Glan yr Afon Viaduct
|Glan yr Afon Viaduct|
|Construction No.||UB 95|
|Operational No.||UB 37.17|
|Stations | Locations | Bridges | Tunnels | Map|
Glan yr Afon Viaduct (sometimes known as Glan yr Afon Bridge or Glanrafon Bridge) carries the Welsh Highland Railway over the deep valley of the Afon Treweunydd ( English: river of the town on the moors) as it runs into Llyn Cwellyn.
Erected as part of the extension of the North Wales Narrow Gauge Railways line to Rhyd Ddu, it was probably built by the Butterley Company (ex inf Jim Wood, a distinguished BR bridge engineer) about 1880 and delivered by rail. The extension which opened in 1881. It may well have been financed by the Glan yr Afon Slate Company who depended on it for economical export of their produce.
The bridge has beams 8ft deep and nearly 100ft long, with a span of 94ft. The height of the bridge over the Afon Treweunydd is 45ft. It is uncertain whether it is of iron or of an early steel, but in either case it shows little sign of rust and little sign of lamination. We allowed in the calculations for it to have been of iron, which is about two-thirds the strength of steel. It is thought that the beams would have been delivered in sections 50ft long, the first such beam being pushed out over the gap and the second rivetted to it by a field splice, then the complete girder slung out to the opposite abutment by sheer legs. The process would be repeated for the second beam. There are triangulated struts, many renewed by the excellent Brunswick Foundry of Caernarfon about 2002. As the original beam was intended for an axle load of 4.5 tons, extra vertical stiffeners were added in 2002 and at a deflection test, with two 62-ton Garratts loaded onto it, it deflected only 10mm. The Butterley Company was pleased.
The fixed ends of the beams are at the north ends; the south ends slide on iron bearings. While the line was shut, from 1937 to 2000, the bearing of the east beam seized and pulled the bearing plate three or four feet out of the south abutment - a more convincing demonstration of the coefficient of linear expansion of steel has never been seen. The beam was lifted, the bearing plate replaced (and greased!), the abutment stitched and all is well. The bridge is an extraordinary survival of Victorian engineering at its best.
Technically the bridge is erroneously described as a viaduct. A viaduct has two or more spans of equal size; Glanrafon Bridge has but one span of 94ft and a subway for a footpath through the south abutment. Although it is rarely seen by members of the public, it is one of the principal structures - dare one say glories? - of the entire line.
It would be pleasant if we could presuade the adjacent landowner to allow us to cut one or two trees on the east side to allow passengers a view of the splendid waterfalls.