Bridges Paper

Research Paper: Bridge Construction Honors Physics

Since the beginning of time, humans have engineered structures to overcome obstacles. Steel, stone, lumber, and even living vegetation have been used in an effort

to reach the places, people, and things we desire. Artists, architects and engineers pour vast time and money into the construction of these structures and, in doing so, reshape the very environment in which we live. These structures are known as bridges, and make it easier to get from one place to another. Without bridges, people would have to walk or drive around barriers and take a boat across water. There are more than half a million bridges in the United States, and people rely on them daily to cross obstacles, such as streams, valleys, and railroad tracks. Engineers must consider many things, such as the distance to be spanned and the types of materials available, before determining the size, shape, and overall look of a bridge. All major bridges are built with the public’s money, and are expected to best serve the public interest. Therefore, the bridge design must be as efficient and economical as possible, as well as elegant and safe. Today’s bridges are built on the basis of careful scientific and engineering studies. After the site of a new bridge has been chosen, studies are made of topography, geology, and meteorology of the location. The most appropriate type of bridge is chosen on the basis of these studies. Bridge engineers calculate the loading capacity of the bridge on the basis of the anticipated traffic and the stresses exerted by wind and weather. An exact stress analysis is done to determine the tensile strength, compressive strength and shearing strength of each member of the bridge structure; computers today play a major role in such calculations. For very large bridges, scale models may be built to verify the results of the studies, especially those of steel, to make sure that they can carry the anticipated load with a satisfactory margin of safety.

Most bridges are held up by at least two supports set in the ground. The distance between two adjacent supports is called a span of a bridge. The supports at each end of the bridge are called abutments, and the supports that stand between the abutments are called piers. The total length of the bridge is the distance between the abutments. Most short bridges are supported only by abutments and are known as single-span bridges. Bridges that have one or more piers in addition to the abutments are called multi-span bridges. Most long bridges are multi-span bridges. The main span is the longest span of a multi-span bridge. The types of bridges vary in total length, the length of their spans, and the weight they can support. Bridges range in length from a few feet or meters to several miles or kilometers. Before deciding which kind to build at a particular place, engineers determine the length of the structure and of each span. They also must consider the maximum load that the bridge will carry and the materials available to construct the bridge. A bridge must be strong enough to support its own weight as well as the weight of the people and vehicles that use it. It also must resist natural occurrences, including earthquakes, strong winds, and changes in temperature. Most modern bridges have a concrete, steel, or wood framework and an asphalt or concrete roadway. The roadway is the part of the bridge on which people and vehicles travel. Humans have been constructing bridges since ancient times. The earliest bridges were made by nature, and were nothing more than fallen trees used to cross rivers or ditches. As civilization advanced, artisans discovered ways to use stone, rock, mortar, and other natural materials to construct longer and stronger bridges. Running water wore away rock and formed an arch, creating the first stone bridge. New principles

underlying bridge construction began to evolve, and today, there are seven main kinds of bridges: arch bridges, girder bridges, cantilever bridges, truss bridges, moveable bridges, suspension bridges, and cable-stayed bridges. As engineers began to design bridges with longer and longer spans, they had to find materials that were stronger and lighter in weight. Most bridges were made of stone or wood until the late 1700’s, when cast iron and wrought iron came into wide use. In the late 1800’s, steel became the main material used for bridges. The first concrete bridge was built in 1869. A short time later, bridge builders began using reinforced concrete. In the 1930’s, prestressed concrete became an important material for bridge building. Steel and concrete are still used in many modern bridges. New kinds of steel have been developed, including silicon steel, nickel steel, manganese steel, chrome-nickel steel, heat-treated carbon steel, and cold-drawn high-carbon wire. Other materials such as iron and aluminum are incorporated into the construction of bridges, as well. In about 2200 B.C., the first bridge known to historians was built in Babylon. It was an arch bridge. Arch bridges are structures in which each span forms an arch. The spans range up to about 1,700 feet, or 518 meters long. Early arch bridges were built by the ancient Chinese, Egyptians, Greeks, and Romans, and consisted of large stone blocks wedged together to form an arch. Arch bridges are still being built, but the arch today generally is made of steel and concrete, because stone is too expensive. The majority of arch bridges that have short spans are made of concrete or wood. Arch bridges with long spans are built of concrete or steel. In the middle of the 19th century, the French developed the hinged arch, which has greater bearing ability.

Engineers must design arch bridges so that the sides of the arch do not spread apart or collapse the bridge. The roadway of some arch bridges lies on top of the arch and is supported by vertical columns called spandrel columns. These columns transfer the load of the roadway to the arch, which bears the weight of the bridge. The roadway of a tied arch bridge is below the curve of the arch. The roadway is supported by girders or other types of beams that hang from the arch. The girders or beams also connect to the ends of the arch to prevent the ends from spreading out. The abutments support the weight of the bridge. The arch that supports the bridge exerts both vertical and horizontal thrust, and therefore needs substantial support at both ends. After the log or piece of wood used to cross a stream or gap, the simplest and oldest type is the pile-and-beam bridge, which consists of a series of wooden beams that support the deck of the bridge, with the ends of the beams driven into the bed of a stream or into the ground. These bridges were common by the Roman Empire, and they still are all over the world for short spans on country roads or railroads. A variation on the theme is the girder bridge, also known as the beam bridge, which include many highway bridges. Girder bridges are made of beams called girders whose ends simply rest on piers or abutments. A girder that is supported just at its ends is a simple span, while one that has three or more supports is a continuous span. The span length of girder bridges range up to about 1,000 feet, or 300 meters. The span is smaller than most bridges because the farther apart the piers are, the weaker the beam becomes. When something pushes down on the beam, the beam bends because its top edge is pushed together, and its bottom edge is pulled apart.

These bridges may be used to cross most areas. There are two main types of girder bridges. In one type, called a box girder bridge, each girder looks like a long box that lies between the piers or abutments. The top surface of the bridge is the roadway. Box girder bridges are built of steel or concrete. In the other type of girder bridges, the end view of each girder looks like a T or an I. Two or more girders support the roadway. This type of bridge is called a plate girder bridge when made of steel, a reinforced or prestressed concrete girder bridge when made of concrete, and a wood girder bridge when made of wood. There are some girder bridges that are supported at just one end; they are known as cantilever bridges. Cantilever bridges consist of one, two, or several independent beams called cantilevers. These bridges may have spans as long as about 1,800 feet, or 550 meters. The most common type has two cantilevers, each extending from opposite banks of a waterway. The two cantilevers are joined together above the middle of the waterway by a beam, girder, or truss. Each cantilever has two sections, an anchor arm and a cantilever arm. The anchor arm extends between an abutment and a pier. One end of the cantilever arm is supported by the pier, and the other end extends freely over the waterway. The free ends of the two cantilevers are joined together by a suspended span. Most cantilever bridges have two anchor spans and one center span. Each anchor consists of an anchor arm. The suspended span and two cantilever arms make up the center span. Many cantilever bridges have truss frameworks, and are made of steel or prestressed concrete.

The great advantage of the cantilever bridge is that it has no pier in the middle, and does not require temporary piers while it is under construction. Cantilever bridges are used over rivers which have a very swift current or which have a very deep channel in the middle. The first modern cantilever bridge was built across the Main River in Germany in 1867. It had a central span of 425 feet, or 130 meters. Truss bridges were developed in the late 19th century. Truss bridges are supported by trusses. A truss is a metal framework that is specifically designed for greatest strength at the points where the load on the bridge exerts the greatest pressure. The parts of the trusses are arranged in the form of triangles. Such bridges are built over canyons, rivers, and other areas. A truss bridge may have a main span that extends more than 1,000 feet, or 300 meters. Each truss consists of steel or wood parts that are connected to form one or more triangles. The simplest truss consists of three parts fastened together at their ends to form a triangle. Most truss bridges have one set of trusses on each side of the roadway. In a single span truss bridge, if the bridge is at the top, it is a deck truss; if it is at the bottom and extends between two abutments or piers, it is a through truss. In a continuous truss bridge, each truss has three or more supports. Some locations are suitable for either a truss bridge or a girder bridge. In such cases, some engineers choose to build a truss bridge because it requires less construction material than the girder type. However, many engineers prefer a girder bridge because it is more attractive and easier to construct and maintain. Most moveable bridges are truss spans.

Moveable bridges have a roadway that is moved entirely or partially to provide enough clearance for large ships to pass. There are four common types of moveable bridges; the bascule, vertical lift, swing, and pontoon bridges. The bascule bridge is a descendant of the medieval drawbridge. Its arms are swung up like the gates at a railroad crossing to allow ships to pass. Some bascule bridges open at one end, and others open in the middle. A vertical lift bridge has a roadway that extends between two towers. The roadway rises between the towers like an elevator, and allows for ships to pass underneath. A swing bridge is mounted on a central pier. The bridge swings sideways to enable ships to pass. Unlike the swing bridge, the bascule and vertical lift bridge designs do not require a central pier. Instead, they both have massive counterweights which move up or down like window weights. A bridge built on boats or floating piers is called a pontoon bridge. This type of crossing is often laid down by armies to get troops across a river quickly. Persian armies used a pontoon bridge to cross the Hellespont on their way to invade Greece in 480 B.C. Pontoon bridges are also used where the water is so wide and deep that pier construction is too expensive. Suspension bridges are perhaps the most impressive type of bridge because of their long main span and especially attractive appearance. Suspension bridges do not cost as much as other long bridges and a skillful bridge architect can make it a thing of beauty. A suspension bridge has two or more towers that have one or more flexible cables that support the bed of the bridge. The Chinese built suspension bridges as early as the sixth century, but they did not appear in Europe until the 18th century, with chains supporting the deck. Today’s familiar suspension bridge, with wire cables

supporting the deck, was introduced by the French in the middle of the 19th century. Suspension bridges are used to span great distances. Most suspension bridges have a main span more than 1,000 feet, or 300 meters long. Some have a main span longer than 4,000 feet, or 1,200 meters. Suspension bridges are used to cross deep water or steep canyons, and in other places where the construction of piers is especially difficult and expensive. These bridges require only two piers, each of which supports a tower. The main span of a suspension bridge stretches between the two towers. Each of two side spans extends between a tower and an anchorage. Most anchorages are huge blocks of concrete set at the ends of the bridge. The use of cables allowed a great reduction in weight. The cables that are supported by the towers are called the main cables. A suspension bridge has atleast two main cables. Each of these cables extends from one end of the bridge to the other and is secured at each end by an anchorage. The main cables are connected to the top end of vertical suspender cables. The bottom end of each suspender cable attaches to the roadway of the bridge. The great fault of the suspension bridge is that it quivers and sways with the wind. To minimize such movement, most suspension bridges have a thick structure that supports the roadway. This type of structure helps stiffen the bridge and is called a stiffening girder or stiffening truss. A body of soldiers, marching in step across a small suspension bridge, sometimes can cause it to swing so violently that it will fall. Long suspension bridges are usually supported by several cables. Some cables are over three feet, or 91 centimeters thick. Many famous bridges, including the Golden Gate Bridge in California and the George Washington Bridge linking New York and New Jersey, are suspension bridges. The rigidity of the deck that is hung from the

cables is maintained by using stiffening trusses that run from side to side of the deck. The newest bridge design is the cable-stayed bridge, and greatly resembles suspension bridges. Both have roadways that hang from cables, and both have towers. In a cable-stayed bridge, however, cables run diagonally from each segment over the tower, and back down to the opposite segment. Such bridges do not require massive anchors at either end of the span, and may be used if its foundation can support only one tower. Most cable-stayed bridges have three spans, but some have one tower and two spans. The most efficient cable-stayed bridges have a main span of about 700 feet, or 210 meters long. The cables of a cable-stayed bridge may be linked from the roadway to the towers in several ways. The cables may extend from various points on the roadway to the tops of the towers, forming a radiating pattern. The cables form a fan pattern, also called a harp pattern, if they are connected from a variety of points on the roadway to several points on the towers. If the cables are attached from one point on the roadway to various points on the towers, they form a star pattern. Compression and tension are present in all bridges. Both are capable of damaging part of the bridge as varying load weights and other forces act on the structure. The bridge design needs to be able to handle these forces without buckling or snapping. Buckling occurs when compression overcomes an object’s ability to endure that force; snapping is what happens when tension surpasses an object’s ability to handle the lengthening force. The best way to deal with these powerful forces is to either dissipate them or transfer them. With dissipation, the design allows the force to be spread out evenly over a greater area, so that no one spot bears the concentrated brunt

of it. In transferring force, a design moves stress from an area of weakness to an area of strength. Different bridges prefer to handle these stressors in different ways.

References:

(1975). Bridge. In The World Book Encyclopedia.(Vol. 2, pp.490-495).Chicago: Field Enterprises Educational Corporation. Billington, D., Billington, P., & Shirley-Smith, H. (n.d.). bridge (engineering) -Britannica Online Encyclopedia. Encyclopedia - Britannica Online Encyclopedia. Retrieved December 26, 2011, from http://www.britannica.com/EBchecked/topic/79272/bridge BUILDING BIG: Bridge Basics. (n.d.). PBS: Public Broadcasting Service. Retrieved December 15, 2011, from http://www.pbs.org/wgbh/buildingbig/bridge/basics.html Hennes, R. Bridge. (2010). In The World Book Encyclopedia.(Vol. 2, pp.600-603). Chicago, IL: World Book, Inc. Trefil, J. S. (2001). Bridge Design. In The Encyclopedia of Science and Technology. (PP.77-78). New York: Routledge. Types and History of Bridges. (n.d.). ThinkQuest : Library. Retrieved December 9, 2011, from http://library.thinkquest.org/CR0210346/history.html

Note: There are a total of six sources. Three of them come from books (all of which are encyclopedias) and the other three come from the internet.