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 Bridge Launching

Bridge Launching


The first prestressed concrete (PC) bridge was built in 1941. From the end of World War II onwards, PC bridge construction developed quickly, thanks to the pioneers of the new technology: Guyon, Freyssinet, Leonhardt, Magnel, Morandi, Mo ̈rsch and Ross, among others.

The design of most of these new bridges was limited to isostatic or poorly redundant systems; because analysis of hyperstatic systems was not possible with the means of calculation available.

For many years, the basic criterion in the design of PC bridges was to ensure the possibility of simple structural analysis. So, multispan bridges cast on falsework utilised the simple support scheme, the first balanced cantilever bridges in the 1950s were hinged at midspan, and the first PC decks built with movable scaffolding systems in the 1960s were supported at the piers or articulated at the counterflexure points. Most of the PC bridges of the post-war period were also designed to minimise the quantities of structural materials.

From the 1960s onwards, the extraordinary progress in computing techniques extended the possibilities of analysis of hyperstatic structures. The technological maturity of prestressing led to lighter and more ductile structures with an enhanced capability of resisting temporary construction stresses. The improved knowledge of materials stimulated technological innovation, and materials with higher performance found intelligent use and adequate processing methods.
Increasing labour costs amplified the labour component of the construction cost of new bridges, and new construction methods were developed to rationalise production, regularise quality,increase the erection rate, and ensure the safety of workers and the public.

The creative thinking during these decades led to the development of several construction methods that ensured the competitiveness of PC over steel construction. At the same time, however, the lower cost of high-quality hot-rolled steel plates, better workshop organisation,new splicing techniques and new field assembly methods extended the use of steel–concrete composite bridges to spans that previously were the domain of PC bridges. New types of box girders combining PC slabs and steel corrugated-plate webs were also developed to address these borderline spans. This further stimulated research and created transdisciplinary connections between different technologies. New construction methods took advantage of the recent advances in technology to widen the field of application of already familiar techniques and to reduce the labour component of the cost of bridge construction. Structures can be moved, but this requires a different way of thinking about them, and the availability of suitable technology. So, although the idea of launching a bridge is not new (think of a tree trunk in Palaeolithic technology) and numerous steel bridges were launched in the 19th century, launching PC bridges was made possible one century later by the availability of a new low-friction materials such as poly-tetrafluoroethylene (PTFE) (Teflon).

For many decades, the light weight of steel structures had permitted their launch by means of winches and lubricated wooden skids, with frictional loads that, although considerable, did not cause excessive stresses in the piers or require expensive launch equipment. The flexural efficiency of the steel girders and the ability of the material to indifferently resist tensile and compressive
stresses facilitated launching and avoided overdesign. Launching does offer several advantages over in-air erection; however, compared with ground crane assembly, the costs of launching were often higher due to the availability of only one working point, and launching was initially limited to bridges to be located high above the ground or in inaccessible areas.In the eyes of the PC bridge pioneers, some of the disadvantages of launch technology were less critical, and others were even promising. Construction duration and yard organisation for a PC bridge are different from those for a steel bridge, and the cost of labour and equipment is so high that every possible alternative must be examined. Launching of a PC deck built on the ground promised savings in both labour and equipment, but practical application was limited by the weight of the deck and the low tensile strength of concrete.

These obstacles were gradually overcome. Advances in prestressing technology lightened the deck, made it more flexible and ductile and less subject to cracking, simplified splicing of tendons, and allowed the introduction and removal of prestressing according to need.The commercial availability of personal computers and structural analysis programs simplified analysis of the continuous beam in the multiple support configurations of launching. The development of steel–Teflon skids offered a substantial reduction in launch friction, and technological advances in electro-hydraulic equipment offered the possibility of moving huge masses with due precision.

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