Cable-Stayed and Suspension Bridges Case dissertation

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It was observed by Cowper that the smallest loads could change the shape of the chain and thus the calculations for the load bearing capacities came to be formulated. Finally the movement of the load itself, where the curve is distorted when the load is at one end and not yet approached the middle, the curve at the middle takes a parabola. Thus the suspension bridges were modified as per Cowper's theory to include an arch in the bridge length that matched the curves of the chains, and it was also a feature that was named the 'inverted arch bridge.' (Cowper, 1847, p. 7)

In many ways the suspended bridge is superior over a span that is very long and for hundreds of years it has demonstrated its superiority over the other types by being consistently selected as the model for bridges. The origin of the suspension bridge was traced by Kawada (2010, p. 29) from the development of the manufacture of chains for the ships. The flat eye bar chain (see pic at appendix 1) was used in brown's suspended bridge and this came after the development of these chains for ships. (Kawada, 2010, p. 29)

Looking at the history of suspension bridges, engineers like Finley and other engineers in Britain did research on the tower -- cable interactions. While the Menai Bridge was built it was the focal point of this type of enquiry. But the concern centered on the tower that could withstand the pull of the cable. It was Navier who approached this quest with mathematical applications and categorized the tower and cable systems and the possibilities of the combination of the two. Thus there was classified the slim vertical tower supported by the central cable and a separate anchor cable. In the second type the massive tower supported a single continuous cable that could slide back and forth over the tower. Naviers formulas for the suspension bridge thus detailed the effects of the cable sag and the concentrated loads relative to the bridge span and size. (Kranakis, 1997, p. 136)

Thus in the times of Navier the suspension bridge was thought of as the ultimate in bridges because it was believed to span very long distances and at the same time it was flexible, massless and using the Navier formula it was easy to calculate the cable curve. On the other hand the second type of bridge that was suspended from the tower with a span of cables for shorter length was the cable stayed bridge. This type of bridge- the cable stayed was developed in Germany before World War II and it was a method adapted to save the use of steel. Some of the early bridges that were built in the post war periods are the Stromsund Bridge in Sweden and Dusseldorf North Bridge and the Marcablo Lake Bridge at Venezuela are some examples of cable stayed bridges. The modern bridge of this type can be seen in India where the Akkar Bridge in Sikkim spans 76.2 Meter and the 457m span Hoogly Bridge are modern examples of the cable stayed type. (Vikctor, 2007, p. 18)

This literature review has not gone on to the details of the comparisons of other type of bridges and has also omitted many technical details. There are some gaps in the literature regarding the modern bridges especially the cable stayed bridges. While exclusive literature and research can be found in the case of suspension bridges, the research on cable stayed bridges is scattered. In looking at the history of these bridge types, it is evident that the cable-stayed bridges can be seen constructed during the 1600's. But its possibilities and attraction came to the notice of engineers very recently as a bridge concept. One of the important features of the cable-stayed bridges is that they are economical and can be used for the intermediate spans and in places where the suspension bridge can be costly. However it is in the long-span range. A comparison between the suspension and cable-stayed bridges was attempted by Podolny, Jr., whose comparison is based on the qualities of cable weight optimization, stiffness and the aerodynamics as a major factor. Podolny, Jr. (1976, p. 295) came to a general conclusion that the cable-stayed system is stiffer and more economical compared to a suspension system of identical span.

Wind effects in the case of suspension bridges and cable-stayed bridges show that the cable-stayed system is more adaptive to wind force than the suspension bridge. These and other factors make the cable bridges more attractive. The literature review has not this far been able to throw light on the possibility of combining both the types to meet a longer height and span demand. That must wait as a specific and new research topic. But it is stressed that it is not impossible and when we compare the types, both the suspension and the chain stay type, they have many similarities and can be combined over vast spaces. However a comparison of the two was done by Croll (1997, p. 348) which can throw more light into the matter. The comparison is done with the sets of bridges. Of the six basic forms of a bridge, suspension bridges and cable stayed bridges are the most used and considered today. In the case of an analysis therefore a case study is in order.

4. Case Studies

The review of literature on cable stayed bridges is not exhaustive and therefore there is reliance both on modern and ancient writers. The reason is that the possible application of this type of bridge system is recognized with the better possibilities only recently. For example, only recently there has been extensive research all over the world in this type of bridges especially with the use of concrete. The bridges are constructed using an orthotropic deck, and with continuous girders supported by stays and these stays are attached to towers that are placed in the intermittent ranges and the cable stay has become possible with the extensive introduction of higher tensile steel and concrete decks. (Dayaratnam, 2000, p. 22)

Cable stay system has an inbuilt space system that has stiffening girders, a steel or concrete deck towers and inclining cables that take the tension. The stiffening girders is the feature of this system and the horizontal thrust of the cables are minimized with the orthotropic deck invented recently. The advantages of the cable stay are that the deflections are very small, and is much stiffer, lesser resources like steel is used, anchorages and heavy systems are not required for the cables as in the span bridge. Further it can be used for future innovations that may be required to find a method of creating a structure without harm to the environment. (Dayaratnam, 2000, p. 25)

The comparison using similar type of bridges done by Croll (1997, p. 351) seem to have stumbled upon the simplicity of the difference in the simplicity of the cable stayed and the suspension bridges when he examined France's longest main span cable-stayed bridge, Porlt de Normandie, "which was completed in 1994, and having a central span of 856m and the catenary suspension bridge at Tancarville, having a central span of 608m.." This was an ideal method of comparison and Croll, says that he was taken in by the slenderness of the Pont de Normandie cable stayed bridge as opposed to the "heavy main cable and stiffening girder of the Tancarville Bridge." (Croll, 1997, p. 352) Croll argues that "while the overall dimensions are the same, the formula LCd=U applied would show a compression above the pier. And the s horizontal force distribution for the material comparisons eliminates the external horizontal reactions as a contributing factor." (Croll, 1997, p. 352)

Like wise in the case of the deck tension for the suspension bridge as against the cable stay, the deck must be able to create a membrane tension to offset components of the inclined cable forces. Owing to the strength and the lesser complexity the cable stayed system can be used universally because of its specific features that are unique to it like high strength steel, the decks, and modern welding and maintenance makes it easy for the type of bridge to be conceived and created at lesser cost. Hence it is most favored. (Dayaratnam, 2000, p. 27) There is no doubt that the bridges have their own merits and demerits but the cable stay type is being favored more by engineers.

5. Conclusions

The difference between the suspension bridge and the cable type are the towers at each end and the towers support and the cable-stayed type of bridge is quite modern and is economical but not useful in long spans. While we can appreciate that fact that the chain stay type is the most useful and favored it is possible to combine the two in such a way that the tower and cable stay and the suspension bridge types can be amalgamated to build…[continue]

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