Bridge Notes 1

Paul Duffy

It is not known just how many bridges there are in Ireland. One estimate puts the number at 25,000 masonry-arch bridges alone. During the Second Corrib Drainage Scheme, some 1519 bridges and culverts were reconstructed. Given that the CorGiven that the Corrib catchment has an area of some 1200 sq. miles, an estimate of 1 bridge per square mile would appear reasonable over the entire country. This would put the number of road bridges at 32,000. Railway and pedestrian bridges would increase this number somewhat. By any reckoning, this is a significant contribution to this country's built environment.

Some of these bridges have well-documented histories, others have little or nothing known about their construction. Many of our bridges are simple, unpretentious, utilitarian structures. Some are bold, significant structures standing out in the land or streetscape. What follows is a fraction of our bridge heritage. Two of the bridges have vanished : the Gobbin's and Flannery Bridges. The other, Amiens St Bridge, remains standing.

Amiens Street Railway Bridge, Dublin

This bridge has a 95-foot span over a very busy street. There are two curved-top main girders, each 10 foot 9 inches deep in the centre and 6 foot 7 inches deep at the ends. Plate cross-girders are placed at 4 foot 6 incherders are placed at 4 foot 6 inches, on which is riveted longitudinal plate-iron troughing, extending across the bridge and terminating at the sides with wing plates riveted up to the web-plate of the main girder, forming continuous stiffeners from end to end of the main girders. A light ornamental, close-cast iron parapet is bolted on to the top of the curved, or upper boom of the main girder, the top line of the parapet being carried on parallel to the bottom of the girder.

The bridge crosses the street below very obliquely, and althoughh cast-iron columns were allowed at the edge of the footpaths, the main spans are unavoidably large. When designing the bridge, a girder had to be adapted that would form a screen to provide a deck, or floorway, which would not only be watertight, but also deaden sound or vibration of passing trains as much as possible. At the same time, the girders and parapets were given ornamental appearance. The bridge carries a constant service of heavy trains, it is perfectly dry underneath and is remarkably free from noise and vibration.�

Flannery Bridge, Carna/Kilkieran, Co. Galway.

This single-spaaae-spaBridge, was, at 180 feet, reputed to be the longest in both Ireland and the British Isles when opened in 1954. The bridge and abutments contained 572 cubic yards of concrete. The reinforcement consisted of 17 tonnes of high-tensile steel, 22 tonnes of mild steel and 33 miles of pre-stressing wire formed in 14,742 feet of cables. Eventually, grouting and concrete cover proved no match for high saline exposure. The 1995 bridge inspection programme carried out by the Co. Council indicated that chlorides, from the tidal inlet spanned by the bridge, had worked their way through the concrete cover and attacked both the post-tensioned cables and their steel sheath coverings. As the bridge was considered an extremely important part of Ireland's engineering heritage, considerable time and effort was expended in evaluating possible remedial measures, but to no avail. The inlet is now spanned by the third Flannery Bridge.

The Gobbin's Tubular Bridge, Co. Antrim

The Gobbin's Tubular Bridge must rank as one of the most unusual bridges built in Ireland. It formed part of a series of footbridges along the Gobbin's Path at Islandmagee, Co. Antrim. It was designed by Berkley D. Wise of the Belfast and Northern Counties Railway. Wise, as engineer-in-chief to the railway was also responsible for the re-design of a number of its stations. The bridge was fabricated in the Permanent Way Yard at Belfast and transported as deck cargo on a scow to the Gobbin's where it was hoisted into position.

The bridge consisted of a steel framework 70-foot long, which spanned a chasm 65-foot wide. The framework was composed of longitudinal angles and bars with tubular elliptical stiffeners. All joints in the bridge were butted and had cover plates. The deck was composed of pitch pine planks, 12x3inches, bolted onto floor brackets at each ellipse.

Paul Duffy is a forensic engineer with Galway Co. Council.