Every Kind of Bridge Explained in 15 Minutes

Practical Engineering
21 May 202417:35

Summary

TLDRThe video script from Practical Engineering explores the diverse and fascinating world of bridge engineering. It delves into various types of bridges, from simple beam bridges to complex suspension and cable-stayed designs, explaining how each manages forces and spans distances. The script highlights the structural and aesthetic considerations in bridge construction, showcasing the ingenuity behind truss, arch, and tied arch bridges. It also touches on moveable and floating bridges, emphasizing the creativity and problem-solving inherent in engineering to overcome geographical challenges and accommodate various transit needs.

Takeaways

  • 🌉 Bridges are essential for overcoming challenging terrains and connecting people and resources across gaps.
  • 📐 The simplest bridge structure is the beam bridge, which consists of a horizontal member across two supports, often made from concrete, steel, or box girders.
  • 🔨 Truss bridges are lightweight and rigid, made of smaller elements that distribute load axially in compression or tension, allowing them to span greater distances.
  • 🏗️ Bailey bridges are portable and easy to assemble truss bridges, originally designed for World War II and still used today for temporary crossings.
  • 🌿 Most covered bridges are timber truss bridges, with roofs and sidings to protect the wood from environmental damage.
  • 📐 Trestle bridges are similar to trusses but have a series of short spans with frequent supports called trestles.
  • 🏰 Arch bridges use a curved element to transfer weight to supports using compression forces, and are popular for their efficiency and aesthetic appeal.
  • 🌉 Cantilever bridges balance the structure's weight over the supports using beams or trusses that project horizontally, allowing for impressive spans.
  • 🚀 Cable-stayed bridges use cables attached to tall towers to support the deck, offering a variety of configurations and dramatic shapes.
  • ⏳ Suspension bridges are iconic for their long spans and slender appearance, supported by main cables and towers, and often require stiffening elements to reduce movement.
  • 🛠 Moveable bridges, like bascule, swing, and vertical lift bridges, are designed to allow for the passage of ships and can be found in various unique designs tailored to specific locations.

Q & A

  • What are the challenges that engineers face when designing bridges?

    -Engineers face challenges such as topography, including wet, steep, or treacherous terrain, as well as areas prone to natural disasters. They must also manage the incredible forces involved in bridge construction, ensuring that the structures can support the weight of traffic and the bridge itself.

  • What is a beam bridge and what are its limitations?

    -A beam bridge consists of a horizontal member across two supports. It can be made from various materials like rolled steel or concrete. Its limitation is that it can't span great distances because the required girders would be too large and heavy to support their own weight, let alone traffic.

  • How does a truss bridge differ from a beam bridge in terms of design and functionality?

    -A truss bridge uses an assembly of smaller elements to create a rigid and lightweight structure. Unlike a beam bridge, truss members experience primarily axial forces in compression or tension, which simplifies the design process. Trusses can span greater distances than solid beams due to their weight reduction and efficient use of material.

  • What is a Bailey bridge and why was it designed?

    -A Bailey bridge is a type of temporary truss bridge designed to be portable and easy to assemble. It was designed during World War II for rapid deployment in military operations, and it is still used today as a temporary crossing when a bridge fails or is closed for construction.

  • How do arch bridges differ from other types of bridges in terms of the forces they use?

    -Arch bridges use a curved element to transfer the bridge's weight to supports using compression forces alone. This is different from beam bridges that are loaded perpendicularly or truss bridges that experience both compressive and tensile forces.

  • What is the keystone in an arch bridge and why is it important?

    -The keystone is the topmost stone in a stone arch bridge. It is crucial for keeping the entire structure standing as it locks the other stones into place, providing the necessary compression to maintain the arch's form.

  • How does a cantilever bridge work and what is its significance?

    -A cantilever bridge uses beams or trusses that project horizontally, balancing most of the structure's weight above the supports rather than in the center of the span. This technique is significant because it allows for longer spans and was used in the Forth Bridge to achieve the longest span in the world at the time.

  • What is a cable-stayed bridge and how does it differ from a suspension bridge?

    -A cable-stayed bridge supports the deck from above through cables attached to tall towers or spars, forming a fan pattern. Unlike a suspension bridge, which uses cables or chains to hang the deck below, cable-stayed bridges attach the deck directly to each tower, providing a unique appearance and structural integrity.

  • What is an extradosed bridge and how does it combine the benefits of a cable-stayed structure with girders?

    -An extradosed bridge is a design that combines the benefits of a cable-stayed structure with girders. It uses internal tendons to keep the concrete in compression and then pulls these tendons out of the girder, attaching them to a short tower. This acts more horizontally to hold the girders in compression, providing the stiffness needed to support the deck.

  • How do suspension bridges manage the forces acting upon them and what are their key features?

    -Suspension bridges manage forces through massive main cables or chains that hang the road deck below with vertical hangers. Key features include towers on either side that prop up the main cables and immense anchorages that transfer the bridge's weight into the foundation, keeping the cables from pulling out of the ground.

  • What are moveable bridges and how do they accommodate for the passage of boats and ships?

    -Moveable bridges are designed to allow the passage of boats and ships by physically moving out of the way when needed. Types include bascule bridges that rotate upward, swing bridges that rotate horizontally, vertical lift bridges that raise the deck upward, and transporter bridges that shuttle a small length of deck across a river.

  • What is a floating bridge and how does it differ from traditional bridge construction?

    -A floating bridge uses buoyant supports, such as hollow concrete structures as pontoons, eliminating the need for a foundation. This differs from traditional bridge construction that relies on fixed supports and foundations. Floating bridges are used in specific applications where space or funding is limited and are less common due to the engineering challenges they present.

Outlines

00:00

🌉 Introduction to Bridge Engineering

The script begins by highlighting the challenges posed by Earth's diverse topography and the ingenious solutions provided by bridges. It emphasizes the functional beauty and structural variety of bridges, which are essential for overcoming obstacles like wetlands, steep inclines, and disaster-prone areas. The narrator, Grady, introduces the series 'Practical Engineering' and delves into the topic of beam bridges, explaining their construction from simple horizontal members to more complex box girders. The limitations of beam bridges in terms of span distances are discussed, leading to the introduction of truss bridges as a more efficient alternative for longer spans. Truss bridges are described as lightweight and rigid structures composed of smaller elements, which simplify the design process by distributing loads axially. The script also touches on different types of truss bridges, such as through trusses, deck trusses, lenticular trusses, Bailey bridges, and covered bridges, each with unique characteristics and applications.

05:00

🏗️ Exploring Different Bridge Designs

This paragraph explores various types of bridges beyond the simple beam and truss designs. It starts with the arch bridge, a time-tested structure that uses compression forces to transfer weight to supports. The construction challenges of arch bridges are noted, particularly the need for temporary supports until the arch is complete. The importance of the keystone in stone arches is highlighted. The paragraph then discusses different arch bridge variations, such as deck arch, open-spandrel, closed-spandrel, through arch, and the unique moon bridge. The discussion moves to tied arch and network arch bridges, which incorporate elements of truss design for increased strength and creativity. Rigid-frame bridges, which integrate the superstructure and substructure into a single unit, are also mentioned. Cantilever bridges, known for their horizontal beams or trusses that balance weight above supports, are explained, with the Forth Bridge in Scotland as a notable example. The paragraph concludes with a look at cable-stayed and extradosed bridges, which utilize steel's tensile strength and innovative designs for shorter spans, respectively.

10:06

⚓ Suspension Bridges and Moveable Bridge Types

The script continues with a focus on suspension bridges, which are iconic for their long spans and slender profiles. It describes how main cables or chains support the road deck from above, with towers and anchorages transferring the weight into the foundation. The paragraph also covers self-anchored suspension bridges, which differ in how they resist tension forces. The need for stiffening in suspension bridges due to their lightweight nature is mentioned, along with the high costs associated with their construction and maintenance. Moveable bridges are introduced as solutions for areas where ships need to pass. The paragraph outlines several types of moveable bridges, including bascule, swing, vertical lift, and transporter bridges, each with unique mechanisms for allowing passage. The uniqueness of every moveable bridge is emphasized, as they are customized to fit specific locations.

15:07

🛶 Floating Bridges and Low-Water Crossings

The final paragraph discusses alternative bridge designs such as floating bridges, which use buoyant supports and eliminate the need for a foundation. These are highlighted for their use in military applications and permanent structures, often employing hollow concrete pontoons. The engineering challenges associated with floating bridges are noted, as is the innovative Moses Bridge in the Netherlands, which has its deck below the waterline. Low-water crossings are introduced as a cost-effective solution for spanning small streams, designed to be submerged during flood events. The disadvantages of low-water crossings, including their impact on fish passage and the safety risks associated with driving through floodwaters, are discussed. The paragraph concludes with a mention of viaducts, long bridges with multiple spans, and the importance of understanding the nuances and creativity in bridge design, as exemplified by various unique bridges around the world.

📰 The Impact of Media Bias in Bridge Reporting

In the concluding paragraph, the focus shifts to the role of media in shaping public perception of events, using the collapse of the Francis Scott Key Bridge in Baltimore as a case study. The script discusses how different media outlets, regardless of their political leanings, can present the story in varied ways based on the details they choose to emphasize. The sponsor, Ground News, is introduced as a service that aggregates major news stories and provides context to help readers understand the biases and factuality of the sources. The paragraph highlights Ground News' features, such as visual breakdowns, tags for political bias and factuality, and the 'Blind Spot' feature that reveals stories covered predominantly by one side of the political spectrum. The narrator encourages a broader perspective on issues and promotes the use of Ground News to gain a more transparent view of the media landscape, offering a discount for their Vantage subscription.

Mindmap

Keywords

💡Beam Bridge

A beam bridge is a simple structural crossing consisting of a horizontal member supported by two supports. It is fundamental to the theme of the video as it represents the basic concept of bridging a gap. The script mentions that beam bridges can take various forms, such as rolled steel beams or plate girders, and are commonly used for overpasses. However, they are limited in the distance they can span due to the increasing weight of the beams.

💡Truss Bridge

A truss bridge is an assembly of smaller elements that form a rigid and lightweight structure, designed to span greater distances than solid beams. The script explains that trusses are efficient because they allow for the load to be distributed axially, simplifying the design process. The video highlights different types of truss bridges, such as through truss and lenticular truss bridges, showcasing the diversity and creativity in bridge engineering.

💡Arch Bridge

An arch bridge uses a curved element to transfer the bridge's weight to supports using compression forces. The script emphasizes the historical significance of arch bridges, as many of the oldest bridges used this design. It also mentions the construction challenges, such as the need for temporary supports until the arch is complete, and the importance of the keystone in stone arches.

💡Cantilever Bridge

A cantilever bridge is a type of bridge where beams or trusses project horizontally, balancing the structure's weight above the supports. The script provides the example of the Forth Bridge, which held the record for the longest span in the world for decades, demonstrating the effectiveness of cantilever design in creating long-span bridges.

💡Cable-Stayed Bridge

A cable-stayed bridge supports the deck from above through cables attached to tall towers, forming a fan pattern. The script describes the simplicity and versatility of cable-stayed bridges, which allows for various configurations and dramatic shapes. This concept is integral to the video's exploration of modern bridge design and engineering.

💡Suspension Bridge

A suspension bridge uses cables or chains to hang the deck below, supported by massive main cables or chains. The script highlights the iconic nature of suspension bridges due to their enormous spans and slender appearance. It also explains the structural components like towers, anchorages, and stiffening elements that contribute to the stability of these bridges.

💡Moveable Bridge

A moveable bridge is designed to allow for the passage of ships by physically moving out of the way. The script categorizes various types of moveable bridges, such as bascule, swing, vertical lift, and transporter bridges. This concept is key to the video's discussion on the adaptability of bridge designs to different environmental and functional requirements.

💡Floating Bridge

A floating bridge utilizes buoyant supports, eliminating the need for a foundation. The script mentions that these are used in military applications and permanent structures, often employing hollow concrete structures as pontoons. This keyword is significant as it showcases an innovative approach to bridge construction in areas where traditional foundations are not feasible.

💡Low-Water Crossing

A low-water crossing is designed to be submerged when water levels rise, typically used in areas prone to flash floods. The script discusses the disadvantages of low-water crossings, such as blocking fish passage and safety concerns during heavy rainstorms. This concept is relevant to the video's broader theme of the challenges and trade-offs in bridge design.

💡Viaduct

A viaduct refers to a long bridge with multiple spans, mostly above land, requiring multiple intermediate supports. The script notes that viaducts can be singular structures or part of elevated expressways in modern cities. This term is important for understanding the scale and scope of certain bridge projects.

💡Ground News

Ground News is a news aggregation service mentioned in the script that provides context to major news stories, including political bias and factuality. The script promotes Ground News for offering a more transparent media landscape, which is relevant to the video's discussion on how stories are framed and interpreted in the media.

Highlights

Earth's challenging topography often requires innovative bridge designs to overcome obstacles.

Bridges are not just functional but can also be aesthetically pleasing and breathtaking.

Engineers classify bridges based on how they manage the forces involved in their design.

Beam bridges are simple with a horizontal member across two supports but have limitations in spanning long distances.

Truss bridges use smaller elements to create a rigid and lightweight structure, allowing for greater spans.

Lenticular truss bridges are visually striking and resemble lenses.

Bailey bridges, designed during WWII, are portable and still used today for temporary crossings.

Covered bridges often use timber trusses and are protected from the elements by roofs and siding.

Trestle bridges are similar to trusses but have short spans with frequent supports.

Arch bridges transfer weight using compression forces and are popular for their efficiency and aesthetic.

Keystone is crucial in stone arch bridges, providing the necessary support for the structure.

Tied arch bridges use a chord to resist thrust forces, blending truss and arch characteristics.

Cantilever bridges balance the structure's weight above supports, allowing for impressive spans.

Cable-stayed bridges use cables attached to towers for support, offering a variety of configurations.

Extradosed bridges combine girders with cable-stayed features for shorter spans.

Suspension bridges are iconic for their long spans and slender, graceful appearance.

Moveable bridges, like bascule and swing bridges, adapt to allow ship passage.

Floating bridges use buoyant supports and eliminate the need for a foundation.

Low-water crossings are designed to be submerged during floods, with their own set of challenges.

Viaducts are long bridges with multiple spans, often used to span wide valleys.

Ground News provides a broad perspective on news stories, highlighting political biases and media landscapes.

Transcripts

00:01

The Earth is pretty cool and all, but many of  its most magnificent features make it tough  

00:06

for us to get around. When the topography is too  wet, steep, treacherous, or prone to disaster,  

00:13

sometimes the only way forward is up: our  roadways and walkways and railways break  

00:18

free from the surface using bridges. A lot of  the infrastructure we rely on day to day isn’t  

00:24

necessarily picturesque. It’s not that we can’t  build exquisite electrical transmission lines  

00:30

or stunning sanitary sewers. It’s just that we  rarely want to bear the cost. But bridges are  

00:36

different. To an enthusiast of constructed works,  many are downright breathtaking. There are so many  

00:42

ways to cross a gap, all kindred in function  but contrary in form. And the typical way that  

00:48

engineers classify and name them is in how each  design manages the incredible forces involved.  

00:55

Like everything in engineering, terminology  and categories vary. As Alfred Korzybski  

01:00

said, “The map is not the territory.” But,  trying to list them all is at least a chance  

01:04

to learn some new words and see some cool  bridges. And honestly, I can hardly think  

01:09

of anything more worthwhile than that. I’m  Grady, and this is Practical Engineering.

01:26

One of the simplest structural crossings is the  beam bridge:

01:31

just a horizontal member across two 

01:33

supports. That member can take a variety of forms,  including a rolled steel beam (sometimes called  

01:39

a stringer) or a larger steel member fabricated  from plates (often called a plate girder). Most  

01:45

modern bridges built as overpasses for grade  separation between traffic are beam bridges  

01:50

that use concrete girders. And instead of a group  of individual beams, many bridges use box girders,  

01:56

which are essentially closed structural tubes  that use material more efficiently (but can be  

02:02

more complicated to construct). Beam bridges  usually can’t span great distances because  

02:07

the girders required would be too large. At a  certain distance, the beams become so heavy,  

02:12

they can hardly support their own weight,  let alone the roadway and traffic on top.

02:17

One way around the challenge of the structural  members’ self-weight is to use a truss instead  

02:22

of a girder. A truss is an assembly of smaller  elements that creates a rigid and lightweight  

02:27

structure. Unlike a beam, the members of a truss  don’t typically experience bending forces. The  

02:33

connections usually aren’t actual hinges that  permit free rotation, but they are close enough.  

02:38

So, all the load is axial (along their length)  in compression or tension. That simplifies the  

02:44

design process because it’s easier to predict  the forces within each structural member. The  

02:49

weight reduction allows trusses to span greater  distances than solid beams, and there are a wide  

02:55

variety of arrangements, many with their own  specific names. In general, a through truss puts  

03:01

the deck on the bottom level, and a deck truss  puts it on top, hiding the structural members  

03:07

below the road. A particularly photogenic  type of truss is a lenticular truss bridge,  

03:13

named because they resemble lenses, which  themselves are named because they resemble  

03:17

lentils! A Bailey bridge is a kind of temporary  truss bridge that is designed to be portable  

03:22

and easy to assemble. They were designed during  World War II, but Bailey bridges are still used  

03:27

today as temporary crossings when a bridge fails  or gets closed for construction. Most covered  

03:33

bridges are timber truss bridges. Since wood is  more susceptible to damage from exposure to the  

03:39

elements, the roof and siding are placed to keep  the structural elements truss-worthy.

03:44

A trestle  bridge is superficially similar to a truss: a  framework of smaller members. Trestle bridges  

03:51

don’t have long spans, but rather a continuous  series of short spans with frequent supports which  

03:56

are individually called trestles, but sometimes  the whole bridge is just called a trestle,  

04:01

so like so many other instances of structural  terminology, it can be a little confusing.

04:06

This next bridge type uses a structural feature  that’s been a favorite of builders for millennia:  

04:11

the arch. Instead of beams loaded perpendicularly  or trusses that experience both compressive and  

04:18

tensile forces, arch bridges use a curved element  to transfer the bridge’s weight to supports using  

04:24

compression forces alone. Many of the oldest  bridges used arches because it was the only  

04:29

way to span a gap with materials available  at the time (stone and mortar). Even now,  

04:34

with the convenience of modern steel and  concrete, arches are a popular choice for  

04:39

bridges. They make efficient use of materials  but can be challenging to construct because  

04:44

the arch can’t provide its support until it is  complete. Temporary supports are often required  

04:50

during construction until the arch is connected  at its apex from both sides. In stone arches,  

04:56

the topmost stone is key to keeping the whole  thing standing, and, of course, it’s called  

05:00

the keystone. When the arch is below the roadway,  we call it a deck arch bridge. Vertical supports  

05:06

transfer the load of the deck onto the arch. The  area between the deck and arch has a great name:  

05:12

the spandrel. Open-spandrel bridges use columns  to transfer loads, and closed-spandrel bridges  

05:19

use continuous walls. If part of the arch extends  above the roadway with the deck suspended below,  

05:25

it’s called a through arch bridge. A moon bridge  is kind of an exaggerated arch bridge, usually  

05:31

reserved for pedestrians over narrow canals where  there’s not enough room for long approaches.  

05:37

They’re steep, so sometimes you have to use steps  or ladders to get up to the top and back down.

05:42

One result of compressing an arch is that it  creates horizontal forces called thrusts. Arch  

05:48

bridges usually need strong abutments at either  side to push against that can withstand the extra  

05:54

horizontal loads. Alternatively, a tied arch  bridge uses a chord to connect both sides of  

06:00

the arch like a bowstring, so it can resist  the thrust forces. That means a tied arch is  

06:05

structurally more of a truss than an arch,  and that provides a lot of opportunities for  

06:10

creativity. For just one example, a network  arch bridge uses the tied arch design,  

06:16

plus criss-crossed suspension cables, to support  the deck. To tell an arch from a tied arch by eye,  

06:23

it’s usually enough to look at the supports. If  the end of each arch sits atop a spindly pier  

06:29

or some other structure that seems  insubstantial against horizontal forces,  

06:34

you can probably bet that they are tied together  and it’s not a true arch bridge. Similarly,  

06:39

a rigid-frame bridge integrates the superstructure  and substructure (in other words, the deck,  

06:45

supports, and everything else) into a single  unit. They don’t have to be arched, but many are.

06:51

Another way to increase the span of a beam bridge  is to move the supports so that sections of the  

06:56

deck balance on their center instead of being  supported at each end. A cantilever bridge  

07:02

uses beams or trusses that project horizontally,  balancing most of the structure’s weight above  

07:07

the supports rather than in the center of the  span. This is such an effective technique that  

07:13

the Forth Bridge crossing the Firth of Forth in  Scotland took the title of longest span in the  

07:18

world away from the Brooklyn Bridge in 1890  and held the record for decades. This famous  

07:24

photograph demonstrates the principle of that  bridge perfectly: The two central piers bear  

07:29

the compression loads from the bridge. And, the  outer-most supports are anchors to provide the  

07:34

balancing force for each arm. This way,  you can suspend a load in the middle.

07:39

The longest bridges take advantage of steel’s  ability to withstand incredible tension forces  

07:44

using cable supports. Cable-stayed bridges  support the deck from above through cables  

07:50

attached to tall towers or spars. The cables  (also called stays) form a fan pattern,  

07:56

giving this type of bridge its unique  appearance. Depending on the span,  

08:00

cable-stayed bridges can have one central  tower or more. Their simplicity allows for  

08:05

a wide variety of configurations, giving rise  to some dramatic (and often asymmetric) shapes.

08:11

For shorter spans, you can combine the benefits  of a cable-stayed structure with girders to get  

08:16

an extradosed bridge. Imagine a concrete girder  bridge that uses internal tendons to keep the  

08:22

concrete in compression, then just pull those  tendons out of the girder and attach them to  

08:27

a short tower. Rather than holding the deck up  vertically like a cable-stayed bridge, they’re  

08:32

acting more horizontally to hold the girders in  compression, giving them the stiffness needed  

08:37

to support the deck. It’s a relatively new idea  compared to most of the other designs I’ve listed,  

08:42

but there are quite a few cool examples  of extradosed bridges across the globe.

08:46

Where a cable-stayed bridge attaches the  deck directly to each tower, a suspension  

08:51

bridge uses cables or chains to dangle the  deck below. In a simple suspension bridge,  

08:57

the cables follow the curve of the deck.  This is your classic rope bridge. They’re  

09:01

not very stiff or strong, so simple suspension  bridges are usually only for pedestrians. A  

09:07

stressed ribbon bridge takes the concept  a step further by integrating the cables  

09:11

into the deck. The cables pull the deck  into compression, providing stiffness and  

09:16

stability so it doesn’t sway and bounce.  This design is also primarily used for  

09:21

smaller pedestrian bridges because it can’t span  long distances and the deck sags in the middle.

09:26

Then you have the suspended deck bridge, the  design we most associate with the category  

09:31

with the longest spans in the world. Massive main  cables or chains dangle the road deck below with  

09:37

vertical hangers. Suspension bridges are iconic  structures because of their enormous spans and  

09:43

slender, graceful appearance. Towers on either  side prop up the main cables like broomsticks in  

09:49

a blanket fort. Most of the bridge’s weight  is transferred into the foundation through  

09:54

these towers. The rest is transferred into the  bridge’s abutments through immense anchorages  

10:00

keeping the cables from pulling out of the ground.  Alternatively, self-anchored suspension bridges  

10:05

connect the main cables to the deck on either  side, compressing it to resist the tension forces.  

10:12

Because they are so slender and lightweight,  most suspension bridges require stiffening with  

10:16

girders or trusses along the deck to reduce  movement from wind and traffic loads. These  

10:22

bridges are expensive to build and maintain, so  they’re really only used when no other structure  

10:27

will suffice. But you can hardly look at a  suspended deck bridge without being impressed.

10:33

Bridges have to support the vehicles and people  that cross over the deck, but they often have to  

10:38

accommodate boats and ships passing underneath as  well. If it’s not feasible to build the bridge and  

10:44

its approaches high enough, another option is  just to have it get out of the way when a ship  

10:49

needs to pass. Moveable bridges come in all shapes  and sizes. A lot of people call them drawbridges  

10:55

after their medieval brethren over castle moats.  A bascule bridge is hinged so the deck can rotate upward.  

11:05

A swing bridge rotates horizontally  so a ship can pass on either side.  

11:10

A vertical lift bridge raises the entire deck upward, keeping  it horizontal like a table. A transporter bridge  

11:16

just has a small length of deck that is shuttled  back and forth across a river. That’s just a few,  

11:22

and in fact, every moveable bridge is unique  and customized for a specific location,  

11:27

so there are some truly interesting  structures if you keep an eye out.

11:31

On the other hand, sometimes there’s no need for  ship passage or a lot of space below, and in that  

11:36

case, you can just float the bridge right on the  water. Floating bridges use buoyant supports,  

11:41

eliminating the need for a foundation.  These are used in military applications,  

11:46

but there are permanent examples too. Many use  hollow concrete structures as pontoons, with  

11:51

pumps inside to make sure they don’t fill up with  water and sink. And actually, a lot of bridges  

11:56

take advantage of buoyancy in their design,  even if it’s not the main source of support.  

12:01

A design like this presents a lot of interesting  engineering challenges, so there aren’t too many  

12:06

of them. Similarly, the pedestrian bridge at  Fort de Roovere in the Netherlands (probably  

12:11

pronounced that wrong) has its deck below the  water, giving it the nickname of the Moses Bridge.

12:17

If space or funding is really tight,  one option to span a small stream is  

12:22

a low-water crossing. Unlike bridges  built above the typical flood level,  

12:27

low-water crossings are designed to be submerged  when water levels rise. They are most common  

12:32

in areas prone to flash floods, where runoff  in streams rises and falls quickly. Ideally,  

12:37

a crossing would be inaccessible only a few  times per year during heavy rainstorms. However,  

12:43

low-water crossings have some disadvantages. For  one, they can block the passage of fish just like  

12:48

a dam. And then there’s safety. A significant  proportion of flood-related fatalities occur when  

12:53

someone tries to drive a car or truck through  water overtopping a roadway. Water is heavy.  

12:59

It takes only a small but swift flow to push a  vehicle down into a river or creek, which means at  

13:05

least some of the resources saved by avoiding the  cost of a higher bridge are often spent to erect  

13:10

barricades during storms, install automatic  flood warning systems, and run advertisement  

13:15

campaigns encouraging motorists never to  drive through water overtopping a roadway.

13:20

You may have heard the term viaduct before.  It’s not so much a specific type of bridge,  

13:25

but really about the length. Bridges that span a  wide valley need multiple intermediate supports.  

13:31

So, a viaduct is really just a long bridge  with multiple spans that are mostly above  

13:36

land. There’s really not a lot of agreement on  what is one and what isn’t. Some are singular  

13:42

and impressive structures. But many modern cities  have viaducts that are, although equally amazing  

13:47

from an engineering standpoint, a little less  beautiful. So, you’re more likely to hear them  

13:53

called elevated expressways. And that gets to the  heart of a topic like this: without listing every  

13:59

bridge, there’s no true way to list every type  of bridge. There’s too much nuance, creativity,  

14:04

and mixing and matching designs. The Phyllis J.  Tilly bridge in Fort Worth, Texas combines an arch  

14:10

and stressed ribbons. The Third Millennium Bridge  in Spain uses a concrete tied arch with suspension  

14:16

cables holding up the deck which is stiffened  with box girders. The Yavuz Sultan Selim Bridge  

14:22

in Turkey combines a cable-stayed and suspension  design. In some parts of India and Indonesia,  

14:28

living tree roots are used as simple suspension  bridges over rivers. There are bridges for  

14:33

pipelines, bridges for water, bridges for animals,  and I could go on. But that’s part of the joy of  

14:40

paying attention to bridges. Once you understand  the basics, you can start to puzzle out the more  

14:44

interesting details. Eventually, you’ll see  the Akashi Kaikyo Bridge on a calendar in  

14:50

your accountant’s office, and let him know  it’s a twin-hinged, three-span continuous,  

14:55

stiffened truss girder suspension bridge with  a double-tower system.

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Or maybe that’s just me.

15:01

We care a lot about bridges. My previous video  covered the engineering that goes into vessel  

15:06

collision design for bridges, focusing on the  recent collapse of the Francis Scott Key Bridge  

15:11

in Baltimore that was a huge story in the news  covered by nearly every major outlet across the  

15:16

globe. Over 400 sources reported the even from  every side of the political spectrum. Since it  

15:23

was so widely reported, there’s a pretty even mix  between left-leaning, center, and right-leaning  

15:28

outlets, but if you look at the headlines, you’ll  see all kinds of ways the story was painted with  

15:33

political and ideological biases from both sides  of the aisle. By focusing on different details of  

15:38

the story - the victims' nationalities, the DEI  policies of the ship operator, the response by  

15:44

prominent politicians - the framing can subtly, or  not-so-subtly, change how you interpret the facts.

15:52

Seeing all this in one place is possible thanks  to my sponsor, Ground News. They aggregate major  

15:57

news stories and add context to make reading the  news easier and more effective. Every story comes  

16:03

with a quick visual breakdowns and tags for  political bias, factuality, and ownership of  

16:08

the sources backed by ratings from independent  news monitoring organizations. For this story,  

16:13

you can see that nearly half of the reporting  outlets are media conglomerates and just over  

16:18

half of those outlets have been rated “High  Factuality.” They also have a feature called  

16:22

the Blind Spot that shows you stories mainly  covered by one side of the political spectrum:  

16:27

stuff you might totally miss if you only  follow a few main sources for your news.

16:31

I don’t necessarily agree with how every  story on the Key Bridge is painted,  

16:35

but it’s important to me to get a broad  perspective on issues like this. It’s  

16:39

not just because I was trying to find  the right way to tell the story myself,  

16:43

but because stories like this are how we shape  our view of the world around us. In that way,  

16:48

journalism has a lot of power over us, and  Ground News hands some of that power back to  

16:53

you. If you’d like a more transparent media  landscape, they’re offering a huge discount  

16:58

right now at the link in the description: 40  percent off the Vantage subscription, which  

17:02

includes unlimited access to all their features.  That’s ground dot news slash practicalengineering  

17:08

or just click the link in the description. Thank  you for watching, and let me know what you think!

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関連タグ
Bridge EngineeringInfrastructureTruss BridgesBeam BridgesArch BridgesSuspension BridgesCable-StayedMoveable BridgesFloating BridgesLow-Water CrossingsViaducts
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