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00:02The Leaning Tower of Pisa, one of the world's most iconic monuments.
00:08For nearly a thousand years, it defies the laws of gravity.
00:15We can see a lot of cracks.
00:18Its incredible tilt triggers a race against time for the world's top engineers.
00:24Structural failure will start here.
00:29Today, investigators use high-tech tests.
00:34That is a lot of load.
00:36To uncover hidden dangers.
00:38The soil cannot take any more stress.
00:42Why does the tower tilt?
00:44Can it survive another earthquake?
00:49And what could finally make it fall?
00:52It would literally explode. The whole tower would just crash down.
00:57To unravel its mysteries, we will deconstruct this marble giant.
01:02We'll explore its structural core.
01:06And expose its roots underground.
01:10To reveal the hidden secrets of this medieval engineering wonder.
01:24The Leaning Tower of Pisa in Italy.
01:28One of the most recognizable buildings on the planet.
01:33It's constructed 800 years ago as a bell tower for the city's cathedral.
01:40The Leaning Tower stands at the heart of the Piazza dei Miracoli, or Miracle Square.
01:49We are in the center of the Piazza of the Cathedral Square.
01:56Every single monument in that square looks like a miracle.
02:01The tower most of all.
02:09This medieval bell tower is an immense column of fine white marble.
02:17Nearly 300 steps spiral around its central shaft.
02:23Ascending eight pillared stories to a lofty belfry.
02:28At more than 180 feet, this medieval skyscraper is defined not by its height, but by its five degree lean.
02:40With a staggering overhang of more than 12 feet, why does this monumental tower lean so precariously?
03:00Valerio Assani dedicates his professional life to unraveling the tower's mysteries.
03:10Starting with what explains its tilt.
03:16Do the medieval architects make a basic building error?
03:22Valerio believes that the answer is hidden in plain sight.
03:26Written into the very stone of the tower itself.
03:33For centuries, people had wondered about those reliefs.
03:38The architects wanted to give us something that could help us to understand the significance of the tower itself.
03:46A bas-relief by the entrance to the tower reveals an unexpected image for a church building.
03:55In front of us, we have a bas-relief showing a lighthouse and two ships coming into a harbour.
04:02This unusual imagery doesn't stop there.
04:07Most bell towers are square in shape. Not this one.
04:12The hollow cylinder of the tower itself also echoes that of another structure.
04:18The tower was like a lighthouse for the city.
04:22A symbolic lighthouse.
04:28Pisa is not on the sea. It's landlocked.
04:34So why a building that looks like a lighthouse?
04:40Could it be a key to solve the mystery of why the tower leans?
04:47The answer may lie in an astonishing discovery just 800 feet from the tower.
04:58Beneath the city, drowned in thick black mud, archaeologists unearth the remains of 30 wooden ships.
05:10Within them, fishing nets, ropes and baskets are frozen in time.
05:18Their cargo gives a clue to their age.
05:23Roman pottery and coins reveal these ships are more than 2,000 years old.
05:32But why are there ancient Roman ships buried beneath the city of Pisa?
05:45Naturalmente, la prima domanda che ci si fa,
05:47per quale motivo, 30 navi sono state ritrovate a circa 15 km dalla costa.
05:54C'è stata molta incredulità .
06:01Domenico Barreca is the archaeologist in charge of rebuilding these extraordinary ships.
06:04Domenico Barreca is the archaeologist in charge of rebuilding these extraordinary ships.
06:11Ci siamo resi conto subito che era diventato uno degli scavi più importanti al mondo.
06:22Two thousand years ago, Pisa sits right on the coast.
06:26A complex system of waterways and lagoons link it to the sea.
06:33Ships use them to sail directly into the heart of the town,
06:38to deliver cargo of wine, oil and fruit.
06:44But gradually the waterways fill up with silt, burying the shipwrecks in a marshy grave.
06:56Over the centuries, the coastline drifts to the west,
06:59leaving Pisa landlocked miles from the sea.
07:08The leaning tower of Pisa resembles a lighthouse,
07:11built literally on its hidden seafaring past.
07:19The tower's foundation sits on a mix of sand and clay.
07:26One of the most iconic monuments on the planet is built on a bog.
07:32Is this why it leans?
07:36The answer is watch some children trying to build a model brick tower on a soft carpet.
07:44John Berland is known for his work on the world's most challenging engineering projects.
07:51Here is some foam rubber which represents this very soft ground.
07:58So let's start building a tower and we try and make it as straight as possible.
08:06Now you can see that at a certain height, that's perfectly stable.
08:10So we'll go a little bit higher, keeping it as straight as we can,
08:16but give it one more brick, like that.
08:20If the tower has a very small disturbance, like a fly sitting on one side,
08:26the center of gravity moves and it just runs away with itself.
08:31The ground is so soft, it's incapable of resisting the momentum of the tower.
08:37We watch it.
08:40And over it goes.
08:46How much longer can the tower of Pisa continue to defy the laws of gravity?
09:15The leaning tower of Pisa.
09:19Built to be the most impressive bell tower in medieval Europe.
09:25It's been fighting gravity for over 800 years.
09:31What could make it collapse?
09:40More than 32,000 marble blocks make up the tower.
09:46Stacked eight stories high, they weigh an immense 16,000 tons.
09:56But it's not solid marble all the way through.
10:01Behind the smooth marble face sits a core of rough limestone and mortar.
10:09This solid backbone keeps the tower standing for 800 years.
10:14But is there a hidden weakness?
10:26Pietro Croce is a structural engineer.
10:30He's an expert on the loads that put buildings under pressure.
10:35He wants to find out whether the tower can survive the stresses created by its own tilt.
10:45Is there a clue in the tower's unusual shape?
10:51If you look to the tower, it's not symmetrical.
10:55The final result is a curved shape which is like a banana.
10:59The tower is not straight.
11:03It's not easy to see with the naked eye.
11:06But the tower bends from the base to the top.
11:12Pietro investigates the unusual banana shape from the inside.
11:18He wants to find out how it affects the loads acting on the tower.
11:25He starts at the sixth floor.
11:28Now I am measuring the height of the wall in order to compare the 8 on the north side with
11:35the 8 on the south side.
11:41The measurements don't seem to add up.
11:4516 feet and 7 inches on the north side.
11:4916 feet and 11 inches on the south.
11:54So it is clear that this floor and the upper floor are not parallel.
12:00These odd measurements are not mistakes.
12:04They're built in.
12:09In 1172, five years into construction, the tower is already leaning to the north.
12:16Engineers add height to one side to counteract its lean.
12:22Construction continues, but Pisa suddenly goes to war with its neighboring states.
12:27And building stops for 100 years.
12:32When construction finally resumes, three more stories shift the center of gravity.
12:38And the tower now leans to the south.
12:44Finally, they build the belfry lopsided to offset the lean.
12:48It's the final curve of the banana-shaped tower.
12:57The tower's peculiar shape is the result of a rescue job.
13:01But it doesn't stop it leaning.
13:06The tower still overhangs by 12 feet.
13:11The more the tower leans, the greater the stress put on its structure.
13:19Pietro investigates whether the stonework can withstand these immense forces.
13:29The tower's stresses are concentrated around the first floor on the south side.
13:35It bears the full brunt of the lean.
13:41The most vulnerable point of all is where the tower's walls intersect with its stairwell.
13:52Now we can see the ceiling of the staircase.
13:56A lot of cracks due to the high stresses, which is a very critical point.
14:05A huge void has been created in the walls to make room for the stairwell.
14:12Here the masonry is at its thinnest, and yet the stresses are at their highest.
14:18The walls buckle under the pressure.
14:22This situation is clearly visible looking at the crack patterns that we can see inside the staircase.
14:37This is the danger zone.
14:43The collapse due to structural failure probably will start here.
14:55One thing we know, if it fails here, at the wall on the first floor, the consequences could be catastrophic.
15:12The marble giant faces monumental threats.
15:18But some are more visible than others.
15:22Is there a hidden threat, high up, in the belfry?
15:45The leaning tower of Pisa in Italy.
15:48The leaning tower of Pisa in Italy.
15:49It's been waging war with gravity for over 800 years.
15:55But a leaning tower can reach a critical point, when even the smallest disturbance can send
16:01it crashing.
16:09Towering 160 feet above the town, the belfry holds seven colossal bronze bells.
16:17The largest weighs almost 8,000 pounds, as much as two cars.
16:27Stone projections on the arches support sturdy wooden beams.
16:33These huge blocks of wood suspend the bells with thick iron straps.
16:40With a combined weight of more than 23,000 pounds, could the bells be a threat to their
16:47own bell tower?
16:58Structural engineer Pietro Croce investigates the forces generated by the bells in the
17:03leaning tower.
17:07He visits the Marinelli Bell Foundry in the Italian town of Agnone.
17:19Established in 1040 AD, the Marinelli Foundry is one of the oldest family businesses in the
17:26world.
17:29One of its bells hangs today in Pisa's landmark leaning tower.
17:45The sheer size and weight of the bells makes just hanging them a risky business.
17:53Armando Marinelli is the 26th generation of his family to run the foundry.
18:17Bell towers do fall down, and none more dramatically than the bell tower of St. Mark's Basilica in Venice,
18:26northeast Italy.
18:29In the 16th century, the bell tower in St. Mark's Square stands over 300 feet tall.
18:36Five swinging bells ring out for 400 years.
18:40But in 1902, without warning, a huge crack appears.
18:44And within three days, it crashes to the ground.
18:53Could the same fate befall the iconic leaning tower of Pisa?
19:03Pietro Croce wants to test what kind of stresses a bell generates when it swings.
19:16Pietro Croce chooses a bell of similar size and weight to bells in the leaning tower.
19:23He places markers on the bell and uses a high-speed digital camera filming at 960 frames per second
19:32to track the motion of the bell as it swings.
19:39This will allow him to calculate the points of acceleration and deceleration when it moves.
19:49How much greater are the forces created by a swinging bell than a static one?
19:59Pietro Croce works with university colleague Maria Luisa to make his calculations.
20:06We start from a certain frame.
20:12The video on this program is able to advance one frame each time.
20:21We go again until the bell reaches its vertical position.
20:31They conclude that the bell generates forces twice its weight when it swings halfway.
20:38What if it swings all the way?
20:42A complete rotation of the bell could have a dynamic force roughly four times the static forces.
20:53A bell that exerts one ton of static load on the tower could create four times that amount with each
21:00swing.
21:04Swinging bells in a bell free generate a horizontal force that acts on the tower through the wooden axle.
21:14When many bells swing separately, the forces cancel out.
21:19But when they swing together, the bells create a dangerous surge in one direction.
21:30How much damage are the bells doing to the leaning tower of Pisa?
21:39Pietro takes a closer look inside the belfry itself.
21:48There is some sign of cracks due to the vibration caused by the bells.
21:58You can see that near the supports there are stones which are broken.
22:05And if we look on the biggest one, we can see again indication of local stresses and local cracks induced
22:17by the bell vibration.
22:21Today, no one is taking any chances.
22:25The bells no longer swing.
22:29Automated clappers create the chimes that ring out over the city of Pisa.
22:36Despite the forces ranged against it, the leaning tower still stands.
22:43But how does the tower survive over 800 years with its precarious tilt?
22:51Could the answer lie with a curious pair of towers 95 miles away?
23:13The leaning tower of Pisa.
23:16It's a marvel of 12th century engineering.
23:21A precarious balancing act that seems to defy the laws of physics.
23:29What hidden secret lies behind the tower's miraculous survival?
23:36Does the answer lie with these two medieval watchtowers 95 miles away in the city of Bologna?
23:49The towers of Bologna are simple brick skeletons with metal braces and iron rings.
24:00The taller tower rises 318 feet into the air and has a 2 degree lean.
24:08But the shorter tower dangerously tilts more than double this amount.
24:14Both towers use the same materials.
24:18And have the same foundations.
24:20In the same dense sand and clay.
24:25If their construction is identical, what accounts for the difference in height and lean?
24:34Could the answer to this mystery also reveal the secret of the leaning tower of Pisa's incredible balancing act?
24:48You might ask yourself, why is it that a shorter tower is leaning more than a taller tower?
24:58Nick O'Reardon is a geotechnical engineer.
25:02It is almost always the passage of time which holds the key to why things are the way they are.
25:11Nick believes a clue to the puzzle lies in the soil the two towers are built on.
25:22In a London laboratory, he investigates.
25:26This force is basically modeling the effect of loading from the tower.
25:32In the cylinder is a bore sample of soil taken from deep underground.
25:40Fellow engineer Christopher Russell helps Nick set up the test.
25:45It's immediately taking load. A lot of load. In fact, that is a lot of load.
25:51The test mimics what happens to the soil underground when a building is erected quickly.
25:58The force is building up and building up.
26:03At first, the soil gets stronger as it becomes more compact.
26:10So, 30% gain in strength.
26:15But the load increases faster than the soil's ability to settle and strengthen.
26:22So, this is as though the tower is being built very rapidly indeed.
26:29The soil can only withstand so much pressure.
26:33It's hit its peak and it's coming over the top.
26:39But that is failing.
26:42That is failed soil.
26:49The experiment reveals the dangers of hasty building.
26:56They examine their soil sample to reveal further clues.
27:03The shearing of the soil causes failure and you get a polished surface like this.
27:09If you can imagine walking through a very heavy clay field with wellingtons and you skid on the surface,
27:16then you get the same sort of shiny surface where your boot sole has skidded, shearing the soil.
27:24You can just see the shiny surface. You can see the actual movement lines.
27:31The soil is pushed to its breaking point.
27:35There comes a point at which the soil cannot take any more stress.
27:39And it's at that point that the builder ordinarily would start to panic and say,
27:45Oh, my tower is on the move.
27:48And I better stop building and maybe I better reduce the height of that tower.
27:54Nick believes that this is the fate of the shorter of the two Bologna towers.
28:00The medieval builders have to knock the top off to save the whole structure from collapse.
28:1112th century Bologna is filled with slender towers, each a symbol of wealth and power.
28:21Two rival families, Garisenda and Assinelli, compete for the tallest tower, right next to each other.
28:33The Garisenda's tower initially outpaces the Assinelli's.
28:37But when it leans dangerously, they are forced to decapitate it.
28:45The slower rising Assinelli tower never tilts more than two degrees.
28:50And eventually wins this medieval tortoise and hare race.
28:56The slow build of the Assinelli tower allows the soil to settle and consolidate.
29:03That's how they learnt empirically that you can't build them too quickly.
29:07You've got to build them in stages.
29:10The story of the leaning towers of Bologna also shines light on the most famous leaning tower of all.
29:19When the Tower of Pisa is being built in the 12th century, local wars halt work for 100 years.
29:29It's this that allows the soil to settle and strengthen and ensures the tower's survival.
29:37If they had built it straight up, it would have probably got up to about the fifth level and it
29:43would have fallen over.
29:44No question about it.
29:46The tower survives.
29:49But 800 years later, it's in peril.
29:53Every month it leans a fraction more, inching to a point of no return.
30:00Will a high-profile rescue mission save it?
30:04Or destroy it?
30:25The Leaning Tower of Pisa.
30:28One of the world's most iconic monuments.
30:33The Leaning Tower of Pisa.
30:35The Leaning Tower of Pisa.
30:35The Leaning Tower of Pisa.
30:35The Leaning curve is increasing every year.
30:39Its incremental, but relentless
30:43And can only end one way.
30:53In 1995, engineer John Berlin joins a high-profile Italian commission to save the tower.
31:03He's got a fine line to tread.
31:07There were two unacceptable outcomes for our work on the tower.
31:11One we don't talk about, namely it falling over.
31:15But the other one, which was equally unacceptable, was a straight tower.
31:19I mean, the Pizans didn't want a tower that was made vertical, because it's no longer a leaning tower.
31:26John develops a computer model to predict every aspect of the tower's tilt.
31:33But his figures reveal an enigma.
31:37The model produced everything on the inclination beautifully, almost perfectly.
31:42It's just that right at the very end, the model told us that the tower was falling over.
31:49According to John's model, the tower will fall over if it leans at more than 4.75 degrees.
31:58But it's already reached 5.5 degrees.
32:03It's a finding with chilling implications.
32:07Our conclusion was that the tower must be very, very close to falling over.
32:20An urgent solution is needed to save this historic monument.
32:26The PISA Commission finally agrees on a rescue plan.
32:35They've fixed 10 steel cables to the foundations on the north side,
32:42which extend 150 feet underground
32:47and attach to massive anchors in the dense sand.
32:55To stop the excavations from flooding, engineers freeze the ground with liquid nitrogen.
33:03But the ground suddenly swells,
33:06and the tower tilts more in one day than it has in a whole year.
33:21It was very, very frightening and dramatic.
33:25That was the time I wrote in my diary,
33:28I fear we may have lost the tower.
33:32In desperation, engineers fix additional counterweights
33:36to the opposite side of the tower's lean.
33:41They were stacked one by one on the north side of the tower here.
33:46So it's a bit like sitting out of a sailing boat.
33:48You sit out and stop it leaning so much.
33:52The emergency fix stops the tower from toppling over.
33:58But its lean is worse than ever.
34:01How can they save the 16,000 ton giant now?
34:15In preparation, John's team secures heavy-duty steel cables around the tower
34:21and anchors them into the ground.
34:28Engineers dig a tunnel up to the foundations on the opposite side of the lean
34:33and carefully remove soil with great precision.
34:40Over the course of a year, the tower settles into the cavity,
34:44reducing the overhang by nearly two feet.
34:49In 2001, the cables and counterweights are removed,
34:54revealing the leaning tower of Pisa in all its glory once again.
35:03The tower, it's just extraordinary.
35:07I mean, it just glows.
35:09So it's a privilege, actually, to go back
35:11and think that you've played a part in conserving this incredible monument.
35:19Today, the tower stands straighter
35:22and safer.
35:24But does that mean it won't ever fall?
35:28Italy is in one of the most notorious seismic zones in the world.
35:34Could the tower still be brought down
35:36by a powerful earthquake?
35:57The Leaning Tower of Pisa.
36:00An 800-year-old medieval masterpiece
36:03saved from tipping over by the efforts of modern engineers.
36:13But the Leaning Tower is in one of the world's deadly earthquake zones.
36:19Is it still at risk from a cataclysmic collapse?
36:33In 2016, a major earthquake, measuring 6.5 on the Richter scale,
36:38hits the town of Amatrice, south of Pisa.
36:45The town is flattened.
36:48Around 300 people die,
36:50and 4,000 more are left homeless.
37:00We are in Amatrice, in the red zone,
37:02and we can see a lot of rubble.
37:05We have a structure which has been completely destroyed,
37:09but we have structures which are still standing.
37:16One of the surviving buildings dominates the skyline.
37:23It's the town's 82-foot-tall clock tower.
37:30Astonishingly, underneath the scaffolding,
37:32this medieval building remains intact.
37:37What is apparently strange here is that we have the tallest buildings
37:43which are still standing,
37:46while smaller buildings are collapsed.
37:58Every structure has a natural frequency.
38:02This is the rate at which it will vibrate when it's jostled or struck.
38:08Tall objects have a low frequency.
38:11They move from side to side slowly.
38:15Short structures have a high frequency.
38:18They move from side to side quickly.
38:23In an earthquake,
38:24a shorter structure will start vibrating at its natural high frequency.
38:31But an earthquake's own high frequency waves
38:34can intensify the vibration of a shorter building.
38:40This can make it shake more and more violently
38:43until it falls apart.
38:49But it's a different story for taller buildings.
38:53The high frequency waves of an earthquake
38:56don't match the low frequency of a tall building.
39:01This is why it can be left unscathed.
39:16The tower of Pisa's tall and slender shape
39:19helps protect it from seismic activity
39:22like the Amatrice clock tower.
39:30But the Pisa tower also leans.
39:35The fact that the tower is leaning
39:37means that it has much higher stresses
39:40on one side than the other.
39:44So the fact that when it vibrates
39:46it is generating high stresses
39:49would mean that it would be more vulnerable.
39:52This presents a puzzle.
39:55The lean of the tower creates such a vulnerability
39:58that even moderate earthquakes
40:00in the region should topple it.
40:05Against the odds,
40:06the leaning tower of Pisa remains standing.
40:10Why?
40:12In reality, nobody knows.
40:18To be honest, nobody knows.
40:22The leaning tower of Pisa still holds on
40:26to some of its mysteries.
40:34It stands today as the true miracle
40:37of Miracle Square.
40:41Capturing the imagination of visitors
40:43from all over the world.
40:47It's thrilling to go back
40:49and just to see everyone enjoying it
40:52and what their reactions are.
40:56The iconic leaning tower
40:58not only still stands,
41:00but looks set to survive
41:02for centuries to come.
41:06The tower, it should be safe
41:09at least for 200 or 300 years.
41:13We will see.
41:18The tower of Pisa survives
41:21800 years of gravity,
41:23structural stress,
41:24and earthquakes.
41:28Through a combination
41:30of incredible coincidences
41:32and blind luck,
41:34the leaning tower still stands.
41:38A testament to medieval
41:40and modern engineering.
41:55The tower of Pisa
42:12You
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