Cantilever bridge
A cantilever bridge ( English cantilever bridge ) is a bridge , in which parts of the structure as a cantilevered beam ( "Boom") are executed. Usually two jibs facing each other and connected at their tips form the central opening of such a bridge. At this point the bridge structure has its lowest height, while it is highest at the beginning of the jib over the river piers.
In the Gerber girder bridge, a separate bridge girder (so-called suspension girder ) is articulated between the free ends of the cantilevers , so that a greater opening width can be achieved. This construction corresponds to the two- hinged tanner beam , the invention of Heinrich Gottfried Gerber , with which he converted a continuous beam lying on more than two bearings into a statically determined support system.
Simple cantilever bridges made of wood with attached intermediate parts (see adjacent figure) already existed in ancient times. They were only used in modern bridge construction after Gerber's reinvention in the second half of the 19th century.
The massive steel bridges that emerged from the chain and rope suspension bridges and which look similar to them are also counted among the cantilever bridges (no central joint, see adjacent figure: Eiserner Steg in Frankfurt / Main).
The original boom as a steel truss later came box girder of prestressed concrete added.
history
Cantilever bridges are likely to be a very old type of bridge, as simple versions can be made with stones and logs. Bridges made from these materials are still being built to cross rivers in Pakistan , Afghanistan , India and the People's Republic of China . For this purpose, trunks must be fixed with stones on both banks so that the longest possible end protrudes towards the river. If the ends do not reach the middle of the river, wooden structures can be placed on the logs protruding from both banks. In Europe such wooden bridges are known from the Gauls . They were used in Savoy until the 18th century.
A proposal for a wooden cantilever bridge from the builder Villard de Honnecourt has come down to us from the late Middle Ages . This construction method only appeared in the sketchbooks and did not seem to have been implemented anywhere. This construction also uses beams that connect the two cantilever beams attached to the banks.
In the middle of the 19th century, the engineer Heinrich Gottfried Gerber helped the cantilever bridge to breakthrough (see model in the picture opposite). He described in a patent of 1866, such as a continuous beam so with joints can be provided that it statically determined is and the so-called Gerber beam is. The most common design consists of two cantilevered girders, which are placed over supporting pillars on both banks and protrude towards the middle of the river. Their ends are connected by a suspension beam , which does not itself rest on any pillars, but is only supported by the cantilever beams.
Tanner girders were mainly used for large steel trussed railway bridges , which could not be designed as suspension bridges due to their span and the higher loads of the trains compared to road traffic . After the invention of the tanner girder, other types of steel truss cantilever bridges emerged, but these did not catch on and were only used in a few structures. The Forth Bridge is an example of this .
In the 20th century only a few cantilever bridges were built in steel framework. In America, the construction method lasted the longest, supported not least by the steel industry. Outside America, the only major structures that were built were the Howrah Bridge in India and the Minato Bridge in Japan .
The steel truss cantilever bridge was mainly due to pre-stressed concrete - girder bridges replaced (see illustration), which were easier, faster and build more cost-effective. The cantilever bridge technology is also used for prestressed concrete bridges when the superstructure is built as a cantilever beam starting from the individual pillars. With this design, at the end of the construction phase, the ends of the individual sections are connected to one another without joints, so that the superstructure becomes a statically indeterminate continuous beam.
See also
literature
- Henry Petroski : The rise and fall of cantilever bridges. In: American Scientist , Volume 106, November – December 2018, pp. 336–339 ( online )
Remarks
- ↑ This direct connection between two i. d. R. statically determined bearing bridge parts is no longer statically determined, even if the connection is articulated.
Web links
Individual evidence
- ↑ B. Nebel: Cantilever bridges. Retrieved February 13, 2013 (with picture of a simple bridge in Nepal).
- ^ Pont Villard de Honnecourt
- ↑ Description of the patent on December 6, 1866 awarded to the engineer Gerber on beam supports with exposed support points. In: Journal of the Bavarian Architects and Engineers Association , 1870, p. 25 (p. 29 in digitized version)
- ↑ Figures on page VI of the patent
- ^ Kurt Hirschfeld: Structural engineering: theory and examples . Springer, Berlin 2006, ISBN 3-540-36772-1 , pp. 41 .