Sunday, August 10, 2008
Menai Suspension Bridge
The many achievements of the Scots engineer Thomas Telford (1757–1834) include bridges over the River Severn at Montford, Buildwas, and Bewdley, all built in the 1780s. In the following decade, as engineer for the Ellesmere Canal Company, he designed and constructed aqueducts over the Ceiriog and Dee Valleys in North Wales. Temporarily returning to Scotland, with William Jessop he built the Caledonian Canal, more than 900 miles (1,440 kilometers) of highland roads, and harbor works at Dundee, Aberdeen, and elsewhere. From 1810 he was engaged as principal engineer—William Alexander Provis was the resident engineer—to construct a highway between the Shropshire county town of Shrewsbury and Holyhead in northwest Wales. It is widely agreed that his masterpiece is the Menai Suspension Bridge (1819–1826), which carries that highway across the Menai Strait, linking Bangor in mainland Wales with the island of Anglesey. It was the first large-scale chain-link suspension bridge and at that time the longest span bridge ever erected.
In 1782 a meeting on Anglesey examined complaints concerning the operation of the ferries at Porthaethwy, Llanfaes, Llanidan, and Abermenai that for centuries had been the only means of crossing the Menai Strait to the Welsh mainland. Increasing traffic across had led to delays and overcharging, and many of the boats were neglected and in dangerously poor condition. Alternatives to the ferries were canvassed, including an embankment and stone or timber bridges. With 4,000 vessels passing through the strait each year, those proposals were met with reasonable objections, and nothing was done. In October 1785 the Irish Mail Coach service was inaugurated between London and Holyhead on Anglesey, where travelers took ship for Ireland. The situation was further exacerbated in 1801, when the Act of Union demanded that Irish members of Parliament travel between Dublin and London, partly via the primitive Holyhead-Shrewsbury road and of course the ferry. Nevertheless, it was not until 1810 that Parliament commissioned Telford to recommend the line for a link across North Wales and Anglesey, including a bridge across the Menai Strait.
Attempts to improve only parts of the existing road were disastrous, so in 1816 Telford was appointed its resident engineer. His 69-mile (110-kilometer) stretch of the 93-mile (150-kilometer) toll highway (now the A5 national road) was probably the best road in Britain. It was up to 40 feet (12 meters) wide, with easy gradients and excellent bridges; moreover, its well-designed construction meant that it could accommodate heavy wagons.
Telford offered three alternative designs for the Menai Strait bridge, and that for a suspension structure was accepted. Finally, after forty years of debate and quibbling, the first stone was laid on 10 August 1819, and in the face of opposition from ferry proprietors and businesspeople in the ferry ports, construction work commenced. Including the approaches the bridge is 1,500 feet (459 meters) long. The approaches, completed in the fall of 1824, were carried on seven stone piers—three on the mainland side and four on the Anglesey side—supporting arches. The 579-foot (177-meter) main span, with its 24-foot (7.4-meter) dual carriageway, was suspended 100 feet (30 meters) above the water by sixteen chain cables hung from 153-foot-high (47-meter) massive battered towers—they were called “pyramids”—at each end, built of limestone from Penmon Quarries at the north end of the strait. Telford designed the piers to stand above the low-water mark, to facilitate inspection of the masonry.
The suspension chains were fabricated in wrought iron from Hazeldean’s foundry near Shrewsbury. Each consisted of 935, 9-foot-long (2.75-meter) eyebar links, about 3.5 inches (83 millimeters) square in cross section, pinned together. To prevent rusting between fabrication and placement, they were immersed in warm linseed oil. Tunnels were excavated in rock to provide anchorage, and the first section of chain was secured at the mainland end, draped over the top of the eastern pyramid and left hanging to water level. The procedure was repeated on the Anglesey side. The central section, weighing nearly 28 tons (25.4 tonnes), was maneuvered into position between the towers on a barge and connected to the end sections before being raised to the top of the tower by block and tackle and the strength of 150 men, thus completing the span. The chains were all placed in ten weeks, by July 1825. Iron rods suspended from them were bolted to iron joists that carried a timber deck. The Menai Strait bridge was opened to the public on 30 January 1826. Its completion and Telford’s Shrewsbury-Holyhead road reduced the travel time between London and the Irish Sea port by a quarter.
Without stiffening lateral trusses, Telford’s bridge soon proved unstable in the winds that swept through the strait, causing the road deck to oscillate. In 1826 a gale caused 16-foot (4.9-meter) deformations in the deck before it failed; although severely damaged, the bridge survived and was strengthened. A more rigid timber deck was incorporated in 1840 and that was replaced by a steel structure in 1893. Further changes were made in a major renovation of 1938–1941, ostensibly to cater for modern automobile traffic (the previous load limit per vehicle was 5 tons [4.6 tonnes], although it might also have been defense related). The arched openings in the towers were widened to allow easier passage of larger vehicles, the carriageway was strengthened, and the chains were replaced with steel cables and realigned. The bridge remains in use.