ISEROI

Article from Newsletter No. 4 November 1997

The 383m long Wexford Bridge has been replaced by a new superstructure during a 10-week road closure period, commencing on the 15th September 1997.

The original seven span prestressed concrete balanced cantilever bridge showed excessive chloride ion content in the concrete resulting in serious corrosion of reinforcement and prestressing anchors together with lamination and spalling throughout the bridge.

The replacement bridge, designed by Barry & Partners, is a continuous steel girder deck with a composite concrete slab. The girders are supported off the reconstructed original piers and abutments. The design philosophy was for maximum prefabrication of structural elements to enable replacement in the shortest possible time. The bridge design was in accordance with BS 5400 Part 3 (1982). Bridge loading was HA and 45 units of HB in accordance with BD 37/88. Structural analysis was carried out using the ENFELA 3D space frame computer programs. The aerodynamic response of the superstructure was checked for compliance with BD 49/93.

Four longitudinal plate girders in the same line and profile as the original prestressed concrete beams are supported by transverse girders at the piers. This enabled the pier reconstruction to take place and the bearing seatings to be constructed before demolition. The seven-span girders have maximum spans of 63.4m, with depths of 4.5m over the piers, reducing to 0.9m at midspan.

The composite concrete deck was precast in 3m long full 12m width sections. Continuity was provided at the transverse joints by looping the longitudinal reinforcement over a 300mm in-situ joint. The concrete was Grade 50 and included a corrosion-inhibiting additive.

Steel Fabrication and assembly was carried out by Construzioni Cimolai Armando SpA. A total weight of 1650 tonnes of plate girder was fabricated in Pordenone in Northern Italy and shipped in lengths of 13 - 23m to site, via Waterford Port. The plate girders were assembled on the 5 acre site beside the bridge, into 63.4m twin girder lengths, ready for erection. Exposed steel was painted to a Marine A 20 year maintenance specification. The interior of the structures between the steel flanges was "enclosed" with metal decking.Defective and laminated concrete in the bridge piers as well as areas showing high levels of electrochemical potential were broken out to fully expose corroding reinforcement. Repairs were carried out using repair concrete or mortar including a corrosion-inhibiting additive. Pier extensions were initially cast between the original concrete piers and then completed after final demolition and erection of steel girders.

The Contractors, Ascon Ltd, employed 150 tonne and 65 tonne Sumitomo crawler cranes on land, and the Marlin 375 tonne ringer floating crane on the water as well as a pontoon-mounted 85 tonne crane.

Demolition was carried out by rock breakers on the bridge deck acting simultaneously from both ends of the cantilever spans working back to the piers. Prestressing was cut in stages as the demolition progressed. The haunch sections over each twin pier, weighing 100 tonnes, were sawed, cut and lifted off by the Marlin floating crane to be finally broken down on the riverbank. Concrete debris from the riverbed was removed by grab after completion of the demolition.

The steel girders were lifted in pairs onto the bearing seatings at the piers. Each span projected 18m beyond the piers providing stability for the next lift. The maximum weight of the twin girders was 145 tonnes.

Precast concrete deck units were fabricated by Banagher Concrete and transported to site for immediate lifting onto the girders. The units included anchorages for parapet posts and lighting standards. Erection of the precast units commenced as soon as the first three spans of structural steel girder were in place.

The bridge was re-opened to the public on the 22nd November 1997, within 10 weeks of the closure.

Barry & Partners