EFFICIENCY OF BRIDGE CONSTRUCTION
A bridge's structural efficiency may be considered to be the ratio of load carried to bridge mass, given a specific set of material types. In one common challenge students are divided into groups and given a quantity of wood sticks, a distance to span, and glue, and then asked to construct a bridge that will be tested to destruction by the progressive addition of load at the center of the span. The bridge taking the greatest load is by this test the most structurally efficient. A more refined measure for this exercise is to weigh the completed bridge rather than measure against a fixed quantity of materials provided and determine the multiple of this weight that the bridge can carry, a test that emphasizes economy of materials and efficient glue joints.
A bridge's economic efficiency will be site and traffic dependent, the ratio of savings by having a bridge (instead of, for example, a ferry, or a longer road route) compared to its cost. The lifetime cost is composed of materials, labor, machinery, engineering, cost of money, insurance, maintenance, refurbishment, and ultimately, demolition and associated disposal, recycling, and replacement, less the value of scrap and reuse of components. Bridges employing only compression are relatively inefficient structurally, but may be highly cost efficient where suitable materials are available near the site and the cost of labor is low. For medium spans, trusses or box beams are usually most economical, while in some cases, the appearance of the bridge may be more important than its cost efficiency. The longest spans usually require suspension bridges.
HISTORY OF BRIDGES
The first bridges appeared in ancient times. They were made by nature itself — as simple as a log fallen across a river or stones in the water. The first bridges made by humans were probably spans of cut wooden logs or planks and eventually stones, using a simple support and cross-girder arrangement. Some early Americans used trees or bamboo poles to get from one place to another across the water. Long reeds or other harvested fibers were woven together to form a connective rope which was used in early bridges.
The Arkadiko Bridge is one of four Mycenaean arch bridges and part of a former network of roads in the Peloponnese, in Greece. Dating to the Greek Bronze Age (13th century BC), it is one of the oldest arch bridges still in existence and use. Several intact (= not damaged) arched stone bridges from the Hellenistic era can be found in the Peloponnese in southern Greece.
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The greatest bridge builders of antiquity were the ancient Romans. The Romans built arch bridges and aqueducts that could stand in conditions that would damage or destroy earlier designs. Some stand today. An example is the Alcántara Bridge, built over the river Tagus, in Spain. The Romans also used cement, which reduced the variation of strength found in natural stone. One type of cement, called pozzolana, consisted of water, lime, sand, and volcanic rock. Brick and mortar (= a mixture of cement, sand and water) bridges were built after the Roman era, as the technology for cement was lost then later rediscovered.
The Arthashastra of Kautilya mentions the construction of dams and bridges. A Mauryan bridge near Girnar was surveyed by James Princep. The bridge was swept away during a flood, and later repaired by Puspagupta, the chief architect of emperor Chandragupta I. The bridge also fell under the care of the Yavana Tushaspa, and the Satrap Rudra Daman. The use of stronger bridges using plaited bamboo and iron chain was visible in India by about the 4th century. A number of bridges, both for military and commercial purposes, were constructed by the Mughal administration in India.
Although large Chinese bridges of wooden construction existed at the time of the Warring States, the oldest surviving stone bridge in China is the Zhaozhou Bridge, built from 595 to 605 AD during the Sui Dynasty. This bridge is also historically significant as it is the world's oldest open-spandrel stone segmental arch bridge. European segmental arch bridges date back to at least the Alconétar Bridge (approximately 2nd century AD), while the enormous Roman era Trajan's Bridge (105 AD) featured open-spandrel segmental arches in wooden construction.
Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America, just prior to European colonization in the 1500s.
During the 18th century there were many innovations in the design of timber bridges by Hans Ulrich, Johannes Grubenmann, and others. The first book on bridge engineering was written by Hubert Gautier in 1716. A major breakthrough in bridge technology came with the erection of the Iron Bridge in Coalbrookdale, England in 1779. It used cast iron for the first time as arches to cross the river Severn.
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With the Industrial Revolution in the 19th century, truss systems of wrought iron were developed for larger bridges, but iron did not have the tensile strength to support large loads. With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel.
In 1927 welding pioneer Stefan Bryła designed the first welded road bridge in the world which was later built across the river Słudwia Maurzyce near Łowicz, Poland in 1929. In 1995, the American Welding Society presented the Historic Welded Structure Award for the bridge to Poland.
THE MILLENIUM BRIDGE (KAZAN)
The Millennium Bridge is a cable-stayed bridge that spans Kazanka River, in Kazan, Tatarstan, Russia. Its name originates from Kazan's thousand anniversary, widely celebrated in 2005 and the shape of its M-like pylon. The construction of the bridge began in 2004, the first part was ready in 2005 and the second part in 2007. The building cost was approximately 94 million euros. The bridge is 831 m long. The main part of this bridge is the 45-m pylon which looks like the letter ‘M’. This form originates from Meñyıllıq (Cyrillic: Меңъеллык), the Tatar for thousand years old, or its Latin variant Millennium. The roadway carries three lanes of traffic and a pedestrian walkway in each direction. The bridge connects Gorky park and Fatix Amirxan Avenue.
THE USAGE OF BRIDGES
A bridge is designed for trains, pedestrian or road traffic, a pipeline or waterway for water transport or barge traffic. An aqueduct is a bridge that carries water, resembling a viaduct, which is a bridge that connects points of equal height. A road-rail bridge carries both road and rail traffic.
Bridges are subject to unplanned uses as well. The areas underneath some bridges have become makeshift shelters and homes to homeless people, and the undersides of bridges all around the world are spots of prevalent graffiti. Some bridges attract people attempting suicide, and become known as suicide bridges.
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To create a beautiful image, some bridges are built much taller than necessary. This type, often found in east-Asian style gardens, is called a ‘Moon bridge’, evoking a rising full moon. Other garden bridges may cross only a dry bed of stream washed pebbles, intended only to convey an impression of a stream. Often in palaces a bridge will be built over an artificial waterway as symbolic of a passage to an important place or state of mind. A set of five bridges cross a sinuous waterway in an important courtyard of the Forbidden City in Beijing, the People's Republic of China. The central bridge was reserved exclusively for the use of the Emperor, Empress, and their attendants.
Some bridges carry special installations such as the tower of Nový Most bridge in Bratislava which carries a restaurant. Other suspension bridge towers carry transmission antennas. A bridge can carry overhead power lines as does the Storstrøm Bridge. In railway network, an over-bridge is a bridge crossing over the course of the railway. In contrast, an under-bridge allows passage under the line.
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