Spaghetti Bridge Project - Essay Example

Arch, suspension and truss bridges May 2014 Introduction Bridges are important structures that serve to overcome barriers that hinder movement from one point to another. The bridges are used in connecting two regions separated by a river or may be used to connect an island to mainland. This report looks at three types of bridges: arch bridge, suspension bridge and truss bridge. Arch Bridge Stone was the common material that was in ancient arch bridges. The principle behind the this type of bridge is that material are put under compression where the material commonly put to use are stone, steel and concrete as this materials have good compressive characteristics (Kutz, Myer 2011). When a material is put under compression the particles are squeezed together. A typical example that that illustrates the ability of a material to withstand compressive stress is where a long thin column is able hold a building. Figure 1.1 Figure 1.2 Daniel Carter Beard Bridge (DCB) which found in Cincinnati is a good modern example. Figure 1.6 shows a Roman bridge where the weight carried by the arch has its origin from stones that are on top of the arch. A modern example is the Daniel Carter Beard Bridge in Cincinnati shown in figure 1.1. In the roman bridge the weight that the arch carries comes from the stones on top of the arch. For the DCB bridge case the weight originates at the road deck, running up through the vertical cables which are subjected to tension and the tension forces are then distributed into the arch. The compression forces exerted to the arches are required to press ultimately on to the ground and in order for this to done there is need to create large abutments. This scenario is clearly illustrated in figure 3. Figure1. 3 Figure 1.4 is a picture of a simple wooden arch bridge in china at adjacent to it is figure1.5 which a diagram illustrating the flow of forces the arch down to the abutment and from which the forces are transmitted to ground. In DCB the abutments are not seen. This means that there is a provision of some stiff elements which are under compression and are able to transmit the forces to the riverbanks. The bridge may also be a tried arch where tension forces are involved that allows each of the two ends of the arch to pull against each other. Figure 1.4 Figure 1.5 For the case of Roman arch bridge, there is balancing of the thrust in the inner supports from one side to the other side with the remaining thrust being experienced only in the last buttress. The thrust which is experienced in a series consisting of short arches would be less the amount that is likely to arise where there is one long arch. Figure 1. 6 Figure 1.6 The New River Gorge Bridge which is found in West Virginia has been reported as being the longest arch bridge having a central span of 1700 feet and its total length being 4224 feet. The Lupu Bridge in Shanghai China is longer by 105 feet. The New River Gorge Bridge having a height of 360 feet and a weight of 88,000,000lbs is still the highest ever bridge (www.architecturebychildren.org,2012). Arch bridges are heavy and to increase their ability to carry loads means that they are to be made deeper. In order to avoid the buckling of arch materials, it requires that more materials are to be used with the end results being the arch being even heavier. More increase in material results to a situation where more of the materials are used to support the weight of the arch itself with too little of the strength being left to bear the intended live loads. Suspension bridge In the ancient suspension bridge the material of choice were chains, ropes and vines as can be seen in Figure 2.1. In current suspension bridges the material which are used are steel plates or super-strong steel cables. The cables subject material into tension. Materials like stones and concrete which were seen to be in common use in arch bridges are not strong when subjected to tension, because of their brittleness and their heaviness (Sigmund, 2007). When a material is subjected to tension forces the particles are pulled apart. When a rope is used to hold a weight at its ends takes a role of a long thin element under tension. Figure 2.2 is an illustration of the major parts of a suspension bridge that include the deck, the cable, the suspender, the tower and the backstay. Figure 2.1 Figure 2.2 In suspension bridge there is a curved tension member as can be seen in diagram 2.1. From diagram 2.3 it can be seen that in a suspension bridge both tension and compression forces are involved. The cables are only subjected to tension forces and when the cables stretch across the towers, they create compression forces in the towers by pulling them down. Figure 2.3 The rope bridges in china and the Roebling Bridge in Cincinnati are some of the good examples of suspension bridges. The construction of Roebling Bridge commenced in 1856, where the two stone towers were the first parts to be constructed. Before the real work on the bridge begun , it was necessary for cofferdams that held back Ohio River water to be built that Ohio River water could be held back enabling construction of foundation on dry riverbed. The bridge construction was halted before completion of construction of towers partly as a result of civil war. Temporary bridge which were built from boat to boat near the Roebling Bridge were used in transferring of troops to Kentucky. By the time the war was coming to end construction of the bridge was over and the bridge was then opened in 1866. The year 1896 saw the modification of the bridge to enable it carry heavier loads where there was addition of cables and the deck stiffened by trusses (www.architecturebychildren.org,2012).. The modification of the bridge can be seen by comparison of pictures where new cable is seen running over the top of the towers in addition of the truss on sides of road deck. Figure 2.4 Figure 2.5 Working of suspension bridge The backstays which are the cables that go from tower top down to the ground where they are connected to concrete piers or rocks buried underground and their function is to ensure that bending does not occur in the towers. Looking at the Roebling Bridge it can be seen that the cables at the center span are curving in upwards direction but the cables which are on the outsides (backstays) are straight. The direction of force runs in the same direction as the cable thus keeping it straight. On the other hand the force on the main cables act at an angle to the cables. Characteristics of suspension bridge Unlike with the case with arch bridge, suspension bridges are very light and this makes it possible for them to span for very long distances. Askashi Kaikyo Bridge found in Japan is the leading world wide in terms of length with a centre span of over a mile (6531). Apart from the bridge having a long span the design of the bridge makes it able to withstand huge earthquakes in the range of 8.5units and hurricane wind forces from wind speed of up to 220MPH. Askashi Kaikyo span is more than six times that of Roebling bridge which has a span of 6531feet. Due to the lightness of the suspension bridges cases of them being damaged by wind have been reported, a good case being that of Tacoma Narrows Bridge in Washington that occurred in 1947. Figure 2.6 Truss Bridge The working of trusses is similar to that of beams where both compression and tension forces are carried but unlike beams trusses are less bulky (not heavy) as compared to beams. Beams have materials in some areas which are not fully utilized in withstanding loads. Builder and engineers have the ability to determine the parts that are liable for removal. The truss which is settled for is able to concentrate the forces into smaller members thus doing away with areas of the beams that are under-stressed. The situation in beams and truss is illustrated in figure 3.1 Figure 3.1 The central Bridge was constructed in 1890 (Figure 3.2) was replaced by the Taylor-Southgate Bridge constructed in 1995 (Figure 3.3). There are some noticeable differences between the two bridges. In The Taylor-Southgate Bridge there is use of fewer and longer spans which is not the case in the Central Bridge. Because of the longer spans there is production of larger forces in the Taylor-Southgate but the bridge does not have a depth that can be equated to that of the Central Bridge. The tubes used in Taylor-Southgate Bridge are subjected to much higher stress level, but have the ability to withstand this stress, because in these bridge there is use of stronger type of steel that were not available in 1890. The Central Bridge is early example of a bridge type referred to as cantilever truss bridge. At the central part of the span there is a small trapezoidal shaped truss which normally is supported by two cantilever trusses of larger size. The tall trusses lying at the of each other rest on stone piers which are in mid the river. This type of construction was common during the type of construction of the Central Bridge. Figure 3.2 Figure 3.3 Conclusion The principles of operation of the three types of bridges have been seen in this report. The arch bridge it was seen that in order to accommodate more load the material of the bridge need to be increased, but at a certain point the weight of the bridge cannot be increased further. The suspension bridge has been found to be the lightest and thus can be used for the longest span. Read More