The Downs – Bridge Beams31st July, 2013 by Gillian Murtagh
Shay Murtagh Precast recently manufactured prestressed bridge beams for an overbridge for the N4 Downs grade separation scheme. The client was the NRA and Westmeath County council and the contractor was SIAC Construction.
The new bridge was a two span deck comprising of prestressed U5 beams which supported a 200mm insitu concrete deck. The new deck had an overall width of 19.1m and each span was 17.173m.
Fourteen number U5 beams were required and these were 17.034m long and weighed 24t. GRC permanent formwork was manufactured to support the insitu deck between the individual beam webs during construction and approximately 160m² of GRC was required.
75mm thick non-participating prestressed permanent formwork was also required to span between the webs of adjacent U5 beams and approximately 308m² was manufactured to complete the deck.
Precast ramp units
Two precast ramp units were also required on this project to span between a support pier on the western approach to a new footbridge over the N4. The first unit spanned from the west footbridge support to an intermediate pier. The second unit spanned from the intermediate pier to a bankseat support on a compacted fill embankment where subsequent ramp approaches were cast insitu on grade.
The ramp units were 3.6m wide and 12.1m long and weighed 30.6t. The cross section comprised of a reinforced concrete deck with four equally spaced 300mm wide longitudinal downstand beams projecting 250mm below the flat soffit of the deck. The deck was 150mm thick at the edge beams and 188mm deep along the centre to provide a 2.5% crossfall. A 100mm high upstand was cast along the deck edges to provide a fixing zone for a pedestrian parapet.
The deck section was thickened locally to the soffit of the downstand beams at the ends of the units. Dowel opes were provided at the ramp unit ends to enable connection to the footbridge support pier and embankment bankseat. A 490mm wide by 280mm deep recess with projecting reinforcement was provided to the ramp unit ends bearing onto the intermediate pier. This enabled the casting of an insitu stitch between the units over this pier.
The Bridge was delivered in by our transport department and installed by the Siac on-site team.