Original subtitles

Narrator: How did engineering innovation allow a bridge

to span this vast valley and become the tallest in the world?

Man: Defying gravity, defying nature.

Narrator: How did engineers rip up the rulebook

to build this hotel upside down in an abandoned quarry?

Man: When a building is flipped like this,

then things that you would normally take for granted

need to be completely reconsidered.

Narrator: And what happens when you attempt a world first,

to build a forest in the sky in the middle of a city?

Woman: That is a huge amount of extra weight

that the building needs to absorb.

Narrator: This is the age of the extraordinary...

Man: Where else can you swim

from one skyscraper to the other 300 feet in the air?

Narrator: Where ingenious engineers

have unleashed unchecked creativity...

Woman: Everything in this building

pushes at the boundaries of what's possible.

Narrator: Building structures so outrageous, they defy logic.

Woman: The forces on this thing

look like it should be torn apart.

Narrator: Now their secrets are revealed.

Discover the incredible stories of their construction...

Woman: These are extraordinary feats of engineering.

Narrator: To try and understand,

how did they build that?

What happens when a road

that carries up to 50,000 cars a day

needs to cross a river?

Corina kwami: Crossing over a road, river, or valley,

that's easy... build a bridge.

Narrator: But what if that river is in a valley

that's nearly 1,000 feet deep, a mile and a half wide,

and located in one of France's

most beautifully unspoiled regions?

The answer is to hire

one of the world's most visionary architects

and assemble a team of brilliant engineers

to construct the most advanced bridge possible.

Nehemiah mabry: You know something is

at the cutting edge of engineering

if you have to design a brand-new system

to deal with it.

Narrator: The millau viaduct is the tallest structure in France

and the tallest bridge in the world.

Michel virlogeux: Nothing has been done exactly like this.

Each big bridge is a new bridge.

Narrator: It cost 300 million Euros

and took 15 years to create a bridge so special

that people travel from around the world to see it.

So, how did they build it?

It's quiet now, but in the late 1990s,

the sleepy town of millau, in the south of France,

was a bottleneck of traffic on a tourist road

between Paris and the mediterranean coast.

Lord foster: Imagine peak holiday,

the route from Paris to the mediterranean,

@and the traffic just slows down.

You've got 20-mile nose-to-tail pollution.

Narrator: Relief, though, was on its way

in the shape of a new autoroute to the mediterranean

that, 150 miles south of Paris,

would finally bypass long-suffering millau.

But there was no way around the massive tarn valley,

where the town lies.

The valley is a vast,

steep-sided one-and-a-half-mile-wide split

in the massif central plateau.

Deciding exactly how to cross it wasn't easy.

Michel virlogeux was on the project

right from the first planning stages in the late 1980s.

Michel virlogeux: The first idea was to go

slightly down in the valley and to come up.

And finally, somebody, a road engineer said,

"why do you go down in the valley?"

We could not answer,

and so we said immediately, we'll look at it.

Narrator: So, architects were asked to design a bridge

to span the whole valley, and in 1996,

the plan from foster and partners won approval.

Foster: Our interpretation as a team

was to march across the valley

from plateau to plateau with the most efficient,

the lightest impact on the landscape

and create a series of spans,

and along the way, you would bridge over the tarn.

Narrator: Building such an extraordinary bridge

would turn the project into a laboratory

of engineering innovation on a massive scale.

The whole bridge weighs 290,000 tons

and is made up of three major components...

Seven giant concrete piers rise up from the valley floor,

on which rests an 8,070-foot-long road deck.

To top it off, seven huge pylons tower above the piers,

with tensioned cables supporting the road below.

Because these cables send the load directly

back to the pylons, which are balanced

by an equal load from the opposite side,

the cable-stayed bridge design is very efficient.

To realize the slim line,

elegant design would take innovation

at every stage of construction.

@hayley loren oakes: That demands a feat of engineering

@on a completely different level, something extraordinary.

Narrator: At the start of construction in 2001,

the first things to be built

were the seven huge load-bearing towers, or piers.

Building a road deck at this height

had never been attempted before.

They were so tall that the engineers

had to turn to skyscraper technology.

♪

The builders used a technique known as climbing formwork.

Each pier had its own platform and mold,

which would create a 13-foot section at a time.

Once one level of concrete had been poured and set,

hydraulic Jacks pushed the mold up

so that the next level could be poured.

But as the piers grew,

getting the concrete to the top to pour

became more and more difficult.

Corina: That's where cranes come in,

but the piers were so high

that the cranes had to be tied to them for stability.

Narrator: With robot-like efficiency,

all seven piers grew 13 feet every three days.

They took 21 months to reach road level, by which point,

each contained 16,000 tons of steel

and three million cubic feet of concrete.

Next came the road.

Eight steel sections were assembled on-site

at the valley's edge, but lifting them into position

with the crane at this height would be too dangerous.

Incredibly, despite weighing 36,000 tons,

these sections would be slid out across the top of the piers,

but this posed a serious problem.

Michel: If you use a classical technique,

you will have to push the piers,

and, in addition, you have friction,

and so the piers would not have resisted these forces

because they are very tall.

Narrator: The friction caused by sliding the sections of road

over the piers could cause them to fall over,

but the team came up with an ingenious solution.

Michel: So, the idea was to develop a launching system

which do not introduce any horizontal force on the piers.

Hayley: What they used to lay the bridge deck

of the millau viaduct was a complete one-off.

Narrator: The team constructed seven temporary metal piers.

On top of all of the piers,

computer-controlled hydraulic platforms

would lift and move each road deck along,

very slowly, out towards the centre,

with the middle road decks already rigged with two pylons.

Launched from both sides,

the road decks moved just 23 inches every four minutes,

five times slower than a snail, until they met in the middle.

Hayley: Each piece would move forward, inches at a time.

It took days to get the whole thing into place.

Narrator: With each deck taking up to three days

to reach its final position,

this process was vulnerable

to one of the region's most unpredictable threats.

Winds in the tarn valley had been recorded

at 125 miles per hour, but even much lighter winds

would cause problems during the critical deck launches.

Michel: Every morning, the site received a weather prediction,

and we decide not to launch

if we have not five days in front of us

with less than these 37 kilometres per hour.

Debbie sterling: Can you imagine what would happen

if a giant piece of this roadúdeck got caught up by the wind?

It would cause really bad damage.

Narrator:

The clock for two days,

the central sections of the millau viaduct,

complete with the two middle pylons,

were finally within touching distance.

Global positioning systems had been used to plot their path.

Despite starting at different heights

and with a gentle curve

throughout the one and a half miles,

when the roads met,

they were within the one centimetre tolerance.

The final component was not entirely crucial

to the engineering,

but was a fitting way to celebrate

an incredible piece of innovation.

Foster: It was inventing how you would make the bridge,

that magic moment when, you know,

from one and a quarter kilometres coming together

that two and a half kilometre, and they magically touch.

Narrator: The road was complete,

but it wasn't designed to solely rest on the seven piers.

It needed to be supported at more regular intervals,

to be strong and rigid enough

to take the loads of the bridge deck

and five million vehicles a year traveling across it.

This would be done by massive cables.

Ellie cosgrave: The millau viaduct

is what we call a cable-stayed bridge.

It makes use of thick steel cables

to support the weight of the bridge

and lock the structure together at the same time.

Narrator: The cables would be hung from 285-foot pylons,

each on top of a concrete pier.

The prefabricated steel pylons, weighing 700 tons,

had to be installed in one piece.

Loaded onto massive transporters which crawled across the bridge,

the pylons were then inserted into a frame

and pivoted into position.

The seven pylons would share the job

of supporting their own 5,000-ton section of road,

each from just 22 tensioned steel cables.

Expected to have a lifespan of at least 120 years,

the bridge needed to be easily maintained,

so a secret tunnel was built in, running under the road.

Camille baudel is a tour guide.

Camille baudel: We are inside the deck

of the millau viaduct... [Thump]

and we are just underneath the roadway joint.

That's why we can hear the noise of the vehicles passing on it...

[Thump]

and the joints are here

to absorb the movement of the deck

because when it's very hot, the deck expands,

and when it's cold, it's retracted, like this,

so the joints are here to open and close all the time.

Narrator: Being a bridge for vehicles,

accidents have to be considered,

especially a crash that causes a sudden weight imbalance,

which could be catastrophic.

Camille: So, these balls are here

because if there is an accident on the traffic

and a lot of liquid pours into the deck,

these balls will float so the liquid can go down,

and there is no more weight inside the deck.

Narrator: Finally, in December 2004,

one month ahead of schedule,

the first cars rolled across this stunning strip of tarmac.

Now, five million vehicles pass over it a year,

and people even take detours just to experience its magic.

Foster: Initially, you couldn't cross

because everybody was stopping their cars and photographing,

because a lot of people make the journey just to see the bridge,

as well as to enjoy the speed of the crossing.

Narrator: The millau viaduct pioneered new ways of building

and did it in record time,

a remarkable achievement for a proud team.

Foster: A three-year adventure for 500 people.

Everybody coming from different disciplines,

different backgrounds,

and wanting to achieve something that was truly outstanding,

something that had not been done before

and something that would turn making a car journey,

a truck journey, across this 2 1/2 kilometres in the sky

a kind of ethereal experience.

Michel: Each bridge is a prototype.

You use ideas which generally already exist.

An engineer must know what was done before

and must be able to reuse good ideas which exist.

A colleague said, engineers are climbing

on each other's shoulders.

♪

Narrator: Most buildings in Shanghai

try to reach for the skies, but just a few miles west,

designers and engineers have flipped this habit on its head.

Here, they've gone down.

Nestled into a 300-foot-deep quarry

is an incredible new building

that offers a true escape from reality.

Corina: This place is part "thunderbirds,"

part bond villain lair.

It's even got a shark pool!

Narrator: But this is no film set.

It's the world's first underground hotel.

Creating this 500,000- square-foot upside-down hotel

with underwater floors

meant turning engineering on its head.

Hayley: When your building's flipped like this,

everything that you take for granted

is just thrown into the air.

Narrator: It took 12 years and $300 million to engineer

a way around earthquakes, floods, and crumbling rock.

The result...

The 336-room, subterranean wonderland hotel.

So, how did they build it?

Back in 2005, before the project started,

this deserted quarry, a 30-minute drive from Shanghai,

was earmarked to become the exact opposite

of a tourist destination.

@hao zeng: The government planned to make this place

as a treatment place for the garbage.

Narrator: But a developer had a vision,

to create a hotel with a feature so unique

that people would travel from around the world

to experience it.

Tasked with turning this site from garbage can to gold mine

was architect Martin jochman.

Martin jochman: When I visited the quarry,

actually it was completely overgrown

and the water level was quite high.

It was more like a lake which was surrounded

by old industrial buildings and overgrown vegetation.

Narrator: Martin used the location's natural assets

as a starting point for his design.

Martin: The inspiration for the design

came from the quarry and from the greenery,

from the rocks, from the waterfalls,

and it needed to span between the ground level

and the bottom of the quarry itself.

This was what would make it

really outstanding and spectacular.

Narrator: Martin was keen to create a building

that fitted naturally and seamlessly into the environment.

What appears to be just a two-story,

grass-covered complex at ground level

is only the tip of the structural iceberg.

Hidden below is the main 16-story hotel room section

of the building,

the design embracing shapes from within the quarry.

One half curves in, the other out.

Between them is a waterfall-like glass atrium,

which houses the elevator shafts and services,

but it's at water level where things are most unusual.

Here, huge 16-foot-deep aquariums

hold over 300 tons of water,

so the lowest two floors of restaurants and guest suites

have huge windows onto an extraordinary underwater world.

Martin: What I tried to do is to create a building mass

that joined with the quarry and became part of it.

It became part of the overall character of the quarry itself.

Narrator: In 2011,

as the 5,000-strong construction team began work,

this challenging site threw up its own unique set of problems.

Before the engineers could start building the hotel,

they had to prepare the quarry itself,

which had been abandoned for years.

Zeng: Before we start building construction,

we had to make sure the condition is okay

for building works, and, as you see,

the cliff here is not very stable,

so we had to solve this problem.

Narrator: So, how do you prepare a giant hole filled with water

for an underground hotel?

First, the team drained the quarry of 30 feet of water

before turning to the rock faces,

which would need to fit the hotel structure.

Martin: The quarry had to follow

the shape of the building itself,

but also, because of the safety requirements,

we needed to do some stabilization

of the rock faces themselves.

Some of the rock faces had to be blasted, but not too many.

Narrator: The quarry is comprised of andesite,

a type of rock formed during a volcanic eruption.

It's solid in places but can be crumbly and unstable,

creating dangerous conditions for anyone below.

Ellie: It is pretty amazing to see behind this building

because it looks from the outside

as if it's all fixed to the wall,

but actually there's a huge cavernous space behind it,

and it's really only fixed at the top and at the bottom.

Narrator: The team came up with a solution.

49-foot steel bolts were driven deep into the rock

at six-and-a-half foot intervals, creating a huge grid.

The compressive force of these 6,000 bolts

knitted together the loose rock.

For added security, a metal mesh was anchored across the bolts

and sprayed with a thin layer of concrete.

All this is still visible from the inner atrium.

Ellie: It is pretty amazing to me how well the concrete

has weathered to colour match the original walls over time.

Narrator: Only once this was all done

could they begin work on the massive foundations.

They would require 2.1 million cubic feet of concrete,

into which steel trusses would be embedded.

But pumping all that concrete from ground level down 300 feet

wasn't as easy as you might imagine.

@joshua macabuag: Until it sets, concrete is a delicate mix

of materials of different sizes and weights,

and it has to be continually mixed.

Martin: Getting the concrete down

was probably the most important and most difficult to solve.

We all know how to pump concrete high to build skyscrapers,

but pumping it down 90 meters is a different story altogether

because gravity can separate the parts of the concrete

and can weaken the concrete.

Narrator: The upside-down structure

of Shanghai's wonderland hotel

meant the foundation's concrete had to be pumped down,

which risked weakening the material.

As its components vary in size and weight,

the concrete could separate out by the time it reached

the bottom of the quarry,

so the team came up with a simple

but incredibly effective solution

to ensure that the concrete stayed mixed.

Martin: The builder had to create remixing stations

on the way down the quarry,

where the mix of the concrete would be collected,

remixed, and sent down again so that when the mix

got to the bottom where it needed to get to,

it was the right consistency.

Narrator: This innovation was just one of the ideas pioneered

as engineers tried to overcome the site's unique challenges.

Martin: This method was patented,

so it's one of the, I think, 38 or so patents

that the, the builder had to create

to, to realize this building.

Narrator: By 2015,

the foundation and substructure were completed

and work had begun on the next levels,

but the build faced another challenge.

Seven years prior,

the devastating sichuan earthquake

that left 87,000 dead and 5 million homeless

prompted the Chinese government

to enforce strict seismic building codes

across the country.

Joshua: But when you have an earthquake,

then what's happening is that the ground

is shaking violently from side to side,

and it's that sideways force that has to be carried down

into the foundations,

so if a structure is in an earthquake zone,

then it has to have a way to resist that sideways motion,

or it will fall down.

Narrator: To prevent a catastrophic collapse,

buildings in earthquake zones

are often set apart from the ground using isolator pads.

Ellie: Base isolators are essentially like suspension

but for a whole building.

This means that when the earth moves because of the earthquake,

the base isolators absorb that movement,

leaving the rest of the building completely undisturbed.

Narrator: The wonderland hotel, though,

couldn't use this technique in the traditional way,

as it had a fundamental difference.

Corina: Normal buildings have one point of contact

with the earth, at the bottom.

Here you need to connect the top, where people enter,

with the bottom, where the weight of the structure is.

The problem is, is that during an earthquake,

these might move in the opposite direction,

and when that happens, well, you have a hotel falling in a hole.

Narrator: To prevent the top of the building

from tearing away from the bottom during an earthquake,

both ends of the structure

needed to move independently of each other,

so a radical solution was needed.

First, a series of inverted l-shaped steel trusses

were embedded into the concrete base,

rooted to the quarry floor.

This created a rigid joint at the bottom of the building

that reduces lateral movement during a quake.

It was, in fact, at the top of the building

where traditional isolator pads would be used.

Martin: The worst-case scenario, if it was fully rigid,

would be that the whole building collapses,

and it's a major disaster.

The way it's been designed was a rigid connection at the base

and then at the top had to sit quite loosely

on the edge of the quarry

so that it would enable any movement

in case of earthquake.

Narrator: So, the engineers turned

seismic isolation technology upside down.

Joshua: They have used a form of seismic isolation,

which means they've taken that top section,

they've sat it on a series of separation joints, steel plates,

which means that the sections can move independently.

Narrator: 16 floors up at the top,

the building isn't fixed onto the rock.

Instead, the upside-down l-shaped trusses

of the structure

rest on the concrete lip that runs around the quarry wall.

A set of steel isolators sit between the trusses and the lip,

allowing the building to slide back and forth

in the event of an earthquake.

Martin: This is to allow for minimal damage to the building

rather than the whole building collapsing.

Narrator: Water was key to the design of this complex,

and in 2018, one of the last phases of construction

was refilling the quarry

with over 50 million gallons of water.

It would take six months but would also pose

one of the biggest threats to the hotel and its guests...

Flooding.

Corina: You've got twice the annual rainfall of London

coming down into a waterproof rock hole.

Plus, there's a waterfall pouring water in

and a river and a canal, all within overflowing distance.

Narrator: Prone to typhoons and monsoons,

this coastal region is one of the world's

most vulnerable to flooding,

so the team came up with a solution

that could instantly address the smallest increase in water.

Martin: In order to maintain this water level,

we have a very powerful emergency pump,

and this will come into operation

if there's too much water coming into the quarry.

It could be through exceptional rainfall

or if there is some, some breakage

into the quarry from the canal,

so this can then deal with that situation.

Narrator: Fully automated and computer-controlled,

the emergency water pumping system is programmed

to keep the water level within a 20-inch tolerance.

Six pumps are at the ready to evacuate

3,500 gallons per minute into the surrounding canals.

The next challenge was to create a suitable ecosystem

for exotic fish like these.

The two underwater floors needed to have spectacular views

to warrant the $3,000-a-night price tag

for its exclusive suites.

But the quarry lake was too dark and vast

to guarantee guests a worthy sight.

Martin: The client envisaged that part of the building

would be underwater,

but no real instructions as to how to deal with it,

so I suggested that we put aquariums

below the water level

to face the guest rooms or the restaurants,

so the client really agreed that we should put

themed tropical aquaria with some sharks in it, as well.

Narrator: Separate from the main body of water in the quarry,

these self-contained aquariums would be huge...

16 feet long and over 16 feet deep,

containing 40 tons of water.

The engineers needed a material that would be strong enough

to withstand the pressure of the water

and clear enough for the guests to see the display.

Normal glass wasn't up to the job.

Joshua: Glass is perfect for small aquariums,

but when you're trying to hold back

many tons of water like you are here,

then it's just too brittle,

meaning that a small crack could be catastrophic.

Narrator: Upping the depth of the glass was pointless.

The thicker the glass, the more opaque it becomes.

The solution was to use acrylic.

Martin: Acrylic can be as thick as 200 millimetres,

300 millimetres, and you can still see clearly through it.

Narrator: The largest of the acrylic aquarium windows

the engineers installed weighed over four tons

but revealed the underwater paradise in all its glory.

♪

In 2018, the intercontinental wonderland hotel

finally opened its doors to the public.

It had taken 12 years of hard graft and amazing engineering

to overcome an array of complex problems,

but against the odds,

Martin and his team created this extraordinary building,

the world's first underground luxury hotel,

an outstanding achievement for a site once earmarked

for a garbage dump.

Corina: This project is an example

of taking a scarred piece of land

and turning it into something beautiful, luxurious,

and even natural.

But above all, it's an amazing piece of engineering.

♪

Narrator: Cities are growing more crowded by the day.

Greenery is being pushed back to make room for concrete,

but what if it didn't have to be this way?

In Italy, radical designers and engineers are pioneering

a new approach to both urban expansion

and environmental regeneration.

Manfredi catella: This idea was a bit crazy,

like having trees on a building.

Narrator: These trailblazers have rethought

how city life could be, lifting a forest into the sky,

creating a stunning, life-giving sculpture.

Debbie: It feels like this futuristic version

of what buildings should be.

Nehemiah: What many of our cities and urban areas

are crying out for is more green space.

This could be the solution.

Narrator: Engineers had to create a structure strong enough

to hold two and a half acres of forest

and tough enough to withstand high winds,

all while battling vibrations from subway trains.

So, how did they build it?

Milan is a city of 70 square miles,

almost all of it dense urban sprawl,

but packing people in has meant sacrificing something important.

Ellie: Green spaces are an essential part

of the city's ecosystem.

Not only do they do the obvious thing

of absorbing carbon dioxide and emitting oxygen,

but they give us humidity, they collect dust,

and create an ecosystem for insects and animal life.

All of these things together make the city

a more hospitable and pleasurable place.

Narrator: Starved of the space to create a traditional garden,

a developer decided to think outside the box.

Manfredi: Developers love challenges, so the big thing was

how can we reintegrate the neighbourhood into the city

and having the opportunity to do it through the nature.

There was a strong commitment by all of us

@in making a sustainable project.

Narrator: The answer was to create a building

that could hold all the plants of a park on the ground

within a vertical structure.

This would be bosco verticale, or the vertical forest.

Structural engineer Luca buzzoni

was brought in to make it possible.

Luca buzzoni: As a structural engineer,

what I was, I must admit I was skeptical about it.

You typically don't put big trees on tall buildings

because of additional weight, because of wind forces

and all sort of things.

Narrator: Luca's doubts were well-founded.

No building like this had ever been attempted.

On these structures, plants came first,

and they would impact the way the buildings were constructed,

so an unusual collaborator was needed.

Laura gatti is an agronomist,

or in layman's terms, a plant whisperer.

Laura gatti: This building has been designed for trees.

We make calculation on what

@the trees is needing to survive,

thinking to be a tree and thinking like a tree.

This is not a building in which the greening

is put just for decoration.

Joshua: Normally the landscaping is the last part of the project,

but here the plants are actually

built into the fabric of the structure,

@and the design of the structure

is led by the needs of the plants,

and that's pretty standard for a greenhouse

or an arboretum, but it's not that usual

for an inner city block of flats.

Narrator: This unusual brief for up to 27-story apartments

that were half giant plant pots

would result in a unique and complex plan.

The ambitious design called for two residential towers

that would rise 360 feet and 250 feet from the ground.

Cantilevered terraces would extend

11 feet from the structures.

These would have to be strong enough to hold 900 trees

and allow them to grow up to 20 feet in height.

In addition, 16,000 other plants

would act as cladding for the building.

Construction began in 2009, with 6,000 workers on site.

Their first challenge came from Milan's public transport system.

Luca: The buildings on this project have been built

either on top or very close to existing Metro tunnels,

so it was quite the challenge to, to build big buildings

on top of those tunnels.

Narrator: 27-story buildings like these

would normally need concrete piles

drilled deep into the bedrock for stability,

but with the subway

carrying 1.4 million commuters every day right underneath,

the engineers had to find a way to give their buildings

a strong base without touching the tunnels,

and they inadvertently struck gold.

Luca: It was possible to design shallow foundations

even if the buildings are pretty tall and pretty big

because the soil in this part of the town is very good.

It's made of sand and gravel,

so the capacity of the ground is pretty high.

Narrator: Although the ceilings of the subway tunnels

were as little as 11 feet below ground level,

the engineers could cautiously build on top of them.

During construction, a 24-hour monitoring station

was set up to keep an eye on any stresses

exerted on the tunnels below.

Though bosco verticale's

wouldn't interfere with the subway,

the vibrations from the trains could still ruin

the living conditions for the buildings' occupants,

so the engineers turned to a solution

often used to protect buildings in earthquake zones.

Luca: So, basically, what this system makes

is very similar to what you have

in a normal car with a shock absorber,

so in the same way shock absorbers take vibrations

from the irregularities in the roads and damp them down.

Narrator: The vast buildings were floated

on a bed of base isolators made up of steel Springs.

The Springs act as a dampers for the vibrations from the trains,

so the earth is free to move below the building,

while the structure remains insulated

from the subway line underneath.

With the buildings and trains

effectively isolated from each other,

the engineers could tackle

the most unique challenge of this project.

They had to design a structure

to cope with the hundreds of plants and trees,

and the soil they live in,

whose weight and size constantly change.

To complicate things, many of these plants

would live around the edges of huge cantilevered balconies.

Cantilevers are a trick of engineering.

These reinforced concrete slabs

look like they're suspended in space,

when they are, in fact, internally fixed

to support columns within the main structure.

But they are usually designed to bear a limited weight.

Nehemiah: So, when I think about these balconies,

I think about the fact that they are essentially

experiencing cantilever action, so not only is it carrying

the weight of the plants and the soil,

but any moisture that's in the air

that gets trapped into the soil is just adding to the load

and therefore increasing the amount of reaction

that the attachment has to resist

on the exterior of the building,

and so the fluctuations of the loads

or the weight on these balconies really does change

as the seasons and the humidity changes.

Narrator: The engineers came up with a way

to make the balconies incredibly strong

to counter the weight of the trees and plants.

Luca: We have post-tensioned slabs,

and what that means is that in concrete slabs

we normally only have steel rebars.

In this case, we also have steel cables,

and those steel cables are pre-stressed

so that the presence of the cables in the slabs

provide additional resistance and additional stiffness.

Narrator: So, as well as predictable, stable

steel and concrete,

these towers would also be shape-shifting

organic structures,

with dynamic loads that needed to be understood

and controlled as much as possible.

Luca: This is a live building.

It changes season after season, and it grows year after year,

so it's not something that's normal on a project,

so what we did first of all,

we tried to understand how to cope

with the changing life of the buildings.

Narrator: It was crucial the experts identified plants

that gave the engineers

a chance to design a building that could cope.

Laura: We select trees that are not allergenic

or not messy tree,

and we understand which are the position on the building

in which the wind is higher than other parts,

and we select the trees to thrive in this condition.

Narrator: That wind was the biggest concern.

A large tree in a strong wind

could put serious strain on the building,

and the wrong trees could badly damage the structure.

The team needed to know as much detail as possible before

choosing these potentially destructive elements.

A selection of trees were tested in a wind tunnel

to assess if they were suitable for high-rise living.

Luca: First of all, we had a set of tests

in the wind tunnel here in Milan to understand the behaviour

of the trees together with the behaviour of the building

to see if the presence of the trees

was introducing additional forces on the building

and to understand the magnitude

of the additional forces on the building.

And then we had a second set of tests in Florida.

Ellie: At Florida international university,

they have built the wall of wind.

Here they can build whole buildings

and blast them with hurricane force wind

to see how they stand up.

Narrator: The tests helped determine which trees

were the perfect weight and shape for the building,

but there was another, more basic worry

that had to be addressed.

If a small tree on the ground is knocked over by the wind,

it can be a small problem.

If a tree falls from a large building,

then you've got a very big problem.

Hayley: If a tree falls from the 27th floor,

everyone is going to know about it.

Narrator: With six-foot trees planted 27 stories high,

no one wanted to take any chances.

Luca: For most of the trees

and all the medium and tall trees,

we have cables that basically provide

an additional anchor for the trees in case they break

so that they cannot fall over during windstorms, for instance.

And then for all the big trees

and especially for all the positions

that we identified as possibly critical

because of high wind velocity during windstorms,

we provided an additional anchoring system

that basically connecting the root ball of the trees

to the concrete structure so that they cannot fall over.

Narrator: It took three years to create this tower block oasis,

which was officially opened in 2014.

These two buildings are proof that people can have it all...

Enjoy life in the Metropolis

and a taste of nature right outside their high-rise window.

Nehemiah: I think it's also great

that a person can look out their window

and see what looks like a forest growing on their balcony,

when, in fact, they're in the middle of an urban area.

Narrator: The engineers who built bosco verticale

overcame new challenges

and achieved the best of both worlds,

pushing sustainable green spaces skyward

and offering a glimpse of the future for city dwellers.

Luca: It's a brave example

that shows that something different is possible,

that healthier cities are possible for us.

@manfredi: The strong commitment and passion by all of us

@made a project that, at the end,

has become today a symbol, an icon for sure.