Original subtitles

Narrator: In this episode...

This is some of the most incredible engineering

that I've ever seen.

This is really something extraordinary.

Narrator: ...The planet's only floating railroad bridge...

We're essentially putting a rail on a marine vessel.

It's extremely exciting.

Narrator: ...And the pioneering historic innovations...

It's impressive. It's really cool to see this.

Narrator: ...That made the impossible possible.

-- Captions by vitac -- www.Vitac.Com

captions paid for by discovery communications

♪

king county, washington --

home to seattle and bellevue,

hubs for the nation's booming tech industry...

...Where the population is exploding

and traffic is gridlocked.

Engineer john sleavin lives

and works in a city pushed to its limits.

There's a lot of major corporations in seattle.

The traffic's getting worse.

The need for transportation is increasing,

and the need for choices are increasing.

Narrator: The solution could be to connect the cities by train,

but king county's unique environment

can make travel difficult.

♪

sleavin: One of the unique features of seattle

is its geographic terrain.

There are a number of lakes that all restrict where

and how you can place transportation services.

In particular, lake washington sits between

downtown seattle and bellevue,

both of which are high-tech areas that need to be connected.

Narrator: In a landscape known for vast bodies of water,

lake washington is the largest and deepest.

Here, traditional bridges just aren't possible.

But engineers in seattle have the answer.

♪

this once impassible lake has now been conquered

by the incredible I-90 floating bridges...

♪

...A concrete mega structure that actually sits

on the surface of the water unsupported by columns.

This project is incredibly unusual in that

we're applying systems that has not been done

by anybody else in the world ever before.

Narrator: These extraordinary buoyant bridges are capable

of carrying 142,000 cars a day.

Nowhere else in the world has this ever been done.

It's floating. It moves.

Narrator: With a massive 357,000 tons of reinforced concrete,

the twin floating bridges weigh more than 52,000 elephants.

All that weight is floating on 38 monster pontoons

with nothing but 210 feet of water below the surface,

crossing a span of over 1.5 miles

and capable of supporting the weight of rush-hour traffic.

And now engineers are entering

the most challenging phase of construction,

adding a state-of-the-art train line

and creating the planet's only floating railroad bridge.

Delalla: Once this system is commissioned and in operation,

this will be an engineering feat like no other.

Narrator: But this ambitious project

poses huge engineering challenges.

Is it possible to connect a railroad from land

onto a floating moving bridge,

if those rails were just attached on either side,

that continuous connection would experience

all those movements at one point

and would probably snap the rail.

Narrator: What happens when a high-voltage current

is introduced to a structure in water?

There is a risk of stray current escaping from the rails,

which could get into the critical bridge structure.

Narrator: And will the bridge be strong enough

to support 300-ton trains?

Sleavin: We can eccentrically load the bridge

and potentially crack it.

That would not be good.

Narrator: But the biggest challenge is keeping

this concrete superstructure afloat.

Stonecipher: It is very important

that if there's any water intrusion,

it won't sink the whole bridge.

Narrator: The first step for seattle's engineers

was to decide whether they had to build a floating bridge

or if they could go with a more traditional design.

Engineer jim stonecipher is very familiar

with the daunting complications of building on this lake.

Stonecipher: The lake is deep

and, being in earthquake country,

we need a good material to set our foundations in,

and that's just not available on the bottom of lake washington.

Narrator: Even if engineers were to sink support columns

through 213 feet of water,

they would then hit a soft lakebed made of silt and clay.

Pillars would need to go through another

164 feet of sediment

to reach a solid footing.

Add the column length needed above the water

and this becomes an incredibly expensive

and unstable structure.

Stonecipher: On the engineering side,

it would be difficult to build the standard cable stay

or other type of bridge in that area.

It takes a unique kind of bridge span

to span lake washington.

Narrator: So the engineers' only option

is to float the bridges.

But how can they ensure

the giant concrete structure doesn't sink?

♪

[ ticking ]

♪

on the caribbean island of curaçao,

local engineer albert zwueste is exploring

how a clever piece of engineering

could help the team at lake washington.

The island's main town, willemstad, was a perfect port,

but by the mid-1800s,

the deep natural harbor was creating a problem.

But the channel is 492-feet wide

and 49-feet deep with a soft sandy seabed,

making most bridges impossible to build,

especially one that allows the passage of ships

into the harbor.

♪

but when american ice merchant leonard burlington smith

sailed into curaçao in 1876,

he had the answer.

♪

♪

affectionately known to the locals

as the swinging old lady,

it's one of the oldest pontoon bridges in existence.

[ alarm buzzes ]

but the brilliant pontoon design

doesn't just allow for transit between each side.

The floating bridge has another trick up its sleeve.

♪

♪

the impressive 548-foot bridge span

is hinged at one corner

and swings open to allow boats into the harbor.

♪

♪

smith's design was brilliant in its simplicity.

And just beneath the pedestrian walkway lie the vital components

that will prove significant to the engineers in seattle.

Pontoon bridges have been around for millennia,

but few can compare to the swinging old lady.

♪

[ ticking ]

♪

now, on lake washington,

engineers are taking the idea of the pontoon bridge

and supersizing it.

♪

♪

narrator: King county, washington,

is one of the nation's fastest-growing regions.

There's a constant battle to keep the population connected.

But with the massive lake washington in the way,

engineers have been forced

to come up with an innovative solution --

a pair of gigantic concrete floating bridges

supported by pontoons.

The pontoons are large enough to support a highway

carrying 50 million cars a year

and the first ever floating bridge railroad.

Engineer jim stonecipher is responsible

for maintaining the bridge.

♪

stonecipher: So our solution to crossing lake washington

was building these pontoon bridges.

We make a concrete pontoon out of very dense concrete

with hollow cavities inside.

The concrete has enough buoyancy in it

to support the bridge and the traffic on it.

♪

narrator: During construction,

38 giant pontoons are positioned end to end,

giving the illusion of one massive bridge base,

each pontoon is divided into cells and sealed

with watertight hatches.

Two overhanging bridge decks

provide enough space for eight lanes of traffic

and two train tracks.

Stonecipher: One of the reasons we have so many pontoons

is for redundancy,

so that if one fails, it won't sink the whole bridge.

Each compartment has its own door

and sealed off, kind of like a ship,

and that way, we don't lose the pontoon bridge

and we can maintain traffic.

Narrator: Keeping these mega bridges afloat

is an impressive feat,

and it takes even more incredible engineering

to keep them from floating away.

Down below us, you're going to see the anchor cables

that help stabilize the bridge

and keep them in place.

And here comes one now.

You can see it just below the water.

The longest anchor cable is about 739 feet

in about 165 feet of water.

Narrator: Buried in the lake bed,

movements from the bridges

put pressure on these anchor cables,

causing them to fray.

Woman: I got a cable here.

Narrator: To prevent catastrophe,

a team of divers working at depths

of up to 165 feet

are currently replacing damaged components.

The anchor cables are very heavy,

and it takes a real big team

to get those anchor cables in place.

Narrator: So far, 32 huge new cables have been installed.

But as the seasons change,

so can the tension of the cables.

Stonecipher: From summer to winter here on lake washington,

as the lake raises and lowers,

anchor cables become slack or tight.

And we don't want increased pressure on the bridge

or we do not want the cables to be slack.

Narrator: A rupture in the cables

could spell disaster for the bridge.

So it's imperative that as the lake's water level changes,

the anchor cables are adjusted to the correct tension.

Stonecipher: So now we're down inside of one of 18 pontoons.

Watch your head. Little rough.

We're walking in through the anchor cables

in one of the segmented compartments of the pontoon.

And this is the anchor cable on the pontoon.

This particular anchor cable

is 579 feet long.

Narrator: Hauling such an enormous cable

in these tight spaces

calls for a compact yet powerful piece of equipment.

So this is a jack that we use to actually make the adjustment.

This is 150 ton ram.

We use this to either extend the cable out a little bit

or bring the cable in to maintain

a 65-ton average on the cable.

Narrator: The jack begins to pull,

all with the press of a button.

Stonecipher: You see the travel of the cylinder right there.

We're actually very slowly pulling the cable in.

Over here in the other room,

you can see we have this air gap between the jacking plate

and the jacking head.

And so we're actually pulling the cable into the pontoon.

So now we've gained about an inch,

so we're gonna put shims in here.

Narrator: These steel plates will bear the load

when the jack is released.

Right now, this time of year,

we only have to move it. Probably an inch.

In the spring and the fall, we'll move it about six inches.

Narrator: Adjusting the 110 anchoring cables is crucial

for keeping the bridge in alignment

and maintaining a safe road surface

for more than 50 million vehicles every year.

Stonecipher: So if we didn't have these anchor cables,

eventually this bridge would float north or south,

depending on which way the wind's blowing.

♪

narrator: Across a combined bridge span of three miles,

these 38 jumbo pontoons support a lifeline

for millions.

Without these incredible floating bridges,

the city would be gridlocked.

The first stage of this mega project is complete,

but engineers will face more impossible challenges

in their mission to create the world's first

floating railroad line.

Sleavin: As we transition to a floating bridge,

it tends to move a little,

and this could potentially disrail a train.

♪

♪

narrator: In seattle, an exploding population

has pushed the transportation network to its breaking point.

In an attempt to defy the impossible,

lake washington's colossal floating bridges

continue to evolve.

The next stage of the project is to install

a one-of-a-kind railroad across the north span.

Over 350 tons when fully laden,

the 55-mile-per-hour commuter trains

will carry more than 18 million passengers

across the bridge every year.

Construction of this groundbreaking project

is underway.

Delalla: To my right is the first set of tracks.

There'll be two sets of tracks here

when the construction is complete.

Narrator: But these new tracks create a unique danger

that engineer craig delalla must overcome.

Delalla: The rail system is powered

by a 1,500 volt d.C. System.

The return path for that current is the rails here.

Narrator: Water and electricity famously don't make a good mix.

If electricity escapes the tracks,

it could lead to disaster.

And surprisingly, the biggest concern is not electrocution,

it's corrosion.

Delalla: So any time you have a steel structure,

the risk of rust or corrosion,

which is the loss of metal, could impact the bridge.

By introducing rail to the floating bridge,

it further increases the risk of corrosion to the bridge

and the bridge structure.

Narrator: When that current discharges into the water,

it can corrode crucial components at the exit point,

threatening the bridge's integrity.

For this unprecedented construction project,

craig's team needed to invent brand-new methods

to eliminate destructive stray current.

Delalla: So you'll see here that there's multiple elements

of isolation, including plastic pieces here

between the track and the fastener.

We also coat the bridge with a special dielectric material

that is also a high insulator for electricity.

And so with these elements,

we are able to protect the bridge

from any stray current ever making its way onto the bridge.

Narrator: But with this mighty structure at stake,

the team isn't taking any risks.

Delalla: Have to move these barriers out of the way.

Should any stray current make it through

the first line of defense...

This is gonna be harder.

...There is a backup plan.

The anode assembly is the one without a tape here.

So we have here is the anode coming out of the water.

There's eight of these that hang 50 feet down into the water.

These are mixed metal oxide anode assemblies.

He's put current into the water, which is drawn into the bridge

and allows the bridge to polarize.

Narrator: Left unchecked,

stray current could enter the lake

through bridge metal,

but over 1,400 anodes

feed another electrical charge into the water.

This protective flow drives into the bridge structure

and holds the stray current at bay,

saving crucial components.

Delalla: Without corrosion control,

the life expectancy of the bridge could be shortened.

Applying it to a floating bridge like this

is really something extraordinary.

Narrator: With the danger of corrosion eliminated,

the team can begin to install the rail.

But now a new threat looms over the project.

So right now, we're about to go across the bridge.

Narrator: Keeping the mission on track

is engineer john sleavin.

Sleavin: As we transition to a floating bridge,

we are on a floating structure,

and just like any marine vessel,

it tends to move a little.

Now, for an automobile with rubber tires,

they can go across an angled point or a bump quite easily.

And this is very difficult for a train

because the steel rails need to be continuous.

They can't have brake points or angle points

that could potentially disrail a train.

Narrator: The floating bridge needs to handle

a range of movement caused by lake levels,

wind, and uneven traffic loading.

This stretching and twisting at the joints

constantly changes the transition angle,

threatening a track misalignment between lake and land.

For 800 passengers on a speeding train

close to heavy traffic and deep water,

this could be fatal.

We need to find a solution across that expansion joint.

That's critical to the operation of the rail.

Narrator: To evade a devastating derailment,

john's team will need to connect

with the innovators of the past.

♪

♪

narrator: In the pacific northwest,

engineers are designing a floating railroad bridge

that will connect the two sides of lake washington,

but changing lake levels, wind, and uneven traffic loads

can cause unwanted movement

and threaten the integrity of the bridge.

To keep things running smoothly,

the team will need to go back in time.

♪

[ ticking ]

♪

norway -- known for its vast fjords.

[ train horn blows ]

civil engineer berthe dongmo-engeland

is on the hunt for a relic

from the golden age of locomotive travel.

Scottish engineer thomas bouch

encountered a similar problem

when extending great britain's railroad lines,

but in 1849, he came up with a solution

that would roll out across the continent...

The train ferry.

Wow, look at that. This is so amazing.

Narrator: Bouch's concept of a ship with inset rails

enables locomotive wagons to float across water.

This ferry in norway follows bouch's design.

But the ferry itself is only part of the story.

Once the wagons have reached the end of the line on land,

there's still the problem of getting them onto the barge.

A misalignment of the track would be catastrophic.

And with lake water levels constantly changing,

bouch needed a clever solution.

The adaptability of bouch's hinged ramp is a concept

that will prove instrumental for the team in seattle.

With the help of an enormous winch system,

the span is lowered and the tracks are perfectly aligned.

♪

connecting these tracks provided a lifeline

for the region's industry, with an amazing roll-on,

roll-off solution.

♪

the groundbreaking train ferry

and hinged ramp configuration kept cargo wagons on the move

and changed locomotive transportation forever.

♪

[ ticking ]

♪

170 years after bouch's inspired idea,

seattle's greatest engineering minds have developed a system

that he could have only dreamed of.

♪

sleavin: We call this a track bridge

because we're bridging over that expansion joint.

This unique design has to contend with conditions

not seen on any other railroad bridge in the world.

Sleavin: We had looked at some other systems,

but this has two more degrees of motion

that don't exist on other bridges.

What we have is a system to try to handle

all those different levels of movement,

but rather than happening at one point on the rail,

we've spread that over a longer distance.

As we go underneath here, we can see some different elements.

Each of these wings have a curve to them.

That means when the bridge goes down

because the lake level goes down,

these wings will rotate up.

And when the opposite happens, these wings rotate down.

Narrator: These curved wings work in unison

with a complex range of components

to bend the rails into a gentle arc

and keep them level over the moving angle points.

Eight of these 43-foot-long track bridges

will cross the four hinges between fixed

and floating segments,

allowing a smooth transition for the trains.

After the complex track bridges are assembled,

the system is thoroughly tested

at a special facility in colorado

to ensure safety, speed, and efficiency.

Sleavin: Our tests revealed that at our designed speed,

our maximum speed of 55 miles an hour,

the track bridges were good.

The stresses in the rails were fine,

and the ride for the passengers was comfortable.

If we didn't have this track bridge,

it probably would have been impossible to put trains

across the bridge. At the very least,

we would have had to stop the trains

and almost just bounce across it.

At its worst condition,

that may have even caused the trains to derail

at that low speed.

Narrator: With this incredible design,

seattle's engineers are one step closer

to conquering the seemingly impossible.

Sleavin: This is a completely new and unique solution

addressed just for this specific location.

Nowhere else in the world

are there any track bridges like this.

♪

narrator: But to realize their dreams

of crossing lake washington by train,

engineers face one final challenge.

Sleavin: We'll have four-car trains,

so when two trains are passing each other,

that puts a lot of stress on the concrete.

Narrator: And to create more impossible engineering,

the team will have to turn to innovators of the past.

Wow. I'm completely awestruck by this building.

It really is impressive.

♪

♪

narrator: Seattle, washington --

home to the world's only twin floating bridges.

And these superstructures

are about to get another world first.

For the project's final phase,

the planet's only floating railroad line

will cross an enormous 1.5 mile span over lake washington,

revolutionizing seattle's transportation network.

♪

but these concrete bridges will need

to support the weight of multiple train cars.

We have thousands of daily commuters that rely on this,

as well as sports fans and university students.

Narrator: Engineer john sleavin is in charge of the project.

Sleavin: So when these trains are fully loaded,

we'll have four-car trains,

and each car will weigh approximately

175,000 pounds.

So when two trains are passing each other,

essentially doubling the load, which is very heavy in one spot,

that puts a lot of stress on the concrete.

Narrator: A massive four-car train at maximum capacity

could weigh 350 tons.

When two trains pass, as much as 700 tons

could bear down on a short stretch of the bridge.

This crushing load can put enough stress on the concrete

to cause catastrophic ruptures.

Sleavin: We've done a lot of structural analysis

on these loads

and realizing it takes a lot of stress into the bridge,

and so to preserve the lifetime,

we need to figure out how to strengthen the bridge.

Narrator: So engineers will have to reinforce

the concrete to withstand

the full force of hundreds of daily train crossings.

It's a challenge that might be impossible

without the innovators of the past.

♪

[ ticking ]

♪

the city of lourdes, southern France --

an important holy site

for catholic pilgrims from around the world.

Civil engineer patric nagle

is going underground in search of a structure

with a capacity for a colossal congregation.

♪

whoa.

[ singing in italian ]

narrator: This is the basilica of st. Pius x.

Nagle: I'm completely awestruck by this building.

It really is impressive.

Narrator: It's built beneath the city

to protect views of the sacred site above ground.

Nagle: What is striking about this magnificent building

is a wide open space -- no central columns, no supports.

And we can see the structural form of the 29 arches

running the length of the building,

and this creates a usable space,

which can accommodate 25,000 people.

Narrator: But this subterranean structure seems to defy gravity.

The flatness of the arches is maybe something

we wouldn't expect. A typical arch is much more like this.

These are very flat arches.

It is clear that something special here

is happening from an engineering perspective.

Narrator: This engineering enlightenment

came from eugène freyssinet.

In 1928, he perfected

a method of concrete strengthening,

using strands of steel cable under high tension.

This technique, known as post tensioning,

provided support for concrete beams of unprecedented spans.

But hidden within the concrete,

it's not easy to see how this system works.

So here we have a simple model.

We have a number of wooden blocks,

which represent a concrete beam, resting on two supports.

And you will see a string running through the beams,

which is simply there to hold together the blocks.

If I apply a load to the beam,

you will see that it is put into bending,

and you can see cracks opening up within the concrete.

So the secret is to put in compression

before the load is applied.

Narrator: To achieve the compression needed,

post tensioning must be introduced into the beam.

Nagle: So in this case, it is provided by string

and a tourniquet to tension the string.

So I have now tightened up the stressing, if you like,

and we put this back on the supports.

So this time, we can apply double the load,

and we can see that there is no movement and the beam

does not go into bending.

This gives a much more efficient use of the concrete

and allows us to provide bigger spans

and more efficient use of the material.

Narrator: By compressing the beam,

its density and strength are increased,

a method that could prove vital for seattle's bridge engineers.

So essentially what we are doing in the beams

and the arches behind me here is applying an external force

to increase the load-bearing capacity of the structure.

The tendons that we see in here

are formed of steel strands

housed within ducts and stressed by hydraulic jacks

after the concrete has hardened.

Narrator: The strengthened concrete provides

an expansive ceiling

without the need for obstructive pillars.

Instead, arches span the chamber

and descend to the floor close to the edge.

Nagle: Looking at the structure today,

there are no cracks. It is very finely designed

to make sure we maximize the capacity of the concrete.

Narrator: This long, shallow vault

would not have been possible

without freyssinet's extraordinary

post tensioning solution.

Without it, we would not be able

to achieve some of the beautiful

and brilliant structures we see around us today.

♪

[ ticking ]

♪

narrator: Back at seattle's floating bridges,

engineers are applying freyssinet's

groundbreaking technique on a record-breaking scale.

♪

♪

narrator: For the final phase of the I-90 floating bridges,

engineers are constructing the planet's first

and only floating railway line

to cross the enormous 1.5-mile span over lake washington.

Just like at the basilica of st. Pius x in France,

they're fortifying concrete through extreme compression.

So what we've done to strengthen the bridge

is put post tensioning cables in the bridge.

♪

narrator: But the super-sized system on lake washington

is using some of the longest

post tensioning cables the world has ever seen.

Sleavin: These cables are approximately 4,000 feet long,

running from one end to the other.

One continuous cable in each one of these conduits.

Very unique in this situation

that we've added 4,000 feet of post tensioning.

Most post tensioning is much shorter --

100 to maybe 200 feet.

Narrator: With a combined length of over 78,000 feet,

20 of these steel super cables

are thread through the pontoons,

spanning the north bridge's floating platform.

Powerful hydraulic jacks then pull them tight.

Sleavin: So what we've done is we've put conduits through the bridge,

those conduits are then used to string the cables through that,

and we pull those cables tight.

Narrator: But keeping a post tensioned mega cable in place

requires oversized anchors.

So here are the reaction frames inside the bridge.

These are the big steel frames that we pull tight against

when we tension the post tensioning cables.

So their job is to hold the post tensioning cables tight

so that we put that force into the bridge to strengthen it.

Narrator: 20 massive reaction frames weighing 7.5 tons each

are pulled inwards by the tensioned cables.

Like huge bookends, they squeeze the bridge

from either side.

Compressing the concrete increases its density

and strengthens the bridge,

allowing it to take an even heavier load.

Applying extreme compression to the structure

has to be executed with pinpoint precision

to within 1.5 millimeters.

Sleavin: These frames are critical.

Without them, there's no way

we could have added post tensioning.

Narrator: The result is a super-strong floating platform

capable of withstanding the 700-ton point load

of two trains crossing simultaneously.

Sleavin: So this is an incredible solution to the problem,

extremely long post tensioning cables added to a bridge

allowing us to add the trains to the surface of this bridge.

♪

narrator: The I-90 floating bridges represent

impossible engineering on a staggering scale.

Every stage of this groundbreaking enterprise

poses extraordinary challenges.

♪

delalla: There are many facets and many people

involved in this design,

and it's been really great working on this.

I'm really proud to see it coming together.

Narrator: By building on the work

of the pioneers of the past,

overcoming huge challenges,

and pushing the boundaries of innovation...

This is some of the most incredible engineering

that I've ever seen.

It's extremely exciting for me and my team

to be able to work on such not only an important project,

but a unique project.

Narrator: ...The engineers are succeeding

in making the impossible possible.