The International Association for Bridge and Structural Engineering (IABSE) has announced that the Outstanding Structure Award for ‘Bridge or Other Infrastructures’ category 2021 has been won by the Rose Fitzgerald Kennedy Bridge over the River Barrow, as part of the N25 New Ross Bypass project. ‘A landmark structure with three-towers and iconic profile that has pushed the boundaries for the span of a concrete extrados bridge.’ 

The award comes on top of one it received last year as Engineers Ireland and ESB celebrated the work behind Ireland’s longest bridge as part of the organisation's Engineering Excellence Digital Series.

The Outstanding Structure Award

The IABSE Outstanding Structure Award (OStrA) was established in 1998. It is one of the highest distinctions awarded by IABSE and recognises, in different regions of the world, some of the most remarkable, innovative, creative, or otherwise stimulating structures.

Rose Fitzgerald Kennedy Bridge at night. © Royston Palmer

Sustainability and respect of the environment is also an important factor. This award recognises the most remarkable, innovative, creative or otherwise stimulating structure completed within the past few years. The awards are restricted to a maximum of one award per year. The Outstanding Structure Award Committee was chaired by Naeem Hussain.

Winner ‘Bridge or Other Infrastructures’ category: Rose Fitzgerald Kennedy Bridge 

The Rose Fitzgerald Kennedy Bridge is one of the most iconic structures in Ireland. This three-tower extradosed bridge has two main spans of 230m each, the longest post-tensioned all concrete extradosed spans in the world.

While there are longer spans in extradosed bridges, they all take advantage of a lighter steel composite section in the central part of the main span. The structure is part of the N25 New Ross Bypass project, which strengthens regional connectivity as well as local connections between the communities of Wexford, Kilkenny and Waterford in the south east of Ireland.

 

At the location of the crossing, the River Barrow is 300m wide and is surrounded by a landscape of gently rolling hills. This modern, 21st century landmark has been designed to be sympathetic and complementary to its surroundings and environment, enhancing the quality of life in New Ross town which was previously subjected to frequent traffic congestion. The structure was conceived during the planning stage as a three-tower extradosed bridge with a central tower higher than the lateral towers. 

Proportioned to the golden ratio in height and span distribution, with a truly shallow (less than 15 degrees) cable arrangement in a harp configuration, the bridge represents a formidable structural design and construction challenge. 

Asymmetrical configuration

Three towers following an asymmetrical configuration give the bridge a unique profile, with a shallow single central plane cable system in a pure extradosed structural system.

The Rose Fitzgerald Kennedy Bridge has two main spans of 230m each which are the longest post- tensioned all concrete extradosed spans in the world to date. Despite the main span lengths, the towers are only 27m and 16m in height, while a deck slenderness of L/65 at the central section and L/35 and L/27 over the lateral and central towers was achieved.

The structure comprises a single central plane of cables supporting a dual carriageway, leading to a 21.9m deck width. The deck is slender, with a 3.5m deep section (span/65) at midspan, 8.5m over the central tower (span/27) and 6.5m over the side towers (span/35).

The tower height limits, with the central tower rising 27.0m above the deck level and the lateral towers 16.2m, result in all cables following the conventional extradosed harp arrangement with minimal spacing in the pylon (1m) and a shallow angle with the deck, which varies from nine to 11 degrees due to the longitudinal slope of the deck.  

Engineers Ireland award

The Rose Fitzgerald Kennedy Bridge was also the recipient of an award from Engineers Ireland in 2020. Last November, the organisation described it as the "construction of the world record-breaking bridge in Ireland’s southeast", and it was among the projects recognised by Engineers Ireland and ESB in the Engineering Excellence Digital Series.

Constructed by BAM-Dragados with the detailed design being undertaken by Arup, in collaboration with Carlos Fernandez Casado, it comprises a 14km bypass which includes an 887m long, nine-span, three-tower extradosed Rose Fitzgerald Kennedy Bridge.

The structure is part of the N25 New Ross Bypass scheme which strengthens connections between the communities of Wexford and Kilkenny, while also enhancing local and regional connectivity.

By reducing congestion, improving road safety and opening economic opportunities, the bridge has improved the lives of residents on both sides of the River Barrow and, indeed, the much wider community.

Constructing the longest bridge in Ireland required significant temporary works, including two temporary piers, a push-pull prop for the balanced cantilever construction and a temporary artificial island to facilitate construction of the main central pier where the piles extend 32m below ground.

2D hydrodynamic modelling was carried out to estimate the potential changes in the flow regime associated with both the temporary island and the permanent in-channel bridge pier.

Detailed ship impact analysis was carried out for the central pier to resist potential loads up to 17MN, taking account of the river bed and bank topography and dissipation of energy of the ship while approaching the foundation, along with the non-linear behaviour of the soil structure displacements during impact.

The bridge includes four extradosed concrete spans; two of which are 230m in length. The construction method, balanced cantilever, in conjunction with the asymmetry of the towers cables on each tower, led to a main cantilever of 140m at the longest stage of construction, a world record for concrete deck extradosed bridges.

Structural analysis tools, such as explicit time dependent creep curves and step-by-step non-linear iterative analysis, were used. The largest cables consist of 125 strands and went through a full-scale fatigue test of two million cycles in a lab in Chicago, one of only two labs worldwide that have the capacity to test cables of this size.

Fire engineering design investigated the effects of various scenarios of vehicle fires on the bridge deck. Thermal blankets were designed for the lower part of the cables.

The consciously chosen asymmetry of the three towers, with the central tower slightly higher than the side towers, provides this structure with unique personality, contributing to the Wexford-Kilkenny skyline.

It will stand as a world-class 21st century structure that enhances this part of the country while also serving as a key link in local and regional infrastructure connectivity.

The opening of the N25 New Ross bypass on January 29, 2020, was a landmark day for the designers and contractors and for Transport Infrastructure Ireland, Wexford County Council, Kilkenny County Council and the technical advisers for the scheme, Mott MacDonald Ireland.