Engineer Explains Every Bridge For Every Situation

[Narrator] This is an example of a suspension bridge.

And this is a cable-stayed bridge,

and an arch bridge, a beam bridge,

a truss bridge, a cantilever bridge,

a movable bridge, a hybrid bridge, a floating bridge.

There are a lot of kinds of bridges,

but the all serve the same function.

So a bridge is a structure that's designed

to be able to carry movable loads

from one side to another.

It needs to be able to traverse or span a distance,

whether it's caused by a body of water

or another road that's beneath it.

[Narrator] This is the guy who decides

what kind of bridges to make.

My name is Dr. Nehemiah Mabry.

I am a structural engineer and educator.

[Narrator] And what is a structural engineer?

A structural engineer is an engineer

that specializes in designing structures

or any types of stable bodies

that's able to resist loads or weights of any kind.

[Narrator] So why are there so many

different types of bridges?

Bridges are designed from the top down,

but they're built from the ground up.

And so we, as engineers begin to think

how much load or what type of weight

does this bridge need to carry?

And how would this deck then transfer that weight

whether it's a moving truck, or a resting parapet,

or concrete divider?

[Narrator] How do you decide what kind of bridge to build?

Right, so there's a couple of things

we look at when we want to design a bridge.

So you look at skew angle.

You're going to look at stationing or work points.

You're also gonna look at what we call a typical section,

which is what is it gonna look like

once we slice this bridge in half,

what do we want that typical section to look like?

And then from there, once we get that,

it's a matter of calculating the loads

that the various lengths of girders

or various length of materials that are necessary.

We're gonna also look at the material that we wanna use

to then be able to produce

that typical section that we desire.

They're primarily two main forces that each component

each member of a bridge is gonna experience.

It's either a tension force or a compression force.

A tension force is basically a force

that will attempt to try and stretch the member

or stretch the component of the bridge.

And a compression force is a force that will attempt

to shrink the member or the component of the bridge.

And so those are the two main forces

that any member of the bridge will have to experience.

Some members will have to experience both at the same time.

For instance, if you have a beam,

now a beam is a long member that is meant to span

from one support to the other support.

In many typical cases,

the beams are supporting the loads

transferred from the deck.

Well, in the middle of the span,

these beams are often bending

or experiencing stresses or a force that causes a bend.

Well, in this case,

you're gonna have the bottom of the beam

that's gonna be tempted to be stretched.

That is a tension force.

However, the top of the beam as it bends

is gonna be experiencing a compression force,

because at the top, it's gonna be shrinking.

An example of a component experiencing

maybe just compression would be the columns.

This is a member or a part of the substructure,

which is connected directly to the foundation.

As loads from the beam rest on the columns,

the columns are gonna be experiencing compression,

or more specifically, axial compression,

along the length of the column.

Another case where tension may be the only thing,

given a case of a suspension bridge.

These are bridges where you see a draping cord or cable

going from one support to the other.

And then hanging from those draping cables

are additional cables

that are then attached to the deck to hold up the deck.

Well, these particular cables are experiencing pure tension,

because it is experiencing a force

that is stretching those cables.

[Narrator] So here's a bunch of different bridge types.

Suspension bridges, they are typically supports,

or towers, connected by a draping cable.

And from those draping cables on both sides of the bridge,

there are vertical cables

that then connect to that draping cable down to the deck.

And the way that the deck is then resisted

by those vertical cables and tension

connected to the draping cables,

that then transfer the load to these towers.

[Narrator] What's the best example of a suspension bridge?

The most recognizable example,

particularly in the United States,

is the Golden Gate Bridge.

The Golden Gate Bridge is that bright, red,

that beautiful landmark out West

that has these two very strong draping cables

going from the north to the south tower.

And from those cables,

they're typically made of steel wires

wound and compacted together,

which make them very, very, very strong in tension,

draping vertically from those main cables down to the deck.

What's so great about this suspension bridge

is that it's in a region

that is known to experience an earthquake.

Being that they are supported by cables,

for the most part,

there is the ability for bridges like this

to be able to have some sort of give and not be so stiff.

And be able to move, even if the ground shakes,

without severely damaging itself.

The north and south ends of a suspension bridge,

particularly the Golden Gate Bridge,

are referred to as piers.

And these piers act as the substructure,

which needs to resist those forces

that is being applied to it by these main cables.

And so these big piers need to be extremely reliable,

because they are essentially resisting

half of the bridges weight.

[midtempo music]

[Narrator] What's another type of bridge?

Another similar or cousin, I like to think of,

to the suspension bridge is a cable-stayed bridge.

In this case, they're similar in that they are towers,

primary supports, that then connect to the ground.

But instead of having two main draping cables,

the cables are then attached directly from those towers,

from those piers, to the deck.

Each side of the tower sort of acts as a counterweight

to the other side.

And so this is not only helpful in the way

that the bridge remains in equilibrium,

but this is also something

that many have used to their advantage

throughout construction of those bridges as well.

By holding things in place as things are being built.

What it allows you to do is use cables

to then act as a resisting force

to the suspended or a hanging deck.

Because the deck in and of itself

is not required to have anything beneath it,

your cables are able to go much, much longer

than you could fabricate anything else

to serve as a super structure.

[Narrator] So cable-stayed would be really useful

for long or tall bridges?

Yes, so the Millau Viaduct

is actually the tallest bridge in the world.

Being that you have each pier

that is about the height of the Eiffel Tower itself.

And from it, you have your cables

that are keeping the deck in place,

making it a very aesthetically appealing bridge,

but also using the counterweights of both sides

to be able to have a much longer span in between piers

than you would with any other bridge type.

There's only but so long you can design a plank or a girder.

And so, because you don't want to have so many supports,

also when you're traversing or you're building a bridge

across a large body of water,

the energy and the effort it takes to drill piers,

or piles, down beneath the surface of the water

and to build these supports,

makes it more of an incentive to have less supports, right?

And because we want less supports,

we're then going to consider a bridge type

that can allow us to have longer spans.

Hence, the suspension bridges and even more so,

the cable-stayed bridges becomes an attractive option.

Not to mention that it just looks cool.

[Narrator] So that means some cable-stayed bridges

might have only one support?

Right, exactly.

So the Langkawi Sky Bridge is a curved pedestrian bridge.

It's just beautiful.

It has the aesthetic value right there.

It's an interesting design challenge when you look at,

How can we put this manmade structure in this place,

but yet at the same time preserve the natural beauty

of that environment?

So it has then advantageous to then say,

How can we have the minimal amount of piers, in this case,

even one pier positioned and act as a support

for the entire bridge?

Which is why this truss pier is singularly located

and then attached to cables, eight cables to be exact,

that are necessary to support the deck in various points,

thus providing the support needed to support the traffic.

[Narrator] Can you combine these different bridge types?

Yes, so the Brooklyn Bridge is a good example

of a hybrid between a suspension bridge

as well as a cable-stayed bridge.

So you have your very, very large towers, your pillars.

Your piers that have existed for a very, very long time

that have these draped cables from one to the other.

From these draped cables there are vertical cables

that are attached to the deck,

as you would see in any other suspension bridge.

But if you look very, very closely,

you will also see that some cables are at a diagonal.

They're also attached from the deck

directly to these tower piers,

which makes it more of a hybrid

between your suspension bridge

as well as your cable-stayed bridge.

The cables of the Brooklyn Bridge

are comprised of 19 strands that are then bundled together.

And then each of these are composed

of about 278 wires per strand.

And together they actually produce

a much stronger tension resistance

than if you just had a single solid strand.

So those towers on the Brooklyn Bridge

are stone towers as well

using limestone, granite,

and other rocks that have proven

to be able to stand the test of time,

because of their durability and strength and compression.

[midtempo music]

[Narrator] What is our third bridge type?

Arch bridges are primarily resisting the load

in axial compression along the arc.

Within arc bridges themselves,

you could have a spandrel arc bridge,

where the deck is above the arch.

Or there are cases where the arch is elevated

and the deck is actually through the arch.

And so we call that a through arch bridge.

Particularly, stone bridges stand the test of time,

because stones are a natural material.

And so using a material

that has already went through the years of weathering,

countless years of reformation,

and really been able to remain what it is over millennia.

So I think that's one of the reasons why stone bridges

and even arch-stone bridges

is able to stand the test of time.

Because it's simply asking of the material,

what the material has already been doing

for many, many, many years.

When it's designed appropriately,

and the geometry is nailed,

you can pretty much have a bridge that is invincible.

[Narrator] What's a good example of an ancient arch

that's still standing today?

So the Aqueduct is another example of a stone-arc bridge.

Here we have series of stone arcs

that allow the force to be transferred

actually throughout the arc.

These smaller arches then, of course,

transfer the load to the bigger arches.

It takes advantage of the opportunity

of breaking up the spans,

such that so much isn't required of a single arch.

It's also redistributing the load along,

across the smaller arches,

so that as they're transferred upon the bigger arches,

the larger arches can then act as piers

that are spaced further out apart giving greater spans.

[midtempo music]

[Narrator] What then is a truss bridge?

[Nehemiah] A truss is a structural type

that is comprised of several different elements.

The triangular shapes are necessary,

such that each element of the truss

is experiencing a pure tension or a pure compression.

[Narrator] Can you give us an example?

[Nehemiah] So the Sydney Harbour Bridge

employs two truss in the shape of an arc.

And it also is the type of arc

that we refer to as a through arc bridge.

Such that the deck is not above the arc,

but it's in line or beneath the apex of the arc.

There are also vertical struts

that then connect from the truss arc

that are to support the deck through tension.

This aligns with the use of the elements

within the truss as well,

which are also designed and put in place

to be able to only experience purely axial forces,

whether it be in compression or in tension.

[Narrator] Aren't truss bridges used by the military

for rapid deployment?

Absolutely, the Bailey Bridge is sort of a military bridge

that is prefabricated, pre-built,

away from the site that it is to be put in place

to transfer vehicular or pedestrian traffic.

It really came out of the need for military units

to be able to quickly build a bridge,

for it to be light enough for them to carry with them

as they move from one place to the other.

And it's sort of like an Erector Set.

Piece by piece, prefabricated, put together,

and can be constructed without the use of heavy machinery,

without the use of a crane or any pile drivers of any sort.

[midtempo music]

[Narrator] what type of bridge is this?

So a cantilever is a structure

that is supported on one end,

where there is a fixed side or support,

whether beams or other types of support members,

that are resisting the rotation of the deck.

And so cantilever it's incredibly important

to make sure that the fixed end is strong enough,

and that the flexing extended member

has the ability to resist the tension on the top

and the compression on the bottom.

Perhaps the longest cantilever bridge

is the Pont de Quebec Bridge, there in Quebec, Canada.

Steel, unlike concrete, is extremely strong in tension.

It can resist forces that attempt to stretch it.

And so while we are making beams

that perhaps are going to experience a lot of bending,

we want something that can resist the compression

at the top and the tension at the bottom, or vice versa,

depending on how it's expected to bend.

[Narrator] So how do you make a material

that resists heavy loads?

We've gotten very sophisticated

in how we are anticipating the types of loads

that these beams are gonna experience.

And so instead of just simply

putting steel inside of the concrete

so that you have that reinforced composition,

there's another type that we call prestressed concrete.

We take the steel, before we can bring the concrete in,

and we stretch out the steel.

And while in that tension position,

we then pour the concrete around it

and create a beam with the tensioned steel.

And after that concrete has hardened,

then we release the tension off of that steel,

such that now the steel wants to then retract back

to its original position.

But because it now has concrete around it,

it is actually prestressing

before it's experiencing any load,

it's already applying a stress load on the concrete.

And so now you have a bow where the concrete or the beam

is gonna want to naturally, without any load on it,

bow up a little bit.

And so that's a way to sort of anticipate

the fact that when loads go

and we put these beams in place,

the loads are gonna wanna make the beam deflect downward.

But because we put what we call a camber,

we put camber into this prestressed beam,

we sometimes are able to achieve

no deflection at all at the beginning of installation.

[midtempo music]

[Narrator] What other bridge types are there?

When we're talking about plank bridges,

then those planks are acting as beams.

And so they have to resist what we call bending moment.

And so a bending moment is what causes these beams,

in this case, to experience both tension and compression.

Compression, being at the top as it begins to bend.

And tension, being at the bottom as it begins to deflect.

So U Bein Bridge is the largest,

wooden footbridge in the world.

It has upwards of 1,000 wooden pillars

that have been driven throughout the water

along the length of the bridge.

It has approach spans

that were originally made of stone or brick,

but have then also been replaced by wood.

Over time, the weathering

and the deterioration of certain pillars

have been replaced with concrete.

But nevertheless, this is primarily

an entirely constructed wood bridge

that offers amazing views there in Myanmar.

Along the length of this bridge

there are four wooden pavilions at equal distance.

I don't know if you would call them maybe checkpoints

along the length of the bridge,

being that it's so long.

[midtempo music]

[Narrator] So what about this bridge

in Enoshima, Japan?

What type is this?

This bridge is another pure example of a bridge type.

There aren't separate members that you're connecting

and transferring load with,

but it can almost be looked at as a frame in and of itself

that put in place.

And you look at it and that's the money shot

right there, right?

Looking at the super, steep incline and decline

that it has when you're approaching the bridge.

That shot definitely it helps exaggerate

the steepness of the grade.

However, it is a lot steeper than your typical bridge

on it's a vertical curve as we call it.

It has about 5%, 6% slope, which is quite the slope

for any passenger or vehicle driver

may typically drive on.

But it's necessary,

because instead of being a movable bridge,

they decided to make it high enough

with enough vertical clearance,

such that the ships can then pass there in Japan

right underneath it,

without any needs of actuators or mechanical devices

to be able to move it.

[midtempo music]

[Narrator] And then there are movable bridges.

[Nehemiah] Movable bridges are things that we see

that are designed to be able

to not only transfer the load sturdily,

continuously using a deck that connects across the chasm,

but it's also designed by some mechanical powered devices

to be able to move and to lift up,

such that the waterway that is traversing

can allow the passage of boats or any other vessels

that need to pass by.

It can then also be lowered back in place,

so that the loads can continue to travel across it.

[Narrator] Let's hear an example.

So the Somerset Bridge is a very, very short timber bridge

to be able to cover a passageway in Bermuda

that connects Somerset to Warwick, Bermuda,

and it's a movable bridge as well.

This bridge doesn't need to be

completely moved out of the way.

There aren't big ship or water vessels

that are passing through.

In fact, this movable bridge is operated by hand,

unlike other movable bridges,

which may use hydraulics or other mechanical means.

It's just hand-crank bridges that allows the deck

to open up just perhaps two or three feet,

just wide enough for a mast of a small boat

to be able to pass through.

[Narrator] What about a bigger example?

Okay, yeah, okay, I got you.

Gateshead Millennium Bridge,

this bridge is actually a pedestrian bridge

not meant to transfer any vehicle traffic,

but walkers, joggers, cyclists are able to travel

that curved, outer semicircle of the bridge.

It's amazing because it has sort of that eye look.

It's referred to as the Winking Eye as a nickname.

It's just incredible.

An incredible just marvel.

Particularly, when it first went up.

It was one of the first of its kind

that we ever seen like that.

Instead of actually moving and separating

two components of the bridge,

it is shaped in a way such that the semicircles

are able to rotate.

And when it is in an upright position,

it then creates enough vertical clearance

for ships and sea vessels to then pass underneath.

And then it's simply rotated back into its place.

[Narrator] What about this sci-fi looking bridge?

The Rolling Bridge, it's pretty cool.

It looks like a little roly-poly bridge.

And what's interesting about this bridge

is that as in a lot of movable bridges,

this has several joints throughout in triangular sections

such that it's able to curl upon itself

with multiple bending locations

throughout the length of the bridge.

So as it curls up, using the power of hydraulics,

being a system that employs the pressure of fluids,

it's able to then, at its size, be deployed very quickly,

rolled up, put back down,

again, nice and neatly.

The triangular segments fold on top of each other

and then roll back out,

just as easy as you would see a roly-poly do it.

[Narrator] Is there one bridge that puts a bunch

of these examples together?

The Tower Bridge is another kind of hybrid bridge.

It's a suspension bridge.

And then it's also a movable bridge in the middle.

On the outsides of the towers,

there are suspension cables vertically supporting the deck.

But then there is a walkway bridge

at the top of the two towers.

Beneath there is a drawbridge that lifts up

allowing the passage of ships.

But what's interesting about the Tower Bridge

is that there is also a walkway bridge at the top,

which in and of itself acts as a connecting member

between the two towers.

Because the outsides of the towers are suspension bridges,

they are experiencing sort of a pulling force

that's attempting to pull them apart.

Therefore, the walkway that is connected at the top

is acting as a connecting member

that's experiencing tension itself,

keeping the top of the towers also aligned and in place.

[midtempo music]

[Narrator] What is the same

about all these different bridges?

All of these different types of bridges

are essentially doing the same thing,

they're carrying load

and they're transferring it to the ground.

They're just accomplishing it in slightly different ways.

[Narrator] So how do you make sure a bridge

stays up for a very long time?

A lot of infrastructure that has been built years ago,

expiring, for lack of a better word,

around the same time,

causes decision-makers to have to determine,

what will we replace entirely

and what can we afford to just simply repair?

It's not uncommon for bridges to be designed

with what we call a 50-year design life cycle.

Bridges incorporate a lot of reliability measures,

which determine, with a high reliability,

this bridge would be safe for this amount of time,

say 50 years.

That doesn't mean that this bridge

can no longer perform its function after that design life.

However, this is just the highest interval of confidence

for which that structure can be designed.

You may see in our country,

particularly in countries

that had large massive infrastructure projects,

50 or so years ago,

now at a point where a lot of our heavily used bridges

and structures are requiring frequent maintenance.

There needs to be a more frequent inspection cycle.

There needs to be perhaps some modifications.

Removal of the concrete that has spalled off,

or deteriorated, or crumbled,

and replaced with something more durable,

such as weather-protected steel, which we have,

or epoxy coated deck.

And so there are rehabilitation efforts

that can just as well reduce or restore

the ability of the bridge,

as opposed to just tearing it down.

Perhaps there may be cases where an engineer

may go and inspect, they analyze,

they'll look at how it may have deteriorated

or fatigued over its life cycle.

And they won't recommend that it be torn down,

but they may recommend that it be posted.

And that instead of carrying a certain amount of load,

now it be determined that it can only carry

a fraction of that load,

and continue to then function safely for the public.

Restoring urban infrastructure is a huge issue,

because of the fact that the users increase

at such a higher rate.

And that's why in New York,

a lot of the bridges,

the age of the bridges

as well as the levels of use that they experience,

require more frequent intervals of repair,

maintenance, and inspection for the public.

We have a high number, an unacceptable number,

of either structurally deficient

or functionally obsolete bridges,

which is why there has been widespread support

for the restoration and improvement

of our nation's infrastructure.

So the choice is either preservation and repair

or completely new construction.

[midtempo music ending]