Caroline Cavanagh looks at the development of a highly controversial bridge in the historic city of Kilkenny.
Kilkenny city is a growing city and its centre depends on good access, strong permeability and proper urban planning in order for it to develop.
The Central Access Scheme was conceived to encourage and facilitate the redevelopment of the heart of the city, consolidating it and reducing the pressure to develop at the edge of the urban area, while providing access to develop in a sustainable and cost-effective manner over 20 acres of brownfield city centre sites.
The Central Access Scheme had been the objective of successive Kilkenny city development plans since 1980; after lengthy legal battles it was finally approved in 2009 and construction began in 2015.
The new access bridge in the heart of Kilkenny city is positioned in a historic and culturally significant urban location that is also a special area of conservation resulting in substantial ecological and environmental challenges.
Following planning objections, the landmark cable-stay bridge originally proposed was replaced with a simpler yet sophisticated structure which is more visually sympathetic to the local surroundings, comprising of a five-span semi-integral 125m bridge crossing the River Nore. Designed by Malone O’Regan and constructed by John Cradock Ltd (JCL) the St Francis bridge was completed in 2016.
It was a requirement that the bridge be simple and elegant, with an architectural requirement for flat soffits and minimal structural depth of a mere 1,200mm due to the major headroom restriction; supporting two 3.5m-wide carriageways, plus cycleways and footways, with a design velocity of 30 km/h and providing access to the riverbank amenity area.
Banagher Precast Concrete collaborated with consultants Malone O’Regan on the overall bridge design using their combined expertise in precast prestressed bridgework to develop an innovative solution and create this aesthetically important structure with flat soffit and innovative ‘invisible crossheads’.
The project began with in-river works where John Craddock Ltd constructed six 15m-long bases, including two cofferdams in the River Nore, each with 14 x 900mm diameter cast in-situ bored piles with average depth of 12m. To add to the project complexity all in-river works had to be completed within the restrictive fisheries season.
Substantial in-situ reinforced concrete works included four piers, each with two cylindrical columns, averaging 8m high, 1,200mm diameter, and two abutments, each 15m wide – total concrete volume of 2,400m³.
While work commenced on site, the unique ‘invisible crosshead’ moulds were designed and manufactured to incorporate the service portals, the 70 prestressed Y beams were poured, and the other major precast element – 1,800m² of precast concrete permanent formworks for the bridge deck was also manufactured.
With a tight one-week window for precast installation, the execution of multiple crane set-ups, deliveries via restricted routes and installation had to be planned in great detail and executed with exact precision.
First, the technically challenging bespoke crossheads were delivered, installed and stitched in situ. Then, during the course of the week, 70 prestressed Y beams measuring up to 26m were delivered to site just-in-time and installed in sequence to mirror the different crane set-ups, followed by the installation of the services and the precast deck.
A major advantage of using prestressed Y beams was that they could be lifted into position safely and quickly, they were immediately stable and provided a safe solid deck from which to work from – very important as the safety of construction workers is always a paramount consideration and especially relevant here when the team were working over water and so many services had to be incorporated into the structure deck and crossheads.
The use of concrete was a considered strategy as the visual appearance of the flat soffit of the bridge was of significant architectural importance in the context of the riverbank amenity parks routed below the bridge on the north and south banks.
The concrete finishes to the precast elements and overall aesthetics were critical to this project in light of the stringent planning conditions and the low headspace above the walkway running alongside the river. By manufacturing the beams offsite in a controlled factory setting, it ensured quality and uniformity in the concrete finish.
To complement the aesthetic, cantilevered structural steel walkways were incorporated on either side of the bridge for cyclists and pedestrians, along with a unique curved and cantilevered viewing platform.
The design also specified the provision of local Kilkenny limestone cladding to abutments, bespoke bronze alloy handrails, exposed aggregate containment barriers and innovative low level lighting to subtly enhance the aesthetics of the structure.
A key aim for Kilkenny County Council was that the bridge should have a positive impact on helping achieve its goals for sustainable development and environmental improvement.
With the opening of the bridge, the perennial congestion has been alleviated with a consequent reduction in local carbon emissions. The provision of a dedicated and safe foot/cycle way has also encouraged non-motorised transport modes.
At the design stage, careful consideration was given to ensure that the risks of adverse environmental effects, such as scouring, were not increased and works were undertaken within the required fisheries season.
The design consultants and precast engineers designed and created an elegant flat soffit structure, using innovative ‘invisible crossheads’ and steel plates cast into the ends of the beams. This bridge is an integrated structure with limited number of movement joints minimising whole life costs.
The use of prestressed Y beams also delivered structural advantages over other beam types, which lead to significant construction savings and reduced work on site.
The concrete structure as it stands today is robust and provides lasting transport resilience. Concrete was the natural choice, ensuring that this transport link can withstand environmental effects and climate change with minimal maintenance.
Winner of the 2016 Excellence in the Civil Engineering Award (€5m-€10m) category by the Civil Engineering Contractors Association, the St Francis bridge facilitates the sustainable development of Kilkenny city centre and in so doing maintains commercial, economic, social and cultural vitality in the historic core of the city.
Author: Caroline Cavanagh