All language subtitles for Impossible Engineering s07e07 Worlds Highest Bridge.eng

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.

captions paid for by
discovery communications

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
region are very soft.

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.

Architect Wendy Fok
has special access

to this engineering marvel.

This bridge is amazing.
This is so exciting.

I'm going to actually
go and climb over this

and take a closer look
at the cable-stay myself.

The view up here
is absolutely insane.

It's actually quite amazing
that this bridge is held up

by the cables
and by these amazing towers

that are on the left
and the right of us.

So there are no anchors
on this bridge.

The only way to build
this bridge in this region

is to have a cable-stay design.

The load
from the 22,000-ton deck

and everything on it

is carried through
224 cables into the towers

and down into the foundations,
removing the need for anchors.

But attaching each cable was
an extensive job for engineers.

It was actually a lot of smaller
cables on the inside

that's bundled up,

that is then connected on to
the anchor point on the bottom.

First, one of the cable strands

is pushed through
a waterproof casing.

It's then attached
to one of the towers

and joined to the bridge deck.

More strands are then
threaded through the casing.

Up to 43 make up
one cable bundle,

the longest of which
is 1,253 feet.

Each bundle is tightened into
a carefully calculated strain.

Getting the cables
and the tension

right in this bridge is crucial.

So inside of each of
these cables is a gauge

that measures the constant
tension on this bridge.

If any
of the strand cables fail,

engineers in a control center

can immediately spot
and replace them.

The marvelous aspect
about this bridge is that

even if you were
to remove one cable,

the bridge is still stable
and that's pretty incredible.

But winter in Guizhou
brings a new onset of problems.

It is one of the most severely
frozen areas in western China,

and because of the bridge's
altitude,

ice could form on the cables.

It's a potentially
deadly problem

that Liu Bo
and his team have to solve.

With the risk of falling ice

causing a potentially fatal
traffic accident,

engineers came up with
a brilliantly simple solution.

Yeah.

Moving the cables to the edges
of the bridge will force ice

to fall safely into the valley

and keep this lifeline
flowing all winter long.

But engineers have even
greater challenges ahead.

In order to build the tower,

the engineers had to think about
getting concrete

all the way up to the top.

It's just mind-boggling.

Guizhou valley, China,

known as the crack in the earth.

To cross it, engineers
are taking on the impossible...

...with the world's
highest bridge.

The Beipanjiang first bridge is
a crucial crossing for millions.

At almost a mile long

and with a staggering
2,362-foot main span,

this is a cable-stay bridge
on an epic scale.

But designing a structure
of this magnitude presents

a terrifying set of challenges.

Architect Wendy Fok
has traveled from New York

to check out
this engineering marvel.

This bridge is incredibly long,
and it's really rare

for a cable-stayed bridge
to be this long.

Devising a super long
cable-stayed bridge

is extremely complex.

Everything is connected.

If one thing changes,

everything else is affected,
too.

Imagine my hands are the cables.

And as the span of the bridge
gets longer,

the angle of the cables
get flatter and that's bad

because it puts the deck
under compression.

There's an optimal angle
for these cables,

which are 30 to 40 degrees.

And in order to build
a bridge this long,

you need a very tall tower.

Basically the longer the bridge,
the taller the tower.

This exceptionally long bridge

needs exceptionally high towers.

In order to build the tower,

the engineers had to think about

getting concrete
all the way up to the top.

It's just mind-boggling.

So how do you build
soaring concrete towers?

The answer lies with
a pioneering innovation

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.

Physicist Suzie Sheehy
is in Hampshire

in the south of England.

330 steps.
I don't even think I'm halfway.

She's getting an exclusive look

at a little-known
but vital piece

of engineering history.

In the 1860s and '70s,
there was a huge fascination

with building stuff
with concrete.

People would build sculptures
and other objects,

but no one had yet dared
to build something as audacious

as a tower out of concrete.

They all thought it would
collapse under its own weight.

But one man was determined

to prove everyone wrong.

Aha!

Finally made it to the top.

This is sway tower.

Wow! What a view.

Topping out at over 200 feet,

when it was built in 1885,

it was the tallest concrete
structure in the world.

I can see the whole forest.
And I can see the sea.

It's incredible!

You can just see for miles
in every direction framed

by these beautiful,
beautiful windows.

Definitely worth the climb.

Sway tower was built
by Andrew Peterson,

a retired judge with
a passion for architecture.

What's incredible
is that all of this

was built by Peterson,

who was just
an amateur enthusiast

and yet he made this huge
impact in civil engineering.

Just like the engineers

at the Beipanjiang bridge
in China,

Peterson needed to build
tall concrete towers.

The ingenious method he came up
with would change the world.

So how do you make
a concrete tower?

Well, obviously concrete
and a mold to form it in

and the concrete
then gets pressed into the mold.

The next step
to build my tower up

is to get another mold
and keep going.

That's looking pretty good.

So I'm going to add a third one.

But in the 1800s,

the molds
were the most expensive part

of building with concrete.

Peterson came up with
a brilliantly simple solution

to use fewer molds
but still build high.

By the time I've poured
in the top two molds,

the bottom one has set.

So then I can remove
the bottom mold

and place it back
on the top and keep going.

And then you could
just keep doing this,

moving the different molds up

and the tower would grow floor
by floor.

And the only limit was
how tall you dared to go.

It's called climbing formwork.

Yes, there we go.

And it's still the go-to method

for building
tall concrete towers.

Oop. Oh, goodness.

There we go.

I think if Peterson saw this,
he'd think I need

a little more practice
before I build a real one.

Incredibly, Peterson
used just three molds

to build
this entire concrete tower.

It's still the tallest
non-reinforced

concrete structure in the world.

What's incredible
is you can still see the lifts

where each of the layers
of concrete was poured

and it would have taken two days
to fill each one

and then two days
to move the bottom

one up to the next layer
and it took six years to build.

Peterson proved the world wrong.

He showed us 130 years ago

that we could build a tall tower
out of concrete

and that paved the way for us

using concrete
in structures today.

Now engineers in China

are using Peterson's

revolutionary
construction method

and taking it to the next level.

The Beipanjiang first
bridge in China

boasts two massive towers,

the tallest reaching 883 feet...

...engineering mega structures
in their own right.

This thing is colossal.

You can see from top to bottom,
it's just so big.

But before these
towers could be built,

engineers first had to make more
than 130,000 tons of concrete.

Zhou Ping, director
of the Beipanjiang bridge,

is tackling this issue.

The answer was to use
the difficult environment

to their advantage.

Crushing the soft rock to
make sand is an ingenious idea,

but it also has
an added benefit.

Finally, construction
of the towers can start.

Thousands of steel bars
are formed into a grid

to reinforce the concrete
once it's poured.

Just like in England's
sway tower,

climbing formwork is the key
to raising these super towers.

In order to build the tower,

the engineers had to design
a mold.

And then they've added
concrete into the mold

and then lifted it up manually,

so that they went up
section by section.

Assembling the molds manually

enables precise control.

So engineers can form
the demanding h-shape design,

but as the towers grow,
so did the challenges.

Initially, concrete is lifted
in enormous vats

by huge tower cranes,

but at greater heights,
a pump is needed.

A high-powered,
gravity-defying pump is used

to propel the thick concrete
to a height of almost 900 feet.

These towers seem to be
going on forever.

You can't even see
the top of it.

It is quite amazing here.

More than 12 million
gallons of concrete

were used to make
this pair of colossal towers.

And you can still see the lines

that are left behind
from the mold

all the way to the top
and that's pretty cool.

These exceptionally tall towers

ensure the cables
are at the optimum angle

to take the weight
of the massive bridge deck.

Because the bridge needs
to be so long,

these towers need to be so high

and the engineers
definitely did that.

But to bridge this mighty chasm,

engineers will face
their greatest obstacle yet.

It's impossible to build
a scaffolding to reach up here.

Guizhou,

the most poverty-stricken region
in China.

This impassible valley
restricts development.

The solution...
the Beipanjiang first bridge,

the highest on the planet.

At 883 feet, the largest tower
is taller

than Rockefeller center
in New York.

The two towers hold 224 cables

with a combined length
of 250 miles,

five times longer than
the Panama canal.

Engineers are now facing
the most dangerous phase

of the project...
constructing the bridge deck.

It's a gorgeous view.

Super deep.

New York-based
architect Wendy Fok

is at one of the project's
highest points.

It's so difficult to even see
to the bottom of this valley.

It's impossible to build
a scaffolding to reach up here,

to build a bridge deck,

so the engineers
had to find another way.

Now engineers need to support

the nearly mile-long bridge deck

while it's being built
without scaffolding...

a feat that might
not be possible

if it weren't for
the innovators of the past.

Engineer Luke Bisby is
at the firth of Forth river,

near Edinburgh in Scotland,
to see a piece of engineering

that could help out
the team in China.

In the early 1800s, engineers
were in a desperate race

to build a rail line
between London and Aberdeen

along the shortest route,
and this meant building a track

along the east coast
of Scotland.

But this huge estuary,
the firth of Forth,

was standing in the way
and presented a major obstacle.

The typical method
of building bridges at the time

was to use temporary scaffolding
to hold up the span

while it was constructed.

Once complete,
the scaffolding was removed,

leaving the finished bridge.

Here, it's completely
impractical to build

a bridge with a scaffold.

Not only is the estuary very
wide, but it's also very deep.

And even if you could get your
scaffolding poles down that far,

the bottom is completely
full of mud and silt,

and so they wouldn't hold.

Here, it would take
an incredibly bold engineer

to build the bridge
without a scaffold

and have the enormous
span lengths required,

which had never been
attempted before.

British engineer Benjamin Baker

risked his reputation
when he took on

this impossible challenge.

I really can't believe
I get to do this.

Normally, only engineers
who work here get to be here.

- Morning.
- Morning.

Baker's solution

is one of
the most astonishing pieces

of Victorian engineering
anywhere in the world.

Now, this is just incredible.

This is the forth bridge...

Now, that is a view.

...a 1.5-mile-long
cantilever design,

the longest bridge in the world
when it was built.

That is amazing.

Engineer's bucket list,
this one.

Constructing such
a long bridge here

was not going to be easy.

However, baker had an idea

that might help the engineers
at the Beipanjiang first bridge.

Cantilever bridges
are all about balance,

with the weight
of the steel work on one side

of the tower balanced out
by the steel work on the other.

Baker also used
that counterbalancing act

to construct the bridge.

The towers were
constructed first

and then the bridge
was built out

symmetrically on either side.

Every time a beam was added
to one side of the bridge,

an identical beam was added
to the other side of the bridge.

And in this way the bridge
became the scaffolding

for its own construction,
and it seemed to defy gravity.

It was an incredible solution,

and Baker's risk paid off.

The Forth bridge opened in 1890.

It now carries 200 trains a day
and 3 million passengers a year.

130 years ago, Baker's
ingenious balancing act

changed the way
that bridges are constructed.

Engineers were no longer reliant
on scaffolding

beneath bridges
during their construction.

At the Beipanjiang
first bridge in China,

engineers have taken Baker's
simple yet brilliant idea

to jaw-dropping new heights.

Like the Forth bridge,

they've constructed the deck
without scaffolding

by balancing it
around the towers using cables.

In order to build a bridge
without temporary scaffolding,

let me show you
what they did here.

The approach span
was already here,

so in order to add a new section
over the gap,

I'm adding
a cable to hold it up.

To keep the forces balanced,

I now need to add a cable
on the other side of the tower.

Adding the cables
symmetrically keeps the forces

around the towers balanced
and supports the deck.

Now I'm adding another section
over the gap,

and every time I do so,

I have to add a cable
to hold it up,

and another one
on the other side of the tower

to keep it balanced.

I add another section, I add
another cable on the front,

and I add another one
in the back.

And there you go...

a perfectly built
cable-stayed bridge.

During construction,
instead of building beam-by-beam

over the deadly drop,

the main span
is built in sections.

Then, for the first time ever,

the deck sections are slid
underneath the growing span.

They're lifted into place...

...then fixed
to the end of the deck.

Construction was twice
as fast as traditional,

beam-by-beam building.

After only 42 months,
the two sides

of this vast valley
were finally connected.

It seems like I'm walking
forever on this bridge.

Can't believe how long
this bridge is.

What amazing
piece of engineering.

But before traffic could cross

the massive main span,

the team of engineers
had one last problem to solve.

After 42 months of construction,

the Beipanjiang first bridge

finally connected the two sides
of the massive Guizhou valley.

Now engineers needed to ensure
the bridge was safe enough

for traffic to cross.

When heavy vehicles
cross the bridge,

if it's not stiff enough,
the deck could sag.

If this happens repeatedly,
it will cause fatigue,

which makes the metal
very brittle,

risking a sudden collapse.

Stopping this huge deck from
flexing is a serious challenge.

The solution...

an orthotropic deck.

The card isn't stiff enough.

The two pieces of card
are still used,

but this time the bottom one
will be shaped differently.

Oh.

Now that the bridge is stiffer,

it doesn't flex
when it takes heavy loads...

...solving the problem
of fatigue.

The orthotropic deck

was installed
in 34-ton sections.

The finished span is so stiff,
it hardly flexes,

even in the middle.

To see it, Liu Bo is taking
his colleagues to an area

off-limits to the public...
inside the bridge deck itself.

The orthotropic deck is
the final piece of the puzzle

for this world
record-breaking bridge.

The Beipanjiang first bridge
is taking engineering

to breathtaking new heights.

I can't imagine how this bridge
could be built,

if it weren't for
modern-day technology.

Spanning
the mighty Guizhou valley,

this astonishing bridge

is engineering
on a legendary scale.

Almost a mile long,
with two landmark towers

reaching more than 2,400 feet
above the river,

it has 250 miles of steel cables
to hold up a 22,000-ton deck.

It's an epic structure,
unlike any other.

The Beipanjiang first bridge
is cutting-edge engineering

on a staggering scale,

finally joining two regions

that have been separated
for centuries.

We're now doing the same
journey again,

but instead of five hours,

we're only taking one hour
to cross this region.

This is an absolutely amazing
bridge, just really elegant.

The view here is breathtaking.

By learning from
the pioneers of the past

and overcoming
terrifying challenges,

engineers have pushed
the boundaries of innovation.

When I look at the bridge,
I'm very proud of it.

To me, it's more like a child.

It's grown up, and now it can
contribute to the society.

And they've succeeded

in making
the impossible possible.

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