INTRO
There's no doubt you've seen a bridge, and it's almost as likely that you've travelled over one. If you've ever laid a plank or log down over a stream to keep from getting wet, you've even constructed a bridge. Bridges are truly ubiquitous -- a natural part of everyday life. A bridge provides passage over some sort of obstacle: a river, a valley, a road, and a set of railroad tracks...
In this article, we will look at the three major types of bridges so that you can understand how each one works. The type of bridge used depends on various features of the obstacle. The main feature that controls the bridge type is the size of the obstacle. How far is it from one side to the other? This is a major factor in determining what type of bridge to use, and by the time you are done reading this project you will understand why
The Basics
There are three major types of bridges:
- The beam bridge
- The arch bridge
- The suspension bridge
The biggest difference between the three is the distances they can cross in a single span. A span is the distance between two bridge supports, whether they are columns, towers or the wall of a canyon. A modern beam bridge, for instance, is likely to span a distance of up to 200 feet (60 meters), while a modern arch can safely span up to 800 or 1,000 feet (240 to 300 m). A suspension bridge, the pinnacle of bridge technology, is capable of spanning up to 7,000 feet (2,100 m).
What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to span a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with two important forces called compression and tension:
- Compression is a force that acts to compress or shorten the thing it is acting on.
- Tension is a force that acts to expand or lengthen the thing it is acting on.
A simple, everyday example of compression and tension is a spring. When we press down, or push the two ends of the spring together, we compress it. The force of compression shortens the spring. When we pull up, or pull apart the two ends, we create tension in the spring. The force of tension lengthens the spring.
Compression and tension are present in all bridges, and it's the job of the bridge design to handle these forces without buckling or snapping. Buckling is what happens when the force of compression overcomes an object's ability to handle compression, and snapping is what happens when the force of tension overcomes an object's ability to handle tension. The best way to deal with these forces is to either dissipate them or transfer them. To dissipate force is to spread it out over a greater area, so that no one spot has to bear the brunt of the concentrated force. To transfer force is to move it from an area of weakness to an area of strength, an area designed to handle the force. An arch bridge is a good example of dissipation, while a suspension bridge is a good example of transference.
The Beam Bridge
A beam bridge is basically a rigid horizontal structure that is resting on two piers, one at each end. The piers directly support the weight of the bridge and any traffic on it. The weight is travelling directly downward.
Compression
The force of compression manifests itself on the topside of the beam bridge's deck (or roadway). This causes the upper portion of the deck to shorten.
Tension
The result of the compression on the upper portion of the deck causes tension in the lower portion of the deck. This tension causes the lower portion of the beam to lengthen.
Example
Take a two-by-four and place it on top of two empty milk crates -- you've just created a crude beam bridge. Now place a 50-pound weight in the middle of it. Notice how the two-by-four bends. The topside is under compression and the bottom side is under tension. If you keep adding weight, eventually the two-by-four will break. Actually, the topside will buckle and the bottom side will snap.
Dissipation
Many beam bridges that you find on highway overpasses use concrete or beams to handle the load. The size of the beam, and in particular the height of the beam, controls the distance that the beam can span. By increasing the height of the beam, the beam has more material to dissipate the tension. To create very tall beams, bridge designers add supporting latticework, or a truss, to the bridge's beam. This support truss adds rigidity to the existing beam, greatly increasing its ability to dissipate the compression and tension. Once the beam begins to compress, the force is dissipated through the truss.
Despite the ingenious addition of a truss, the beam bridge is still limited in the distance it can span. As the distance increases, the size of the truss must also increase, until it reaches a point where the bridges own weight is so large that the truss cannot support it.
Types
Beam bridges come in dozens of different styles. The design, location and composition of the truss is what determines the type. In the beginning of the Industrial Revolution, beam-bridge construction in the United States was developing rapidly. Designers were coming up with many different truss designs and compositions. All-iron or wood-and-iron combinations were replacing wooden bridges. The different truss patterns also made great strides during this period. One of the most popular early designs was the Howe truss, a design patented by William Howe in 1840.
His innovation came not in the pattern of his truss, which was similar to the already existing Kingpost pattern, but in the use of vertical iron supports in addition to diagonal wooden supports. Many beam bridges today still use the Howe pattern in their truss.
The Arch Bridge
An arch bridge is a semicircular structure with abutments on each end. The design of the arch, the semicircle, naturally diverts the weight from the bridge deck to the abutments.
Compression
Arch bridges are always under compression. The force of compression is pushed outward along the curve of the arch toward the abutments.
Tension
The tension in an arch is negligible. The natural curve of the arch and its ability to dissipate the force outward greatly reduces the effects of tension on the underside of the arch. The greater the degree of curvature (the larger the semicircle of the arch), however, the greater the effects of tension on the underside.
Dissipation
As we just mentioned, the shape of the arch itself is all that is needed to effectively dissipate the weight from the center of the deck to the abutments. As with the beam bridge, the limits of size will eventually overtake the natural strength of the arch.
Types
Arch types are few -- after all, an arch is an arch is an arch. The only real subcategories come in the form of cosmetic design. There are, for example, Roman, Baroque and Renaissance arches, all of which are architecturally different but structurally the same.
Arches are fascinating in that they are a truly natural form of bridge. It is the shape of the structure that gives it its strength. An arch bridge doesn't need additional supports or cables. In fact, an arch made of stone doesn't even need mortar. Ancient Romans built arch bridges (and aqueducts) that are still standing, and structurally sound, today. These bridges and aqueducts are real testaments to the natural effectiveness of an arch as a bridge structure.
The Suspension Bridge
A suspension bridge is one where cables (or ropes or chains) are strung across the river (or whatever the obstacle happens to be) and the deck is suspended from these cables. Modern suspension bridges have two tall towers through which the cables are strung. Thus, the towers are supporting the majority of the roadway's weight.
Compression
The force of compression pushes down on the suspension bridge's deck, but because it is a suspended roadway, the cables transfer the compression to the towers, which dissipate the compression directly into the earth where they are firmly entrenched.
Tension
The supporting cables, running between the two anchorages, are the lucky recipients of the tension forces. The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage. The anchorages are also under tension, but since they, like the towers, are held firmly to the earth, the tension they experience is dissipated.
Almost all suspension bridges have, in addition to the cables, a supporting truss system beneath the bridge deck (a deck truss). This helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple.
Types
Suspension bridges come in two different designs: the suspension bridge, recognized by the elongated 'M' shape, and the less-common cable-stayed design, which has more of an 'A' shape. The cable-stayed bridge does not require two towers and four anchorages, as does the suspension bridge. Instead, the cables are run from the roadway up to a single tower where they are secured.
The tower in a cable-stayed bridge, like its counterpart in a suspension bridge, is responsible for absorbing and dealing with the compression forces. In both bridges, the cables are under tension.
