Under-Slung and Overhead Gantries for Span by Span Erection of Precast Segmental Bridge Decks

摘要

Decks that are made self-supporting span by span are by far the most common structural systems used for precast segmental viaducts. Unlike other systems the contractor has little choice than to use overhead or under-slung gantries when erecting such viaducts, unless it is possible to utilize ground-supported falsework. Decks are activated span by span by stressing all segments of a span together with longitudinal prestressing cables, which run the full span length. Because such gantries need to be able to support the weight of an entire span, they are among the heaviest erection equipment used for bridge construction. The method by which segments are joined and activated to self-supporting spans has a direct impact on gantry's performance criteria and configuration. The designer should understand the detail of how such bridges are built, as many design and detailing aspects are governed by construction considerations and not by in-service requirements. The contractor, when ordering a gantry, needs a good understanding of how the bridge, he/she has to build, works structurally to enable him/her to order the right erection equipment. This paper starts with a classification of the precast segmental bridge decks built span by span. It subsequently explains the main features of typical under-slung and overhead gantries and compares their application ranges. The interaction between erection equipment and permanent works is highlighted followed by comments about design/detailing of main girders, mechanization and work access, risks and cycle times. It concludes with remarks about reusability and guidelines for ordering such gantries.