Original subtitles

ANNOUNCER: This program is rated PG, and contains

ANNOUNCER: This program is rated PG, and contains

ANNOUNCER: This program is rated PG, and contains

mature subject matter.

Viewer discretion is advised.

(DRAMATIC MUSIC PLAYING)

(HELICOPTER BLADES WHIRRING)

(SIRENS WAILING)

(WOMAN SPEAKING)

(MAN SPEAKING)

NARRATOR: It's one of the deadliest bridge collapses in history...

It looks like something out of a disaster movie.

NARRATOR: But why did it fail?

Was it terrorism?

Was it heavy loads?

Was it corrosion?

NARRATOR: Now...

(CAPTIVATING INSTRUMENTAL MUSIC PLAYING)

...engineers unlock the clues to the world's biggest disasters...

(TRAIN HORN BLOWING)

(WOMAN SCREAMING)

...to find out what happens When Big Things Go Wrong.

These are the ultimate fears that keep structural engineers awake at night.

(PEOPLE SCREAMING)

MAN: Oh, my God!

(SUSPENSEFUL MUSIC PLAYING)

NARRATOR: It's 6:01 in Minneapolis

on a scorching hot summer afternoon.

Crossing the I-35 bridge is an everyday commute for thousands of drivers,

like Lindsay Walz.

I had the air condition full blast,

my windows were up

so that I could stay as cool as possible.

NARRATOR: Approaching from the other direction,

businessman Andy Gannon.

I grew up in South Minneapolis, probably about five miles away from the 35W bridge.

I crossed it, on average during the week, about four or five times.

I knew the bridge was going to be busy

because that's considered rush hour in Minneapolis at that time of day.

It's probably the worst time to be driving in downtown Minneapolis.

(DRAMATIC MUSIC PLAYING)

NARRATOR: The I-35 bridge

runs north-south across the Mississippi.

Opened in 1967, it's one of the busiest in Minnesota.

JOHN LAURITSEN: It's a workhorse bridge.

It's a bridge that gets you to and from where you need to be.

And it is something that you wouldn't think twice when you drive on it.

You wouldn't think about it at all.

(SUSPENSEFUL MUSIC PLAYING)

ROB BELL: Most people don't think of this bridge as special at all.

But 140,000 cars and trucks drive cross it every day.

That's more than the Golden Gate Bridge.

NARRATOR: A workhorse built for punishment,

the I-35 is a truss bridge.

Made of steel beams joined together

to form two large trusses resting on four concrete piers.

It's a strong, load-bearing bridge

that carries eight lanes of interstate traffic.

What nobody knows is that the bridge is fatally weakened

and each extra car is taking it closer to catastrophic collapse.

(CLOCK TICKING)

I am creeping forward, just,

like, car length by car length.

I wasn't even putting my foot on the pedal, you know, to put the gas on.

It was just un-brake and re-brake.

I got to probably about the middle of the bridge

and there was a really loud

clank.

(EERIE MUSIC PLAYING)

I can hear the... The actual sounds of failure,

mechanical failure, loud, you know, cracks and breaks.

I'm thinking, "This isn't good.

"I've got to get off the bridge."

NARRATOR: But time runs out for the drivers on the bridge.

(CLOCK TICKING)

The bridge collapses, taking 111 vehicles with it.

It's as though someone has just kicked the legs out from a table

and the whole thing just drops down into the river below.

Over 10,000 tons of steel and 1,000 feet of bridge

come crashing down into the water.

You are waiting for this... This monster,

this wave of concrete to come and take your life.

(POLICE SIRENS WAILING)

Because my car was nosediving,

the water just came up through the engine,

and through the ventilation system,

and right up into the cabin of my car.

So, by the time the car actually, like,

settled and stopped moving, I was drowning.

(THRILLING MUSIC PLAYING)

I thought that was it. For sure.

(SIRENS WAILING)

(HELICOPTER BLADES WHIRRING)

(DRAMATIC MUSIC PLAYING)

NARRATOR: Shocked by multiple casualties,

Minneapolis confronts one of the deadliest bridge collapses in U.S. history.

It looks like something out of a disaster movie.

NARRATOR: But what caused it? And how?

ANDREW HERRMANN: Was it terrorism? Was it heavy loads?

Was it corrosion?

You just have all these questions

that you want to try to answer to find out why it actually did come down.

NARRATOR: The answers to this mystery could lie in

decisions from before the bridge was even built.

And the trail of clues starts in 1956, in the nation's capital.

(DRAMATIC MUSIC PLAYING)

With the Cold War at its peak, President Eisenhower embarks

on one of the biggest engineering projects in history,

building the interstate highway.

The plan is to build 48,000 miles of road

so that cities can be quickly evacuated

if the Russians drop an atom bomb.

(EXPLOSION)

And so he sets out on this very ambitious campaign to build a system of highways.

NARRATOR FROM ARCHIVE: Chicago, Dallas, Baltimore, Miami.

An efficient system of highways and interstate freeways.

You've got a lot of rivers, you've got a lot of valleys,

you've got a lot of mountains, and they all have to be crossed,

and that can't all be done just with highway.

You need bridges that close the lengths.

If you want to build a strong bridge and over a big obstacle,

the go-to design is the truss bridge.

Truss bridges are some of the most simple bridges to build.

An ideal situation when it comes to

putting up a lot of bridges in a short amount of time.

NARRATOR: They seem the perfect choice

for Eisenhower's enormous network.

But they come with a big problem.

A design flaw that's the first clue to the I-35 disaster.

By 1964, it's Minneapolis' turn to join

one of the longest highway networks in the world.

But first, engineers must build a bridge

across the mighty Mississippi River.

(SURREAL MUSIC PLAYING)

They choose a truss design.

A framework of steel beams and posts

arranged in a series of triangles.

Triangles are very stable structural shapes.

What they do is,

they take triangles

and they bolt them together

as they try to bridge the span.

BISBY: One of the reasons a truss bridge makes sense here,

is it's a very rigid type of bridge.

So, if you're carrying very heavy traffic loads, four lanes of traffic,

you don't want your bridge moving down too much under those loads.

And a truss bridge is a very rigid structural form.

NARRATOR: But for all its strength,

this design has a crucial weakness.

Most bridges can support the loss of one key part,

but not a bridge like this.

HERRMANN: When you're using a truss,

you generally only have two trusses crossing the river.

So if one fails, the entire bridge will potentially collapse.

NARRATOR: In a perfect world, the I-35 would be built

with a safety feature called redundancy,

where if one truss breaks, there are backup supports in between to prevent collapse.

But the I-35 doesn't have that.

It only has two trusses.

And if either of them fails,

the entire bridge will be destroyed.

BISBY: If you fracture one piece of the bridge,

the whole structure comes down like a house of cards.

NARRATOR: A big structure without a backup plan is a huge risk.

A lesson learned the hard way in Florida.

(DRAMATIC MUSIC PLAYING)

Miami.

A new pedestrian bridge has just been built to connect a college campus to town.

It's only been standing for five days, when disaster strikes.

Just like the I-35 bridge, the collapse is instant and shocking.

But that's not the only similarity.

Because a clue to the destruction of the Miami bridge

is in its design.

The designers want to wow people with this bridge,

so it's got an asymmetric zig-zag pattern,

which gives it an interesting look,

but at heart, it's a truss bridge with triangular sections,

just like the I-35.

NARRATOR: But it's built with just one truss.

If this fails, the bridge is in danger.

It's being constructed on the side of the road in one piece,

and then, in an overnight lift, will be brought up,

put in place and set down.

One day, you have no bridge,

the next day, there's a pedestrian bridge present.

NARRATOR: From the moment it's up,

the bridge is in trouble and starts cracking.

(EERIE MUSIC PLAYING)

Now, for concrete bridges, a certain amount of cracking is not that worrisome,

but these cracks were much, much larger.

NARRATOR: Five days later,

workers are sent in to carry out emergency repairs.

But it's too late.

(DRAMATIC MUSIC PLAYING)

(HORNS HONKING)

(SIREN BLARING)

(WOMEN TALKING)

NARRATOR: The collapse starts when a diagonal beam breaks,

and with no backup, the entire bridge follows like a house of cards.

Six die, including one worker.

Like the Miami bridge,

the I-35 bridge in Minneapolis has a flawed design.

And the scary truth is, there 18,000 other bridges in the U.S. built the same way

with no backup.

But they're not falling down.

So there must be other reasons

for the mysterious collapse in Minneapolis.

Coming up...

More clues uncovered.

BELL: Unknown to anybody, not the designers,

not the builders, a mistake has been made.

NARRATOR: And a Milwaukee mega crane shows what can happen

when big things get overloaded.

(DRAMATIC MUSIC PLAYING)

(CAPTIVATING INSTRUMENTAL MUSIC PLAYING)

(DRAMATIC MUSIC PLAYING)

NARRATOR: When big things go wrong,

engineers have to find out why.

MAN: (ON RADIO) I have serious doubts about making the airport.

(PEOPLE SCREAMING)

NARRATOR: In the case of the I-35 bridge in Minneapolis...

The first clue is its flawed design.

But there are thousands of bridges just like this,

and they're still standing.

So what else explains this failure?

More answers may lie in its construction

and a deadly error in a crucial component.

1967, and the bridge is taking shape.

Engineers think they're building it strong enough to ensure

it never fails, whatever it faces.

When engineers design things,

they incorporate something called the factor of safety.

Basically, a truss bridge like I-35 is made twice as strong as it needs to be.

NARRATOR: The key to that strength,

flat plates of steel called gusset plates.

So the function of a gusset plate is to glue

the members of the truss bridge together.

NARRATOR: These crucial components

join all of the steel beams on the bridge together.

They attach at the ends of each triangle along the truss,

with bolts and rivets on both sides

to create a very strong joint.

And there are 56 of these joints

on both east and west sides of the bridge.

As the last steel plates are riveted on,

the I-35 bridge opens.

Here we go.

(PEOPLE CHEERING)

NARRATOR: But there's a problem.

Unknown to anybody, not the designers, not the builders,

not the first people driving over the bridge,

a mistake has been made.

NARRATOR: The engineers miscalculate the size of 24 of the key plates

holding the bridge together.

BISBY: And some of the gusset plates

that are in critical locations in this structure

are about half as thick as they ought to be.

NARRATOR: They should be one inch thick.

Instead, they're half inch.

This compromises the bridge

because they're holding together critical members of the truss.

And if one of the joints fails, the entire bridge has the potential for collapse.

NARRATOR: It's incredible.

The structural integrity of a huge bridge

weakened by 24 small steel plates that are simply too thin.

But a small part can bring down a big thing,

as construction workers in Wisconsin discover.

(DRAMATIC MUSIC PLAYING)

In Milwaukee, the Brewers' baseball stadium is getting an upgrade,

a new roof.

We're gonna have a roof on next year.

Whoo!

The roof is created with steel sections

that are a million pounds each. Heavier than a 747.

NARRATOR: This huge job requires a mega crane with a fitting name.

Big Blue.

But as it lifts the roof, there's a sickening crack.

(METALLIC CREAKING)

MAN: What the hell is that?

Watch it! Watch it!

NARRATOR: The clue to Big Blue's destruction

lies in the failure of a single, small part.

Just like the I-35 bridge,

Big Blue relies on a key component

to hold its mighty frame together.

The kingpin.

A foot wide and 11 feet long,

it's 1/50th the size

of the massive lifting arm it holds in place.

The kingpin is strong, but not unbreakable.

The crane is designed to be used in winds under 20 miles per hour,

and on this day, the winds were at 26 miles per hour and more.

The load can act like a big sail or a wing.

NARRATOR: As it lifts the roof, that cracking sound...

(LOUD CRACK)

...is the kingpin breaking.

MAN 1: What the hell was that?

MAN 2: What's going on here?

NARRATOR: With the crane arm acting as a massive crowbar,

the force on the kingpin builds up until it ruptures,

shattering the huge washers that hold it in place

and snapping off the end.

It's a fatal failure for Big Blue.

(LOUD CRACK)

MAN: Watch it! Watch it! Watch it!

(DRAMATIC MUSIC PLAYING)

NARRATOR: One small piece, one giant disaster.

The lesson for the I-35 bridge,

big things are only as strong as their smallest parts.

And in Minneapolis, some of these are too weak to do their job.

But what caused them to break?

(GENTLE INSTRUMENTAL MUSIC PLAYING)

(GENTLE INSTRUMENTAL MUSIC PLAYING)

(DRAMATIC MUSIC PLAYING)

NARRATOR: In Minneapolis,

unraveling clues to the I-35 disaster is an engineer's top priority.

You just have all these questions

that you want to try to answer to find out

why it actually did come down.

NARRATOR: And for the victims, a terrifying ordeal.

I can hear the cracks and breaks

and I'm looking out and I see the bridge actually drop.

I thought I was going to die a bloody mess.

NARRATOR: Despite a flawed design and a dangerous mistake,

the I-35 bridge still stands for 40 years.

So what else explains its sudden collapse?

The next clue lies on top of the bridge.

1977. The bridge has been open for a decade.

In that time, traffic across the interstate system doubles.

Bad news for the already compromised I-35 bridge.

Traffic over the bridge has increased dramatically since it opened,

increasing the load it regularly has to take.

NARRATOR: And it's not just cars.

Every 18-wheeler causes as much road damage as 9,000 cars.

But increased traffic isn't the only extra weight on the bridge.

So in 1977, the roadway of the bridge was deteriorating.

And it was decided that a new concrete surface

would be added to extend its life.

Over the entire length of the bridge,

this added three million pounds of additional weight.

NARRATOR: That's not all.

Over the next 20 years,

extreme weather puts even more stress on the already strained bridge.

(SIREN WAILING)

REPORTER: On 35W and St. Paul, a school bus and car collided.

No one was injured.

The I-35 bridge has a problem with black ice.

It's so close to the Falls, that mist is coming over

and with the extreme conditions in Minnesota, it freezes.

So, they put this de-icing system on the bridge

to combat that and make it safer.

But this adds over 1.2 million pounds of additional weight.

Three million pounds thanks to the new decking system,

and 1.2 million pounds due to the de-icing system.

None of that additional weight will have been considered

by the bridge's original designers.

NARRATOR: A big structure compromised by bad weather.

It's a recipe for disaster, as horrified music fans learn in Indiana.

(CROWD CHEERING)

12,000 fans are gathered to watch country duo Sugarland...

Just as a storm rolls in.

(PEOPLE SCREAMING)

How did a grandstand that's been used countless times before

collapse, killing seven people?

(FIRST RESPONDER SPEAKING ON RADIO)

(SUSPENSEFUL MUSIC PLAYING)

NARRATOR: Like the I-35 bridge,

this structure's built with trusses.

And also,

like the I-35 bridge,

it doesn't take much to overload it.

BISBY: Now what's important to understand

about a tent structure like this grandstand

is that the posts that hold the roof up need to stay vertical

and the design is intended to make sure that that's true.

NARRATOR: The posts are tied to concrete blocks

with steel wires to keep them upright.

Each block is heavy. Four thousand pounds.

So the structure can withstand winds as high as 43 miles per hour.

But increase the wind by just another 15 miles per hour,

and the extra load on the posts is enough to bring them down.

(PEOPLE SCREAMING)

As a result, seven people are killed.

Just like the stage in Indiana,

the I-35 bridge in Minneapolis is heading for disaster.

But it's not there yet.

There's still a chance.

If clues are spotted,

catastrophe can be avoided.

(FAST-PACED MUSIC PLAYING)

Coming up...

WALZ: Who was that?

Who took a picture of that bent gusset plate?

NARRATOR: Will engineers spot a critical warning?

All this bridge needs is one extra nudge.

NARRATOR: And a Philadelphia refinery

pays the price for failed inspections.

(EXPLOSION)

(DRAMATIC MUSIC PLAYING)

(CAPTIVATING INSTRUMENTAL MUSIC PLAYING)

NARRATOR: In Minneapolis,

engineers are trying to learn why the I-35 bridge collapsed.

Some answers are clear.

A key component is weak.

The gusset plates are half as thick as they need to be.

NARRATOR: A new road surface adds weight.

You've got a new bridge deck that's added an enormous amount of mass to the structure.

NARRATOR: But is there a moment when the bridge could have been saved?

(SUSPENSEFUL MUSIC PLAYING)

In June 2003,

engineers are carrying out a routine inspection on the bridge

when they notice something disturbing.

At a pair of joints called U10 east and U10 west,

they see the steel plates

holding the bridge together

are bending.

Why wasn't this clue seen before?

BISBY: In the structural engineer's psyche,

the joint is always stronger than the beam.

So you don't have to check gusset plates

because they're always much stronger than the elements, right?

So, you don't look at them.

(CAMERA SHUTTER CLICKING)

NARRATOR: They photographed the bent plates.

But these warning signs are somehow ignored,

because the engineers believe they're unbreakable.

Gusset plates don't fail, so seeing a gusset plate bowed,

it may have been determined that maybe it was during erection or something

that it got bent a little bit.

DICKRELL: The engineers say it's probably not a big deal

because it's been there for so long.

"We'll deal with it further down the road."

NARRATOR: A critical chance to repair the faults is missed...

Four years before the bridge ultimately fails.

(DRAMATIC MUSIC PLAYING)

But miss a warning sign, however small,

and it can have dire consequences,

as a huge disaster in Philadelphia proves.

The biggest oil refinery on the east coast,

spread across a massive 1,400-acre site.

If you think about a theme park like Six Flags,

it's like then times the size of Six Flags.

The really crazy thing to think about

is the amount of energy they process.

It's bigger than the biggest

nuclear weapon in the U.S. arsenal.

It's bigger than that.

NARRATOR: The potential for disaster is terrifying.

So engineers regularly inspect its 950 miles of pipes for faults.

But just like at the I-35,

they miss a vital weakness at a critical joint.

It is a chemical engineer's worst nightmare.

NARRATOR: A leak of highly flammable propane gas.

HENRY: My God. We use it to do barbecuing, right?

Lots of energy in a small volume.

(SUSPENSEFUL MUSIC PLAYING)

NARRATOR: Forty seconds after the gas appears...

(EXPLOSION)

The blast is so powerful that a 19-ton steel tank

is blown 2,000 feet away across the other side of the river.

And the shock is felt as far away as New Jersey.

MAN: Holy (BLEEP)!

Look at that, it's a (BLEEP) bomb!

NARRATOR: Miraculously, no one dies.

The failure is initiated at one single piece of pipe,

an elbow joint.

And critically, this is a piece of the processing plant

that has not been inspected over the years.

NARRATOR: This faulty pipe

has been slowly corroding for over 40 years,

until it's worn down to half the thickness of a credit card.

Then it reaches breaking point.

In these types of heavily engineered environments,

one small factor, if it goes wrong and goes unchecked,

can cause catastrophe.

NARRATOR: Coming up...

With the warning signs ignored,

a perfect storm is brewing for the I-35.

All this bridge needs is one extra nudge.

(DRAMATIC MUSIC PLAYING)

NARRATOR: In Minneapolis, the I-35 bridge is in trouble.

Through its 40-year life, a series of engineering errors

has pushed it to the brink of collapse.

But in its final hours, what tips it over the edge?

(SUSPENSEFUL MUSIC PLAYING)

With just three hours to disaster,

a construction crew, loaded with materials and equipment,

heads onto the bridge to make routine repairs.

They start to stockpile construction equipment

and gravel on the surface of the road deck.

Unfortunately, they've placed this load

right on top of the joint at U10.

NARRATOR: The exact place where the plates are bending to breaking point.

It's a very unfortunate occurrence

because you have all of this additional load

now concentrated directly on top of the weakest spot on the bridge.

NARRATOR: Over 300 tons of equipment,

including cement trucks and heavy machinery

is now stacked up,

and the final blow is on its way.

(CLOCK TICKING)

A little after 6:00 p.m., right at the height of rush hour,

cars are streaming across the bridge.

NARRATOR: With the construction crew blocking two lanes,

traffic builds up and comes to a standstill.

There are over 100 buses, cars, trucks,

slowly filling up each inch of the bridge.

NARRATOR: One hundred vehicles

adds another half million pounds to the strained structure.

(HORN HONKS)

(CLOCK TICKING)

Midway across the bridge, Lindsay Walz.

The traffic started to slow down.

I'm hot, I'm tired at the end of a long day,

and I just wanted to get home.

The load on the bridge is almost as big as it's going to get.

(CLOCK TICKING)

NARRATOR: Approaching from the opposite direction, Andy Gannon.

I knew they were working on both the outside lane

and the inside lane of the bridge.

So I knew it was going to be crowded.

This is the most weight that this bridge has ever had in its 40 years.

(CLOCK TICKING)

NARRATOR: Now the 111th car rolls onto the bridge,

and it's the final straw.

(MACHINERY RATTLING AND WHIRRING)

(CLOCK TICKING)

(DRAMATIC MUSIC PLAYING)

I see waves of concrete lifting up cars

and dumping some of them over into the river.

I'm waiting for it to come to me

and we all go down as one.

My car doesn't actually really stop moving

until it lands at the bottom of the Mississippi.

I was drowning.

I had to get out of my car or that would be where I died.

I knew that my time was absolutely running out.

And I kicked and I swam and I surfaced.

A construction worker brought a broom handle to fish me out.

I assumed that I was one of a handful of survivors at best.

NARRATOR: Meanwhile, Andy Gannon's car has fallen 50 feet

onto an unstable island of concrete.

He scrambles from his silver Chrysler to escape.

GANNON: I thought to myself, we're going to continue

to drop in the water at any minute.

NARRATOR: But he stops in his tracks

when he sees a bus full of terrified school kids.

I'm hearing people, and the screams from the school bus is what I hear.

-(SIRENS WAILING) -(CHILDREN SCREAMING)

Holy smokes, I've got to go help.

(MAN SPEAKING ON RADIO)

A couple seconds later, there's probably four or five adults,

we took the kids off the school bus

and handed them down to individuals that were on the River Road.

(INDISTINCT CHATTER)

(HORNS HONKING)

Then I see black smoke coming out of the cab of the truck.

(MAN SPEAKING ON RADIO)

I could hear him... Hear him screaming.

NARRATOR: As smoke turns to fire,

there's nothing Andy can do.

He didn't make it and...

And, you know, that's, uh... That's probably the...

The worst part about the whole thing.

(AMBULANCE SIREN WAILING)

NARRATOR: The collapse takes a grim toll.

Thirteen dead, 145 injured.

Minneapolis is a city in shock, and a city looking for answers.

(SOMBER MUSIC PLAYING)

(GENTLE INSTRUMENTAL MUSIC PLAYING)

(GENTLE INSTRUMENTAL MUSIC PLAYING)

(PEOPLE SCREAMING)

(DRAMATIC MUSIC PLAYING)

NARRATOR: Big disasters can have obvious causes.

But sometimes, engineers have to search for the answers.

(911 OPERATOR SPEAKING)

(WOMAN SPEAKING)

I'm in a fog, I'm in shock.

I started to look up above me, where the bridge had been,

and I couldn't understand what had happened to me just then.

NARRATOR: The sudden collapse of the I-35 bridge leaves everyone guessing.

But what really happened?

Seen through the eyes of engineers, the clues are clear.

A flawed design...

If you fracture one piece of the bridge,

the whole structure comes down like a house of cards.

NARRATOR: ...a deadly error...

Unfortunately, some of these gusset plates

are only half as thick as they should be.

NARRATOR: ...extra weight added to the bridge...

A large concentrated force above the weakest point of the bridge.

NARRATOR: ...and warning signs ignored,

until finally, failure is inevitable.

(CLOCK TICKING)

(DRAMATIC MUSIC PLAYING)

At the gusset plate that fails,

the stresses become so high

they start overcoming the capacity of the gusset plate itself.

And the truss beam that's connected to that gusset plate pushes through the joint

and completely destroys the node at U10 west.

NARRATOR: The plates at U10 are ripped apart.

DICKRELL: With those gusset plates gone,

there is no way for the truss bridge to hold itself together.

PROCTOR: The whole center section of the bridge snaps

and begins to drop down.

BISBY: And in the CCTV footage,

we can see that the north end of the bridge collapses slightly later.

There's this sort of domino effect

as the structure fails from south towards the north.

BELL: In the space of about seven seconds,

the whole bridge, that's stood there for 40 years,

is completely gone.

(SIRENS WAILING)

NARRATOR: It's an engineering catastrophe

no one in the Twin Cities will ever forget.

That was our 9/11.

No disrespect, I'm not comparing them,

but that was our bad day, our failure, our, um, loss.

Sometimes, I look back and think, you know, this shouldn't have happened.

I've thought a lot about that.

I've thought a lot about who put that concrete there,

who measured those gusset plates,

who was that inspector who took a picture of that bent gusset plate?

GANNON: What's the next thing that's going to break?

Can I trust anything anymore?

How do we make sure it doesn't happen again?

Just 10 weeks after the I-35 was destroyed,

construction of a new bridge begins across the Mississippi.

NARRATOR: In September 2008,

the new I-35 West, Saint Anthony Falls Bridge, opens.

It's a symbol of doing what we can to put the past behind,

yet never forgetting the people that were impacted by this,

and still are impacted by this to this day.

NARRATOR: This bridge learns from past mistakes.

A state-of-the-art smart bridge with sensors everywhere.

So as cars cross, their safety is guaranteed.

The bridge can actually tell the people who are operating it how it's feeling,

what kinds of strains and stresses and forces is it being exposed to.

NARRATOR: It's built with reinforced concrete.

And if one part of the bridge fails, the rest will stay standing.

It's a bridge for the 21st century.

LAURITSEN: The old bridge before the tragedy,

nobody ever thought about it.

The new bridge, people think about it all the time.

And when you drive over the new 35W bridge,

you're like, "Wow, I can't believe what happened here."

You don't feel like it's going to go back to that.

You feel like we learned from our mistakes.

NARRATOR: But not all of our bridges are new.

And despite engineers' best intentions,

nothing is ever 100% safe.

Between 1980 and 2012,

over 1,000 bridges failed across the country.

And when these big, strong structures go wrong,

they can quickly turn into catastrophe.

Unlike a surgeon, who may cause the death

of a single person if they get the surgery wrong,

an engineer can very easily cause the deaths

of hundreds or thousands of people,

and that's something that all structural engineers

need to keep very clear in their minds.

(PEOPLE SCREAMING)

BELL: Bridges will collapse,

ships will sink and airplanes will crash

if mistakes are made.

(DRAMATIC MUSIC PLAYING)

But everyone should rest assured

that right at the top of an engineer's list

is to learn from those mistakes.

NARRATOR: This season on When Big Things Go Wrong...

MAN: Oh, my God.

...a horrific building collapse in New Orleans...

When big things are pushed beyond their limits,

the results are catastrophic.

NARRATOR: A devastating plane crash in San Francisco...

BELL: It's amazing to think that something as small as a grain of sand,

can bring down something as big as a major airliner.

NARRATOR: A mysterious cruise liner collision in Italy.

There's a problem with the engine.

The throttle is turned on full thrust.

NARRATOR: Engineers unlock more clues

behind the world's biggest disasters.

When something goes wrong, it goes really, really wrong.