Bridges
A bridge is a structure built to span a gorge, valley, road, railroad track, river, body of water, or any other physical obstacle. Designs of bridges vary depending on the function of the bridge & the nature of the terrain where the bridge is to be constructed.
History
The Si-o-se Pol bridge over Zayandeh
River is the epitome of Safavid dynasty (1502-1722 ) bridge design. Isfahan, IranThe
first bridges were spans made of wooden logs or planks & eventually stones,
using a simple support & crossbeam arrangement. Most of these early bridges
were very poorly built & could rarely support heavy weights. It was this inadequacy
which led to the development of better bridges. The arch was first used by the
Roman Empire for bridges & aqueducts, some of which still stand today. These
arch based bridges could stand in conditions that would previously have swept
any bridge away.
An English 18th century example of a bridge in the
Palladian style, with shops on the span: Pulteney Bridge, BathAn example is the
Alcántara Bridge, built over the river Tagus, near Portugal. Most earlier
bridges would have been swept away by the strong current. 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, & volcanic rock.
Brick & mortar bridges were built after the Roman era, as the technology for
cement was lost then later rediscovered.
Although large Chinese bridges existed in wooden construction since the ancient 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, & others. The first engineering book on building bridges was written by Hubert Gautier in 1716.
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.
Etymology
This Y-shaped bridge in Zanesville,
Ohio, United States is the only one of its kind.The Oxford English Dictionary
traces the origin of the word bridge to an Old English word brycg, of the same
meaning, derived from a hypothetical Proto-Germanic root brugjo. There are cognates
in other Germanic languages (for instance Brücke in German, brug in Dutch,
brúgv in Faroese or bro in Danish, Norwegian & Swedish).
The word for the Pope, pontiff, comes from the Latin word pontifex meaning "bridge builder". It can also mean just "builder".
Types
There are six main types of bridges: beam bridges, cantilever bridges, arch
bridges, suspension bridges, cable-stayed bridges & truss bridges.
By
use
A bridge is designed for trains, pedestrian or road traffic, a pipeline
or waterway for water transport or barge traffic. In some cases there may be restrictions
in use. For example, it may be a bridge carrying a highway & forbidden for
pedestrians & bicycles, or a pedestrian bridge, possibly also for bicycles.
A Road-rail bridge carries both road & rail traffic.
The area underneath several bridges have become makeshift shelters & homes to homeless people.
The undersides of bridges all around the world are spots of prevalent graffiti.
An aqueduct is a bridge that carries water, resembling a viaduct, which is a bridge that connects points of equal height.
Decorative or ceremonial
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, & their attendants.
Structural & evolutionary
taxonomy
A bridge taxonomy showing evolutionary relationshipsBridges may
be classified by how the four forces of tension, compression, bending & shear
are distributed through their structure. Most bridges will employ all of the principal
forces to some degree, but only a few will predominate. The separation of forces
may be quite clear. In a suspension or cable-stayed span, the elements in tension
are distinct in shape & placement. In other cases the forces may be distributed
among a large number of members, as in a truss, or not clearly discernible to
a casual observer as in a box beam. Bridges can also be classified by their lineage,
which is shown as the vertical axis on the diagram to the right.
Efficiency
A
bridge's structural efficiency may be considered to be the ratio of load carried
to bridge weight, given a specific set of material types. In one common challenge
students are divided into groups & given a quantity of wood sticks, a distance
to span, & glue, & 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 & determine the
multiple of this weight that the bridge can carry, a test that emphasizes economy
of materials & efficient glue joints (see balsa wood bridge).
A bridge's economic efficiency will be site & 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, & ultimately, demolition & associated disposal, recycling, & replacement, less the value of scrap & 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 & 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.
Special installations
Some bridges
carry special installations such as the tower of Nový Most bridge in Bratislava
which carries a restaurant. On other suspension bridge towers transmission antennas
are installed.
A bridge can carry overhead power lines as the Storstrøm
Bridge.
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