Published April 5, 2021
Bridges come in all forms and sizes. They can be short, long, straight, arched, fixed, suspended, have waterways below them or even a cafe. But their design and structure aren’t just for aesthetics. Different types of bridges serve different purposes.
Technically, a bridge is a structure built over a span of physical obstacles like valleys and rivers to provide passage from one side to the other. In other words, it’s a structure that allows us to cross landscapes that are otherwise unpassable. It shortens travel time, makes moving goods easier and helps promote commerce.
Though this definition sounds really simple, building a bridge is much more complex than that. There are many factors that need to be considered before deciding what type of bridge should be built. This includes the terrain, the expected traffic and the materials available. It must be able to withstand the elements and bear the load of both pedestrians and vehicles passing through it regularly.
Though there are many different kinds of bridges around the world, their structure can be traced to seven main types. To help you understand better, here’s a quick guide to the different types of bridges.
1. Beam Bridges
This is one of the simplest and most basic types of bridge. As the name suggests, it uses one or several beams laid horizontally over the span of an area between abutments. These beams support a deck area that is usually made of wood planks or stone slabs. In most cases, you’ll also find other beams supporting the main beams to provide additional stability. Though they’re less costly and easy to build, this type of structure doesn’t allow for the transfer of stress and often has a limited span.
Because they’re very simple, it’s one of the oldest bridges built by men. The first beam bridges were built by laying logs over rivers and streams. In fact, the most common example of beam bridges is the basic log bridges you often see in the countryside.
Today, most beam bridges are made of metal beams, steel boxes, and pre-stressed construction concrete.
2. Arch Bridges
An arch bridge is just as its name suggests. It’s a type of bridge with abutments on both ends shaped like a curved arch. This design enables the bridge to sustain vertical loads by conveying the force along the curve of the arch to the abutments on each end. As such, the entire load of the bridge rests on these abutments. They are also responsible for holding the arch in a precisely fixed position.
Aside from being able to bear heavy loads, arch bridges are very visually striking. This makes them very popular in the ancient world. From Roman aqueducts to ancient Chinese timber arch bridges, this type of bridge is very common in historical sites around the world. They used to be built of mortar and stones but in modern times, the stones are replaced with concrete, iron, and structural steel.
3. Truss Bridges
Truss bridges got their name from the wooden or metal triangles (known as trusses) that serve as their main load-bearing structure.
Since trusses are resilient to stress, truss bridges are much more stable than other types of bridges. This design principle also allows the distribution of load all throughout the entire structure. They require fewer building materials too making them more cost-efficient. These make truss bridges very popular in modern construction.
4. Cantilever Bridges
Cantilever bridges rely on the load-bearing capacity of cantilevers. This is a type of rigid structure supported only on one end and the rest extends horizontally into space. Cantilevers work by carrying the structural load from its unsupported end back to the structure supporting it. This allows it to bear overhanging structures (like a bridge deck) without additional support.
This type of bridge design first appeared in the 19th century. At that time, there was a high demand for longer bridges to traverse wide bodies of water. Engineers at that time discovered that increasing the number of supports will distribute the loads among them allowing for a longer bridge.
Some of the most popular cantilever bridges in the world are the Pont de Québec Bridge in Canada and Minato Bridge in Japan.
5. Suspension Bridges
Just like cantilever bridges, suspension bridges don’t rely on an understructure for load-bearing. Instead, the roadway is hung from a suspension cable using a vertical suspender. The main cables are suspended between towers and connected to the anchorage. This structural design transfers vertical loads from the deck to the towers through curved cables in tension.
Though suspension bridges are aesthetically pleasing and can span longer distances, it’s also very costly to build. They are also usually built for automobile traffic though they are also strong enough to support freight trains.
6. Cable-Stayed Bridges
Cable-stayed bridges use a similar design principle to that of suspension bridges. The main difference is the tension cables are diagonal and connect the deck directly to one or more vertical towers. The tensile forces in these cables keep the deck in horizontal compression.
The first cable-stayed bridges use few but very heavy cables to support the roadways but modern construction uses thinner cables in large quantities. This technique allows the use of thinner decks which reduces construction cost.
7. Tied-Arch Bridge
If you’ve ever crossed the Fort Pitt Bridge in Pittsburgh, Pennsylvannia, then you’ve just seen an example of a tied-arch bridge. Often touted as a cross between a suspension bridge and an arch bridge, tied-arch bridges feature an arch rib with vertical ties on both sides of the deck. The tips of the arch are then tied together with a bottom chord.
This design uses the principle of arch bridges where the load from the deck is translated as tension in the vertical ties to the curved section of the arch. The tensile force tends to flatten the arch and push its ends into the abutments. Since the force that passes to the abutments is lesser, tied-arch bridges are suitable for areas with less solid foundations.
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