Mary McAleese Bridge, M1, Drogheda
The River Boyne is approximately 150m wide and consists of two channels separated by Yellow Island. The river is tidal at the crossing. Due to environmental constraints the new bridge was not allowed to have any supports within either river channel.
The motorway crossing point was about 20m above the normal water level of the river and the distance between the points where it departs from ground level on either side of the valley is approximately 400m.
The bridge was designed to be cable stayed to span the river. The deck spans, centre to centre of bearings, are as follows:
- Back span 42.5m Main (cable stayed) span 170.0m
- Side Span 1 45.0m
- Side Span 2 40.0m
- Side Span 3 30.0m
- Side Span 4 25.0m
The pylon is supported on two bases with 16 piles each 1.5m diameter into rock and is an inverted ‘Y’ in elevation, extending some 93m above the top of the pile cap and is constructed in reinforced concrete. The legs and the pylon head are hollow being generally rectangular in section and tapering in both front and side elevations. The pylon legs vary in section from 5.0m x 4.8m at the base to 4.2m x 3.865m at the underside of the pylon head.
Full access for both personnel and equipment is provided to the pylon head through the hollows in the pylon legs and head.
The “legs” were constructed in 6m tall pieces using “climbing” formwork platforms fixed to already completed heights of the pylon.
The steel beam deck was built above the river valley and omnia planks placed for permanent falsework. The completed deck framework was then craned into position and “launched” using sliding plates, and jacks and multi-axle SPT’s. As it was launched, cable strands from the pylon, both temporary and permanent, were connected to the deck in a planned sequence.
The cable stays were enclosed in HDPE wrapping. Each stay consists of between 32 and 68 no. 15.7mm diameter galvanised high-tensile 7-wire strands. The outer sheathing varies between 180 and 250 mm diameter.
- Structural Concrete, 19,000 cubic meters
- Reinforcing bar, 2,500 tonnes
- Structural Steel, 2,200 tonnes
- 15.7mm diameter strand in stays, 333 tonnes, 283,000m total length