Bridge Design and Engineering

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 of Fribourg in Switzerland was another famed carpenter who worked with cable-stayed bridges. His 32-meter cable-stayed bridge of 1784 was made entirely out of timber, even using timber stays.

In 1817, Redpath and Brown began further development of this bridge type, but later two of their built bridges would collapse, gaining a lot of bad press for this type of bridge. In the United States, some small cable-stayed bridges were constructed during the 1920's, and stays were utilized in the construction of the famed Brooklyn Bridge, but unfortunately the bad reputation earned by the collapsed cable-stayed bridges of days past mostly kept bridge designers away from this kind.

The first of the modern cable-stayed bridges was created by F.Dischinger, a German engineer who spent time reconstructing destroyed bridges in postwar Europe. During that time he discovered the positive aspects of this type of bridge, and he would create the Strosmund bridge in Sweden, in 1955, beginning a new era of bridge engineering. In 1988, the very noted Sunshine Skyway bridge -- a cable-stay bridge, of course -- in Tampa, Florida would win the Presidential Design Award from the National Endowment for the Arts.

A cable-stayed design was also chosen for a new bridge across the Charles River in Boston, Massachusetts. Elements of cable-stay bridges now make their way into the design for many other bridge types as well, giving birth to the cable-stayed hybrid bridge. The cable-stayed bridge has at least one pillar in the middle of the span of the bridge, with cables supporting the roadbed. Bridges with one, two, and three pillars have been constructed.

When the bridge has two pillars, it will look very similar to a suspension bridge, though the actual functionality of the bridge is quite dissimilar. In a suspension bridge, a huge cable is strung between at least two pillars, which is what bears the majority of the total load. The cables experience tension from crossing the gap between pillars, and the weight of the cable is the primary load. Smaller cables and rods will suspend from the main cable to hold the roadbed.

In the cable-stayed bridge, however, the pillars themselves, not the cables, form the primary load-bearing structure. Often a cantilever-type approach is used to support of the roadbed near the pillars, but further from the pillars cables run from the roadbed directly to the top of the pillar for support. Cable-stayed bridges can be identified by the number of spans, the number of towers, the girder type, and the number of cables.

Typical towers used are single, double, protal, and A-shaped towers; typical cable varieties are mono, harp, fan, and star arrangements. The typical cable-stayed bridge spans 110m to 480m, and the longest bridge of this type is the Tartara Bridge in Japan, which is 1,480m long. The cable-stayed bridge has some complications that accompany its design and use, like all other bridge types do.

For example, the cables that run from the roadway directly to the top of the pillars pull to the side as opposed to pulling directly up they way they do in suspension bridges. This requires the roadbed to be much stronger to resist the sideways-pulling loads of these cables. The cable-stayed bridge is also made very lightweight and with very flexible cables, which makes them highly susceptible to swaying in the wind during storms. They also cannot cover nearly the length that suspension bridges can.

The structure is overall very complex to design, and calculations for today's large bridges would be impossible without the aid of computers. On the plus side, cable-stayed bridges require far less roadbed material than would be needed for a cantilever bridge, and can be used for much longer gaps than many other bridge designs. The major amounts of material used are the cables, which are very economical in themselves. Also, while these bridges may not lend well to heavy winds, they have remarkable staying power during an earthquake.

The cable-stayed bridge is a great type of bridge. Although it has received a lot of bad press because of a few highly publicized collapses many years ago, the modern cable-stayed bridge is a wonderful answer to the needs of many large gaps. More economical and wide-reaching than many bridge designs, the cable-stayed bridge is more interesting than many of the other options. THE REINFORCED CONCRETE BRIDGE The reinforced concrete bridge is another standard bridge type.

Unlike both the suspension bridge and the cable stayed bridge which are similar to each other in design, the concrete bridge does not utilize cables to connect the bridge bed to towers for support. However, steel cables have previously been used inside of the concrete for reinforcement.

The three principal parts that make up the reinforced concrete bridge are the main supporting structure, which includes the columns, piers, arches, and abutments), the bridge deck, and ancillary features (like parapets and services.) This type of bridge is most commonly associated with arch bridges, but it is not exclusively used for arches at all. Concrete is a mixture of sand, stone, and cement which hardens into a solid form when mixed with water in the right proportions. Although concrete was used by the.