The Earthquake Proof Bridge Richard Hammond's Engineering Connections

This is the Rion-Antirion Bridge in Greece, one of the longest bridges in the world.

It crosses some of the most active earthquake fault lines in Europe.

It also sits in a powerful natural wind tunnel

and there's nothing solid at the bottom of the sea to build it on.

To make matters worse, this coast is moving away from that coast every day.

How did they ever build it?

They faced what would have been an impossible challenge

if it weren't for some unusual connections.

The hammock...

Go on, lads, let it rip.

..a fizzy-pop can...

Oh, no. There's a fire!

This is lovely.

..a toboggan...

..Indian incense...

These contain an oil that really does smell lovely.

..and a steel chimney.

The Gulf of Corinth slices deep into mainland Greece, but at its Western end it's very narrow.

To avoid a 280-mile detour around the mainland of Greece,

you only need to cross this small stretch of water. Sounds simple.

On one side of the Gulf of Corinth, Rion.

On the other, Antirion. You can see what they did. Rion, Anti-Rion.

100 years ago, the Greek Prime Minister dreamed of a bridge to connect the two places.

Without some unprecedented engineering, that bridge would still be a dream.

Nearly two miles in length,

the Rion-Antirion Bridge is really long, but most incredibly, it's practically earthquake proof.

Which is reassuring, as it sits in one of the most active seismic zones in all of Europe.

It can withstand shocks measuring 7.4 on the Richter scale,

enough to completely destroy your average bridge.

Back in 2008 the bridge was hit by a tremor.

The shaking ground had caused buildings to collapse less than 15 miles away,

killing two people and injuring many more.

Amidst the chaos, the bridge survived unscathed,

allowing emergency traffic to pass safely and quickly over the gulf.

But surprisingly,

shaking ground wasn't the first challenge earthquakes posed to the bridge builders.

Before they could start, they had to deal with what lay at the bottom of the sea.

For the engineers, this was always going to be a bridge over troubled waters.

Yeah, I know, sorry. But it was!

Just getting started was going to be a huge engineering challenge.

In fact, when they began, they still didn't know how they were going to lay their foundations.

The sea here is 65 metres deep,

a challenge in itself, but at the bottom there's nothing but sand and silt for hundreds of metres down.

There's no solid bedrock and no solid bedrock means no solid foundations.

Solid foundations are, of course, crucial to any structure,

especially when they're shaken by frequent earthquakes.

Combining a sandy seabed with seismic activity results

in a relatively little-known danger from earthquakes called liquefaction.

In a tremor, soft, wet ground literally turns to liquid, which really is as bad as it sounds.

Nobody in the world had ever before set out to build a bridge in these conditions.

The engineers came up with the solution that seems to defy logic.

But first, why does a tremor turn sand and water into a liquid?

I've created an earthquake machine to replicate the conditions

at the bottom of the Gulf of Corinth.

This engine is going to play the part of an earthquake.

The sand and water can play themselves, and to get a bone-shaking feel for the problem,

I'll be the bridge itself.

If you add an earthquake to loose, wet ground,

something pretty strange can happen, as I shall now demonstrate.

Here we have the loose, wet ground.

It's sand with water but it's solid enough.

Standing in here quite happily.

OK, if we're ready then, chaps, take it away.

Now, my earthquake is starting.

It is really very loud and very shaky.

Straightaway I see water coming to the top.

In fact, I think I'm starting to sink.

That's a lot of water.

Clearly, what's happened is - apart from being deafened - I've sunk.

It isn't just that the water's on the top. I mean, I really have... I'm in the ground.

So I need to find out what's happened.

To do that, I need to get out and that's not very easy.

Hold on!

Yep.

Obviously, in the Western movie version of this, I'd get my horse

by the side of the quicksand and he'd pull me out. OK.

The ground holds me up fine before the earthquake, then it turns into quicksand.

As it's shaken, the whole thing changes from solid to liquid.

It liquefies and I sink.

'Geo-technical engineer Stuart Haigh explains that the key is that the sand is wet.'

When the sand particles are shaken, the spaces in between them,

or the pores, try and get smaller, the water in there gets squashed and the pressure goes up.

Once that happens, whatever's standing on top of it sinks into the ground.

Combine wet sand with seismic shocks and it's a killer.

When a huge earthquake hit Kobe Harbour, Japan,

in 1995, over 6,000 people died.

It measured 7.2 on the Richter Scale.

Sure enough, the ground was sandy and full of water.

When the mix was shaken and stirred, the result was catastrophic liquefaction.

Literally the previously solid ground became liquid.

Ordinarily, engineers can drain the sand to get rid of the water or compact it

so that the water is squeezed out.

None of this was possible in the Gulf of Corinth

and the sand to compact up to 1,640 feet deep.

An impossible task.

There was no solution in the bridge builders' manual,

so the Rion designers had to come up with their own, which they did,

thanks to incense.

This is vetiver grass and it's used to make incense thanks to a sweet-smelling volatile oil

in the roots.

But it wasn't the oil they were after.

These are the roots, hanging down in here.

These contain an oil that really does smell lovely.

Now, this grass originates in swamps in India, so harvesting it was horribly difficult.

Snakes and awful conditions.

The growers soon learned to grow it in places more convenient for harvesting - on riverbanks.

They then noticed that these roots, which can grow up to seven metres long in fully-grown plants,

stabilised the soft ground they were growing in.

That's what interested the bridge designers.

The way that the long, thin roots of the vetiver grass stabilised riverbanks

worked for the bridge engineers too.

With my own earthquake rig, I'm going to put their solution to the test.

Here you go, Jim. Is this it?

-That's it.

-And just this many? I've got, what, six of them?

-Six will be fine.

And it's going to be an earthquake of the same strength as before?

OK, well, I'm going to trust you and your theory and I'm not going to put my waders back on.

I'm going to go in. I shall wear just my new boots.

-You'll be fine.

-Really?

-Yep.

It's not that I can't sink - the rods don't actually touch the bottom of the tank -

They work because they reduce movement in the sand.

So they should stop my board from sinking and my new boots should stay new.

All right. Let's do it.

Don't ruin my new boots.

Go.

Whoa!

I'm not going any further.

After some initial settling, it does the job.

So by sinking steel rods into the seabed, the engineers managed to stabilise it.

Actual vetiver grass is used to stabilise river banks and cliffs edges across the world,

from Africa to Fiji. Those long, thin roots hold the soil together.

Beneath three of the four piers of the Rion-Antirion Bridge

are 200 metal piles driven into the sand.

They're just like my small ones in the skip,

but here each pile is huge - at least 25 metres long.

They still don't reach the bedrock.

Even more surprising, the bridge doesn't rest on these piles.

The tops of the piles are almost a metre beneath the base of the piers,

but they're enough to keep the sand from liquidising, even in an earthquake.

The bridge won't sink. This solution had never been used before.

Until the construction of this bridge,

building on wet sand in an earthquake zone was obviously a big no-no.

But thanks to a fragrant gift from Mother Nature, the engineers came up smelling of roses.

Well, vetiver grass.

Even though the entire weight of the bridge is held up by just these four piers,

the engineers had succeeded in keeping them from sinking into the ground in an earthquake.

As soon as they'd solved one problem here, another one came up.

They could stop the piers from sinking in an earthquake,

but how could they stop them from toppling over?

In an earthquake, the earth moves vigorously from side to side

and would take the gigantic piers with it.

That could be a big problem for a big bridge.

Phil Corbett has offered to take me a bit closer to the problem.

The piers are actually hollow, so I'm going to go right down to the bottom of them.

This is just the first floor down and I'm starting to get

a real sense of what I've taken on.

Ooh!

That's the centre of the earth. It must be.

Well, it is a nerve-racking 110 metres down in total.

There can't be that much further to go now, surely?

Then, just when I thought I'd reached the bottom...

Well, I've been climbing down ladders and stairs for what felt like weeks

and this is all I've done. Sea level.

There's a whole lot more to come because this thing goes on and on and on down there.

33 flights of stairs later...

This is it, then. Bottom of the sea.

65 metres above is the surface of the sea. Under there, the bed.

These are the biggest bridge piers of their kind in the world.

This one weighs 171,000 tonnes and from down here to the top, it's about 30 storeys.

90 metres across at the base of this thing.

In an earthquake - it is almost impossible to imagine -

to keep it from falling over, this whole gigantic tower needs to move freely from side to side.

If the base got stuck, the whole thing would topple over.

The engineers need to make sure that each pier can move freely.

So what to do?

The answer to that lies in an age-old winter pastime.

The Rion-Antirion Bridge has one thing in common with winter sports.

They both involve moving freely - sliding.

Of course, tempting as it is...

I didn't come here to play in the snow.

Just like a toboggan, the bridge piers have to slide freely over the sea floor which is made of sand.

But they don't slide well on sand because of their shape.

To show you what I mean, this board is flat, like the bottom of the bridge pier.

Yeah. The problem is the leading edge is digging in, or toeing in, as they say in the trade.

It won't go anywhere.

That could have been a real problem for the Rion Bridge.

Sliding down a hill of loose sand is one thing, but does a pier

on the flat seabed really have the same problem?

I've got myself an accurate scale model of a bridge pier.

Well, give or take a few hundred metres.

It's currently being supported without a problem

by this ultra-smooth track of fine sand.

This time, to create the effect of the ground moving sideways,

or laterally, the role of the earthquake will be played by a 4x4.

I've also brought along Ian Price, an expert in stabilising soils.

So even though we're not at the bottom of the sea, by any means, you reckon this is

-a good representation of what might happen 65 metres down?

-Yeah, we're attempting to simulate that effect.

OK. So I'm going to drive away because this is a lateral force we're talking about?

We're talking about large lateral forces.

-This is a large lateral force.

-I'm going to stand well clear.

OK, I'm going to create an earthquake.

Ah.

My pier fell over.

-So this is the toeing in you're talking about?

-That's right.

Why did it fall? Why didn't it just keep following me?

The weight overcame the ability of the sand to support the structure.

In an earthquake, the piers of the Rion Bridge would be pushed sideways, like this.

As it moves, the leading edge puts more pressure on the sand beneath it

until it's more than the sand can support.

It digs or toes in, and then it topples over.

Really not what you want.

So the key is how much pressure from the leading edge a material can support.

In winter sports, the solution is simple.

Take away the leading edge altogether.

Make your skis, snowboard or toboggan curl up at the front.

That's fine for a toboggan, but making mammoth pier bases

that curl up like giant sleds would be virtually impossible to construct.

They needed another solution.

The one they chose was extreme.

Rather than change the shape of the piers, they decided to change the surface of the seabed.

For their solution, the bridge engineers needed to understand why you toe in to soft material.

I've got to ask, why is there a paddling pool

full of what appears to be mashed potato?

Well, this demonstrates what the engineers found on the site of the Rion-Antirion Bridge.

What we have there is silts and clays which are very, very soft, very analogous to mashed potato.

There's no way that sands will support the weight of the structure. Try it yourself, Richard.

-What, get in?

-Yeah. Why not?

-All right, fair enough.

This is not something I've done before. So I'm the bridge.

Clearly, mashed potato isn't going to be up to the job.

Yeah, mash is obviously too soft to support me.

But there is a way that exactly the same stuff can support more pressure.

-These, then, are just potatoes?

-They are.

And they work perfectly They'll support me.

So obviously those potatoes are mashed and these aren't,

but they're still a paddling pool of potatoes. What's the difference?

The difference is particle size.

Potatoes are a larger particle size and consequently you will get increased bearing resistance.

So just the bigger particle size.

Same material, but smaller particles of it over there and I sink through them,

-bigger particles here and they can hold up more weight?

-That's right.

So the answer to the bridge builders' problems was potatoes.

Well, not exactly potatoes, but a larger particle size - gravel.

So gravel was just what the engineers needed.

Instead of changing the shape of the piers to make them curl up at the edges,

they changed the seabed with a layer of gravel. A huge job.

I'm going to create another 4x4 earthquake and put the pier on a gravel base.

It should support the leading edge so it won't toe in and fall over.

So the only change this time is we're replacing sand with gravel?

-Yes.

-This will make all the difference?

-Yes.

-You reckon?

-I think so.

-I'll bring on an earthquake.

Stand back.

-That made all the difference.

-It did.

-So it does work.

The answer for the engineers was to build their bridge on un-mashed potatoes. Well, no. Gravel.

At the bottom of the Gulf of Corinth,

the Rion Bridge rests on a thick layer of gravel above the steel rods.

This allows the bridge piers to slide from side to side

without digging or toeing in.

They used enough gravel on the seabed

beneath the Rion-Antirion Bridge to cover two football pitches three metres deep.

These massive piers are amazing.

They stand strong but without being fixed to the seabed,

so they can move and withstand earthquakes.

The piers really are awesome but they're obviously only part of the story.

Their purpose is to hold up the bridge deck, which is what it's all about.

The deck has four lanes as well as two safety lanes

that run for almost two miles in each direction.

It's mammoth.

If the engineers fixed it to any of the piers and those piers then

moved in an earthquake, which the gravel allows them to do,

it could buckle

or break. Not good.

Right, what we need now is another one of my earthquake demonstrations.

This one is going to require a little imagination.

Bear with me. This is my road deck, which I'm going to fit to this boat.

But for the purposes of this, the boat represents the piers of the bridge.

So that's the piers supporting the road deck, concrete and steel, fixed to it solidly.

Now we're going to have an earthquake.

I need muscle so I'm going to break out the Herefordshire Special Ballet Squad.

In you come, then, lads, please. Come on.

They'll be the muscle to make the earthquake.

I'm going to be the traffic on the road deck, fixed solidly to the piers.

So, getting into place. I'm the traffic on the deck.

Oh, no, I hope there isn't an earthquake shortly.

Go on, lads, let it rip.

As an experience, it's unpleasant,

but if this were a road deck on a bridge and I were heavy traffic on it,

clearly the stresses that the materials would be subject to would be catastrophic.

It's not going to work.

One earthquake, the whole thing could collapse.

To save the bridge deck from the violent and chaotic movements of an earthquake,

the engineers needed to build it in a way that allowed it

to move independently of the piers, which is where sailors and their sleeping arrangements come in.

Christopher Columbus first encountered hammocks in South America

and he brought them back to Europe.

Early sailors very quickly discovered that they were quite handy things

for making life more bearable on board because, well,

ships weren't the most sophisticated of devices then. Things could get pretty rough,

but no matter how rough the seas, the hammock would isolate them from the movements of the ship

and make life just about bearable.

They didn't know it, but in fact a hammock

is a kind of pendulum and it has a steady rate it wants to move at,

which it will do regardless of what's going on around it.

And it meant that sailors were more comfortable.

What's good enough for them is actually good enough for the bridge.

So just as a pendulum, or rather a hammock, swings gently on board a ship in rough seas,

it ought to make my ride on the earthquake simulator a bit smoother, too.

Here is my modified bridge.

Remember, the boat represents the bridge piers, the structure.

This hammock now represents the deck slung between them.

Right, I need to break out the boys to hold the bridge steady first of all, please.

Then I hope there isn't another earthquake.

Right, I'm getting onto the road deck, which now is slung, pendulum style, just like a hammock.

Goodness, what if an earthquake were to occur right now when I'm on the bridge?

Actually it is already.

Straightaway, what's happening here is the hammock,

the pendulum suspension, is isolating me from the movement of the piers.

Even when things get pretty bad.

I mean, this is a pretty disastrous earthquake we're experiencing right now.

The pendulum is turning chaotic movement into something a bit more predictable

and a lot more gentle.

I can imagine, straightaway, how this would stay standing.

The Rion engineers borrowed that same pendulum principle to help earthquake-proof their bridge.

Just like a hammock, the deck of the Rion Bridge is fully suspended from the top of the piers.

It's unique in all the world.

Incredibly, as the piers move in an earthquake, the deck swings independently.

It seemed, yet again, the engineers had tamed the earthquake,

but their free-swinging solution might itself have created a problem.

The deck can swing freely,

but if it swings too far in either direction,

it would smash into one of the four thin arms of the piers.

That's 75,000 tonnes of road hitting the concrete,

a big moving mass, and it could destroy the bridge.

The engineers couldn't allow it to happen,

but once an object as big as the road deck starts moving, it takes something extraordinary to stop it.

Enter, the viscous damper, an ultra-powerful braking system.

To experience a viscous damper first hand, I've devised a little experiment, a demonstration.

First, I'm going to show what happens with no brakes, no viscous damper.

That in front of me is a teeter-totter.

Basically it's a giant seesaw.

But this is no playground and that is a mighty big drop at the other side.

And this, in my car, is a roll cage, just in case my confidence outstrips my skill.

Right, all I've got to do is drive up there.

It's my job to drive up it, past the pivot point and then let gravity do its worst.

Of course, the higher I go, the further I'm going to fall.

I'm not enjoying this at all.

The pivot point must be somewhere about here.

This is just horrible! I can't see a thing. I don't know where I'm going.

At some point, I know that something bad is going to happen.

That much is guaranteed.

I can smell clutch. Ooh!

That's not very...

For the purposes of this demonstration,

what just happened to my car as it hit the tarmac is the equivalent of the free-swinging bridge deck

violently hitting the pier that holds it up during an earthquake.

If the deck swung hard enough, it could break the bridge.

The engineers needed to slow the deck movement and I need to stop the seesaw banging down so hard.

We both need some brakes, which is where viscous damping comes in.

It's a braking system where you use a liquid to resist movement.

If you've ever tried to run in a swimming pool, you'll know all about the resistance water can create.

This black box is going to be full of water and there's also a fan in there

which will have to move through it, absorbing all the energy from the seesaw. It's a brake.

This is a viscous damper. It's basically a liquid - in this case,

water - that slows the movement.

It should give me a softer landing.

Only one way to find out.

OK, any minute now we should reach balance point.

Oh! Oh, this is lovely.

That really does work perfectly.

What happens is the vertical motion of the teeter-totter, of the seesaw,

is slowed down by the rotary motion of the fan inside the liquid.

Essentially what it does is turn some of that energy, that kinetic energy, into heat

inside the damper, which gives me a much, much softer landing, kinder to my car.

Viscous dampers are used all over the place to soften movement.

These planes are coming in to land on an aircraft carrier in the middle of the ocean.

They approach at over 100 miles an hour, but they can stop in such short distances

thanks to viscous dampers that are attached

to the catch wires stretched across the runway.

And the bridge is fitted with its own incredible liquid safety system.

These things that look like pistons are the viscous dampers and they're just like my system, in principle.

This footage shows the dampers being tested.

Instead of a fan spinning in water, a piston has to move through oil

which is much thicker, or viscous, than water,

so it offers much more resistance to movement.

In infra-red you can see it heating up, so the liquid is

turning all that kinetic energy - that movement - into heat.

The viscous dampers on the bridge are the biggest in the world.

Slinging the road deck like a hammock allows the bridge deck to move

and the viscous dampers stop it from moving too much so it doesn't

hit the arms of the piers and shake itself to bits.

The designers made this extraordinary bridge uniquely earthquake-proof,

with unprecedented solutions.

But all this flexibility created a further problem

because earthquakes aren't the only natural hazard in this region.

Mother Nature has another trick up her sleeve.

This narrow stretch of the Gulf of Corinth, with mountains on either side, makes a natural wind tunnel.

A flexible bridge is good for withstanding earthquakes but it's bad when it's windy.

Even with the dampers to soften the movement, near-constant winds in the gulf

mean the bridge would be swaying constantly.

An impossible choice, then, for the engineers. Make the bridge flexible enough to cope with an earthquake,

and on normal days when there isn't an earthquake, it would swing so violently,

it would be uncrossable thanks to the winds in the Gulf of Corinth.

But make it strong enough to cope with the wind by fixing it firmly in place,

and when there is an earthquake,

it would snap and crumble into an 800-million-Euro pile of rubble.

The answer to their conundrum lies in this, a fizzy-pop can.

Engineers don't usually design things to fail.

They build them to withstand the daily challenges they will face -

to be strong and rugged and to stay that way.

Helpfully for the Rion Bridge, an inventor threw away the usual rules of engineering

after a frustrating picnic. He needed a new way of opening a can.

The humble tin can has been around for 200 years.

It can keep produce fresh for decades and transformed the food industry.

They weren't designed for easy access.

Getting into them required some muscle.

That changed when inventor Ermal Fraze came up with a new way of opening them,

which revolutionised cans and protects the Rion Bridge.

Yep, this is a real picnic, just on my own.

Ermal Fraze found himself on a picnic with a canned drink

but someone had forgotten to pack the can opener.

Being a resourceful sort of chap, he managed to wrestle it open on his car bumper

but that took all the chrome off the bumper.

So he went away and thought about the problem

and he devised the pull tab, precursor to today's stay tab.

This weak section here is designed to be strong enough to keep the fizzy drink contained within,

but weak enough so that by hand, without any sort of machine,

you can just rip it open. This has to fail at a set limit.

It has to fail predictably.

Ah, yep. And it does. I have made a bit of a mess of my picnic though.

In the right place,

making something fail predictably can be the difference between life and death.

For instance, you might recently have reorganised your workshop

and tidied away a load of cardboard boxes into a corner

and then, inexplicably, soaked them in petrol, for some reason,

and then decided to catch up on that bit of angle grinding you've been meaning to get to for so long.

Oh, no. There's a fire!

That could be a real problem without predictable failure,

which is what's just happened.

This is a sprinkler system.

I installed it myself, which is fortunate because I'd forgotten where I'd put the fire extinguisher.

It's pretty much exactly the same as the sprinkler system

in schools and buildings across the world but it's a lot uglier.

It works in exactly the same way.

Disaster averted and here's how it works.

This is the sprinkler system and the job it's got to do sounds pretty complicated.

It's got to automatically detect a fire and then automatically put it out.

But the way it does that is pretty simple.

This is the kind of thing you will have seen sticking out of the ceiling

in offices, shops and schools.

These are connected to a water pipe along here.

Now, before they've been triggered, they feature this little glass vial.

This has been designed so that at 68 degrees it breaks.

The red liquid boils at exactly 68 degrees.

That makes the pressure inside high enough to break the glass.

That's the predictable failure.

They can predict - 68 degrees, that goes.

When that happens, that falls away and it opens up the hole,

here, so the water can just flood out, hit that disk and form the sprinkler effect,

putting the fire out automatically.

And it's all thanks to a predictable failure in there.

Engineers have adopted the pop-can principle of predictable failure

all over the place to help us go about our daily lives safely.

Think circuit breakers,

such as fuses that cut out when electrical circuits overload, or car airbags that inflate in an accident.

Life goes on normally, but we are protected the moment something changes.

So how does predictable failure help the bridge against winds?

You stop the bridge moving unless it's struck by a massive shock.

You don't allow wind pressure to swing it, but it breaks free and protects itself in an earthquake.

Dr Papanikolas showed me how the combination of the fizzy-pop can theory

and the viscous dampers keep the bridge safe.

So where are the predictable failures in here, then?

Because it all looks massive and solid.

OK. What you have here is the deck. It is laterally supported

by the dampers. In the middle one you have what we call a strut.

Oh, so that solves the problem of the wind?

If you think, every day, you don't want the bridge to swing back and forth.

Inside each strut, there's a so-called fuse.

It's designed to break at a set limit, just like a ring-pull.

When the load gets too big, it fails.

The fuses can take the load of even the strongest winds,

but when an earthquake hits and the load exceeds

the pre-set limit, they break and those huge viscous dampers start to work.

This isn't just theory.

This CCTV footage caught the action in June 2008.

A magnitude 6.5 earthquake hit the bridge, the fuse snapped,

and the dampers sprang into action. It worked.

So as designers, as engineers, seeing your creation actually work

and do what you always knew it would do, but actually seeing it for real,

must have been amazing.

Yes, it's an unbelievable feeling.

This is the feeling of engineering.

An earthquake- and wind-proof bridge, all thanks to predictable failure,

courtesy of Ermal Fraze's easy-open drinks can.

Well, that covers just about everything. Foundations?

Earthquake-proofed. Check.

Piers, the deck? Earthquake- and wind-proofed. Check. Oh, no, hold on, we've forgotten something.

The cables.

Believe it or not, the same cables that hold the road deck

could pose a mortal threat to the bridge.

Not because of earthquakes, not even because of gale-force winds. Just a gentle breeze can be lethal.

Over 6,000 miles away, in Baytown, Texas, the cables of the Fred Hartman Bridge

began to look a lot less than solid.

Spring 1997, two years after the bridge opened.

In winds as low as 10 miles an hour, the cables bounced up and down ferociously.

This happened time and time again.

The structure suffered 100 separate stress failures, which could have been disastrous.

Fortunately, the Texas Department of Transportation had been alerted

and fixed it by tying all the cables together,

holding each other down.

The same conditions that threatened the Fred Hartman Bridge

are a risk to any lightweight, round metal structure

such as a cable, a tower or a chimney.

The Emley Moor TV mast in Yorkshire was constructed in 1964,

one of a new generation of lightweight structures built of metal rather than brick.

In 1969, in light winds, the tower collapsed without warning.

Ice buildup was blamed initially, but British aerodynamicist Kit Scruton

knew it was due to a bizarre aerodynamic effect.

It's called vortex shedding.

That's nowhere near as difficult to understand as it sounds like it's going to be.

To demonstrate, I've got a fan and that metal pole over there.

So let's make it a windy day.

That vibration is caused by swirls of air.

This specially shot footage shows exactly what's happening.

You can see the swirls moving from side to side.

That's called vortex shedding.

Any round structure creates these swirls of air which can

pull it one way and then the other.

You've got to remember, of course, that that's not the wind buffeting it,

it's not gusting that's causing that shaking to happen,

it's one vortex being shed after another, after another, that makes

it vibrate like that. A further problem there is that if this goes on long enough, shaking and shaking

and shaking, the metal itself will fatigue. It can be a catastrophic failure.

It moves towers from side to side and it can move cables on bridges up and down,

as the Fred Hartman Bridge in Texas can testify.

Fortunately, British aerodynamicist Kit Scruton worked out

how to overcome the lethal threat from vortex shedding.

He invented the helical strake.

A helical strake is a strip of metal that winds like a big coil spring around the top

of tall steel towers today.

It keeps them safe

from the powers of vortex shedding.

Yep. Now, I know what it looks like I've done is tied

blue string around another tower, but this is more than that.

This is a helical strake.

The theory runs, what this does is, as the wind arrives at the structure,

instead of a vortex forming the length of it,

rolling around and shedding, and then another one and then another one,

which sets up that movement, it breaks those vortices up.

That simple act, breaking them up and changing the time at which they come around,

evens the whole process out.

No vibration, no failure of the structure.

That's the theory. Now I'm going to make it windy and test it.

So my blue string - helical strake - works.

And now you can see them everywhere -

just take a look next time you're out.

And back on the Rion-Antirion Bridge, every cable is fitted with one of Kit Scruton's

helical strakes. They break up the wind

hitting the cables to protect the bridge from vortex shedding.

On top of everything else, to accommodate for the fact that Rion

is moving away from Antirion, the bridge has the largest

expansion joints in the world, allowing for the two coasts to drift

five metres away from each other.

Now, what have they done to protect against possible meteor strikes?

Well, it's a thought.

The Rion-Antirion Bridge achieved what was previously impossible.

In an active earthquake zone and natural wind tunnel,

and without bedrock for foundations,

the engineers created a magnificent design that overcame each of these hurdles.

Remarkably, despite the need for unprecedented engineering solutions,

the bridge opened four months ahead of schedule, fulfilling a century-old dream.

Now it's part of the daily life of the people of Rion and Antirion,

but the effect has been felt far beyond these two small towns

because it has effectively redrawn the map of Greece.

None of it would have been possible without a hammock...

Go on, lads, let it rip.

..some Indian incense,

a steel chimney, the principles of tobogganing...

and a fizzy-pop can.

This is lovely.

Oh, no. There's a fire!

Subtitles by Red Bee Media Ltd

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