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Bridge Design and Engineering
Bridges are an integral but often overlooked part of today's commuting society. Most drivers feel completely secure and grounded when on any well-designed bridge, even though they may in fact be hundreds of feet in the air above a large gap or body of water. Bridges are not only functional for travel, but may be the key to growth and survival of many areas that would otherwise remain in isolation. Bridges also have a way of becoming important historical and artistic landmarks, and many bridges receive engineering and even artistic awards. Many cities like San Francisco, for example, are as recognizable by the their bridges as by any other city landmark. There are many different kinds of bridges used today. Three of the more common and interesting types of bridge in common usage today is the suspension bridge, the cable stayed bridge, and the reinforced concrete bridge.
THE SUSPENSION BRIDGE
One of the engineering marvels of the early 19th century was the development of the suspension bridge. This new type of bridge was welcomed into modern use with open arms because of the many advantages this design offered over its predecessors. The advantages of a suspension bridge are that the center span of the bridge may be extremely long so that it can span very wide and very deep space, that it can be built very high over water so that tall ships may pass under it, and that "the economics of the suspension bridge still make it the most efficient form [of bridge] for very long spans." (Lux) The suspension bridge is a fascinating topic to discuss because of its unique engineering and design, as well as the complex history in both the 19th and 20th centuries.
The suspension bridge is comprised of two pairs of pillars at either end of the span of the bridge, with two or many more cables connecting between them. The main part of the bridge is suspended from these cables, the largest of which will eventually anchor into the ground. The tension in the main cables and the compression in the pillars are the main forces that keep them standing. Some identifying parts of the suspension bridge are the anchor, which actually anchors the bridge to the ground; the main span, which is the space between the two towers; the towers and tower pier, which mount the towers beneath the water; the cable band, which fits the suspenders to the main cable; the camber, which is how flexible the roadway is; and the stiffening truss, which prevents the bridge from flexing or sagging too much. The typical span lengths of suspension bridges are between 2,000 to 7,000 feet today.
The suspension bridge was originally designed by John Roebling in 1845, and has in fact been inducted into the Inventor's Hall of Fame for this contribution to engineering. His first experience with construction of a suspension bridge was actually the Pittsburgh Aqueduct which was the first of its kind. While the aqueduct was still under construction, be began to build a suspension bridge across the Monongahela River in Pittsburgh. These first projects earned Roebling a reputation for building bridges within projected costs and always on time. In 1847, Charles Ellet would win the bid over Roebling for building a suspension bridge across the Ohio River at Wheeling, which was to be a major link in the National Road which had reached Ohio. Built for an incredibly low $5,000, the bridge would span 1,010 feet, and would be the longest suspension bridge in the world at the time. In 1848, Ellet would begin the railroad suspension across the Niagara Goge and amaze crowds as he used theatrical showmanship to begin construction with a flare. In 1854, however, Ellet's reputation would take a pitfall when his famed Wheeling bridge would buckle and collapse under high winds. Roebling learned much from this tragedy, and improved his own designs.
Into the 20th century, by 1926, the Benjamin Franklin Bridge would connect Philadelphia to New Jersey, and at 1,750 feet long it would have the longest clear span of any suspension bridge in the world, and it was the first bridge with towers fabricated of steel cells. In 1929, the Ambassador Bridge connecting Detroit and Windsor, Ontario, would become the longest suspension bridge, with a clear span of 1,850 feet, and since that time all of the longest bridges have been suspension bridges. In 1936, the Transbay Bridge connected San Francisco and Oakland in California, a first because it was actually two suspension bridges connected by a central anchor with a clear span of 2,300 feet each. In 1937, the four-year construction of the Golden Gate Bridge in San Francisco was completed. Designed by Joseph Strauss, this would be the longest bridge in the world (4,200 feet) for many years to come. This bridge would bring in a considerable profit that would maintain both the structure itself and the highways around it. In 1939, the Bronx-Whitestone suspension bridge would be constructed, although over the course of the next several years engineering flaws would be discovered and repaired, so it wound up being something of a patchwork project with a very unique look. In 1940, the Tacoma Narrows Bridge, the third longest suspension bridge at the time, was both completed and collapsed. It had been built with a very narrow deck and without any stiffening trusses in order to save money, and it was known by the nickname "Galloping Gertie" even before it opened because it flexed and moved in the wind. This collapse would bring engineers to examine the role of aerodynamics in bridge building. In 1950, a new, safer suspension bridge would cross the Tacoma Narrows. In 1951 the Delaware Memorial Bridge near Wilmington, Delaware opened with a clear span of 2,150 feet. In 1957, the Mackinac Straights Bridge, or "Mighty Mac," (designed by David Steinman) opened to connect the Upper and Lower Peninsulas of Michigan, and while the central span of this bridge was only 3,800 feet, this bridge did have the longest span between approaches, and also the longest cables every spun. The heavy tresses and aerodynamic structure of this bridge, as well as the concrete-pouring methods used, made this an innovative project. In 1957, The Walt Whitman Suspension Bridge would connect Philadelphia with Camden, New Jersey; this was the fifth longest bridge when it opened, and is still one of the 20 longest in the world today. In 1964, the Verrazano Narrows Bridge was 60 feet longer than the Golden Gate bridge at 4,260 feet, and the designer, Othmar Ammann, used laser sighting devices to set the massive towers. This bridge is also much wider than the Golden Gate. In 1968, the New Delaware Memorial Bridge would become the twin of the 1951 Delaware Memorial Bridge across the Delaware River. In 1981, the Humbar Estuary Bridge opened, again breaking all previous records, at 4626 feet long. Unfortunately, the high tolls charged to cross this bridge discouraged use and the debt has today soared to a completely unrepayable cost.
The exciting ups and downs of suspension bridges make them an integral part of Western landscape today. In the commuting and constantly moving world of our society, the rapidly growth of size and number of suspension bridges over the past two centuries reflects the growth and development of the humans for whom and by whom they are designed.
Famous bridges like the Golden Gate Bridge not only provide much needed transportation, but also stand as landmarks and icons of modern engineering and culture; failures such as the Humbar Estuary Bridge warn against future mishaps financially, and disasters like the Wheeling and Tacoma Narrows bridges would stand as testimony to the vulnerability of even the greatest of mankind when faced with terrible winds of change.
THE CABLE-STAYED BRIDGE
Although most people are more familiar with the suspension bridge, the cable-stayed bridge is also a very common bridge type that is able to span long distances. In fact, many times the cable-stayed bridge is mistaken for being a suspension bridge because of the somewhat similar design. Although they may seem bizarre in appearance to some, many people consider cable-stayed bridges to be quite beautiful and impressive works of functional art. Many people also mistake them for being a very modern sort of structure, when in fact they have a very long documented history. The cable-stayed bridge is of great historical interest, with an extremely unique design that both advantages and disadvantages over other bridge types.
The earliest design of a cable-stayed bridge appears in a book called Machinae Novae published in 1595, but as far as is known today it was not constructed at that time. The earliest bridge similar to today's cable-stayed bridges that was actually built was most likely early 1600's by a Venetian carpenter names Verantius. His bridge was far more primitive than our cable-stayed bridges of today, built with timber wood and multiple chain stays. In the late 1700's, C.J. Losher…[continue]
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