Humans have tamed steel, stone, lumber, and even living vegetation, all in effort to reach the people, places, and things that we desire. Although the concept of bridges is as simple as a tree falling across a creek, bridge design and construction requires very serious ingenuity. Artists, engineers, and architects pour vast resources into bridge construction so that they can reshape our daily environment for the better. When building bridges you’ll need help from BATS which are the key structural components of bridge construction such as beams, arches , trusses, and suspensions.
Various combinations of these four technologies make it possible for numerous bridge designs, ranging from some bridges as simple as beam bridges, arch bridges, truss bridges, and suspension bridges to more complicated bridges like side-spar cable-stayed bridges. Some of the key differences between these four types of bridges is the lengths that they can cross a single span, which is the total distancve between two of the bridges supports. Bridges supports can take the forms of columns, towers or even the walls of nature around the bridge like canyons.
Beam bridges range up to 200 feet , while modern arch bridges can reach up to 800-1000 feet safely. Suspension bridges on the other hand are able to extend from 2000-7000 feet across. Compression and tension are present in all bridges and they are capable of damaging parts of the bridge as varying load weights and other forces act on the structure of the bridge. It is the job of the bridge design to handle these forces without buckling or snapping. Buckling occurs when a compression is able to overcome a objects ability to endure that certain force.
Snapping is what happens when tension surpasses an objects ability to handle the lengthening force. The most effective way to deal with these powerful forces is to either dissipate them or transfer them. With the dissipation the design allows the force to be spread out over a greater area so that no one certain spot has to endure to much pressure. In transferring force, a design moves stress from an area of weakness to an area of strength. Beam bridges, bridge building isn’t more simple than this. When building a beam bridge all you need is a rigid horizontal structure and two supports, one at each end, to rest it on.
These components directly support the downward weight of the bridge and any traffic traveling over it. Many beam bridges use steel or concrete to handle their certain loads. The size of the beam, and the certain height of the beams, determines how far that the beam can span up to. By increasing to height of the beam, the beam has more material to lower the tension. To create taller beams the designer of the bridge adds supporting latticework, or a truss, to the bridge’s beam. The support from the truss adds rigidity to the existing beam, greatly increasing its ability to dissipate the compression and tension of the bridge.
Once the beam begins to compress, the force spreads through the truss. Yet even with a truss a beam bridge is only good for a max-limited distance. To make the bridge have a greater distance you need to build a bigger truss, until you have reached the point where even a truss cant support the bridges weight. During the industrial revolution, beam bridges were developing in the United States rapidly. Engineers gave many different truss designs in order to try and perfect it. All the different truss patterns also factored into how bridges were being built. ome designs had the truss under the bridge ,while some designs had the truss above the bridge. A single beam spreading any distance undergoes compression and tension. At the very top of the beam has the most compression and at the very bottom of the beam has the most tension. In the middle of the beam has very little compression or tension. This is why beams are built with bridges, they provide more material on the tops and bottoms of beams to better handle the forces of compression and tension. 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 truss design, which is a variant of a triangle, creates both a very rigid structure and one that transfers the load from a single point to a considerably wider area. After being used for 2000 years of architectural use, the arch continues to feature prominently in bridge designs. Its semicircular structure elegantly distributes compression through its entire form and diverts weight onto its two abutments, which are the components of the bridge that directly take on the pressure being exerted onto the bridge. The tensional forces in arch bridges are virtually negligible.
That is because 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 greater the effects of tension on the underside of the bridge. If you build a big enough arch, the tension will eventually overtake the support of the bridges natural structure. While there is a fair amount in variety in arch bridge construction, the basic structure of every arch bridge is the same. For example there is Roman, Baroque and Renaissance which are all architecturally different they all have the same basic structure.
It is the individual arch itself gives its namesake bridge its strength. An arch made of stone doesn’t need a mortar. In fact the ancient Romans built arch bridges and aqueducts that are still standing today and are made of stone. The tricky part , however is building the arch, as two converging parts of the structure have no structural integrity until they meet in the middle, which mean additional scaffolding or support systems are typically needed. The modern materials such as steel, and prestressed concrete allow us to build far larger arches than the ancient Romans ever were able.
Modern arches typically span between 200 and 800 feet. There is one bridge in West Virgina named the New River George Bridge and it measures an impressive 1700 feet. Suspension bridges, as the name implies its suspend the rail the railway by cables, ropes, or chains from two towers. These towers support most of the bridges weight as compression pushes down on the suspension bridges deck and then travels up the cables, ropes, or chains to transfer compression directly into the earth. The supporting cables receive the bridges tensional forces. The cables of the bridge run horizontally between the two far flung anchorages.
Bridge anchorages are essentially solid rock or massive concrete blocks in which the bridge is grounded. The tensional forces pass through anchorages and into the ground. In addition to all the cables almost all the suspension in bridges feature a supporting a truss system beneath the bridge is called a deck truss. This often helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple. Suspension bridges can easily cross distances such as 2000 to 7000 feet and this enables them to reach distances that other bridge designs cannot.
Because of this bridges complexity and of their design they require a lot of materials , they are the most costly bridge to build. But not every suspension bridge is made out of steel andother costly materials. It can be as simple as twisted grass. When the Spanish conquistadors made their way into Peru in 1532, there they discovered an incan empire connected by hundreds of suspension bridges, achieving spans up to 150 feet or more across deep mountain gorges. Europe on the other hand wouldn’t see a suspension bridge for atleast 300 more years. At a first glance the cable-stayed bridge may look like just a variant of the suspension bridge, ut don’t let their similar towers and hanging railways confuse you. Cable stayed bridges are different from suspension bridges because they don’t require anchorages, nor do they need two towers. Instead the cables run from the railway up to a tower that bears the weight alone. The tower in a cable stayed bridge is responsible for absorbing and dealing with all the compression forces. The cables attached to the bridge run to the tower in a variety of ways. For example, they can run in a radial pattern, cables can extend from several points on the road to a single point at the tower.
They can also be in a parallel pattern, the cables attach to both the roadway and the tower at several separate points. The first cable strayed bridges were constructed in Europe after world war 2, but the basic design dates back to the 16th century and Croatian inventor Faust Vrancic. A contemporary of astronomers Tycho Brache and Johannes Kepler, Vrancic produced the first well known sketch of a cable stayed bridge in his book “machinae Novae. ” Today cable stayed bridges are a popular choice as they offer all the advantages of a suspension bridge but at a leser cost for spans, up to 500 to 2800 feet.
They require a less steel cable, and are faster to build and incorporate more precast concrete sections. Most of humanities build bridging legacy is a story of artificial structures crafted out of the natural elements. Build a bridge out of woven vines or hewn boards and nature will certainly turn it into compost. Building a living bridge takes patience of course. The war-khasis people for example create root-guided systems from hallowed halves of old betel nut tree trunks to direct strangler fig roots in the desired direction.
They simply direct the roots out over a creek or river spanning and only allow the roots to dive into the earth on the opposite bank. The larger living bridges boast lengths of up to 100 feet and care bear the weight of 50 people. There are things that engineers such as torsion which occurs when high wind causes the suspended roadway to rotate and twist like rolling a wave. Also there is shear stress which occurs when two fastened structures are forced in opposite directions. If a bridge has sighs of shear stress and it is unchecked the bridge can literally rip the bridge in half.
A simple shear force would be to drive a long stake halfway into the ground and then apply lateral force against the side of the upper portion of the stake. With enough sufficient preasure youd be able to snap the stake in half. Resonance, you can think of this as simply a vibrational equivalence of a snowball rolling down a hill and becoming an avalanche. It starts off relatively small and periodicly stimulus of a mechanical system, such as wind buffeting a bridge. These vibrations however are more or less in harmony with the bridges natural vibrations.
If unchecked the vibrations traveling through the bridge can form torsional waves. The best example of this occurred in 1940, when resonant vibrations destroyed the Tacoma Narrows Bridge in Washington. The innocent was especially shocking at the time as the structure was designed to withstand winds up to 120 miles per hour and collapsed in a mere 40 mile wind. When there was close examination of the innocent it suggested that the bridges deck-stiffing truss was insufficient for the span, nut this alone couldn’t bring such a structure down.
As turned out, the wind that day was at just the right speed and hit the bridge at just the right angle to set it off the deadly vibration. Continued winds increased the vibrations until the waves grew so large and violent that they broke the bridge apart. This simple effect is just like a singer breaking glass with their voice. Wind isn’t the only thing that is a threat to bridges. For example when an army marches across a bridge, the soliders often “break step” so that their rhythmic marching will start resonating throughout the bridge. A sufficient large army marching at the right cadence could set the deadly vibration into motion.
In order to mitigrate fully the resonance effect in a bridge, engineer incorporate dampeners into the bridge design to interrupt the resonant waves and prevent them from growing. Another way to halt resonance is to give it less room to run wild. If a bridge boast a solid roadway, then a resonant wave can easily travel the length of the bridge and wreak havoc. But if a bridge roadway is made up of different sections with overlapping plates, then the movement of one section merely transfers to another to another via the plates generating friction. The trick is to create enough friction to change the frequency of the resonant waves.
Changes the frequency prevents the waves from building. While wind can certainly induce destructive resonant waves, whether a whole host of destructive assaults on the bridges we build. In fact, the relentless work of rain, ice, wind, and salt will inevitably bring down any bridge that humans can erect. Bridge designers have learned their craft by studying their failures of the pass. Iron has replaced wood and steel has replace iron. Pre-stressed concrete now plays a vital role in the construction of highway bridges. Each new material or design Technique builds off the lesson of the past. Torison, resonance.