The Assassin (Sha ren zhe Tang Zhan)

In this episode...

The view up here is absolutely insane.

...the world's highest bridge...

...and the groundbreaking innovations from the past...

I'm pretty excited to be here.

This is a really special piece of engineering history

and not many people get a chance to come here.

...that made the impossible possible.

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Guizhou valley in China.

At 1,854 feet below the ground, this massive crack in the earth

has separated the local community for centuries.

New York-based architect Wendy Fok

has traveled to this remote region

to see how extraordinary engineering

is pushing the boundaries of what's possible.

Canyons are pretty dramatic.

It's beautiful around here.

She's attempting the dangerous journey

across the valley.

It's kind of crazy ride.

Roads are really bumpy,

and the only way to get from one side of the county

to the other is through this road.

It's very frustrating because it's a 5-hour drive.

The long trip down the steep slope

and up the other side is treacherous.

It's dangerous... the road...

because there's a lot of sharp rocks,

lots of steep gorges.

You also see a few landslides,

so piles of rocks on the side of the road.

So drivers have to be very careful.

This natural divide has a devastating effect

on the community.

There's 35.8 million people in this region of Guizhou.

With all these windy roads and steep valleys,

it's very difficult for the local farmers

to get the goods out of this county to sell them.

So as you can imagine, it is one of the poorest regions in China.

A solution is desperately needed.

But with a nearly half-mile drop to the valley floor,

nobody has ever bridged a gorge this deep.

The solution engineers in China have come up with

is breathtaking.

This is the Beipanjiang first bridge.

With a deck nearly 2,000 feet above ground,

it's the highest bridge in the world.

It is remarkable how high this bridge is

and how long it extends.

And at almost a mile long,

it's one of the longest cable-stayed bridges

on the planet.

Liu Bo is deputy chief engineer of the bridge.

Yeah. Yeah.

It's normally off-limits to pedestrians,

but he's giving two colleagues a unique tour.

The bridge is made up of a pair of massive concrete towers,

the tallest reaching 883 feet high.

The 22,000-ton steel bridge deck is the length of five Titanics.

It's so high, one world trade center

in New York could fit underneath.

The deck is attached to the towers

with 250 miles of cables,

enough to stretch from New York city to Washington, D.C.

But to create this unprecedented structure,

the team need to solve many tough engineering challenges.

How do you construct

terrifyingly high concrete towers?

How do you assemble a super-long bridge deck

in such a dangerous environment?

The valleys here are so deep, you can't even see the bottom.

This is too high to build

temporary scaffolding for a bridge deck.

And what type of bridge do you build

on vertical cliffs

full of hidden caves and crumbling rock?

Before engineers could even get started,

they would have to find a way to work

with the area's deadly geology.

The rocks around this

Also, it's very steep.

To make things even more difficult,

there are a lot of hidden caves and cracks along the mountain.

This poses a huge challenge

for the engineers to design the bridge.

Most types of long-span bridges

need support anchors built into the rocks.

For an arch bridge, weight pushes down

and the bridge's curved shape moves the force sideways.

So they need gigantic anchors in the banks.

On a suspension bridge, the deck hangs from two cables,

which also need huge anchors

to keep the bridge from collapsing.

But the soft, crumbly, landslide-prone rock

in this region isn't suitable to hold massive bridge anchors.

Liu Bo is on-site surveying the rock.

The only option is to find a bridge design

that doesn't need the support of anchors,

which means the team will have to draw inspiration

from the pioneers of the past.

This is amazing.

A tour of London on a beautiful, sunny morning,

on a boat, on the river Thames.

Physicist Andrew Steele is exploring London

from a unique perspective.

Look at this. Absolutely beautiful structure.

It's the iconic tower bridge.

From down here, this structure just looks absolutely enormous.

There's an incredible range of bridges

in the British capital,

which could help inspire the team in China.

This is Blackfriars bridge.

It's an arch bridge. It was constructed in 1869.

And from underneath,

you can see these beautiful wrought-iron ribs,

which are holding the whole structure up.

Here, we have Chelsea bridge, a suspension bridge.

You can see that big red cable running along the top,

that's the main cable.

Among these famous giants,

there's a lesser-known bridge

that's actually one of the most important in the world.

This is a piece of engineering history,

an entirely new type of bridge

that people didn't think at the time could even be constructed.

Built in 1873, this is Albert bridge.

From an engineering point of view,

there is an awful lot going on here.

You can see we've got these support columns,

we've got the curved cables, the straight cables.

Pretty daring for the time.

Albert bridge is the brainchild

of British engineer Rowland Mason Ordish,

who was determined to build the impossible.

The Victorians had managed to build loads

of different kinds of bridges,

but there was one particularly desirable design

that remained elusive.

It's called the cable-stayed bridge,

and the idea actually dates from centuries before.

It was in the 1600s that the first cable-stayed designs

were proposed, even before the existence of actual cables.

The design of a cable-stayed bridge

means all the weight is carried up through the cables

and down through the towers,

so it doesn't need anchors on the banks.

But in the 19th century,

building a cable-stayed bridge had never been successful,

because the forces are so complex.

The problem was, the maths was just too hard.

Imagine trying to calculate all the forces

on one of these cable-stayed bridges.

You've got loads of different cables, all at different angles,

pulling on the bridge, pulling on each other.

It's a trigonometry nightmare.

Over-tensioning even a single cable

could lead to a catastrophic failure.

For 200 years, people thought the cable-stayed bridge

was never going to be built.

Ordish knew the risks

of building a cable-stayed bridge.

So to ensure Albert bridge didn't fail,

he ingeniously combined it with a suspension bridge.

From up here you can really see

what's going on with this bridge.

Since we're in the middle, this is the main cable,

the suspension bridge aspect of this structure.

And we've then got these smaller suspenders,

and what these do is connect the main cable

to the span of the bridge,

allowing it to support the bridge

on multiple points along its length.

And then finally, this is what makes this bridge so special.

Here, we find the cable-stays,

and it's really clear from this angle what they do.

They're taking some of the load from this span,

and then transferring the force up into those towers there.

This is two bridges in one,

and that is the genius of Ordish's revolutionary design.

Albert bridge was a major step towards building

a pure cable-stayed bridge that didn't need anchors,

exactly what the engineers in China are looking for.

And today, cable-stayed bridges are commonplace.

So when you see a modern cable-stay bridge,

it's all thanks to pioneers like Ordish who, over 150 years ago,

showed this elegant and deceptively complex design

could be done.

In China, engineers are building

on Ordish's groundbreaking work

and supersizing it for the 21st century.

The Guizhou and Yunnan regions of China,

separated for centuries by this cavernous abyss, until now.

This is the Beipanjiang first bridge.

The bridge stretches almost a mile across the ravine.

The tips of the towers reach 2,461 feet

above the valley floor, higher than two Eiffel towers

and the statue of Liberty combined.

And it's the first-ever cable-stay crossing

to hold the title of world's highest bridge.