Bridges

For transport infrastructure upgrade projects, bridge widening is typically a more economical and time effective solution compared to bridge replacement. The design and construction of bridge widenings is uniquely challenging. The design approach includes information collection, load rating, establishing design criteria, and structural analysis and design. At the start of the project, as much information as possible should be gathered from as-built drawings, past inspections, condition assessments and load ratings, site survey, etc. The load rating should assess the unaltered state, final widened state and temporary construction states. The load rating outcome informs the overall design solution. The design criteria are not so simple to establish because many existing bridges being widened were not designed for current design actions. In the structural analysis, particular matters that require careful attention include differential creep and shrinkage, differential settlement, construction staging, and load distribution between new and existing elements.

This paper presents some main considerations, challenges and solutions for the design and construction of short to medium span bridge widenings, with reference to two bridge widenings from the Warringah Freeway Upgrade (WFU) in Sydney. The WFU is an enabling works project for the current Western Harbour Tunnel project. WFU includes the widening of the Falcon Street and Mount Street overpasses, which are two steel I-girder bridges with concrete decks constructed in the 1960s. The widenings of these bridges, using composite steel I-girders, consists of two main girders placed parallel to the existing girders, and four splayed girders bolted to the new outer main girder. The connection of the new and existing decks, bridge articulation, steelwork details, and construction methods are discussed. Finally, recommendations drawn from the design and construction processes of these bridge widenings are presented, offering valuable insights for future projects.