What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to span a distance nearly 7 times that of a an arch bridge? The answer lies in how each bridge type deals with two important forces called Compression and Tension:
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 them is to spread them out over a greater area, so that no one spot has to bear the brunt of the concentrated force. To transfer them is to move the forces from an area of weakness to an area of strength, an area designed to handle the forces. An arch bridge is a good example of dissipation, while a suspension bridge is a good example of transference.
For the bridge builder program you will be constructing a beam bridge with a span of 24 meters. So let's look at how the beam bridge deals with tension and compression.
A trubeam bridge is basically a rigid horizontal structure which is resting on two piers, one at each end. The weight of the bridge and any traffic on it is directly supported by the piers. The weight is traveling directly downward.
The force of compression manifests itself on the top side of the beam bridge's deck (or roadway). This causes the upper portion of the deck to shorten.
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.
If you take a two by four and place it on top of two empty milk crates, you would have 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 kept adding weight, eventually the two by four will break. Actually the top side will buckle and the bottom side will snap.
Many beam bridges that you find on highway overpasses, like the ones pictured below, use concrete or steel 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 lattice work, 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 bridge's own weight is so large that the truss cannot support it.
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 US was developing rapidly. Designers were coming up with many different truss designs and compositions. Wooden bridges were being replaced by all iron or wood and iron combinations. 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.
If the beam were designed so that there was more material on the top and bottom, and less in the middle, it would be better able to handle the forces of compression and tension. For this reason I-beams are more rigid than simple rectangular beams.
A truss system takes this concept one step further. Think of one side of a truss bridge as a single beam. The center of the beam is made up of the diagonal members of the truss while the top and bottom of the truss represent the top and bottom of the beam. Looking at a truss in this way we can see that the top and bottom of the beam contain more material than its center (corrugated cardboard is very stiff for this reason).
In addition to the above mentioned effect
of a truss system, there is another reason why a truss is more rigid than
a single beam. A truss has the ability to dissipate a load through the
truss work. The design of a truss, which is usually a variant of a triangle,
creates both a very rigid structure and one which transfers the load from
a single point to a considerably wider area.
We have so far touched on the two biggest forces in bridge design. There are dozens of other forces working on bridges which need also be taken into consideration when designing a bridge. These forces are usually specific to a particular location or bridge design.
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, or 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, 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 which 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 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 starts resonating throughout the bridge. An army large enough, 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 which 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, each of these alone can bring down a bridge, 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 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 which will eliminate or even mitigate these forces. The only deterrent is preventive maintenance.
Now you are ready to start building your bridge.
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