Civil Engineering heritage: Scotland - Lowlands and Borders

Date 2007

Event ID 589061

Category Descriptive Accounts

Type Publication Account

Permalink http://canmore.org.uk/event/589061

Civil Engineering heritage: Scotland - Lowlands and Borders

Union Bridge, near Paxton: Although projected after Telford’s Menai Bridge, which was on a much larger scale, Union Bridge erected over the Tweed four miles west of Berwick in 1819–20 by Capt. Samuel Brown RN was completed first. It has an 18 ft wide deck and for five years was the largest wrought-iron span suspension bridge in the world carrying vehicular traffic, which it still does. It is believed to be the oldest suspension bridge still carrying vehicles and is now subject to a two ton weight limit. The bridge was a triumph of the newly-emerging technology made practicable by Cort’s improvements in ironmanufacture. Brown used his patent wrought-iron chains to achieve a clear span of 361 ft (437 ft between supports), a span several times greater than was practicable by means of a stone arch. The main ironwork consists of individual chains formed of 15 ft 2 in. diameter eye-bar links in three pairs, one above the other, at each side of the deck, with a dip of about 26 ft. The bridge has a supporting tower only on the north bank, being anchored into the rock face at its south end.

Brown’s expertise was in chain manufacture rather than bridge design and his original proposals for the 60 ft tall tower and abutments were considerably improved on the advice of John Rennie. Nevertheless, this application of his eye-bar chains, and its development by Telford at Menai Bridge, undoubtedly exercised an important influence in suspension bridge practice. The bridge was erected in the remarkably short time of 12 months and cost approximately £7700 which was compared at the time with the sum of at least £20 000 for a multi-span masonry arch bridge. Knowledge of Brown’s achievement and of other innovative Scottish wrought-iron suspension bridges was widely spread by Robert Stevenson in his authoritative Edinburgh Philosophical Journal article in 1821, notable for its early description of deck undulation. By 1824 translations of Stevenson’s article had appeared in German, French and Polish publications.

In 1903 the bridge was strengthened by the addition above the chains at each side of a single steel cable and

hangers which will only come into play and support the deck if an original chain fails. In 1974 the bridge was reconditioned including the replacement, in spheroidal graphite iron, of defective short interconnecting links between the original main eye-bars which were retained in service. An original worn link has been preserved in the ICE Museum at Heriot-Watt University, Edinburgh. Shirley-Smith states in his book that the bridgewas blown

down six months after its erection but this is untrue and he may have been confusing it with the fate of the first

Dryburgh suspension bridge. Although not as influential as Menai Bridge, Union Bridge nevertheless represents a landmark in the development of long span bridges and its custodians through the years are to be congratulated for its tasteful preservation and maintenance.

R Paxton and J Shipway 2007

Reproduced from 'Civil Engineering Heritage: Scotland - Lowlands and Borders' with kind permission from Thomas Telford Publishers.