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1. INTRODUCTION There's no doubt you've seen a bridge, and it's almost as likely that you've traveled 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, a set of railroad tracks... In this edition of HowStuffWorks, 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 article you will understand why. So let's get started!
2. BASICS
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 a an arch bridge? The answer lies in how each bridge type deals with two important forces called compression and tension:
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 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.
3. BEAM BRIDGE
Compression
Tension
Example
Dissipation 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 bridge's own weight is so large that the truss cannot support it.
Types
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.
4. ARCH BRIDGE
Compression
Tension
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 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.
Compression
Tension
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
Forces on a cable-stayed bridge
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.
Torsion, which is a rotational or twisting force, is one which has been effectively eliminated in all but the largest suspension bridges. The natural shape of the arch and the additional truss structure of the beam bridge have eliminated the destructive effects of torsion on these bridges. Suspension bridges, however, because of the very fact that they are suspended (hanging from a pair of cables), are somewhat more susceptible to torsion, especially in high winds. All suspension bridges have deck-stiffening trusses which, as in the case of beam bridges, effectively eliminate the effects of torsion; but in suspension bridges of extreme length, the deck truss alone is not enough. Wind-tunnel tests are generally conducted on models to determine the bridge's resistance to torsional movements. Aerodynamic truss structures, diagonal suspender cables, and an exaggerated ratio between the depth of the stiffening truss to the length of the span are some of the methods employed to mitigate the effects of torsion. Resonance (a vibration in something caused by an external force that is in harmony with the natural vibration of the original thing) is a force which, unchecked, can be fatal to a bridge. Resonant vibrations will travel through a bridge in the form of waves. A very famous example of resonance waves destroying a bridge is the Tacoma Narrows bridge, which fell apart in 1940 in a 40-mph (64-kph) wind. Close examination of the situation suggested that the bridge's deck-stiffening truss was insufficient for the span, but that alone was not the cause of the bridge's demise. The wind that day was at just the right speed, and hitting the bridge at just the right angle, to start it vibrating. Continued winds increased the vibrations until the waves grew so large and violent that they broke the bridge apart. When an army marches across a bridge, the soldiers are often told to "break step." This is to avoid the possibility that their rhythmic marching will start resonating throughout the bridge. An army that is large enough and marching at the right cadence could start a bridge swaying and undulating until it broke apart. In order to mitigate the resonance effect in a bridge, it is important to build dampeners into the bridge design in order to interrupt the resonant waves. Interrupting them is an effective way to prevent the growth of the waves regardless of the duration or source of the vibrations. Dampening techniques generally involve inertia. If a bridge has, for example, a solid roadway, then a resonant wave can easily travel the length of the bridge. If a bridge roadway is made up of different sections that have overlapping plates, then the movement of one section is transferred to another via the plates, which, since they are overlapping, create a certain amount of friction. The trick is to create enough friction to change the frequency of the resonant wave. Changing the frequency prevents the wave from building. Changing the wave effectively creates two different waves, neither of which can build off the other into a destructive force. The force of nature, specifically weather, is by far the hardest to combat. Rain, ice, wind and salt can each bring down a bridge on its own, and in combination they most certainly will. Bridge designers have learned their craft by studying the failures of the past. Iron has replaced wood and steel has replaced iron. Pre-stressed concrete is used in many highway bridges. Each new material or design technique builds off the lessons of the past. Torsion, resonance and aerodynamics (after several spectacular collapses) have been addressed in better designs. The problems of weather, however, have yet to be completely conquered. Cases of weather-related failure far outnumber those of design-related failures. This can only suggest that we have yet to come up with an effective solution. To this day, there is no specific construction material nor bridge design that will eliminate or even mitigate these forces. The only deterrent is preventive maintenance.
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