Suspended bridge
The traditional suspended bridges are conceived according to a technique of very old construction, that in fact of the Pont S in lianas, but with range and steel wire ropes much longer. The principle of the suspended bridges is to maintain the weight of the apron by two carrying cables firmly fastened with the banks.
The control of steel allowed the construction of the first suspended bridges as of the 19th century, the oldest suspended bridge always in service seems to be the bridge of the Ile Beard with Lyon, built in 1827 (originally with a suspension with chain). According to a first calculation, approximately 400 bridges were built during this 19th century, a great majority between 1825 and 1850. Number are still the achievements always in place. Apart from France, one can also quote the Pont of Brooklyn to New York (1883).
The structure of a suspended bridge enables him to have important ranges but n the other hand, a certain number of disadvantages presents:
- It requires the presence of imposing and heavy, essential anchorage pillars to retain the considerable forces which are exerted, which functionally binds it to the geology of the ground which will support it.
- the replacement of the cables becomes a very hard and tiresome work requiring several months as well as the closing of the bridge lasting this time.
- the catch with the wind badly studied can cause its collapse, the Tacoma Narrows Bridge inaugurated on July 1st, 1940, crumbled on November 7th, 1940.
History of the suspended bridge
In civilizations Chinese, incas, or African, the suspended bridge was very early a means of traditional crossing, mainly in the mountainous regions where the difficulty of throats arised of crossing. Thus it is estimated that one found more than 200 suspended bridges at the Incas at the 16th century on arrival of the Spaniards, showpieces of the vast network of ways of the Amerindian empire. They usually reached the 50 meters length, probably more, that is to say longer than any arch of European masonry of the time. Only the appearance of steel-trussed bridge will make it possible to exceed this distance from crossing without intermediate pillar. If Incas were only Amerindian civilization to develop such suspended bridges, they existed in other civilizations of mountainous regions of the World, in the Himalayas and in old China. One already found in China of the suspended bridges with steel chains to the IIIe front century J. - C.. But these ancient bridges were generally composed of lianas, and a Tablier out of wood, allowing the passage of a modest load with a light structure of bridge.It is in America that will be born the modern suspended bridge. A judge, James Finlay, with the idea of a suspended bridge with chain S in Wrought iron. The Jacob Creek is completed in 1802, in the west of the Pennsylvania. James Finlay, in front of the success of this formula which allows a bridge inexpensive and easy to build, will deposit a patent. A first generation of bridges sees the day starting from 1810. The range ranges between 15 and 50 meters maximum. But the use of the bridges reveals a problem of Oscillation: the bridge enters easily in Résonance, and the pressure which is exerted on the chains make it yield. Actually, American know-how in Engineering and in the quality of the wrought iron is well too weak. The development of the bridges is limited in the face, and load. Many accidents stop success incipient from the suspended bridge.
The technique then will cross the Atlantic, to find new followers at the Britanniques, which have an enormous advance in the Métallurgie. The chain S are improved considerably. Consequently, the suspended bridges become very ambitious. The first British bridges are built towards 1815, dimensions do not cease growing. In 1826, celebrates it engineer Thomas Telford builds the Suspended bridge of Carried out (Carried out Bridge) , of 125 meters range, which allows the passage of the sailing boats. It is then the largest bridge of the world, the majority of the bridges of the time ranging between 70 and 100 meters of range. The suspended bridge is the only means to await such lengths, the bridge becomes monument with the glory of progress, into full Industrial revolution European.
It is precisely the European rise of this one which exports the suspended bridge on the continent. In France, technology is known through the British exploits reported in the newspapers. A mission of study of the Bridges and Chaussées is carried out in 1821, without results. The territory contains one of the rivers passable at the time with more difficulty, the the Rhone. The bridges are far from numerous, 3 including one broken (the bridge of Avignon) between Lyon and the estuary. Indeed the river is broad, very powerful, and does not know a notable fall of its flow since undergoing the snow melt. Without “dry” season, it is thus impossible to build piles according to the tested method. A company, Seguin Brothers (Annonay, Ardeche), directed by Marc Seguin, thus proposes a project innovating in 1822: the Suspended bridge of Tournon. The company understands very quickly that a traditional suspended bridge is impossible in France because of poor quality of the chains. One then tries to replace them by beams of wire of iron. It is the birth of the cable. After several tests, a refusal of the Highways Departments, the project is finally accepted. To the innovation of the cables is added the hydraulic use of Béton for the foundations, of the Reinforced concrete (25 years before the first patents) for the superstructures, and of the structures of reinforcements rigidify the Tablier out of wood. The suspended bridge took its modern form. One of the advantages of the suspended bridge is that it can support loads definitely higher compared to stayed girder the Bridge.
Some suspended bridges
(all dimensions are in Mètre S.)