All language subtitles for Unearthed.(2016).S07E05.Twin.Towers.The.Hidden.Secrets.1080p.AMZN.WEB-DL.DD+2.0.H.264-Cinefeel_track3_[eng]

Narrator:
Manhattan, new york city.

Once it is home to two of
America's

Most iconic buildings --

The world trade center
Twin towers.

They were giants
Of their time.

When constructed, they are
An engineering triumph --

The tallest buildings on earth.

The skyscraper has always been
An engineering marvel,

But it was the twin towers
That really set the precedent.

Narrator:
Remembered across the globe
For their tragic final moments,

The story of their origins
Is rarely told.

Today, investigators use
Pioneering technology

To examine the twin towers'
Surviving footprint

And secret history...

We can see details you can't see
From the human eye.

Narrator:
...And put the materials they
Are built with to the test.

They really broke records
In structural design innovation.

Narrator: how do engineers push
The boundaries of building tall

To erect these towering giants
In the heart of new york city?

To solve this mystery, we'll
Digitally reconstruct the towers

And reveal the hidden secrets
Behind their iconic design

That once defined
New york's skyline.

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Narrator:
New york city...

Today over 6,000 high rises
Dominate the horizon,

But back in the 1960s,
Only a handful of buildings

Stand higher than 40 stories.

Darton: something radical needed
To be done to bring new york,

Really, into the 21st century,
And in our culture,

Bold gestures
Are often made architecturally.

Narrator:
So new york's leaders invest
Hundreds of millions of dollars

In a spectacular
Building complex.

The architect, minoru yamasaki,

Went through literally
Hundreds of plans,

And eventually he succeeded

By coming up with the idea
For the twin towers.

Narrator: the world trade center
Design features

Five lowrise buildings
Surrounding a central plaza.

Rising from the center,

The most ambitious skyscrapers
Of all time,

Each soaring more than
1,300 feet

Above lower manhattan.

Over four times taller
Than the statue of liberty.

Together they will deliver
10 million square feet

Of real estate.

Every day, 50,000 workers will
Flock to this city in the sky,

But no one has ever tried
Erecting a building this tall,

So how do they do it?

Eric darton is an expert
On the history of new york city.

He investigates the challenges
The engineers face.

The idea was to acquire land
And to build on a 16-acre site,

Which is not a lot,

Enough office space
To accommodate 50,000 people.

That's why the towers had to be
As tall as they were.

Narrator:
The architects look to what
Is then

The world's tallest skyscraper
For inspiration --

The iconic empire
State building.

No skyscraper had come near
To surpassing it in 40 years.

The empire state building
Is the high point

Of skyscraper design --

A steel grid reinforced
With concrete

And encased in strong limestone.

It weighs 365,000 tons,

But using this building model
For the twin towers

Would limit the internal space.

Darton:
The empire state building
Has 2 million square feet

Of floor space, office space.

The program for the world
Trade center

Was 8 million square feet
Of office space.

You can't just scale up
The empire state building.

There'd be so many columns,

You couldn't put anybody
In the building.

Narrator:
So experts have to figure out
How to build skyscrapers

That each have double

The floor space
Of the empire state building.

Professor adrian brugger
Investigates

The empire state building
And twin towers structures.

Brugger:
Here I have a 3d print

Of one of the floors
Of the empire state building,

And you can see that we have
This very thick core

In the middle here.

Narrator:
The model shows how the weight
Of the empire state building

Is spread
Across a thick central core

And hundreds of columns,

But in order to beat
The height of

The empire state building and
Accommodate

The 50,000 office workers,

Japanese american architect

Minoru yamasaki
And american engineer

Leslie robertson
Will have to come up

With a revolutionary solution.

Minoru yamasaki had the idea
Of this stress skin

Architecture from nature,

Which is generally
A very good place

To get your motivation from.

And he thought of bamboo.

Narrator: bamboo is hollow
And its entire weight

Is supported by its outer skin.

It's a very strong plant.

In japan it's also
A symbol of prosperity.

The idea is the perfect solution
For the twin towers.

Instead of spreading the load
Across a grid,

Yamasaki creates a super strong
Hollow structure.

All the weight is supported
Around the edge.

It's a breakthrough
In skyscraper design.

Each tower is made up of
236 steel columns.

They stand just 26 inches apart
Around the perimeter.

More than 43,000 windows
Complete the outer skin.

Inside, steel plates stretch
Across the void,

Topped with nearly
A million tons of concrete

To create 110 floors.

The north tower is crowned
With a final feature --

A 360-foot communications mast.

Now they have a new task --
To make this outer skin

Strong enough to support
The weight of the tower.

Adrian has rare access

To the surviving structures
Of the twin towers,

Carefully conserved
In the basement of

The 9/11 memorial museum.

He examines
The surviving columns

From the outer skin
Of the building.

So here you can see one of those
Mega columns

That is landing the forces

From 110 stories
Into the ground.

And if we look up at the column,
We'll see that one of these

Large columns goes through
This beautiful trident structure

That then picks up
Three smaller columns.

Narrator:
The hollow building structure is
A breakthrough in engineering,

But this revolutionary concept
Goes even further,

To the skeleton itself.

This column is actually hollow
On the inside,

And that's purposely done

So that we can
Increase its stiffness.

So the material is all
Along the perimeter.

Narrator:
Not only is the building hollow,
But the columns are hollow, too.

Finally the designers use
An extra trick

To reduce the overall weight
Of the twin towers.

The metal is 4 inches thick
At the base,

But tapers to just 1/4-inch
At the highest point.

Brugger:
They get lighter and lighter
As you go up to the top.

And that is essentially to make
The building

As efficient
As humanly possible.

So this is how
Structural engineers

Are able to build this high.

Narrator:
Structurally the design
Is perfect.

It breaks through
The constraints

Of contemporary
Skyscraper building.

With this method, the height
Seems almost limitless.

It will allow the twin towers
To rise taller

Than any other building
On the planet,

Reaching
An astonishing 1,368 feet,

But engineers face
An additional challenge...

Darton:
How do you bring people in?

The vast majority of people
Going into

The world trade center
Came in underground.

Narrator:
Office workers arrive at
The towers via the subway

And then go straight
To the elevators,

But in the mornings,
There are 50,000 people

Coming in at the same time.

How can they all travel
Through the tower?

In the empire state building,

Elevators occupy almost 3/10s
Of the buildings floor area.

If engineers follow a similar
Design,

The twin towers would
Lose vital floor space.

The taller a building is,
The more elevators it needs

To service it,

But the shafts take up
Valuable floor space.

The subway system in new york
Provides inspiration

Where local trains
Stop at every station

And express ones fast track
To key hubs.

So engineers add two sky lobbies
To the twin towers.

These are special floors
Where people can switch

From a large capacity
Express elevator to a local one.

So multiple elevators can share
The same shaft,

Increasing the usable
Floor space.

Engineering masterminds designed
These iconic towers

To break the record for the
Tallest building in the world,

Ready to steal the crown
From the empire state building.

But when construction begins,

Engineers face an unexpected
Challenge beneath the ground.

Could it become a serious danger
For the building's foundations?

Narrator:
The twin towers are once

The most recognizable
Skyscrapers in the world

And the tallest buildings
Of their time.

The engineers who build them
Face huge challenges

From the very start
Of their construction.

That's because much
Of lower manhattan

Is built on reclaimed land.

When engineers start to dig
Their foundations,

They make a surprising
Discovery.

Hidden below the pavement,
Engineers

Encounter water-logged mud
And rubble,

Including a 400-year-old ship
Preserved in mud.

As they dig deeper, a modern
Obstacle emerges --

An active railway tunnel

That carries
100,000 passengers per day.

And even further down,
70 feet below street level,

They finally reach bedrock.

This exceptionally hard material
Is the perfect base to build on,

But in order to secure
The twin towers' foundations

Into the bedrock,

The engineers first
Have to reach it.

Structural engineer rick ellman
Investigates how it's possible

To build a skyscraper
On reclaimed land.

These tall towers that are all
Around us,

If they were not
Constructed properly,

They would lean
And very likely topple over.

Narrator:
The most famous example of the
Dangers of unsteady foundations

Is the tower of pisa in italy.

It's build on marshy land,

Similar to the reclaimed land
Of manhattan.

Over time, the soft ground
Causes the tower

To tilt dangerously.

But unlike pisa, manhattan has
An incredibly tough bedrock

Just 70-feet underground.

The twin towers, foundations
Can be firmly anchored

Into this solid rock,
But there's a huge problem --

Reaching it.

Ellman:
Excavating in reclaimed land
Can be a real challenge.

Because of the weak soils
And the high ground water,

You're unable
To just simply excavate.

Narrator:
If they excavate, water and mud
Just slide back into the hole,

So engineers come up
With an ingenious solution --

They create a wall
To surround the foundations.

The wall acts as a protective
Barrier.

First, they dig long trenches
Around the edge of the site

And fill them with
A liquid slurry

To stop them collapsing inwards.

Then they drop reinforced steel
Cages into the trenches and pump

In concrete
To displace the slurry.

Once these concrete walls
Set solid,

Engineers can excavate
Everything inside them,

Creating a giant
Underground bathtub.

Instead of holding water in,
The bathtub keeps it out

So the foundations
Can now be built.

At the 9/11 memorial site,
Rick ellman has rare access

To the remains of the bathtub.

Ellman:
Here we are, 70-feet below
The ground surface

In the area that was once
Occupied

By the original
World trade center foundations.

The space was created using the
Slurry wall construction method.

Narrator:
The wall holds back the
Surrounding mud and ground water

To protect
The towers' foundations.

The wall itself is probably
3 to 5-feet thick,

And this system, amazingly,

Is holdings thousands
Of pounds of force.

Narrator:
The twin towers engineers
Are some of the first

To use this pioneering technique
In new york.

In 1968, the bathtub is complete

And the main
Building work can begin.

But engineers face
Additional challenges.

The twin towers are the world's
First skyscrapers

Built without stone or brick.

Their flexible metal structure
Causes them

To sway in high winds.

How can engineers
Stop new yorkers

From getting sea sick inside?

Narrator:
April 1973...

In the heart of manhattan,
Two skyscrapers are set

To emerge on the horizon --
The twin towers.

Their hollow metal structure
Is a revolutionary design.

They are the first skyscrapers
Ever built

Without stone or brick.

The engineers use
A new technique

To construct these steel giants.

They click the pieces together
Like lego.

Engineer sawteen see

Is the managing partner
Of the company

That built
The world trade center.

All the design of
The twin towers

Allowed for faster and safer
Erection and construction,

So it went up really fast.

Narrator:
Building the pieces off site
Allows engineers

To complete them
To the highest standard,

And it means the structure can
Be assembled quickly on site.

See: in a sense it was like
An assembly plant.

Narrator:
The twin towers' external skin
Goes up first.

59 steel columns per side,
Each just 14 inches wide.

They bear most of the towers'
Weight.

In the center of each tower
Sits a core of 47 steel columns

To give the tower
Extra rigidity.

Thin steel plates 60-feet
Wide bridge the gap,

Connecting the inner
And outer columns,

Anchoring the entire
Structure together,

And creating the vast
Open floor plans.

It's a futuristic lightweight
Structure,

But will this flexible
Design be a match

For new york's high winds?

At the top of the buildings,
Wind speeds

Can reach 100 miles per hour.

To make it very tall,
The building

Had to be very stiff
To resist the wind loads.

Narrator:
If the building sways too much,
It could sheer the steal,

Or the elevators could even
Get stuck in the shafts.

So the twin towers become the
First skyscrapers in the world

To be evaluated
For wind dynamics.

A new field of research is born.

Scale models on the twin towers

And the complex,
The surrounding areas

Were built and tested
In colorado.

Narrator:
In the 1960s, the twin towers'
Engineers

Create the fist ever wind tunnel

And put their models
To the test.

They shoot 150 mile per hour
Winds across it.

The tests show they will hold
Strong structurally --

A success,
But there's a problem...

Even though the models
Can withstand the wind loads,

They sway 1 inch
From side to side,

Equivalent to 40 feet
On the full size buildings.

Workers at the top will sway
With the buildings.

Gazing out of the window
At the second tower

Swaying will make the sense
Of movement worse.

On the 110th story,
A gust of wind

Will trigger instant nausea.

It would be impossible
To work there.

So the wind tunnel test provided
Wind loads,

But we didn't know whether those
Accelerations were acceptable

To people who are
In the building, the occupants.

Would they feel sea sick?

We don't know.

There were no guidelines around.

Narrator:
How much sway humans can endure

Is a critical question
For skyscraper engineers.

To investigate, structural
Engineer antony darby

Launches the "V" simulator,

A state of the art virtual
Reality skyscraper

At the university
Of bath, england.

Heavy duty hydraulics
Suspend a custom made office,

Simulating the sway motion of
A skyscraper in gusts of wind.

Inside, a volunteer wears
3d glasses

To experience
A virtual reality view

Out of the skyscraper windows.

Antony's team records the
Volunteer's physical reactions

As the skyscraper's
Sway intensifies.

We can measure their response,
Their heart rate,

Their eye blink rate,

Things that measure stress
Or tiredness.

We can also assess their work
Performance,

How that degrades
Over time as well.

Narrator:
Antony's team collects data
From hundreds of volunteers

To determine
The breaking point of humans.

He measures the movement
In g-force.

The typical g-force felt on
A roller-coaster is 4g

Compared to just
0.01g for walking.

What we've shown here
Is that below

About 0.005g,
People don't feel the motion.

Above 0.005g, you can perceive
The motion,

But it might not affect
You that much.

But as we steadily get higher up
Around 0.04g,

Then it does become intolerable.

Narrator:
The simulator tests reveal that
0.04g is the upper limit.

Anything above this force
Is intolerable.

Today, this research is used
For high-tech skyscraper design

And rigorous testing.

But in the 1960s, tests
Like these don't exist.

The twin towers team

Has to invent
The very first test themselves.

The twin towers engineers build
Their own test rig

And hang it inside a ventilation
Shaft in the lincoln tunnel.

Volunteers sit inside
The fake office

While two engineers push it
From side to side

And ask them to report
When they feel sick.

Even these crude tests
Are enough to confirm

What the twin
Towers engineers feared --

People aren't able to work
In these conditions.

In order to reduce the 40-foot
Sway, the architects

Create special shock absorbers
To reduce

The movement between the wall
And the floor.

They install 10,000 of these
Dampers across the two towers,

Absorbing huge amounts
Of energy,

Reducing the sway
To just 3 feet.

They are a success.

They are the first skyscraper
Dampers in the world.

See: everything in the original
Twin towers

Was innovative
And ground breaking --

In the design,
In the materials,

In the construction,
And erection.

Narrator:
The twin towers are a modern
Masterpiece

Created with exquisite attention
To detail.

By creating these technological
Solutions, the engineers

Keep the 50,000 people
Inside safe

From these foreseeable dangers.

But the threat that lies ahead
Is unimaginable

To its creators --

The deadly impact of the
Terrorist attack on 9/11

Surpasses any force engineers
Could ever predict.

♪♪

Narrator: in 1973,
The twin towers

Become the tallest
Buildings on the planet.

These engineering marvels
Rise 1,368 feet into the sky,

But no one could foresee
The challenges

They would face
Over their lifetime.

In 1975, an electric short
Circuit failure

Set six stories
Of the north tower on fire.

Firefighters manage to control
The blaze.

Fortunately the building
Was unoccupied

Or there would have been,
Potentially,

Some serious consequences.

Narrator:
20 years later, a bigger impact
Strikes --

A terrorist plants
A 1,200-pound bomb

In the north tower garage.

It explodes
With devastating force,

Yet the tower stands strong.

But on september 11, 2001,
A tragedy bigger

Than anything imaginable
Strikes the towers.

The world watches in disbelief.

Over 17,000 people are working
Inside the twin towers

When suddenly a
Terrorist-hijacked boeing 767

With 92 people on board crashes
Into the north tower.

The impact creates
A burning hole

Between the 94th and 99th floors
Of the 110-story skyscraper.

The 10,000 dampers absorb
The energy from the impact,

And the hollow skeleton design
Supports the weight

Of the building
Despite the damage.

17 minutes after
The first plane hits,

A second
Hijacked boeing 767

With 65 passengers and crew
On board...

...Crashes into the 77th to 85th
Floor of the south tower.

There was a tremendous outburst
Of smoke, debris.

Debris from both impacts,
Particularly the second one,

Rained down on the whole area.

Narrator:
Over 1,000 people perish
In these sudden crashes.

While the towers are still
Standing,

Over 15,000 people
Manage to evacuate.

But the tragedy isn't over.

The flammable jet fuel
From the aircraft

Makes the crash much more deadly
Than anyone could have foreseen.

When engineers designed the twin
Towers, they protect

Against a crash from the
Largest aircraft at the time --

A boeing 707.

But the hijacked aircraft
Are much large boeing 767s

Carrying 20,000 gallons
Of kerosene.

This jet fuel pours down through
The 12 miles of elevator shafts

Onto other floors,
Spreading like wildfire.

Inside the towers, there are
Thousands of tons of office

Furnishings,

Many of them highly flammable

So the fire spreads
Unusually quickly.

The firefighters soon realize
That there was no controlling

This blaze,
And from then on,

It was concentrating
On not saving the building

But on rescuing
People inside.

Narrator: at exactly 9:59 a.M.,
Almost two hours

After the first collision,

The south tower collapses...

...In just 10 seconds.

Half an hour later,
The north tower goes down, too.

More than 2,000 people
Are still inside the buildings.

Darton:
It was impossible, I think,
For anyone --

Engineer, tourist, new yorker,

To imagine that these towers
Could possibly come down.

Narrator:
After standing strong for nearly
Two hours,

What ultimately causes
The towers to collapse?

The twin towers'
All-steel structure

Is exceptionally strong,

But its innovative design
Hides potential weaknesses.

At the base of each tower,

The steel columns
Are 4 inches thick

To bear the weight
Of the building above.

But near the top, these columns
Are only 1/4-inch thick.

To protect the steel from fire,
Engineers coat it

In a fireproofing spray...

...But this protective layer
Is thin and easily dislodged.

Could this be the downfall
Of these iconic buildings?

♪♪

Structural engineer,
Adrian brugger investigates

If the fire could be
The cause of the collapse.

Brugger: the ensuing fire raged
Inside the building.

The impact caused massive swaths
Of windows to be torn out

And this extra damage
Created winds

That were actually
Fanning the flames.

Narrator:
Although the fireproof coating
Can withstand the flames,

The impact of the aircraft
Leaves it compromised.

The aircraft entered
The building

Between 400
And 550 miles per hour

And the impact
Lasted only 0.7 seconds.

And of course,
During that impact,

We actually had all of
That fireproofing stripped.

Narrator:
Adrian's team investigates
How fire affects steel

That has been stripped
Of its fireproofing.

His team uses floor beams

Identical to those
From the twin towers.

They load them with 1.4 tons
Of weight

To simulate the weight
Of each floor of the tower.

They prepare to heat up
The steel beam.

We will apply heat
By simulating a fire.

Narrator:
The temperature of the fire
Climbs

To 1,800 degrees fahrenheit --

Equivalent to that
Inside the towers.

Now we're heating up the beam
With flame throwers,

And you can see
The temperature is going up

On our infrared camera.

We're reaching about 500 degrees
Celsius,

So that's 800 degrees
Fahrenheit.

Narrator:
At 800 degrees fahrenheit,
The trusses hold strong,

But then they start
To show signs of weakness.

Now we see a cherry red spot
On the beam that's indicative

Of about a temperature
Of 750 celsius,

Which is equivalent
To 1,300 fahrenheit.

The beam is actually
Deflecting significantly.

Narrator:
The steel beam gives way after
Just 11 minutes

At 1,300 degrees.

Adrian's investigation shows
That without fireproofing,

The floor beams
Cannot withstand the high heat

And lose their strength.

The extreme heat weakens
The steel floor plates

Causing them to sag,

Pulling the walls
In on each other.

The heat also weakens the steel
Bolts

That hold the floors in place
Causing each floor

To collapse down
Onto the one below --

A process known as pancaking.

This catastrophe goes down
In history

As america's darkest day.

The wreckage burns for 100 days.

♪♪

Rescue operations salvage
The structural remains

Of the twin towers.

Senior vice president
Of the 9/11 memorial museum,

Anthony gardner,

Leads the mission to preserve
The twin towers structures.

1.8 million tons of debris fill

Ground zero
In the aftermath of the attacks.

Essentially it was 7-story
Smoldering pile of material

That stretched up
From street level

And then also down seven stories
To the bedrock level of the site

Where I'm standing today.

Narrator:
The scale of destruction
Is overwhelming.

But despite the force,

Some structures
Survive the collapse.

These elements are conserved
On the 9/11

Memorial site to this day --

A display of respect and a place
To remember

Those that lost their lives.

One of the columns
Of the twin towers

Was found intact
In its original position.

So this is the last column that
Stood, despite the destruction.

Rescue and recovery workers
Adorned it

With all these inscriptions

And tributes
To their fallen colleagues.

Narrator:
As the clean up operation
Evolves, the twin towers

Deep foundations
Come into view.

Buried by rubble, can the
Damaged bathtub wall

Continue to hold back
The hudson river?

What will it take to keep this
Area of lower manhattan safe?

♪♪

Narrator:
September 11, 2001...

After months of wreckage
Clean up,

Just a handful of structures
From the towers remain --

A reminder of the fight
The skyscrapers

Put up against the attacks.

Can any part of these buildings

Possibly stay
Standing to this day?

Above ground,
Very little survives --

A single staircase.

A steel column from the core
Of the south tower.

And steel tridents that

Once formed the lower facade
Of the towers.

But below ground, something
Far bigger survives --

The vast concrete bathtub

Built to protect
The site's foundations.

But deep cracks span the wall

And many anchoring bolts
Begin to leak ground water.

Without repair, the wall
Could fail,

Jeopardizing nearby buildings.

What will it take to keep this
Area of lower manhattan safe?

Head of conservation,
Lisa conte,

Leads the preservation mission
At the 9/11 memorial.

Her four-person team
Works all year round

To keep this
Historic structure standing.

Here we are looking at a portion
Of the original bathtub.

Just beyond it is
The hudson river.

Narrator:
Following the events of 9/11,
The damage to the bathtub wall

Threatens new york city
With a potential catastrophe.

If the bathtub wall fails,

The area will rapidly flood
With groundwater

And water from the nearby
Hudson river.

This water would then inundate
The subway tunnel

That runs
Through the bathtub...

...Threatening widespread
Flooding of

The underground rail network.

With the bathtub breached,

The surrounding ground
Would weaken, risking subsidence

Or even collapse
Of surrounding buildings.

To detect any weak spots
In the wall,

Lisa's team turns
To cutting-edge technology.

Every year scanning specialist
Scott lee

Laser scans the entire wall
To millimeter accuracy.

His lidar scanning works like
Echo location,

Bouncing beams off the surface

And recording the distance
With incredible accuracy.

By doing the lidar or laser
Scanning, we create

A very accurate millimetric
Model of the entire slurry wall.

Narrator:
The team also takes super high
Resolution photos of the wall

To record the surface detail.

A platform raises the team
Into the air

To capture the wall
From every angle.

Every detail matters.

Scott then
Combines the lidar scan

And the thousands of photos

Into a super high resolution
3d model of the wall.

We project those really high
Resolution photos

Onto that solid surface model

To create a very detailed
Photo realistic model.

We can see
Every nook and cranny.

Often we can see details you
Can't see from the human eye.

Narrator:
Armed with this 3d model,
Lisa's team can hone in

On the weak parts of the wall
That need attention.

The brown that you see here
Is corrosion staining related

To the interaction of the water
And the steel elements.

Also present,
But less easy to see, are salts.

And those are the areas
Of white here

And coming down the face here.

Narrator:
The 3d scan pinpoints areas

Where new water
Starts to seep through.

It's really important that we're
Working to preserve the wall

Because everything needs
To be cared for

In order to preserve
Its integrity over time.

Narrator:
The wall shows great strength
And resilience.

With the help of lisa's team,
It will remain standing for many

More decades to keep
The surrounding city safe.

This colossal underground
Structure

Is a reminder of
The innovative engineering

That made these radical
Skyscrapers a reality.

How have the twin towers'
Innovations

And their tragic end

Changed the design
Of skyscrapers forever?

Narrator: the twin towers,
Revolutionary skyscrapers

That have changed
The world forever.

What can we learn from
The towers

And the events
That lead to their collapse?

The impact of two
Hijacked airplanes

Creates massive gashes
In the towers

And an unprecedented fire
That causes them to collapse.

2,753 people die,

But the building's put up
A remarkable fight

In the face of the attack.

On the north face
Of the north tower

2/3 of the columns
Were taken out,

But yet, the towers stood
For more than an hour,

Which allowed occupants

Who were below
The force of impact to escape.

Darton:
The death toll was horrific,

But had they not been as well
Built or engineered

In the wonderful way
That they were,

They probably would have
Collapsed much earlier

And the death toll would
Have been exponentially higher.

Narrator: the twin towers are
Revolutionary structures,

Revered by engineers
All over the world.

They are the first to break
Through the ceiling

Of skyscraper height,

Passing this legacy
Of innovative

Engineering onto
Their successors.

The skyscraper has always been
An engineering marvel,

But it was the twin towers
That really set the precedent

With their revolutionary
Technology,

With all the different
Extraordinary inventions

That had to be created
To even make it happen

That set the precedent
For the design and engineering

Of the even taller buildings
Around the world today.

Narrator:
Two of the most recognizable
Skyscrapers of all time.

Two of the first buildings
To use a bathtub design.

They paved the way
For skyscrapers

On reclaimed land
All over the world.

The hollow tube design
Is now adopted

By thousands of buildings
Worldwide.

With subway inspired express
Elevators paving the way

For people to travel
Through cities in the sky.

And their lightweight flexible
Structure triggers

A whole new field of research
Into skyscraper wind dynamics,

Setting the stage for cutting
Edge research for years to come.

13 years after the 9/11 tragedy,

A new building stands at
The world trade center site --

The new one world trade center.

Rising 1,772 feet tall, it is
New york's tallest skyscraper.

It is designed with features
To remember the people

And the original twin towers
With protection

As a top priority.

See: like world trade center one
And the other one's here

Are designed
With every safety feature

That the engineers
Could come up with.

Since 9/11, more public
Buildings

Like the world trade center
And other office buildings

Have adopted
Bomb blast criteria.

♪♪

Narrator: today at ground level,
Massive voids

Show the footprints
Of the towers.

♪♪

So this is the north tower
Memorial pool,

And the memorial pools mark
The location

Of where the towers stood
At the street level of the site.

And here, etched in
The bronze parapets

Are the names
Of the nearly 3,000 people,

Including my brother,
Who perished on 9/11.

Narrator:
The twin towers and those
Who lost their lives in 9/11

Are never to be forgotten.

♪♪

The twin towers rose
On new york's skyline

As a powerful message
Of american innovation,

Breaking boundaries
And reaching higher

Than anyone
Had ever gone before.

Their fall was the darkest day
In america's history,

But their legacy lives
On through other skyscrapers.

A legacy never to be forgotten.

♪♪

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