Term Paper: Bridge Design and Engineering Bridges

Pages: 7 (2956 words)  ·  Bibliography Sources: 1+  ·  Level: College Senior  ·  Topic: Engineering  ·  Buy This Paper

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[. . .] His 32-meter cable-stayed bridge of 1784 was made entirely out of timber, even using timber stays. In 1817, Redpath and Brown began further development of this bridge type, but later two of their built bridges would collapse, gaining a lot of bad press for this type of bridge. In the United States, some small cable-stayed bridges were constructed during the 1920's, and stays were utilized in the construction of the famed Brooklyn Bridge, but unfortunately the bad reputation earned by the collapsed cable-stayed bridges of days past mostly kept bridge designers away from this kind. The first of the modern cable-stayed bridges was created by F.Dischinger, a German engineer who spent time reconstructing destroyed bridges in postwar Europe. During that time he discovered the positive aspects of this type of bridge, and he would create the Strosmund bridge in Sweden, in 1955, beginning a new era of bridge engineering. In 1988, the very noted Sunshine Skyway bridge -- a cable-stay bridge, of course -- in Tampa, Florida would win the Presidential Design Award from the National Endowment for the Arts. A cable-stayed design was also chosen for a new bridge across the Charles River in Boston, Massachusetts. Elements of cable-stay bridges now make their way into the design for many other bridge types as well, giving birth to the cable-stayed hybrid bridge.

The cable-stayed bridge has at least one pillar in the middle of the span of the bridge, with cables supporting the roadbed. Bridges with one, two, and three pillars have been constructed. When the bridge has two pillars, it will look very similar to a suspension bridge, though the actual functionality of the bridge is quite dissimilar. In a suspension bridge, a huge cable is strung between at least two pillars, which is what bears the majority of the total load. The cables experience tension from crossing the gap between pillars, and the weight of the cable is the primary load. Smaller cables and rods will suspend from the main cable to hold the roadbed. In the cable-stayed bridge, however, the pillars themselves, not the cables, form the primary load-bearing structure. Often a cantilever-type approach is used to support of the roadbed near the pillars, but further from the pillars cables run from the roadbed directly to the top of the pillar for support. Cable-stayed bridges can be identified by the number of spans, the number of towers, the girder type, and the number of cables. Typical towers used are single, double, protal, and A-shaped towers; typical cable varieties are mono, harp, fan, and star arrangements. The typical cable-stayed bridge spans 110m to 480m, and the longest bridge of this type is the Tartara Bridge in Japan, which is 1,480m long.

The cable-stayed bridge has some complications that accompany its design and use, like all other bridge types do. For example, the cables that run from the roadway directly to the top of the pillars pull to the side as opposed to pulling directly up they way they do in suspension bridges. This requires the roadbed to be much stronger to resist the sideways-pulling loads of these cables. The cable-stayed bridge is also made very lightweight and with very flexible cables, which makes them highly susceptible to swaying in the wind during storms. They also cannot cover nearly the length that suspension bridges can. The structure is overall very complex to design, and calculations for today's large bridges would be impossible without the aid of computers. On the plus side, cable-stayed bridges require far less roadbed material than would be needed for a cantilever bridge, and can be used for much longer gaps than many other bridge designs. The major amounts of material used are the cables, which are very economical in themselves. Also, while these bridges may not lend well to heavy winds, they have remarkable staying power during an earthquake.

The cable-stayed bridge is a great type of bridge. Although it has received a lot of bad press because of a few highly publicized collapses many years ago, the modern cable-stayed bridge is a wonderful answer to the needs of many large gaps. More economical and wide-reaching than many bridge designs, the cable-stayed bridge is more interesting than many of the other options.

THE REINFORCED CONCRETE BRIDGE

The reinforced concrete bridge is another standard bridge type. Unlike both the suspension bridge and the cable stayed bridge which are similar to each other in design, the concrete bridge does not utilize cables to connect the bridge bed to towers for support. However, steel cables have previously been used inside of the concrete for reinforcement. The three principal parts that make up the reinforced concrete bridge are the main supporting structure, which includes the columns, piers, arches, and abutments), the bridge deck, and ancillary features (like parapets and services.) This type of bridge is most commonly associated with arch bridges, but it is not exclusively used for arches at all.

Concrete is a mixture of sand, stone, and cement which hardens into a solid form when mixed with water in the right proportions. Although concrete was used by the ancient Romans and other early civilizations for many building structures, it did not prove to be very useful in the building of bridges until steel was available for reinforcement. Concrete behaves very much like stone for building; it is very resistant to compression, but it is not very bendable or tensile, which gave it a significant weakness. However, with steel imbedded within the concrete, it is given tensile strength, making it perfect for bridges.

The first reinforced concrete bridge in America was built in 1889, but it was an experimental project and engineers continued to use trial and error methods to improve the materials and designs. Concrete varies in strength and other properties depending on the precise proportions of the ingredients, and it takes a while to perfect the ratios to be used for various projects. Engineers and contractors welcomed use of this material, however, because it was already being used in dams, roads, and buildings in the early 1900's. Many methods of reinforcement were being used and tested, including steel beams, twisted bars, cables, and ridged rods; the ridged rods are the most commonly used method today.

The advantages of reinforced concrete bridges are many. First and foremost the material in very inexpensive, and the sand, stone, and water needed can usually be found locally or in nearby areas of the project at hand, which means that there are no costly importation.

In comparison with metal trusses, concrete is also low-maintainance, as it does not need to be repainted often, and there is no worry about rusting.

There has been a boom of interest in old concrete bridges recently because of concerns that they are not safe for modern use due to the approach alignments and lane widths on the road, and also due to concerns that the concrete should be reinforced further to support heavier loads. Rehabilitation of very old reinforced concrete bridges can prove to be very expensive, however, compared to completely replacing the bridge, but there is a great deal of historical interest surrounding concrete bridges. One of the difficulties in restoring old concrete is that ratios for the ingredients was not standardized when many of the beautiful historical concrete bridges were built, which means that matching the existing concrete for patches can be nearly impossible.

Often times it makes more financial sense to simply preserve the supporting structure, but to reconstruct the deck and parapets completely from scratch.

IN CONCLUSION

All three types of bridges discussed here have their own sets of advantages and disadvantages. Cost and availability of materials, complexity of engineering, length capabilities, and artistic and aesthetic choice all go into the decisions that an engineer, designer, or contractor must consider when choosing the appropriate bridge for each necessary location and use. All of these bridges have historical merit and interesting design and usage facts.

Bibliography

Cable Stayed Bridge." Super Bridge. NOVA. 1997. http://www.pbs.org/wgbh/nova/bridge/meetcable.html

Caprani, Colin. "Cable Stay Bridge History." Cable Stay Bridges. Thesis. http://www.clubi.ie/ccaprani/thesis/

Christien, et al. "Suspension Bridge." Wikipedia. 2003-2004. http://en.wikipedia.org/wiki/Suspension_bridge

Concrete Arch Bridges."

Connecticut's Historic Concrete Bridges. http://www.past-inc.org/historic-bridges/concrete-right.html

Davis, Allen; Michols, Kevin; Olson, Carlton. "Evalutation of Historic Reinforced Concrete Bridges. http://www.germann.org/Pages/Download/download/asce2001.pdf

Invention Factory. "Roebling's Bridge Division." Roebling History Archive. September 2002. http://www.inventionfactory.com/history/RHAbridg

Lux, David. "Bridge Chronology." The History of American Technology. Fall 2002. http://web.bryant.edu/~history/h364material/bridges/brdgs_1.htm

Matsuo Bridge Co. "Cable Stayed." The Basic Bridge Types. http://www.matsuo-bridge.co.jp/english/bridges/basics/cablestay.shtm

Minnesota Historical Society. "Minnesota's Historical Bridges: Cedar Ave. Bridge." Historic Significance. 1996. http://www.mnhs.org/places/nationalregister/bridges/nrhecab/sign.html [END OF PREVIEW]

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