedly wanted to build a bridge at this spot, but never got around to it because Greece had no money at the time to justify it. It still doesn’t, but, y’know. (We’re sorry, Greece, you’re a low-hanging fruit when it comes to money.)

What Trikoupis didn’t know at the time was that the Gulf of Corinth is a seriously hostile environment to the construction of any bridge, and when the project to build one was planned and approved in the 1990s, they would have to rack their brains to overcome that environment.

The first and most obvious issue was the length – 3 kilometers, as we said before. That’s a challenge, but not unprecedented or insurmountable; longer bridges did exist at the time. But here’s where we run into novel issues. First, the Gulf of Corinth is growing. Because of plate tectonics, the area where the bridge was planned to be built is expanding at a rate of around 30mm a year, or just over one inch. This tectonic movement, by the way, is why the Gulf of Corinth exists in the first place. For our purposes, this poses a serious problem to the existence of our bridge, since after a certain amount of time this movement is just going to pull the whole thing apart.

And, of course, where there are plate tectonics, there are earthquakes, and the Gulf of Corinth is one of the most active earthquake zones in Europe, producing quakes up to about magnitude 6.5. Another problem we have is that the seabed of the Gulf of Corinth is not solid – it’s almost entirely made up of sand. As it turns out, Anakin Skywalker isn’t the only one who doesn’t like sand; bridge engineers don’t like it, either, because it makes it really hard to get a stable foundation for your structure.

But that’s not even the worst part. Either of these problems on their own is a serious challenge, but combined, they are worse than the sum of their parts. When an earthquake strikes, the sand and silt at the bottom of the Gulf is prone to a phenomenon known as “liquefaction”, whereby the ground, despite being a solid, instead starts to behave more or less like a liquid. For our purposes, anything sitting on top of that sand would sink into it, in this case a bridge foundation. How can we deal with it? Well, we can’t get rid of the water; it’s the sea, after all. You can’t get rid of the sand, either; the bedrock is as much as 500 meters below it.

How on earth are we going to deal with all of this? Well, let’s tell you what they did.

Modern Solutions

Let’s talk first about the liquefying sand. The engineers found a natural phenomenon having to do with plant roots. They found that when a plant’s roots wound into the soil, the soil itself was an order of magnitude more stable. Anyone who has ever had to weed their garden can attest to this fact.

So the engineers took this natural idea, and did the engineering version of it: they sunk over 200 giant metal piles, at least 25 meters long, into the sandy bottom of the Gulf. By placing these bars into the sand, they would prevent the sand from liquefying in the event of an earthquake, and the bridge foundations could rest on these spots. With just some metal rods, they took an incredibly unstable foundation, and made it stable enough to support a bridge.

So that’s the first problem dealt with, what’s next? Well, the liquefying sand isn’t the only problem in an earthquake; there’s, of course, the earthquake itself. The side to side movement would cause the bottom of the bridge towers to dig into the sand, which would restrict the towers’ ability to move with the earthquake. This is how engineers create earthquake-proof buildings, by the way: design them to move with the shaking of the ground, so that the materials bend instead of break.

How to solve this, then? Well, the designers really couldn’t alter the structure of the bridge, because that would create structural instability. So instead of changing the bridge, they changed the floor. Above the layer of soft sand where they had driven in the metal piles, the engineers dumped a 3 meter thick layer of gravel which the bridge towers would rest on top of. This, as it turns out, makes all the difference; gravel has a larger particle size than sand, and so the bridge towers can slide across it more easily than wet sand.

Alright, we should have everything on the bottom of the sea worked out, now let’s get into the actual bridge design. The bridge towers, also called pylons, are absolutely enormous; 90 meters across at the base, and the outer and inner towers are 141 meters and 164 meters tall, respectively. From the top of these towers run a series of cables which fully suspend the bridge deck from the towers; this is done because again, an earthquake could seriously stress the materials. So instead of having the towers support the bridge normally, they made a hammock-like structure where the bridge deck can move almost entirely independently. In addition to this, they threw in some powerful liquid-based dampers to prevent the bridge deck from smashing into the towers. Side note, a similar technology is used on aircraft carriers to slow down planes after they’ve landed on the deck. Some people have called this bridge the most earthquake-proof bridge in the world, and it’s not hard to see why.

But now we have yet another problem to deal with. All this flexibility that makes the bridge so resilient to earthquakes opens it up to another force of nature – the wind.

Gone With The Wind

Rio-Antirrio Bridge construction
Rio-Antirrio Bridge construction by Michael Paraskevas is licensed under CC-BY-SA

The spot where the bridge was to be built has mountains either side of it, creating a natural channel for wind to flow through. As we’ve demonstrated in our Tacoma Narrows Bridge video, wind is a very real threat to bridges. That particular bridge was destroyed by winds around 40 mph. Well, in the Gulf of Corinth, they can reach up to 70 mph. Oh dear.

This is a manageable problem; many unremarkable bridges are rated to withstand wind speeds even higher than that. But here we run into a Catch-22. If you make the bridge more solid to resist the wind, you end up exposing it to the shaking movement of an earthquake. So we have a real conundrum here – make the bridge earthquake resistant, and it’s vulnerable to wind. Make the bridge wind resistant, and its vulnerable to an earthquake. We can’t have it both ways… can we?

Turns out, we can. What the engineers did was this – on those same liquid dampers that keep the bridge deck from hitting the towers, they attached large metal “fuses” to lock them in place. That seems odd, you might think; aren’t the dampers supposed to move to absorb the energy from an earthquake? You’re right – they are supposed to do that, in an earthquake, and these fuses are designed to fail when one strikes. Not even the strongest wind gusts can push the bridge enough to break the fuses, but when an earthquake hits, the energy overpowers the fuse, breaking it. Then, without the lock to keep them in place, the dampers begin to work, absorbing the energy of the earthquake. It’s a controlled failure, and it’s rather genius.

But the wind doesn’t just affect the bridge deck; there’s also the cables holding it up to think about. When wind hits a circular object, a phenomenon known as vortex shedding occurs. It’s rather complicated, but all you need to know is that it tends to shake the object back and forth really hard. This is bad for things made of metal, since over time this can fatigue it and eventually cause it to fail. Thankfully, there’s a really simple solution called a “Scruton strake”, which is quite fun to say. Basically take some metal, wind it around the sides of the object in a helix, and it will break that vortex shedding process. Consequently, every cable on the bridge has a Scruton strake on it. You can actually find these all over the place if you look for them.

So that’s the earthquakes and wind dealt with, now we have just one more problem – the slow, inexorable advance of plate tectonics. The Gulf of Corinth is still growing, so what do we do about that? Well for such an interesting-sounding problem, we have a rather bland solution called expansion joints. Basically, as the coast widens, parts of the bridge will lengthen with it, up to a total of 5 meters. They are the largest expansion joints in the world, so, that’s something. The bridge has a design life of 120 years, so these joints should cover it until it needs to be replaced for more normal reasons.

And that’s it. That’s every design feature this bridge has – all the novel ways its engineers dealt with a truly extreme environment. The bridge was started in mid-1998, and was finished six years later in time for the 2004 Summer Olympics in Athens, with the first people to officially cross it being the Olympic torchbearers. With all of that done, the bridge was officially named the “Charilaos Trikoupis Bridge” in honor of the prime minister who dreamed it up all those years ago. Certainly, he’d be proud of such a feat of engineering. And then he’d probably be annoyed at having to pay 14 euros to drive across it. Oh yeah, it’s a toll bridge, guess we forgot to mention that part.

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