[Note that this article is a transcript of the video embedded above.]

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.