Throsk Viaduct
Also referred to as Alloa Bridge, Alloa Swing Bridge, Forth Bridge, Alloa.
See Canmore entry and Railscot entry for information, photographs and maps.
This was an impressive viaduct, 1600 ft long, with multiple wrought iron bowstring spans, now reduced to an impressive array of sandstone pylons with their iron and steel bracing, the girders having been removed in the early 1970s. There was a swing bridge section part way across, towards the Alloa end.
1882 Laying of the memorial stone: '.... The swing pier will consist of a group of six cast-iron columns or cylinders 5 feet in diameter, placed hexagonally. ... The engineers for the railway and bridge are Messrs Crouch & Hogg, C.E., Glasgow, who are represented on the works by Mr Joseph Wilson, resident engineer, and Mr R. B. M'Kay, the company's inspector of ironwork. Messrs Watt & Wilson, of Glasgow, are the contractors for the whole, and they have entrusted the ironwork to the well-known firm of Messrs P. & W. MacLellan, ...'[1]
1885 'ALLOA RAILWAY AND BRIDGE. The opening of the Alloa Railway, which takes place on the lst October, is a suitable occasion for giving some account of the line, and the bridge across the River Forth, which is the most important work connected with it. .... The bridge over the River Forth, which is a work of considerable magnitude, is a wrought iron structure, 1615 feet in length, divided into 21 spans arranged as follows — Commencing from the south side of the river there are 13 spans of 68 feet, one span of 80 feet, and one span of 100 feet. There are then two opening of 60 feet in the clear, another one of 100 feet, and one of 80 feet, then two more spans of 68 feet up to the north bank. .... The superstructure consists of wrought-iron lattice girders, ... As before mentioned, there are two opening spans for the passage of vessels, and these are provided by a swing span, which swings round its centre, and so opens both passages when required. The central pier for carrying this swing span consists of a cluster of six cast iron cylinders braced together and carrying a circular wrought iron box girder upon which is placed a cast roller path, on which rollers run when the swing span is moved. There is also an upper roller path resting on these rollers, and by suitable arrangements the girders of the swing span, which are 150 feet in length, rest on these rollers. On the top of the swing girders a cabin is erected in which the steam engine for opening and closing the bridge is placed, and from this suitable shafting and gearing is connected with the opening machinery. ..... The iron-work was executed by Messrs P. & W. Maclellan, and the engines have been supplied by Messrs A. & P. Steven. [2]
1885 'THE ALLOA FORTH AND RAILWAY. OFFICIAL INSPECTION. ... There are two opening spans, swinging on a massive central pier, composed of six cylindrical columns. This remarkable gateway is in the fairway of the channel of the river, only navigable by anything but the smallest craft at high water mark as the spans are only 24 feet clear. The “swing span" is rather a ponderous piece of mechanism, and is wrought by a pair of inverted engines, situated in a neat cabin on top of span — the engine, boiler, and cabin have been constructed and fitted by Messrs A. & P. Steven, Provinside Engine Works, Glasgow. The engines are capable of working from 15 to 20 H.P., and are connected with gearing adapted to swing the span, lock the same, and swing it into position. They can be wrought to a pressure about 60 lbs., the water for the boiler being raised by means of a steam lifter from a tank-waggon on the rails. The span, we learn, can be wrought by hand levers should this be found necessary. Two spans of 60 feet each are opened free by the use the swing bridge, and on either side are first the 100 feet and next the 80 feet spans, there being only these two on the north and the south side of the swing bridge. The piers are surmounted by cast iron caps, and the bridge protected and strengthened by strong malleable iron girders proportioned to the length the spans, thus the swing bridge girder is 12 feet 6 inches, the 100 feet spans, 9 feet; 80 feet spans 8 feet, and the 68 feet spans 7 feet in height. Large rubble stones are deposited in the river round the cylinders of the swivel pier. The utmost precautions were taken m the specifications to ensure the best material for the stability of the bridge. The main, cross, and over girders are of wrought iron, with flooring buckle plates inverted. Each of the girders before being put up were tested with a pressure reaching 12 cwt. each foot of span, the weight being as nearly as possible at the centre of the girders. None of them, during the first test, indicated a deflection of 1-80th of inch over each foot of span, exceeding which the girders were to be rejected. All are malleable iron, and were constructed to bear a tensible strain of tons per square inch before fracture and 11 tons per square inch when in permanent way. The signals and signal cabin arrangements seem very complete — the two cabins "Forth Bridge" and “Throsk," being fitted with “Tyler's Patent Tram Tablet apparatus for single lines", a contrivance which supersedes the staff system. By this arrangement the line can effectively blocked to prevent the possibility of collision on a single line of rails, and to insure that only one train will be on the bridge at once. Electric comnunication, as a matter course, exists between the two cabins, and the wire communications by an ingenious contrivance can be entirely cut off at the swing spun, as said span opens. As it adjusts itself again, the electric appliances also become united, and can be resumed at once.
'Messrs Crouch & Hogg, C.E., Glasgow, are the engineers of the railway and bridge, the principal contractors, as already stated, being Messrs Watt & Wilson, Glasgow, Mr John Band, manager. Messrs P. & W. MacLellan, engineers, Glasgow, were the sub-contractors for the iron work -the manager for the firm being Mr James E. Wilson. Messrs Kinnear & Moodie, Leith, erected the signal cabins. Engine, boiler, and cabin for swing bridge, Messrs A. & P. Steven, Provinside Engine Works, Glasgow. Signals — Messrs Steven & Son, Glasgow. Telegraphic arrangements carried out under the superintendence Mr A. S. Gunn, of the Caledonian Railway telegraphic department — the manager of the work being Mr Bowman. The inspection .....'[3]
Some clarification: The reports refer to a pair of inverted engines. 'Pair' in this context meant a pair of cylinders on a single bedplate and with a single crankshaft. Inverted meant that the cylinders were at the top (the earliest steam engines had the cylinder at the bottom, working an overhead beam). In passing, mention will be made of the engine working the Kingston Dock Swing Bridge, which was a horizontal engine which was literally inverted (compared with normal practice) and bolted to the underside of the bridge girders! Regarding the number of bridge spans, the newspaper reports represent the swinging portion as two spans (single girder, three piers).
The steam engine from the swing bridge is on display at the Scottish Maritime Museum, Irvine.