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This article is about the edifice. For other meanings, see Bridge (disambiguation).
Pedestrian bridge over the River Windrush at Bourton-on-the-Water (south west England)
A bridge is a structure built to span a gorge, valley, road, railroad track, river or other body of water, or any other physical obstacle.
The purpose of a bridge is to allow passage of people, cars, trains or ships over an obstacle.
A moveable bridge gives space for tall items such as ship's masts to pass through, or may be used to span distances that are
variable in height or distance.
History
Stone bridge "Ponte dei Salti" at Lavertezzo (Ticino) near Locarno
The first bridges were simple wooden logs or planks spanning a stream or such; the next examples found use stone, but again as
a simple support and crossbeam arrangement. The arch was first used by the Roman Empire for bridges, and many Roman bridges and aqueducts still exist today.
The Romans also had cement, which reduced the variation of strength found in natural
stone. Brick and mortar
bridges were built after the Roman era, as the technology for cement was lost.
With the advent of the Industrial Revolution of 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, with its high tensile strength, much larger bridges were built, many
using the ideas of Gustave Eiffel, which were first shown at the
Eiffel Tower in Paris France.
Etymology
The Oxford English Dictionary traces the
origin of the word bridge to an Old Norse word brygga, meaning
"landing stage, gangway, or movable pier".
Types of bridge
Bridges may be classified by their use or by the structure employed in their design.
By use
A bridge is usually either designed for trains, pedestrian or road traffic or may carry 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 and
forbidden for pedestrians and bicycles, or a pedestrian bridge, possibly also for bicycles. A bridge which has a series of spans, typically
arches, is called a viaduct. An aqueduct is a kind of bridge that
carries water, resembling a viaduct. Sometimes a bridge carries a pipeline only. When a bridge spans a road or railroad track, it is often called an overpass (US) or flyover (UK). A skyway is typically a fully-enclosed bridge between two buildings.
By structure
Log bridge
The earliest bridges were opportunistic use of logs that fell naturally across streams. The first manmade bridges were
probably intentially felled trees. The use of emplaced logs is now sometimes used in temporary bridges used for logging roads,
where a forrest tract is to be harvested and the road abandoned. Such log bridges have a severely limited lifetime due to soil
contact and subseqent rot and wood-eating insect infestation. Long lasting log bridges may be constructed by providing well
drained footings of stone or concrete combined with regular maintenance to prevent soil infiltration.
Beam bridge
The direct ancestor of the log bridge, this is now made from steel "I" beams, reenforced concrete, or post-tensioned concrete
(concrete with tubes for cable tenons). It is frequently seen in pedestrian bridges and for highway overpasses and flyovers.
Truss bridge
Compared to a log or wood beam bridge, a truss bridge can be much more economical in
its use of materials, partcularly when wood truss members are combined with metal fittings to transfer tension through the
structure. Trusses can carry heavier loads over longer spans using shorter elements than can log bridges. Conceptually, simple
trusses are considered to be pin-joined triangulated structures, where the engineering analysis of the structure depends upon the
assumption that there is no bending stress placed upon the truss members. Such structures may be analysed using simple methods.
The assumption of bending stresses require a more sophisticated analysis, yet well within the capablilities of pre-computer
engineering. Since most early bridges of this type were constructed largely of wood, it is typical that they are either formed
from treated wood (typically using creosote or are protected with a roof and
enclosed in building-like siding, forming a covered bridge. There is a substantial variation in the design of trusses, but all
use the principle of alternating tension and compression elements (some elements perform both tasks under dynamic loading). Where
elements are known by engineering analysis to carry only tension, they may be made of slender steel rods. The upper chord of a
truss is generally in compression, while the lower is in tension.
Bridges generally use a pair of trusses joined with diagonally braced horizontal elements to form a box structure. The roadbed
may form part of the upper or lower elements or may be suspended somewhere in the middle. Where a bridge must span an canyon at
an angle the side trusses may be offset. This is usually seen where a highway or railroad through a canyon is carried over a
stream to take advantage of more favorable grading or construction opportunities presented by the opposite bank.
Truss bridges may be made from almost any material with sufficient rigidity and strength, including peeled logs, wood beams,
reenforced concrete, and metal beam structural elements.
The concepts of the truss are used in other types of bridges and in components of bridges such as the deck structures of some
suspension bridges.
Compression arch bridge
Reenforced concrete arch bridges
This form was the earliest type of bridge that could form long spans using stone, brick, or simple concrete, materials which
while strong in compression and somewhat so in shear, cannot resist much force in tension. The roadway rides on or above
the supporting structure. Early roman aquiducts use a vertical stack of such bridges to obtain height while maintaining rigidity
in the supporting structure by avoiding tall, thin vertical elements. This type is still used in canal viaducts and roadways as
it has a pleasing shape, particularly when spanning water, as the reflections of the arches form a visual impression of circles
or elipses.
Compression arch suspended-deck bridge
Made from modern materials such as steel or reinforced concrete, the compression arch soars above the roadbed. Cables connect
the roadbed to the arch. One of the most famous bridges of this type is found in Sydney, NSW, Austrailia. Where the arch cannot
be supported from below it is brought out from each landing point using temporary cable stays supported from temporary towers,
kept from tipping inward by stays downward to earth anchors.
Simple suspension bridge
Suspension bridges are another early bridge type are still formed from native materials, cheifly grass rope, in some areas of
South America. These must of course be periodically renewed owing to the limited lifetime of the materials. A more permanant
variation, suitable for pedestrian and sometimes equestrian use, may be made from simple wire rope. In such bridges, the roadbed
of the bridge may follow the downward and upward arc of the load bearing cables, with additional light ropes at a higher level
used to form handholds. Owing to practical limitation in the grade of the bridge deck
this type is quite restricted in its load carrying capacity relative to its span. To walk on such a bridge at a reasonable pace
requires a particular gliding step, as the more normal walking step will induce traveling waves that can cause the traveler to
pitch (uncomfortably) up and down.
Suspended-deck suspension bridge
Portion of a plate deck suspension bridge
Modern vehicle-carrying suspension bridges use towers to
support the main load bearing cables. These cables must be securely anchored at each end of the bridge, since any load applied to
the bridge is transformed into a tension in these main cables. The roadway is supported by vertical suspender cables. This type
of bridge is the only practical type usable for longer spans, where it is impractical or hazzardous to maritime traffic to add
central supports. This kind of bridge is particularly pleasing to the visual senses, with the most beautiful of the type being
generally acknowleged as the Golden Gate Bridge at the entrance
to San Francisco Bay.
Cable-stayed bridge
Portion of a three tower cable stayed bridge
Cable-stayed bridges are relatively modern. The design
uses a number of individual cables from the tower to the roadway. These do not arc (except under their own weight) but proceed in
straight lines to multiple attachment points along the roadway. Towers may hold a single set of cables that tie to the center of
the roadway as shown at left, or may contain two sets tied to the edges of the roadway. This type of bridge is usually composed
of pairs of towers, but the method is not restricted to this form. Triple tower bridges have been constructed and a single tower
bridge has been designed (but not yet constructed) as the main replacement span for the easter section of the San
Francisco-Oakland Bay Bridge. Perhaps the most esthetically pleasing single tower bridge is the recently completed (opened July
4, 2004) Sundial Bridge, a glass decked pedestrian bridge crossing the
Sacramento River at Redding, California
Cantilever bridge
This is employed mostly to overcome construction difficulties where it is not practical to temporarily support the bridge from
below during construction. An advantage of the cantilever is that it can be constructed by working only from the support caissons
- this is done by building each side of the cantilever in sychronization to ensure the balance of the structure. Typically, two
or more such cantilevers upon completion will be securely anchored to massive supports at their outer spans to resist the inward
tipping of the cantilevers, with a substantial gap between the two cantilevers. A prefabricated central span will then be lifted
into place using cables. One of the most famous of such bridges is the Firth of Forth bridge in Scotland, notable for its
innovative use of tubular structural elements. The eastern span of the San Francisco Bay Bridge uses this structure, soon to be
replaced by a more modern (and suposedly more earthquake resistant) span.
Moveable bridges
To allow ships to pass which can not pass under it, a bridge may be constructed such that it (or part of it) can be turned up
(drawbridge; either one part or two) or sideways (swing bridge). A third
method is that the bridge deck is lifted while staying horizontal (lift bridge or lifting bridge). (Alternatively, if road
traffic is very light, a transporter bridge may be used.)
The tilting Gateshead Millennium Bridge
spanning the river Tyne between Gateshead
on the south bank and Newcastle upon Tyne on the north
(see image below), is a pedestrian bridge with two huge hydraulic rams at each side that tilt the structure back
allowing small boats, etc. through.
Gateshead Millennium Bridge
For small bridges these movements may be enabled without the need for an engine. Some bridges are operated by the users,
especially those with a boat, others by a bridge-person, sometimes remotely using video-cameras and loudspeakers.
There are often traffic lights for the road and water traffic, and
moving barriers for the road traffic.
Smaller moveable bridges, called jetways, are used in airports to allow passengers to cross the variable distances and heights between the terminal building and aircraft of varying sizes.
See also
Works of art
featuring bridges or using a bridge metaphor
Related topics
External link
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