Modern Bridge Construction Technologies - Coursework Example

The Construction of Bridges Introduction Definition of a bridge Humankind has, over the ages felt the need for crossing obstacles that are faced by all of us, while we are negotiating any pathways. As bridges are the answer to such problems, they can be defined as any structure that can enable us to crossover a culvert, stream, river, railway track or a part of the ocean. A few logs thrown across a small stream, is the simplest form of the bridge construction. As bridges provide us the passage to cross over the obstacles, we all can find any type of a bridge, in our daily life. The construction of flyovers to allow the traffic pass over at several layers, is a fine example of bridges, that can be seen currently almost everywhere....(1) Types of bridges Given below are several types of bridges, with brief details about the each type. While explaining the bridge construction techniques and requirements in detail, this essay will discuss on the technical parameters involve in it. 1. A beam bridge is a simple form of bridge construction, using steel beams, girders reinforced by concrete. The forces of tension and compression in the beam bridge are designed keeping in view that the weight, both live and dead weight of the bridge is directly transferred downwards through the piers. While the steel or concrete beams handle the load, the size and the height of the beam will distance the span length of the bridge. For example, the beams of greater heights have more capacity to distribute the forces of tension involved. Sinmilarly, sometimes the beams are supported by the addition of trusses, which increases the beam strength and its ability to dissipate more effectively, while dealing with the forces of compression or tension. 2. A truss bridge is built to connect obstacles on a straight pathway, using timber logs and iron rods. These were constructed commonly during the earlier days. Sometimes trusses are added to the existing beam structure for increasing its rigidity. 3. An arch bridge is constructed in such a manner that its load is transported to the two ends of the bridge, while this type of the bridge is always in the shape of an arch. Arch bridges have the compressive forces acting along the curve or the arch so that the load is pushed forward towards the ends or the piers of this bridge. However, the more curve in the center may cause activate greater effects of tension, down below....(2) 4. A cantilever bridge is supported only at one end of the bridge, while such structures project horizontally as a cantilever. 5. A suspension bridge uses suspension cables that are anchored at both ends of the bridge, as the bridge hangs on such cables, without having any other support like pillars etc. The cables are strung with tow towers at the ends of the bridge, which take the total load. Most of the suspension bridges have a truss system placed beneath the deck of the bridge. This reduces the bridge sway movements, while strengthening the bridge deck. 6. A cable-stayed bridge is similar to the suspension bride, as both employ cables to support the bridge structure. However, shorter cables are used in such type of bridges, thus allowing for towers of lesser height.........(3) An overview of bridge construction The bridge construction will include layout plans, organization of construction controls and technology involved in the construction of the bridge. However, certain basic construction techniques and terms will be used while planning for the construction of any type of the bridge. Superstructure As the term suggests, superstructure is the top portion of the bridges, which generally relates to the bridge construction above the level of obstacle involved. It will broadly involve construction of a bridge deck, its structural members and the parapet walls along both sides of the bridge. This can be in the form of side railings. Further the final touching of the bridge will also come under the superstructure like side walk footpaths or cycle tracks, lighting etc.. The bridge deck is the part of the bridge that provides the roadway for the bridge path, as it includes the bridge ends as well. Commonly constructed bridge decks use cement concrete reinforced with high quality steel bars. However, in rural areas, the timber logs or planks may be used for such decks. In case of fast bridge construction requirement, like defense bridges, open steel girders can be used as bridge decks. The latest technology has also innovated the use of polymer reinforced fibber decks for certain special bridge deck applications. The bridge deck technology takes into consideration the slope and grade of the road in relation to the obstacle. However, there are certain important factors that may cause a deck to fail. These include primarily the use of poor mix concrete, mostly adopted by scrupulous contractors to save on the high cost of cement, improper air segregation and vibration, as well as improper curing after the laying of concrete. Other major factor for cause of such failures can be improper bonding of the steel reinforced with the concrete, because of pre setting of concrete due to slow pouring of the same and air traps thus formed. There are other structural members like pre-cast concrete beams, pre-stressed and reinforced concrete T-beams. or steel I beams and girders. Similarly, the decks can be supported by pre-cast and pre-stressed as well as reinforced concrete I beams. The superstructure will also have secondary members used as cross beams. These are used between two main structural members. This is mainly to strengthen further the bridge superstructure, while distributing its load uniformly. Other superficial features like lighting and railings along the bridge deck path have nothing to do with the structural design of the bridge. However, it will require aesthetic sense while designing these utilities. ......(4) Substructure The superstructure of the bride will require many assemblies and components to support it. These are part of the bridge substructure and these include pilings, piers, abetments, bridge footings etc. Abetment designs should be such that it supports the bridge deck at its extreme ends. However, for the long bridges, the intermediate piers and interior bents support the abetments. A pier can be a solid concrete wall while the intermediate bents are built with columns. Bridge seats are the area on the top of these bents or abetments, which are platform like areas, built on the top of the cap of such abetments and piers. Special rubberized bearings called shoes connect the structural members of these bridge seats. For uniform distribution of bridge load, spread footings form the base of such abetments and piers. As the name suggests these are large solid base areas spread uniformly to make large blocks of concrete. This will be done, in most of the cases, after the pile foundation is over. The spread footings take the entire load of the substructure, while transmitting it to the foundation material. Spread footings are designed as an anchor against the lateral movements of the bridge substructure, while taking into consideration the wind and other loads that are faced by the bridge superstructure In case of certain substandard soiling and loose soils, the spread footings may not be able to withstand the substructure load and it can cause the bearing failure. As a result, the foundation buckles under the bridge load, which causes damage to the structure apart from its movement away from the designed parameters. Hence, for such locations the pile foundation is advised. As the piling operation transmits the load of the bridge structure to the deep underlying hard soil or rock, such foundation piling members are driven deep into the ground and they act as load bearing members for the complete bridge structure. The whole operation of pile foundation is meant to stabilize the foundation of the footings, which take the load of the bridge substructure.....(4) Bridge spans A span is the portion of the bridge that goes from one substructure unit to the other, in a bridge. As a particular bridge may have several abetments and piers, one or more spans will be required to connect the various bridge segments formed between two abetments or piers and between a pier and abetment or vice-versa. Span lengths are sometimes referred as the total length of the bridge. However, technically, the span length is the length of the individual span of a bridge that may have many such spans. These span lengths are measured, in most cases, as the length from the center point of one bearing to the center point of the other bearing, while going from one substructure unit to the next. Spans are laid out with proper design and careful technique to avoid any undue bumps while the traffic transmits from one portion of the bridge to the other. Sometimes the filling material is used to allow for such smooth transaction. However, this requires periodic maintenance as well. Although the bridges are classified as per their basic designs like a cantilever or a cable bride, arch bridge or a suspension bridge, trussed bridge etc.; sometimes the bridges are also referred as single span, two, three or multiple span bridges. The span bridges can be of simple or continuous span type. A simple span bridge has a fixed bearing on the one end and a flexible bearing at the other end, while crossing from one substructure unit to the other substructure unit. Further, any bridge built on abetments only, is a simple span bridge. When the single span extends from one abetment to a pier or pier to an abetment, it is also a simple span bridge. Simple spans are usually connected on the road surface by using certain coating and filling materials. However, a continuous span bridge will have one or more bridge segments, whose structural members will cross over number of substructure units without a break. For the purpose of continuity, these structural members may require splicing for meeting the length requirement. Continuous spans will require one fixed bearing and a number of expansion bearings to ensure that these spans are properly anchored to the substructure Many bridges will have both continuous as well as simple spans .......(4) Bridge design terminology The bridge design will determine the span and type of the bridge. For example, a suspension bridge can span a distance many times than an arch bridge, which in itself can span more distance than a beam bridge. Accordingly, there is a difference in the design parameters of the specific bridges. These will include forces like tension, compression, torsion and shear that play an important role in the design of the bridge meant for particular application. Tension The application of force of tension to the bridge construction layout will result in more bridge length as the tension applied to a spring will make it longer. Compression Similarly, the force of compression acts in the opposite manners that result in the shorter bridge length, as we can understand in the terms of a nonprofessional. The example here again is the spring being shortened when compressed. The forces of tension and compression should be properly calculated while designing the specific bridge requirements. However, the designers have to look at some aspects very carefully as more compression may result in the buckling of bridge weight under excess compressive forces. Similarly, the excess force of tension may cause snapping, like in a cable or suspension bridge. Therefore, the need is to distribute the weight and these forces uniformly so that their effect is not concentrated on a certain point, but is spread over a wider area. This makes the handling of these forces easier, while designing a safer bridge. Other design job could be the transfer of forces from one area to another, instead of distributing their effect over a greater area. For example, while designing a suspension bridge; these forces are transferred from one area of the bridge to its other area......(5) The forces of torsion and shear are similar to the buckling and snapping characteristics of the bridge design. Conclusion Although there are most modern bridge construction technologies and design software available, the bridge designers always look for the location and site parameters before designing and construction of a bridge. The soil testing and rock formation as well as ground conditions play a major part in determining the type of the bridge to be constructed. Similarly, the bridges over the sea or deep river beds, will require specific foundation technique to build a strong and safe bridge The other important aspect for bridge construction will be to determine the current and future usage of bridge pathways. A rural bridge that does not have any traffic can be a simple beam type bridge supported by wooden piers, with concrete or naked steel girders as bridge deck. BIBLIOGRAPHY In-text citation references (1) The Effect of Bridge Design on Weight Bearing Capacity http://www.sciencebuddies.org/science-fair-projects/project_ideas/CE_p011.shtml?fave=no&isb=cmlkOjI2MjE1NzMsc2lkOjAscDoxLGlhOkNF&from=TSW-- accessed on 16 th. March 2009 (2) Morrisey, Michael—How Bridges Work http://science.howstuffworks.com/bridge5.htm Aceesed on 15th. March 2009 (3) http://www.buzzle.com/articles/types-of-bridges.html (4) Bridge Construction Overview .http://www.in.gov/indot/files/bridge_chapter_01.pdf (pages 1-1 and 1-5) ( accessed on 15th March 2009) (5) http://science.howstuffworks.com/bridge7.htm Sources http://architecture.about.com/od/bridgegallery/Bridge_Construction_.. (accessed on 15th March 2009) http://architecture.about.com/od/bridgegallery/Bridge_Construction_.....( accessed on 10th March 2009) http://science.howstuffworks.com/bridge.htm ( accessed on 11 th March 2009) Bridge Construction Overview .http://www.in.gov/indot/files/bridge_chapter_01.pdf (pages 1-1 and 1-5) ( accessed on 15th March 2009) Read More