Nova (1974–…): Season 46, Episode 17 - Why Bridges Collapse - full transcript

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."

Major funding for "NOVA"
is provided by the following:

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.

It was a masterpiece,

and everybody knows
it was a masterpiece.

But 50 years later,

the bridge fails
catastrophically.

Did Morandi's pioneering design
play any role

in causing it to collapse?

To find out
what caused the disaster,

investigators need to discover
which part of the bridge

failed first.

They look for CCTV cameras
in the area

that would have recorded
the moment the bridge collapsed.

Rain obscures the view
in almost all the cameras.

But in this scrapyard,

they find a video
that captures the disaster

from beginning to end.

The recording is part

of an ongoing
criminal investigation,

and is impounded.

Francesco Cozzi
is the state's prosecutor

in charge
of the criminal investigation.

Sources familiar
with what happened

have provided clues

that allow a reconstruction
of the probable sequence

of the bridge collapse.

At just after 11:36 a.m.,
something happened

that caused the southeast
cable stay on pylon nine

to break.

Unsupported, that side
of the road deck sags.

This unbalances
the whole structure,

and increases the load
on other components

beyond their breaking points.

This sends the road deck
plummeting to the ground,

and causes the giant tower
to collapse.

It looked like
the unthinkable had happened.

Some of the steel cables
Morandi had embedded in concrete

to protect them from corroding

had snapped.

But why?

The bridge had stood firm
for over half a century,

carrying millions of travelers.

So what caused it to fail
the very moment it did?

Like most road bridges
built during the building boom

of the 1960s and '70s,

the Polcevera Bridge
had been experiencing

steadily rising traffic loads.

By 2009, it carried
four times as many vehicles

as 30 years earlier.

Over 25 million vehicles
crossed it each year.

Could the weight of traffic
on the bridge

have brought it down?

Like all major bridges in Italy,

the Polcevera Bridge
was designed

to carry the load of a convoy
of heavy military vehicles.

This meant it should have been
able to carry the weight

of modern traffic.

But a small error
in design or construction

can have
catastrophic consequences,

as happened to this bridge
in the United States.

On August 1, 2007,

Kelly Kahle was driving
towards the I-35W bridge

in Minneapolis.

It's rush-hour traffic, 6:00.

I'm going north,

and we get
right on the interstate.

All of a sudden,
there's a rumbling.

And then all of a sudden,
the bridge in front of me,

far in front of me,

looks like it rose up.

And it almost looked
like it was oscillating.

Looking back on it,

I can see that we were
on a point that was failing,

and we were starting to fall.

This is Kelly's car.

It narrowly escaped
plunging into the river.

I climbed onto the hood
of the car,

and skimmied
onto this concrete island,

and just looking around
and seeing other people

kind of getting their wits
about them.

111 vehicles were on the bridge
when it gave way.

13 people died.

145 were injured.

When investigators
reassemble the pieces

of the collapsed bridge,

their attention focuses
on a part known

as a gusset plate.

Gusset plates
are flat plates of steel

that hold the girders
of a truss bridge together.

The key pieces
of the I-35W bridge

have been preserved.

These pieces are all connected
to the single gusset plate

that failed
when the bridge collapsed.

During the collapse,

this folded in on itself
and was completely mangled

by the weight
coming down on top of it.

At the time of the collapse,

workers were preparing
to re-lay the road.

Trucks had dumped piles
of sand and gravel

on the closed lanes.

There were 800,000 pounds
of construction equipment

on the top of the bridge,

as well as over 160 people
in cars,

because it was the very end
of rush hour that day.

That combined load

proves too much
for one of gusset plates.

The gusset plate
split along a line of rivets.

You can see
that it's completely torn apart.

The bridge was overloaded.

But its designers
had also made a mistake.

The gusset plate
was only a half an inch thick,

and it should have been
one inch.

So it should have been
double the thickness

that it, that it was,

and that's what led
to the failure.

Did Genoa's Polcevera Bridge

collapse because
of a similar fault...

a failure to make it
strong enough

to carry heavy loads?

When Morandi planned the bridge,

he designed it
to support a lot of weight.

The ideas of Morandi was,

let's build a bridge
with very, very strong cables

to where it's overdesigned,
very much overdesigned.

So the design was
incredibly safe for the time.

When the bridge was built,

the cable stays were capable
of carrying

over twice the weight
of the vehicles on it.

The bridge should have been
easily strong enough

to carry the light traffic
on it when it fell.

What is well known

is that the total load
that the bridge could support

was well beyond the loads
that was on the bridge.

So if the weight
of the vehicles on the bridge

didn't cause it to collapse,

what did?

Expert sources suggest

that the southeast cable stay
snapped

near the top of the pylon.

If it was this cable stay
that broke first,

something had gone
seriously wrong.

This is the kind of fault
that all engineers fear.

It's known
as a single-point failure,

when one component breaks

and brings down
an entire bridge.

It's a lesson driven home
by an earlier disaster.

In 1928, the people

of Point Pleasant
in West Virginia

celebrated the opening
of their new bridge

over the Ohio River.

It was a really exciting day.

There was 10,000 people in town.

There's pictures that show
the bridge was completely

covered with people.

They were very proud of it,

you know, to have a nice,
big, shiny bridge like that.

Well, it was the talk of the,
talk of the area,

the East Coast, you know?

The Silver Bridge.

It was nicknamed
the Silver Bridge

because of its shiny coat
of aluminum paint.

But unknown to the people
that used it,

its design contained
a serious flaw.

On December 15, 1967,

Peggy Huber was driving
towards the Silver Bridge.

Traffic was heavy.

It was Friday
and near Christmastime.

I got behind a dump truck.

And it was moving so slow,

it stopped right at the light
where the bridge was.

And I was behind it.

And I had reached down
to turn the radio dial,

and I heard this awful noise,
and when I looked up,

the bridge was just like
a tinker toy, waving,

and it just fell.

It just sounded
like a bunch of metal.

It was very loud.

It collapsed, I think, at 4:58,

so it was a busy time.

There was 31 vehicles that fell.

The collapse killed 46 people.

It remains the worst road-bridge
disaster in American history.

To find out what had caused
the bridge to fail,

investigators searched
the riverbed for wreckage.

They brought in
a whole bunch of divers,

and they would bring pieces up,

and of course,
they would examine all of it,

see if it would fit in any way.

The bridge was
an eyebar suspension bridge.

Giant bars, up to 55 feet long,

with a hole called an eye
at each end,

were linked together
with steel pins.

Crucially, each link
of the suspension chain

contained only two eyebars.

And they finally found

one of the eyebars
that was broken... split.

And when they examined it,

they found that there had been
a, a hairline fracture in it.

Nearly 40 years of corrosion
in the steel

had triggered a fracture

that ripped the eyebar
from its pin.

The weight of the bridge
proves too much

for the remaining eyebar,

pulling it with an extreme force
that tears the link apart.

The failure of this single point

triggers
an unexpected domino effect,

causing the entire bridge
to collapse.

In the United States alone,

around 18,000 bridges
have been identified

as "fracture-critical."

This means
if one component fails,

part or all of the bridge
would probably collapse.

Many were built
in the 1960s and '70s

during the rush to complete
the interstate highway network.

Today, engineers aim
to design bridges

so that
if a single component fails,

it won't bring down
the whole structure.

Modern cable-stay bridges
are built with a fan of cables

to spread the load.

This design is very different

to that of Morandi's
Polcevera Bridge in Italy.

Morandi bundled
all his cables together

and covered them in concrete

to create single, giant stays.

His design was meant to protect
the cables from corrosion.

But he inadvertently made
the bridge vulnerable

to a single-point failure.

If just one
of the huge stays broke,

the result would be disastrous.

Sadly, Morandi's design
was completed

a few years before
the Silver Bridge disaster.

Luckily, it has not been
widely copied.

Other engineers realized
the potential problems,

and only a handful of
similar bridges were ever built.

The Polcevera Bridge

was routinely inspected,
maintained, and monitored.

For it to collapse, something
must have gone seriously wrong.

But what?

Four months after the disaster,

workers remove
the last of the debris.

Forensic engineers now have
several key pieces of evidence.

Their initial inspections reveal
that some of the steel cables,

that were supposedly
well protected

inside the
concrete-covered stays,

have suffered from corrosion.

Some cables appeared broken.

When there is corrosion,

the corroded cable are brittle.

So if you just bent them,
you break them,

which is not usual for steel.

But when you have corrosion,
this may happen very easily.

The investigators identify
17 key sections of the bridge

for further investigation.

Their prime suspect
is part number 132.

This is believed to be

the part of the cable stay
near the top of the bridge

that broke.

They ship the pieces
to a laboratory in Switzerland,

where investigators will attempt
to find out

what caused the steel cables
to corrode.

When Morandi covered the cables
with concrete,

they should have been protected.

Concrete is very alkaline,
the opposite of acidic.

This produces a thin layer
of stable iron oxide

around the steel,

and should almost completely
prevent the steel from rusting.

Thousands of bridges rely
on this protection.

In most cases, it works fine.

But just occasionally,
it goes badly wrong.

London.

The Hammersmith Flyover.

It's one of
the city's busiest bridges.

Hammersmith Flyover is critical
to the London road network.

It provides one
of the main routes

from Heathrow Airport
into central London,

and it's also one of
the major routes out

to west London in itself.

In December 2011,

just months before
the 2012 London Olympics,

engineers discover

this vital bridge
is at risk of collapse.

The overpass was built
in the early 1960s

from giant concrete sections.

The problem lay in the way
they were held together.

To build
the Hammersmith Flyover,

construction crews use a form
of prestressed concrete

made using a technique called
posttensioning.

They hoist giant, prefabricated
sections of the bridge

into position,

then thread 64 steel tendons
through holes in the structure.

They then stretch
and tension the wires

to hold the concrete sections
of the bridge together.

To prevent the steel cables
from corroding,

workers cover them
with a concrete grout.

Like Morandi, the engineers
of the Hammersmith Flyover

thought the concrete would
prevent the cables ever rusting.

But in the early 2000s,

inspections reveal
they were wrong.

Water is seeping into gaps
in the concrete grout,

causing the steel tendons
that held the bridge together

to rust.

As you can see here,
they're all encased in grout.

And that means
it's very difficult

for the water to escape.

Um, and it's also very difficult
for us to inspect the tendons

and make sure
that everything is okay.

Unfortunately,

the concrete grout
that was meant to prevent rust

trapped water in cavities

and hid any corrosion
that did occur.

Transport for London calls in

structural-integrity engineer
Jon Watson

to try to identify
where the corrosion is worst.

He installs over 500 microphones
inside the bridge.

We put a microphone
about every three meters

along the structure,

to listen out
for anything that's distinct,

like a wire break.

Each of the cables buried
in the concrete

contains 19 separate strands
of wire.

The cables themselves are built
up of individual wires.

When these wires corrode,
a small crack grows in the wire.

Eventually, the wire snaps

with a very distinct
snapping sound.

And it's this energy,
this sound wave,

that's detected by the
microphone on the structure.

It enables us to determine
the position of the wire break

on the cable itself.

Over the period of three-
and-a-half years of monitoring,

we detected in excess
of 1,100 wire breaks.

So typically one or two a day.

Within a couple of months,

we were able to determine
exactly where the hotspots were,

where most wire breaks
were occurring.

This told engineers

where the corrosion
was most severe,

allowing them to focus
their inspection.

We were able to break out
the grout

around full sections
of those bundles

of highly stressed steel cables

to see just how deep

the corrosion had penetrated
those cables.

What they discovered
was alarming.

The bridge was at risk
of collapse.

Although they thought
the risk small,

to be safe,
they decided to close it.

To repair the bridge,

they attached four miles
of new cables to the outside

and stretched them,

just like a super-sized version
of the original posttensioning.

It's estimated
that this rescue operation

has extended the bridge's life
by 60 years.

The close shave
with the Hammersmith Flyover

shows how cables buried
inside concrete

can rust.

So did a similar problem
with corrosion

cause the Polcevera Bridge
in Italy to fail?

In Switzerland,
scientists examine

the pieces of the bridge
brought from Genoa.

They need to find out

what caused the steel cables
that held up the bridge

to corrode.

Like the Hammersmith Flyover,

the Polcevera Bridge relied
on posttensioned steel cables.

They held the road deck together

and gave the cable stays
their strength.

Back in the 1960s,
to construct each stay,

workers cover its 52 cables
with steel ducts,

then cast the concrete
around them.

Once this sets,

engineers stretch the cables
to posttension the stay.

Finally, they inject
a concrete grout into the ducts

to cover the cables and
prevent the steel from rusting.

When scientists examine

the steel cables
inside the stays,

they also carry out
a forensic examination

of the ducts.

In some, they find cavities...

places where
the grout is missing,

leaving the steel cables
unprotected.

Once moisture entered
these cavities,

the steel wires would corrode,
become brittle, and break.

Investigators have not yet
announced the cause

of the bridge's collapse.

But one theory is that
corrosion of the cable stay

played a major role.

Autostrade, the company
that managed the bridge,

maintains that a failure
of the cable stays

was not the primary cause
of the collapse.

It says that,

with the amount of corrosion
the Zurich laboratory found,

the cables would still have had
enough strength

and would not have broken.

It also says
the ducts were present

and doing their job
of containing the cement grout.

If the bridge's collapse was, in
fact, due to its poor condition,

could Autostrade have detected
that it was about to fail?

Corroded steel is easy to see...
if it is exposed.

But when steel is embedded
in concrete,

it's very hard to check
that it isn't corroding.

Concrete is very difficult

to inspect non-destructively.

It's very difficult
to detect actual wire breaks

and really quantify
the extent of corrosions.

On the Polcevera Bridge,

engineers used
different techniques

to assess the level of corrosion
of the cables

inside the concrete stays.

One they appear to have relied
on heavily

is called reflectometric impulse
measurement technique,

or RIMT for short.

This method aims
to detect corrosion

by sending short electrical
pulses up the steel tendons

and recording
the reflected signal.

At the beginning,

everybody was very optimistic
with this method.

In 1994, '95,
some technicians from Autostrade

published some papers
where they said,

"Oh, the method is fantastic,
you can find everything."

But in 1997,

independent scientists
conducted a study

and found that RIMT could not
reliably detect faults.

Part of the prosecution case

is likely to question the use
of the RIMT method.

Autostrade says that since 2000,

engineers have used
a new version of RIMT

called RIMT2.

It says this is
completely different,

and does enable experts
to accurately assess corrosion,

and that its reliability
has been verified

by multiple international
scientific studies.

RIMT2 is not widely used
in the industry.

In 2017, Autostrade submitted
a detailed plan

to replace the cable stays.

But before the work could start,

the bridge collapsed.

The Polcevera Bridge

is one of the worst road-bridge
disasters to hit Europe

in over a century.

It raises troubling
and pressing questions

about how our aging bridges
are inspected and maintained.

The highway-building boom
of the 1960s and '70s

has left us with thousands of
bridges that are growing old,

and we don't know
how many contain hidden flaws

that have yet
to reveal themselves.

Prestressed concrete bridges,

with their concealed
steel wires,

are now a particular concern.

Berlin, Germany.

This is the Elsen Bridge.

It was built in the mid-1960s.

In the summer of 2018,

inspectors find
a 90-foot-long crack

in its concrete span.

Fearing that some of the
steel cables inside the bridge

have corroded and snapped,

they restrict traffic.

Many aging prestressed-concrete
bridges

have similar problems.

Some, like the
Hammersmith Flyover,

can be saved by
retrofitting external cables.

Others, like the Elsen Bridge,

will have to be demolished
and replaced.

So how likely is
another bridge collapse

like the one in Genoa?

You know, it's hard to say

how likely a collapse like that
would be,

but we have had
over two dozen bridges fail

in the United States since 2000,

for various reasons.

So there are concerns
these are bridges that...

We have a number of them
that need

to be repaired and replaced.

I think we have
a significant challenge.

We have underinvested
in our transportation system

for decades.

But safety has a price.

We could probably double
or triple what we're spending

across all levels of government

to really make those repairs
that need to be made.

Maintaining
this aging infrastructure

is a huge challenge,

but engineers are rising to it.

Modern cable-stay bridges
are designed

so that the cables can be
removed and replaced one by one.

And engineers have even
developed a solution

to one of the
most challenging problems

in bridge maintenance.

The main cables that run between
the towers on suspension bridges

are extremely difficult
to replace.

So what do engineers do

if they discover
that these cables are corroding?

Pittsburgh, Pennsylvania.

It has been called
"the City of Bridges,"

as it has over 400.

Richard Connors manages
some of them,

including the city's
Philip Murray Bridge.

Now, this bridge gets
about 17,000 vehicles a day.

It's very important.

This is a major physical asset
in the city.

The bridge depends
on its main cables,

which are over 85 years old.

This cable's about 13 inches
in total diameter.

It's made up of 4,864
individual wires, steel wires,

that were installed in 1933.

This film, of San Francisco's
Golden Gate Bridge,

shows how the
Pittsburgh bridge's cables

would have been made.

Individual wires were stretched
across the river,

bundled together,

then wrapped
inside a waterproof skin

to prevent moisture
seeping inside.

But when engineers opened up

the cables
of the Pittsburgh bridge

in 2009,

they found that water
had gotten inside.

We noticed, upon inspection
of the main cable,

there was significant corrosion,

to as much as 21%
to 23% loss in certain sections.

Most of the corrosion was
at the center of the bridge,

the lowest point,

where the water would just sag
to the low point.

To stop the corrosion,

engineers first seal the cable
in a new waterproof skin.

Then they install a dehumidifier

to blow dry air
through the cable.

This should reduce the humidity
inside the cables

to less than 40%.

Below this level,
steel won't rust.

The dry air travels up
to the top of the main cables.

And then it's blown through
the cable at very low pressure,

one to two pounds
per square inch.

And the dry air collects
moisture along the way,

inside the cable.

And the moisture comes out
right here.

Removing the moisture
from the cables

will stop them corroding

and hopefully extend
the life of the bridge

for years to come.

This ingenious system prolongs
the life of suspension bridges.

Engineers are using
other techniques

to help preserve structures
built of prestressed concrete.

Innovative technologies
like these

should enable our aging bridges
to remain safe

into the future...

if their faults are spotted
in time.

The sudden collapse
of the bridge in Genoa

is still felt in the city.

The loss
of this vital transport link

continues to cause traffic jams
and delays.

To get the traffic moving again,

planners are racing to build
a new bridge.

Its designers have
a big advantage

over previous generations
of engineers.

They have had the chance
to learn from earlier mistakes.

Bridges now are rarely built

with components that are prone
to single-point failure.

Regulations
on designing gusset plates

now require engineers
to calculate their strength.

And when building
posttensioned bridges,

greater attention is paid

to ensure cavities can't form
in the concrete grout.

We may never completely design
all weaknesses

out of the structures we build.

But every disaster
is a lesson we can learn from,

to make bridges safer
in the future.

Major funding for "NOVA"
is provided by the following:

To order this "NOVA" program
on DVD,

visit ShopPBS
or call 1-800-PLAY-PBS.

This program is also available
on Amazon Prime Video.