All language subtitles for Ancient.Engineering.S02E10.1080p.AMZN.WEB-DL.DDP2.0.H264-chr00t_track3_[eng]

(dramatic music)

- [Narrator] The world's
greatest structures

push the boundaries
of engineering

all fueled by a constant
desire to innovate.

- Without engineering, there
will be no modern world.

- [Narrator] Gigantic buildings,
complex infrastructure

and ingenious inventions.

- Engineering is the key that
turns dreams into reality.

- [Narrator] Many of today's
incredible achievements

rely on breakthrough
technologies

first devised by
ancient engineers.

- It's astounding how
they achieved this.

- [Narrator] Early civilizations

built on an unimaginable scale

and with incredible precision.

- They raised the
bar for engineering

in a way that no one
thought possible.

- These are some of the
finest engineers in history.

- [Narrator] Redefining
the known laws of physics

and dreaming up the impossible.

They constructed engineering
wonders from colossal stadiums

to mighty waterways
and complex machines,

all with the simplest of tools.

- You cannot imagine the
skills people would've needed

to build like this.

- [Narrator] By
unearthing the mysteries

left by these ancient engineers,

we can now decode their secrets.

- That so many of their
creations still survive

is testament to their
engineering prowess.

- [Narrator] And ultimately
reveal how their genius

laid the foundations for
everything we build today.

(dramatic music)

Of all the structures
on the planet today,

few are as awe
inspiring as bridges.

- I think people relate to
them. They do something to us.

They talk about human ingenuity.

- They're almost
impossible things.

They span enormous
natural features.

They get lost in the mist.

- [Narrator] With the longest
over 100 miles in length

and some reaching heights
of more than 1000 feet

spanning rivers, seas
and even continents.

They are the structural
giants of the modern world.

- Many of the world's great
cities have founded by water

and very often, bridges
are the grandest structures

of those cities.

- Bridges are so important
cause they actually capture

the scale and the skill
at any point in history.

- [Narrator] In the modern age,

bridges have had to bear ever
increasing loads of traffic

and remain essential for travel,
communication and commerce.

- Bridges are our critical
connectors of society.

They really are the lifeblood
of our transport system.

- [Narrator] But they are
more than just conduits

of our global economy.

When it comes to human
creativity and ingenuity,

little can compare with
the inspired variation

that goes into building bridges.

From the Helix
Bridge in Singapore

which mirrors our body's DNA

to Vietnam's Golden Bridge
held up by faux stone hands

or the distinctive configuration

of the gates at
Millennium Bridge

which spans the river
Thyme in England.

- It's unique in the
way that it moves

which is really unusual
and it's terribly clever.

- [Narrator] But innovation in
bridge design and engineering

goes as far back as our
first human ancestors.

(light music)

Ever since early humans
first walked the Earth,

we've needed to find ways
to span gorges, rivers

or sometimes just the
smallest of obstacles.

- It's entirely possible that
the first bridge builders

were people in the
Neolithic period

because at that point,
they've got stone tools

and with those stone
tools, they can fell trees.

- Those early humans put in
that first log across the river

where if you like the kind
of very first engineers.

They had a problem and
they find a solution.

In a way that was the
birth of engineering.

(light music)

- [Narrator] For
thousands of years,

humans assembled
rudimentary bridges.

But around 4,000 BC,

as farming settlements
became established

across Northern Europe,

communities needed more reliable
methods to cross waterways

or natural barriers.

- As early humans began to
explore their environment,

they were seeking ways to
bend nature to their will.

And one of the
ways of doing that

was to make a quick
route from A to B.

- [Narrator] For the
ancient engineers

of the Somerset Levels
in Southwest England,

the challenge was how to
cross the low marshy peatlands

to reach their agricultural
and foraging grounds.

- This is a wetland landscape
which most of the year

is damp and boggy,

but sometimes is almost
completely covered by water.

So not only is it
difficult to traverse,

to actually build something
in this landscape,

is equally challenging.

- In some ways it's the
most difficult kind of thing

to bridge over

because you've got
nothing to stand on.

You can only build
out from one end

and you can stand on the thing
that you've already built

to build the bit
in front of you.

- [Narrator] The other
key challenge they faced

was how to reliably support
any kind of structure.

- The obvious solution
was to introduce some form

of vertical pile.

But the problem is that
piles will sink into the mud.

So another solution
had to be found.

- [Narrator] In the 1970s,

archeologists determined exactly
how they solved this issue.

Preserved in the peat
was a raised wooden track

acting as a low bridge
to cross the marsh

which has been
dated to 3,806 BC.

- This track is
over a mile long.

So this is significantly
big bit of infrastructure

that would've taken thought
and planning and organization.

- [Narrator] The result

was a design that would
raise these farmers

above the marsh,

allowing them to cross much
more easily and reliably.

- They came up with a design
which featured crossed stakes

driven into the mud

which supported a series
of wooden platforms.

- By inclining those wooden
pegs into the ground,

it forms a very stiff section

which doesn't settle
down into the bog.

It's a very ingenious solution.

- [Narrator] Through
clever engineering,

Neolithic people had devised
a successful solution

to traverse difficult terrain.

Though technically a trackway,

it displays some of the
essential structural ideas

that would lead to
bridges of the future.

(dramatic music)

From the ancient world
through to the modern day,

bridge engineers have
needed to consider

several key elements

to make their structures viable.

- So bridges have to deal
with the self weight,

the thing itself.

Live load which is the
imposition of traffic

whether it be road,
rail, pedestrians,

it changes, it varies.

It could be nothing. It
could be very intense.

- It's gotta be able to
withstand high winds,

maybe snow loads for example.

These are all very
complex things.

So the engineers
designing the bridges

have to factor that in
to their calculations.

- [Narrator] The first
people to fully understand

the forces at play on a bridge

were masters of mass scale
engineering, the Romans.

- The Romans were
incredible engineers.

They built astonishing things.

They built aqueducts.
They built coliseums.

They built harbors.
They built roads.

There is nothing they
wouldn't try and build.

- [Narrator] They would
take bridge building

to another level,

transform the map of Europe
and make a mark on our planet

that would last to this day.

From 509 BC onwards, the
Romans established a Republic

that would eventually stretch
across Southern Europe,

Africa and some of Asia.

- The Roman Republic
is predicated on
constant expansion.

And if you've got
any designs on power

certainly in the
first century BC,

what you have to do as either
a governor or a general

is to be seen, to be going to
the far reaches of the empire.

- [Narrator] And one of
the most essential pieces

of infrastructure to span the
divide and conquer territory

were bridges.

- Bridges are really
integral to the Roman project

because roads are when those
roads encounter rivers,

they have to go over them.

But they're also symbolic

and they're demonstrating
Rome's power

to reach beyond natural
frontiers or natural barriers.

- [Narrator] One man who was
determined to make a mark

on the Republic

was the most famous
of all Roman generals.

- Around the time the Roman
started looking to the North,

a new figure emerges who proved
to be the greatest warlord

of the Roman Republic

and this is Gaius Julius Caesar.

He had enormous ambitions

and this included
making himself famous

by being the first
Roman to conquer Gaul.

- [Narrator] From 58 BC,

Caesar was engaged
in a war with peoples

occupying a region
of Western Europe.

But to cement his foothold,

he needed to stop its
neighbors from Germania,

from their regular incursions.

- So you now have German tribes

crossing over the River Rhine,

are starting to interfere in
Caesar's conquest of Gaul.

This is going against Caesar's
plan and he doesn't like it.

- [Narrator] But
lying between Caesar

and his desire to quell
the Germanic rebellion,

lay a formidable obstacle that
was 1000 foot wide in places,

the River Rhine.

To transport his army across it,

he needed to build a bridge
and not just any bridge,

one strong enough to
support his 10 Legion Army

of over 40,000 men.

- This is a challenge that is
really going to cement Caesar

as someone who has the
engineering prowess

as well as the military
prowess to take on Germania.

- [Narrator] But the Rhine
was potentially as challenging

an adversary as his enemies.

Undeterred in 55 BC,

Caesar launched his mammoth
construction project.

Archeological evidence is scant,

but Caesar's own words
in his campaign diaries

allow us to paint a vivid
picture of its construction.

100 piles, each one and
a half foot in diameter,

were driven at an oblique angle.

The piles were braced
together in pairs

to create piers
set 40 feet apart

at the top of which
logs could be inserted

to create the
foundation of the deck.

Heavy supporting posts
were tied to the piers

at an acute angle

to act as buttresses
on the downstream side

to help resist the
forces of the river.

Further cross bracing
may have been added

to strengthen the structure.

Planks were then laid to
create the deck and roadway.

- And this technology enabled
Caesar to build this bridge

which could span this
enormous, fast flowing

continental river.

- [Narrator] Remarkable
for a bridge of its size,

it was completed
in just 10 days,

allowing Caesar and his
army to cross into Germania.

But Caesar's next
engineering move

was even more astonishing.

- He completely takes apart
the bridge that he made

because you don't want to
leave an open back door

to the Germanic tribes.

- He marches across,
does what he wants to do,

marches back, pulls
down the bridge

and it's all in a
day's work really.

- [Narrator] Romans may have
built other military bridges,

but for sheer
engineering audacity,

nothing matches
the Rhine Bridge.

- This is Caesar reminding
people back in Rome

that he was still on the scene.

He could do really what
he wanted with his troops.

Where Roman wants
to go, it will go

is a message of this bridge.

- [Narrator] Caesar had
perfected speed engineering.

And in the mid 20th century,

a man who tinkered with
model bridges as a hobby

would turbocharge this concept

to help change the
course of history.

(bomb explodes)

(dramatic music)

At the outbreak of
World War II in 1939,

Donald Bailey, a
British civil servant,

devised an idea on the
back of an envelope

for a military bridge.

So ingenious it is
credited with contributing

to help end the conflict
six years later.

- The Bailey bridges were
an essential component

in keeping the Allies moving

and the German and Italian
forces were systematically

blowing up bridges
as they retreated,

And what the Allies
needed was a solution.

- [Narrator] But bridge
technology at the time

wasn't fit for purpose.

- The engineering
elements of the armies

did have bridging solutions,

but they're often ready
made quite heavy units

that had to be crane into place.

- The other issue they
were confronted with

is that many of the
bridges they were using

couldn't withstand the weight

of some of the heavier
equipment, especially tanks.

- [Narrator] Enter
the Bailey bridge,

a sectional pre-fabricated
bridge that was
easy to transport

and could be erected
in a matter of hours

with only minimal assistance
from heavy equipment.

Built from regular steel
alloys and modular in form,

the bridge was
assembled using a series

of crossed braced
rectangular side panels

each up to 10 foot in length.

These could be
carried by six men

and were connected
using interlocking lugs.

19 foot wide transoms were
then run between each side.

After one section was complete,

it was pushed
forward over rollers

and another section
built behind it.

They were then
connected together with
pins in the corners

of the panels.

To create the floor,

10 foot long stringers
were added for support.

Wooden planking would then be
placed to top the stringers

to provide a road bed.

- It's a very light way
to create a rigid frame

and it's bolting together
those sections of frame

that allow you to create
quite long bridges

capable of withstanding
the weight of a tank.

- [Narrator] But it was
the quick modular assembly,

often without the need for
support piers in the water,

which gave them the
edge over the enemy.

- It's a bit like the
flat pack furniture

you can buy these days.

It's modular. It's
mass produced.

It goes together in
a fairly easy way.

It could be put
together pretty quickly,

moved and dismantled as needed.

- And so to have a
kit of very few parts

which could quickly
be replicated and
shared with the Allies

meant that the Americans and
the Canadians and the English

were all making components
for the same bridge.

- [Narrator] A Bailey
bridge could easily span

up to 230 feet

and if required, could
reach over a thousand feet.

Once the troops had
used the bridge,

they then took a leaf out of
the Roman engineering playbook.

- In the same way
that these bridges

could be put up very quickly,

just like the Romans
with their Rhine Bridge,

they could be taken down very
quickly and used elsewhere.

- [Narrator] By
the end of the war,

over 3000 Bailey bridges
covering more than 55 miles

had been constructed.

Leading British commander field
Marshall Montgomery to say,

"Without the Bailey bridge,

"we should not
have won the war."

- I think the real impact
of the Bailey bridge

is that it allowed
the Allied forces

to maintain that
speed of advance.

That was absolutely critical.

And in that way,

these bridges played a
really important role

in the defeat of Nazi Germany.

(dramatic music)

(sheep bleating)

- [Narrator] By 27
BC, the Roman Republic

had transitioned into an empire.

As it expanded, they required
more permanent infrastructure

that would last longer than
wooden bridge technology.

- Wood can be washed away
or get damp and rotted.

So that was a good temporary
bridge building material.

To build a permanent bridge,

you want to build in masonry,
in stone, brick and concrete.

- [Narrator] They
turned to a design

which would change the
face of construction

not just in the Roman
era, but for all time,

the arch bridge.

- The Romans, they really
were the sort of past masters

of taking their technological
innovations of their day

and making them their own.

And in the case of the arch,

this is something they'd
inherited from Greece,

but this was a key component
in many of their structures.

If we look at aqueducts,
stadiums, basilicas

and especially bridges.

- [Narrator] The ability of
an arch to span a large area

is largely due to the
force of compression.

- When things are
pressed together,

obviously that is compression.

Tension is the precise opposite

where things are pulled apart.

And this is the things that
engineers have to deal with

when they design
and build bridges.

And they have to choose
their materials carefully

because some materials
are good in compression,

some materials are
good in tension.

- [Narrator] Needing
their bridges

to span further
than ever before,

the Romans had upgraded
to stone arches.

- Stone is a good
material in compression.

And an arch is known as
something which carries its load

predominantly in compression
down to the foundation.

- [Narrator] To support
arches over longer spans,

an innovative
approach was required

to construct the
piers mid river.

- The problem with
that is you're building
in flowing water.

You've got to find a way
of keeping the water out

while you're working.

And one way of doing this is
something called a cofferdam.

- [Narrator] The Romans
developed a temporary barrier

so that the workers could
build within a dry space

in the middle of
a flowing river.

Vertical timber stakes were
driven into the river bed

long enough so that
a significant portion

was higher than the water
level forming a walled dam.

The water in the cofferdam was
emptied using either buckets

or an Archimedes screw.

Gaps between the stakes
were sealed with clay,

providing a watertight area
in which the foundations

and piers could be constructed.

Once the piers were built,

the cofferdams could be
deconstructed and removed,

but the flow of water around
the piers raised another issue.

- One of the biggest
enemies of bridges

is actually the river scour.

That is where you've got
a support in the water

and the river is constantly
flowing around that support.

And if you don't shape
the support properly,

the flow of water will actually
erode around the foundation

and it's a very real risk.

- [Narrator] To help mitigate

this potentially
catastrophic erosion,

the Romans devised a
particular shape to the piers.

- You would tend to build each
pier with a point at each end

called a cutwater or styling.

- It made the flow around
the pier much more smooth.

So today likewise,

we shape our river
piers carefully

so that the scour doesn't
happen or it's minimized.

- [Narrator] Constructing
the bridge's heavy arches

also required some
innovative thinking

as they required
considerable support

so that the growing span
could be held in place

before being finally
secured with a keystone.

A platform was built
at the base of the pier

using a measurement
called a springing line.

Temporary wooden support
known as falsework

was erected in an arch
shape from this line.

Separate wedge-shaped
stones called voussoirs

were placed incrementally
over the falsework frame

until a final trapezoidal
keystone was inserted.

The sides of the
keystone must be tapered

so that the force
of the weight load

is channeled out and down.

Once the keystone had been
set in place with concrete,

the falsework could
then be removed

because compression and friction
keep the structure stable.

- The keystone is the
critical piece of the puzzle.

This allows the whole arch
to work in compression.

And without that, you
have no structural system

therefore you can't
support any load.

- [Narrator] Buttresses were
then added to stiffen the wall

against overturning forces.

Finally, the bridge was
laid with a brick deck

and paved to carry
soldiers and chariots.

- The Romans are rightly
revered for their audacity

building on the scale that
no one had seen before.

So the longest Roman
bridge in their empire

was at Merida in Spain.

It's almost 2.5
thousand feet long.

- [Narrator] The arch
bridge was such a success

that the Romans crossed
almost every major river

in their empire

and many examples still
stand to this day.

(dramatic music)

While the cofferdam
allowed the Romans

to construct piers in rivers,

a development based on it that
came almost 2000 years later,

allowed construction to take
place not only in water,

but under it.

Completed in 1883,
the Brooklyn Bridge

and it's almost 1600
foot long main span

linking New York's two
most important boroughs,

Brooklyn and Manhattan,

broke the record for the
longest suspension bridge

in the world.

But during its construction,
catastrophe was never far away.

- The project suffered
all manner of setbacks,

not least when the original
engineer, John Roebling,

contracted and died from tetanus

as a consequence of an
injury he suffered on site.

- [Narrator] The baton was
passed to his son Washington,

who was now tasked to build
the bridge's foundations

in the deep, fast flowing
waters of the East River.

The solution was the caisson,
which like the cofferdam,

was a technology also
used by the Romans.

- To build really deep
foundations for bridges

that are in deep water,

using a cofferdam is no
longer really feasible.

So caissons were designed

that is essentially
an upturned box

that can go down into the water

and displace the water from
it with high pressurized air.

- [Narrator] Washington
Roebling's pressurized caissons

were some of the
first of their kind,

designed to ensure
that the mud and water

below the water line

couldn't infiltrate the chamber

while the men dug out the
area for the pier foundations.

- I can't imagine what
it would've been like

to work in one of
these caissons.

Damp, claustrophobic,
struggling to breathe.

I mean it must have been
like a hell on Earth.

- You're also in a
pressurized environment

and the science of understanding
what that pressure did

to the human body

and the change between
surface level pressure

and caisson pressure
wasn't properly understood.

- [Narrator] A local doctor
noticed a worrying trend.

the deeper and longer the
workers called sandhogs

stayed under water,

the more they suffered
agonizing symptoms.

- Many of the diggers
were suffering

from these sort of terrible
pains and cramps in the bodies.

And they weren't entirely
sure what it was.

They were bent
double if you like

and that's the word that
we now use to describe

a form of decompression
sickness, the bends,

and this was a terrible
problem for the engineers.

- [Narrator] At least 20
men died a horrible death

from Caissons disease

and it affected
Roebling himself.

Eventually, confined
to his home,

the chief engineer's job was
passed to his wife, Emily.

- So for 10 years,

Emily was essentially
acting as chief engineer.

And over that time,

she learned how to do
calculations herself.

She understood the
stresses and the loads

the bridge was under

so that she could ensure that
the completion of this bridge

was done successfully.

- [Narrator] Over 10 years
later at the opening,

Emily Roebling carried a
rooster across the bridge

as a sign of victory

while her husband
Washington could only watch

from his bedroom window.

- Emily Roebling was really
a role model at the time.

Women weren't even getting
education as engineers

let alone running these
incredible projects.

So women engineers today
have a lot to thank her for.

- [Narrator] Around 1485,

The great Italian Renaissance
master Leonardo da Vinci,

had drawn plans for an
extraordinary wooden bridge

which could support itself
without bolts, ropes or nails.

But around 500 years earlier

in a very different
part of the world,

ancient engineers had already
devised a similar concept

that utilized their
exceptional carpentry skills.

(light music)

Goods have been traded and
transported from across China

for millennia.

Historical documents
suggest that

the bridges traders
needed to cross

suffered from
structural problems

associated with
their central pillars

being damaged by the
force of flooded rivers.

Plank bridges, which
removed the need for piers,

were even less reliable.

- A simple strat
bridge of three planks.

It comes with all
sorts of problems.

You can't span
particularly wide distances

and the whole structure is also
open up to twisting as well.

- There was too much
movement in the bridge.

It becomes too flexible.
It distorts under load.

So there was a need to
do something different.

- [Narrator] The answer
emerged 1000 years ago

during the Song Dynasty,

when it said a prison guard
devised a new approach

which addressed these issues

through a unique
construction method

that more resembled weaving,
the woven timber arch bridge.

- So the way in which the
timbers were placed together

in that interlocking form

gave the bridge a
degree of flexibility.

Now all bridges need
flexibility to stay up

but this was a sort
of a really unique way

in which the forces
could be dissipated

as the traffic passed over
the top of the bridge.

- [Narrator] And they
were built entirely

by using traditional
woodworking techniques

and used no glue,
screws or fasteners,

a method that was first
practiced in Neolithic times.

- You could compare it
to the skilled woodwork

in cabinet making for example,

where you've got
mortise and tenon joints

that keep that
structure together.

In these bridges, it's
the same skilled woodwork

that makes the bridge so strong.

- [Narrator] Abutments
usually made from stone

were built on rock to secure
the bridge at each end

before construction
began in three sections.

The first, a three-sided arch
made up of three long beams

and two crossbeams.

The second, a five-sided
arch of five long beams

and four crossbeams

was interlocked with the first.

And the result is
a weaving effect

which gives the bridge its
strength and stability.

Wooden planks were then
laid over the substructure

to form a deck

and a roof was sometimes
built over the top

for protection
from the elements.

- It's so ingenious.

What the woven arch bridge
does is it almost takes

two bridges that in themselves

are not quite
structurally sound,

but by combining them,
by interposing them,

actually you create an
incredibly strong bridge

that can span wider distances.

- [Narrator] All Chinese
bridges of this type

were thought to have disappeared

and the skills it took to
construct them lost forever.

In the 1980s however,
there was a shock discovery

when 100 were found in a
remote mountainous corner

of Southeast China,

which inspired later designs.

- It's just such a wonderful
moment to look on something

that you thought had gone

and to have that connection
for past culture,

past civilization

and see in this case what
engineers in the past

are really capable of.

- And what I find personally
really fascinating

is that these bridges still
continue to be constructed today

as part of an intangible
cultural heritage

that's actually even
recognized by UNESCO.

- [Narrator] They remain proof
that the engineering skills

of these ancient
master craftsmen

are still as pertinent today.

But the Chinese were not
the only ancient engineers

to build bridges which
would defy belief.

(light music)

The Inca rulers of their
South American civilization

in the 15th century

used their skills
to build perhaps the
most vertigo inducing

bridges on the planet.

The Inca Empire stretched
3,400 miles from north to south

and extended along the Pacific
Coast and Andean highlands

from the Southern border of
Columbia to Central Chile.

It lasted from the 12th
to the 16th century

where the Incas were
overthrown by the Spaniards.

- But within that
short timeframe,

their authority, administration,

the way that they organized
groups of linguistically

quite varied people
under one leadership

meant that they created
really monumental things

and that's reflected in
some of their structures.

- [Narrator] Key
to this expansion

where bridges spanning
up to 200 feet

but constructed incredibly
using little more than grass.

- When we think of bridge
building, we think of stone.

We think of wood.

Well imagine spanning
vast caverns with grasses,

with plants,

and that's what
these bridges do.

That's quite something.

- [Narrator] For their
plant-based suspension bridges,

the Incas use their
crafting skills

to twist lengths of
grass, vines or leather

into short sections measuring
up to a few feet in length.

These were then spliced
with similar sections,

creating the length of
multistrand rope required.

- So when you're twisting
two ropes together

to make a rope of
double the thickness,

it always wants to unlock.

But if you turn that rope on
itself and then twist it again,

what you have is those
two parts of the rope

working against each other.

They wanna unlock against
each other but they can't.

- You can think about
it like braiding hair

where you take two, three pieces

and you weave them together

and that increases
the tensile strength

and this is what
they were doing.

- [Narrator] Lengths
of multistrand rope

could then be woven together

to create the thickest,
strongest rope cables.

- So some of these
ropes are huge.

They're the width
of a human torso

and they're capable of taking
really quite extreme weight.

- This is a huge undertaking.

There'll be whole communities,

whole villages coming
together, weaving these cables.

- [Narrator] Each bridge
contains up to 10 miles of rope,

each one weighing
up to 200 pounds,

making a combined weight
approaching a ton.

So how could these
cumbersome materials

be hoisted up steep
mountainsides?

- That would fall on some
of the younger members

of the community,
the most athletic.

They would have to get
down, cross the river

with a stretch of this cord

and then climb all the
way up the other side.

- It requires real
bravery, audacity

to be over steep
ravines with rocks,

but it became a way
of life for the people

of the Incan Empire.

- [Narrator] The critical
factor in this bridge's design

was how to secure the supporting
ropes to the cliff itself.

The walkway was constructed
of up to four braided ropes

while two smaller ropes
were used for the handrails

with vertical suspenders
joining them to the deck

spaced at around
four inches apart.

These six ropes were then
wound around boulders

which were wedged into
slots in the cliff face

and acted much like bollards

to securely tether the
bridge to the mountain side.

- It's absolutely critical

that these things were
tethered correctly

because if they weren't,

you run the risk of losing
the work of a whole community

and potentially lives with it.

- [Narrator] A 100-foot
bridge could easily take

7,500 pounds or 56 people,

three or four times the
load that was required,

a similar ratio used
to build bridges today.

- You may get entire
communities, armies

moving across these bridges
because the rope is so strong.

- [Narrator] It's estimated
that a large rope bridge

would take around two
weeks to construct

and could last as
long as two years

dependent on conditions

but regular maintenance
could extend their life.

- In that moist
jungle environment,

they absorb the humidity.

They stretch through
the forces put on them.

You can have
friction wearing away

at the ends where
they're tethered.

You can have rot as well.

- [Narrator] These
entirely organic bridges

were strong enough that they
could take an army on horseback

marching two by two,

which provided the
perfect platform

for the oncoming invaders.

- The problem for the Incas

is of course that the same
bridges that had opened up

vast ways of landscape
to their kingdom

were the same bridges that
were used by the Spanish

during the conquest.

- [Narrator] In 1492,

the Spanish Army known
as the Conquistadors,

arrived in the Americas.

And by 1531, had
reached the Inca Empire,

making full use of
the Inca engineering.

- These bridges had
facilitated free access

and free movement

throughout the Inca's landscape.

But of course because of that,

it was also part
of their downfall

because these were the
routes that were used

by the Spanish
conquest of the area.

And this was probably
not the original function

that the Incas had in mind.

- [Narrator] By 1572, Inca
rule had completely ended

but it hadn't all been plain
sailing for the Spanish.

They had to suspend
their disbelief

when they first saw
the swaying bridges.

- I would feel the same way

if I'd stumbled across one
in the middle of nowhere.

I'd take some convincing
to walk across it.

(dramatic music)

- [Narrator] Stability
under all conditions

is something modern
bridge builders

have also had to
take into account.

But even today, engineers
learn as much from failure

as they do from success.

The image of the Tacoma
Narrows Bridge in America,

buckling violently in 40
mile an hour winds in 1940

remains a disturbing reminder

that sometimes things
can go very wrong

even with the best
engineered projects.

- The Tacoma incident was one
of those moments in history

where it stopped us
dead in our tracks

while we tried to work out
what an earth happened there.

- One of the difficult
things to realize

is that bridges are
mobile structures

because they're
constantly being moved,

buffeted by wind.

Then unless you build
in a certain mobility,

a flexibility into their
design, it will be a disaster.

- [Narrator] Despite being
designed and built by engineers

with a strong track record
in bridge construction,

a then little understood
natural phenomenon

set off vibrations caused
by whirling masses of air

known as vortices.

- So wind blowing on the
bridge, creating vortices,

which creates a fluctuating
change in pressure.

And if it coincides with
the natural frequency

of the bridge,

then the bridge response
starts to amplify

and get bigger and
bigger and bigger

until you get to that
horrible collapse point.

- [Narrator] The solution
to eliminate the possibility

of such vibrations escalating

comes through
engineers installing

specifically designed
devices called dampers,

which counter the effect

of this potentially
catastrophic movement.

- There are different
types of dampers

but essentially what they're
doing is dissipating the energy

of the vibration
of the structure.

If you think about the
shock absorbers on your car

or a mountain bike,

it's reducing the magnitude
of that movement or that shock

that you would feel.

- [Narrator] One type is
called a tuned mass damper.

- We have this weight
supported on sprung system

so that it has a frequency

which is tuned to the same
frequency as the bridge.

- [Narrator] As
the bridge moves,

the weight moves in the opposite
direction or out of phase,

which counteracts the forces,

helping to reduce any
movement in the structure.

As well as canceling
out the vibrations

caused by wind vortices,

dampers can also reduce
damage and metal fatigue

caused by other phenomena
including earthquakes.

And as with all technologies,

new understandings
continue to be made.

- Recently, there's
been more work done

in trying to understand
the complex nature

of the interactions of air
with structures like bridges.

This helps us to ensure
that any future design

will be strong and resilient

to even the most
extreme loading.

- [Narrator] But these
breakthroughs were
only made possible

by the enormous
technical advances

made almost two
centuries earlier.

(dramatic music)

It can be argued that
bridge building came of age

with the birth of the
Industrial Revolution.

- Casting with iron
had been around

for really quite some time

but it was time consuming
and done mostly by hand.

What we see in the middle
of the 19th century though,

because of innovations in
both chemistry and furnacing,

is that whole process
can be scaled up

to produce both more and
objects of a much bigger size.

It was this upscaling
in material outputs

that fueled arguably the
greatest increase in production

the world had ever seen.

- [Narrator] From
1760 to around 1840,

these advances propelled
industrial economies

through the designing
and constructing

of massive infrastructure.

In England during this time,

one man stood head and
shoulders above all others

when it came to
engineering prowess.

- Isambard Kingdom Brunel

is probably one of the most
renowned engineers of all time,

whether it was ships, whether
it was railways, bridges.

You know this is one
of the key figures

of the Industrial Revolution.

- If you literally thumb
through his notebooks,

you can see that
this is a genius man.

- [Narrator] In 1831 at age 24

and while still an
engineering novice,

he won a competition to build
a bridge over the River Avon

in Bristol,

which would make travel in
both directions much easier.

To span the formidable 245
feet high 700 foot wide gorge,

Brunel came up with a radical
idea in Britain at the time

to put Bristol on the
map, a suspension bridge.

- In a suspension bridge,

you've got the perfect balance
of tension and compression.

So you've got the
tension in the cable

and the suspenders that actually
connect the catenary cable

to the deck itself.

But as the cable
goes over the towers

and is anchored into
the ground either side,

those cables are actually
transmitting compressive loads

through the towers.

- [Narrator] On the
Clifton Suspension Bridge,

six rod iron chains, three on
either side, span the bridge

and hang over two
86-foot tall towers.

Metal saddles sitting atop
the towers are in compression

as they take the weight of the
chains as well as the deck,

which is attached to them
via 162 vertically hung

iron suspender rods.

Construction began in 1831
and Burnel's first hurdle

was to choose the
most suitable rock

for the foundations to carry
the bridge's enormous weight.

- However, the problem
with the Avon Gorge

is that the rocks were
unstable on either side.

So the solution that
Brunel came up with

was to build two enormous
abutment chambers

which he then used to
support the pylons above.

- [Narrator] Already
beset by a shortfall

in construction funds,

tragedy struck in 1859
when Brunel passed away

having never seen
his bridge completed.

It wasn't until 1862 with
new engineers in charge

that work finally resumed

but Burnel's replacements
faced a considerable challenge.

- Whenever you build
a suspension bridge,

you need to build some
means of getting the cable

or in this case the
chains across the gap

cuz until you've got those up,

you can't bring up the
bridge deck to hang off it.

- They had to build a
platform across the gorge

in order to carry the
weights of these chains

which were enormous.

- [Narrator] This
came in the form

of a temporary wooden platform
secured to lighter cables

which was curved to match
the hang of the chains.

A movable carrier
traveled along the cable

to secure each of the
4,200 links into position.

- So once these chains
have been coupled up,

they're going to support
the bridge structurally.

What you need to
do is tether them

to either side of the gorge.

- [Narrator] The chains were
anchored through hollow shafts

tunneled into the bedrock.

A land saddle angles
them into the tunnel

while a brick anchor plate

attached to the
ends of the chains

prevents them from
being pulled out.

- So it doesn't matter how
much you pulled on the chain,

it would bang itself
against the bedrock.

- [Narrator] With
the chains tethered,

the temporary platform
could be removed

and vertically hung
iron suspender rods

could be connected to them
to support the deck below.

Once roof caps were put on
to cover the chain saddles,

the Clifton Suspension
Bridge was finally opened

33 years after
Brunel had begun it.

- The completion of the bridge

largely to Brunel's
original designs

was seen as a memorial
to the great engineer.

It is one of the
great iconic symbols

of the Industrial Revolution.

- [Narrator] It
remains to this day

an amazing achievement of
both Victorian engineering

and Brunel's incredible vision.

(dramatic music)

The development of new
materials paved the way

for much larger and
stronger bridges

to meet the new demands
of the 20th century.

- It's amazing to think

that in the course of
around a 100 years,

we'd gone from roads
that were in essence

on a single horse track

to having 12 lanes
super highways.

At some point, this traffic
needs to come together

to cross an expansive
water or a river.

So these bridges need
to be future proof

to take these excessive loads.

- [Narrator] Opened in 1932,

Australia's iconic
Sydney Harbor Bridge

took the arch to
a whole new level,

replacing rot and cast
iron with a much improved

and stronger material, steel.

Spanning 1,650 feet,

it is one of the longest steel
arch bridges in the world.

And at 440 feet
high, the tallest.

It is held together by six
million hand driven rivets

and carries loads of more
than one million cars,

trucks and trains every week.

- This really is an
absolutely monumental bridges.

It's a tremendous sight
and you can only really

imagine the challenges
that engineers would face

by trying to get across
this enormous span of water.

- You're building this huge arch

from opposite
sides of the harbor

and they've got to
meet in the middle.

Without our modern
day computer systems,

you think how on Earth
did they do that?

- [Narrator] The deep
water of Sydney Harbor

made building pier
supports impractical.

Somehow during the construction,

the mammoth steel arch
spans needed to be supported

as they were being
assembled toward each other

from opposing banks.

- What they did was they
attached very high strength,

steel cables to the construction
as they were working

so this to stop the
thing falling over

and these cables were
anchored in the ground.

So they would then be able to
incrementally grow the arches

a little bit by little bit.

- [Narrator] Huge steel pivots
at the base of each arch

allowed the cables
to be tightened or
slackened as necessary

to remain on course.

- Right from the very beginning,

every time they put a piece on,

they would've had
to do a survey,

making sure that they
were following the line

that the arch has to follow

so that it was going to
meet in the right place

in the middle.

- [Narrator] After seven
years of construction,

the gap was closing.

- So as those two arches
come close to each other,

you could begin to see
whether one's high or low

or whether if one
was slightly twisted.

- [Narrator] The
engineer's foresight

proved vital to this
critical moment.

- There was a need to
pull out on one side

and pull in on the other side
and make a bit of adjustment.

In fact, it was quite surprising

that the amount of adjustment
was as small as it was.

- This was a period
when they didn't have

sort of laser sighted
surveying equipment.

They were working with pen
and paper and slide rule.

And when you take all
of that into account,

the fact that they were
just over an inch out

is really quite remarkable.

- [Narrator] This relatively
minuscule adjustment

saw the two arches finally
meet up as designed.

- There's a very
special occasion when
two parts of a bridge

meet for the first time.

And of course at
Sydney, would've been
terribly significant,

that meeting of the arch
and the great celebration.

- [Narrator] The
Sydney Harbor Bridge

owes its existence to the 1400
laborers and many engineers

that brought two sides
of Sydney together

by manipulating solid
steel to maximum effect.

(dramatic music)

Modern bridges have not only
united both sides of a city,

but even countries themselves.

The mile long Bosphorus
Bridge in Istanbul

links the continents
of Europe and Asia.

While at five miles long,

the Orison Bridge is the
longest combined road

and rail bridge in Europe

joining Sweden to Denmark.

Orison may look like
a suspension bridge

but it's appearance
is deceptive.

It uses cable state technology,

a design feature of some
of the longest bridges

in the world.

The most remarkable
of these is in France,

the Millau Viaduct.

- I gotta say Millau
is probably one of my

favorite pieces of engineering.

I mean it's elegant.

It's sleek. It's slender.

It's French.

- [Narrator] At over 1000
feet, the Millau Viaduct

is the tallest
bridge in the world,

standing higher than
the Eiffel Tower.

Completed in 2004,

its design utilizes the
cable stay principle.

- So you've got a mast and
the cables run from the mast

well like a coat hanger and
support the bridge deck.

And normally, a conventional
cable stay bridge

would have two towers and
one main span in between.

In the case of Millau Viaduct,
it's a little different

because there are
seven of these towers.

- [Narrator] Building a bridge

one and a half miles in length

with a deck almost
900 feet above ground

was never going to
be straightforward.

And like with all
enormous structures,

fluctuations in temperature
had to be taken into account.

- As you build a big bridge,
the thing moves around.

In the morning, it
will be, it'll be cold.

And as the sun comes out
and it gets warm, it moves.

It grows. It expands.

So as bridge engineers,
we get used to this

this amount of movement

and it's quite a lot
quite surprising,

alarming to somebody
who's not used to it.

- [Narrator] This movement,

especially of the deck
which sits on the piers,

can cause destabilization.

But the Millau engineers
had a unique solution

that had never
been tried before.

- These very tall
concrete piers are split.

The top part is in two
well like a tuning fork.

As the bridge changes
in temperature,

the bridge will
expand and contract.

And that means the tops of
the piers would move with it.

And if they were very stiff,

those peers would then
generate very large forces.

But in fact by splitting it,
making them very flexible,

those forces are not generated.

And so it's a very clever
way of dealing with

what we call the
articulation of the bridge.

- [Narrator] Millau's
projected 124 year lifespan

is due to such
innovative design.

In many ways, making it the
shape of bridges to come.

(light music)

From their earliest days,

when humans overcame the
challenges of crossing rivers

to trade and link communities,

we have continued to invent
ever more extraordinary ways

to build bridges

from the death defying to
the sublime to the iconic.

Bridges continue to serve a
vital function in societies:

that of uniting people,
enabling travel and commerce

and acting as conduits
to our future prosperity.

But none would've achieved
these lofty ambitions

without the skill,
daring and ingenuity

demonstrated through innovations
by ancient engineers.

(dramatic music)