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00:02Manhattan, New York City.
00:06Once it is home to two of America's most iconic buildings,
00:11the World Trade Center Twin Towers.
00:15They were giants of that time.
00:19When constructed, they are an engineering triumph,
00:22the tallest buildings on Earth.
00:25The skyscraper has always been an engineering marvel,
00:28but it was the Twin Towers that really set the precedent.
00:34Remembered across the globe for their tragic final moments,
00:38the story of their origins is rarely told.
00:42Today, investigators use pioneering technology
00:46to examine the Twin Towers' surviving footprint and secret history.
00:50We can see details you can't see from the human eye.
00:54And put the materials they are built with to the test.
00:59They really broke records in structural design innovation.
01:03How do engineers push the boundaries of building tall
01:07to erect these towering giants in the heart of New York City?
01:13To solve this mystery, we'll digitally reconstruct the towers
01:17and reveal the hidden secrets behind their iconic design
01:22that once defined New York's skyline.
01:34New York City.
01:36Today, over 6,000 high-rises dominate the horizon.
01:41But back in the 1960s, only a handful of buildings stand higher than 40 stories.
01:47Something radical needed to be done to bring New York really into the 21st century.
01:53And in our culture, bold gestures are often made architecturally.
01:58So New York's leaders invest hundreds of millions of dollars in a spectacular building complex.
02:07The architect, Minoru Yamasaki, went through literally hundreds of plans.
02:12And eventually, he succeeded by coming up with the idea for the Twin Towers.
02:19The World Trade Center design features five low-rise buildings surrounding a central plaza.
02:27Rising from the center, the most ambitious skyscrapers of all time.
02:33Each soaring more than 1,300 feet above Lower Manhattan.
02:38Over four times taller than the Statue of Liberty.
02:42Together, they will deliver 10 million square feet of real estate.
02:48Every day, 50,000 workers will flock to this city in the sky.
02:53But no one has ever tried erecting a building this tall.
02:56So how do they do it?
03:01Eric Darden is an expert on the history of New York City.
03:05He investigates the challenges the engineers face.
03:08The idea was to acquire land and to build on a 16-acre site, which is not a lot.
03:16Enough office space to accommodate 50,000 people.
03:20That's why the towers had to be as tall as they were.
03:25The architects looked to what is then the world's tallest skyscraper for inspiration.
03:30The iconic Empire State Building.
03:34No skyscraper had come near to surpassing it in 40 years.
03:42The Empire State Building is the high point of skyscraper design.
03:47A steel grid reinforced with concrete and encased in strong limestone.
03:52It weighs 365,000 tons.
03:56But using this building model for the Twin Towers would limit the internal space.
04:04The Empire State Building has 2 million square feet of floor space, office space.
04:10The program for the World Trade Center was 8 million square feet of office space.
04:17You can't just scale up the Empire State Building.
04:20There'd be so many columns you couldn't put anybody in the building.
04:24So experts have to figure out how to build skyscrapers that each have double the floor space of the Empire
04:31State Building.
04:35Professor Adrian Brueger investigates the Empire State Building and Twin Towers structures.
04:42Here I have a 3D print of one of the floors of the Empire State Building.
04:47And you can see that we have this very thick core in the middle here.
04:52The model shows how the weight of the Empire State Building is spread across a thick central core and hundreds
04:58of columns.
05:00But in order to beat the height of the Empire State Building and accommodate the 50,000 office workers,
05:08Japanese-American architect Minoru Yamasaki and American engineer Leslie Robertson will have to come up with a revolutionary solution.
05:18Minoru Yamasaki had the idea of this stressed skin architecture from nature,
05:23which is generally a very good place to get your motivation from, and he thought of bamboo.
05:29Bamboo is hollow, and its entire weight is supported by its outer skin.
05:35It's a very strong plant. In Japan, it's also a symbol of prosperity.
05:40The idea is the perfect solution for the Twin Towers.
05:45Instead of spreading the load across a grid, Yamasaki creates a super strong hollow structure.
05:51All the weight is supported around the edge. It's a breakthrough in skyscraper design.
06:00Each tower is made up of 236 steel columns. They stand just 26 inches apart around the perimeter.
06:10More than 43,000 windows complete the outer skin.
06:15Inside, steel plates stretch across the void, topped with nearly a million tons of concrete to create 110 floors.
06:25The north tower is crowned with a final feature, a 360-foot communications mast.
06:34Now they have a new task, to make this outer skin strong enough to support the weight of the tower.
06:43Adrian has rare access to the surviving structures of the Twin Towers, carefully conserved in the basement of the 9
06:51-11 Memorial Museum.
06:53He examines the surviving columns from the outer skin of the building.
06:57So here you can see one of those mega columns that is landing the forces from 110 stories into the
07:04ground.
07:05And if we look up at the column, we'll see that one of these large columns goes through this beautiful
07:09trident structure,
07:11and then picks up three smaller columns.
07:13The hollow building structure is a breakthrough in engineering.
07:17But this revolutionary concept goes even further, to the skeleton itself.
07:23This column is actually hollow on the inside, and that's purposely done so that we can increase its stiffness.
07:29So the material is all along the perimeter.
07:32Not only is the building hollow, but the columns are hollow too.
07:39Finally, the designers use an extra trick to reduce the overall weight of the Twin Towers.
07:45The metal is four inches thick at the base, but tapers to just a quarter of an inch at the
07:51highest point.
07:53They get lighter and lighter as you go up to the top.
07:56And that is essentially to make the building as efficient as humanly possible.
08:00So this is how structural engineers are able to build this high.
08:05Structurally, the design is perfect.
08:07It breaks through the constraints of contemporary skyscraper building.
08:13With this method, the height seems almost limitless.
08:18It will allow the Twin Towers to rise taller than any other building on the planet, reaching an astonishing 1
08:26,368 feet.
08:30But engineers face an additional challenge.
08:32How do you bring people in?
08:35The vast majority of people going into the World Trade Center came in underground.
08:42Office workers arrive at the towers via the subway, and then go straight to the elevators.
08:48But in the mornings, there are 50,000 people coming in at the same time.
08:53How can they all travel through the tower?
08:59In the Empire State Building, elevators occupy almost three-tenths of the building's floor area.
09:06If engineers follow a similar design, the Twin Towers would lose vital floor space.
09:16The taller a building is, the more elevators it needs to service it.
09:20But the shafts take up valuable floor space.
09:26The subway system in New York provides inspiration, where local trains stop at every station, and express ones fast-track
09:35to key hubs.
09:37So engineers add two sky lobbies to the Twin Towers.
09:41These are special floors where people can switch from a large-capacity express elevator to a local one.
09:48So multiple elevators can share the same shaft, increasing the usable floor space.
09:57Engineering masterminds designed these iconic towers to break the record for the tallest building in the world.
10:04Ready to steal the crown from the Empire State Building.
10:10But when construction begins, engineers face an unexpected challenge beneath the ground.
10:17Could it become a serious danger for the building's foundations?
10:31The Twin Towers are once the most recognizable skyscrapers in the world.
10:37And the tallest buildings of their time.
10:42The engineers who build them face huge challenges from the very start of their construction.
10:48That's because much of Lower Manhattan is built on reclaimed land.
10:53When engineers start to dig their foundations, they make a surprising discovery.
11:00Hidden below the pavement, engineers encounter waterlogged mud and rubble.
11:06Including a 400-year-old ship preserved in mud.
11:12As they dig deeper, a modern optical emerges.
11:16An active railway tunnel that carries 100,000 passengers per day.
11:23And even further down, 70 feet below street level, they finally reach bedrock.
11:30This exceptionally hard material is the perfect base to build on.
11:34But in order to secure the Twin Towers' foundations into the bedrock, the engineers first have to reach it.
11:44Structural engineer Rick Ellman investigates how it's possible to build a skyscraper on reclaimed land.
11:53These tall towers that are all around us, if they were not constructed properly, they would lean and very likely
12:00topple over.
12:01The most famous example of the dangers of unsteady foundations is the Tower of Pisa in Italy.
12:09It's built on marshy land, similar to the reclaimed land of Manhattan.
12:14Over time, the soft ground causes the tower to tilt dangerously.
12:21But unlike Pisa, Manhattan has an incredibly tough bedrock just 70 feet underground.
12:27The Twin Towers' foundations can be firmly anchored into this solid rock.
12:33But there's a huge problem.
12:36Reaching it.
12:37Excavating and reclaimed land can be a real challenge.
12:41Because of the weak soils and the high ground water, you're unable to just simply excavate.
12:48If they excavate, water and mud just slide back into the hole.
12:53So engineers come up with an ingenious solution.
12:57They create a wall to surround the foundations.
13:01The wall acts as a protective barrier.
13:07First, they dig long trenches around the edge of the site and fill them with a liquid slurry to stop
13:13them collapsing inwards.
13:17Then they drop reinforced steel cages into the trenches and pump in concrete to displace the slurry.
13:29Once these concrete walls set solid, engineers can excavate everything inside them, creating a giant underground bathtub.
13:40Instead of holding water in, the bathtub keeps it out, so the foundations can now be built.
13:49At the 9-11 memorial site, Rick Ellman has rare access to the remains of the bathtub.
13:57Here we are, 70 feet below the ground surface in the area that was once occupied by the original World
14:05Trade Center foundations.
14:07The space was created using the slurry wall construction method.
14:12The wall holds back the surrounding mud and ground water to protect the tower's foundations.
14:19The wall itself is probably three to five feet thick, and this system amazingly is holding thousands of pounds of
14:27force.
14:29The Twin Towers engineers are some of the first to use this pioneering technique in New York.
14:38In 1968, the bathtub is complete, and the main building work can begin.
14:46But engineers face additional challenges.
14:51The Twin Towers are the world's first skyscrapers built without stone or brick.
14:57Their flexible metal structure causes them to sway in high winds.
15:02How can engineers stop New Yorkers from getting seasick inside?
15:17April 1973.
15:19In the heart of Manhattan, two skyscrapers are set to emerge on the horizon.
15:24The Twin Towers.
15:26Their hollow metal structure is a revolutionary design.
15:31They are the first skyscrapers ever built without stone or brick.
15:36The engineers use a new technique to construct these steel giants.
15:40They clip the pieces together like Lego.
15:44Engineer Sautine C. is the managing partner of the company that builds the World Trade Center.
15:50Well, the design of the Twin Towers allowed for faster and safer erection and construction.
15:58So it went up really fast.
16:02Building the pieces off-site allows engineers to complete them to the highest standard.
16:08And it means the structure can be assembled quickly on-site.
16:12In a sense, it was like an assembly plant.
16:19The Twin Towers external skin goes up first.
16:2359 steel columns per side, each just 14 inches wide.
16:28They bear most of the tower's weight.
16:32In the center of each tower sits a core of 47 steel columns.
16:37To give the tower extra rigidity.
16:41Thin steel plates 60 feet wide bridge the gap, connecting the inner and outer columns.
16:47Anchoring the entire structure together.
16:49And creating the vast open floor plans.
16:53It's a futuristic, lightweight structure.
16:56But will this flexible design be a match for New York's high winds?
17:01At the top of the buildings, wind speeds can reach 100 miles per hour.
17:06To make it very tall, the building had to be very stiff to resist the wind lows.
17:13If the building sways too much, it could shear the steel.
17:17Or the elevators could even get stuck in the shafts.
17:20So the Twin Towers become the first skyscrapers in the world to be evaluated for wind dynamics.
17:27A new field of research is born.
17:30Scale models of the Twin Towers and the complex surrounding areas were built and tested in Colorado.
17:41In the 1960s, the Twin Towers engineers create the first ever wind tunnel and put their models to the test.
17:49They shoot 150 mile per hour winds across it.
17:54The tests show they will hold strong structurally.
17:57A success.
17:58But there's a problem.
18:01Even though the models can withstand the wind loads, they sway one inch from side to side.
18:07Equivalent to 40 feet on the full-size buildings.
18:12Workers at the top will sway with the buildings.
18:15Gazing out of the window at the second tower swaying will make the sense of movement worse.
18:21On the 110th story, a gust of wind will trigger instant nausea.
18:26It would be impossible to work there.
18:29So the wind tunnel tests provided wind loss, but we didn't know whether those accelerations were acceptable to people who
18:38are in the building.
18:38The occupants.
18:40Would they feel seasick?
18:42We don't know.
18:43There were no guidelines around.
18:49How much sway humans can endure is a critical question for skyscraper engineers.
18:56To investigate, structural engineer Antony Darby launches the V Simulator.
19:02A state-of-the-art virtual reality skyscraper at the University of Bath, England.
19:09Heavy-duty hydraulics suspend a custom-made office, simulating the sway motion of a skyscraper in gusts of wind.
19:18Inside, a volunteer wears 3D glasses to experience a virtual reality view out of the skyscraper windows.
19:27Antony's team records the volunteers' physical reactions as the skyscraper sway intensifies.
19:34We can measure their response, their heart rate, their eye blink rate, things that measure stress or tiredness.
19:44And we can also assess their work performance, how that degrades over time as well.
19:50Antony's team collects data from hundreds of volunteers to determine the breaking point of humans.
19:57He measures the movement in G-force.
20:00The typical G-force felt on a roller coaster is 4G, compared to just 0.01G for walking.
20:08What we've shown here is that below about 0.005G, people don't feel the motion.
20:16Above 0.005G, you can perceive the motion, but it might not affect you that much.
20:21But as we steadily get higher, up around 0.04G, then it does become intolerable.
20:29The simulator tests reveal that 0.04G is the upper limit.
20:34Anything above this force is intolerable.
20:39Today, this research is used for high-tech skyscraper design and rigorous testing.
20:46But in the 1960s, tests like these don't exist.
20:50The Twin Towers team has to invent the very first test themselves.
20:57The Twin Towers engineers build their own test rig and hang it inside a ventilation shaft in the Lincoln Tunnel.
21:08Volunteers sit inside the fake office, while two engineers push it from side to side and ask them to report
21:16when they feel sick.
21:19Even these crude tests are enough to confirm what the Twin Towers engineers fear.
21:25People aren't able to work in these conditions.
21:30In order to reduce the 40-foot sway, the architects create special shock absorbers to reduce the movement between the
21:38wall and the floor.
21:40They install 10,000 of these dampers across the two towers, absorbing huge amounts of energy.
21:47Reducing the sway to just three feet.
21:52They are a success.
21:54They are the first skyscraper dampers in the world.
21:58Everything in the original Twin Towers was innovative and groundbreaking.
22:03In the design, in the materials, in the construction and erection.
22:10The Twin Towers are a modern masterpiece.
22:13Created with exquisite attention to detail.
22:16By creating these technological solutions, the engineers keep the 50,000 people inside safe from these foreseeable dangers.
22:27But the threat that lies ahead is unimaginable to its creators.
22:32The deadly impact of the terrorist attack on 9-11 surpasses any force engineers could ever predict.
22:53In 1973, the Twin Towers become the tallest buildings on the planet.
22:59These engineering marvels rise 1,368 feet into the sky.
23:06But no one could foresee the challenges they would face over their lifetime.
23:11In 1975, an electric short circuit failure set six stories of the North Tower on fire.
23:19Firefighters managed to control the blaze.
23:21Fortunately, the building was unoccupied or there would have been potentially some serious consequences.
23:28Twenty years later, a bigger impact strikes.
23:31A terrorist plants a 1,200-pound bomb in the North Tower garage.
23:37It explodes with devastating force.
23:40Yet the tower stands strong.
23:44But on September 11, 2001, a tragedy bigger than anything imaginable strikes the towers.
23:55The world watches in disbelief.
23:59Over 17,000 people are working inside the Twin Towers.
24:05When suddenly, a terrorist hijacked Boeing 767, with 92 people on board, crashes into the North Tower.
24:14The impact creates a burning hole between the 94th and 99th floors of the 110-story skyscraper.
24:23The 10,000 dampers absorb the energy from the impact.
24:27And the hollow skeleton design supports the weight of the building despite the damage.
24:3617 minutes after the first plane hits, a second hijacked Boeing 767, with 65 passengers and crew on board,
24:47crashes into the 77th to 85th floor of the South Tower.
24:51There was a tremendous outburst of smoke, debris, debris from both impacts, particularly the second one,
25:01rained down on the whole area.
25:06Over 1,000 people perish in these sudden crashes.
25:10While the towers are still standing, over 15,000 people manage to evacuate.
25:19But the tragedy isn't over.
25:23The flammable jet fuel from the aircraft makes the crash much more deadly than anyone could have foreseen.
25:32When engineers design the Twin Towers, they protect against a crash from the largest aircraft at the time,
25:39a Boeing 707.
25:41But the hijacked aircraft are much larger Boeing 767s, carrying 20,000 gallons of kerosene.
25:49This jet fuel pours down through the 12 miles of elevator shafts onto other floors, spreading like wildfire.
25:59Inside the towers, there are thousands of tons of office furnishings,
26:04many of them highly flammable, so the fire spreads unusually quickly.
26:10The firefighters soon realized that there was no controlling this blaze.
26:14From then on, it was concentrating on not saving the building, but on rescuing the people inside.
26:20At exactly 9.59am, almost two hours after the first collision, the South Tower collapses.
26:31In just 10 seconds.
26:37Half an hour later, the North Tower goes down too.
26:44More than 2,000 people are still inside the buildings.
26:49It was impossible, I think, for anyone, engineer, tourist, New Yorker, to imagine that these towers could possibly come down.
26:59After standing strong for nearly two hours, what ultimately causes the towers to collapse?
27:09The Twin Towers' all-steel structure is exceptionally strong, but its innovative design hides potential weaknesses.
27:17At the base of each tower, the steel columns are four inches thick, to bear the weight of the building
27:23above.
27:26But near the top, these columns are only a quarter of an inch thick.
27:32To protect the steel from fire, engineers coat it in a fireproofing spray.
27:40But this protective layer is thin, and easily dislodged.
27:46Could this be the downfall of these iconic buildings?
27:55Structural engineer Adrian Brueger investigates if the fire could be the cause of the collapse.
28:03The ensuing fire raged inside the building.
28:06The impact caused massive swaths of windows to be torn out, and this extra damage created winds that were actually
28:13fanning the flames.
28:15Although the fireproof coating can withstand the flames, the impact of the aircraft leaves it compromised.
28:23The aircraft entered the building between 400 and 550 miles per hour, and the impact lasted only 0.7 seconds.
28:32And of course, during that impact, we actually had all of that fireproofing stripped.
28:37Adrian's team investigates how fire affects steel that has been stripped of its fireproofing.
28:44His team uses floor beams identical to those from the Twin Towers.
28:48They load them with 1.4 tons of weight, to simulate the weight of each floor of the tower.
28:55They prepare to heat up the steel beam.
28:58We will apply heat by simulating a fire.
29:01The temperature of the fire climbs to 1,800 degrees Fahrenheit, equivalent to that inside the towers.
29:09Now we're heating up the beam with flamethrowers, and you can see the temperature is going up on our infrared
29:14camera.
29:15We're reaching about 500 degrees Celsius, so that's 800 degrees Fahrenheit.
29:22At 800 degrees Fahrenheit, the trusses hold strong, but then they start to show signs of weakness.
29:29Now we see a cherry-red spot on the beam that's indicative of about a temperature of 750 Celsius, which
29:37is equivalent to 1,300 Fahrenheit.
29:39The beam is actually deflecting significantly.
29:42The steel beam gives way after just 11 minutes at 1,300 degrees.
29:52Adrian's investigation shows that without fireproofing, the floor beams cannot withstand the high heat and lose their strength.
30:02The extreme heat weakens the steel floor plates, causing them to sag, pulling the walls in on each other.
30:12The heat also weakens the steel bolts that hold the floors in place, causing each floor to collapse down onto
30:20the one below, a process known as pancaking.
30:25This catastrophe goes down in history as America's darkest day.
30:31The wreckage burns for 100 days.
30:40Rescue operations salvage the structural remains of the Twin Towers.
30:45Senior vice president of the 9-11 Memorial Museum, Anthony Gardner, leads the mission to preserve the Twin Towers structures.
30:551.8 million tons of debris fill ground zero in the aftermath of the attacks.
31:02Essentially, it was a seven-story smoldering pile of material that stretched up from street level and then also down
31:10seven stories to the bedrock level of the site where I'm standing today.
31:15The scale of destruction is overwhelming.
31:20But despite the force, some structures survive the collapse.
31:25These elements are conserved on the 9-11 Memorial site to this day.
31:31A display of respect and a place to remember those that lost their lives.
31:37One of the columns of the Twin Towers was found intact in its original position.
31:43So this is the last column that stood despite the destruction.
31:48Rescue recovery workers adorned it with all these inscriptions and tributes to their fallen colleagues.
31:58As the cleanup operation evolves, the Twin Towers deep foundations come into view.
32:04Buried by rubble, can the damaged bathtub wall continue to hold back the Hudson River?
32:10What will it take to keep this area of Lower Manhattan safe?
32:25September 11th, 2001.
32:29After months of wreckage cleanup, just a handful of structures from the towers remain.
32:36A reminder of the fight the skyscrapers put up against the attacks.
32:41Can any part of these buildings possibly stay standing to this day?
32:47Above ground, very little survives.
32:51A single staircase.
32:53A steel column from the core of the South Tower.
32:58And steel tridents that once formed the lower facade of the towers.
33:03But below ground, something far bigger survives.
33:08The vast concrete bathtub, built to protect the site's foundations.
33:14But deep cracks span the wall, and many anchoring bolts begin to leak groundwater.
33:20Without repair, the wall could fail, jeopardizing nearby buildings.
33:25What will it take to keep this area of Lower Manhattan safe?
33:31Head of Conservation, Lisa Conte, leads the preservation mission at the 9-11 Memorial.
33:38Her four-person team works all year round to keep this historic structure standing.
33:44Here we are, looking at a portion of the original bathtub.
33:49Just beyond it is the Hudson River.
33:53Following the events of 9-11, the damage to the bathtub wall threatens New York City with a potential catastrophe.
34:03If the bathtub wall fails, the area will rapidly flood with groundwater and water from the nearby Hudson River.
34:13This water would then inundate the subway tunnel that runs through the bathtub.
34:20Threatening widespread flooding of the underground rail network.
34:25With the bathtub breached, the surrounding ground would weaken, risking subsidence or even collapse of surrounding buildings.
34:36To detect any weak spots in the wall, Lisa's team turns to cutting-edge technology.
34:44Every year, scanning specialist Scott Lee laser scans the entire wall to millimeter accuracy.
34:53His LIDAR scanning works like echolocation, bouncing beams off the surface and recording the distance with incredible accuracy.
35:02By doing the LIDAR or laser scanning, we create a very accurate millimetric model of the entire slurry wall.
35:10The team also takes super high-resolution photos of the wall to record the surface detail.
35:19A platform raises the team into the air to capture the wall from every angle.
35:25Every detail matters.
35:27Scott then combines the LIDAR scan and the thousands of photos into a super high-resolution 3D model of the
35:35wall.
35:36We project those really high-resolution photos onto that solid surface model to create a very detailed photo-realistic model.
35:45We can see every nook and cranny. Often we can see details you can't see from the human eye.
35:52Armed with this 3D model, Lisa's team can home in on the weak parts of the wall that need attention.
35:59The brown that you see here is corrosion staining related to the interaction of the water and the steel elements.
36:07Also present, but less easy to see, are salts.
36:10And those are the areas of white here and coming down the face here.
36:20The 3D scan pinpoints areas where new water starts to seep through.
36:26It's really important that we're working to preserve the wall because everything needs to be cared for in order to
36:33preserve its integrity over time.
36:35The wall shows great strength and resilience.
36:39With the help of Lisa's team, it will remain standing for many more decades to keep the surrounding city safe.
36:47This colossal underground structure is a reminder of the innovative engineering that made these radical skyscrapers a reality.
36:57How have the Twin Towers innovations, and their tragic end, changed the design of skyscrapers forever?
37:15The Twin Towers. Revolutionary skyscrapers that have changed the world forever.
37:22What can we learn from the towers and the events that lead to their collapse?
37:28The impact of two hijacked airplanes creates massive gashes in the towers, and an unprecedented fire that causes them to
37:36collapse.
37:392,753 people die.
37:43But the buildings put up a remarkable fight in the face of the attack.
37:47On the north face of the north tower, two-thirds of the columns were taken out.
37:52But yet, the towers stood for more than an hour, which allowed occupants who were below the floors of impact
37:59to escape.
38:02The death toll was horrific, but had they not been as well-built or engineered in the wonderful way that
38:09they were,
38:10they probably would have collapsed much earlier, and the death toll would have been exponentially higher.
38:16The Twin Towers are revolutionary structures, revered by engineers all over the world.
38:22They are the first to break through the ceiling of skyscraper height,
38:26passing this legacy of innovative engineering onto their successors.
38:31The skyscraper has always been an engineering marvel,
38:35but it was the Twin Towers that really set the precedent with their revolutionary technology,
38:41with all the different extraordinary inventions that had to be created to even make it happen,
38:47that set the precedent for the design and engineering of the even taller buildings around the world today.
38:57Two of the most recognizable skyscrapers of all time.
39:01Two of the first buildings to use a bathtub design.
39:05They paved the way for skyscrapers on reclaimed land all over the world.
39:09The hollow tube design is now adopted by thousands of buildings worldwide,
39:15with subway-inspired express elevators paving the way for people to travel through cities in the sky.
39:22And their lightweight, flexible structure triggers a whole new field of research into skyscraper wind dynamics,
39:30setting the stage for cutting-edge research for years to come.
39:3513 years after the 9-11 tragedy, a new building stands at the World Trade Center site.
39:42The new One World Trade Center.
39:47Rising 1772 feet tall, it is New York's tallest skyscraper.
39:52It is designed with features to remember the people and the original Twin Towers,
39:56with protection as a top priority.
39:59Like World Trade Center one and the other ones here,
40:03are designed with every safety feature that the engineers could come up with.
40:09Since 9-11, more public buildings like the World Trade Center and other office buildings have adopted bomb blast criteria.
40:25Today, at ground level, massive voids show the footprints of the towers.
40:35So this is the North Tower Memorial Pool.
40:37And the memorial pools mark the location of where the towers stood at the street level of the site.
40:43And here, etched in the bronze parapets, are the names of the nearly 3,000 people, including my brother, who
40:52perished on 9-11.
40:57The Twin Towers and those who lost their lives in 9-11 are never to be forgotten.
41:10The Twin Towers rose on New York's skyline as a powerful message of American innovation.
41:17Breaking boundaries and reaching higher than anyone had ever gone before.
41:22Their fall was the darkest day in America's history.
41:25But their legacy lives on through other skyscrapers.
41:29A legacy never to be forgotten.
41:55The Twin Towers and the people that they died,
41:57and that was just a great effort to keep the money loose at home.
41:58The Twin Towers were also the most famous as well.
42:04The Twin Towers were the most famous for their lives and the most famous for their lives.
42:04Well, here's the only thing with the world and the most famous,
42:05The Twin Towers and the World Trade Center can be found in L&M.
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