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Fatal Engineering
Fatal Engineering (2025) S01E02
Fatal Engineering (2025) Season 1 Episode 2
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00:00Every day, around the world, millions of motorists take to the roads and bridges.
00:07These marvels of design have pushed the boundaries of physics and engineering since their creation.
00:14We're engineers. We design things to survive. We don't design things to fail.
00:20But when things go wrong, their collapses are spectacular and unforgiving.
00:26In 2018, 43 people lost their lives when torrential rain caused the Mirandi Bridge in Genoa, Italy, to collapse.
00:36Their journey to the peace and tranquility of the Italian coast will become a nightmare.
00:44In 1940, in the state of Washington, the Tacoma Narrows Bridge, just months after its inauguration,
00:51broke apart due to an oscillating motion never before observed on a bridge.
00:57You don't have to be an expert to see that it's only going to end in one way.
01:03In 2007, a single structural element of the I-35W Highway Bridge in Minneapolis failed,
01:10causing the entire structure to collapse, killing 13 people.
01:15It's just like a house of cards. You pull one card out, the whole thing falls down.
01:20What lessons did we learn from these catastrophic tragedies?
01:24How did the engineering community and the institutions responsible for maintaining these impressive structures respond?
01:31And what has been done to ensure that bridges designed today avoid fatal engineering?
01:37When it was inaugurated in Genoa, Italy in 1967, the Polchevera Viaduct, more commonly known as the Mirandi Bridge,
02:04was widely celebrated as a masterpiece of engineering.
02:10To the north, it is supported by a series of fairly standard piers.
02:15In the south, the pillars are very different.
02:18It has these three pylon structures which are very, very visually dominant in the city.
02:27Each tower has four connection points with two stays stretching in either direction of each pylon.
02:33This unique triangular design is the trademark of its designer, the Italian engineer, Ricardo Morandi.
02:42He had already experimented with it on the Maracaibo Bridge in Venezuela a few years earlier.
02:48A bridge eight times larger than the one in Genoa, which has already proved its worth.
02:53He's a brilliant man. He's already well known for doing, you know, pretty unusual designs.
03:00It's not just an ordinary bridge.
03:02But on the 14th of August 2018, it's one of these, the vital component to the collapse of the bridge.
03:15It's 1135 AM. In the middle of August, many motorists traverse the Mirandi Bridge, part of one of the busiest routes in Italy.
03:29Whether on holiday or simply going about their daily lives, disaster is imminent.
03:34The Mirandi Bridge is destined to fall.
03:43No one, not even the engineers and inspectors responsible for ensuring the safety of the bridge, foresaw the tragedy that was about to unfold.
03:51People inside their cars, as they're driving along, minding their own business, suddenly, seemingly out of nowhere, dropping 40 odd meters into the valley.
04:071136 AM. A stay cable on Pylon 9 breaks, causing 250 meters of deck to collapse, just under a quarter of the total length of the bridge.
04:23Tons of concrete and the vehicles on the bridge plunge into the Pochavera River, also falling into the railway tracks and buildings below.
04:32The collapse killed 43 people and injured many others.
04:51For the people of Genoa and those close to them, it has become clear that this visionary bridge, emblematic of the city, has become a symbol of a fatal engineering flaw.
05:00This is another one of those cases where the engineering community around the world looks in horror, and in this case, sadly, many people died, and asks themselves, why? Why has it happened?
05:28Why does it happen?
05:33To understand why the Mirandi Bridge collapsed, it is essential to consider the materials used in its construction.
05:40After the Second World War, with the production and availability of steel considerably reduced, architects and engineers turned to other materials for their projects.
05:55For Ricardo Mirandi, it was concrete.
05:59Ricardo Mirandi was fascinated by something called pre-stressed concrete.
06:04Pre-stressed concrete is subjected to compressive forces by means of cables tensioned before any loads are applied.
06:12This unique construction technique invented in the 1920s by French engineer Eugene Fresenet requires real craftsmanship to work perfectly.
06:25But the thing that is particularly important in the story we are going to be talking about is the cable stays.
06:38With a length of 86 meters between the top of each pylon and the deck, these pre-stressed concrete stays, hailed for their elegance and unfailing strength, are responsible for keeping the deck in the air.
06:51However, it does not have the same properties as steel.
07:04Concrete as a material is very good under compression.
07:06It can resist being squished very, very well.
07:09But if you put tension in concrete, it's going to crack, and that's no good.
07:16To prevent the concrete from cracking, smaller cables are placed in a sheath around the other cables inside the concrete.
07:25But as motorists drive over the Mirandi Bridge, they have no idea that one of these stays is severely compromised.
07:32The pre-stressed concrete would be tested beyond its limits.
07:39In its first year of existence, the Mirandi Bridge was used by around 5 million vehicles.
07:55Fifty years later, a year before its dramatic collapse, traffic had increased five-fold to 25 million vehicles a year.
08:05Not only the volume, the weight of traffic on the bridge has also increased dramatically.
08:12Cars are bigger, more people are moving, but also the increase in the amount of freight from the nearby port.
08:17The Port of Genoa is the largest port in Italy, and one of the most important in continental Europe.
08:25Heavy goods vehicles have to cross the Mirandi Bridge in order to leave the port.
08:32Every time a vehicle crosses the Mirandi Bridge, the structure and its concrete stays are subjected to varying loads.
08:40These changes in weight have the effect of deforming the structure.
08:43As the Mirandi Bridge continued to strain against the ever-increasing demand of traffic, cracks began to form.
08:53The cracks allow a silent and relentless threat capable of destroying even the strongest structures made of concrete and steel.
09:03Genoa is a coastal city where salty sea air, precipitation and heat are all part of the climate.
09:20Any metal exposed to these conditions would rust, and if left untreated, could fail.
09:25The problem with these cables inside that concrete sheath, you can't inspect them apart from just the outside surface, and you just don't know what's going on inside.
09:37The first to express concern was Ricardo Mirandi himself.
09:42Twelve years after the inauguration of the bridge, he is publicly declaring that he is concerned about the level of deterioration he is observing on his pre-stressed concrete stays.
09:53The Italian engineer proposed solutions to prevent any further corrosion on or inside the structure.
10:02The problem is nobody did anything.
10:05It was not until the 1990s that repairs were carried out on pylon number 11.
10:11Aware of the deterioration of the materials over time, authorities commissioned a detailed report and analysis of the condition of the stays.
10:19It was discovered that 30% of the cables inside the concrete casing on pylon 11 had corroded, resulting in a 50% loss of load capacity.
10:36But it's not the only one.
10:39Pylons 9 and 10 are in an equally damaged and worrying condition.
10:43Restoration work is being carried out to reinforce the pylons.
10:48Steel reinforcements were added to the cables on pylon 11 to bring back that load capacity.
10:56But in doing so, it ruins the clean aesthetic that Mirandi was after.
11:01A few years later, more modifications were made to the cable stays on pylon 10, this time steel cradles installed at the top of those cables.
11:13But nothing was ever done to pylon 9.
11:16The plan is we're going to do those in a few years' time.
11:23Unfortunately, we never got there.
11:26Pylon 9 was left as originally designed.
11:30This was a fatal error.
11:33A fatal error.
11:35The gravity of which the whole world will realize on August 14, 2018.
11:41The time is 11.35 a.m.
11:43It's lashing with rain and there's water on the roads, adding even more weight.
11:56The torrential rain on the Mirandi Bridge pummels the pre-stressed concrete stays.
12:02The water is spreading inside, relentlessly damaging the steel cables, which have no waterproof protection.
12:08The cables have corroded to such an extent, they just could not cope any longer.
12:16This is when the next event comes into play.
12:21One vehicle is on the approach to pylon 9, a 44-ton truck.
12:2611.36 a.m., the heavy truck arrives at the pylon and triggers the start of a chain reaction that will be fatal for the Mirandi Bridge.
12:39When the wheels of such a truck hit a step, it creates a big impact.
12:50So that truck is most likely to be the cause of what started the collapse.
12:57But ultimately, it didn't need very much.
12:59At that moment, one of the cables snaps.
13:02They break one after the other, leading to the total failure of the shroud.
13:08But it's creating a new phenomenon.
13:10If one of the cables snaps on one side of the tower, the tower itself becomes unstable and imbalanced.
13:16It's pulled the other way.
13:18So almost immediately after the snapping of the cable, the tower collapses and the bridge decks breaks.
13:24A section just under 250 meters in length, centered on pylon 9, gives out.
13:34Any car on that section at the time was going down with it.
13:46The vehicles plummeted 45 meters, the height of a 13-story building.
13:54Some died from the impact of the collapse.
14:00Others survived the drop, but were then crushed by falling pieces of concrete from the tower.
14:07A total of 43 people were killed when the Mirandi Bridge collapsed.
14:13This led to a one-year state of emergency in the Genoa region.
14:19It highlighted the catastrophic state of Italy's infrastructure.
14:22The company in charge of the bridge, Autostrade, underwent a partial nationalization, offering to settle the legal case against the company for 1 million euros in damages.
14:35And a further 26 million euros in compensation.
14:38Although pylons 10 and 11 remained intact after the collapse of the Mirandi Bridge, they were demolished on June 28, 2019.
14:53A replacement bridge, the Genoa St. George Bridge, was inaugurated 13 months later on August 4, 2020.
15:02A single moment brought about by years of negligence and a refusal to fully address evident issues can be catastrophically fatal.
15:13Wednesday, August 1, 2007, in Minneapolis, Minnesota.
15:27It's 6 p.m.
15:29Rush hour on Interstate 35 West, commonly known as I-35W.
15:33As part of that road, there's a bridge.
15:42A very, very busy bridge, crossing the river, carrying thousands of vehicles every day.
15:49The I-35W highway bridge has three main spans, made of steel and concrete.
15:54And a truss is a bridge where the elements are usually diagonals, and all the pieces are working in tension or compression.
16:04And this truss carries all the load, enables the thing to support all the traffic.
16:10It's a very efficient kind of structure.
16:12But this bridge will become one of the most famous, or should I say infamous, in American history.
16:17It's 6 p.m.
16:24Rush hour on Interstate 35 West, commonly known as I-35W.
16:33What they don't know is that this routine drive will turn into a major catastrophe.
16:396.05 p.m., 40 years after its inauguration, the I-35W highway bridge collapses.
16:45Happens in the blink of an eye.
16:48The bridge gives way, throwing all vehicles, passengers, construction workers and equipment, down into the cold waters of the Mississippi River.
17:00The accident kills 13 people and injures 145.
17:06Though authorities initially investigated a terror attack, this was a disaster that had been 40 years in the making.
17:13At first glance, the I-35W highway bridge is a solid structure perfectly adapted to its environment.
17:23It's made up of interconnected steel beams, ensuring that loads are distributed evenly.
17:27But all of those members have to be joined together to create the overall truss.
17:32And usually, when we're doing this in a system where everything is bolted or riveted together, we use a thing called a gusset plate.
17:39The gusset plate, this small steel sheet, will be one of the key elements that will make the Minneapolis Bridge one of the most tragic bridge collapses.
17:48More than 200 of these are riveted on both sides of the lattice junction points.
17:54Although small, they create essential links in the bridge.
17:58Each one must transfer the weight from one element to another.
18:02The load transfers out of the member, into the rivet, from the rivet, into the gusset plate, from the gusset plate into the rivet of the other member, and then from the rivet of the other member into the other member.
18:14The gusset plate that will take the stress of all of those forces together.
18:20So getting the right thickness of gusset plate is absolutely crucial to the strength of not only that joint, but to the strength of the structure as a whole.
18:29Investigators discovered that the builders of the bridge neglected the thickness of these plates.
18:35At just 13mm thick, the gusset plates on the I-35W bridge were not strong enough to support the bridge's weight and that of the traffic.
18:46It was designed to cope with the heavy traffic of a region like Minneapolis with a population of 3 million.
18:54But in 2007, 100,000 vehicles cross it every day.
18:58As commuters head home on August 1st, the safety of the I-35W does not enter their mind.
19:08Having been a key route through the heart of Minneapolis since it was reopened in 1967, there's no reason for fear.
19:16But some warning signs were already apparent, because on that day, traffic on the bridge is disrupted.
19:22On the day in question, there are new modifications being made to the bridge.
19:29Heavy machinery and building materials ready to resurface the bridge's deck.
19:34Four of the eight lanes of the 35m wide deck were once again closed for renovations.
19:41Renovations that don't worry drivers.
19:44They've seen it for months though, it's nothing out of the ordinary.
19:47What is out of the ordinary is what's happening beneath the bridge deck.
19:50The renovations are underway to combat an almost hidden threat.
19:56A dangerous process that is slowly but surely eating away at the structure.
20:01Another bridge falling victim to corrosion.
20:07This relentless enemy is fed not only by the river that the bridge crosses,
20:12but also by the only natural waterfall on the Mississippi, 875 meters downstream.
20:17St. Anthony Falls.
20:21The spray from the falls directly attacks the concrete slab on which the cars drive.
20:27But it's not the only factor damaging the bridge.
20:30If you've ever spent a winter in Minnesota, you know that the winters get brutal, the temperatures plummet, there's ice, there's lots of rain.
20:42And so the authorities, as we do all over the world, use salt on the roads to melt the ice and make it safer for drivers.
20:49While cold and water are real threats, salt adds its corrosive power to the concrete slab and, more importantly, to the steel bars that reinforce it.
21:01To avoid this, they add concrete to the deck. Several tons will be added to the initial weight.
21:07The bridge is designed to support this increased weight. But this isn't the first time weight has been added.
21:14Because when it started life, it was only six and a half inches thick, 165 millimeters, and that's super thick.
21:21Moisture and salt used during the winter months to de-ice the road would slowly seep into the concrete and corrode the metal within.
21:27In the years following its inauguration, the concrete deck began to crack. To address this issue, engineers took a major decision in 1977, only 10 years after the bridge's construction.
21:41They poured more concrete on top to protect and renew that surface so that the bridge deck was structurally sound, despite all the environmental factors that were trying to constantly attack it.
21:52However, this added 512 tons to the bridge's initial weight. They don't know it yet, but this is the beginning of a nightmare scenario that will slowly but surely doom the I-35W highway bridge.
22:09Drivers crossing the bridge couldn't know that the structure was now much heavier than when it was built.
22:15But the experts should have realized this during a previous inspection.
22:19It's really important that engineers are sent out to do detailed inspections and analyses of structures like the I-35 West Bridge.
22:29And that happened, in fact, it happened four years prior to this disaster.
22:34And there is evidence from those engineers' reports of the smoking gun in this disaster.
22:41This is the photograph that showed signs of the already impending catastrophe.
22:46This image shows gusset plates and shows signs of warping.
22:50As an engineer, when you see a component that started to bow, like this gusset plate, that tells you that the forces within your structure are doing something different to how they were originally designed.
23:03The consequences would be dramatic because, although very solid, the I-35W highway bridge has an Achilles heel. This is known as a non-redundant bridge.
23:12It means that there are no redundancies built within the structure.
23:18If one member in the truss fails, then the whole bridge can fail.
23:23To avoid that kind of disaster scenario, all parts of a bridge like this must be perfectly calibrated and inspected.
23:30However, the bending of this gusset plate was not considered a danger by inspectors.
23:34The rivets and the buckling plate experience fluctuating stresses all the time.
23:40And that causes something called fatigue, which eventually creates a crack.
23:44And if the loading continues like that, that crack will grow.
23:48And as the crack grows, eventually there will be a point when bang.
23:51During the fateful day of August 1st, 2007, construction workers prepared the materials needed to add the additional layer that evening.
24:05571 tons of sand and gravel ready for concreting and a 36-ton tanker are now on the bridge.
24:15In total, more than 600 tons of material were added to the structure above the bent gusset plate.
24:21After years of development, which increased the dead load of the I-35W, this added weight is placed directly above a weakened pressure point.
24:32At 6pm, everything was in place for the bridge workers to continue with the deck renovation work.
24:41They have to start their shift at 7pm.
24:44What they don't know is that by 7pm, there'll be no bridge left to resurface.
24:496.03pm.
24:54It's rush hour.
24:55People are making their way home from work.
24:57They're crossing the I-35 bridge in huge numbers.
25:02But that gusset plate has now got to the point where it can't take it any longer.
25:12The micro-cracks within that gusset plate have grown.
25:15The gusset plate fails and it fails catastrophically.
25:216.05pm. The beginning of the end for the I-35W highway bridge and tragedy for those on it.
25:28The members that are connected to it, detached, they're no longer attached to each other.
25:32The force has got nowhere to go.
25:35So, it just collapses.
25:37And it collapses instantly.
25:38The central span collapses, sending vehicles, people and building materials plummeting 35 metres.
25:5317 vehicles fall into the Mississippi River.
25:55Some are crushed by the deck's pieces falling on them.
25:59But there's more to come.
26:03After the central span collapses, the two adjoining spans follow.
26:08Everything falls into a railway yard and the Mississippi River.
26:13111 vehicles, including 25 construction machines and a school bus loaded with 63 students, were on the bridge when it fell.
26:24While many miraculously come out of the disaster alive, for others, a new nightmare begins.
26:31Emergency services take six minutes to reach the accident scene.
26:38In the meantime, solidarity is building among the survivors who are trying to rescue people trapped in their cars, which are filling up with water.
26:47The collapse of the I-35W highway bridge injures 145 and leaves 13 people dead.
26:54The I-35W bridge became fatal for three main reasons.
26:57Firstly, the design of the gusset plate has to be up there as being a major problem.
27:04Number two is the question of maintenance.
27:06Maintenance is vital.
27:09And number three, actually, is that the bridge is carrying more load than it was designed for.
27:14As it turns out, that problem was not located purely within Minnesota.
27:20In 2007, the Federal Highway Administration counted 19,273 non-redundant bridges in the United States alone,
27:31465 of which had a steel truss similar to that of the I-35W.
27:36Of those, 31%, or 145 bridges, were classified as structurally deficient.
27:42On all these bridges, if a key structural element were to fail, the bridge would present a high risk of partial or total collapse.
27:51In memory of this tragic accident, the defective gusset plate is now exhibited in a museum in Minneapolis.
27:59It's a haunting reminder that failures like this could become all too common.
28:03What is unique is what happened on the other side of the country, in Washington State, when, in 1940, a bridge ripped itself apart.
28:17The famous, iconic Tacoma Narrows Bridge.
28:33Tacoma Narrows Bridge represents one of those moments in history where everything changed.
28:40There's a before Tacoma, and there's an after Tacoma.
28:4511.09am, on November 4th, 1940.
28:49A little more than four months after its inauguration, the cameras of the reporters on site capture these exceptional and unprecedented images that leave people speechless.
28:59It's overcast, and the wind is already causing the bridge to showcase its wave-like motion.
29:0611.10am, the brand new Tacoma Narrows Bridge collapses spectacularly in this inlet of the Pacific Ocean.
29:15The deck, now twisting in unpredictable motions, continue to shed concrete and steel, with successive portions of the bridge falling into the water of the narrows below.
29:25Hailed as a triumph of human perseverance when it was inaugurated, the Tacoma Narrows Bridge would become a symbol of contemporary engineering failure.
29:41Why was such an important bridge destined for disaster?
29:44In 1937, the local authorities decided to build a bridge to link the cities of Tacoma and Gig Harbor.
29:57It's intended to help commuters traverse the cold waters of the Puget Sound, providing a vital economic and military route.
30:04The way to get from one side to the other was a very long way around, so this is a really good example of where a bridge is going to make a massive difference to people's lives.
30:15An initial design was proposed by a local engineer, Clark Eldridge.
30:19Clark Eldridge is proposing a steel truss bridge, of which there are thousands at this time in the United States.
30:29Its design incorporates 7.6 meter deep trusses to reinforce the rigidity of the bridge.
30:34But at a cost of US$11 million, or at today's value $250 million, the Eldridge project is considered too expensive.
30:43On the other hand, you have Leon Moisef, who is a steel rope expert, and he is proposing a sleek suspension bridge.
31:03But crucially, the suspension bridge will do that job cheaper.
31:07At US$8 million, $3 million less than the Eldridge project, the proposal by this experienced engineer, who has worked on the Manhattan Bridge in New York,
31:19and the Golden Gate Bridge in San Francisco, also has other advantages.
31:24It was using less material. It was going to be easier to build, and so that design was selected.
31:31The Leon Moisef Bridge is supported by two 130 meter high towers, linked by cables that suspend the deck 60 meters above the waters of the Tacoma Narrows.
31:45The longest span is 853 meters, making it the longest suspension bridge in the United States, and the third longest in the world.
31:54The engineer did away with the usual steel trusses, and replaced them with two 2.4 meter high girders along the deck.
32:04It's very, very thin, like a ribbon, but also because it's very narrow.
32:09With a deck only 12 meters wide, the length of three cars, the bridge's visionary elegance was to become the weak point of this revolutionary work of art.
32:26July 1st, 1940. The cameras and the public flocked to the inauguration of what is regarded as a masterpiece of engineering.
32:33What locals don't know is that the bridge is already doomed. In his efforts to create a visually pleasing cheaper bridge, Moisef made fatal miscalculations.
32:44Very early on, the workers are regularly subjected to high winds that are funneled down this valley.
32:52The wind. One of the main enemies of a bridge that absolutely must be taken into account when designing it.
33:03If it was a truss bridge, then obviously the wind would pass through it to an extent.
33:09Unlike trusses, these girders don't allow wind to pass through.
33:13Although visionary, Leon Moisef's proposal to replace the trusses with seal girders on the sides of the deck
33:19would be the first mistake that would prove fatal to the Tacoma Narrows Bridge.
33:28The wind hits the side of the bridge and it's presented with an eight foot deep girder, which is a solid object.
33:37And the wind, of course, has to go somewhere.
33:40As the wind hits the truss, it creates small vortices.
33:44These vortices move along the truss and find a completely new direction when they reach the edges.
33:51Where it goes over the top and underneath, the wind separates from the structure.
33:58And that creates a little vacuum, a little area of low pressure every time one of those vortices forms.
34:04That pressure creates a sort of suction which draws the bridge down.
34:10If it's on the bottom and if it's on the top, it'll draw the bridge up.
34:20You may think of bridges as rigid, stationary structures.
34:24In fact, all bridges are designed to have some movement, some flex in them.
34:30Despite these worrying signs, the bridge was open to traffic.
34:37The bridge's strange behavior earned it the nickname Galloping Gertie.
34:43From day one, the bridge deck sways, it moves, it dances in the wind.
34:50Some people enjoy the process of driving across it, almost like being on a roller coaster.
34:56Engineers are concerned because this amount of movement was not part of the design process.
35:06They are attempting to stabilize the deck by securing it at both ends using cables connected to 50-ton concrete blocks on the bank.
35:17But they break.
35:24Two months after the inauguration of the bridge, hopes were pinned on a team of engineers and scientists led by Professor Farquhar-Harson from the University of Washington.
35:35They are building this miniature replica of the bridge and carrying out tests to reproduce its exact behavior when the wind blows.
35:46They have come up with two proposals to stop its undulations.
35:50The first proposal is to drill holes in the steel girders along the side of the bridge so that the wind has a passage through and doesn't have to go up and under the bridge.
36:08The second is to add deflector vanes and fairings along the deck to increase the bridge's aerodynamic profile.
36:17The first option is rejected because of the irreversible nature of the solution, but the second must be implemented.
36:24Unfortunately, the Tacoma Narrows Bridge would not hold out long enough for work to begin.
36:29November 7th, 1940. The time is 10.30 a.m.
36:38Winds of 68 kilometers per hour blow across the Tacoma Narrows Bridge.
36:43It's waving up and down, oscillating up and down and side to side, as it has done for months before.
36:50But the bridge starts to move in a slightly different way.
36:52A phenomenon never before seen on a superstructure.
37:00The very thin deck, praised for its elegant design, is proving to be the second weak link of the Tacoma Narrows Bridge.
37:09This bridge, which had longitudinal girders down each side, had very little transverse stiffness.
37:18The bridge deck has started to twist, and that twisting movement was the fatal difference on this day.
37:28Faced with the terrifying sight of this deck twisting along its entire length, the engineers are increasingly worried.
37:35The bridge is immediately closed to traffic.
37:41The time is 10.31 a.m.
37:44Unfortunately, one driver has already been allowed past and is on a deadly route towards the bridge.
37:52Leonard Coatsworth, it's just him and one passenger in his car, his dog named Tubby.
38:01Tubby and his master find themselves stranded in the middle of the raging bridge.
38:05As the bridge sways almost uncontrollably, a major failure goes unnoticed.
38:14A cable band at the bridge's mid-span snaps.
38:20Leonard can't go any further, and he can't go back.
38:23He decides to abandon his car.
38:25He climbs out, desperate to make it to safety with Tubby, but is knocked from his feet by the powerful twisting motion of the concrete, throwing him first to one side of the bridge and then back to the other.
38:39Tubby, still sitting in Leonard's car and terrified by the movement, refuses to leave the vehicle.
38:45Leonard has to accept the inevitable if he wants to save himself.
38:4811 a.m. Professor Farquharson came to observe the new torsion behavior of the bridge.
38:57He's about to witness something completely new and even more dangerous.
39:02As the bridge continues to sway, it draws its power from the winds that continue to batter the deck.
39:10Not only is Galloping Gertie twisting out of control, it's also building energy.
39:19There is a critical point where the speed of the eddies being shed coincides with the natural frequency of the bridge.
39:28It is no longer just the wind that creates this twisting movement along the bridge, but the transfer of energy within the structure itself, known as resonance.
39:39It's a runaway train bound to the end of the track.
39:46To make matters worse, the two ends of the bridge are twisting in opposite directions.
39:52The bridge twists violently up to 45 degrees in one direction and then the other.
39:58This creates a propeller effect with only the middle of the bridge locked in place while the two sides twist violently.
40:0511.02 a.m.
40:10A girder bulges out on the Gig Harbor side of the bridge, unable to endure this violent twisting motion.
40:16The weight that is now put on the hangers next to it immediately is too much. They fail and the whole thing falls like a domino rally.
40:24Professor Farquharson and Leonard can do nothing more than watch the car and poor Tubby fall with the rest of the bridge's main span deck.
40:34Takoma is one of those moments in history when everybody just stops and says, wow, now we've learnt something we didn't know.
40:47It was a phenomenon that had just not been seen before.
41:04Following the collapse of the Takoma Narrows Bridge, many other suspension bridges have been built and have become benchmarks of structural engineering.
41:10Bridges are icons within our cities and cityscapes around the world.
41:23But when things go wrong, their collapses are spectacular and unforgiving.
41:29We're engineers. We don't design things to fail. We design things to survive. Sometimes mistakes happen. Sometimes humanity has made the same mistake twice.
41:41Whether it's a design error, an unknown phenomenon or negligence, when a bridge collapses, the whole world is in shock.
41:48When these errors are not caught, then a collapse can become a tragic case of fatal engineering.
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