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]
The steam engine from the swing bridge is on display at the Scottish Maritime Museum, Irvine.