DEMM Engineering & Manufacturing - - NEWS -

The 18-arched stone and cast iron Pont­cy­syllte Aqueduct took ten years to build and was fi­nally fin­ished in 1805. The UNESCO World Her­itage Site car­ries the Llan­gollen Canal across the River Dee in Wales. It is the old­est and long­est nav­i­ga­ble aqueduct in Bri­tain and the high­est in the world. It was built by Thomas Telford and Wil­liam Jes­sop, al­though Jes­sop was mostly in a su­per­vi­sory role leav­ing the de­tails in Telford’s hands. Be­ing a pi­o­neer in the use of cast iron for large scaled struc­tures, Telford had to in­vent new tech­niques, such as us­ing boil­ing sugar and lead as a sealant on the iron con­nec­tions.

The bridge is 307m long, 3.7m wide and 1.60m deep. It com­prises a cast iron trough sup­ported 38m above the river on iron arched ribs car­ried on eigh­teen hol­low ma­sonry piers ( pil­lars). Each of the 18 spans is 16m wide.

The mor­tar used lime, wa­ter and ox blood. The trough was made from flanged plates of cast iron, bolted to­gether, with the joints bed­ded with Welsh flan­nel and a mix­ture of white lead and iron par­ti­cles from bor­ing waste. Af­ter twenty-five years the white lead was re­placed with or­di­nary tar. The plates are not rec­tan­gu­lar but shaped as vous­soirs, sim­i­lar to those of a stone arch. There is no struc­tural sig­nif­i­cance to their shape: it is a dec­o­ra­tive fea­ture only, fol­low­ing the lines of the stiff­en­ing plates in the cast­ings be­neath. The supporting arches, four for each span, are in the form of cast iron ribs, each cast as three vous­soirs with ex­ter­nal arches cast with an un­pierced web to give greater strength, at the cost of ex­tra weight. Us­ing cast iron in this way, in the same man­ner as the stone arch it su­per­sedes, makes use of the ma­te­rial’s strength in com­pres­sion. They also give an im­pres­sion of greater so­lid­ity than would be the case were the webs pierced. This im­pres­sion is en­hanced by the ar­range­ment of strips of thicker stiff­en­ing in­cor­po­rated into the cast­ings, ar­ranged in the man­ner of joints be­tween vous­soirs. Cast plates are laid trans­versely to form the bed of the canal trough. The trough is not fas­tened to the arches, but lugs are cast into the plates to fit over the rib arches to pre­vent move­ment. The aqueduct was left for six months with wa­ter inside to check that it was wa­ter­tight. A fea­ture of a canal aqueduct, in con­trast with a road or rail­way viaduct, is that the ver­ti­cal load­ing stresses are vir­tu­ally con­stant. Ac­cord­ing to Archimedes’ prin­ci­ple, the mass (weight) of a boat and its cargo on the bridge pushes an equal mass of wa­ter off the bridge.

The tow­path is mounted above the wa­ter, with the in­ner edge car­ried on cast- iron pil­lars in the trough. This ar­range­ment al­lows the wa­ter dis­placed by the pas­sage of a nar­row­boat to flow eas­ily un­der the tow­path and around the boat, en­abling rel­a­tively free pas­sage. Pedes­tri­ans, and the horses once used for tow­ing, are pro­tected from fall­ing from the aqueduct by rail­ings on the out­side edge of the tow­path, but the holes in the top flange of the other side of the trough, ca­pa­ble of mount­ing rail­ings, were never used. The trough sides rise only about 15 cm above the wa­ter level, less than the depth of free­board of an empty nar­row boat, so the helms­man of the boat has no vis­ual pro­tec­tion from the im­pres­sion of be­ing at the edge of an abyss. Ev­ery five years the ends of the aqueduct are closed and a plug in one of the high­est spans is opened to drain the canal wa­ter into the River Dee be­low, to al­low in­spec­tion and main­te­nance of the trough.

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