La técnica del pretensado exterior en los puentes modernos

External prestressing technique in modern bridges

In this paper, the more important of external prestressing which are related to the great bridge structures, are presented. All this taking into account papers published by french engineers mainly, who have contributed to the revival of a constructive technique already know, but up till now forgotten. Bridges named as "mixed tendon layout" (Cantilever and Incremental Launching Methods), in which classical bonded prestessed layout coexists with external prestressing, are presented. New typologies of bridges structures in which the use of external prestressing is almost inescapable are also presented: concrete bridges with steel folded webs. The incremental launching method with provisional cable-stayed tower is related to external prestressing, which is only a external prestressing where the eccentricity on supports is lead to its greatest posible value, thanks to concrete deviators placed on outside of the deck. The different ways of implementing external prestressing are discussed and one of the more extended applications of external prestressing is not forgotten: the post-tensoioned repair of concrete structures. Finally a review of advantages and disadvantages of external prestressing is presented with special reference to the ultimate-state design problem.