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Today on Impossible Engineering, the biggest construction project in the

of New York City.

When you think about all the things that we had to do on this site to make it

possible, I mean the project is just astounding.

Over 16 million square feet of space conjured out of thin air. Every week we

come up with challenges that I've never faced as an engineer.

And the pioneering historic innovations. That flame gets pulled into the

cylinder. It causes an explosion.

And that throws the ocean up.

These boats are gigantic.

And these are concrete boats. Can you believe that?

That made the impossible possible.

Manhattan.

26 square miles of some of the most valuable land in the world.

Demand for space is high.

But surrounded by water, spreading out is not an option.

The only way is up.

It's extremely difficult to get a large plot in Manhattan because everything's

been spoken for for over 100 years.

So basically, when you do find a large plot, chances are it's because it's a

railroad yard.

These vast expanses may be free of buildings, but they're strewn with

Any construction seems impossible.

But now on the west side of Manhattan, ambitious engineers are taking that

challenge on.

Hudson Yards, America's largest real estate development, is being built

on top of a working rail yard.

The engineering on this site is incredible.

But it's not one individual piece, right?

There's a whole series of these things added together.

I mean, not just you're designing a building, but then you're designing a

complete specialty underlying structure to be able to support that. And unless

you look out at the Western Rail Yards and see the trains there, you have no

idea what's underneath us.

Changing the skyline is not just one new skyscraper, but six.

With more towers to come, Jeff Butler is the senior project manager on site.

Each of these buildings is an amazing building.

We have over a million square feet of retail space. We have almost 5 million

square feet of residential space, 10 million square feet of commercial space.

It has no comparison in scale.

There have been some great developments in New York, but I'm very excited that

this is the biggest and best.

Built almost entirely over a giant base platform above a working rail yard,

Hudson Yards covers a 28 -acre site on the west side of Manhattan.

Multiple megastructures are being constructed simultaneously, including

third tallest tower in New York, an arts venue that can appear from nowhere, and

a brand new park constructed from scratch.

When Hudson Yards is complete, we'll have 125 ,000 people here working,

and playing every day. It's an entire city within a city.

But this ambitious $25 billion project poses immense engineering challenges.

How do you build New York's highest external observation deck?

We're about 1 ,050 feet up. So the combination of engineering and

the construction sequence was really complicated.

In a city where blackouts can be catastrophic, how do engineers keep

Yards running 24 -7?

The firms at Hudson Yards absolutely cannot live with the threat of power

outages.

But it's the specific site that poses the biggest problem of all.

Jay Cross has masterminded the project from the start.

The actual plot of land is highly valuable because there's just only so

it. There's rivers all around us.

But the West Side Yards was always like the hole in the donut because it was all

train tracks.

Spanning seven city blocks, the West Side Yard was used solely as railway

sightings for Penn Station.

The audacious plan to construct on top of the tracks while the trains continue

to run.

The engineering challenges were significant, not least of which we have

over an operating rail yard. That logistically is a challenge unlike any

How is it possible to build directly over 30 active train tracks?

Part of the answer is to be found 715 miles west of Manhattan.

Architect Marty Sandberg is in Chicago.

Discovering how ambitious engineering helped a newspaper company overcome the

confines of this crammed city.

In the 1920s, the Chicago Daily News was one of the best -selling papers in the

city, and as a result, they had outgrown their current building and were on the

search for a new home.

Unfortunately, there was just simply no open land left at that time.

The problem was the same as that faced by the Hudson Yards team, the railways.

At the beginning of the 20th century, Chicago was the busiest railroad hub in

North America, with more lines radiating out than any other city.

The downside to being such a rail hub was, at the time, Chicago was literally

choked off by rail yards on every side of the city.

But the Daily News decided to attempt an extraordinary engineering feat.

So this is the Daily News building.

I'm a sucker for a gorgeous old limestone building, and this is a great

of it.

In a city hemmed in by train lines, how did they manage to find the space to

build this?

It was down to a new law known as air rights.

The Daily News had bought from the rail company the right to build in the air

above their tracks.

Essentially, the trains were only so high and they weren't really getting any

higher, so they recognized that they were wasting a whole lot of space up

and they saw this as a pile of money just waiting to be had.

Pouncing on this opportunity were architects John A. Holabird and John

Root, Jr.

Incredibly, they built the 26 -story tower block directly on top of active

tracks. So when we look at the building now, you can't tell there's still a mess

of train lines running back and forth underneath.

Everyone coming in there day to day would never have any idea that they're

sitting over an active rail line.

This is what the engineers at Hudson Yards need to achieve.

So how is it done here?

This is our sandbox of Chicago right here.

All we need are areas where we can sink a couple of key foundations in between

the trains.

So we start by seeking out a couple of areas where we can sink these deep

foundations down, trying to aim for the bedrock if possible.

From there, we go ahead,

drill our foundations down nice and steady, and all of a sudden ground level

no longer down here. We're up here 20 feet above.

The building could then be constructed on top of this platform, with the weight

being supported directly by the pillars.

And we've just turned a train yard into some of the most valuable and useful

land in Chicago.

The city is happy.

The railroad is happy. The person walking down the street is no longer

next to a train. It really turned into a great solution for everybody.

Holabird and Root's incredible achievement led to an explosion in air

Railroad plots, once shunned by developers, were now worth millions.

In New York City, the Hudson Yards development is thought to be the biggest

rights deal ever.

A billion dollars paid for fresh air above Manhattan's Western Rail Yard.

Tracks covering 28 acres must somehow be straddled to build multiple

skyscrapers, a supersized steel sculpture,

an assortment of public buildings, and 14 acres of open space.

And throughout construction, the trains below have to keep running.

We work basically at the mercy of rail traffic.

So think of it as they own the basement and you own everything above the

basement. And so of the 30 tracks, we are only allowed to close four tracks at

time for the purposes of construction.

What we had to then do is weave all of the foundations for the buildings

the track to go down to rock.

The foundations for this massive 28 -acre site draw on techniques used in

Chicago over 90 years ago.

The first step is to drill holes nearly 150 feet deep into the bedrock, which is

78 feet below the ground.

Then, insert the supersized steel and concrete support column known as

300 caissons support the entire development, carefully located between

tracks. They take the weight of the steel frames for the tower blocks, as

as the giant 430 ,000 square foot base platform that surrounds the buildings.

This concrete and steel platform creates a new ground level above the tracks.

The platform doesn't support anything except the open space where we're

right now.

Where the platform is underneath the buildings, the buildings go right down

between the tracks. They go all the way to rock. So the trick is in the

foundation. That's where all the load transfer goes.

The platform just fills in the space in between.

But while boxing in the active rail yard creates acres of new usable space, the

incoming trains below are now confined underground, creating a fresh challenge.

When the trains come and bring commuters in, they drop them off in Penn Station

and they park here for the day. We have engines running and just all the train

activity is creating heat below us. It can get up to 150 degrees Fahrenheit,

which is very hot.

The team had to find a way to keep the rail yards ventilated.

To utilize the record airspace above, they will need a solution to keep the

city's commuters cool.

Building on top of a major transportation hub yields serious

challenges, but it also presents the problem of temperature control.

If the heavy train traffic were left unchecked, the temperature below the

Yards project could reach up to 150 degrees.

Below you right now are about five jet engines.

They're about 40 feet long, maybe five feet in diameter.

They're actually lined up right like this, and they exhaust through that

It's pretty exciting to be in here and to be able to see that this is something

that nobody ever gets to see and to experience when we go down here, right?

actually see down into here and be within the structure and to see the fan

systems that are operating on either side.

Twin ventilation shafts at the plaza connect to 15 giant fans sandwiched

the tallest sections of the platform.

Principal engineer Eli Gottlieb has special access to the restricted area

the platform itself.

The fans are installed at the plaza directly above the trains.

Welcome to the central fan plant underneath the yards.

This is one of three fan plants that are actually down here.

These fans are really part of the lungs of the facility that actually allow it

to breathe.

So these fans allow us to exhaust.

any of the heat generated by the trains out and pull in new, fresh, cooler air

to maintain a tenable client for everybody who's down there.

These fans can fully exhaust the entire facility in seven minutes with all the

air that's here and completely replenish it.

But for Hudson Yards engineers, more impossible problems could jeopardize the

project.

Six skyscrapers will provide 18 million square feet of commercial and

residential space, including a school, over 200 -room hotel, and retractable

center, all perched on 300 caissons squeezed between a network of train

But in one critical section, this isn't possible, where the tracks converge in

an area called the throat.

Engineer Eli Gottlieb is one of the team charged with solving the problem.

From here, you can actually see that there's going through a lot of

and this switching allows them to fan out from the four tracks that they come

at to the 30 storage tracks. And you can imagine that if the tracks are spaced

far apart, there's space in between them to be able to build a caisson and a

column coming up. But as the tracks converging together, you can just

that you're just running out of space in between the two of them to actually be

able to install anything.

This area of the development is earmarked to have a tower block and a

mall built on top of it.

If engineers can't span the 148 -foot wide gap and support the buildings

Hudson Yards will be left with a massive hole in its plans.

This is really a key problem on the site.

All of that underneath us really narrows down the number of possibilities of

what we can do to be able to support everything above.

Can a game -changing innovation from the British railway boom be the missing

piece of the puzzle for the team at Hudson Yards?

Almost there now.

Wow.

It's a long way down.

Physicist Dr.

Andrew Steele is getting up close with the Worcester Railway Bridge in order to

find out.

Wow.

What a beautiful view.

In the early 20th century, the UK railway system was undergoing a huge

expansion. Not only were there more tracks, but there were also more trains,

these trains were much faster.

And as they were getting faster, they were also getting heavier, and that

that bridges on the railway network were being put under increasingly large

strains.

Like the team in New York, engineers needed to find something strong and

reliable to bridge the gap.

James Warren was a merchant with no formal training in engineering.

In 1848, he registered a patent that would revolutionize bridge design, the

Warren Trust.

This is Warren's design, this repeating pattern entirely made of equilateral

triangles, so triangles which are the same length along each side.

Now, triangles like that have the advantage that they're incredibly

The railway track is above my head right now. And what that means is that as a

train passes over the top of us, then its weight is distributed through all

these different struts. And every single one of them can share that load. That

means that all these individual beams are either in tension, so being pulled,

compression, so being squashed.

Previously, many bridge engineers had used vertical struts.

So how much stronger was the triangle?

So they can both take one brick.

Two model bridges are being tested to the braking point. So in this simple

design, all that load is mainly going through, honestly, just this vertical

strut here.

Whereas on the Warren truss over here, all of that load with this triangular

pattern of struts is being spread throughout the whole bridge.

We're going to go for brick number three.

All right, brick number four.

Bit nervous about this one, to be honest.

The question is, can it take a fifth?

But a sixth. I think this is going to be it.

Surely this time.

Oh!

Total structural failure there. You can see there's lollipop sticks everywhere.

But the Warren Trust, it's still supporting that load pretty strongly.

Because no individual lollipop stick is taking that much weight. And so,

incredibly, it can hold all of that mass.

The Warren Trust really was an incredible engineering innovation.

And, of course, it's not just bridges. Wherever a weight needs to be borne and

spread over a structure, you'll see these equilateral triangles holding them

This incredibly time -tested design.

So how can engineers at Hudson Yards supersize this 170 -year -old creation

support their mammoth project?

Engineers at Hudson Yards are supersizing a 170 -year -old creation to

part of their seven -story shopping mall and skyscraper.

Where piled foundations are impossible, sandwiched between the retail space

above and the converging rail tracks below, lie a series of 13 -foot -high

Warren trusses.

So we're inside the platform underneath retail.

Over all the switching of Long Island Railroad, we're standing basically

a bridge structure, right, where we're standing on the bottom of the bridge and

there's the top of the bridge above us, and these warrant trusses are those main

elements of the bridge that are spanning across.

In total, 14 giant trusses are used to span the crucial 148 -foot gap.

By being able to use this truss structure and build a nice stiff bridge

that's actually really efficient for being able to support the buildings

The tower supported by the truss is not only the highest on the project, but

once it's complete, will be the second tallest office building in New York.

Engineer Jay Cross has special access to its most incredible feature on the

100th floor, a 10 -minute ride away in the temporary lift elevator.

They need to change it so dramatically as you get higher and higher.

It's amazing.

And it's the view from the top that inspired another phenomenal piece of

engineering.

At first we designed the building without an observation deck.

And we realized how high we're going to be.

So we challenged the architects to say, come up with the idea of an observation

deck. We, of course, thought they were going to come up with something that was

inside the tower.

Instead, they came up with a protrusion, which was pretty darn dramatic.

This is the highest man -made outdoor observation area in the Western

Hemisphere.

Almost 1 ,100 feet up, it offers a brand -new bird's -eye view of the city

below.

But constructing a platform in the air calls for more impossible engineering.

At first, we weren't quite sure how to construct it.

We thought maybe it would be like a bridge where you just kind of build a

and you inch your way out from the building.

But the problem was we felt that we would have to have a lot of supporting

scaffolding to do that.

So our steel fabricator came up with the idea of making 14 pieces.

that would come up pre -assembled, and you would just bolt them on as you go

out.

Built out from the building, section by giant section, some weighing up to 50

tons.

The result is an 800 square foot sky deck.

But the final piece of the puzzle will be an immense triangle made of

glass.

We felt that in addition to walking towards the peak, which I think will be

exciting enough, we thought, why not make it a little bit more exciting and

create a glass floor?

Right now, there's nothing there. There's no glass. We're just looking

,000 feet.

And it's pretty exciting, don't you think?

It felt a lot more comfortable when the orange netting was across here. Now that

it's down to like two or three wires, there's no need to get too close.

Four 1 ,200 -pound triangular sections of glass will complete the sky deck.

Two and a quarter inches thick, it will allow visitors to take in the vertigo

-inducing views from this mind -blowing vantage point.

I'll walk on it, I'm sure. I'll be goaded onto it, but I'll be nervous

But this impossible project is attempting more audacious engineering,

further challenges.

We're moving the equivalent of about 600 elephants stacked up on top of each

other, wearing eight roller skates.

Can their innovative construction be taken to the next level?

It's very difficult to figure out that solution from an engineering standpoint.

New York's biggest ever impossible engineering challenge.

Hudson Yards is a 17 million square foot real estate project constructed over a

fully functioning rail yard.

Perched on a specially constructed base platform above the tracks, this is a

multi -tower mini -city.

Multiple skyscrapers, a super -sized mall, 14 acres of public land, all

to life simultaneously.

But perhaps the most inventive engineering on site is a unique arts

as The Shed.

Engineer Eli Gottlieb is overseeing its build.

The Shed is built up of two parts.

One is what we've always thought of as the fixed building, which is the

that we're standing in right now.

And then it has the sleeve that slides over the fixed building so that when

not in use, it's actually retracted. But then when they have larger events and

they need the space, they'll be able to roll the shed out to enclose the plaza

and turn it into this larger venue that we can see here in front of us. There's

really no other places like this.

The team is using a special inflated plastic to keep the shell as lightweight

possible.

so the power required to move it is kept to a minimum.

The drive system on the roof generates just 180 horsepower.

So we're up on the roof of the fixed building of the shed. You can see here

the main teeth from the drive mechanism that the actual shed motors will then

roll against and drive the assembly back and forth.

We're moving the equivalent of about 600 elephants stacked up on top of each

other. In this case, wearing eight roller skates.

The roller skates are, in fact, eight pairs of six -and -a -half -foot -high

wheels running along heavy -duty rails.

And in 2016, this retractable building was put to the

test for the first time.

To see the shed move was actually fantastic, but it was also really

because it is so quiet.

And in fact, a whole number of people actually kind of missed it because it

happened and they sort of didn't hear it or realize that it was going on in that

moment.

The vast 28 -acre Hudson Yards is a series of separate construction

With many being built over an active rail yard, the challenges are

huge.

Although the site's southwest corner is clear of the main tracks, it posed an

equally challenging problem.

A massive tower had to be perched directly on top of a $2 .4 billion

transportation hub.

Just three years old, the 34th Street Hudson Yard Station is one of the city's

newest. With one and a half million commuters coming into Manhattan every

it's a key selling point for the development.

but it presented the engineers with a challenge.

We've limited opportunities to put weight on the subway station and limited

footprint to put weight outside the subway station. So we're looking for

putting as big a building as we can with the limited options we have at the

foundation.

Lack of space for new foundations meant 40 % of the tower block's weight had to

be carried by the foundations of the subway station itself.

Overloading was a real danger.

The tower had to shed some weight without shrinking in size.

Our building is 51 stories of office space, and that's a lot of building to

on limited foundations.

So the engineers needed to find solutions to lighten up the building as

possible. It seems impossible, but can engineers of the past shed any light on

lightweight materials?

It's beautiful.

Kip to Peak State Park in Virginia.

Engineer Megan Hart is off to discover how a material used to solve a wartime

problem could help the team in New York.

During World War I and World War II, there was a massive manufacturing

and that took a lot of raw material.

The main one was steel.

Steel production went into overdrive to supply the military.

This led to a shortage.

But the U .S.

Navy needed to build a fleet of supply ships.

So how did they manage it?

These are the inspired solution, made out of an innovative version of a

surprising material.

These are concrete boats. Can you believe that?

And they actually float.

Not only do they float, but they carried supplies during the war effort.

The man behind it is Stephen J. Haidt.

He was working in the Kansas City building trade in the early 1900s when

a eureka moment.

He noticed that shale stones left too close to the kiln during the firing

process would bloat up when they overheated.

The heat caused air bubbles to form in a honeycomb -like structure inside the

stones, making them lightweight.

So Haidt added them to cement and water to create his revolutionary new

concrete, an invention that could help out Hudson Yard's engineers.

So Stephen J. Haidt, instead of using traditional aggregate, which is heavy,

it's just rock, he used an expanded shale. And that expanded shale is much

dense.

During World War II, 24 of these 330 -foot -long concrete ships were made,

unexpected solution to the Navy's problem at sea.

In New York, engineers draw on Hayes' discovery to solve a skyscraper's

weight issue.

55 Hudson Yards is a whopping 780 feet high, and its sizable mass bears down on

a major transport hub.

To avoid overloading, the team had to save weight on the inside.

We're on the 47th floor of 55 Hudson Yards, and lightweight concrete helped

achieve a light building to go over the limited foundation we had around the

subway station.

So if we look above and below us, slab above, slab below, every floor on this

building is made out of lightweight concrete.

And we saved about 20 to 30 percent in weight overall.

It made a huge difference.

If we save 20 percent in weight, we can make the building 20 percent higher. So

20 percent more buildings, quite a bit.

The concrete floors are also lightweight because they're only nine inches thick.

But spanning the tower's vast open plan areas is problematic for such a thin

floor. To keep floor weight down, meet the expectation of vast open floor

and maintain structural integrity, engineers must employ a revolutionary

innovation from the past.

Engineers must find a way to maintain the lightweight qualities of Hudson

concrete floors while also preventing deflection or bowing under pressure.

The team's solution is a pioneering technique known as post -tensioning.

Historically, post -tension technology in New York City specifically hasn't

used regularly. And this is the first building where it was used regularly in

the slabs.

So overhead we can see white painted areas that say PT zone.

That's where we're using post -tension concrete.

Simply put, it's a high -strength cable that's pulled through the concrete that

helps limit deflection.

A network of steel cables are threaded through liquid concrete.

Hydraulic jacks apply massive tension to pull them taut.

Anchored in place.

The compressive force of the concrete is increased from 7 ,000 to 8 ,500 PSI,

reducing deflection, allowing a thinner, lighter slab.

With our high -strength lightweight concrete and using the post -tension

it makes for the most robust system we can for a thin slab.

And while post -tensioning allows the team to build bigger and higher over the

east side of the project, they have yet to break ground to the west.

the other half of the $25 billion Hudson Yards development.

A second, even bigger steel and concrete platform will be built above the tracks

on the other side of the 11th Avenue viaduct.

Eight colossal buildings will be created.

This city within a city will double in size.

We have six giant buildings around us, and we have a lot more coming in very

short order.

We're bringing more office space to New York City than there is in downtown San

Diego.

But the engineers of this brand new neighborhood face another hurdle.

They must defy New York's notoriously unpredictable weather.

To be a success, Manhattan's newest business quarter must stay online 365

year.

But maintaining a steady power supply in New York is a huge challenge.

Hurricane Sandy caused the latest and most devastating in a long history of

power cuts affecting the city.

When we lose power here in Manhattan, it's eerie, right? It's a large city.

We're used to seeing it lit up, and we're used to seeing the city alive 24

a day.

Frank Norcross is in charge of Hudson Yards Energy Supply Team.

He needs to find a way to protect the new development from power cuts.

The customers that will call Hudson Yards home can absolutely not afford to

power at any moment.

The things that we maintain to keep safety and security in a large city are

under threat when we lose power.

So, in the face of hurricanes or other disasters, how do you keep the power

supply running?

The team needs to turn to the past to prevent history from repeating itself.

Physicist Dr.

Andrew Steele is at the Anson Engine Museum in the north of England in search

a historic innovation that would blow steam power away.

It was Nicholas Otto, a German salesman with a passion for engineering, who came

up with an invention that transformed the 19th century.

This is what Otto came up with. It's called an atmospheric gas engine, and

got a number of advantages over the steam engines that existed at the time.

Firstly, you don't need a huge supply of water to run this thing.

Secondly, you don't need a team of people. You can do it with just one

And thirdly, you can have a load of these small engines scattered around the

factory to operate individual machines, rather than having one massive steam

engine to drive all the equipment in the whole place.

More powerful for its size than its steam engine counterpart.

Sorry, that's a bit of physical labor.

It's not like starting your car.

This new breed was fueled by natural gas.

Oh wow, there we go.

The way this atmospheric gas engine works is it's got a massive piston

this cylinder.

So then when that piston starts moving upwards, it draws in a mixture of gas

air. And when it gets to a certain point, somewhere around here, that pilot

light, a little bit of that flame gets pulled into the cylinder.

It causes an explosion, and that throws the piston up through the cylinder.

Essentially a miniature power station, Otto's single -acting cylinder was

successful.

But it was his newly improved gas -powered model that could provide the

for the engineers of Hudson Yards.

19th century German engineer Nicholas Otto had a stroke of genius while

on the early atmospheric gas engine.

This is the next iteration of Otto's design, and it's truly revolutionary.

It uses something called the four -stroke cycle.

Unlike his previous engine, this makes four strokes of the piston for each

ignition.

First, the piston moves downwards, sucking air and gas into the chamber.

Secondly, as the piston rises, the air and gas are compressed.

Thirdly, it's ignited and the piston forced down.

And on the fourth stroke, the exhaust gases are pushed out.

The result is an incredibly efficient, reliable and quiet engine.

The principle behind this engine, the four -stroke cycle, is also known as the

Otto cycle after the man who invented it.

Engines like this were quickly deployed in factories, but they also rapidly

found much more widespread use, particularly in the emerging automobile

industry.

Nicholas Auto's innovation found its fame in light industry.

Engineers at Hudson Yards are using his game -changing design to provide an

immense power backup system.

In 2015, four of the biggest four -stroke gas engines in the world arrived

site in Manhattan.

44 -ton, 20 -cylinder, 4 ,600 -horsepower beams.

What we're looking at here is very comparable to what you'd find in the

end of any souped -up hot rod.

It's an internal combustion engine that operates on a four -stroke cycle. It

just happens to be about 30 feet long and 10 feet tall.

This is really the beating heart of the energy infrastructure here at Hudson

Yards. If New York's electricity fails and the city is plunged into darkness...

The natural gas supply will kick in, and these monumental machines will take

over the job of running the Hudson Yards power plant.

This plant protects Hudson Yards from power outages because it has the ability

to operate in microgrid mode.

What that means very simply is if there were an issue out on the utility grid,

we could simply disconnect from the grid, then start restoring power that we

source from our plant here.

When called upon, these monster engines will produce over 13 megawatts of power.

It is exciting to hear one of these engines fire up.

It is the same sort of teenage boy excitement that you would find, you

turning over any engine.

several fold because of the size of these engines.

This engine has a heartbeat.

This engine breathes. This engine creates energy, and you get a very

sense of that when the plant is running.

The Hudson Yards project represents impossible engineering on a staggering

scale.

A brand new 28 -acre neighborhood built over a working rail yard.

Everybody knows the Manhattan skyline of New York, so when you change it, that's

actually in some ways the most exciting thing. All of a sudden there's this

monumental cathedral in the middle.

Every stage of this trailblazing enterprise poses extraordinary

The engineering on this site is such an example of incredible teamwork and

incredible vision from so many people to be able to bring this to fruition.

Inspired by the work of the pioneers of the past, supersizing and breaking

the mold themselves.

The engineers are succeeding in making the impossible possible.

Coming on to the construction site, you get to appreciate it every day.

But still, every single day, it's a wow moment.

I love this project.

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