DOLDOWLOD FOOT BRIDGE 1867 (also known as the Ystrad Bridge) nr. Rhayader, Wales HEW 1245 SO 003 617 N52o 14' 38.3" W3o 27' 44.6"
PRIVATE LAND This bridge was designed by James Dredge (1794-1863), a brewer in Bath who became a successful self-educated engineer. He developed a very distinctive type of suspension bridge which combined the principles of suspension and cantilever. Each half of the bridge works as a right angle triangle, the hypotenuse of which is formed by the wrought iron rods extending along the deck at oblique angles up to the top of the pylons - . The bottom tips of these two triangles meet at the center of the bridge. If you cut a Dredge bridge in two at the center, both halves would still stand whereas if you did that with a conventional suspension bridge, it would collapse.
The chain of rods that loop from the top of each tower down to touch the outer edges of the deck at the center are simply positioning the rods which are actually supporting the bridge rather than having the chain itself provide its support as in a conventional suspension bridge. "Each chain (in a Dredge bridge) increased from a single wrought iron rod at the centre of the bridge to the requisite number over the … (towers) by progressively adding an extra rod at each link position. This resulted in a significant reduction in the quantity of iron and in the self-weight of the suspension system. … Connected to each joint in the main chains was a pair of inclined bars which gave support to the deck. These 'oblique' rods … were all set at different angles to the horizontal. … This introduced compression into the deck: a fact which Dredge argued in his favour, since the conventional suspension bridge decks of his era were greatly affected by wind-induced oscillatory motion." (Don Mc Quillan "Dredge Suspension Bridges in Northern Ireland", THE STRUCTURAL ENGINEER Vol. 70, No. 7 April 1992)
Of the fifty or so bridges Dredge built in the British Isles, only eight survive today: this one, the Victoria Bridge in Bath, the Stowell Park Bridge in Wiltshire and two in Scotland, at Aberchalder and at Inverness plus three in Northern Ireland. His client in this case was the nephew and heir of the inventor James Watt. This bridge on the River Wye was built to Dredge's patented design by his son William four years after Dredge's death.
The bridge is used daily on a private estate off the A470 in Wales. From two 14 foot cast iron pylons, the 7' 4" long wrought iron rods, five on each side at the pylon, descend to the center in decreasing number. At each rod link, hanger rods run down at varying angles to support the wooden deck which has a span between pylons of 125 feet. In this Dredge bridge, there is a slight but unique variation in his 'taper' principle The iron work was made by the Llanidloes Railway Foundry.