The Assassin (Sha ren zhe Tang Zhan)

Today, on "Impossible engineering,"

the Shanghai tower,

the most technologically advanced skyscraper

on the planet...

...built to withstand earthquakes and typhoons...

...using technology never before seen in a skyscraper...

...to make the impossible possible.

captions paid for by Discovery communications

Shanghai is a megacity.

With 24 million inhabitants and rising,

it's one of the most densely populated cities in the world.

With a population of 10,000 people per square mile,

the only place left to build is up.

By the mid 1990s,

development reached an area of farmland

on the east river bank known as Pudong.

And in 2008, work began

on the most impressive building of them all...

The Shanghai tower.

Over 2,000 feet tall,

it's the second tallest building in the world

and the tallest ever built in a seismic zone.

It's the most technologically advanced skyscraper

on the planet,

with 128 floors and 9 indoor gardens,

where 16,000 people will work, sleep and play.

It truly is a city in the sky.

For lead structural engineer Dennis Poon,

the project was an exciting challenge.

The engineering challenges for the Shanghai tower

began at its foundations because, not only

is Shanghai in an active seismic and typhoon zone...

...it's also sinking.

The land under this massive Metropolis

is deflating like a giant air mattress.

It's shallow-water table

is collapsing under the weight of the city's modern buildings.

Solid bedrock is 650 feet down.

Above the bedrock is a soft layer of sand, clay, and soil.

Without a solution,

the 850,000 ton Shanghai tower would surely sink.

The engineers only have one shot.

There's no room for error

when you're building a tower this tall.

A modern world full of skyscrapers

was once inconceivable.

Early man was limited by what nature provided.

Ah!

Aw... For the ancient Egyptians,

taller meant wider and an awful lot of manpower,

unless you believe in some of the more otherworldly theories.

When it comes to building towers,

the ground below has frequently thrown a wrench in the works.

Mamma Mia!

In order to create a skyscraper 11 times

taller than Pisa's famous learning tower,

the designers of the Shanghai tower

will need to draw inspiration

from the work of a 19th-century engineering pioneer.

Chicago is home to some of the world's most iconic skyscrapers.

But just over a century ago,

building anything taller than just a few stories

was thought to be impossible.

You wouldn't know it to look at it,

but Chicago is actually one of the least logical places

that you could ever attempt to build a skyscraper.

We've got soil here that is incredibly squishy.

Underneath the kind of shorter, smaller layer of earth,

we've got a very, very deep layer of soft, squishy clay.

This is a really difficult thing

to build a skyscraper on of course because,

when you load the building, the building's going to sink.

As Chicago began to boom

and property downtown became more valuable,

the demand to grow higher and maximize space

posed a problem for city planners.

Taller buildings meant heavier buildings.

And, as they had discovered,

there was a limit to weight the soil could handle.

This is the auditorium building of Roosevelt university,

designed by Adler & Sullivan in 1889.

It's a national landmark

and so important to Chicago as well.

The auditorium building at Roosevelt university

is 236 feet tall.

When it was completed in 1889,

it was the tallest building in the city.

The radical idea that made this building possible

came from engineer Dankmar Adler.

We're headed down to see Adler's specially designed foundations

to deal with the soil here in Chicago.

Previously, the weight of a building

would bear down on its walls.

It would sink into the clay like a cookie cutter.

Adler's idea was to use wood and steel crossbeams

encased in concrete to create a reinforced concrete raft

for the 110,000 ton building to sit on.

I'm gonna illustrate what's happening here with this piece

of modeling clay used to represent the clay

that's under our feet here in Chicago, this sort

of peanut-butter-jelly-like substance.

The clay is really quite deep,

and the bedrock doesn't start in Chicago

until 75 to 100 feet below the surface.

So I've got my chopstick here to sort of illustrate.

And I take my structure,

and I go through the hardpan.

And they just didn't have the technology in the 1880s

and the 1890s to excavate that far down to bedrock.

So instead, architects and engineers here in Chicago

are coming up with new ways to solve this problem.

The method that we're standing on here

is sort of the equivalent

of this kind of penny that I'm gonna illustrate.

So this pad or this raft is put down,

and then the building, the column on top of that

is put on top of that.

And you can see that I can push it down a little bit

into the clay, but not really that far.

This is distributing the load.

Adler's revolutionary engineering solution

proved that building on substandard soil was possible.

Today, Chicago's tallest building,

the Willis tower, is 1,450 feet tall.

That's more than six times the height

of Adler's auditorium.

But despite its influence on the world of skyscrapers,

Adler's design wasn't perfect.

So, on the north edge of the building,

we've got this large, heavy exterior walls

that are much heavier than what...

What's happening on the interior of the building.

And we can see the challenge with that,

the fact that this is much heavier

and this is much lighter in the building

with this long crack here in the floor.

We can also illustrate that pretty well with these marbles.

Dennis Poon and his engineering team

may owe a debt to Dankmar Adler,

but they can't afford to have the Shanghai tower subside

like the Chicago auditorium.

They'll have to take Adler's innovative idea

and supersize it.

At 2,073 feet tall, the Shanghai tower

is the second tallest building in the world.

Engineers can't afford to have it sink unevenly

like Dankmar Adler's 19th century Chicago auditorium.

So Dennis Poon and his team of engineers

have taken Adler's concrete raft design and supersized it.

In 2008, the two-year operation

to build the Shanghai tower's foundation begins.

First, they sink hundreds of deep supporting piles

into the soil.

Then, they pour the concrete raft or mat foundation.

It takes 60 hours of continuous pouring

to create the concrete raft.

2,000 workers and 450 concrete trucks

are needed to complete the operation.

The mat soaks up over 2 million cubic feet of concrete,

breaking a world record.

Without the revolutionary work of Dankmar Adler,

building a tower this tall on the soft Shanghai soil

would be impossible.

But the foundation of the Shanghai tower

is just the beginning.

The tower's structure must be strong enough to support

128 floors,

each weighing in at around 6,500 tons.

To design a structure with the strength

to support over 800,000 tons,

Dennis and his team look to past engineering solutions

for the answer.

At one time, the height of a building

was determined by the thickness of its walls

because the weight of each story

had to be supported by the floors below.

Tall buildings were the realm of the rich and powerful.

Medieval religious architects figured out a way

to design thin walls with large stained glass windows.

But they needed external supports

called flying buttresses.

These made the buildings

grow sideways and were exclusive to the wealthy church.

Bless you, my child.

But there was a revolution on the horizon.

And this picturesque region surround the river severn

in england was at the center of it all.

Shropshire was at the heart of the industrial revolution.

It was the silicon valley of the 18th and 19th century.

It's engineers were at the cutting edge of technology

because they had mastered the production of iron.

This area gave birth

to some of the greatest engineering pioneers of our time

and enabled the construction of iconic structures

like this... iron bridge,

the first cast-iron bridge of its kind in the world.

But just a few miles upriver is a less iconic

but much more significant building.

It wasn't until about 20 years ago

that the historical importance of this building