Every Kind of Bridge Explained in 15 Minutes

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

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

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

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

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

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

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

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

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

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

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

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

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

One of the simplest structural crossings is the  beam bridge:

just a horizontal member across two 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

or some other structure that seems  insubstantial against horizontal forces,  

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

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

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

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

deck balance on their center instead of being  supported at each end. 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 is such an effective technique that  

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Or maybe that’s just me.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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