The design of this innovative Precast Concrete Viaduct Bridge structure known on M50 Upgrade Contract as ‘Structure 17’ is a large curved viaduct bridge spans over a live railway, canal and existing N3 dual carriageway. This structure will ensure free flow of traffic east bound on N3 to access the M50 Motorway South bound. Main Contractors for the overall NRA funded project M50 (D&C) commissioned Spanish company Alvisa and Shay Murtagh Precast to undertake a design, production and installation of this structure.
For this viaduct bridge project the general scheme of the structure is a group of six precast post-tensioned curved U-shaped bridge beams departing from one abutment and splitting to connect the first abutment with two more abutments, one receiving two of the six beams and the other the remainder.
Bridge Beams & Bridge Design
The bridge design used for the S17 is a new precast concept for Ireland because of the use of very large, curved, precast concrete bridge beams connected at the project site by means of post-tensioning systems. This hypostatic structure consists of different radius beams with varying depth from 1.60 meters to 1.75 meters. The bridge beams are internally post-tensioned at the factory and when erected at the project site the interface joint between beams is also positioned. Lengths of the precast concrete bridge beams range from 15 to 34 meters with beam weights of 130 to 200 tonnes. The joints between beams do not occur over the piers. This characteristic design allows spans between piers of 34.5 meters and 42 meters between pier and abutment. Another characteristic of this viaduct bridge is that all the piers were precast and manufactured by Shay Murtagh Precast and were fitted in place on preformed foundation pads on site.
After all the joints have been filled with special mortar and sealed, then post-tensioned, the precast slabs and cantilever slabs are placed over the bridge. The deck area is then finished with ready mix concrete in situ. This last step of the process gives continuity to the whole structure. Concrete used for the precast elements has a compressive strength of 60 kN/mm2 and includes ground granulated blast furnace slag (GGBS) in order to enhance concrete durability.