All language subtitles for Britains.Greatest.Bridges.Series.1.4of6.The.Britannia.Bridge.1080p.HDTV.x264.AAC.MVGroup.org.Eng

These vast, vertical grooves in the towers here,

designed and built into the towers themselves,

to lift the huge tubes into place.

The jack, a simple cylinder with a piston inside,

was connected by chains to the box girder below.

A steam engine would inject water

at thousands of pounds per square inch into the jack.

That high-pressure water pushed the piston upwards,

the chains raising the beam about two metres at a time.

Then the tubes would be secured, the chains shortened,

and the whole process would start again

for the next couple of metres' lift.

Stephenson decided to use hydraulic jacks,

one here and one on the other side,

they run right down to the water - the whole height of the tower.

They're what guided the ends of the tubes

as they were slowly lifted up.

The pressure needed to lift the 1,500-tonne tubes

would've been immense, even by today's standards.

In many ways, it was like a bomb waiting to explode.

And on the 17th of August, 1849, that's exactly what happened.

The very first beam was only seven metres up the column

when, bang, the entire jack exploded.

And to show you just how big a bang that was,

hauling on ropes to try and save the tube.

but the risks were much higher.

They would only be able to launch the tubes at high tide,

but this would only give them an hour of relative calm

before the vicious currents would sweep through the channel.

If the tubes were not secured by then, disaster would strike.

Thousands of people turned out to witness the event of a lifetime.

All was going well until one of the ropes got tangled up.

One of the anchor points on shore got ripped clean out of the ground.

In a move that would put

a modern health and safety officer into a cold sweat,

the spectators were called upon to help,

this is the remains of that very jack.

Incredibly, it worked,

and the first tube drifted into position

at the base of the towers.

And then came the next challenge -

lifting the 1,600-tonne tube 40 metres up

to its final resting place near the top of the towers.

So how did they do it?

Well, the secret is hidden

in the remaining sections of the bridge.

It's just that very few people get the chance to see it,

as you need to climb below the modern railway track.

absolutely no sign of Fairbairn.

at the Institution of Civil Engineers in London...

..in this great painting.

It's a fictional scene.

You've got Britannia Bridge in the background there,

Stephenson right in the middle

holding court,

surrounded by his contemporaries.

And even his good friend Brunel

makes an appearance,

who had absolutely nothing to do with the bridge.

But, astonishingly, there's

The first clue to the dispute can be found here

It's as if he'd been airbrushed from the whole story.

And there's a reason for his absence.

Fairbairn and Stephenson fell out publicly

over ownership of the box girder.

It was a fight that even today

is considered one of the biggest bust-ups in engineering history.

Hidden within the institution's own library

are a series of correspondence between the two men

which clearly sets out the disagreement.

And this is a real privilege here.

These are the original letters, Fairbairn's letters to Stephenson.

The setback meant it took another eight weeks

The centre, that would be hollowed out.

That's where the piston would be driven up and down by the hydraulics.

But it's the thickness of the walls

of the cylinder that gets me.

You can see this is where it's broken,

this is where it's failed,

but that thickness, it's about a foot there.

It just goes to show the phenomenal pressures involved

to break this.

One man died in the accident,

and the damage caused brought the entire bridge build to a halt.

these monsters would be floated into position,

to get the tube positioned at full height,

but it was soon joined by another tube on the other side,

and the Menai Strait was spanned.

Stephenson himself hammered in the final rivet,

and his revolutionary plan for a railway tunnel in the sky

had worked.

And for the next 120 years, trains would thunder across

Robert Stephenson's Britannia Bridge day and night.

But the success was also to lead to a very public and bitter fight -

one that would take almost 170 years to resolve.

to bear the weight of the massive box girders -

Tie bars provide lateral support to the external walls,

stopping the external walls from spreading.

It travels from one wing wall to the other and keeps them in situ.

It just amazes me that after all this time,

almost 170 years,

all of this is still doing its job, taking the weight of the trains,

and now the cars as well.

It's amazing.

The foundations for these towers were laid in 1846,

and it took Stephenson three years to finish them.

But by 1849, the towers were ready

that'd be needed to fill this sort of space.

thousands of wrought iron plates

held together by over two million rivets.

But no-one had ever made a bridge like it before -

and the risks were phenomenal.

The legendary engineer Isambard Kingdom Brunel,

a rival and friend of Stephenson,

is said to have told him that, "If your bridge succeeds,

"then mine have all been magnificent failures."

Which is one of the reasons why Stephenson decided on a rehearsal

and built this.

The Conwy tubular bridge,

I did not expect, when I walked in, that this was what was awaiting me.

Are you ready, Rob? Here we go. Yeah.

And that means you can go inside.

These magnificent structures are normally closed to the public,

but I've been given the chance to join Network Rail,

owners of the bridge,

as they give the 176-year-old towers a health check.

Gary, I'm still getting over how surprising, magnificent...

Kind of lost for words just how amazing it is in here.

Yeah, Rob, this is Anglesey railway abutment.

It's the biggest of the two aptly named cathedral abutments.

As you can see, the sheer size of it. It's enormous.

about 20 miles down the line from the Britannia Bridge,

It's huge. 33 metres high,

55 metres long from abutment face to back wall,

and around 20 metres wide.

This cathedral abutment is held up by three enormous arches,

which in turn have 14 smaller arches sitting on top of them

holding up the rail track.

It's like a giant wedding cake,

with each tier supporting the one above.

Essentially, this big space is saving on material.

You can imagine the volume of material

so Stephenson decided to cross it in two sections,

it was nothing compared to what faced him

when he tried to cross the Menai Strait.

Unlike the relatively calm waters at Conwy,

the currents in the Menai Strait are much fiercer

because the strait isn't a river, it's sea.

Twice a day, tidal currents sweep through this narrow strait

in both directions,

creating dangerous and unpredictable conditions.

Admiral Horatio Nelson described this

as one of the most treacherous stretches of sea in the world.

Not only is it more dangerous, it's also much wider than at Conwy,

But if Stephenson thought that was hard,

each one 140 metres long.

Early on a June evening in 1849,

they started to float the first tube down the strait

on pontoons controlled by ropes from the bank.

It was launched from the riverbank right behind me here,

and at 140 metres in length,

it was longer than any ship that had been built at the time.

About half as wide as the Menai Strait itself,

it would span the gap between the Anglesey side

and the central tower here.

Just like the tubes at Conwy,

almost 170 years after this was built.

which opened in 1849.

The crossing here is less than half the span of the Menai

and not nearly as high,

but it was, in many ways, a dry run for the Britannia Bridge.

Passengers travelling by train will barely notice it,

little more remarkable than passing through a rather short tunnel.

But from the outside, you can appreciate its magnificence.

From underneath here, along the length of the bridge,

you can see where those cells, developed by Fairbairn, run,

giving the bridge its strength.

It's still taking the weight of around 50 trains a day

And this is engineering history right here.

In many ways, there's no better testament

to the box girder than this.

The massive girders were constructed on the banks,

then the huge 400-foot structures were floated on rafts downriver

to be jacked up to their final resting place.

The idea of floating an 1,100-tonne beam along the river

and then lodging it exactly in place between these two huge stone towers

seems bonkers even by today's standards.

Against all the odds, it worked,

and the Conwy Bridge was up and running.

You were down by the water. Yeah, as close as practical.

Wow.

There was no way at all you'd go into the tunnel.

Mm. At all. You'd have been roasted alive.

Also there on that night were two local cameramen

who rushed to the scene to cover the fire.

So, you were both there with your cameras?

Yes, indeed. Yes, indeed. Yeah.

So, filmed it and took photographs.

There's the pictures of the actual fire.

You can see the amount of material that's falling off it. It was incredible.

So, you were underneath the bridge then, were you?

it turned into steam, so it didn't do much work.

Very, very noisy. Was it? Incredibly noisy.

Absolutely, yeah.

Bits flying down. What kind of noise?

Just crackling and burning.

This is debris just dropping down. That's falling off it.

That's the debris that's on fire. Absolutely, yeah.

And this was the most spectacular fire.

Debris was falling off in great big chunks,

and where it landed in the strait, of course, it was still alight.

The next morning, the bridge was still smouldering

and the damage was obvious.

we could have tackled it from four corners and perhaps from above,

and once the highly flammable mix of tar and wood was ablaze...

..the fate of the bridge was sealed.

Two of the firemen who were on duty that night still live in the area,

and it was a night they'll never forget.

When you first arrived there and both saw what was in front of you,

what were your reactions?

What did you think? "Wow." (LAUGHS)

There was no saving the bridge.

Really? You could tell straightaway?

It was like fighting a chimney fire, if you like, on its side.

If it was building on fire,

It burnt well into Sunday, about four o'clock, didn't it?

but because of the location of the bridge,

you could only tackle it from this end of the tunnel

to the Anglesey side.

In-between, it just burnt.

But that fire was spreading away from you? Yes.

There was no saving the bridge at all. It was just an inferno.

Could you feel how hot it was?

Could you kind of sense how hot it was?

Very, very. The radiated heat was terrible.

When you projected water into the tube

designed and opened in less than two years.

a plan was agreed and was rushed into construction.

And it's that bridge that still stands here today.

The new bridge is not spectacular or revolutionary,

but practical and efficient.

It carries both trains and cars, split on two levels.

It uses the same towers built by Stephenson almost 170 years ago,

but now its weight's taken by its two great arches.

Ironically, a more traditional way of crossing a river,

but as there are no tall naval ships around any more,

it was the simplest option.

The new bridge was commissioned,

In a matter of weeks,

As a rushed job, it doesn't look too bad,

and despite the arches,

it still bears considerable resemblance

to Stephenson's masterpiece.

As an engineer, I feel sad that the key component

of the Britannia Bridge - the tube that changed the world -

was sent to the scrap heap in the 1970s.

And for me it's important to remember its inventors,

Robert Stephenson,

and William Fairbairn - one of the forgotten greats of engineering.

Captions by Ericsson Access Services (c) SBS Australia 2017

but with it gone, a more immediate nightmare was unfolding -

Oh, yes. It burnt itself out.

And all you could do was... Just keep damping down.

It was a token gesture, wasn't it? Yes, that's all.

Stephenson's revolutionary tubes were sagging by over half a metre,

suggesting temperatures inside the tube

must have exceeded 1,000 degrees Celsius,

and they were now resting so precariously in the towers,

it was feared they could fall at any moment.

The historic box girders were utterly destroyed.

The destruction of the bridge

was without doubt an historical engineering tragedy,

but these embers caused Clark's roof to catch fire,

one that threatened the livelihood of the whole island of Anglesey.

Thanks to the success of the railway,

an important and prosperous container port

was operating at Holyhead.

A new power station and aluminium smelter

was under construction.

The island was booming.

But without this bridge,

Anglesey would become an economic disaster zone.

A replacement bridge had to be designed - and quickly.

Fairbairn formally resigned,

"I had satisfied myself that the thing was practicable,

"and I stood by it.

"In order most thoroughly to test experimentally

"the theory I had formed..."

It's all beautiful Victorian language.

"..it was then that I called in the aid of two gentlemen -

"eminent, both of them, in their profession -

"Mr Fairbairn and Mr Hodgkinson.

"They were well qualified to aid me in my research."

For Fairbairn, the amount of "me" and "my" and "I" in that speech

must have been just too much.

And he goes on.

and spent the next year writing a book to set the record straight.

Straightaway, on page one,

he sets out his intention,

"To establish my claim to a considerable portion

"of the merit of the construction of the Conwy and Britannia bridges.

"The various public statements which were made at different times

"by different individuals which either entirely passed over

"or concealed the real nature of the services I had rendered."

The whole thing was a right old ding-dong between the two of them.

Not necessarily what you'd expect from two well-mannered,

proper Victorian gentlemen.

Now, in it, he refers to Hodgkinson, the mathematician he worked with,

This letter here is the first time

that he sets out the results of his tests,

and where he suggests the idea of the cellular tubes

along the top and on the bottom as his idea,

his solution for the Britannia Bridge.

So here we go. Written in Millwall, September the 20th, 1845.

"It is more than probable that the bridge in its full size

"may take something of the following sectional shape."

And there it is.

There is his cellular idea,

with those smaller tubes across the top and the bottom.

Almost 200 years later,

but there's absolutely no doubt Fairbairn was instrumental

to this idea.

But three years later on, on the 17th of May, 1848,

Robert Stephenson made a speech that seemed to make plain

that the idea was his

and that Fairbairn had played little part in it.

And this is a copy of Stephenson's speech here.

Stephenson's words.

"It is now upwards of six years since I entertained the idea

"of constructing bridges with wrought iron plates riveted together."

and was left in charge for the final few weeks of construction,

connecting the rest of the UK to the strategic port at Holyhead

for 120 years.

But on the 23rd of May, 1970, that link was severed,

putting the whole port out of action.

It was 9:43 in the evening when the fire brigade received reports

that Stephenson's great bridge was ablaze.

The fire was destroying one of the most significant innovations

in engineering in over 200 years.

But how could a wrought iron bridge catch fire?

The answer is because of a mistake made by this man, Edwin Clark.

He was one of Stephenson's key engineers on the project

It towered over the Menai Strait,

and it was his bright idea to add a roof.

He added an arched timber roof covered in a tarred hessian,

which he hoped would protect the iron from the weather.

My guess is it was some kind of early roofing felt,

but it perhaps wasn't the cleverest of ideas,

because, unlike iron, this stuff burns really well.

On that fateful night,

a group of local lads entered the tube.

Without any torches they lit some paper,

and the burning embers dropped to the floor.

It's not exactly clear how,

Fairbairn deserves his recognition.

many in the Institution of Civil Engineers

have come to realise Fairbairn's true importance.

In this, the Stephenson Room,

Fairbairn's portrait now hangs opposite the scene

he should be portrayed in.

Now I like to think this helps set the record straight.

Equal status for them both.

We'll never know why Stephenson felt so sure he should take major credit

for the box girder and the cells across the top

that gave the Britannia Bridge its great strength,

but, in my view,

may look solid, but in fact they're hollow.

He ought to be remembered amongst the great Victorian engineers

alongside the likes of Brunel and Stephenson.

For over a hundred years, trains hurtled over their bridge,

keeping the vital link to Ireland open day and night.

But on the 23rd of May, 1970,

something happened that no-one could've imagined.

The bridge caught fire.

Right from the very start, Stephenson's Britannia Bridge

was more than just another bridge crossing.

It was a vital link between London and Dublin.

..it just about takes my weight,

was to rely on the oldest type of bridge in the book -

the simple beam bridge.

It doesn't get much simpler, really.

Two supports and a beam across the top.

As a solution, it would keep the Royal Navy happy because

it'd be the same height above the water

right the way across its length.

But it has a fairly major drawback. It's not very strong.

I'll show you what I mean.

If I stand roughly in the middle here...

Ooh!

The only other option

but only just.

You can see how much that's bending.

If I add another plank, let's see what happens now.

There we go. Instantly much stronger.

But with each plank I put on, it gets heavier.

And that's the problem.

My bridge here is a couple of metres in length

and I'm adding about 80 kilograms on top.

And you can see the amount of wood we need to make it strong.

Scale that up to what Stephenson had planned,

beams to span over 450 metres in length,

They demanded that any bridge left two clear channels,

was the bridge over this treacherous stretch of water.

The obvious solution would be to build a bridge

with two big arches -

one arch from the mainland

to the rocks in the middle of the strait,

then a second arch over to Anglesey.

Stephenson himself came up with a design to do just that.

But there was a problem - the Royal Navy.

Back in the 1840s, the Navy ruled the waves...

..and they weren't going to have a railway bridge

stopping them from ruling the Menai Strait.

taking weights of hundreds of tonnes at a time.

each over 400 feet wide and a whopping 100 feet high

to allow tall sailing ships to pass through unhindered.

You might think that Stephenson

could have simply copied Telford's suspension bridge,

which lies about a mile to the east

and built almost a quarter of a century before.

But that's a road bridge. This needed to be a rail bridge.

Suspension bridges and trains simply do not mix.

The massive weight of the train can cause it to flex and sway so much

that it becomes a real danger the train itself could derail.

So the suspension bridge was out.

let's see what happens.

you can see it's actually hollow through the middle.

And it's that hole all the way through

that gives the plant its strength and rigidity.

And it's this clever trick of nature that the Britannia Bridge exploited.

This is a solid aluminium rod.

This is aluminium, it's exactly the same length,

but it's a hollowed out tube.

Now, they both contain the exact same amount of material,

they weigh exactly the same,

so you might expect they'd have the same strength.

But if I add on equal weights in the middle,

Now if I cut open the stem of this plant,

Look at that.

Look at that bend under the weight of that

almost-full bucket of water.

Right, watch this.

I'll now add on exactly the same weight to the tube.

Something quite remarkable happens.

The tube hardly bends at all compared to the solid rod.

And it was this phenomenon that was the key to designing the bridge.

Rather than a simple solid beam,

the whole bridge would become a massive tube.

Three towers would support two 140 metre-long tubes

wasn't supported by these huge arches we see today,

The sheer volume, and thus weight of material required,

just made it a nonstarter.

The solution he came up with wasn't just clever,

it would change the world.

And it was a solution borrowed from nature.

At 460 metres long and around 40 metres high,

nowadays a bridge like the Britannia here

is nothing special really,

but around 170 years ago

it pushed engineering to its absolute limits.

Back then, this great span

Possibly the hardest part of the entire 260-mile route to construct

because the Navy had demanded nothing should obstruct

the masts of their tall ships.

Instead, the entire bridge was to be formed from great beams

which ran as a straight line right across the Menai Strait.

As a concept, it sounds simple,

but the entire span of the bridge would need to be 460 metres long,

and it would need to support trains weighing hundreds of tonnes.

No-one had done anything like this before.

Luckily, the solution to the problem was right in front of them.

They just needed to know where to look.

constructed using a straight wrought iron beam

had a span of over 460 metres and weighed more than 4,500 tonnes.

It was built to cross the Menai Strait,

a dangerous tidal channel between North Wales

and the island of Anglesey -

a stretch of water so hazardous

that even Admiral Nelson was said to fear it.

Sadly, much of what you see today is a rebuild.

A lot of the original bridge was lost to a devastating fire.

The first Britannia Bridge didn't have these magnificent arches.

In fact, it didn't even have a road.

It was just a rail bridge,

Opened in 1850, this game-changing wrought iron bridge

supported by three great towers.

And it's that beam which made the Britannia Bridge so special.

But why?

Well, it's the way it was built,

using this unlikely-looking lump of iron - the box girder.

Unglamorous as it may appear,

this small piece of what was once a giant, long tube

is one of the most important advances in engineering

in the last 200 years.

It changed the modern world in ways that Robert Stephenson,

the designer of the bridge, could never have dreamed of.

Stopping traffic.

spanning our most dramatic landscapes,

have not only linked our island, but made it great.

These are the bridges that are known around the world,

built by visionaries like Stephenson and Brunel,

who are famous even today.

Look at this!

From the banks of the Tyne to the mighty Thames,

from the Firth of Forth to the Menai Strait...

..I'm on a journey to discover how those great bridges were built...

Here we go.

..and the sweat and sacrifice that went into their constructions.

Your car, your TV, the sofa you're sitting on,

I'll uncover the huge egos, flawed geniuses and jealous rivalries

behind their creation.

It's as if he'd been airbrushed from the whole story.

These are Britain's Greatest Bridges.

Britain's great bridges changed our country,

shrinking distances and boosting trade.

But there's one bridge that not only changed Britain,

it changed the world,

and it's a bridge that you may never have heard of.

This one - Robert Stephenson's Britannia Bridge.

which back in the 1840s

not just to the history of engineering

but to the history of the modern world?

Well, the answer is twofold - the Royal Navy and politics.

In 1845, Robert Stephenson,

son of the railway engineer George Stephenson,

was employed to build the Chester to Holyhead railway.

Part of the job was to connect the island of Anglesey

to the Welsh mainland.

But it's what lay to the west of Anglesey

that made the bridge so important.

Beyond Anglesey is the Irish Sea and then Ireland itself,

ended up with a bridge that's so incredibly important,

was a turbulent part of the British Empire.

The need for a fast connection

wasn't so that tourists could paddle on the beaches of Anglesey.

It was to speed up the vital strategic route

between London and Dublin,

which prior to the Britannia rail bridge

had to stick to the slow routes via Thomas Telford's suspension bridge.

But with the Industrial Revolution taking hold,

faster railways were expanding across the width and breadth of the UK.

But to reach the port at Holyhead,

they needed to cross the dreaded Menai Strait.

And call me a geek, but as an engineer,

perhaps even your clothes,

there's a good chance they were transported

across the globe in a giant floating box girder...

..because it was this innovation, pioneered on the Britannia Bridge,

that helped us to build the huge cargo ships

that crisscross the world's oceans, in effect shrinking the planet.

Today, box girder technology is everywhere,

making even the most unexpected structures a reality, like this.

The Infinity Bridge here in Stockton-on-Tees.

Just look at it. It's fantastic, isn't it?

It kind of flows and skips across the water.

40 metres in the air, each weighing 1,500 tonnes.

I love a structure like this because it does its job.

It's robust and it gets people from one side of the river to the other,

but it does it with grace and style.

Visually, it's about as far as you can get

from the Britannia tubular bridge,

but at its heart is the very same simple box girder.

It's smaller and more crafted,

but it still relies on the same hollow tube technology

pioneered by Stephenson almost 170 years ago.

So how was it that a far off corner of Wales

The results were always disappointing.

Together, their task was to experiment

with different designs of tubes

until they discovered ones strong enough to span the strait.

I say together,

but Stephenson was actually away a lot,

doing other gigantic projects across Europe.

Still, the other two got stuck in.

Taking their inspiration from nature,

the team started with cylindrical and elliptical tubes,

building lengths almost 10 metres long and loading them with weights

until they broke.

was a mathematician by the name of Eaton Hodgkinson.

Try as they might, their oval beams were never going to span the straits.

But then they switched to oblongs,

which is when Fairbairn had the idea that would change the world.

And this is it.

Rather than just one big tube, and it is enormous,

Fairbairn suggesting using multiple cells.

These smaller tubes

across the bottom and the top

sandwiching the main central tube,

and the effects of this relatively small change were incredible.

Suddenly they were experimenting

Iron was Fairbairn's forte.

But there was an even bigger problem.

Up until now, the longest wrought iron span was under 10 metres,

more than 14 times shorter than the 140 metres

Stephenson was proposing.

Stephenson was in unchartered waters,

using a new technique and a type of iron

never used before on this scale,

so to help him he brought in two of the unsung heroes

of Victorian engineering -

a move that would result in a bitter feud.

The first was William Fairbairn.

with hollow tubular beams 20 metres in length

His background was in shipbuilding,

and he had a works on this site here in Millwall

near the River Thames in London.

If anyone could find out the best way

to make gigantic tubes out of wrought iron, it was him.

Fairbairn stood out from his peers

because he was one of the first engineers

to actually try and understand and analyse why structures failed,

which, believe it or not, was unheard of at the time.

The second member of the Britannia team

Back in 1849, Stephenson's Britannia Bridge

navigate the world's oceans.

Some are as long as the Britannia Bridge itself.

And it was only possible because of the humble box girder,

designed to cross a small strait in North Wales.

But coming up with the means to cross the Menai Strait

was only part of the solution.

Designing a beam that could span these great distances was one thing.

Building it across this deadly stretch of water would be far harder,

and in the end, it would result

in one of the biggest bust-ups in engineering history,

denying William Fairbairn the limelight he so deserved.

Today, thousands of massive cargo ships and oil tankers

was one of the biggest civil engineering projects

in the world.

Huge stone towers holding up

a 460-metre-long hollow beam of wrought iron

across one of the most dangerous stretches of water

in the British Isles.

One of the truly brilliant features of the Britannia Bridge

was that Stephenson and his colleagues found a way

of making that beam incredibly strong but also incredibly light.

But that weight-saving concept didn't end there.

The central towers and huge stone structures at each end

those tubes along the bottom and on the top -

that could support around 80 tonnes in weight.

It was a giant leap forward,

but there's a big difference between a 20-metre prototype

and a 460-metre bridge.

This is just a small piece of the full-size beam

they ended up making.

You can just imagine it stretching right out across the Menai Strait.

It was revolutionary.

Of course, modern bridges look nothing like this

great lump of iron,

but it was Fairbairn's simple innovation,

making it possible for the first time to use wrought iron on a large scale.

a key to so many bridges around the world.

And it would prove to be even more important than that.

In the 1840s,

the longest ships in the world were only around 80 metres.

Any longer and they tended to snap in the middle in heavy seas.

But the box girder helped change all that.

It gave us the ability to construct a rigid beam hundreds of metres long,

and it's that that's allowed us to build the huge ships

that have shrunk the world.

And by huge, I mean HUGE.

And that's exactly what happens on the cast iron as well.

Just watch what happens when I give this bar here

even a mild tap with the hammer.

(LAUGHS) Look at that.

That's gone right through.

I can demonstrate what happens using this twig.

Now, if I bend it down in the middle,

the bottom surface is being pulled, it's being stretched.

It's what's called coming under tension.

And it stretches and stretches until eventually it cracks

and then breaks all the way through.

Here we go.

It's actually extremely brittle.

This deadly characteristic

was to result in the deaths of five people,

and almost stopped the Britannia Bridge in its tracks.

On the 24th of May, 1847,

a year after construction had started on the Britannia,

the bridge over the River Dee, another Stephenson design, collapsed,

causing a train to derail and crash into the water below.

The accident almost cost Stephenson his career

when he was hauled before the court on a negligence charge.

He was eventually cleared, as the collapse was blamed

on the insufficient strength of the girders, not his design.

In many ways, this bridge kicked off the Industrial Revolution.

Two smaller spans of 70 metres

would then sit between the outside towers

and the abutments on the banks.

When completed, the tube would be large enough for trains

to run inside them, across the entire 460 metre span.

Whatever that tube was made from, it had to be very, very strong.

Today, we take metal as a construction material for granted,

but in Stephenson's day it was still something of a novelty.

This is the famous Iron Bridge near Telford.

Built in 1777, it was the first really large construction

made from cast iron.

But the disaster made people realise that cast iron

Beforehand, cast iron was mainly used for small things, like pots and pans

and knives and hinges, but this bridge changed that.

Suddenly it announced to the world

that cast iron was the building material of the future.

Compared to wood or stone,

you get a lot more strength per pound of weight,

but iron's main advantage over its traditional rivals

was that you could cast it into any shape you like.

It really was revolutionary.

But as strong as it is, it does have a fundamental weakness.

Let's see what we can do here.

crystalline structure that cast iron has.

In fact, it's almost got a grain to it.

It's got a lot more structure, and, as such,

it behaves very differently to its cousin, cast iron.

Let's give it the same test here with the hammer.

(CLANGING)

Oh! Yeah, that was very different.

The sound's different, the feel's different in the hammer.

Now I'm really giving that some wellie,

and I've put a bit of a kink in it.

I mean, if I really want to go for it, I can pop it in the vice here.

It's got virtually no carbon in it and it's lost that gritty,

Alright, if I really try and bend this...

..let's see if I can actually break it.

(GRUNTS)

It's bending.

You can see I'm putting a lot of effort into that.

This behaves extremely differently to cast iron.

Of course, when you're building a bridge,

the sheer volume of wrought iron needed to be vast.

But, thankfully, this was the Industrial Revolution,

and there was access to massive steam hammers

and rolling mills to work the iron,

yet crucially without reducing its strength.

wasn't the wonder material they'd thought.

But there is a way of stopping cast iron from shattering,

and that's to hit it hard - turning cast iron into wrought iron.

Traditional blacksmiths Duncan and Jack

have set me up with a furnace,

a hammer, an anvil, and a white-hot piece of cast iron -

everything I need to show how wrought iron was made.

Cor, look at that. That's a beaut. MAN: Give it a brush.

Heating cast iron, then hitting and rolling it,

realigns the internal structure of the metal,

making something that's rigid and brittle more flexible

ROB BELL: Britain's iconic bridges,

It also helps remove impurities like carbon and sulphur

that weaken its structure.

So, when you see a blacksmith hammering away

on the anvil here in the workshop,

it's not just about shaping the material.

It's giving it strength, adding structure.

At the end of this incredibly intensive process,

you end up with a piece of iron with very different properties.

Now, this is a piece of wrought iron.

It's been through that hammering and rolling process.