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Today on Impossible Engineering, the world's toughest structure, the largest

hydroelectric power station in the world. I think it's the greatest project

ever seen.

And a football stadium of record -breaking proportions.

It's the longest continuous single arch span in the world and really an

incredible engineering feat.

It took revolutionary engineering to make the impossible.

China, the world's most populous country.

At 1 .3 billion people and rising, the country's infrastructure is under

pressure.

In downtown Shanghai, it's so easy to see how much energy is consumed on a

basis. In this city alone, population has grown to 24 million people.

To sustain this many people, the country consumes almost as much coal as the

rest of the world.

But China needs a more sustainable way to keep the lights on.

Their solution, the Three Gorges Dam, the largest hydroelectric power station

the world.

It's over 7 ,500 feet long. That's 21 football fields and

holds back a 400 square mile reservoir.

Construction began on this incredibly tough structure in 1994.

Getting the build right is a matter of life and death for the millions of

living further along the Yangtze River.

pushing against the wall.

The residents downstream are really depending on this wall to stay up.

Any imperfections and the consequences could be catastrophic.

A concrete structure of this magnitude would be impossible without one of

America's greatest engineering achievements.

than had ever been built.

Even today, roughly 80 years later, it takes your breath away.

Weighing in at 6 .6 million tons of concrete, this was an

unparalleled engineering marvel.

The dam harnesses the power locked within the mighty Colorado River.

The dam stands 700 feet tall and has a base thickness of 660 feet.

The biggest problem and the biggest challenge was one of sheer scale.

But the severe southwest heat makes building a structure as big as the

Dam extremely difficult.

If engineers poured all of the Hoover Dam's concrete in one go, it would take

125 years for it to cure and cool.

meaning uneven setting and potentially catastrophic cracking.

Hoover Dam Project Supervisor Frank Crow came up with an ingenious solution, one

that can still be seen deep within the old inspection tunnels running through

the dam.

The solution was to pass extremely cold water through one -inch pipes.

And the amazing thing is we can actually still see evidence of those pipes here.

In 1931, thousands of workers began building the Hoover Dam using gigantic

blocks, cooling the concrete with ice water produced by a refrigeration plant.

As an engineer, this is an incredible sight to see.

Over 80 years later, the Hoover Dam still provides electricity to three

The Three Gorges Dam is five times bigger and generates an incredible 11

more power than the Hoover Dam.

The height of the dam is 181 meters and the total length of the dam is

2 ,309 meters.

Building it requires almost a billion cubic feet of concrete.

In 1998, pouring begins.

To accelerate the curing process and reduce the risks of cracking.

Engineers take techniques pioneered at the Hoover Dam to a whole new level.

The ingredients are air cooled before they're mixed.

High -speed conveyors take the concrete from mixing zone to site in just 15

minutes.

Workers pour an average of 700 ,000 cubic feet every day.

More than 30 ,000 people took part in the construction.

Water cooling is supplemented with a mist sprayed over the working area to

reduce the effects of the hot summer weather.

It takes eight years to pour all the concrete.

In 2006,

the whole dam was completed.

It's the most important milestone of the project.

This massive structure harnesses the clean hydroelectricity potential of

largest river, the Yangtze.

The Yangtze River is the third largest and longest in the world.

And the river is now the lifeblood for the thousands of people who live along

it.

But blocking ship traffic on this busy waterway with a giant dam is simply not

an option.

100 million tons per year.

To get the ship through, the Three Gorges team is employing a technique

dates back to medieval times.

Here is the Three Gorges ship lock.

It's the largest ship lock of this kind in the world.

The lock is almost a mile long.

It raises and lowers river traffic 370 feet.

through five giant steps.

But there's a catch.

It takes about three hours and 40 minutes or even four hours to pass

ship lock.

Taking four hours to pass through the locks is simply too slow for most ships

traveling down the river.

So Mr. Ding and his fellow engineers must come up with another solution, and

fast.

Tough enough to hold back the mighty Yangtze River, the Three Gorges Dam is

largest hydroelectric power plant in the world.

But getting vessels through the dam's massive shiplocs quickly.

would have been impossible without the pioneering engineers of the past.

In 1870, designer Edwin Clark was asked to solve a particularly tricky problem

facing the small village of Anderton in the north of England.

He was asked to link the busy Trent and Mercy Canal with the adjacent Weaver

River to speed up journey times for barges carrying valuable commercial

of salt.

This was a pretty stiff challenge for Clark in the late 1800s because the

between the canal and the river was about 50 feet.

So Clark developed an ingenious solution.

Known as the Iron Spider, the Anderton is the oldest operating boat lift in

the world. It's extraordinary.

It's a three -story high marvel of Victorian engineering.

Clark's creation scoops up barges and the water they are floating in,

transferring them in one smooth action.

At the time, this was revolutionary, but the Iron Spider is based on a

fundamental principle of water pressure.

If you apply a pressure on a liquid in a closed system, then that pressure is

distributed throughout the whole liquid in the system.

And Edwin Clarke used this to great effect with his boat lift at Anderton.

He started off by making two watertight caissons. These are the tanks which held

the boats and the water in which they floated.

And he supported those caissons on top of two hydraulic rams.

And there was a liquid in those rams and a pipe joining them both together.

So as I exert some additional force to this hydraulic ram,

I can start to see that the pressure is being pushed through

the adjoining pipe and lifting up my other.

hydraulic ram.

It's brilliant.

At the Three Gorges Dam, designers are building a shiplift similar to Edwin

Clark, but on an epic scale.

big massive swimming pool and being pulled up.

It's an engineering piece of beauty in so many ways and super impressive.

The Three Gorges ship lift can carry a 3 ,000 ton passenger liner.

Reinforced concrete towers support the lift's 433 foot steel pool.

Instead of hydraulics, the lift uses massive counterweights and pulleys to

the pool and vessels floating in it, a vertical distance of 370 feet.

I think it's the best ship lift in the world.

The counterweight of the ship lift weighs nearly 16 ,000 tons.

This world record -breaking lift allows ships to pass through the dam quickly

and easily.

Stretching nearly a mile and a half across the Yangtze River, it holds back

almost 400 square mile reservoir.

But excess water can produce serious consequences.

Now, if we imagine a flood situation where we have lots and lots and lots of

water flowing over this dam, we've got all that water hitting the bottom of the

dam.

As we can see here, the integrity of the dam becomes very questionable and it

starts to collapse. So there it goes.

To prevent it, engineers had to look to the path for inspiration.

The Mares Dam in central France is almost 300 feet tall.

Its builders feared overflowing water would destroy the foundations,

leading to its collapse.

To prevent this, engineer Andre Coyne developed something novel.

This is a ski jump spillway.

So cold because it has a lip at the bottom just like a ski jump.

And the ski jump prevents that water carrying all the way down to the base of

the dam, where it can seriously erode the foundations.

It gets flicked into the air, and all that water turns into droplets as it's

mixed with the air, and the energy is dissipated.

Koi changed the way dams were designed forever.

The colossal Three Gorges Dam uses 46 ski -jump spillway that push floodwaters

300 feet away from the dam's foundations.

But how do engineers harness the power of the Yangtze River to make the Three

Gorges Dam the most productive hydroelectric power station on Earth?

The monumental Three Gorges Dam is the most productive hydroelectric power

station on the planet.

Tough enough to harness the power of the Yangtze River, this feat would have

been impossible without the engineers of the past.

Designers look to the innovations of 19th century engineer James B. Francis

his work in the busy textile town of Lowell, Massachusetts.

The problem was that the mills here in Lowell were being driven by simple water

wheel systems.

These are relatively inefficient.

They're driven by water falling into the buckets to make them turn.

Only using a portion of the energy available here in the canal system.

What Francis designed changed the world forever.

His original invention can still be found deep within Lowell's Canal

Wow.

So this is it.

This is the site of the very first James Francis turbine.

This turbine right here started the hydropower revolution.

James Francis took the idea of a water wheel and turned it on its side.

He enclosed the turbine so water was in constant contact with the wheel.

He added a series of vanes to direct the water at the optimum angle.

The sum total of all those enhancements led to nearly 90 % efficiencies of the

turbine.

At the biggest hydroelectric project on Earth, the Three Gorges Dam uses the

world's largest Francis turbine.

This is one of the two turbine buildings, and just listen to that.

That is the sound of huge quantities of water traveling at up to 80 miles per

hour, turning the 32 largest ever Francis turbines built.

Installing the giant turbines was an engineering feat in its own right.

Each turbine weighs 450 tons.

The crane needed to install them had to be factored into the design of the dam.

These 450 ton turbines can generate the equivalent...

of 25 million tons of crude oil, and wait for it, 50 million tons

of coal.

Water enters through a series of huge inlets and falls 260 feet towards the

Francis turbine.

With a flow rate of up to 33 ,000 cubic feet a second, each turbine rotates at

75 revolutions a minute, driving the generator above.

The potential for China is huge, but the dam's location is a daunting 600 miles

away from Shanghai.

Engineers needed a system that would deliver that energy, huge distances with

minimum losses.

And they wouldn't be able to do it without one of history's most

inventors.

Engineering genius Nikola Tesla.

In 1895, the first large -scale generating plant in the world housed his

groundbreaking, super -efficient transformer.

To demonstrate what a transformer does, we have the simple racetrack set up

here. And I'm going to start by supplying the same amount of voltage to

track.

That's exactly what we expected.

It does heat.

Now let's do the same experiment again, but step up one tracks voltage with a

transformer.

So now, let's rake.

Sure enough, the stepped -up voltage produced a faster car.

A step -up transformer works by sending power into a primary coil.

creating an alternating magnetic field.

When a secondary coil that has more windings on it is placed beside it, the

alternating field is transferred and, thanks to those extra windings, creates

output with a higher voltage.

Instead of shooting cars down a track, Tesla's transformers enabled the power

company to shoot electricity great distances across the country.

Engineers at the Three Gorges Power Station are supersizing Nikola Tesla's

innovative ideas.

This is the main transformer of the Three Gorges Power Plant.

This is to change the voltage into a high voltage.

The reason we stop the power is we need a long -distance transmission to

Shanghai.

The Three Gorges power plant will generate 84 .6 billion kilowatts of

annually, supplying nine provinces and two cities.

This incredibly tough dam produces more hydroelectricity than any other facility

on Earth.

For engineer Kiwa Ding, it represents the culmination of a lifetime's

dedication.

Harnessing the power of a totally different force.

Every day I walk around and I'm just marveled by the feet that this building

actually is.

Is one of the toughest dome stadiums in the world.

The roof structure itself.

is equivalent to the weight of 99 Boeing 777s.

The Dallas Cowboys is one of the most popular football teams in the NFL.

In 2009, America's team wanted a new home.

The idea from the very beginning was really to create the finest stadium of

kind anywhere on the planet.

Architect Brian Truby was asked to build the largest dome structure in the

world.

The result is the tough structural masterpiece.

The epic AT &T Stadium.

AT &T Stadium can host a jaw -dropping 105 ,000 fans.

The massive structure is almost 1 ,300 feet long, with the world's largest

sliding glass doors on each end.

The building spine is two of the longest unsupported arches on the planet.

They support a state -of -the -art roof that can be opened and closed at will.

But building a roof over a football stadium is no easy feat.

The huge forces that come into play to keep 104 million cubic

feet column -free are enormous.

So how do you hold up one of the largest single -span roofs in the world without

any support?

The massive AT &T Stadium can hold 105 ,000 fans, but supporting its gigantic

dome structure without columns requires tough infrastructure.

The solution comes from the 19th century Portuguese city of Porto.

In 1875, engineers were building a new rail line between Lisbon and Porto.

But when they arrived here on the banks of the Douro River, their progress

ground to a halt.

At the time, a span of this size was considered extremely challenging.

And so a bold and innovative new approach to bridge design was required.

French engineer Gustave Eiffel was up for the challenge.

Now for bridges, the arch is actually a really brilliant shape, but it does have

its limits.

And I can illustrate that using this piece of card.

If I place this card between two stones that represent the bridge abutment, and

I place a load on it, it supports that load using compression, and the

compression flows down through the arch and into the abutment.

But of course, in a location like we have here, where we need a much larger

span, we have to increase the length of our span, and our arch becomes much more

shallow, and then the forces of tension start to take over. And as my structure

is loaded, you can see that it struggles to support that load.

And this was precisely the problem that Eiffel faced here at the Douro.

Luke is heading into the heart of Gustav Eiffel's solution.

So here I am dangling halfway up of Eiffel's magnificent arch structure, and

have to say it's a real privilege to be able to do this.

By making use of a simple system of triangles, Eiffel was able to create a

structure that was both light and very strong and stable, and had a bigger span

than ever before. And it's a real marvel of engineering.

Simply genius.

So to understand how this truss system works, we can make a simple comparison

between two shapes that are commonly found in engineering.

If we look first at the square, you can see that if I push down on it, it

doesn't take long until that square deforms and collapses. And that's

the square lacks inherent stability and rigidity.

But you can see if I take this triangle and I apply a vertical load to it, you

can see that it's able to take that load. And that's because these two side

elements go into compression, this bottom element goes into tension, and

equilibrium created at the point where I'm applying the load.

By using this truss system of triangles, Eiffel left his mark on engineering

history when his brig spanned the greatest distance of its age.

Eiffel's experiences here ultimately provided him with the engineering

and techniques to go on to create his most famous structure, the Eiffel Tower.

The designers of AT &T Stadium are using Eiffel's innovative arch truss design

in record -breaking proportions.

The stadium's twin arches soar over 300 feet above the playing field.

Each one weighs over 3 ,000 tons, distributing 19 million pounds of thrust

four colossal concrete abutments.

But designers need these arches to support yet something else.

One of the largest challenges that we had is it's not natural for a

structure to be on top of that. It creates a very steep slope.

At the very steepest level, we're at a 23 -degree angle, which is really the

steepest retractable roof that's ever been done.

How do you move an 837 -ton roof panel up and down such a steep incline?

It's a task that would be impossible had it not been for a high -altitude

innovation.

150 years ago, businessman and avid hiker Sylvester Marsh devised an

solution.

After getting trapped on Mount Washington in a storm, he decided to

railroad so tourists could visit the top of the mountain.

The problem Marsh faced, of course, is that this grade is simply too steep for

regular railway.

In a typical car, the wheels would have spun without any friction, and the train

would have slid right back down the mountain.

To overcome the extreme gradient, Marsh built a train that can literally grip

the track, a system known as rack and pinion.

The pinion is a toothed cogwheel positioned centrally on the underside of

railroad car.

The pinion engages with a rack, which lies between the track's running rails.

You can hear the pinion engaging into the rack, providing that extra

and the friction that's needed to overcome this steep grade.

Marsha's design was an engineering marvel.

When it opened on July 3, 1869, tourists flocked to take the train and come up

to the top of this mountain and enjoy this incredible vista.

Engineers at AT &T Stadium in Texas are using Marsha's rack and pinion system on

an epic scale.

A cutting -edge track guides 1 ,600 -pound panels up and down the stadium's

steep -inclined roof.

You can see the rack here, and then the gear pin here will interlock with this,

so it's an extremely robust connection that very directly connects the roof to

the retractable portion.

The gigantic roof needs surprisingly little power to open and close.

The retractable roof is powered by a series of 7 .5 horsepower motors.

Each side is powered by 64, so it's roughly a Corvette engine pulling and

pushing the roof as it moves along.

When combined with the world's largest sliding glass doors, this awesome

structure offers a unique open -air experience.

When everything is open, you really have almost an outdoor experience that you

would get in an outdoor venue.

I think that flexibility and that uniqueness is what sets the stadium

But when the doors and roof are closed,

keeping all 105 ,000 spectators comfortable in the boiling summer months

Texas is a huge engineering challenge.

There were two big influences, the external heat hitting the building and

heating it up. And you've got 80 ,000 plus fans generating heat. You've got

the technology, including video and sports lights.

All those things create a particular challenge.

So how do you keep such a bat stadium cool?

What you'll see as you look across the top of the upper bowl are huge ducts.

Many of these ducts are about six feet in diameter, so you can actually walk

through the middle of them.

The system can throw out 11 ,000 tons of cooling capacity.

What they're doing is delivering a curtain of air that washes down over the

surface of the seating bowl, so we're really not air conditioning.

all 104 million cubic feet. We're actually air conditioning a zone about

12 feet tall.

This downward flow of cool air blankets spectators.

It remains in place because of its density.

Cold molecules are packed close together, increasing their weight.

Low -density hot air rises, accumulating in the vast space in the dome roof.

The size of the interior is one of our key physical properties that allows us

separate the cooled air from the heated air and keep the seating bowl cool.

But to truly be a multi -use venue, engineers need a surface that can suit a

variety of needs, not just football.

that it needed to be a venue that would not just house the Dallas Cowboys. We

wanted to be able to be flexible so that we could move our field out to grass or

grass to concrete or concrete to wood.

To transform the stadium, engineers must seek inspiration from a structural

masterpiece of the past.

The epic AT &T Stadium is one of the toughest dome structures in the world.

But to transform this football stadium into a multi -use venue, engineers must

look to the past.

Developer Roy Hoffines built an indoor baseball stadium that could comfortably

house the Houston Astros away from the glare of the Texan sun.

It was the world's largest indoor arena with a circumference of nearly half a

mile. At 350 ,000 square feet, it made it the largest air -conditioned room on

Earth.

But a significant problem emerged when it opened.

Players used to complain that the lucite in the dome roof would reflect light,

making it virtually impossible for them to catch a fly ball.

Some of the panels were painted white, but then that led to another problem.

The stadium grass started turning brown and eventually died.

The stadium needed a solution fast.

Two American engineers, James Faria and Robert Wright, had the answer.

The pair developed chemgrass, a synthetic turf consisting of a carpet of

grass blades stretched across a thin rubber base.

And in 1966,

the Houston Astros played their first ballgame on a fully artificial field

in honor of the stadium, it was renamed to AstroTurf.

Its portability allowed the Astrodome to host anything from basketball to

racing.

AstroTurf changed sports architecture around the world.

And the Astrodome provided the blueprint for an indoor multipurpose menu.

We have this surface for the Dallas Cowboys. We have a completely different

field for college football.

We've had grass in here for soccer.

And we have had basketball here as well.

I certainly think the multi -use aspect of it will be one of the legacies of

this stadium.

AT &T Stadium can house over 105 ,000 spectators.

But to fill all those seats, the stadium needs an unrivaled viewing experience.

You can set up one of the finest TV, surround sound, high -tech environments

your own house. So what we had to do here was be better than that.

How do you make every seat the best seat in the house?

Watching sports on television at home is a national pastime, but to compete with

that experience, the engineers of AT &T Stadium had to develop an unrivaled

viewing experience.

The ability to record a moving image onto film was achieved by the late

but transmitting an image so it can be viewed in another location is a much

greater challenge.

Russian engineer Vladimir Zvordkin provided a starting point in 1931.

He patented a camera tube known as the Iconoscope.

Within the tube, a captured image was projected onto a mosaic of light

-sensitive material, breaking it into thousands of picture elements known as

pixels. A sweeping electron beam charged the pixels before they were fed out of

the camera.

Rorykin's invention allowed us to turn a picture into an electrical signal.

But it was how that moving image was transmitted and then replicated on the

screen at the other end that's the really incredible part.

The Emetron was the British Broadcasting Company's first industry standard

camera. By 1937, it was possible to broadcast live into people's homes.

It was here, at Wimbledon, that the BBC's first live broadcast of a sporting

event would take place, with the men's final being beamed into homes across

London. And the hefty young man from across the Atlantic defeated the good

-looking Continental 6 -3, 6 -4, 6 -2.

The key to transmitting a television picture is in how the images are

from the camera.

So imagine this is a single still image of a tennis champion. It's a bit hard to

believe in this case, I know.

What happens is that inside the camera, that image is scanned into individual

lines, broken down like this.

And then these lines are taken out of the camera up to the transmitter,

transmitted through radio waves, which are picked up by the aerial on top of

your home.

Then they're brought down into your television, where these lines are

reassembled again into a full image.

And obviously, this has to happen very quickly.

But that's just a single frame.

To create a moving image, TVs needed to do this at least 24 times every second,

fast enough for the image to appear to be moving.

This illusion of movement is much the same as you get with a flick book.

By looking at a number of still images in very quick succession, you get the

illusion of movement.

Transmitted from London through Alexandra Palace's television tower,

like the Wimbledon final mark the arrival of one of history's greatest

innovations.

Engineers in Texas are redefining live broadcast using one of the world's

largest video boards.

It doesn't matter where your seat is. You are a part of the action that you

see the passion and the inspiration on the participants' faces.

These monumental arches are once again providing a critical engineering

for this extraordinary building.

We were all out here and they hoisted that board up. That's such a feat to

marvel over and to appreciate just how complicated the engineering was.

Two.

One.

Since opening its doors in 2009, AT &T Stadium has hosted over 10 million

visitors.

By learning from the great pioneers of the past, adapting, upscaling, and

innovations of their own.

The engineers of the Three Gorges Dam and AT &T Stadium have created some of

toughest structures on the planet.

This venue proves that if you have huge aspirational goals, you can actually

attain something that's never been attained before.

They've made the impossible.

Possible.

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