Railway bridges are a fascinating subject. The engineers who develop railway bridge types have the incredibly challenge of creating a thin piece of ground that can support thousands of tons of material moving at an incredibly fast pace sometimes hundreds of feet above the ground, with zero room for error.
On top of those impossible conditions, the particular area in which the bridge is needed presents its own set of requirements. Some railway bridge types involve shoring systems to meet a temporary needs. Some railway bridge types have to provide a means for foot traffic in addition to railway traffic. Some bridges support railways above, but also must be movable so that large boats can go underneath. With these vast requirements, there are endless amounts of bridge designs, however, they generally fall under a few overall railway bridge types, or at least a combination of types. Let’s consider these overall bridge designs:
Four Common Railway Bridge Types
- Truss Bridges
If you’ve ever been assigned to create a bridge out of Popsicle sticks and Elmer’s glue, you know what a truss bridge is. A truss bridge supports the magnanimous amount of weight that is going to be going over it through a complicated system of diagonal posts. The diagonal posts distribute the weight that the bridge must support, which adds to its strength and total weight capacity.
There are two common types of truss bridges:
- King Post Truss Bridges have one vertical post for every two diagonal posts, making a triangle between each section of the truss system.
- Queen Post Truss Bridges have two vertical posts for every two diagonal posts, creating a trapezoid shape in each section of the truss system.
- Arch Bridges
The arch bridge is one of the oldest ways to create a reliable bridge, and so any of the arch bridges that you recognize are historic bridges. An arch bridge works by creating a network of wedged stones underneath the bridge that creates the shape of an arch. Each wedge presses against the others, multiplying the amount of weight it can support. Arch bridges are so efficient at supporting the bridge above them that the stones of many of the original arch bridges are not held together with any type of mortar; the power of gravity alone grips them tightly and immovably together. The more weight they support, the tighter the wedges fit together, and the stronger the arch bridge is.
Since the infrastructure of the United States is relatively younger than the rest of the world, and many of our bridge systems were created during the industrial age, when cable bridges and truss bridges were popular, we don’t have a lot of popular arch bridges here. One you might recognize is the Hell Gate Bridge in New York.
- Suspension Bridges
We can’t have an article about the types of bridges in the United States without talking about suspension bridges. The most iconic bridge in our country is arguably the Golden Gate Bridge in San Francisco. While many bridges incorporate more than one bridge design in their structure, the Golden Gate Bridge is a textbook suspension bridge.
A suspension bridge works through a system of vertical towers that are connected together by horizontal cables. The cable supports the deck of the bridge with a series of vertical suspender cables. To prevent the section of the bridge that is further from the structural towers from sagging, the horizontal cables that connect the towers curve in the middle, so the furthest point between the support towers is closest to the deck, requiring the shortest suspension cable.
- Beam Bridges
Beam bridges are about as straight-forward as they come. A beam bridge is simply a series of vertical bridges that supports the horizontal deck. The beams are usually broader at the top than the base, to provider greater support where the burden of weight is. As you can imagine, this simplistic bridge approach is on appropriate for very short bridges.
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