Original subtitles

[narrator] Every day, around the world, millions of motorists take

to the roads and bridges.

These marvels of design have pushed the boundaries

of physics and engineering since their creation.

[Ian] We're engineers.

We design things to survive. We don't design things to fail.

[Rob] But when things go wrong, their collapses are

spectacular and unforgiving.

In 2018, 43 people lost their lives when torrential

rain caused the Morandi Bridge in Genoa, Italy, to collapse.

Their journey to the peace and tranquility

of the Italian coast will become a nightmare.

In 1940, in the state of Washington,

in the USA, the Tacoma Narrows Bridge, just months after

its inauguration, broke apart due to an oscillating motion

never before observed on a bridge.

You don't have to be an expert to see that it's only going

to end in one way.

In 2007, a single structural element of the I-35W highway

bridge in Minneapolis failed, causing the entire structure

to collapse, killing 13 people.

[Ian] It's just like a house of cards.

You pull one card out, the whole thing falls down.

What lessons did we learn from these catastrophic tragedies?

How did the engineering community and the institutions

responsible for maintaining these impressive

structures respond?

And what has been done to ensure that bridges designed

today avoid fatal engineering?

[electronic theme music playing]

[suspenseful music playing]

When it was inaugurated in Genoa, Italy, in 1967,

the Polcevera Viaduct, more commonly known as

the Morandi Bridge was widely celebrated as a masterpiece

of engineering.

To the north, it is supported by a series of fairly

standard piers.

In the south, the pillars are very different.

It has these three Pylon structures, which are very,

very visually dominant in the city.

Each tower has four connection points, with two stays

stretching in either direction of each Pylon.

This unique triangular design is the trademark

of its designer, the Italian engineer

Riccardo Morandi.

He had already experimented with it on the Maracaibo Bridge

in Venezuela a few years earlier, a bridge eight times

larger than the one in Genoa, which has already

proved its worth.

He's a brilliant man.

He's already well known for doing, you know,

pretty unusual designs.

It's not just an ordinary bridge.

But on the 14th of August, 2018, it's one of these,

the vital component to the collapse of the bridge.

[cracking, rumbling]

[suspenseful music playing]

It's 11:35 a.m.

In the middle of August, many motorists traverse

the Morandi Bridge, part of one of the busiest routes in Italy.

[Shini] Whether on holiday or simply going about their daily lives,

disaster is imminent.

The Morandi Bridge is destined to fall.

-[thunder rumbling] -[suspenseful music playing]

No one, not even the engineers and inspectors responsible

for ensuring the safety of the bridge, foresaw the tragedy

that was about to unfold.

People inside their cars, as they're driving along,

minding their own business,

suddenly, seemingly out of nowhere, dropping 40-odd meters

into the valley.

11:36 a.m.

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.

[cracking, rumbling]

Tons of concrete, and the vehicles on the bridge

plunge into the Polcevera River, also falling

into the railway tracks and buildings below.

[shouting in Italian]

[sirens wailing]

[helicopter whirring]

The collapse killed 43 people and injured many others.

For 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.

[speaking Italian]

[in Italian]

[Ian] This 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?"

To understand why the Morandi Bridge collapsed,

it is essential to consider the materials used

in its construction.

[airplane humming]

[explosions blasting]

After the second world war, with the production

and availability of steel considerably reduced,

architects and engineers turned to other materials

for their projects.

For Riccardo Morandi, it was concrete.

[Rob] Riccardo Morandi was fascinated by the material properties

of concrete.

Prestressed concrete is subjected to compressive forces

by means of cables tensioned before any loads are applied.

This unique construction technique,

invented in the 1920s by French

engineer, Eugène Freyssinet,

requires real craftsmanship to work perfectly.

[Ian] But the thing that is particularly important

in the story we're going to be talking about is

the cable stays.

With a length of 86 meters, between the top

of each Pylon and the deck, these prestressed concrete

stays, hailed for their elegance and unfailing

strength, are responsible for keeping the deck in the air.

However, it does not have the same properties as steel.

Concrete as a material is very good under compression.

It can resist being squished very, very well.

[Ian] But if you put tension in concrete, it's gonna crack,

and that's no good.

To prevent the concrete from cracking, smaller cables are

placed in a sheath around the other cables

inside the concrete.

But as motorists drive over the Morandi Bridge,

they have no idea that one of these stays is

severely compromised.

[Shini] The pre-stressed concrete would be tested beyond its limits.

-[cracking, rumbling] -[shouting in Italian]

In its first year of existence, the Morandi Bridge was used

by around 5 million vehicles.

50 years later, a year before its dramatic collapse,

traffic had increased five-fold to 25 million vehicles a year.

Not only the volume, the weight of traffic

on the bridge has also increased dramatically.

Cars are bigger, more people are moving,

but also the increase in the amount of freight

from the nearby port.

The port of Genoa is the largest port in Italy

and one of the most important in Continental Europe.

Heavy goods vehicles have to cross the Morandi Bridge

in order to leave the port.

Every time a vehicle crosses the Morandi Bridge,

the structure and its concrete stays

are subjected to varying loads.

These changes in weight have the effect

of deforming the structure.

As the Morandi bridge continued to strain

against the ever-increasing demand of traffic,

cracks began to form.

The cracks allow a silent and relentless threat

capable of destroying even the strongest structures made

of concrete and steel.

Genoa is a coastal city where salty sea air,

precipitation, and heat are all part of the climate.

Any metal exposed to these conditions would rust

and if left untreated, could fail.

The 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.

The first to express concern was Riccardo Morandi himself,

12 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 prestressed concrete stays.

The Italian engineer proposed solutions to prevent any

further corrosion on or inside the structure.

The problem is nobody did anything.

It was not until the 1990s that repairs were carried out

on Pylon number 11.

Aware of the deterioration of the materials over time,

authorities commissioned a detailed report and analysis

of the condition of the stays.

[Rob] It was discovered that 30% of the cables inside

the concrete casing on Pylon 11 had corroded,

resulting in a 50% loss of load capacity.

But it's not the only one.

Pylons 9 and 10 are in an equally damaged

and worrying condition.

Restoration work is being carried out

to reinforce the Pylons.

[Rob] Steel reinforcements were added to the cables on Pylon 11

to bring back that load capacity.

But in doing so, it ruins the clean aesthetic

that Morandi was after.

A 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.

But nothing was ever done to Pylon 9.

[Ian] The plan is...

we're gonna do those in a few years' time.

Unfortunately, we never got there.

[Shini] Pylon 9 was left as originally designed.

This was a fatal error.

A fatal error, the gravity of which the whole world

will realize on August 14th, 2018.

The time is 11:35 a.m.

It's lashing with rain, and there's water on the roads,

adding even more weight.

[suspenseful music playing]

The torrential rain on the Morandi Bridge pummels

the prestressed concrete stays.

The water is spreading inside, relentlessly damaging the steel

cables, which have no waterproof protection.

The cables have corroded to such an extent, they just could

not cope any longer.

This is when the next event comes into play.

One vehicle is on the approach to Pylon 9, a 44-ton truck.

11:36 a.m.

The heavy truck arrives at the Pylon and triggers

the start of a chain reaction that will be fatal

for the Morandi bridge.

When the wheels of such a truck hit a step,

it creates a big impact.

So that truck is most likely to be the cause of what

started the collapse.

But ultimately, it didn't need very much.

At that moment, one of the cables snaps.

They break one after the other,

leading to the total failure

of the shroud.

But it's creating a new phenomenon.

If one of the cables snaps on one side of the tower,

the tower itself becomes unstable and imbalanced.

It's pulled the other way.

So almost immediately after the snapping of the cable,

the tower collapses and the bridge deck breaks.

A section just under 250 meters in length, centered on Pylon 9,

gives out.

Any car on that section at that time was going down with it.

[sirens wailing]

The vehicles plummeted 45 meters, the height

of a 13-story building.

[somber music playing]

Some died from the impact of the collapse.

Others survived the drop, but were then crushed

by falling pieces of concrete from the tower.

A total of 43 people were killed

when the Morandi Bridge collapsed.

This led to a one-year state of emergency in the Genoa region.

It highlighted the catastrophic state

of Italy's infrastructure.

The 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

and a further 26 million euros in compensation.

[cracking, booming]

Although Pylons 10 and 11 remained intact

after the collapse of the Morandi Bridge,

they were demolished on June 28th, 2019.

A replacement bridge, the Genoa St. George Bridge,

was inaugurated 13 months later on August 4th, 2020.

A single moment brought about by years of negligence

and a refusal to fully address evident issues can be

catastrophically fatal.

Wednesday, August 1st, 2007, in Minneapolis, Minnesota.

It's 6:00 p.m.

rush hour on Interstate 35 West, commonly known as I-35W.

As part of that road, there's a bridge.

[suspenseful music playing]

A very, very busy bridge crossing the river,

carrying thousands of vehicles every day.

The I-35W Highway Bridge has three main spans

made of steel and concrete.

And a truss is a bridge where the elements are usually

diagonals, and all the pieces are working

in tension or compression.

And this truss carries all the load, enables the thing

to support all the traffic.

It's a very efficient kind of structure.

[Rob] But this bridge will become one of the most famous,

or should I say infamous, in American history.

It's 6:00 p.m. Rush hour on Interstate 35 West

commonly known as I-35W.

What they don't know is that this routine drive

will turn into a major catastrophe.

6:05 p.m., 40 years after its inauguration,

the I-35W highway bridge collapses.

Happens in the blink of an eye.

[Shini] The bridge gives way, throwing all

vehicles, passengers, construction workers,

and equipment down into the cold waters

of the Mississippi River.

The accident kills 13 people and injures 145.

Though authorities initially investigated a terror attack,

this was a disaster that had been 40 years in the making.

At first glance, the I-35W Highway Bridge is a solid

structure, perfectly adapted to its environment.

It's made up of interconnected steel beams, ensuring that

loads are distributed evenly.

But all of those members have to be joined together to create

the overall truss.

And usually, when we're doing this in a system where

everything is bolted or riveted together, we use a thing called

a gusset plate.

The 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.

More than 200 of these are riveted on both sides

of the lattice junction points.

Although small, they create essential links in the bridge.

Each one must transfer the weight

from one element to another.

[Ian] The 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.

It's the gusset plate that will take the stress of all of those

forces together.

So 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.

Investigators discover that the builders of the bridge

neglected the thickness of these plates.

At just 13 millimeters thick, the gusset plates on the I-35W

bridge were not strong enough to support the bridge's weight

and that of the traffic.

It was designed to cope with the heavy traffic

of a region like Minneapolis with a population

of 3 million, but in 2007,

100,000 vehicles cross it every day.

As commuters head home on August 1st, the safety

of the I-35W does not enter their mind.

Having been a key route through the heart of Minneapolis since

it was reopened in 1967, there's no reason for fear.

But some warning signs were already apparent,

because on that day, traffic on the bridge is disrupted.

On the day in question, there are new modifications

being made to the bridge.

[Shini] Heavy machinery and building materials

ready to resurface the bridge's deck.

Four of the eight lanes of the 35-meter-wide deck were

once again closed for renovations, renovations

that don't worry drivers.

[Rob] They've seen it for months, though.

It's nothing out of the ordinary.

What is out of the ordinary is what's happening beneath

the bridge deck.

The renovations are underway to combat an almost hidden threat,

a dangerous process that is slowly but surely eating away

at the structure, another bridge falling victim

to corrosion.

This relentless enemy is fed not only by the river

that the bridge crosses, but also by the only natural

waterfall on the Mississippi, 875 meters downstream.

Saint Anthony Falls.

The spray from the falls directly attacks the concrete

slab on which the cars drive.

But it's not the only factor damaging the bridge.

If 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.

And 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.

While 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.

To avoid this, they add concrete to the deck.

Several tons will be added to the initial weight.

The bridge is designed to support this increased weight,

but this isn't the first time weight has been added.

Because when it started life, it was only six and a half

inches thick, 165 millimeters, and that's super thin.

[Shini] Moisture and salt used during the winter months to de-ice

the road would slowly seep into the concrete and corrode

the metal within.

In 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.

They 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.

However, 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.

Drivers crossing the bridge couldn't know

that the structure was now much heavier than when it was built.

But the experts should have realized this during

a previous inspection.

It's really important that engineers are sent out to do

detailed inspections and analyses of structures like

the I-35 West Bridge. And that happened.

In fact, it happened four years prior to this disaster.

And there is evidence from those engineers' reports

of the smoking gun in this disaster.

This is the photograph that showed signs of the already

impending catastrophe.

This image shows gusset plates and shows signs of warping.

As an engineer, when you see a component that's 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.

The 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.

[Rob] It means that there are no redundancies built

within the structure.

If one member in the truss fails,

then the whole bridge can fail.

To avoid that kind of disaster scenario, all parts of a bridge

like this must be perfectly calibrated and inspected.

However, the bending of this gusset plate was not considered

a danger by inspectors.

The rivets and the buckling plate

experience fluctuating stresses all the time, and that causes

something called fatigue, which eventually creates

a crack, and then if the loading continues like that,

that crack will grow and as the crack grows

eventually there'll be a point when...bang.

During the fateful day of August 1st, 2007,

construction workers prepared the materials needed to add

the additional layer that evening.

571 tons of sand and gravel ready for concreting,

and a 36-ton tanker are now on the bridge.

In total, more than 600 tons of material were added

to the structure above the bent gusset plate.

After years of development, which increased

the dead load of the I-35W, this added weight is placed

directly above a weakened pressure point.

At 6:00 pm, everything was in place for the bridge workers

to continue with the deck renovation work.

They have to start their shift at 7:00 pm.

What they don't know is that by 7:00 pm,

there'll be no bridge left to resurface.

6:03 pm...

It's rush hour.

People are making their way home from work.

They're crossing the I-35 bridge in huge numbers.

But that gusset plate has now got to the point where

it can't take it any longer.

The micro cracks within that gusset plate have grown.

The gusset plate fails, and it fails catastrophically.

6:05 p.m., the beginning of the end

for the I-35W Highway Bridge and tragedy for those on it.

The members that are connected to it detach.

They're no longer attached to each other,

the force has got nowhere to go,

so it just collapses and it collapses instantly.

[cracking, rumbling]

The central span collapses, sending vehicles,

people, and building materials plummeting 35 meters,

17 vehicles fall into the Mississippi River.

Some are crushed by the deck's pieces falling on them,

but there's more to come.

After the central span collapses,

the two adjoining spans follow.

Everything falls into a railway yard,

and the Mississippi River.

111 vehicles, including 25 construction machines

and a school bus loaded with 63 students, were on the bridge

when it fell.

While many miraculously come out of the disaster alive,

for others, a new nightmare begins.

Emergency services take six minutes to reach

the accident scene.

In the meantime, solidarity is building among the survivors

who are trying to rescue people trapped in their cars,

which are filling up with water.

The collapse of the I-35W Highway Bridge injures

145 and leaves 13 people dead.

The I-35W bridge became fatal for three main reasons.

Firstly, the design of the gusset plate has to be

up there as being a major problem.

Number two is the question of maintenance.

Maintenance is vital.

And number three, actually, is that the bridge is carrying

more load than it was designed for.

As it turns out, that problem was not located purely

within Minnesota.

In 2007, the Federal Highway Administration counted 19,273

non-redundant bridges in the United States alone,

465 of which had a steel truss similar to that of the I-35W.

Of those, 31%, or 145 bridges, were classified

as structurally deficient.

On all these bridges, if a key structural element

were to fail, the bridge would present a high risk

of partial or total collapse.

In memory of this tragic accident, the defective gusset

plate is now exhibited in a museum in Minneapolis.

It's a haunting reminder that many people that failures

like this could become all too common.

[Rob] What is unique is what happened on the other side

of the country, in Washington state, when, in 1940,

a bridge ripped itself apart.

The famous, iconic Tacoma Narrows bridge.

[Ian] Tacoma Narrows Bridge represents one of those moments

in history where everything changed.

There's a before Tacoma and there's an after Tacoma.

11:09 a.m. on November 4th, 1940.

A 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.

It's overcast and the wind is already causing the bridge

to showcase its wave-like motion.

11:10 a.m.

The brand-new Tacoma Narrows Bridge collapses spectacularly

in this inlet of the Pacific Ocean.

The deck, now twisting in unpredictable motions,

continued to shed concrete and steel, with successive

portions of the bridge falling into the water

of the narrows below.

Hailed as a triumph of human perseverance when it was

inaugurated, the Tacoma Narrows Bridge would become a symbol

of contemporary engineering failure.

Why was such an important bridge destined for disaster?

In 1937, the local authorities decided to build a bridge

to link the cities of Tacoma and Gig Harbor.

[Shini] It's intended to help commuters traverse the cold waters

of the Puget Sound, providing a vital economic

and military route.

[Ian] The 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

gonna make a massive difference to people's lives.

An initial design was proposed by a local engineer,

Clark Eldridge.

Clark Eldridge is proposing a steel truss bridge, of which

there are thousands at this time in the United States.

Its design incorporates 7.6 meter-deep trusses

to reinforce the rigidity of the bridge.

But at a cost of 11 million US dollars, or at today's value,

$250 million, the Eldridge Project is

considered too expensive.

[suspenseful music playing]

[Rob] On the other hand, you have Leon Moisseiff,

who is a steel rope expert, and he is proposing a sleek

suspension bridge.

But crucially, the suspension bridge will do

that job cheaper.

At $8 million US dollars, $3 million less

than the Eldridge project,

the proposal by this experienced engineer,

who has worked on the Manhattan Bridge

in New York and the Golden Gate Bridge in San Francisco,

also has other advantages.

[Ian] It was using less material.

It was gonna be easier to build.

So that design was selected.

The Léon Moisseiff 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.

The longest span is 853 meters, making it the longest

suspension bridge in the United States and the third longest

in the world.

The engineer did away with the usual steel trusses

and replaced them with two 2.4-meter-high girders

along the deck.

It's very, very thin, like a ribbon,

but also because it's very narrow.

With 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.

July 1st, 1940.

The cameras and the public flocked to the inauguration

of what is regarded as a masterpiece of engineering.

[Shini] What locals don't know is that the bridge is

already doomed in his efforts to create a visually pleasing

cheaper bridge, Moisseiff made fatal miscalculations.

[Rob] Very early on the workers are regularly subjected

to high winds that are funneled down this valley.

The wind...

one of the main enemies of a bridge, that absolutely must

be taken into account when designing it.

If it was a truss bridge, then obviously the wind would

pass through it to an extent.

Unlike trusses, these girders don't allow wind

to pass through.

Although visionary, Leon Moisseiff's proposal

to replace the trusses with steel girders on the sides

of the deck...

would be the first mistake that would prove fatal

to the Tacoma Narrows Bridge.

[Ian] The wind hits the side of the bridge,

and it's presented with an eight-foot-deep girder,

which is a solid object.

And the wind, of course, has to go somewhere.

As the wind hits the truss, it creates small vortices.

These vortices move along the truss and find a completely

new direction when they reach the edges.

[Ian] Well, it goes over the top and underneath.

The wind separates from the structure, and that creates

a little vacuum, a little area of low pressure every time one

of those vortices forms.

That pressure creates a sort of suction which draws

the bridge down if it's on the bottom,

and if it's on the top, it'll draw the bridge up.

[Rob] You may think of bridges as rigid, stationary structures.

In fact, all bridges are designed to have some movement,

some flex in them.

Despite these worrying signs, the bridge was open to traffic.

The bridge's strange behavior earned it the nickname,

"Galloping Gertie."

From day one, the bridge deck sways, it moves, it dances

in the wind.

Some people enjoy the process of driving across it,

almost like being on a roller coaster.

Engineers are concerned because this amount of movement was not

part of the design process.

They are attempting to stabilize the deck by securing

it at both ends using cable connected to 50 ton concrete

blocks on the bank.

[suspenseful music playing]

But they break.

Two months after the inauguration of the bridge,

hopes were pinned on a team of engineers and scientists led

by Professor Farquaharson from the University of Washington.

They are building this miniature replica of the bridge

and carrying out tests to reproduce its exact behavior

when the wind blows.

They have come up with two proposals

to stop its undulations.

[Rob] The 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.

[Shini] The second is to add deflector veins and fairings along

the deck to increase the bridge's

aerodynamic profile.

The first option is rejected because of the irreversible

nature of the solution,

but the second must be implemented.

Unfortunately, the Tacoma Narrows Bridge would not hold

out long enough for work to begin.

November 7th, 1940. The time is 10:30 a.m.

Winds of 68 kilometers per hour

blow across the Tacoma Narrows Bridge.

It's waving up and down, oscillating up and down

and side to side, as it has done for months before,

but the bridge starts to move in a slightly different way.

A phenomenon never before seen on a superstructure.

The very thin deck, praised for its elegant design, is proving

to be the second weak link of the Tacoma Narrows Bridge.

[Ian] This bridge, which had longitudinal girders

down each side, had very little transverse stiffness.

The bridge deck has started to twist, and that twisting

movement was the fatal difference on this day.

Faced with the terrifying sight of this deck twisting along

its entire length, the engineers are

increasingly worried.

The bridge is immediately closed to traffic.

The time is 10:31 a.m.

[Shini] Unfortunately, one driver has already been allowed past

and is on a deadly route towards the bridge.

Leonard Coatsworth.

It's just him and one passenger in his car.

His dog, named Tubby.

[dog barking]

Tubby and his master find themselves

stranded in the middle of the raging bridge.

[Shini] As the bridge sways almost uncontrollably, a major failure

goes unnoticed.

A cable band at the bridge's mid-span snaps.

[dog barking]

Leonard can't go any further and he can't go back.

He decides to abandon his car.

[Shini] He 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.

Tubby, still sitting in Leonard's car and terrified

by the movement, refuses to leave the vehicle.

Leonard has to accept the inevitable if he wants

to save himself.

11 a.m.

Professor Farquharson came to observe the new torsion

behavior of the bridge.

He's about to witness something completely new, and even

more dangerous.

As the bridge continues to sway, it draws its power

from the winds that continue to batter the deck.

Not only is Galloping Gertie twisting out of control,

it's also building energy.

There is a critical point where the speed of the eddies being

shed coincides with the natural frequency of the bridge.

It 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.

It's a runaway train bound for the end of the track.

To make matters worse, the two ends of the bridge are

twisting in opposite directions.

The bridge twists violently up to 45 degrees in one direction,

and then the other.

This creates a propeller effect, with only the middle

of the bridge locked in place while the two sides

twist violently.

11:02 a.m.

A girder bulges out on the gig harbor side of the bridge,

unable to endure this violent twisting motion.

The 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.

[cracking, rumbling]

[Shini] Professor 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.

Tacoma is one of those moments in history when everybody just

stops and says, "Wow, now we've learned something

we didn't know."

It was a phenomenon that had just not been seen before.

Following the collapse of the Tacoma Narrows Bridge,

many other suspension bridges have been built and have become

benchmarks of structural engineering.

Bridges are icons within our cities and cityscapes

around the world.

But when things go wrong, their collapses are

spectacular and unforgiving.

We'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.

Whether it's a design error, an unknown phenomenon,

or negligence, when a bridge collapses, the whole world is

in shock.

When these errors are not caught, then a collapse can

become a tragic case of fatal engineering.