NOVA

For more than 50 years,

the Polcevera Bridge stood as a landmark

in the city of Genoa.

It was a masterpiece,

and everybody knows it was a masterpiece.

It was one of the most famous bridges in Italy.

It was the bridge of the future.

The bridge was really beautiful.

But on August 14, 2018,

disaster strikes.

A huge 800-foot section of the bridge,

carrying four lanes of traffic,

collapses.

27 vehicles plummet into the valley below.

43 people die.

It is one of the worst road bridge collapses in Europe

for more than a century.

And yet there was no warning.

Why did this seemingly sound bridge

suddenly fall down?

And is anyone to blame?

In the last ten years,

more than 60 bridges have collapsed

around the world.

How many more are about to give way?

In the United States, over 47,000 bridges

are classified as structurally deficient.

We have had over two dozen bridges fail

in the United States since 2000.

Can engineers guarantee the safety of our aging bridges

before it is too late?

"Why Bridges Collapse," right now on "NOVA."

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Genoa.

One of Italy's busiest ports.

Over 6,000 ships a year

carry 54 million tons of cargo through here,

nearly all of it placed on trucks.

The trucks fan out in all directions,

but many end up on this highway,

the E80.

It's a critical link

between the South of France and Rome,

shuttling more than 70,000 cars and trucks past Genoa

every day.

August 14, 2018.

Firefighter Davide Capello drives his S.U.V.

along route E80,

heading east towards Genoa.

Gianluca Ardini,

a foreman for a furniture company,

heads in the opposite direction along the E80 highway.

Both Gianluca and Davide

are heading towards the Polcevera Bridge,

locally known as the Morandi Bridge.

This iconic landmark connects the two sides of Genoa.

At 11:36 a.m.,

the world in front of them begins to come apart.

A massive 800-foot section of the bridge,

carrying four lanes of traffic,

collapses.

Gianluca's truck plummets 100 feet.

27 vehicles fall with the bridge.

This is Davide Capello's S.U.V.

It falls 130 feet

and lands inside a cavity formed by a section of the bridge.

Across from Davide's car,

trapped inside the remains of the collapsed bridge tower,

rescuers make a shocking discovery.

Hanging inside this truck is Gianluca Ardini.

Firefighters begin the precarious job

of rescuing him from the mangled cab.

It takes 40 minutes to cut Gianluca free

and carry him to safety.

400 firefighters from all over Italy

arrive with specialized equipment

most often used to rescue people from earthquakes.

Sniffer dogs search for survivors.

After four days, firefighters finally recover the last body.

The collapse of the Polcevera Bridge

kills 43 people and injures 15.

It's far and away Italy's worst road bridge disaster.

The catastrophe sounds a chilling alarm.

Engineers around the world want answers...

why was there no warning?

What caused the sudden collapse?

And most important, how many other bridges are at risk?

The stakes are huge.

If the bridge failed due to age,

that could mean disaster for thousands of other bridges

built at the same time.

All products of a highway construction boom

that began more than a half a century ago.

Across many countries, the Second World War

destroyed thousands of roads and bridges.

In the years that followed,

engineers set about rebuilding the bridges,

while also constructing new, super-fast highways.

This was the dawn of the golden age of the car.

And nowhere more so than in the United States.

In 1956, President Eisenhower

signed the Federal Aid Highway Act

to create 41,000 miles of interstate highways.

As far as the eye can see, in almost every city,

concrete and asphalt are changing the patterns

of land and land use.

Soon, traffic will flow smoothly

in, around, and between every major city and town in America.

Today in America,

just as in Europe,

many of these concrete bridges are growing old.

About two-thirds

of our interstate highway system bridges

were built in either the 1960s or 1970s,

so a number of those structures are coming to the end

of what's called their useful design life.

The United States has more than 600,000 road bridges.

Over 47,000 of them are classified with the term

"structurally deficient."

What that means is that when the bridge is inspected,

one of the key structural elements

is rated in poor or worse condition.

So it means they're bridges that need to be repaired.

Are these bridges,

located all across America and the world,

ticking time bombs?

And particularly, if they share the same faults

as the Polcevera Bridge in Genoa,

then lives could be at stake.

Within hours of the disaster in Italy,

the site becomes the focus of a major investigation.

Engineers need to find out

what caused the bridge to collapse.

Camillo Nuti is an independent engineer

who worked on the Italian government inquiry

into the disaster.

One should be very cautious before removing anything.

Obviously, you have to remove the debris,

but then be very careful,

because you have to take the pieces,

then try to understand if there was failure somewhere.

If there was failure, you should find those pieces broken.

Finding those pieces is a daunting challenge.

Sifting thousands of tons of debris,

investigators need to identify the single part of the bridge

that failed and triggered the disaster.

They move any suspect pieces to this secure warehouse.

The operation will take months, so at the same time,

they pursue a parallel line of inquiry.

They turn their attention to the way the bridge was built

and its original design.

The planning of the Polcevera Bridge

began in the early 1960s.

Its designer, Riccardo Morandi,

was a star engineer of Italy's postwar reconstruction boom.

His work included astonishing modern structures,

like this aircraft hangar in Rome,

and dozens of bridges.

He was the best bridge design engineer in Italy.

There is no Italian engineer who built what Morandi built.

To construct the Polcevera Bridge in Genoa,

Morandi faced a huge challenge.

The bridge had to span the Polcevera River,

multiple railway lines, and four apartment buildings.

His plan was to build three enormous trestles,

each with a tower nearly 300 feet high.

Then, to hold up the road deck,

he would use giant supports called cable stays,

made of steel and concrete.

When complete, the vast bridge would be over half a mile long.

To build it, Morandi used

a relatively new method of construction,

one that he had helped develop.

Before this time, most concrete bridges

used simple steel reinforcement.

But by stretching steel cables

threaded through hollow guides in the concrete,

engineers found they could build longer spans.

The tension in the wires acted to compress the concrete,

making it less likely to bend and crack.

This was called prestressed concrete,

and Morandi relied on it for the Polcevera Bridge.

In his design, he does something radical.

In most cable-stay bridges, the road deck is supported

by many cables, spread out across the span.

But to support his deck,

Morandi bunches all the cables together

to create massive cable stays,

each one made of 52 steel cables

wrapped in concrete to protect them from corrosion.

The technique was a novelty.

He put just four cables for each pier,

but they were incredibly strong cables.

After four years' construction,

in 1967, Italy's president opened the bridge

to an enthusiastic reception.

The Morandi Bridge was the most important bridge in Italy.

The biggest, the more beautiful, the most...

The most famous.

Absolutely.