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Menai Suspension Bridge
Menai Strait, Gwynedd, north Wales, UK
Menai Suspension Bridge
associated engineer
Thomas Telford
Sir Benjamin Baker
Guy Anson Maunsell
Sir Alexander Gibb & Partners
date  1818 - 30th January 1826, 1839, 1893, 1938 - August 1941
era  Georgian  |  category  Bridge  |  reference  SH555714
ICE reference number  HEW 109
photo  engineering timelines collection
Regarded as Thomas Telford's finest work, the Menai Bridge takes the A5 trunk road across the treacherous Menai Strait. It was built to cope with the increased traffic to Holyhead for ferries to Ireland, and once had the longest span in the world. It remains the only direct road connection between mainland Wales and the island of Anglesey.
Until this bridge was built, travellers depended upon ferries battling the dangerous currents and high winds of the ‘Swellies’ (a section of the strait). The crossing was part of the link between Ireland and Parliament in London. Once Telford’s bridge was complete the journey time from Holyhead to London reduced by nine hours, to a mere 27 hours.
Telford was not the first engineer to design a bridge to cross tthis strait. In 1801 John Rennie Snr was asked for his ideas but his design was thought too expensive — at £259,140 — by a government engaged in war with Napoleon. Telford’s 1810 estimate of £127,331 was much more acceptable, though the final cost of the suspension bridge was closer to Rennie’s tender at £231,500.
The enabling Act was passed in 1815, with a second in 1819 that stipulated completion by July 1823. A third Act extended this to July 1825, though the bridge was not quite finished when it opened to the public on 30th January 1826. Some preliminary works took place in 1818 but resident engineer William Alexander Provis laid the first stone on 10th August 1819.
The bridge is 304.8m long and 45.7m high overall, and carries the road 30.5m above sea level, allowing tall sailing ships to pass beneath.
The suspended central span of 176.5m is flanked by stone approach spans of 15.8m, three on the Bangor side and four at the Menai Bridge (Porthaethwy) end. The arches and piers are constructed in limestone from Penmon Quarries, and the two masonry towers supporting the chains are faced with Anglesey marble. Stonework began in 1820 and was completed in 1824. Stone was delivered to site by boat.
The bridge deck has two carriageways that pass through round-topped arches in the towers, with a pavement between. Both carriageways were suspended from four tiers of wrought iron chains on either side, sixteen in total, each weighing 23.4 tonnes and having a dip of 13.1m. Chains comprised 935 eyebar links, 2.4m long, connected five abreast by fishplates and bolts. Wrought iron suspension rods 25mm square — 796 in all — hung from the fishplates some 1.5m apart and were bolted to iron bars that supported the timber roadway.
The four tiers of chains were supported on cast iron saddles that rested on roller bearings — four saddles atop each tapering tower. The chains were hauled into place via pulleys and teams of 150 labourers, working between tides to the beat of a fife band so they all pulled as one. The first chain link from one side to the other was completed on 26th April 1825, and all sixteen were fixed by 9th June. The pulley system was later reused for installing the chains on the Conwy Suspension Bridge (1822-6).
Secure anchorage of the chains is vital to the integrity of a suspension bridge. Telford decided to anchor them inside tunnels driven 18m into the bedrock on either side — three tunnels each end, two for the outer tiers and one for both inner tiers. The chains were fixed into iron clamps held with 3m long bolts.
A week after the bridge opened to traffic, the timber deck was damaged in a gale and some of the cross beams and suspender rods were broken. To prevent future storm damage the tiers of chains were tied together in places by cross rods and the deck was stiffened with oak cross-beams.
On 23rd January 1836, another gale snapped six of the suspender rods and caused further damage to the deck's beams. The bridge-keeper estimated that the deck oscillated vertically by almost 5m. During construction Provis had produced a detailed record of the work, and he now recommended that the deck needed proper strengthening but no action was taken. Consequently the deck was destroyed during a storm on 6th January 1839. Some of the timber fell into the strait and some dangled from the two central chains. Most of the suspender rods on the outer chains snapped.
This time Provis' advice was heeded and he designed a more robust timber deck that weighed 132 tonnes more than the first. The ends of this deck, as in the original, were floating rather than fixed and the dead load of the whole suspended span was 633 tonnes.
In the 1840s, the Chester & Holyhead Railway looked at conveying trains over one of the carriageways using horse traction rather than locomotives, but this plan was abandoned and the railway used Robert Stephenson's purpose-built Britannia Bridge (1850) instead.
In 1893 the timber deck was replaced with steel troughing under the supervision of Sir Benjamin Baker, which increased the suspended span's dead load to 1,016 tonnes. The ends of this deck were fixed to the masonry abutments but were torn free frequently in high winds. The heavier deck also caused increased stress in the chains.
By the early 20th century, stress in the chains was becoming a problem and there were worries about metal fatigue. There was corrosion in the eyes of the chain links, the fishplates and the hangers that had not been repaired during the four-yearly maintenance painting.
By 1912 the suspender rods had new screw couplings, the lattice parapet had been renewed and the fixed ends of the deck were fitted with bolts that would shear if subjected to large sideways forces, allowing the deck to move. This work was designed by Edward George Rivers.
The Ministry of Transport took over care of the bridge after World War I (1914-8). Further reports on its condition by Mott & Hay (1917) and Tudsbery & Gibbs (1924) concluded that the yield stress in the chain links was, after a century of use, some two thirds of the breaking stress. Weighbridges were installed at both ends of the bridge and no vehicle weighing more than 4.6 tonnes was permitted to cross. Speed was limited to 6.4km per hour with 15m gaps between vehicles.
Reconstruction works began in 1938, designed by Sir Alexander Gibb & Partners with Guy Maunsell, Consulting Engineer. The Ministry of Transport stipulated that there must be two carriageways and two 1.5m wide footways, traffic must flow unimpeded throughout the works, and the structure’s original appearance should be maintained.
The wrought iron chains were replaced with two tiers of steel ones over the full length. Their eyebar links are 4.9m long, 45mm thick and 254mm wide at the shank with pin holes of 254mm diameter. Half of the links were forged and half were cut from high tensile steel plate. Each chain consists of six links abreast pinned together without fishplates. To prevent future corrosion all the links were shot blasted, sprayed with two coats of hot zinc (Schori process) and then painted four times.
Where the chains enter the tunnels to the anchorage points they are made from steel plates riveted together instead of eyebar links. The anchorages and the plates are encased in concrete. Saddles on roller bearings support the chains at the transition from plates to links. The suspender rods were replaced by spirally wound wire ropes with screw buckles for adjusting tension.
A new steel deck was designed to cope with heavier traffic loading and side winds, with movement joints at the ends where it is fixed to the abutments. New footways are cantilevered out on either side of the deck and their parapets conceal the stiffening trusses that run along the outer edges of the roadway. To keep traffic access open, the new deck was constructed 1.2m below the original and later raised into position.
The masonry wall between archways through the towers was underpinned with steel stanchions and rebuilt to be 1.2m wide instead of the original 1.8m, giving an elliptical rather than circular top to the arches and allowing wider carriageways. Reinforced concrete corbels cast in the same style as the masonry allowed the bridge approaches to be widened sufficiently to accommodate the new carriageways and footways.
The roadway on the approaches has 25mm of non-skid asphalt surfacing over 50mm of tarmacadam on a concrete bed. On the suspended span the roadway has a heavily reinforced concrete pad between the two layers of surfacing and the steel deck. The footways have a 25mm thick layer of asphalt over a concrete or steel base, as appropriate. Surfacing work began in October 1940 and was completed in August 1941.
Originally a toll bridge with a toll house at either end, some 457m apart, the toll charges were abolished on 1st January 1941. On 14th April 1953, the three-masted naval training ship HMS Conway foundered below the bridge while being towed to Birkenhead for a refit. On 30th October 1956 she caught fire and was irreparably damaged, and her remains may still be seen at low tides.
In autumn 1999, the road surface was replaced and the bridge strengthened. It was stripped back to bare metal and refurbished with four layers of new paint in 2005 — the first major repainting since 1939.
Resident engineer (1819-26, 1839): William Alexander Provis
Resident engineer (1938-41): J.B.B. Newton
Contractor (1819-20): Straphen and Hall
Contractor (1820-6): John Wilson
Contractor (1938-41): Dorman Long & Co
Original chain links: William Hazeldine, Upton Forge
Ironwork supervisor: Thomas Rhodes
Architect (1938-41): Percy Thomas
Re-surfacing (1938-41): Penmaenmawr & Trinidad Lake Asphalt Co Ltd
RCAHMW_NPRN 415326, 43063
Research: ECPK
"The Menai Bridge" by Thomas Telford, in ICE Proceedings, Vol.1, pp.217-229, London, January 1838
"Menai Bridge Reconstruction" by Guy Anson Maunsell, in ICE Proceedings, Vol.25, Issue 3, pp.165-193, London, January 1946
"Discussion: Menai Bridge Reconstruction" by Guy Anson Maunsell et al, in ICE Proceedings, Vol.25, Issue 3, pp.193-206, London, January 1946
"On the Construction of the Britannia Bridge" in The Engineer, Vol.200, pp.647-648, London, November 1955
"Obituary: Guy Anson Maunsell" in ICE Proceedings, Vol.22, Issue 3, pp.347-348, London, July 1962
reference sources   CEH W&W

Menai Suspension Bridge