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Photo, Sir Alexander Gibb and Partners
ONE END OF THE SWING SPAN of Kincardine Bridge contains the apparatus for securing the bridge in a closed position in perfect alignment. Just below the level of the roadway a box, 5 feet long, contains photoelectric cells. The bolt in the centre and the wedges at the sides can be driven only when the photoelectric apparatus is in alinement (sic) with its counterpart on the end pier of the fixed part of the bridge.
In 1890 borings were made in the river bed at Kincardine during the survey for a railway bridge proposed by the former Caledonian Railway (now L.M.S.). This bridge was not built, but the borings later proved of value to the engineers of Kincardine Bridge. Solid rock was available for foundations on the north side of the river, but the borings indicated a fault in midstream, with rock at too great a depth to be of any value to the builders. It was found that below the layer of mud on the southern bank there existed a belt of firm gravel at a depth of about 50 feet. The whole of the southern section of the bridge thus rests on reinforced concrete piles driven deep into the gravel.
The setting out of the site was accomplished from two positions, 2,000 feet apart, on the centre line of the bridge. The distance was determined by triangulation from a base line established on the L.N.E.R. embankment on the north bank of the river. This setting out was necessary to permit simultaneous building from either end. When, later, the staging had been completed on both sides, up to the central gap for the swing span, the opening was found to be one inch too wide. This small error was easily rectified on the midstream end piers.
With the position of Kincardine Bridge duly determined, the first task was the building of a temporary structure from which the main work could be carried out without delay. The temporary bridge had to be built substantially to serve for a period of three years, during which time its two sections, each a quarter of a mile long, had to carry a double line of standard-gauge railway track for the safe conduct of gantries, wagons and cranes across the fast-flowing Forth.
The bridge was built out from either bank on timber piles, a foot square in section, driven deep down to the solid rock. The pile heads were joined together by massive timbers and heavy steel joists carried the track. The north bank bridge staging carried two lines of railway track. One track ran along the centre line of the main bridge site, the other being arranged parallel to it, on the downstream side. This arrangement was adopted for the building of the north piers, as most of them were built with the help of caissons. Excavated material was loaded into wagons on the centre line track and the cranes ran along the outer or downstream set of metals.
For the southern piers of the main bridge, built entirely all piles, other methods were used. The two tracks surmounting the temporary staging were both situated outside the bridge site at a distance of 62 feet between centres. Each track carried the ends of a travelling gantry on which was...
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