Basic Bridge Information
"In civilized countries bridges are littered about the landscape in generous numbers and in a rich variety…one virtue of bridges is that both the structure and the way it works are clear for all to see" 1 By examining the history of various bridges students can see how this technology and structure has evolved. Students will see the variety of some major bridges as they are reviewed by type and materials used.
Early Bridges
"Basic bridge designs are developed from natural bridges-a tree trunk that has fallen across a stream, vines hanging over a river, or stones that make a stepping-stone path across a shallow stream." 2 From these natural bridges people undoubtedly created his own. One of the earliest types of bridges is the stone "clapper", which means "heap of stones". 3 Before 2000 B.C., it is believed early suspension bridges, floating pontoon bridges and cantilevers could be found in China. 4 These bridges were built to enable man to hunt, gather food, or trade by exploring new lands. Some early bridge achievements are found in Babylon, Egypt, and Persia. In the 7 th to 6 th century B.C. the Babylonian leaders, Nabopolassar and his son, Nebuchadrezzar II, built a bridge that crossed the Euphrates River near the Tower of Babel. It was 120 to 200 meters long with seven brick, stone, and timber piers. "The Greek historian Herodotus (c.485-425 B.C.) described how Persian King Darius I and Xerxes I connected Asia and Europe using pontoon bridges in 513 and 543 B.C. respectively" destroying the Spartans and Athenians. 5 Darius I had a pontoon bridge built across the Danube and the Bosphorus. In 480 B.C. Xerxes I, his son, crossed the Hellespont invading Greece. 6 As impressive as these bridges are, it is the Romans who refined and brought engineering into a new era.
Roman Bridge Builder
As the Roman Empire spread, roads were built to connect the lands, and bridges were built to connect the roads. They brought sophistication and engineering savvy to bridge building. Several major contributions the Romans made to bridge building are the arch cofferdams, cement, and building bridges in sections. The arch is an important contribution because it allowed the Romans to build longer and stronger aesthetically beautiful bridges that did not hinder water traffic. The arch has four essential parts: the abutment, the centering (wooden form), the voussoirs (wedge shaped stones), and the keystone. The keystone is the center stone. Arches are built from the abutments (supported sides) up. The voussoirs are added to each side, building up, and then the keystone is added, in the center, joining the voussoirs. Once the keystone is in place, the centering is removed.
Cofferdams were used as a method to build the piers for the foundation of a bridge. A bridge is only as strong as its foundation, and the cofferdam allowed for a strong foundation to be built. They were made with double walls creating a "water tight box built of piles". 7 It was built around the place to be excavated and water inside was pumped out. Cofferdams were later used in shallower water while caissons were used for deeper water. Although the Romans built some bridges without mortar, for others it was necessary. Using the volcanic ash, pozzalana, from Vesuvius, they created a natural cement by combining it with, water, and lime. This cement being waterproof was ideal for building piers.
One technique that the Romans used was sectional building. This allowed for bridges to be built from the banks out, instead of across the river and up. This process is described in the following quote:
Roman arches were semicircular, which had important consequences. In a semicircular arch, more of the thrust goes directly downward than in a shallow arch, which in consequence requires strong side-bracing at the abutments. This meant that, if the piers were wide enough (one-third of the span), any two could support a complete arch without further propping from the sides. Thus it was possible for Roman engineers to build bridges out from the shore a span at a time-cofferdam, then foundations, then piers, then arch-rather than having to go through the much more difficult operation of putting the entire bridge substructure in position first. 8
The Romans are also known for using stone for their bridges, but in truth, they also created wooded bridges. These wooden bridges have not survived. The oldest Roman bridge, identifiable by name, is the Pons Sublicius which crossed the Tiber River. Built in the 6 th century B.C., Horatius Cocles defended it against the Etruscans. "Sublica" means "pile or stack" It probably was made of wood. 9 Another wooden bridge, built in 55B.C. for military purposes, was one that Julius Caesar wrote of in Gallic War. It is described as a quarter mile long timber trestle bridge built in ten days to cross the Rhine. One stone bridge still standing is the Ponte Fabricio in Rome built in 62 B.C. This bridge connected the bank of the Tiber River with an island in the middle of the river. The distance was 200 ft. The bridge was not to impede river traffic, but to allow the infirmed access to the hospital on the island. The bridge was built with two stone arches. It allowed for ships to pass underneath and the sick to be moved across it. What made this bridge unusual were the three smaller arches in the spandrels above the center pier and the abutments, "designed to reduce weight and provide additional channels for floodwaters".10
Stone bridges built without mortar like the Pont du Gard in Nimes, France were built by the Romans. It is described as "the most famous surviving section of a Roman masonry aqueduct-and the highest bridge the Romans ever built-it is an extraordinary piece of engineering that combines functionality and mathematical precision with great beauty." 11 One of the largest bridges built in the Roman Empire, and still standing, is the Alcantara Bridge in Spain built by the engineer, Caius Julius Lacer (cAD100). It is a granite arch bridge crossing the Tayus River, 57 meters high, and 194 meters long, with 6 semicircular arches. It too was built without mortar. The aforementioned bridges, built in Rome, France, and Spain are prime examples of Roman contributions to bridge building. Roman beliefs in gods were so strong that before any crossing could be built the priest must be called. "The Latin pontifex, priest, originally meant 'builder of bridges". 12 This fact foretells the relationship between the clergy and bridges in the Middle Ages.
Chinese Bridge Builders
China has a diverse topography that necessitates different types of bridges. The Chinese were innovative in several of these designs. One such innovation came about because of a dilemma. Li Chun brilliantly solved how to build a bridge over the Xiao River in the 6 th century. A Roman semi-circular arch would not have worked. In order for it to be long enough, it would have been too high, and too heavy. He needed a new plan. More than one arch wouldn't work because the river was too fast to build a pier. So, Li Chun designed a shallow low arch 7 meters in height, with more advanced spandrels, smaller arches on either side, than those that the Roman were using. This design of a segmented arch was not seen in Europe until the 19 th century, 800 years later. 13 It received, in 1989, the International Historic Civil Engineering Landmark award by the American Society of Engineers. 14 Not only did the Chinese build improved arch bridges, they also diversified the types of bridges being built. In the book, Rivers, LiDao Yan (472-527) describes a wooden cantilever bridge in the Gansu Province built in 405-418 that is 13 meters long. 15 This design allowed for bridges to be built across wider spans. Suspension bridges that crossed gorges like the Anlan Bridge at Guan Xian in Sichuan Province was first constructed in the 3 rd century B.C. It has been rebuilt many times, most recently in 1975 with steel cables. 16
Middle Ages
The church built bridges, as well as, cathedrals. Some of these bridges had chapels constructed on them, such as the bridge d'Avignon which became famous as a children's song. Special monastic groups, the "Fratres Pontifices", were used to maintain and protect these bridges from thieves and robbers. These monks saw the building of bridges as symbolic for crossing over into the afterlife. People donated money to the churches for bridge construction and were promised rewards in the afterlife. The possibility is that the church began overseeing bridge building and bridge maintenance because there is evidence of human sacrifices incorporated into the foundations of early bridges! "At least one child's skeleton has been discovered immured in the foundation of a bridge." 17 Another reason the church was involved in bridge building is that it promoted trading which in turn made towns and cities wealthier. The church would benefit directly from this prosperity. Bridges also aided in communication between the church and the people, and among the clergy.
The Railroad and the Industrial Revolution
"Leonardo da Vinci and Galileo developed theories about the strength of building materials" 18. Sadly, these and other theories were not understood until much later in history. How materials react to forces was not understood until after the Industrial Revolution. During the railroad's race to connect the east and west by rail, many types of rail bridges were built, and the loads greatly increased. Stronger and more bridges were needed quickly. After several railroad bridges collapsed the strength of materials began being studied.
The railroad was essential for the Industrial Revolution. Industry depended on the railroad to bring it materials for manufacturing and to transport the finished product to markets. Trains also accelerated the transportation of goods and people and thus helped to create this economic boom. The railroad obviously needed bridges to connect the rails. The first type of bridge the railroad used was the brick arch. In this semi-circular Roman and Medieval arch bridge, "the rise of the arch must be about half its span". 19 This formula presented several problems. From the flat ground to the bridge, the arch aspect of the bridge would force the train to rise at an incline thus the bridge would have to be "humped-back" or had to be built with a long and sloping approach. 20 Railroads could not traverse the hump. In 1837, Isambard Kingdom Bruel built a bridge with two brick arches, each arch being a 128 ft. span with a 24ft. rise across the Thames at Maidenhead. Skeptics doubted its design, but the bridge held. It is still there carrying even greater loads today. These flatter, or multiple arch bridges solved this problem and were used very successfully.
Railroad bridges in America were often made from a wood truss design. Wood was plentiful and inexpensive, with little or no transportation cost. As trains became larger, they became heavier, and a moving train would shake a wooden bridge apart, thus a stronger type was necessary. "In 1841, Squire Whipple built his first iron bridge, The Erie Canal Bridge in New York State, using two kinds of iron, cast iron and wrought iron." 21 Whipple became the "Father of Iron Bridges" in America. He used math to calculate the forces of tension and compression leading the way to test bridge design before construction.
The cast iron arch bridge was a great success. The cast iron beam bridge was not. Cast iron has similar properties to stone in that it has strong compression strength, but is weak in tension. It weighed less than stone, was less expensive to transport, and created less force on the abutments. Some cast iron beam bridges were built in the 19 th century, but they had to be replaced. The beam relies on tension in which the cast iron is weak. It was confused with the compression strength of the arch, in which cast iron is strong when in compression. Cast iron went out of vogue when a more economical material was found. Today steel plate girders are used for short railroad spans, and box girders made with steel plates may be used for longer beam bridges. 22
James Finlay of Pennsylvania invented the modern suspension bridge of today in 1796. Wrought iron chains began being used after 1790. Suspension bridges were cheaper and more practical to build. The foundation required less construction, comparatively two towers to several arches or piers. Thomas Telford completed the Menai Strait Bridge in 1825. The 550 ft bridge used wrought iron chains. This type of bridge building was suspended for about 100 years because of the railroad. In England during the "Railway Mania" from 1835-1845, it is estimated that between 30,000- 60,000 bridges were built to accommodate the railroad. In other words, about 3,000-6,000 were built a year! Most of these were masonry arches. Samuel Brown constructed a rigid suspension bridge over the River Tees. It was destroyed in months because the weight of the locomotive exceeded the tensile strength of the bridge. 23 Suspension bridges were not thought to be able to carry the loads. Several hundred high tensile steel wires are now woven to make steel cables. These cables connect the roadway to the towers.
Bridges of Today
The material that most bridges today are made of is pre-stressed concrete and steel. Remember, the Romans developed concrete from volcanic ash. It began being used again in the 18 th century. 24 The advantages of reinforced concrete are that it is fire resistant, inexpensive, can be molded into desired shapes, and resists compression and tension. 25 The newest type of bridge is the cable stayed. These bridges were used to rebuild Germany after World War II because they could be inexpensively built on existing piers.
The type of bridge used for a specific site is often predetermined by the location and other environmental factors. Some of these other factors will be discussed in the later sections. A beam bridge's single span is usually 30 feet, an arch bridge spans 800-1000 ft. and a suspension bridge spans more than a mile. The New River Gorge Bridge in West Virginia is a 1700 ft. steel and concrete arch. The world's longest suspension bridge is on schedule to be completed in 2011 connecting Sicily and Italy. New bridges are often pushing the limits of physics, materials and technology.
Comments: