Cantilever construction is a method of construction in which successive sections are added to the free end of a cantilever. It is used particularly for bridges with long spans. In balanced cantilever construction, work advances on both sides of a supporting pier.
History
The method was used for steel bridges before it was applied to concrete. Its first application to reinforced concrete was the central span of the , built in 1930 over the Rio do Peixe between Herval d'Oeste and Joaçaba, in Santa Catarina (Brazil). The span was approximately long. In 1951, the Lahn bridge at Balduinstein, designed by , became the first bridge built by cantilevering in cast-in-place prestressed concrete.
Two years later, the same method was used for the Nibelungen Bridge at Worms, the first prestressed concrete bridge across the Rhine, with a maximum span of . The Stolma Bridge in Norway, completed in 1998 using balanced cantilever construction and lightweight concrete in the central part of its main span, had a span of . At completion, this was a world record for a segmental concrete box girder bridge.
Construction process
In conventional balanced cantilever construction, the superstructure is built out symmetrically from a pier, to which it is rigidly connected during construction. The deck is generally deeper above the pier than at midspan, to resist the greater bending moments near the support.
In cast-in-place construction, a form traveller supports the projecting formwork and the weight of the new concrete. Sections, typically long, are cast on each side of the pier. Once a section has gained sufficient strength and has been post-tensioned, the traveller advances to the next position. Construction continues until the cantilever reaches approximately halfway across the span. The other half is built from the adjacent pier. A closure section joins the two cantilevers to form a continuous structure; earlier bridges often had a hinge at midspan.
Auxiliary girders can also be used. At the Kocher Viaduct, an overhead steel girder carried materials to the form travellers and allowed them to be moved to the next pier. Multi-span bridges can be assembled similarly from precast concrete segments, using large movable launching gantries.
Reinforced-concrete arches can be built by cantilevering with temporary stays and towers. The decks of cable-stayed bridges are also commonly built by cantilevering, with the stays providing support as construction advances.
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