Cantilever Bridges Page 4 Advantages Essay

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In cable-stayed bridges are attached to the towers, which "alone bear the load"; but in the design of suspension bridges the cables "…ride freely across the towers, transmitting the load to the anchorages at either end" (NOVA, 2003).

The original idea for cable-stayed bridges goes back to 1595, according to the NOVA article; in a book called Machinae Novae, published in that year, a sketch of a cable-stayed bridge is clearly presented. Apparently no engineer took that sketch to be a possible bridge solution for hundreds of years because the first cable-stayed bridges were not constructed until the twentieth century (NOVA). A very good example of a cable-stayed bridge is the Sunshine Skyway Bridge in Tampa, Florida, which was built in 1988 and won the prestigious Presidential Design Award from the National Endowment for the Arts.

Robert Lamb and Michael Morrissey explain in the Discover Company's publication, Science / How Stuff Works, that the author of Machinae Novae was Croatian inventor Faust Vrancic. Lamb and Morrissey note that a cable-stayed bridge may, "at first glance," appear to be a cousin of the suspension bridge; but even though they both have "similar towers and hanging roadways" they are not the same. Cable-stayed bridges don't need anchorages and they don't need two towers either, the authors explain.

In a cable-stayed bridge, the cables run from the roadway "…up to a single tower that alone bears the weight," and the tower has the responsibility to absorb the "compressional forces" (Lamb, et al., 2011). Cables may be connected to the roadway in several places and all spiral up to a single point in the tower; Lamb compares the fact of several cables attached at different places on the roadway below to a high point above on the tower to "…numerous fishing lines attached to a single pole" (Lamb).

There are two basic types of bridges in the cable-stay category, according to Aileen Cho writing in Engineering News-Record (Cho, 2012, p. 2). There is the "fan" type and there also is the "harp" type in the cable-stay genre, Cho writes. Of the top 10 cable-stay bridges, all use the modified fan configuration instead of the harp configuration, Cho explains, because in the harp configuration there tends to be "…increased compression in the superstructure" (p. 2). In the fan type of cable-stay bridge the cable stays "…are spaced out over the top portion of the pylon" to allow more room "…to be individually anchored near the pylon top" (Cho, p. 2). While the harp model uses cable stays "…in equal spaces over much of the height of the pylon" and hence, it offers a "pleasant aesthetic appearance" albeit the harp style is not as efficient structurally (Cho, p. 2).

Advantages of Cable-Stayed Bridges

According to Robert Lamb and Michael Morrissey, cable-stayed bridges offer all the positives that go along with suspension bridges, but for cable-stayed bridges that have spans of 500 to 2,800 feet, they cost less to build. Moreover, cable-stayed bridges do not need as much steel cable as other bridges, they are quickly to construct and they "incorporate more pre-cast concrete sections" (Lamb, 2011).

The Top Five of the World's Longest Cable-Stayed Bridges

The Sutong Bridge -- which crosses the Yangtze River in China, is 1,088 meters, Aileen Cho explains. The Stonecutters Bridge spans the Rambler Channel in Hong Kong Harbor; it is 1,018 meters and was completed in 2009. Another long Chinese bridge, the Edong Bridge, which is 926 meters long, spans the Yangtze River at the Port of Huangshi; the Edong Bridge is aesthetically beautiful with harp-like cables connecting the highway with the pylons. It was completed in 2010. The Tatara Bridge, in Japan, crosses the Inland Sea of Japan to connect the main island in Japan, Honshu, with Shikoku. It is 890 meters and was completed in 1999; at that time it was the longest cable-stayed bridge in the world. The fifth longest cable-stayed bridge in the world, as of 2012, is the Pont De Normandie Bridge in Northern France; it is 856 meters and it spans the River Seine.

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(Cho, 2012).

Comparing Cable-stayed Bridges with Cantilever Bridges

For one thing, a cable-stayed bridge is less costly than a cantilever bridge (Weeks, 1996). Indeed, author Mark Denny explains that cantilever bridges "…tend to be massive and therefore expensive" (Denny, 160). Denny goes on to say that cable-stayed bridges "…have much in common with cantilever bridges"; in fact, Denny asserts, a cable-stayed bridge is really "…a cantilever bridge with cables added to relieve the load" (164). Since the cables of a cable-stayed bridge are "…distributed symmetrically about each tower," and hence the weight of the deck (highway or railroad tracks) supported by the tower is also symmetrical, Denny writes (164).

That having been said, Denny adds that there will be some asymmetrical loading with cable-stayed bridges because traffic differs from morning to afternoon. In the morning there may be heavy traffic on one lane of the bridge's road, and in the evening the traffic is heavy on the opposite lane of the road. This "…exerts a torsion upon the bridge deck," and that dynamic can produce "bending in the relatively flexible decks" of some bridges, but not cable-stayed bridges (Denny).

Similar to a cantilever bridge, the tower on the cable-stayed bridge does not experience any "horizontal loading," which greatly enhances the structure's stability, Denny explains (164). Looking at additional similarities between the two types of bridges, the cable-stayed bridge is actually constructed in the same way as cantilever bridges in the sense that each tower is built independently (with both types); and the deck is "cantilevered out symmetrically on both sides" (Denny, 164).

Great care must be taken when building both kinds of bridges, Denny asserts, because the forces exerted "upon the deck can be considerable" (164). On 165 Denny explains that both cantilever and cable-stayed bridges to not require massive "abutments or terminal piers" due to the fact that horizontal loading is very small.

In his conclusion on the comparison between the two kinds of bridges, Denny writes what is obvious to any observer of Cable-stayed bridges. That is, the cable-stayed bridge can be "elegant…in a way that cantilever bridges just cannot"; that is due to the slender look and graceful lines of the cable-stayed bridge. The cables on cable-stayed bridges can and do fan out from the very highest points of the towers, or they may, as Denny writes on page 165, "emerge parallel to each other, harp fashion, from different points up the towers."

Leonardo Fernandez Troyano (Troyano, 2003, p. 592) explains in his book that the history and development of cable-stayed bridges is very different from the history of cantilever bridges, albeit cantilever and other bridge types came well before cable-stayed bridges. "This lag in their origin is being recovered in giant steps," Troyano points out (592). In fact the evolution of cable-stayed bridges has happened far quicker than cantilever bridges, and moreover, it took less than 40 years for the maximum span of cable-stayed bridges to "multiply by almost five times" (Troyana, 592).

The author goes on to assert that the most "representative bridge of the second half of the twentieth century is the cable-stayed bridge," giving the cable-stayed bridge "a value of novelty, which has made them the privileged bridge of present times" (592). Additionally, cable-stayed bridges have a "freedom of design" -- unlike cantilever bridged -- that allows cable-stayed bridges to be more lightweight, to be more suitable for shorter spans. Also, a cable-stayed bridge can have two towers and be "symmetrical," or it can have just one tower "from where the whole main span is stayed" (Troyana, 593).

Conclusion

For engineers there are many advantages to knowing and understanding the various types of bridges. This paper has provided a number of references that point to the strengths of both cantilever and cable stayed bridges. It is the opinion of this paper that cable-stayed bridges are in general the best choice between the two, because the cable stayed bridge is.....

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