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William Edwards Bridge (Pontypridd Old Bridge)
River Taff, Pontypridd, Rhondda Cynon Taff, Wales, UK
William Edwards Bridge (Pontypridd Old Bridge)
associated engineer
William Edwards
date  1755 - 1756
UK era  Georgian  |  category  Bridge  |  reference  ST073904
ICE reference number  HEW 27
photo  Varitek (own work) [CC-BY-SA-3.0], via Wikimedia Commons
The famous old bridge at Pontypridd, north of Cardiff, represents a milestone in the history of masonry arch construction. Its stone arch was the longest then built in Britain and remained so for 40 years. Its distinctive design is still studied today. The bridge no longer carries road traffic but is well used by pedestrians.
The bridge we see was William Edwards' (1719-89) fourth attempt to span the River Taff at this spot. Edwards was a self-taught mason and a non-conformist minister at Groeswen chapel, some 7km east of Pontypridd, from 1752 until his death. His bridges replaced an earlier timber structure.
In 1746, Edwards entered into a contract with the Hundreds of Miskin and Eglwysilan to build a bridge over the river and maintain it for seven years, for the sum of £500. His first bridge, a stone structure with at least three arches, was washed away by a flood after about two and a half years.
For his second bridge, he removed the flood-vulnerable river piers and decided on a single masonry span of 43.9m from bank to bank. When it was almost complete, the centring (formwork) was swept away in a flood and the unsupported arch collapsed.
Undeterred, Edwards rebuilt his single arch with better centring but it too was to fail. With a span of 42.7m and rise of 10.7m, his design had a disproportionate weight ratio between the span’s haunches and its crown. During construction of the spandrel walls, reputedly in 1754, the excessive loading near the abutments forced the crown upwards, displacing the voussoir keystones. It was not a sudden collapse — contemporaneous reports suggest the bridge stood for some six weeks — so Edwards had time to observe the failure mode and learn from it.
He remained convinced of the benefit of a single span over the river, and rebuilt the present arch in 1755-6 to the same overall dimensions (42.7m span and 10.7m rise). Its soffit is an almost perfect circular arc of 27.1m radius. The arch ring is only 760mm deep.
To prevent the bridge’s crown, consisting of little more than the arch ring and parapet, from being forced upwards by the pressure of the massive haunches, the weight of masonry at the ends of the span had to be reduced. Edwards constructed the spandrels with three graduated cylindrical voids in each side, of 1.2m, 1.8m and 2.75m in diameter. He also used charcoal as infill for the spandrels, lightening the load even more. To distribute the weight, it is possible that there was heavier infilling at the crown.
However, the roadway was only 3.35m wide between parapets and the relatively large rise of the arch gave it steep slopes on either side of the crown, making it difficult for heavy vehicles to use. Apparently, a “chain and drag” were used to help descending wagons.
Edwards eventually spent £1,154 on completing his bridges and finished in debt. He is commemorated in the chapel at Groeswen, where a bronze plaque reads, "A builder for both worlds … Adeiladydd I'r Ddeufydd". He is buried in the churchyard at Eglwysilan and, in 1956, a slate plaque was erected in the church porch to mark the bicentennial of Pontypridd Old Bridge.
To ease traffic congestion, in 1857 a new road bridge was constructed adjacent to the south side of the original bridge. Known as the Victoria Bridge, it has three masonry arches and was designed by Thomas Jenkins. Edwards’ bridge is reserved for pedestrian use and its steep roadway now has steps. It has been Grade I listed since 1962 and is a Scheduled Ancient Monument.
Research: ECPK
"William Edwards Bridge, Pontypridd, UK" by T.G. Hughes, in Proceedings of the ICE Bridge Engineering, Vol.158, pp.71-80, London, June 2005
reference sources   CEH W&WBDCE1

William Edwards Bridge (Pontypridd Old Bridge)