Original subtitles

Narrator: How did Dutch engineers

build a giant arch full of apartments

on top of a swamp?

Woman: This one had three stories' worth of mud,

to the point where the workers actually had to wear wetsuits.

Narrator: What happened when a rock-shaped opera house

was built using the most modern and ancient techniques?

Woman: Anyone can build a symmetrical auditorium.

But here they went weird and wobbly

to build something exceptional.

Narrator: And how was a piece of British history

re-engineered for the 21st century?

Woman: That is 315 tons of glass and 478 tons of steel.

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

Man: This house always is on the verge of falling down.

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?

Rotterdam in the Netherlands

is one of the biggest ports in the world.

Criss-crossed with canals, rivers, and docks,

water is the lifeblood of this city.

However, if you're a builder, this water is much less welcome

and something for which you'd better have a plan.

So when the city wanted to build

a massive new horseshoe-shaped structure,

with four floors of underground parking,

right by the docks,

Dutch engineers had to use all the tricks in the book

to make it possible.

Corina kwami: This building not only has

the visible engineering marvel of a huge unsupported arch,

but equally impressive is that they built this into a swamp.

Narrator: Specialist builders had to dive deep down

in a muddy lake to build foundations.

Clever concreters created an incredible 12-story-high arch,

which engineers crowned with extraordinary apartments,

apparently built on thin air.

These penthouses cover dozens of buzzing market stalls,

restaurants, and shops.

And at each end are the largest cable net windows in Europe.

This is Rotterdam's markthal, or market hall.

So how did they build it?

Following German bombing in world war ii,

Rotterdam's flattened city centre had been redeveloped,

but somehow it had lost its soul.

Jacob Van rijs: Rotterdam was planned after the war

in a very functional way,

where they separated living

and working and shopping.

Nathalie De vries: And business-like.

Jacob: Exactly. So they realized,

hey, maybe we should combine it,

because it's not that great to separate.

Narrator: To liven things up, the city challenged architects

to design a permanent covered market

with hundreds of new homes on either side.

Winy maas: So the program demands

for two slabs of housing,

and somewhere in between a market hall.

I said to them, yeah, but that's boring.

So instead of having this, with the two slabs,

and then a market hall in between,

I said, why don't we turn it up, upside down,

so that you have slabs on either side,

and you have more penthouses here,

and you have a big hall,

like a space where you can breathe,

and where you can monumentalize good food.

Narrator: The final approved design

was an ingenious merging of housing and markets,

where not a square inch of space was wasted.

It would all be built on massive four-story-deep foundations

the size of two football fields,

dug out of waterlogged ground, and waterproofed with concrete.

On top, 228 apartments, cast in concrete,

would be stacked like huge bricks,

creating the arch walls with a final top floor of penthouses

that lock it all together and create the roof.

Cladding inside the huge 111-foot high tunnel

would create the largest work of art in the world.

Finally, at each end, a grid of cables

that would support hundreds of panes of glass,

protecting the market stalls and cafeés inside from the weather.

Joshua macabuag: They've made an entire building

in this arched form,

and they've managed to dot apartments and rooms

throughout it.

It's a really interesting andúquite fun piece of engineering.

Narrator: Building the markthal

would require exceptional engineering skills,

right from the beginning.

The ground is so waterlogged here,

anything built below ground level

acts like a boat on water.

Maurice hermens: The basementúis actually like a bath at home,

and you would take a bucket,

@and try to push the empty bucket into the water,

you have to apply a lot of force,

and if you push it really down, and you let it go,

it will jump upwards, and the piles,

they have to prevent the basement

actually jumping upwards.

Narrator: To counter this extraordinary upward force,

engineers had to rethink the role of the foundations.

Rotterdam is completely waterlogged

just 6 and a half feet below the surface.

A four-story deep basement car park

@the size of two football fields

creates vast water pressure underneath it.

So long concrete foundation piles would be built down

to grip into the soil below,

stopping the structure from rising.

But there's also pressure on the sides;

@to counter this, a huge concrete grid would be built

to hold the walls back.

This extreme water pressure

and the sheer scale of the building's footprint

meant these foundations would be some of the most complex

ever built in the Netherlands.

Work began in 2009, as the site was cleared

and a retaining wall was created using a combination

of interlocking steel sheets and tubes,

making a strong waterproof perimeter.

Then, inside the perimeter, the main foundation piles

were made by first driving huge pipes

to a depth of 100 feet.

An impact hammer slowly, but noisily,

pushed each pile into the soft ground.

With over 2,000 piles needed,

it was a slow and disruptive process for the locals.

Man: It takes about 2,500 concrete posts

to build the foundation,

and there was a lot of noise, you know...

[Banging]

So they make a wall of, I think it was about 200 containers,

to reduce the noise.

Cracks in the walls, surrounding buildings,

it had a lot of troubles before it got here.

Narrator: With the pipes hammered the 100 feet down,

concrete was poured in,

giving the building its strong, deep roots.

Once set, the tips of these piles were revealed,

and a massive concrete grid built on top.

This will perform a temporary engineering function,

by bracing the retaining walls

against the huge pressure from outside,

as the engineers dig down a further 36 feet.

And this is when building in the Netherlands

gets really difficult.

Maurice: As soon as you dig like two meters deep,

in this location, you will find water.

The whole basement, we're four stories underneath ground level,

is like building something underwater,

it's a kind of underwater building.

Narrator: As the winter of 2010 progressed,

the site became flooded,

but instead of being a disaster, it was all part of the plan.

The huge grid, now underwater,

was supporting the walls at the top.

But as they dug down,

the lower part of the walls, which aren't supported,

could collapse.

This is where the flood water comes in.

Maurice: The water inside the basement,

it gives a kind of counter-pressure,

actually it gives exactly the same counter-pressure

as the pressure from the outside.

Narrator: If the excavators dug down without water inside,

pressure from outside would have pushed

against the lower dam walls,

and eventually the sides would have caved in.

The water stopped that, but made for hard work.

For months, excavator operators worked blind,

only computer screens telling them where to dig.

Eventually, all 5.5 million cubic feet of material...

The equivalent of 64 olympic swimming pools...

Had been excavated.

Now the engineers had to prepare the bottom of the foundation pit

for waterproof concreting,

with 50 feet of muddy water still in place.

It was time for a very specialized team

to get involved.

Corina: They continued building, but with builders in wetsuits.

This is the most unusual but ingenious method

for building foundations.

Narrator: Highly skilled construction divers swam down

to install steel at the bottom of the lake.

The piles that formed the foundations

needed reinforcing steel attached.

This steel would knit together all the piles

and the first layer of concrete.

Once this was set, it would lock together with the grid above,

creating a super strong waterproof box...

Rigid enough to resist the water pressure from the sides.

Maurice: Construction workers

are putting that reinforcement in place,

making the connections between the underwater concrete

and the piles, and doing all stuff like that.

They were specialized divers,

which could stay down for a couple of hours,

and then had to come up again, and be refreshed.

Narrator: Like the excavators,

the divers couldn't see what they were doing.

Diver: We see very little under the water,

a lot of our job is done by feel,

so it's basically touch and movement,

and generally we have to navigate by,

it's a mental map in our mind

rather than using points of reference.

Narrator: With the steel reinforcement in position,

the next crucial step was to pour the waterproofing concrete

so the site could be drained.

Extraordinarily, this was achieved

by pouring wet concrete into the water.

Hayley loren oakes: It might seem surprising,

@but properly mixed concrete can set just as quickly underwater

as it can on land, as long as it's poured gently

and it's not agitated during setting.

And that's because it's nothing to do with drying;

it's actually a chemical reaction

that naturally expels water.

Narrator: For 72 continuous hours,

1,500 loads of waterproof concrete were delivered,

laying down a 5-foot-thick layer

to seal the floor of the foundation tank.

Maurice: After the concrete is set,

you can pump out the water,

then you have your empty building pit.

Narrator: It took two weeks

to pump out the 3.6 million gallons of water.

Then, the four layers of car park floors were poured,

and the massively strong 3-dimensional foundation grid

was complete.

Now the 12-story markthal could be built.

Right at the start, the architects had the vision

that a roof for the market would be provided

by building a huge arch...

An idea that won approval from the structural engineers.

Joshua: Arches are a very efficient structure.

They span a gap,

and they convert whatever load is on that arch

into pure compression within the arch

and take it down to the ground.

Maurice: The romans were already using that.

They made concrete and brick arches for Bridges and domes.

When it's there, it really works well,

the trick is how to make it.

Narrator: Building an arch means creating a curved shape,

which is good for strength,

but bad when 228 apartments

have to fit neatly inside the arch itself.

To start with, the engineers took a conventional approach.

Maurice: In Holland, the construction technique

used for apartment buildings

is often something called

a tunnel-form construction technique.

It's a set of walls and floors,

in which you can pour the concrete,

and thus realize the walls and floors in one go.

Ellie cosgrave: Tunnel shuttering uses

these large boxes that act as molds,

what we call formwork.

And these molds, you pour the concrete around,

and once the concrete is set,

you can just slide these molds out,

and use them again in another room.

So this means that we're saving time,

we're saving money

not having to create new pieces of formwork every time.

Narrator: To speed things up even more,

the engineers use technology to keep tabs

on exactly when they could move the build forward.

Maurice: There is a system monitoring

the setting of the concrete,

and you can read out the results through Wi-Fi

or mobile network,

so the contractor could see the setting of the concrete

from his office

and know when he could take the next steps in the construction.

Narrator: This technology,

and the repeated shapes of tunnel-form construction,

allowed the floors to be built up very quickly.

But how did the engineers make the arch walls curve out

and then in as they went up?

Maurice: We thought of using the apartments

as kind of bricks in the arch.

Putting the bricks every step, putting the bricks a bit closer,

closes they gap already for a large part.

Narrator: The structural effect

of this stepped stacking of apartments

is especially plain to see

for residents who live higher up in the curve of the arch.

Woman: Yeah, you cannot hang stuff

in particular maybe on these things.

You always have to kind of find a way,

so shelves are not really working that well,

but pictures still do.

Narrator: By mid-2013, with the sides of the arch complete,

the last apartments to be built

would be the exclusive penthouses,

forming the flattest part of the arch,

and the roof of the covered market.

Up until now, the build had relied on the apartment below

supporting the one on top.

But in order to create the arch, engineers had to figure out

how to support these penthouses while they built them...

120 feet in the air

and over the whole 390-foot length of the hall.

Maurice: Filling this whole space

with a temporary structure,

would be a project on its own.

It would be very costly, it would take a lot of time,

so that was, that was the reason to try to eliminate it.

So the whole temporary support structure

was put on wheels on a crane.

Ellie: This structure is essentially a reinforced tower

set on some tracks.

This means that as the roof sections are poured

and allowed to set,

we can move the structure along the building.

Narrator: One at a time, the floors were craned in,

and then concrete poured

to build up the penthouse structures.

When complete, they made up the most crucial engineering blocks

in the entire building.

Maurice: If you have an old-fashioned arch,

you will have bricks coming up,

and then in the end you have a kind of closing stone

filling that gap.

The penthouses are actually that kind of closing stone.

Narrator: Today, the residents of these roof-section penthouses

enjoy peerless city views

and can literally look down on their neighbours.

Woman: It's very special

because you see all the shops downstairs,

and you can see the people around you, so, yeah, it's nice.

Hi there.

You see, he likes it.

Narrator: By early 2014,

the project had reached its final stage.

Cladding smoothed the outside walls,

and the 4,000 aluminum panels inside were each printed

with part of a giant mural.

The "horn of plenty" was created by Dutch artist arno coenen

and at 118,000 square feet,

lays claim to being the largest artwork in the world.

All that was left to complete

was to weatherproof it with a wall of windows at each end.

The designers were determined

to retain the openness of an outdoor market

and keep the incredible artwork on show from outside.

The problem was, Rotterdam's windy weather

meant traditional windows this big

would need thick frames to remain rigid

and prevent the glass from shattering.

So the engineers found another, more flexible way.

Winy: So, it has to be closed, and, so no columns,

how can I do that?

I said, well, you can make cables.

Narrator: An array of windows, supported by a grid of cables,

is called a cable net facçade.

At 6,800 square feet each,

the ones they built here are the biggest in Europe.

Hayley: The principle of a cable net facçade

is very similar to a tennis racquet,

where you have steel cables that are like strings,

and then you have glass mounted between them,

and this gives the strength but also the flexibility

that's needed for covering an area of this size.

Narrator: Like the strings on a tennis racquet,

the cables in the facçade must be extremely tight to work.

With the 48 cables in place, they were then stretched tight,

so they could withstand pressure from the wind.

Each 1-inch thick cable was pretensioned by around 30 tons,

causing it to lengthen by up to 6 inches.

Then, specialized joints were mounted

onto the cable intersections.

Here the glass was clamped on at the corners.

These joints, the cables,

and the silicone between the glass panels

allow for an extraordinary amount of flex

in this giant window.

Winy: I'm aware, it was quite a work for the engineers,

to make the joint, because when there is a storm,

say wind force 11 or something like that,

then it goes in, and out, for more than a meter.

Narrator: In December 2014,

markthal's doors were finally open to the public.

And they've kept on coming ever since,

at a rate of around 20,000 people every day.

Jacob: It's now five years old,

and the amount of visitors keeps more or less steady,

around at 8 million,

which is actually more than the Eiffel Tower,

I can tell you.

Narrator: By making this multi-purpose

market hall possible,

its builders have helped start something big in Rotterdam.

Man: It's a real piece of art that they've put here together,

and we really like it.

Nathalie: It's an amazing piece of construction work.

It's also an amazing civic place,

because it's for everybody.

Narrator: It may have taken five years to rise from the mud,

but markthal now stands as rock-solid evidence

of how great engineering and design

can be a powerful force for change.

♪

75 miles northwest of Hong Kong, in southern China,

the brand-new mega city of Guangzhou

wanted to get noticed.

It decided the answer was an incredible new building

that would draw the attention of the world.

They turned to a legendary architect

with a reputation for designing the impossible.

Zaha hadid didn't disappoint...

Dreaming up one of the most advanced buildings

in the world...

Building a gigantic steel frame

with techniques that are millennia old...

Mimicking natural forms with mathematical wizardry...

And redefining the traditional shape of a concert hall

that has gone unchallenged for decades.

Corina: Anyone can build a symmetrical auditorium,

but here they went weird and wobbly

@to build something exceptional.

Narrator: This radical design is the Guangzhou opera house.

So how did they build it?

The city of Guangzhou is a giant metropolitan centre,

home to 25 million people...

Over three times the population of New York.

In 2002, the regional government commissioned a new centrepiece

that would hold its own

amongst the city's gleaming skyscrapers...

A $200 million opera house.

It was designed by the late zaha hadid,

an architect whose visions

pushed the boundaries of engineering.

She found her inspiration just a stone's throw away,

on the banks of the Pearl river.

Simon yu was the project architect

overseeing its construction.

Simon yu: We had this concept of these two pebbles by the river,

almost like placed on the riverbanks,

and with a sort of water that sort of washes over it

or these pebbles which are dragged along the riverbank.

Narrator: The inspiration may have been simple,

but it meant creating

one of the most advanced buildings in the world.

Corina: When you're emulating a natural shape,

the first things that go out of the window

are symmetry and repetition.

Narrator: The complex would be made up of two auditoriums...

A smaller 400 seater,

and next door, the larger opera house,

able to seat 1,800 people in its acoustically perfect hall.

Both buildings would be cloaked in a net

of 12,000 tons of steel-framed triangles

joined together by 59 huge specially designed connectors,

creating a strong enough frame

for the building's exterior coat.

Each is then covered

in over 75,000 unique hand-cut granite triangles,

completing the stunning vision of two pebbles on a riverbank.

The engineers started with the simple part...

Making the two concrete structures

that would house the concert halls.

These went up quickly using tried and tested techniques.

The first major obstacle came

in the form of the giant steel enclosure

that wraps the cement structures...

A vital part in creating the building's shape.

Nehemiah mabry: The Guangzhou opera house is very clever,

because it feels like something that's fluid, light,

very free flowing,

much like its pebble inspiration.

But underneath its curvy skin, it's packing

some very, very sophisticated geometry and engineering.

Narrator: The answer to building the extraordinary

140-foot-tall, 390-foot-wide, pebble-shaped shell

lay in the humble triangle.

Nehemiah: Despite competition from circles,

triangles are actually the strongest shape.

They're able to distribute a load evenly

throughout all three sides,

and so because of this, here is really the best shape

for evenly distributing the load,

carrying it all the way down to the ground of the structure.

Narrator: To create these triangles,

the engineers needed to design and build

59 unique star-shaped joints, called knuckles.

The incredibly complex design

relied on the power of the computer.

But for manufacturing them,

they turned to an ancient technique

called sand-casting.

Simon: The way they were made

was actually quite old-fashioned.

It was very, very simple in the sense

that they actually would make a 1-to-1 version

of the steel members in wood.

Narrator: To create steel joints using sand-casting,

the engineers first made the exact shape out of wood.

In a foundry, this wooden shape

is sandwiched between two layers of sand,

the wood is then removed,

leaving a perfect imprint and creating the mold.

Then, molten steel is poured in, it hardens,

and when the sand is removed,

a steel version of the wooden shape is revealed.

Simon: These knuckles were actually all absolutely unique,

and they have a very, very tough job

of keeping and bracing all these,

a lot of the steel together, you know,

they all work together.

Narrator: All the knuckles combine

to form the super-strong outer shell.

Ellie: Sand casting, it sounds like really old-fashioned,

right, but in this instance, it really is a case

of if it ain't broke, don't fix it.

The shang dynasty were using this technique in 1300 bc,

and it's still in use today to make one-off pieces.

Narrator: With the frame in place,

the next challenge was creating the pebble-like exterior.

The solution lay in yet more triangles,

each individually cut to cover the 12,000-ton steel frame.

It would be an impressive effect,

but an engineering nightmare.

To mimic the natural shape of a river-washed pebble,

every triangle had to be cut to exact specifications

that changed constantly.

To make things even harder,

the clients loved the idea of the pebbles so much,

they wanted the building clad in super heavy, super hard granite.

Ellie: The total granite facçade cladding area

is 24,700 meters squared, with 75,422 separate pieces.

That is essentially the Jigsaw puzzle from hell.

Narrator: The engineers had all 75,000 pieces of stone

cut to size off site.

Every tile had a tolerance of 5 millimetres,

and lasers were used to make sure each was precisely placed.

With the outer shell complete,

the engineers turned their laser sights on the sky lobby...

An area between the concert halls

and the outer shell,

where the designers wanted to create a futuristic space

with floating walkways with no visible support.

The solution hidden inside these impossible balconies

are a secret collection of steel boxes,

anchored to the building's sturdy concrete core,

that support the structures,

giving the illusion of weightlessness.

They are covered with white organic cladding,

created in huge computer-designed molds

that were filled with white gypsum plaster,

and anchored to the steel and concrete frame, piece by piece.

Ellie: These two separate structures,

this self-supporting outer ring

and the inner concrete structures,

creates this space in the middle

that appears to be completely unsupported,

and this allowed the architects to play around

with these floating staircases,

and these cantilevered levels

that appear to completely defy gravity.

Narrator: The real challenge

lay in creating the concert hall itself,

to create perfect sound,

concert halls are traditionally perfectly symmetrical.

This, however, is anything but.

The key lay in controlling

how the sound bounces around inside the hall.

Hayley: The art of acoustics

is getting the reverberation just right.

If there's too little a gap,

then everything sounds quite flat,

but if there's too long a gap, then you get echoes,

and it sounds quite muddy.

Narrator: They turned to one of the world's

leading acoustic engineers to pull it off.

Peter fearnside: Getting the reflection sequences right...

That's the sound that hits the wall or hits the ceiling

and ends up at the audience...

Is vitally important,

because it adds to the strength of the sound.

It can add to the clarity of the sound.

Narrator: The way you do that

is by building what's called a shoebox shape.

The sound bounces off the walls and back to the audience.

The problem is the bigger the auditorium,

the further the audience is from the stage

and the worse the view.

But if you fan the audience out,

the quality of sound deteriorates.

With an 1,800-seat capacity, this space would be huge.

So the team had to find a way

to keep the audience close and the sound perfect.

Peter: One of the alternatives is asymmetry,

where you can use the side walls of the asymmetrical hall

to reflect the sound in a way that is similar to a shoebox,

that you enforce the lateral reflections.

Narrator: So the plan was to create

curved walls and surfaces within the auditorium,

to direct the sound exactly where the engineers wanted.

But figuring out how to do this was so complicated,

they decided to build a model of the hall,

where the team could test it out.

Peter: In the scale model, you can't test at every seat,

but you use dummy heads and microphones to test at a sample,

and also you test

where you think you potentially are going to have a problem.

Narrator: By analyzing the sound recorded on the tiny microphones

in the different positions,

the team was able to build up a picture

of exactly how to shape the inside of the hall.

Peter: What we were looking for in the model

was strong, focused reflections, from concave surfaces,

but also the presence of multiple reflections

which will give you the effect of an echo,

and such complex space,

that this was the only way we felt at that stage

we could do it.

Narrator: By bending and curving the walls,

the team was able to create an amazing sound

and not compromise the view.

The result of all these complex solutions

of engineering problems

is a building inspired by nature

that is truly out of this world.

Ellie: This building for me is a perfect example

of what happens when architects and engineers

and all sorts of other experts

work together to challenge the ordinary and the simple.

Narrator: In the heart of London,

engineers and architects faced a problem...

How to help a 180-year-old building

find its place in the modern world.

The solution was a breathtaking revamp,

which meant demolishing two acres of the old structure

and, using mathematical precision,

replacing it with an 800-ton roof of steel and glass.

This is the great court of the British museum.

So how did they build it?

At the turn of the century,

the British museum was in dire need of modernization.

The centre of the museum

was a cluster of small and confusing rooms,

and with nearly 7 million visitors a year

struggling to see its 50,000 items,

the time had come for change.

But transforming a building that's almost 200 years old

is a very different proposition from building one from scratch.

Corina: Working with an old building can be a nightmare.

You never know what you're gonna find

or what problems you might run into.

Narrator: The man tasked with the challenge

was one of the world's leading architects,

responsible for the millennium bridge

and the millau viaduct.

Lord foster: To get from one gallery to another,

you would have to plow through a gallery,

so it was a constant traffic jam,

and it wasn't very pleasant.

Narrator: Lord foster's solution to this maze

was to knock it down and create an enormous open space

between the exterior facçades and circular library in the middle.

Over that would go a show-stopping glass roof

to create the largest covered square in Europe.

Lord foster: If you then imagine, kind of surgically,

you take all that stuff around the great circular library,

and you've suddenly got breathing space.

You've got a vast space.

How do you put this glass umbrella

so it will evaporate, and it's like an artificial sky?

Narrator: The plan?

To demolish the mass of rooms

filling the area outside the central round library,

then, to reinforce its foundations

before building a protective shell around the reading room

and strengthening the exterior walls.

Then it could receive the 800-ton roof,

made up of almost 5,000 pieces of steel

and over 3,000 panes of glass.

No two would be the same.

In 1998, with the exhibits moved into storage

and the books from the reading room

moved to the British library,

work began on demolition

and shoring up the 180-year-old foundations.

Mike cook: At the time, the great court project

was a complete rebuild inside of the courtyard.

Narrator: Pulling this off was full of risks.

They had to strengthen the almost non-existent foundations

by pumping concrete two stories underneath the reading room,

creating a reinforced slab for it to sit on

and excavating a new basement underneath.

Mike: The reading room is a very early, delicate cast iron frame

with a very beautiful and listed papier-maâcheé ceiling.

Papier-maâcheé's dry and very, very delicate,

very prone to cracking, so, as we dug,

you didn't want any movements,

or you could easily have cracks appear.

Narrator: The library sat on shallow foundations

that would never take the weight of the roof,

so stage one was to underpin it

by pumping in thousands of tons of concrete.

Even this was a delicate operation.

Knowing the work could disturb the structure above,

alarms were installed.

And it turns out, for good reason.

Mike: I remember the message coming back, you know,

oops, the alarms have gone off,

we seem to be lifting the reading room,

we'd better stop,

and the pumping had got a bit carried away

and started moving the reading room upwards.

Joshua: Can you imagine being the engineers at this point,

where you've got a precious piece of English history

that's stood there for hundreds of years,

and now it's at risk of being quite seriously damaged.

Narrator: The next major engineering challenge

was that the very delicate central reading room

was nowhere near strong enough

to carry the weight of the new roof.

Mike: In order to keep the heavy weight

off the central reading room,

we didn't want to load the roof onto the central reading room,

so we had put in a ring of columns.

They had never got a proper skin on the reading room.

It was very ordinary brick.

It was never designed to be seen, so it needed a new skin.

What foster's cleverly did was leave a little gap

between the new stone and the existing brick,

and in that gap, we put 20 new columns,

and a whole load of cross-bracing

to stabilize those 20 columns.

And that gave us a beautiful place

to take the weight of the roof,

and by putting the cross-bracing in,

it also wasn't gonna wobble sideways,

so it was a really stable core,

and nobody even knows it's there.

It's all hidden behind the stone.

Narrator: So with the foundations in place,

the engineers were able to build a hidden steel structure

around the central library

that would carry the weight of the roof.

The library itself would sit safely inside,

with the steel structure hidden in the new external finish.

Finally, the old building was ready to take the weight

of the new glass roof.

However, designing a curved glass roof

that would join the square outer walls

to the circular library

was a serious mathematical headache.

Mike: I think you have to go and see the roof

to kind of understand why it's complicated.

Ellie: It looks floaty, light, effortless,

but that is 315 tons of glass and 478 tons of steel.

Narrator: The team needed help to realize the design.

They turned to computer power.

Mike: At a time when we were

only just getting used to our computers, you know, really,

in terms of defining complicated geometries,

you know, now, 20 years on, it's pretty easy,

but it wasn't then.

Lord foster: The computer, it enables you

to see the implication of one or several variables,

making it possible to create something

that is at one and the same time

extraordinarily complex,

but to make it look incredibly simple

and, in the end, poetic.

Narrator: With the geometry of each piece

of glass and steel calculated,

the engineers got to work transferring it

from the computer to the courtyard,

starting with building the two acres of steel mesh.

Lord foster: So, it's a new structure

at the circular heart of this,

and then all that very delicate tracery of steel members,

11 kilometres in total length,

making over 3,000, 3,300 triangles,

and that whole thing can move, it can expand, it can contract,

it can absorb snow loads, it can move,

and it's only delivering vertical loads

onto the historic fabric.

Narrator: The team then installed

the 3,312 panels of glass.

With no two pieces the same,

it was an enormous Jigsaw puzzle to complete.

Lord foster: It's like an iceberg...

You are seeing the tip.

What you're not seeing is that extraordinary body of work

by so many individuals, the people who made it.

Narrator: Finally, on December 6, 2000,

the great court opened to the public.

Corina: It's the largest covered public square in Europe,

an incredible piece of engineering.

Narrator: The new courtyard with its floating glass roof

is the result of thousands of man-hours

from lord foster's team

of designers, builders, and engineers.

The result is an extraordinary modern structure

that has transformed

one of London's most important buildings.