Rose Fitzgerald Kennedy Bridge

General

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.

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.

Rising 36m above the water to provide navigational clearance for access to the Port of New Ross, the structure is impressive in both scale and slenderness. The river’s eastern bank is lower than the west, with a sharp rock slope on the west and a significant flood plain on the east. For most of the deck, and particularly the main spans which are on a 5% slope, this topography was an important consideration during the design.

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 9 to 11 degrees due to the longitudinal slope of the deck.

This difference in tower height is also reflected in the differing numbers of cables between the central and lateral towers, with central tower having 18 cables, and the lateral towers having 8 cables each. This feature along with the slope of the deck, climbing from the wide floodplain in the east bank of the river towards the higher and steeper levels in the west bank, a place popularly known as “pink rock” provides the bridge its unique profile and has become already a landmark in its own right.

Design

Design and construction commenced in early 2016, with substantial completion in late 2019. The bridge opened to traffic on 29 January 2020.

The bridge is designed following the Eurocodes and the Irish National Annexes, which are particularly stringent in some key sections such as the stress criteria in Service of posttensioned structures. Additional project specific requirements were catered for in line with the principles of maximising durability and minimising long-term maintenance requirements.

Resilience and robustness were the main goals when designing the structure, requiring state of the art analysis of extreme event scenarios such as ship impact, fire (in any location in the deck) and wind. Additionally, special environmental analysis including extensive hydrodynamic modelling to determine riverbed scour and sedimentation patterns were developed. This analysis ensured that the effects in the natural river environment and the navigational channel were minimised during construction and in the permanent situation.

As a direct consequence of a central plane of cables in an extradosed configuration, the shallow cables cross the top slab in its mid-section, interrupting it for more than 3m for a 6.5m cable spacing. This required specific consideration of the transversal behaviour, resulting in transversal post-tensioning in addition to the longitudinal and main cable prestressing. The transversal behaviour is also optimized by the use of internal steel props connecting the central anchor with the webs at their bottom corners. The webs are spaced 8m which leads to a 6.5m long cantilevers, supported by a precast panel that provides the deck a closed appearance.

The design also took careful consideration of minimising the embedded carbon footprint within the constraints specified by the client, which included providing an all concrete structure. High strength concrete (up to C80/95) was utilised in the deck, which allowed for a slender 3.5m deep box in the constant depth area with minimal web and slab thicknesses. The cross section was optimised and the project requirements – a closed cross section for a deck over 20m wide– were solved with only two webs 8m apart and the use of slender inclined precast panels to support the cantilevers. The GGBS content was maximised as much as possible in all concrete elements to reduce the quantities of cement and improve durability. Stainless-steel reinforcement was also used in the main pier in the river to ensure long-term durability. In addition, the project sourced 90% of the constituent materials locally.

The largest cables consist of 125 strands. These 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. Also, and in order to minimize the pylons that were only 1.60m wide in the transversal direction, the anchor system at the pylons was provided by saddles, which led to a maximum cable length during construction of 278m between active anchors in the deck.

The central pier foundation, which supports the central tower and is integrally connected with the deck, is located within the tidal range of the River Barrow. The pile cap is 27m long by 14m wide by 5.4m in depth, and is supported by 42 bored reinforced concrete piles, 1.2m in diameter and 32m in length.

Construction

After a detailed study of the Design and construction team, two construction methods were used in order to optimize the programme and materials. The side spans up to the lateral pylons on both sides were built with scaffold to the ground for the central box of the deck (8m wide) and a wing traveller in a second stage for the cantilevers and precast panels that close the section.

For the main span four form travellers, two starting in balanced cantilever from the central pier and two on single front from the lateral piers were specifically designed for the project. The form travellers were able to build the full width section, which was casted in a single stage, including the precast panels that complete the close appearance of the deck and reduce the cantilever transversal bending.,

Despite the relative complexity of the construction cycle, with several complex activities in themcritical path (transversal posttensioning prior to stressing of main cables, longitudinal posttensioing, form traveller setting out and loading, precast panels installation, reinforcement including internal transversal steel props installation and concreting), a cycle of 7 days was achieved in the best cases.

The construction method of the main spans allowed for all construction activities to take place without any interference with the river’s navigational channel. Due to the difference in height and number of cables between the central tower and the side towers, the meeting point of both cantilevers is not located at midspan (115m).

The cantilevers from the central tower were 140m long, while the cantilevers towards the main span from the two lateral towers were each 90m in length. Together, these formed the two main spans of 230m each – the longest post-tensioned concrete spans of their type in the world. And from the point of view of the central tower, equated to a span of 280m.

The central tower cantilever during construction, a world record for extradosed construction, would equate to a 280m span in a symmetrical configuration. This, with a deck of only 3.5m deep and shallow cables, represented a significant challenge during construction.

In order to minimize the bending of the central tower during construction, a temporary tower, located between span 4 and 5 at the edge of the river, and designed to take both tension and compression was connected to the deck after the construction of segment no 7 out of the 23 cycles that were required to reach the connection point of both cantilevers.

State of the art structural analysis, including explicit time dependent (test-based) creep and shrinkage curves and step-by-step non-linear iterative analysis, were used to predict the behaviour of the structure during construction, allowing for the prestressing and form-traveller strike to occur only 36 hours after the previous pour.

The structure proved to be very flexible, with deck deflections up to 700mm for a single stage (concrete pour or cable stressing) during the latest stages of the central tower cantilever.

Finally, and in order to facilitate the future inspection and maintenance of the structure, a complete system of Structural Health Monitoring, including continuous reading of cable forces, bearing displacements and loads, and strain deformations at key locations have been implemented along with monitoring of environmental sensors (such as temperature both inside and outside the box and wind loads at deck level and the top of the central pylon).