Design and Engineering
The decision to rebuild Carlisle Bridge (now O’Connell) necessitated a new bridge, downstream, to take the diverted traffic. The chosen bridge was a structural steel swivel bridge designed by the Port Engineer, Bindon B Stoney. It was built in the period between 1877 and 1879 by William Doherty for the sum of IR£44 662 6s 11 d
The bridge was officially opened in August 1879. A condition in its approval to be built was that it should permit vessels to pass and berth in the river as far upstream as the Carlisle Bridge. The design consisted of a single masonry approach span of 11.28m from each bank with a central swing span of 38.71m, which provided two 12.19m wide navigable openings each side of the central pier support structure. The carriageway was 5.64m, and there was a 1.68m footpath on each side. The swing section, made of wrought iron and weighing 200 tons, was supported on and ran on a series of cast spoke wheels. It was powered by a steam engine, which was housed on a timber pier at the downstream end of the central pier.
To minimise encroachment of the temporary works on the navigable width of the river the abutments and central pier were constructed using a single line of piles rather than the double or greater number of rows that had been the approach with earlier upriver bridge constructions. The swing bridge was decommissioned in 1888, mainly as a result of the construction of the Loopline Railway Bridge which restricted the height of ships that could pass through. The restricted road width and gradients created a need for the bridge to be replaced so designs were sought.
A masonry arch of four spans was rejected because of the cost of cut stone, the interruption of river traffic and the thickness of arch needed would cause an unacceptable gradient. Steel was not considered because of appearance considerations. The erection of the bridge directly on the old timber piles was rejected due to the unknown condition of these and the bridge would be skewed to the nearby streets. Navigation clearance was to be no less than the upstream bridges. A cantilever structure was eventually chosen, as an arch without the interruption of barge traffic would be a problem. In addition because of the proposed fixed span navigational rights for seagoing vessels were being extinguished upriver of the bridge. The enabling legislation included a proviso to construct a transporter bridge downstream. This latter bridge has not been built.
The new bridge, built in the same location as the original Butt Bridge, was designed by Joseph Mallagh, chief engineer to the Port & Docks Board, and Pierce Purcell, Consultant. The contractor was Gray’s Ferro Concrete (Ireland) Ltd. It has a central span of 34m and a single approach 12.2m span each side of the central span. The central span was influenced by the span of the Loopline river piers immediately downstream, as to repeat this span would provide a fixed channel for river traffic. The bridge is 20m wide across the spandrels on the central span and carries a 12m roadway and two 3m footpaths.
Before construction proper commenced, the removal of the original swing bridge, approach spans and piers had to be completed. A temporary bridge for pedestrians was constructed on the piers of the Loopline and a trestle of timber piles in the river. The foundations for the piers and abutments consist of mass concrete bases constructed within a cofferdam of sheet steel piles. The piers and abutments were constructed in two stages i.e. first the north pier and abutment was constructed in one large cofferdam while the south side was built using two cofferdams.
However, the excavation revealed sound calp limestone which provided a good key. The foundations consisted of mass concrete with large stones from the previous bridge being used as displacers. The cantilevers are of cellular reinforced concrete on mass concrete piers, balanced by solid reinforced concrete approach spans and abutments. The central and approach spans are integral with the pier. The parapets over the central span consist of an open balustrade.
After completion of the bridge it was tested with various rollers, steam wagons and lorries to see if it conformed to standards. The total overall cost is recorded as IR£65 500.