Construction of Bridges
A bridge is a structure that spans a depression or provides a passage between two points which are at a height above the ground affording a passage for pedestrians, vehicles, and also built to span a valley, road, body of water, or other physical obstacle, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge and the nature of the terrain where the bridge is constructed.
It is one thing to design a bridge. It is another thing to build it. Planning and executing the construction of a bridge is often very complicated, and in fact may be the most ingenious (ingénieur - engineer) parts of the entire enterprise. An incomplete structure is often subjected to stresses and oscillations that would not arise after completion. The construction work is potentially a grave hindrance to existing traffic and to normal life in the area, especially when large local fabrication works have to be installed.
Even before any actual construction is done, substantial work may needed in the form of tests. Boreholes will be made to check the condition of the ground, in conjunction with any available geological maps. Records of wind speed and direction will be consulted, and new measurements made if necessary. In the case of a river or sea crossing, records of water levels and velocities will be needed. Models of the bridge or of parts may be tested aerodynamically and hydrodynamically, and of course mechanical tests will be made. Computer simulations will supplement these tests, enabling a great variety of applied forces to be investigated. There may also be investigations into the effects on people and on the natural environment. It may even be necessary to overcome opposition to the construction, from a variety of objectors.
These are some construction for bridge building that commonly need and used by people :
a. Construction of Arches
An arch is in compression throughout, and it cannot stand except as a whole. It therefore requires temporary support, or falsework, until it is complete. The type of falsework depends very much on the material of which the bridge is made, and on the size of the bridge.
Masonry arches, being made of relatively small voussoirs joined by mortar cannot take tension, need continuous support during construction from below. This type of falsework is called centring, and is often of the general form shown below. Concrete bridges may be supported in a similar way, except that the centring will support the formwork in which the concrete is poured. For large bridges, the centring will be a substantial structure in its own right, and will be expensive.
For a multi-arch bridge, it is desirable to re-use the centring for each span. In the case of a bridge across a river, the centring may be floated from one arch to the next, and will be in the form of a tied arch to maintain it integrity. The centring for the illustrated arch would have been impressive.
Concrete arches also may be built using the cantilever method, as in the schematic example shown below. For very wide and exposed sites, this is preferable to lightweight formwork which could be vulnerable to high winds.
b. Construction of Beam Bridges
Beam bridges are generally in the form of plate girders, box girders or trusses. In all cases, a common construction method is to build the beam away from the final position and slide it or lift it into place as a complete unit. Bowstring or tied arches may be built in the same way.
There is nothing especially complicated about the lifting process, and the stresses in the beam are more or less as they will be in the final position. Nevertheless, a heavy object suspended in space is potentially dangerous, and accidents do happen.
Nevertheless, a slight distortion of the beam did occur. The incident shows the importance, not only of correct instructions, but of good communications. Accidents have happened because instructions were not received, or if received, modified or ignored.
An exception to the method of lifting may be adopted in the case of the suspended span of a cantilever bridge, as in the case of the Forth railway bridge. Here, the beams were built out from the ends of the cantilever arms until they could be joined in the middle. Only then were the temporary rigid connections removed. Such an operation has to be done with great care, because large stresses are released and created.
c. Construction of Cable-Stayed Bridges
One advantage of cable-stayed bridges over suspension bridges is that they can be built out symmetrically from the towers, though long spans are rather flexible until the time of closure.
d. Construction of Cantilever Bridges
A great advantage of a cantilever is that it can be built out from a support without hindrance to traffic or navigation below. The technique varies according to the type of bridge. It will also depend on whether the cantilever is attached rigidly to a free standing tower. Some towers are hinged at the base, in which case falsework will be needed. Even with fixed piers or towers the span may be attached only on a pivot rather than a fixed connection - falsework will be needed here too.
New parts may be taken along the existing part of the bridge, or they may be lifted up from below. Sections of a segmented prestressed concrete bridge may be lifted by crane, or they may be manipulated by means of a launching girder which is slowly advanced along the span. An alternative may be moving formwork which progresses with the construction. The idea is explained in more detail in the section about beam construction.
e. Construction of Truss Bridges
Truss bridges are generally arches, beams or cantilevers, though the decks of cable-stayed bridges and suspension bridges may be in the form of trusses also. Construction methods are as described under arches, beams and cantilevers above.
f. Construction of 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 bridge builder, like the mammals, has to nurse the embryo structure through difficult stages. Very often, the stresses differ considerably from those of the complete structure, and can be more concentrated. The variation in stresses during construction may be so severe that jacking must be provided. Even after completion, a concrete structure may be subject to creep, and the ground may settle, jacks are therefore provided for later adjustment when this behaviour is foreseen.
Excavation for foundations may have to be taken to great depths, through unsuitable ground, often below water level, before solid rock is reached. Keeping out water and preventing diggings from collapsing can require major feats of engineering in themselves. Tunnels in particular are subject to unforeseen problems and disasters.
Refrences :
Whitney, Charles S. Bridges of the World: Their Design and Construction. Mineola, NY: Dover Publications, 2003. ISBN 0-486-42995-4.
Brown, David J. Bridges: Three Thousand Years of Defying Nature. Richmond Hill, Ont: Firefly Books, 2005. ISBN 1-55407-099-6.
Sandak, Cass R. Bridges. An Easy-read modern wonders book. New York: F. Watts, 1983. ISBN 0-531-04624-9.
http://www.brantacan.co.uk/construction.htm
id.wikipedia.org/wiki/Bridge
http://www.answers.com/topic/bridge
It is one thing to design a bridge. It is another thing to build it. Planning and executing the construction of a bridge is often very complicated, and in fact may be the most ingenious (ingénieur - engineer) parts of the entire enterprise. An incomplete structure is often subjected to stresses and oscillations that would not arise after completion. The construction work is potentially a grave hindrance to existing traffic and to normal life in the area, especially when large local fabrication works have to be installed.
Even before any actual construction is done, substantial work may needed in the form of tests. Boreholes will be made to check the condition of the ground, in conjunction with any available geological maps. Records of wind speed and direction will be consulted, and new measurements made if necessary. In the case of a river or sea crossing, records of water levels and velocities will be needed. Models of the bridge or of parts may be tested aerodynamically and hydrodynamically, and of course mechanical tests will be made. Computer simulations will supplement these tests, enabling a great variety of applied forces to be investigated. There may also be investigations into the effects on people and on the natural environment. It may even be necessary to overcome opposition to the construction, from a variety of objectors.
These are some construction for bridge building that commonly need and used by people :
a. Construction of Arches
An arch is in compression throughout, and it cannot stand except as a whole. It therefore requires temporary support, or falsework, until it is complete. The type of falsework depends very much on the material of which the bridge is made, and on the size of the bridge.
Masonry arches, being made of relatively small voussoirs joined by mortar cannot take tension, need continuous support during construction from below. This type of falsework is called centring, and is often of the general form shown below. Concrete bridges may be supported in a similar way, except that the centring will support the formwork in which the concrete is poured. For large bridges, the centring will be a substantial structure in its own right, and will be expensive.
For a multi-arch bridge, it is desirable to re-use the centring for each span. In the case of a bridge across a river, the centring may be floated from one arch to the next, and will be in the form of a tied arch to maintain it integrity. The centring for the illustrated arch would have been impressive.
Concrete arches also may be built using the cantilever method, as in the schematic example shown below. For very wide and exposed sites, this is preferable to lightweight formwork which could be vulnerable to high winds.
b. Construction of Beam Bridges
Beam bridges are generally in the form of plate girders, box girders or trusses. In all cases, a common construction method is to build the beam away from the final position and slide it or lift it into place as a complete unit. Bowstring or tied arches may be built in the same way.
There is nothing especially complicated about the lifting process, and the stresses in the beam are more or less as they will be in the final position. Nevertheless, a heavy object suspended in space is potentially dangerous, and accidents do happen.
Nevertheless, a slight distortion of the beam did occur. The incident shows the importance, not only of correct instructions, but of good communications. Accidents have happened because instructions were not received, or if received, modified or ignored.
An exception to the method of lifting may be adopted in the case of the suspended span of a cantilever bridge, as in the case of the Forth railway bridge. Here, the beams were built out from the ends of the cantilever arms until they could be joined in the middle. Only then were the temporary rigid connections removed. Such an operation has to be done with great care, because large stresses are released and created.
c. Construction of Cable-Stayed Bridges
One advantage of cable-stayed bridges over suspension bridges is that they can be built out symmetrically from the towers, though long spans are rather flexible until the time of closure.
d. Construction of Cantilever Bridges
A great advantage of a cantilever is that it can be built out from a support without hindrance to traffic or navigation below. The technique varies according to the type of bridge. It will also depend on whether the cantilever is attached rigidly to a free standing tower. Some towers are hinged at the base, in which case falsework will be needed. Even with fixed piers or towers the span may be attached only on a pivot rather than a fixed connection - falsework will be needed here too.
New parts may be taken along the existing part of the bridge, or they may be lifted up from below. Sections of a segmented prestressed concrete bridge may be lifted by crane, or they may be manipulated by means of a launching girder which is slowly advanced along the span. An alternative may be moving formwork which progresses with the construction. The idea is explained in more detail in the section about beam construction.
e. Construction of Truss Bridges
Truss bridges are generally arches, beams or cantilevers, though the decks of cable-stayed bridges and suspension bridges may be in the form of trusses also. Construction methods are as described under arches, beams and cantilevers above.
f. Construction of 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 bridge builder, like the mammals, has to nurse the embryo structure through difficult stages. Very often, the stresses differ considerably from those of the complete structure, and can be more concentrated. The variation in stresses during construction may be so severe that jacking must be provided. Even after completion, a concrete structure may be subject to creep, and the ground may settle, jacks are therefore provided for later adjustment when this behaviour is foreseen.
Excavation for foundations may have to be taken to great depths, through unsuitable ground, often below water level, before solid rock is reached. Keeping out water and preventing diggings from collapsing can require major feats of engineering in themselves. Tunnels in particular are subject to unforeseen problems and disasters.
Refrences :
Whitney, Charles S. Bridges of the World: Their Design and Construction. Mineola, NY: Dover Publications, 2003. ISBN 0-486-42995-4.
Brown, David J. Bridges: Three Thousand Years of Defying Nature. Richmond Hill, Ont: Firefly Books, 2005. ISBN 1-55407-099-6.
Sandak, Cass R. Bridges. An Easy-read modern wonders book. New York: F. Watts, 1983. ISBN 0-531-04624-9.
http://www.brantacan.co.uk/construction.htm
id.wikipedia.org/wiki/Bridge
http://www.answers.com/topic/bridge
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