Bridges span all sorts of impediments to roadways, railways, pedestrian and bicycle paths, and even utility lines. The type of bridge chosen depends on the bridge’s necessary span length and proposed function. Oftentimes larger, higher profile bridges are not merely functional, but are also highly aesthetic. Bridges typically shorten the pathway between two points – rather than detour the facility to avoid the impediment, bridges span the impediment. Herein we will discuss several types of bridges, their useful functionality, and aesthetics.
The beam bridge is a rather straightforward evolution of a simple log spanned over a stream. Beam bridges can be constructed from rolled steel beams or steel plate girders, cast-in-place concrete beams or box girders, or precast and pretensioned concrete beams. Beam bridges are the most commonly used bridge type, and are exemplified in highway flyovers and overpasses. Beams are supported on each end and can utilize simple span construction (where the beam does not extend beyond any one support), cantilever construction (where one end of the beam is held in place and the other is free to move in any direction), or continuous (where the beam continues over at least one support and also starts and ends on a support). The beams are generally stiff, so that any flexure, or bending, is neither felt nor visible. This flexure creates tension at the bottom and compression at the top of a simple-span beam. Continuous beams are more complicated in their stress distribution.
The longer a beam is, the more stress it experiences. As a result, beam bridges are the shortest of all modern bridge types.
Arch bridges are, as the name suggests, in the shape of an arch. There are supports on both ends of the arch, bearing the weight of the bridge and, oftentimes, the outward thrust caused by the arch shape. If, however, it is not practical for the bridge foundation to carry this outward thrust, a horizontal tie between the end supports can be added, as a tension member, to carry the thrust. This is called a tied arch bridge.
Probably the oldest arch bridge in existence today is the Mycenaean Arkadiko Bridge located in modern day Greece and dating back in 1300 BC. This particular bridge is an example of a corbel arch, or an arch in which successive levels of brick or stone are placed along the curvature of the arch wall, extending further and further outward until the last remaining opening is filled with what is known as a keystone.
Both the ancient Greeks and Etruscans understood the concept of the arch, but the Romans were the first to really harness arches in the construction of bridges.
Historically, arches have progressed from a circular shape to an elliptical shape to, finally, the parabolic shape. The parabolic shape is the most efficient arch shape, as it results in only axial loads – those along the axis of the arch.
There are many different kinds of arches in use today. These include the aforementioned tied-arch and corbel arch bridges, a deck arch bridge (in which the deck on which vehicles travel is above the arch), and through arch bridges (in which the deck is contained at some point within the vertical geometry of the arch). Examples of these kinds of bridges are:
- Tied Arch: Gateway Bridge, Detroit Michigan – built in time for the 2006 Super Bowl held at Ford Field in Detroit, this bridge features oval-shaped cross bracing, evoking the idea of footballs
- Corbel Arch: Mycenaean Arkadiko Bridge – built circa 1300 BC. Corbel arches aren’t constructed much these days
- Deck Arch: New River Gorge Bridge, Fayetteville, WV – completed in 1977, this bridge stood for many years as the longest steel arch bridge in the world
- Through Arch: The Sydney Harbor Bridge, Sydney, Australia – completed in 1923
The largest arch bridge in existence is the Chaotianmen Bridge located in Chongqing, Southwest China. It is 1741 meters (1.1 mi) in length.
By its simplest definition, suspension bridge is one in which the bridge deck is suspended from large cables via vertical hangers. The cables are suspended from towers. The vertical hangers are attached to both the suspension cables and the deck.
The vertical load on the bridge, from the weight of the bridge itself and vehicular traffic (among other possible load types) is converted into tension in the cables, which requires anchorage at the ends of the bridge. The cables also impart a vertical force on the pillars, putting them into compression.
This bridge type yields the longest main spans of any bridge type. Span lengths can exceed one mile. Suspension bridges are also more able to withstand earthquake loading as they are relatively flexible. They also require less material than other bridge types that may be able to achieve similar span lengths. Construction of the bridge may be completed entirely from above, which is useful in environmentally sensitive areas. However, as evidenced by the failure of the Tacoma Narrows Bridge in Washington, these types of bridges are incredibly sensitive to harmonic vibration under wind loading.
The most famous suspension bridge in the United States is the Golden Gate Bridge in San Francisco. The longest suspension bridge exceeds 1 mile in length and is located in Japan.
Cable-stayed bridges are similar to suspension bridges in that the deck is suspended by cables anchored into the ground; however in a cable-stay bridge, the cables that support the deck are directly attached to the towers at an angle, and they are not anchored in the ground. The loads are transmitted to the ground only through the towers and not through the cables. One disadvantage of this design is that the deck must be more rigid in the horizontal direction in order to resist the horizontal loads imparted by the angled cables.
There are two types of cable-stayed bridges – harp and fan. Harp type cable-stay bridges utilize cables that are essentially parallel, so that the height of attachment to the tower is proportional to the distance of attachment to the deck. The cables of fan type cable-stayed bridges connect when they pass over the top of tower. This is a more structurally sound design, but can be impractical in many situations. As such, a modified fan design is often used. In this cable layout, the cables are grouped and pass close to the top, but not over the top, of the tower.
Cable-stayed bridges are optimal when the span length exceeds those of a cantilever bridge, but are shorter than suspension bridges.
Although most cable-stayed bridges utilize one or more center towers with cables splaying outward on each side, many bridges have been designed utilizing one tower with cables only on one side. Also, a special case of cable-stayed bridge is the extradosed bridge, which utilizes a much stiffer deck and a shorter tower, such that the deck acts as a continuous beam in the vicinity of the tower and a cable-stayed bridge beyond those limits. This kind of bridge is not generally economically feasible or materially efficient.
Trivia: The most famous cable-stayed bridges in the United States are the Sunshine Skyway over Tampa Bay and the Leonard P. Zakim Bridge in Boston – part of the Big Dig Project. The longest cable-stayed bridge in the world is the Russky Bridge in Russia, and it has a 1,104 m (just under ¾ mi) long main span.