A bridge is a structure built to span a gorge, valley, road, railroad track, river, body of
water, or any other physical obstacle, for the purpose of providing passage over the
obstacle. Designs of bridges will vary depending on the function of the bridge and the
nature of the terrain where the bridge is to be constructed.
Types of bridges
Beam bridges
Beam bridges are horizontal beams supported at each end by piers. The earliest beam
bridges were simple logs that sat across streams and similar simple structures. In
modern times, beam bridges are large box steel girder bridges. Weight on top of the
beam pushes straight down on the piers at either end of the bridge. [7]
Cantilever bridges
Cantilever bridges are built using cantilevers — horizontal beams that are supported on
only one end. Most cantilever bridges use two cantilever arms extending from opposite
sides of the obstacle to be crossed, meeting at the center. The largest cantilever bridge is
the 549 feet (167 m) Quebec Bridge in Quebec, Canada.[8]
Arch bridges
Stone arch bridge in Shehara, Yemen
Arch bridges are arch-shaped and have abutments at each end. The earliest known arch
bridges were built by the Greeks and include the Arkadiko Bridge. The weight of the
bridge is thrust into the abutments at either side. Dubai in the United Arab Emirates is
currently building the largest arch bridge in the world, which is scheduled for
completion in 2012.[9]
Suspension bridges
Suspension bridges are suspended from cables. The earliest suspension bridges were
made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables
hang from towers that are attached to caissons or cofferdams. The caissons or
cofferdams are implanted deep into the floor of a lake or river. The longest suspension
bridge in the world is the 12,826 feet (3,909 m) Akashi Kaikyo Bridge in Japan.[10]
Cable-stayed bridges
Like suspension bridges, cable-stayed bridges are held up by cables. However, in a
cable-stayed bridge, less cable is required and the towers holding the cables are
proportionately shorter. [11] The first known cable-stayed bridge was designed in 1784
by C.T. Loescher.[12] The longest cable-stayed bridge is the Sutong Bridge over the
Yangtze River in China.
Truss bridges
Continuous under-deck truss bridge
Truss bridges are composed of connected elements. They have a solid deck and a lattice
of pin-jointed girders for the sides. Early truss bridges were made of wood, and later of
wood with iron tensile rods, but modern truss bridges are made completely of metals
such as wrought iron and steel or sometimes of reinforced concrete. The Quebec Bridge,
mentioned above as a cantilever bridge, is also the world's longest truss bridge.[13]
By use
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.
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, and their attendants.
Even the most primitive human communities must often have created bridges from most
common materials. A fallen tree can be dragged into position to serve as a board. Forest
tendrils may be intertwined as an elementary suspension bridge.
But bridges over more than a narrow width require an architectural device which arrives
relatively late on the scene.
Bridges are as much part of the Roman architectural achievement as aqueducts, and they
present even greater constructional problems.
An equally remarkable feat of Roman construction is the building of bridges across
rivers where no rock or island emerges from the water. An example survives in Rome -
the Sant'Angelo bridge, built for Hadrian in AD 134 as an approach to his great circular
mausoleum, now the Castel Sant'Angelo.
The building of such bridges is made possible by the Roman perfection of cement and
concrete, and by their invention of the cofferdam.
But many years later, in the space of a few months in 1779 the world's first iron bridge,
with a single span of over 100 feet, is erected for Abraham Darby (the third of that
name) over the Severn River just downstream from the location of Coalbrookdale.
And talking about metallic structures in bridges, we are going to present the Millenium
Bridge in London UK.
The Millennium Bridge springs from a creative collaboration between architecture, art
and engineering. Developed with sculptor Anthony Caro and engineers Arup, the
commission resulted from an international competition. London’s only pedestrian
bridge and the first new Thames crossing since Tower Bridge in 1894, it links the City
and St Pauls Cathedral to the north with the Globe Theatre and Tate Modern on
Bankside. A key element in London’s pedestrian infrastructure, it has created new
routes into Southwark and encouraged new life on the embankment alongside St Pauls.
Structurally, the bridge pushes the boundaries of technology. Spanning 320 meters, it is
a very shallow suspension bridge. Two Y-shaped armatures support eight cables that
run along the sides of the 4-meter-wide deck, while steel transverse arms clamp onto the
cables at 8-metre intervals to support the deck itself. This groundbreaking structure
means that the cables never rise more than 2.3 meters above the deck, allowing
pedestrians uninterrupted panoramic views of London and preserving sight lines from
the surrounding buildings. As a result, the bridge has a uniquely thin profile, forming a
slender arc across the water, and spanning the greatest possible distance with the
minimum means. A thin ribbon of steel by day, it is illuminated to form a glowing blade
of light at night.
The bridge opened in June 2000 and an astonishing 100,000 people crossed it during the
first weekend. However, under this heavy traffic the bridge exhibited greater than
expected lateral movement, and as a result it was temporarily closed. Extensive research
and testing revealed that this movement was caused by synchronized pedestrian footfall
- a phenomenon of which little was previously known in the engineering world. The
solution was to fit dampers discreetly beneath the deck to mitigate movement. This
proved highly successful and the research undertaken by the engineers has resulted in
changes to the codes for bridge building worldwide.