Design and Engineering
The refurbishment of the city centre and Dublin City Council’s policy of increased facilities for pedestrians resulted in a competition for a pedestrian bridge over the River Liffey, 125 metres upstream of the existing Liffey Bridge (Ha’penny Bridge). This latter bridge is internationally recognised as a symbol of Dublin, and the winning design was seen as complementing and respecting it. The Ha’penny Bridge was at capacity with over 2000 pedestrians per hour at peak time and access to it was difficult because of its location and the steps on the bridge. In addition the city council wanted to create another north-south access route west of the prime city centre thoroughfare. The winning team were Howley Harrington (Architects) with Price & Myers as Consulting Engineers. The successful contractor was Ascon Ltd.
At the point of crossing the river is 51 meters wide, although the central span of the bridge is 41 meters between the new projecting abutments (4 metres wide). The truss is designed as an asymmetrical parabolic arch, the booms of which are made of solid steel rod. These curve gently inwards in plan and section from the abutments on each bank. It has a gentle gradient of 1 in 20, to allow easy access to all. The shallow curves of the truss create a highly efficient structural solution, through which the distribution of forces in the bridge are expressed clearly. Terminating in the large concrete haunches contained within the granite-clad shell abutments, the bridge structure is a simple and highly efficient portal frame. The truss was prefabricated off site by Thompson Engineering of Carlow, transported 90 km to Dublin and, despite weighing 60 tons, was placed in position by a single crane.
This arrangement permits the use of very slender structural members, creating a sense of lightness and transparency in the bridge, which is echoed in the balustrade of the deck and in the abutment railings. The sensuous curves of the truss are also expressed in section and plan in the abutments, which are shaped to respond to the movement of people on and off the bridge and the flowing river below. The curved, stone-clad concrete shells of the abutments are designed to act as spread-waters to allow the smooth passage of water around them at all tide levels. From the quay wall, the pavement sweeps out over the river, opening up the bridge in width at each end. This relieves pressure at the busiest pedestrian points and provides a place to congregate and linger before crossing the adjoining busy roads. The abutment parapets reflect the detail of the existing quay walls by stepping down the granite stonework in semi-circular sweeps to allow views through to the river and the new bridge. The slotted aluminium deck is supported off a series of secondary ribs running between the cross members. These are integral with the top booms of the truss, and continue upwards to provide supports for the balustrade and leaning rail.
Lighting for the new bridge was designed to be an integral part of the whole concept. The aluminium deck is lit using hidden emitters supplied by fibre optic harnesses running inside the specially shaped aluminium bronze leaning rails. Along the centre of the deck are a series of small uplighters, similar to runway lights, which guide you across the bridge, and are a great favourite with children who skip from light to light. Along with small glass globes at the top of the light standards gradually change colour from white at dusk to dark blue at midnight to suggest the bridge and the city, going to sleep. Finally, narrow beam floodlights, mounted under the pavement of the abutments, shine through the structure of the truss highlighting the filigree nature of its delicate members.
The contract programme was for six months, which the contractor achieved. The estimated cost was IR£1m, but the lowest tender was for IR£1.6m. Because of the location and prestige attached to this particular project, the contractor agreed to fix the price at the tender figure, and it was completed for this sum. Such was the quality of the design and construction that the bridge received awards from the Institution of Structural Engineers (UK), RIAI, RIBA, Construction Industry Federation (IRL), Emerging Architecture Award and Opus Plan Expo 2000.