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Irish Railway Bridges-Part 2

NIALL V. TORPEY C.Eng, F.I.Struct.E, F.IEI

TIMBER BRIDGES

Timber as a structural material has been used as a bridging material since the dawn of history. Timber for bridges was used by the Romans, for example that over the Rhine at Mainz about 90AD; a millennium and a half later; timber was used to bridge the river Aar at Berne (1535).  Two centuries later timber was again used to bridge the Rhine at Schaffhausen (1758). These examples are merely indicative of the widespread use made in Europe of timber for bridging purposes. In Ireland about a generation after the Schaffhausen bridge, timber was used for road bridges in the 1790s at Wexford, Waterford , Mountgarret, Youghal and at Derry , built by an American engineer, Lemuel Cox, another man whose story of his Irish activities needs to be told. Timber was used by the early Irish railway builders for both under and over bridges, and some of these were quite substantial in scale. The largest were those built over the river Nore at Thomastown Co. Kilkenny, and over the river Bandon at Innoshannon, Co. Cork11. The Dublin & Drogheda Railway built timber bridges at Malahide and at Rogerstown in Co. Dublin and the original GS&WR bridge over the Liffey near Sallins Co. Kildare was also a built in timber. All of these bridges were in service for a few decades, before the inevitable biodegradation occurred and replacement became necessary.

 

MALAHIDE

The railway bridge at the Broadmeadow, Malahide, Co. Dublin is a section of the long embankment (ca 6,000 ft or 1,800 m) carrying the Dublin to Belfast line over this tidal inlet. The bridge (ca 600 ft or 180m) is the third main reconstruction since it was originally built by the D&DR in 1844 with the numerous spans and supports in timber. William Anderson described the difficulty of both scour at the foundations and decay in the superstructure experienced with this first bridge in a paper12 as follows:

The obstruction to the free flow of the tide, presented by the embankment, caused a very powerful current to rush through the bridge, and it was very soon found that the soil into which the piles were driven was rapidly washing away, and the bridge settled down in consequence. To arrest this evil large quantities of stones were, from time to time, shot  into  the  stream  till  a  bank  was formed, averaging 130 feet broad, by 30 feet high, extending, in continuation of the embankment, right across the tideway. By this means, and by constantly packing up the rails, in some places to nearly three feet, the viaduct was made to do its duty till the year 1859, when, serious signs of decay becoming apparent. The Directors ordered their engineer, Mr. Marcus Harty, to prepare plans for the reconstruction of the bridge.

The bridge was replaced in 1860 by Courtney, Stephens of Dublin with wrought iron spans on masonry piers and the work was carried out under Harty's personal supervision. The superstructure of this bridge served its function until it required strengthening to carry the new GNR(I) 4-4-0 compound locomotives proposed for the Dublin-Belfast route and which entered service in 1932. The masonry piers of the bridge continued to require attention throughout much of its service life; the need to provide for the effects of storm damage by further dumping of stone ballast for many years (up to the 1960s) is recorded by Holloway and Waters13. A successful programme of grouting of the piers was carried out in the 1960s.  Finally, following the observed deterioration in the wrought iron in this marine environment it was further replaced in the period 1966-1968 with the current prestressed concrete structure and in line with recent practice, the tracks were laid on ballast.

 

THOMASTOWN

The present railway bridge at Thomastown, Co Kilkenny is the second main span carrying the Kilkenny to Waterford line over the River Nore. The complete bridge was originally constructed in the period June 1846 to May 1850, and its construction is described in a paper14 by the engineer to the W&CIR Captain William Scarth Moorsom; the contractors for the masonry construction was Hammond and Murray, Dublin and various others and the designer and contractor for the timber span was Robert Mallet of Dublin whose impressive engineering and other technical work is described in Robert Mallet 1810-187115.

The bridge was built sufficiently wide to carry a double line of rails, though only one was ever laid.

The main span at Thomastown of 200 ft was, in 1850, the longest timber span in Britain or Ireland . The construction of the timber girders was described at the time as follows…

The timber for the arch, or beam, was specified to be of the best quality of Memel and Archangel fir. Some American pitch pine was introduced, (by the permission of the Engineer,) in the walings, but experience has shown, that it should be avoided in future. Every part of the timber, within view, was painted with four coats in white lead and oil, and was also specified to be subjected to Sir W. Burnett's process of saturating with chloride of zinc, but this part of the stipulation was subsequently modified.

As soon as the masonry had been carried up to the courses necessary for receiving the iron and timber bands which steady the abutting pieces of the lattice beam, these bands were inserted in the masonry, and the construction of the beam itself was shortly afterwards commenced, on platforms placed on the shore of the river and covered with the flooring boards, which were afterwards taken up and used for the flooring of the bridge.

The bridge unfortunately exhibited some lively tendencies from the time of its construction and testing which were not finally overcome until its rebuilding nearly a generation later. The timber span appears to have been quite complex, consisting of a double web of lattices, forming six systems of triangulation, with five horizontal chords ... The rails were carried on a complicated system of trusses, resting on the four lower strings... The depth of the main girders was about 20 feet at the centre and 21 feet at the ends, and a camber of 20 (variously 12) inches at the centre. On 1 May 1850 , after removing the wedges a deflection of 3 inches was observed. In the next few weeks after the passage of some trains, a further deflection of almost 2 inches was measured. When tested on the 12 May 1850 , under a test train of 146 tons there was a deflection of 2½”. In the following year or so, the total set of over 7” was measured.

After some thought in the subsequent years there followed in late 1858/early 1859 a timber pier … erected in the centre... of the river giving the bridge some needed if inelegant support. At the end of 1867 decay in parts of the bridge were noted by the then Engineer of the line, Charles B. Galwey, and also in the centre pier, and some repairs were carried out. In 1872 serious structural distress becoming apparent, it was decided as an interim measure to provide additional piers close to the masonry abutments. This was not the end of the engineer's problems, for he reported that after severe gales it was always found that the rails had gone more or less out of line,... the total deviation from the straight at the centre of both top chords was fully 18 inches.

In 1876, the contract for the replacement of the timber construction was given to Courtney, Stephens & Bailey of Dublin, and between Autumn 1876-August 1877 the present wrought iron bridge to carry a single line of rails was built to the design of Charles Galwey16. In the course of its construction, on or about 30 January 1877, a violent south-westerly gale caused so much of the new bridge as had been erected… to be… blown down... As this ... blocked the railway, steps were at once taken to ...clear a foot passage across the bridge... By this means the passenger traffic was carried on... the passengers walking across.(Health & Safety Authority of today please note!)

On Friday 3 August 1877 , the bridge was tested with five engines and tenders covering the whole span and weighing in all nearly 200 tons, and under this load, the deflection at the centre was ¾ inch. This is the bridge, which is still in service today.

 

STONE & BRICK MASONRY

Stone as a bridging material has been used since the dawn of history. It was later found that the arching effect of masonry in compression between firm abutments could be used to increase the span of stone materials. The Romans brought this to impressive use in the numerous bridges and aqueducts, which still exist today throughout the former Roman Empire . In Ireland stone masonry was used in the construction of bridges for many centuries17 and from the earliest days of railways the inherent strength and durability of the material was familiar to the engineers and contractors many of whom had spent much time on the construction of road and canal bridges.

MALLOW

The railway bridge at Mallow, Co. Cork carried the GS&WR main line over the River Blackwater. The bridge was originally constructed in 1848-49 with ten masonry arches each of 60 ft span and came into service in October 1849. The engineer was Sir John Macneill and the Contractor was William Dargan. Given its innate durability, there is little to record by way history in the more than 72 years of service. Unfortunately, on 9 August 1922 during the Civil War, that followed the Anglo-Irish Treaty of December 1921, explosives and the remaining arches afterwards collapsed destroyed the most southerly arches. The effect of this, in terms of railway operation, was the opening of Mallow South as a new temporary railway station (this allowed the renewal of rail services towards Cork and Kerry), and  the  reconstruction of the

bridge was put in hand. The designers for the replacement bridge were J. F. Crowley & Partners of Dublin and London and a steel superstructure was decided upon. The contractors were Armstrong Construction Co. Ltd, London . In view of the national importance of this railway at the time, speed was of the essence and the reconstructed bridge was opened for service by the President of the Executive Council, W. T. Cosgrave on 15 October 1923 , and Mallow South station was closed. The viaduct was supported on slender steel trestles, and following the detection of fatigue cracks at the bearings, these were modified and some of the trestles encased in concrete in the 1950s. In line with current practice, ballasted track has, in recent years, replaced the track fixed to the bridge structure, which was the construction used in 1923.

 

CRAIGMORE

The railway viaduct at Craigmore, Co. Armagh carries the Dublin-Belfast railway over a stream and valley on some 18 arches of 60 ft span. The tallest piers of the bridge are over 100 ft in height. The viaduct is easily the most impressive of the numerous examples of stone masonry arch construction used in the early years of railway building. It was built of granite stone blocks (many of which are quite enormous) from the nearby Goraghwood quarry, which for many years supplied ballast to the Dublin & Belfast Junction Railway’s lines. Craigmore viaduct was built in the period 1851-52 by the D&BJR which company was at the same time building the Boyne viaduct, and is now over 150 years in service.

CONCRETE ARCHES

The use of plain unreinforced concrete for arch bridge construction (faced in brick or stone) may be regarded as a subset of masonry arches. Its use goes back to at least the building of the bridge over the river Erne at Belturbet on the Cavan & Leitrim Railway.

 

STEEL BRIDGES

Steel as a structural material emerged from the ferrous metal industry as a direct continuation to the production of wrought iron, which it superseded in production terms in about 188518. It had improved structural properties from wrought iron, if not the same resistance to corrosion. The first major railway bridge built using the new material was the Forth    Bridge     in     Scotland ,     which    was Dublin , the City of Dublin Junction Railway completed the railway connection from Westland Row (Pearse) station to Amiens Street (Connolly) station, which allowed through running of trains to all main Irish destinations from Dun Laoghaire . This relatively short line is on masonry and steel construction with the track level at about 25 feet above the streets. The longest of its bridges is the Liffey Viaduct. Substantial renewals to this construction have been necessary and in the period 1958-60 the cross-girders and flooring were renewed. While probably the most used bridge on the Irish railway network, certainly in terms of passenger usage, this bridge has always suffered from a ‘bad press’, because of its hiding of the view of the Custom House from the west which was a fine view indeed.

The longest steel railway bridge built in Ireland was that over the Barrow at Great Island . This was built by William Arroll & Co. for the Fishguard & Rosslare Railways & Harbours Co. in the construction in 1902/6 of the line from Rosslare to Waterford and the development of the line from Waterford to Mallow, which give direct access from Britain to the South and South-west of Ireland. The Barrow Bridge ’s opening span allows shipping to access the port of New Ross , and although the line lost its connection from Waterford to Mallow 1967, its other part, from Waterford to Rosslare is still used for some freight movements, particularly beet, and a limited passenger traffic.

The construction of Cork City railways in 1911 allowed the connection of the GS&WR with CB&SCR at Albert Quay and, indirectly, with the C&MDR. The Cork City railway’s track was laid on the city streets and crossed both channels of the river Lee on bridges which had lifting spans of the Scherzer type. With the closure of the west Cork railways in 1961, these bridges became little used by rail except for freight movements to Albert Quay, and when these stopped the ownership of the bridges passed to Cork Corporation who converted them to fixed spans.

Riveted construction was used up to about the late 1950s, after which welded fabrication was used. High strength friction grip (HSFG) bolts were introduced in about 1956

The remainder of this article appears in IRRS Journal number 154, published June 2004.

Copyright © 2004 by Irish Railway Record Society Limited
Revised: November 06, 2015 .

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