A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle. There are many different designs that all serve unique purposes and apply to different situations. Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed and anchored, the material used to make it, and the funds available to build it
TYPES OF BRIDGES
Bridges can be categorized in several different ways. Common categories include the type of structural elements used, by what they carry, whether they are fixed or movable, and by the materials used
1. Arch Bridge
Ancient arches were made of stone. Arches work by putting the material into compression. Stone (as well as steel and concrete) work well in compression. A material is in compression when its particles are being pushed together. A column holding up a building is a long thin compression element.
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PARTS OF ARCH BRIDGE
The compression forces in an arch have to press ultimately against the ground. To receive those large forces large abutments have to be created
The longest arch bridge in the world (until last year) was the New River Gorge Bridge in West Virginia, built in 1977. It has a central span of 1700 feet and a total length of 4224 feet. The Lupu Bridge in Shanghai now exceeds it by 105 feet. The New River Gorge Bridge is still the highest bridge; it rises 360 feet above the river and weighs 88,000,000 lbs
New River Gorge Bridge
Arches are often heavy. They can carry more load by getting deeper. With its full length in compression, the material can buckle. One way of overcoming buckling is to use more material, and make the arch heavier. At some point too much of its strength is used to support just its own
weight and too little strength is left to carry the superimposed loads of traffic.
2. Suspension Bridge
Ancient suspension bridges were made of rope, vines or chains. Newer suspension bridges use steel plates or super-strong steel cables. Cables work by putting the material into tension. Stone and concrete do not work well in tension; they are too brittle and usually too heavy. A material is in tension when its particles are being pulled apart. A rope holding a weight at its end is a long thin tension element.
Parts of a suspension bridge
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A suspension bridge has a curved tension member. Look back at the diagram of “curved tension” back in the forces section. Examples of suspension bridges include rope bridges like those in ancient China, or the Roebling Bridge in Cincinnati Suspension bridges use a combination of tension and compression. The cables can only carry tension loads. By stretching across the towers, they pull down and create compression in the towers.
How a suspension bridge works
The cables that go from the top of the towers down to the ground are the backstays. The backstays are connected to huge rock or concrete piers buried in the ground. The backstays keep the towers from bending in. There are some experiments later in this tutorial that will let you see what happens if the bridge doesn’t have backstays. Look at the second black and white photo of the Roebling Bridge. Can you see that the cables in the center span curve upward to the towers, but the outer cables, called the backstays, are straight? Can you determine the direction of force on the backstays? It is always in the same direction because the force must run in the same direction of the cable. What is the direction of force on the main cables? What makes them curve?
Suspension bridges are very light. This allows them to span very long distances. The longest suspension bridge in the world is the Askashi Kaikyo Bridge in Japan. In addition to the long span, this bridge was designed to resist huge earthquakes (8.5) and hurricane force winds (220 MPH).
3. Beam Bridge
Ancient beam bridges were made primarily of wood. Modern beam-type bridges are made wood, iron, steel or concrete. How a beam operates is more complex than a cable or an arch. In the cable all of the material is in tension, but in a beam part of the material is in tension and part of the material is in compression. Look at the example of the Royal Albert Bridge design by I. K. Brunel in England.
A beam needs to be made of material that can work well under both compression and tension forces. Wood is a good material for this. Stone is not a good material for a beam - it is strong in compression, but weak in tension. That’s why it is good for arches but bad for beams. The same is true of concrete. To make a concrete beam, we need to add steel rods or cables at the bottom (in the tension area.) Long-span beams came into great use after 1850 when the production of large batches of steel became possible. None of the big bridges crossing the Ohio River are beam-types because the span is too long and their weight would be too great. Beams are more often found in shorter spans such as those at many overpasses. Next time you are driving with your parents on the highway, look at the structure beneath the overpasses as you travel beneath them, and you will more often than not see steel wide-flange beams supported by concrete columns.
4. Truss Bridge
Trusses work much like beams: they carry a combination of compression and tension forces. The main difference is that trusses are less bulky (heavy) than beams. Beams use extra material in some areas; these areas don’t use the full strength available to them. Engineers and builders can determine which portions of beams can be removed. The resulting truss concentrates the forces into many smaller members and eliminates the under-stressed areas of beams.
The Taylor-Southgate is a modern example (1995) of a truss bridge over the Ohio River in Cincinnati. It replaced the Central Bridge built in 1890.
Taylor-Southgate Bridge
5. cable-stayed
The newest type of bridge to be developed is the cable-stayed bridge. They have gained great popularity in recent years because of their great beauty and economy. They cannot be used for truly large spans like a suspension bridge, but they are very good for the more moderate spans that trusses have been used for.
The closest cable-stayed bridge near Cincinnati is the William H. Harsha Bridge near Maysville, KY. It has a main span of 320 meters, or about 1,050 feet. It was completed in October 2000.
William H. Harsha Bridge
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