River Suir Bridge, N25 Waterford Bypass

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Architect

Yee Associates

Lead Engineer

Ove Arup & Partners Ltd

Structural Engineer

Carlos Fernandez Casado, SL

Steelwork Contractor

Mabey Bridge Ltd

Main Contractor

Bam-Dragados Joint Venture

Client

CRG Waterford Ltd

For over 40 years Waterford City Council had considered providing a second bridge for the City of Waterford over the River Suir. In 1997 it was concluded that, in addition to a river crossing, a new bypass was required which would connect Cork to the port of Rosslare.

A cable stayed bridge was the favoured choice of construction for the crossing as there would be no need for piers. It also provided the option to use reduced thickness decks offering a navigation clearance of 14m – a necessary consideration in a busy shipping area. Time and cost would also be reduced as the decks could be progressively suspended on the cables rather than using temporary supports in the river.

The asymmetrical twin fan of cables used to support the 230m main deck would enhance the 100m centerpiece, an inverted Y-shaped tower planned to stand on the west bank of the river. This 465m long bridge would become the largest span bridge in Ireland and a landmark structure for Waterford City.

Value engineering input assisted with design and build issues faced during the manufacture stage. Changes were recommended, eg all connections of the cross to main girders from welded to bolted joints, which proved to be substantially more cost-effective and reduced the site programme from 12 to eight weeks; erection changes, from using a floating barge craning all the front span decks into position to the decks being erected from behind at deck level reducing installation costs.

The ladder deck structure carrying the dual carriageway road was made up of 80 main girder sections, each approximately 18 tonnes. The fabrication of the main girders for the cable supported section included some elements of complex geometry in the hanger locations. With large welds and complex angles, dimensional control in these areas became critical. Two box sections at the north abutment, each approximately 170 tonnes, were manufactured in the workshop in segments and welded into full lengths at ground level on site. 102 cross girders, each approximately 10 tonnes, were made in full lengths of approximately 20m. Due to the width of the road deck, and to maintain as low a deck weight as possible, the tops of the cross girders were profiled to follow the road camber.

To protect the 2,800 tonnes of steel required for the decks, a Glass Flake Type II Epoxy treatment was recommended and is ideally suited to bridges.

The structure was erected in two main stages. Firstly the back span, erected from ground level with mobile cranes onto trestles up to the central pylon. The deck was subsequently completed with a precast concrete slab up to the pylon which enabled the front span to be erected in cantilever from the pylon using a modular technique. Each module, comprising two main girder sections and cross girders would be delivered piece-small up to the erection crane sited on the cantilevered end of the deck. Over a seven day cycle each module was erected, the cables installed, pre-stressed and the precast concrete deck positioned. In conjunction with the modular build, a supported section of the front span would be erected of which the largest components were the box sections at the north abutment. These were lifted into position using the UK’s largest available floating crane. These sections were supported on an arrangement of trestles and header beams prior to the front span connection being made and subsequently jacked down into the final position and the trestling removed.

The N25 Waterford Bypass was opened 10 months ahead of schedule on 19 October 2009.

Judges’ Comment

A high-profile cable-stayed road bridge, this helps to generate pride in the community.

The complicated arrangements of the multi-national design team presented a real challenge to the experienced steelwork contractor. Perseverance and the pursuit of practicality in the detail design and construction have resulted in a fundamentally elegant bridge which satisfies client and users.