NOVA

NARRATOR: It's one of the great architectural wonders of the world-- China's Forbidden City, a vast collection of buildings, all made mainly of wood.

It's the largest complex of palace buildings anywhere in the world.

NARRATOR: Said to have had more than 9,000 rooms it was a place of staggering power.

Home to China's rulers for 500 years, it was filled with dazzling treasures.

Planned over a decade, major construction took just four years.

But it raises many questions.

MAN: How did they build it?

Where did the resources come from to create something so vast?

NARRATOR: Behind this grand edifice was a great superpower... Ming Dynasty China.

They had the most powerful economy and technology much more in advance of the West.

NARRATOR: But can scientists decode this ancient wonder, unravel its engineering secrets... And discover how it withstood one of nature's great forces-- earthquakes.

(creaking) (translated): Earthquakes are frequent here, so they pose a huge threat to our heritage.

(rumbling) What's it going to take to knock this thing down?

(rumbling) NARRATOR: Closed to the world for centuries... Now open to reveal the "Secrets of the Forbidden City."

Right now on "NOVA."

NARRATOR: The Forbidden City, in the heart of Beijing.

It's about the size of about 140 football fields and has at least 800 buildings.

Completed in year 1420, it was a pleasure palace, an impregnable fortress, and a technological marvel.

With the top craftsmen, top carpenters, and top materials, so it's the best example of the technology at that time.

NARRATOR: A seat of immense power... And mystery.

ROBIN YATES: Economically important, politically important, and militarily important.

The most important site in the whole of Asia for the last 500 years.

NARRATOR: That power was embodied in the Forbidden City's awe-inspiring grandeur.

JONATHAN DUGDALE: This is the largest palace complex in the world, and not just the largest palace complex that survives, but the largest palace complex built at any time in history.

The scale of what we see here is unparalleled in the world at the time.

And everyone was struck by this because they would never, ever see anything like this in their lives.

And they knew this.

They knew that this was unique in the world.

NARRATOR: Built primarily of wood, the complex follows a clearly laid-out plan, aligned on a north-south axis.

It was home to vast halls... ...the private quarters of the imperial family; temples reflecting the many forms of worship observed there, including Buddhism, Daoism, and Confucian philosophy.

And it was built at an incredible pace.

Major construction of the original Forbidden City buildings took just four years.

In comparison, the great cathedrals of Europe took decades, even centuries, to complete.

There are two pressing questions at the heart of the Forbidden City.

How was it built on such a massive scale so quickly?

And how has it has survived the powerful earthquakes that have repeatedly struck the region over the centuries?

(stone rumbling) China's extensive imperial documents go back thousands of years, recording everything from battles to earthquakes, they enable researchers to build up a detailed picture of China's seismic history.

AN YIN: This is an earthquake fault map of northern China.

And Beijing is located here, right by this fault.

All the red stars are earthquakes that occurred in the past 3,000 years or so, recorded in the historical books.

So, since the construction of the Forbidden City, we can see four large earthquakes.

They really jump around from fault to fault.

NARRATOR: In 1976, a massive earthquake shook Beijing.

At magnitude 7.8, it was centered in Tangshan City, around 90 miles east of the capital.

In just 15 seconds, it killed at least a quarter of a million people... ...the highest death toll of any earthquake in the 20th century.

But in this earthquake-prone region, the Forbidden City has never been brought down.

What is the secret of its resilience?

No Ming dynasty plans for the original great structure have survived.

Builders and engineers have gained a wealth of knowledge as they have restored this aging complex.

All of the main buildings have a supporting wooden structural frame at their heart with walls made of brick and great tiled roofs.

But why and how was the Forbidden City built?

It was the idea of one man-- a military commander, emperor, and visionary known as Yongle.

(galloping, riders yelling) YATES: Without the Yongle Emperor, you simply would not have had a Forbidden City that we see today.

Chinese history would have taken a completely different path and China would not have a civilization that lasted down to the present day.

NARRATOR: Ruling from 1402 to 1424, he was the third emperor of the Ming dynasty.

The Ming drew on earlier technologies unique to China.

Most importantly was printing, paper... (horses whinnying, gunshot) gunpowder, and also nautical technology.

These were all things that were unknown in the West.

Even lavatory paper.

NARRATOR: The vast Ming empire stretched from what is now Vietnam almost to Mongolia.

Both a scholar, and a military commander, Emperor Yongle built the Forbidden City as the center of his new capital at Beijing.

It was an historic decision, built to shift control into his own northern powerbase and to be close to the troublesome border plains occupied by the Mongols.

But unbeknownst to him, Beijing is in an earthquake zone.

Not only are earthquakes here frequent, their causes are not clearly understood.

By contrast, a massive 20th-century earthquake took place in 1906 in San Francisco, followed by a deadly fire.

The city was virtually flattened, and the reason is clear.

San Francisco sits beside the San Andreas fault, which penetrates deep into the earth between two tectonic plates: the North American and Pacific.

The two plates are moving against each other, in particular the Pacific Plate is moving northward relative to North America.

NARRATOR: These plates move past each other, but when they become stuck, tension builds up.

When it's released, the result is an earthquake.

(crashing) YIN: If you look around the world, 90% of the earthquakes occurred along boundaries between plates.

NARRATOR: So North China, which sits in the middle of a plate, should be stable, but it's not.

You have to keep in mind that this area is far away from plate boundaries.

So this is a very peculiar place to have earthquakes inside the plate.

NARRATOR: In North China, the earth's crust is unusually thin and weak, giving rise to faults within the plate itself.

These are known as intra-plate faults.

The North China plate is cut into three pieces by three long faults.

So sometimes the energy is released on this fault.

Sometimes on that one, sometimes on this one.

NARRATOR: Over the centuries, the Beijing region has been hit by several quakes of similar size to the San Francisco killer and scores of smaller ones.

Dr. Zhou, the Forbidden City's resident seismologist, believes the Forbidden City was designed with earthquakes in mind.

(translated): They had a lot of experience with earthquakes, which shaped the building of the Forbidden City.

NARRATOR: In the centuries before the Forbidden City was built, the Chinese had learned to create resilient structures using wood.

(sawing) (buzzing) American woodworker Richard Wiborg...

This is incredible.

NARRATOR: ...has long made a study of China's ancient wooden structures.

(tapping) I saw a photograph of a man sitting on the roof of a pagoda, and, in comparison to the size of the timbers, he looked tiny.

And just the massive quality of the wood, the intricacy, the number of pieces connected together.

A light went on and I said, "I've gotta find out more about this."

A resident of San Francisco, Richard wants to understand how the buildings of the Forbidden City have withstood earthquakes.

Today, he's visiting Tai Miao Temple, the temple of ancestral worship, built by Emperor Yongle in 1420.

This huge hall was where sacrifices were made to honor the emperor's ancestors in the hope of bringing good fortune.

Richard climbs up to the ceiling to investigate some of the key structural elements of Ming architecture.

Here we are in a place I never imagined I'd be, 20 feet off ground in a Ming Imperial building.

NARRATOR: High up, Richard gets right next to a set of brackets that link the timber frame to the roof.

WIBORG: These are called dougong, and they support the roof and stabilize the building.

A bracket set is a complicated puzzle, many different parts that fit together to make a whole, and what we're hoping to discover is A) how they're made and B) why they were made.

NARRATOR: Simple dougong brackets go back 3,000 years.

They gradually became more and more complex.

By the time the Forbidden City was built, they had become essential structural elements.

Every building has a forest of dougongs, linking the columns and beams to the roof.

In Beijing's architecture museum, Richard Wiborg takes one of these brackets apart.

Now, after taking a deep breath, we'll try and put this back together again.

NARRATOR: The only things holding it together are precise carpentry and layers of interlocking pieces.

You can see this isn't-- even for somebody who's done it a few times-- it's complicated.

One piece goes in the front here.

There you have it: a bracket set.

So, you can see it's a little bit flexible.

It's very strong-- pressing down, it can take a tremendous amount of weight.

It ties together with other parts of the building and is a beautiful, creative feature of Chinese architecture.

NARRATOR: A key function of the dougong brackets is to distribute the heavy weight of the tile roof.

They do that by channeling the load evenly onto horizontal beams, and down through the narrow columns supporting the roof.

DINA D'AYALA: The dougong has the role of connecting the columns to the purlin and the rafter that forms the roof.

The dougong is like an element that funnels in all the loads and redistributes it on the top of the column in a very uniform way.

NARRATOR: But what role do the dougong play in protecting the buildings of the Forbidden City from the region's powerful earthquakes?

A unique scale model of a classic Forbidden City structure, one-fifth the original size, will reveal the answer when tested under simulated earthquake conditions.

(men chatting) (sawing) The model has four key structural elements: vertical columns and horizontal beams that make a timber frame.

WIBORG: So we're seeing a column and a horizontal beam go together.

(applauding) Well done!

NARRATOR: The frame holds the dougong that will support the large tiled roof.

This is the base block of a bracket set.

NARRATOR: And like the brackets in the Forbidden City itself, this set is assembled without any fasteners.

There are no nails, screws, or glue.

It's sort of like a wooden puzzle.

It's so fabulously complex.

(man speaking Chinese) WIBORG: This is a wonderful combination of form and function.

Very artistic, very imaginative, and very strong.

(hammering) It's a very sophisticated piece of woodworking, and we're going to see what will happen to it when we subject it to the extreme forces of the earthquake test.

NARRATOR: This model is patterned on a building in the private area of the Forbidden City: the Palace of Longevity and Health.

It's a typical Forbidden City structure.

DUGDALE: What you immediately notice is this huge golden roof that curves outwards away from the building in a way that you just don't see in European architecture.

Looking up here, what we have is these elegantly colored crossbeams decorated with imperial dragons and phoenixes, and they're there to support these huge red columns that bear the structural load of that massive roof.

And if you look up here in between the crossbeams and the roof, underneath that mesh, there's these flower-shaped dougong.

NARRATOR: Together, these four structural components-- columns, beams, dougongs, and roof-- somehow protect these buildings from earthquakes.

But how do they work together?

To find the answer, the scale model is re-assembled in a seismic lab... ...ready for a series of shake table tests that will subject it to dozens of simulated earthquakes.

These will push the columns, beams, brackets, and roof to their limits.

Wow, the roof is so big, it's amazing.

DR. ZHOU (translated): Yes, it is.

This is a very authentic, traditional timber frame building.

NARRATOR: The model has traditional, Ming-style brick walls, two layers of brickwork with rubble between.

DR. ZHOU (translated): Yes, we are ready to start.

WIBORG: Okay, let's go.

NARRATOR: Altogether, it weighs over three tons; and could endanger anyone standing nearby.

So, a safe zone is created around the shake table in all directions.

DR. ZHOU (translated): Test one, start!

NARRATOR: As the test begins, the shake table moves horizontally, replicating the energy and impact of an earthquake.

Gradually, the acceleration increases.

Finally, three tons of Ming-style architecture starts to move.

The engineers have never tested a model like this before.

No one knows how long it will last.

(creaking) An earthquake's size is defined by its magnitude: the amount of energy released.

(creaking) Magnitude 4 is generally considered a weak earthquake that most modern structures can survive.

But what about the model, based on an ancient design?

The magnitude rises gradually.

The model survives a 4, and then a 4.5.

(creaking) The sound was like the creaking of a ship, the wood compressing, twisting, groaning, but no obvious damage.

NARRATOR: Next is magnitude 5, a massive increase, with over 30 times the energy of a 4.

(man speaking Chinese) NARRATOR: The movement piles pressure onto all the structural components.

(creaking) As the magnitude progresses beyond 5, the sideways movement proves too much for one of the brick walls.

(crashing) (crumbling) Maybe we'll take one step back.

(translated): Yes, we are worried.

It's unpredictable.

NARRATOR: While the timber frame flexes, the walls are just too brittle.

(creaking) The second wall topples.

WIBORG: There it goes.

(crashing) D'AYALA: In traditional Chinese building, the walls have only the role of defining and dividing the space.

They don't really have a structural role.

Because they are not supporting the roof, their failure does not threaten the overall stability of the structure.

(crashing) WIBORG: That was pretty exciting.

DR. ZHOU (translated): We can see the walls have completely collapsed.

But the traditional timber frame is still standing strong.

NARRATOR: As the test proceeds to magnitude 7, the model is shaking, but it remains intact.

What role do the dougong brackets play in this.

The timber frames move during the earthquake because of the dougong, and because of this very particular connection.

NARRATOR: Through their ingenious design, the dougong disperse the energy of the tremors.

So as the vibration comes through the columns, and into the dougong, each of the dougong parts move relative to each other.

So, through friction, this movement will dissipate the energy, reducing the overall effects of the earthquake on the integrity of the structure.

NARRATOR: In addition to structural elements, to build the Forbidden City, Ming architects and engineers had to come up with ideas and innovations on an enormous scale.

Before the main building phase could begin, a decade of preparation was needed to set up a vast and complex system of logistics, to conscript a huge labor force, and create a massive supply chain across China.

Because Beijing lacks natural resources, virtually everything had to be brought here, including much of the wood.

The Forbidden City's palaces, temples, and the Emperor Yongle's great halls are dominated by tall wooden columns.

In the ancestral hall at Tai Miao temple, 42-foot-tall columns sparkle as if speckled with gold.

Look at this magnificent column, so round and so smooth.

Imagine how big the tree was that this came from.

In fact, we have 68 columns in total for this hall, and they're supporting the structure.

And they look beautiful as well.

NARRATOR: This is a single log of a nanmu tree.

Today, one of the rarest Chinese woods, it's straight, dense, and resistant to rot.

Just one of these great logs weighed over 40 tons.

But nanmu trees grow more than a thousand miles from Beijing, in Sichuan to the southwest, hidden deep in mountainous forests.

(birds chirping) Felling the trees, and then dragging the heavy logs to the Yangtze River for transportation, was treacherous.

(translated): According to a local saying: "1,000 people go into the mountains, and only 500 come out."

The mortality rate was terrible.

NARRATOR: Despite the cost and risks, the emperor demanded nearly 100,000 logs to be imported from the southwest.

These alone could take four years to reach Beijing.

(chatter) The Forbidden City also needed golden-colored floor tiles, made with special clay in Jiangsu Province 600 miles away.

Perfectly polished, each tile took more than a year to make.

Eighty million bricks were produced in kilns on an industrial scale outside Beijing.

And piles of gold leaf, hammered by hand... (hammering) ...to one-five-hundredth the thickness of a human hair, produced in specialized workshops in distant Nanjing.

But how could all these supplies reach Beijing at the right time?

To accomplish this, the emperor's engineers extended China's ancient Grand Canal.

A simpler version was constructed over a thousand years earlier.

But Yongle revitalized and expanded it into a water super-highway able to carry everything needed to Beijing.

JIM GRIFFITH: The Grand Canal is both longer and older than the Panama Canal and the Suez Canal.

They really solved a very large problem of canal engineering at the time.

NARRATOR: The Grand Canal ran over 1,000 miles from commercial Hangzhou across the Yangtze and Yellow rivers before reaching Beijing.

This is the main channel of the Grand Canal.

To the south is Hangzhou; to the north is Beijing.

NARRATOR: Today, this section is mostly dry, but it's still possible to see the remains of the Ming's water engineering.

This is what it would have looked like 600 years ago.

It would have been full of barges carrying workers, soldiers, materials and grain supplies north, to feed the huge Beijing building site.

But before this could happen, the Ming faced an enormous engineering challenge.

Standing in the way of the canal was a series of hills.

The high point of the canal was at Nanwang, 300 miles south of Beijing.

GRIFFITHS: This point here is approximately the watershed between the north and south section of the Grand Canal.

So if we were to drop water here, water on this side of the line would flow north, water on this side of the line would flow south.

NARRATOR: To get the canal traffic over the high ground at Nanwang meant that millions of gallons of water had to be diverted from nearby rivers.

GRIFFITHS: Here we are at the center of the Nanwang water diversion project, as part of the Grand Canal system.

NARRATOR: This Ming dynasty map shows how rivers were dammed to feed water into huge locks at Nanwang.

These large enclosures could be flooded and drained to raise and lower barges, allowing them to get over the high ground.

To fill the locks and raise the barges, water had to be diverted from the Wen River and stored in three huge man-made lakes.

Barges travelling up the canal to Nanwang entered a series of locks over three miles long.

The river water poured in to raise the barges, so they could proceed, step by step, over the high ground toward Beijing.

GRIFFITHS: As a modern-day engineer, I think the main reason why this is so impressive is partly because of the size, but also because of the complexity of trying to manage the water over such a large area.

That's a huge achievement, even in modern-day engineering terms.

For just this section of the canal there was 300,000 laborers conscripted, working for a period of 200 days constantly.

And they created this with only the most rudimentary hand tools.

(clanging) NARRATOR: But how could the Ming afford this?

The cost was borne by the Emperor's subjects in money, materials, and lives.

YATES: Tens of thousands of people starved to death.

He worked convicts, he worked the military, he worked ordinary peasants, he squeezed as much as he could out of the taxes, in order to build the capital.

NARRATOR: The result was an engineering masterpiece.

Much of the Grand Canal is still in use today.

(dramatic music playing) This massive infrastructure project changed China forever.

Just as the Forbidden City altered the landscape of Beijing itself.

This hill gives us an amazing aerial view of the Forbidden City.

What's more incredible is that prior to the construction, this hill didn't even exist.

It was actually built using the earth excavated from the moat.

NARRATOR: All this was produced through the meticulous planning Emperor Yongle demanded.

He insisted on controlling each step of the vast project himself.

But there were some forces neither he nor his successors could control.

(loud crumbling) Back at the seismic lab, Dr. Zhou and Richard Wiborg are ready to continue the shake table tests on the scale model of a Forbidden City building.

They're still there, too.

NARRATOR: Having reached magnitude 7 with only the brick walls falling, they now want to see if the model can withstand the force of an even greater earthquake-- magnitude 7.8.

This matches the size of the 1976 Tangshan City earthquake that killed a quarter of a million people east of Beijing.

YIN: It's a massive human disaster.

And many, many, the whole families not a single person survived this event.

NARRATOR: In just 15 seconds, 85% percent of the structures in the city were destroyed.

YIN: The Tangshan area has this traditional stone type of buildings.

So basically all the walls, all the houses, they were all constructed by very heavy rocks.

Good for insulating heat and cold, but they're not very good at resisting earthquakes.

NARRATOR: If the heavy stone buildings of Tangshan failed to resist a magnitude 7.8 earthquake... how will the wooden model perform under a similar assault?

(machine whirring) WIBORG: Get ready to jump.

(creaking) Whoa!

It scared me, I'll tell you.

NARRATOR: Will the columns and beams, the dougong brackets, and the heavy tile roof stand up to such powerful shaking?

(creaking) Amazingly, even after a period of 30 seconds-- longer than many real-life earthquakes-- they do.

(loud rumbling) (creaking) What's it going to take to knock this thing down?

NARRATOR: When Dr. Zhou and Richard examine the model, they find one more secret to its resilience-- the columns.

DR. ZHOU (translated): This red line shows the column's position before the earthquake.

We can see that after the earthquake the column has moved over here.

NARRATOR: The model replicates the way the columns stand in the Forbidden City itself, resting freely on stone foundation plinths.

If they were pounded deep into the stone, they would break if overwhelmed by the shaking of an earthquake.

Instead, by being left free to move, they can ride out the tremor.

The model shows how the columns move, dispersing energy, and helping the whole building withstand the shaking.

What we see here is the effect of the shaking on the columns.

We can see that all the columns rotate slightly.

This helps to dissipate energy, and to isolate the movement to the roof.

In that way the structure is still surviving.

NARRATOR: But what about the large, heavy roof?

Its curved shape is a tradition that dates back 2,000 years.

To build the roof, timber boards are laid onto the rafters.

On the outside, layers of red clay are used to waterproof it before glazed tiles are pushed into the clay and overlapped.

The colorful roof tiles and mythical beasts that adorn them are still produced in kilns outside Beijing using traditional methods.

The area's unusual clay produces long-lasting tiles.

But they are heavy.

The roof of a big hall in the Forbidden City is estimated to weigh 2,000 tons... all that weight resting on the dougong brackets connecting it to the columns below.

Exactly the same tiles and construction technique have been used on the one-fifth scale test model.

(rumbling) Look at the roof moving (translated): It's swings back into place, helping it to recover.

NARRATOR: Because of the interlocking construction, the dougong, roof, and frame are connected but can move independently, isolated from each other.

D'AYALA: The timber frame is moving at a higher speed, while the roof is moving at a much lower speed.

(rumbling) NARRATOR: Slow motion reveals how the movement of the roof lags behind the timber frame.

And while the bottom of the structure shifts in one direction, the roof moves the opposite way, countering the effect of the shaking.

There is almost like a re-centering action here because as the timber frame tends to move out in one direction, and the roof is still moving in the opposite direction, it pulls it back towards the resting position.

NARRATOR: The roof acts as a counterweight, balancing the structure, and saving the building from collapse.

But state of the art engineering, carpentry, and logistics alone did not satisfy the ambitions of the Ming Emperor Yongle.

He also demanded artistry.

The centerpiece of the Hall of Preserving Harmony is a magnificent grand staircase.

It's the largest stone sculpture in the Forbidden City, showing dragons playing in the clouds.

Hand-carved from a single vast block of marble more than 50 feet long, it weighs over 300 tons.

More than two jet airliners.

This marble came from quarries 40 miles from Beijing.

But getting the raw stone from the quarry to the Forbidden City was a major challenge.

(translated): After the stones were quarried, they were too large for human labor to move.

So, what did they do?

NARRATOR: Beijing's climate is harsh, and in the winter, the area freezes over.

Ming records describe an ice road, shaped by engineers to cross any obstacle.

But how did the Ming move the massive marble blocks along the ice roads?

Rob Dwyer-Joyce is an engineer who researches friction.

He has designed an experiment to answer this question.

What we have is a wooden sled.

And we've loaded it with stone weighing about a ton, 1,000 kilograms or 2,200 pounds.

And we're going to do an experiment to try to pull this along by hand on the ice.

This is going to be quite difficult.

Because it's... (light grunting) No, can't get it shifted.

NARRATOR: As they struggle, a whole ice hockey team comes to the rescue.

NARRATOR: But even these strong athletes have a hard time getting it moving.

Whoa!

NARRATOR: So how much force does it take to get a one-ton sled moving across the ice?

DWYER-JOYCE: So now we put a load cell in front of the sled, and we're going to measure the force on this load cell as the guys start pulling.

And now let's have some more skaters.

Again.

What we can see here is the force versus time graph.

As we added a player you can see it increases in steps.

And then it gets to a certain force and then drops.

And this drops because the sledge starts to slide.

And when we do that, the force required to pull the sledge along plateaus.

NARRATOR: It takes a team of players to get the sled moving.

But only one to keep it moving.

So the challenge is to get it started.

We know from historical sources that the Chinese had water supplies along the route that they were moving the stones.

And it's not quite clear what they used that water for.

NARRATOR: Ancient documents describe wells dug at 550-yard intervals along the ice road.

Rob has a theory that the wells provided water to help get the huge marble blocks moving along the road.

I'm going to put some water down there.

And see how many we need when we've slightly wetted the ice.

Go on, pull it, pull it, guys.

NARRATOR: The water acts as a lubricant, between the ice and the sled, making it easier to get it to start.

And once it's moving, continuing pressure melts the ice, keeping the sled going.

The Chinese moved these large lumps of stone on a much bigger scale than what we've done today.

I rather admire them for having done that several hundred years ago to realize that once the sled was moving it generated friction, but they probably needed water to form that film between the sled and the ice to get started in the first place.

NARRATOR: This level of engineering skill made it possible for artists to carve the magnificent marble staircase at the Hall of Preserving Harmony.

But similar artistry is found throughout the Forbidden City.

This is epitomized by the use of a vast array of colors-- turquoise, blue and green-- both inside and outside the buildings.

New discoveries about the Forbidden City's paint are now coming to light.

The Hall of Mental Cultivation was built in a quiet, private area of the Forbidden City, which is being restored.

Dr. Lei and his team are working to identify the precise pigments used in the paints.

DR. LEI (translated): The paint is falling off.

So first of all we need to find out what type of paint it is.

NARRATOR: The scanner emits x-rays, which are reflected back at a unique frequency for each element in the pigment.

Back in his lab, Dr. Lei uses both the scan data and the microscopic examination of paint fragments to identify the the precise pigments used.

DR. LEI (translated): It looks like a man-made pigment.

It's called smalt.

NARRATOR: Blue smalt is not a naturally occurring pigment.

And it wasn't made in China.

It's a synthetic color, made by adding cobalt oxide to molten glass.

It was made in Europe after China's Ming period.

So these layers of paint were likely applied during a later restoration, and may be evidence of international trade and exchange.

It also suggests that no expense was spared to decorate the Forbidden City with the world's finest materials.

Today, the Forbidden City is a World Heritage Site that attracts 16 million visitors a year.

But 600 years ago these halls and courtyards were reserved for the elite.

The name itself, the Forbidden City, indicates that ordinary people were not welcome here.

It took over a million laborers to build the expansive complex.

In December 1420, it was complete.

Diplomats and dignitaries arrived from across Asia for its grand opening ceremony.

Visiting ambassadors describe how elephants flanked the main gate.

A hundred thousand people from the Ming empire and beyond filled the great square.

They have never seen this level of craftsmanship, they've never the scale of Beijing or the Forbidden City.

They're overwhelmed by just the physical space.

Of course that is the intention of building the Forbidden City.

It's this statement of political power-- the power of the dynasty but also the power of the emperor, the man who sits on the throne.

And for most people of the time, they were persuaded.

Yeah, that's right, this is the most powerful man in the world.

NARRATOR: The Emperor's day typically began at the Palace of Heavenly Purity.

This courtyard, and the palace behind me, is the most sacred location in the entire Forbidden City.

NARRATOR: This was Yongle's residence.

YATES: And every morning he would rise, at about 4:00.

And he would come down this central pathway, only he could pass along the central line for the dawn audience.

NARRATOR: At daybreak, hundreds of officials would gather in silence to kowtow before his majesty... Filling the courtyard in front of the city's grandest structure, the Hall of Supreme Harmony.

In the very center is the throne.

And above you have the symbol of the heavens, and the throne is directly underneath the heaven.

Why?

Because the emperor is the son of heaven.

NARRATOR: China's traditional belief system demanded that the emperor constantly prove he had the mandate of heaven.

And he had to demonstrate as well that he was worthy to rule by maintaining a careful balance between the earthly and cosmic orders.

But that harmony could be disrupted by nature.

(lightning crackles) Fire, caused by accidents or lightning strikes, was a constant threat to these mainly wooden structures.

The Hall of Supreme Harmony itself burned down many times.

But another great fear was earthquakes.

On the morning of September 2, 1679, Beijing was struck by disaster.

The city was hit by one of the biggest earthquakes in its history.

(loud rumbling) Imperial records suggest it killed around 15,000 people.

China's detailed historic records hold valuable clues, which Professor An Yin uses to work out the magnitude and nature of ancient earthquakes, including the one in 1679.

There is some kind of tombstone, which is supposed to face to one direction, usually faced to the south.

But that tombstone rotate.

So that is very important to geologists because we can use those historical records to determine the way the earthquake was generated-- whether it's moving up and down or sideways.

So for this one it has to be moved by the sideways.

NARRATOR: Imperial Records suggest the 1679 quake was a magnitude 8.

It destroyed thousands of stone structures, including the original white pagoda, or stupa-- for Buddhists, a place of meditation.

So here we see the white stupa, constructed originally in 1651 and then destroyed in a massive earthquake, but then almost immediately rebuilt.

NARRATOR: The restored stupa still towers over the Imperial Park and Beijing.

But while the original stupa collapsed in 1679, records indicate that the Forbidden City, just 500 yards away, suffered only minor damage, mainly to the brick walls.

Most descriptions focus on the city walls and the city gates, it seems like the major palaces' architecture are not damaged.

(squeaking) NARRATOR: But can the model, in its final seismic test, withstand a force even greater than the 1679 earthquake?

The scientists will now expose the model to magnitude 8.2, the power of the largest earthquake ever to hit the Beijing region.

And stronger than the 1906 earthquake that destroyed San Francisco.

DR. ZHOU (translated): Magnitude 8.2, begin!

DR. ZHOU (translated): Now the quake is getting stronger.

The frame is shaking badly.

WIBORG: Pretty darned strong.

NARRATOR: The test moves to magnitude 8.6, the strongest earthquake to hit this part of Asia.

It shook Tibet in 1950, destroying over 70 villages and causing massive landslides.

As the magnitude increases, and the shake table delivers more energy, the structure sways dramatically.

But it remains standing.

This is incredible.

NARRATOR: After a series of escalating shocks, the test now moves beyond 9.5... the largest quake in recorded history.

This devastated Chile in 1960, causing a tsunami as far away as Hawaii.

(rumbling) DR. ZHOU (translated): Magnitude 10.1, begin!

(creaking) NARRATOR: In this final phase, the shake table peaks at magnitude 10.1, as high as the technology can go.

(creaking) The test must come to an end.

DR. ZHOU (translated): Test finished!

We wanted to break this building and it wouldn't break, so, wonderful test.

WIBORG: What was most surprising to me was how much the building can move, how flexible it was.

(rumbling) This is a fabulous proof of the genius of Chinese traditional architecture.

NARRATOR: To build the Forbidden City 600 years ago, the Ming Chinese had to meet a vast array of challenges... Mastering logistics on land, water, and ice...

Applying engineering ingenuity and carpentry skill, and demonstrating artistry of color and design.

It cost thousands of lives and the toil of a nation, but they built a city unique in all the world, at least 800 buildings, mainly all out of wood.

It was the Ming dynasty's most enduring creation, the beating heart of their empire, the Forbidden City.