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Clifton Suspension Bridge
River Avon, Clifton Gorge, Bristol, UK
Clifton Suspension Bridge
associated engineer
Isambard Kingdom Brunel
Sir John Hawkshaw
William Henry Barlow
date  1831, 1836 - 1843, 1860 - 1864, opened 8th December 1864
era  Victorian  |  category  Bridge  |  reference  ST563731
ICE reference number  HEW 129
photo  under construction 1863/64, courtesy Brunel Institute
Bristol's dramatic suspension bridge over the Avon had the longest span in Britain for 97 years. Its construction was interrupted by financial difficulties and the death of its designer, engineer Isambard Kingdom Brunel (1806-59), and it was completed by two eminent colleagues, Hawkshaw and Barlow, as a memorial. The bridge is Grade I listed and continues to carry road traffic over the Clifton Gorge.
Without Bristol alderman and wine merchant William Vick (1707-54), the story of the Clifton Suspension Bridge might have been quite different. In his will he left £1,000 to the Society of Merchant Venturers to fund the construction of a toll-free stone bridge across the deep limestone gorge of the River Avon between Clifton Down and Leigh Woods. The money was to be invested until it reached £10,000 — a sum then thought sufficient for the work.
By September 1829, the bequest had increased to around £8,000. However, constructing a stone bridge at that time would probably have cost in the region of £90,000. A suspension bridge of iron, or a structure of iron and masonry, though cheaper would still need toll income to cover its cost and maintenance.
On 1st October 1829, a trustee committee launched a competition for the design of an iron suspension bridge, with a prize of 100 guineas (£105). Cost estimates were to accompany entries. By the closing date (19th November), 22 entries had been received, including four by Brunel, who was only 23 years old, and others by William Hazeldine (1763-1840), Captain Samuel Brown (1776-1852), William Tierney Clark (1783-1852) and James Meadows Rendel (1799-1856). Among Brunel’s ideas were a span of some 293m and a bridge without towers, with suspension chains sunk directly into stone abutments on the cliff tops.
Five designs were shortlisted, including one by Brunel. All were assessed and rejected by competition judge Thomas Telford (1757-1834). Telford thought the gorge too wide for a single suspended span and believed such a span would be destroyed in high winds. In January 1830, he produced a design of his own, a three span bridge with tall Gothic piers rising from the river banks, and a suspended central span of 110m. His bridge did not attract funding.
An Act of Parliament authorising the construction of a tolled suspension bridge was passed on 29th May 1830. Later that year a second competition was announced. Submissions were received from 12 engineers, with Brunel again presenting four ideas. Five schemes were shortlisted: by Telford, Brown, Rendel, Brunel and William Hawkes (1800-62) of the Eagle Foundry in Birmingham. The judges were Davies Gilbert MP (1767-1839, President of the Royal Society 1827-30, Member of Parliament for Bodmin 1806-32) and civil engineer John Seaward (1786-1858).
Gilbert and Seaward favoured Hawkes' design, with Brunel in second place. However, Brunel made some alterations and convinced the judges to adopt his proposal, which was the only one to satisfy the competition condition that "the weight of the bridge with its greatest possible load should not produce a strain upon the chains of more than 5½ tons per inch" (8,661 tonnes per sq m). He reduced the span from 293m to 214m by moving the Leigh Woods pylon eastwards, supported by an abutment 33.5m high on the west side of the gorge.
On 16th March 1831, Brunel was appointed engineer. In April, the trustees agreed his fee: £2,500 plus £500 expenses, with £800 for a resident engineer and £400 for an assistant resident engineer. Brunel described the project in his journal (26th December 1835) as "my first child, my darling". His estimate of £57,000 included much detailing in the then fashionable Egyptian style, with a sphinx topping each tower. The 194m cross-river carriageway was to be 74.7m above high water, suspended by a pair of double chains.
On 21st June 1831, Lady Elton, wife of Sir Abraham Elton (1755-1842, 5th Baronet of Clevedon Court), laid the foundation stone at St Vincent’s Rocks to mark the site of the Clifton abutment (ST565731). However, little progress was made before work was halted by the Bristol riots of 29th-31st October 1831 and a lack of sufficient loan capital.
On 27th January 1835, William West (1801-61), creator of the Clifton Observatory (ST565732), suggested that using wire rather than bar iron would save money. He cited as an example the 1834 Sarine Bridge in Fribourg, Switzerland, designed by Joseph Chaley (1795-1861). Brunel commented on 3rd February that he had considered using wire but reminded the committee that “It does not follow because wire is light and strong that it is cheap. Wire being upwards of four times as dear as bar-iron, and not being anything near four times as strong, you may have the same strength in a chain of bar-iron for much less expense ...”.
At the committee’s request, Brunel prepared a cheaper design (estimated at £35,000) with smaller plainer towers, a pair of single suspension chains, a timber deck and no separate pedestrian footways. Though initially approved in December 1835, the reduced design caused much ill-feeling and the trustees reversed their decision. They opted to proceed with Brunel’s original plan.
Construction recommenced, and at 7.15am on Saturday 27th August 1836, Spencer Compton (1790-1851, 2nd Marquis of Northampton) laid the Leigh Woods (west) abutment foundation stone. Looking like a solid block of masonry, this abutment is built into the sloping bank and faced with red ashlar sandstone, providing a construction platform for the west tower. The Clifton (east) abutment is much smaller and is founded on the St Vincent’s Rocks cliff top.
The two tapering masonry towers that support the suspension chains are not identical in design, though similar in size. The Leigh Woods tower has chamfered edges and a slightly pointed upper arch (below the capping), while the Clifton tower has angular edges, side cut-outs and a rounded upper arch. Both are faced with local pennant stone, with limestone dressings.
The towers are not level with each other, but they are the same height above their abutments. The west tower stands 26.2m above the roadway, the east one 27.1m, making a roadway gradient of about 1 in 233, though the deck appears level. Brunel did this deliberately to avoid the optical illusion caused by differences in the height and shape of the gorge sides. The cap of the Leigh Woods tower carries the Latin inscription SUSPENSA VIX VIA FIT (a suspended way [road] made with difficulty).
To minimise the effects of wind loading, Brunel intended to install inclined bracing chains above the roadway and "two curved chains lying horizontally" beneath the deck structure, in addition to the main chains. He also devised horizontal roller beds for the moveable saddles on the towers, which hold the suspension chains at their highest point. Making the rollers horizontal ensured the chain loading acted vertically (theoretically producing only compression) on the towers, and enabled the approach span chains to leave the towers at a steeper angle than for the main span. The approach chains are therefore shorter than they would have been and exert less thrust on the anchorages.
During construction, a perilous system of transportation across the gorge was adopted. A 38mm diameter iron bar 244m long was slung between the partly-constructed towers. A basket was suspended from it and pulled across the chasm by ropes. This soon attracted public attention and intrepid souls paid up to 5 shillings (25p) to make the crossing, raising money for the works.
In 1840, the chain supply contract was signed and the masonry work finished, after a change of contractor. By 1842, as well as the abutments and towers, the four anchor tunnels were complete, and the wrought iron suspension chains had been fabricated at Copperhouse Foundry in Hayle, Cornwall. However, by February 1843, the available funds (£45,000, including Vick’s bequest) ran out. Around £30,000 was still needed for the outstanding ironwork, erection of the chains and deck, construction of the toll houses and installation of the Egyptian embellishments.
Attempts to raise the money were unsuccessful and the ironwork contractor demanded payment. In October 1851, Brunel was instructed to suspend work indefinitely and sell the materials and plant to pay the creditors. The time stipulated in the original Act for completing the bridge was 23 years, which elapsed in May 1853. The chains were sold, returning to their home county. Some 1,150 links meant for Clifton were used in the construction of Brunel's Royal Albert Bridge, Saltash. The Clifton Suspension Bridge works were abandoned in July 1853.
Four years later, proposals to erect a cheaper lightweight suspension span to the design of American engineer Lieutenant Colonel Edward Wellman Serrell (1826-1906) incurred disapproval from Brunel and the trustees, and were discarded. No further work was done before Brunel’s death in September 1859.
In 1860, a number of members of the Institution of Civil Engineers joined forces and formed a company to complete the bridge as a memorial to Brunel. The Clifton Bridge Company had share capital of £35,000 and was headed by chairman Captain Mark Huish (c1809-67), a civil engineer, and secretary Captain Christopher Claxton of the Royal Navy. The company’s committee included many influential people, among them Sir Joseph Paxton (1803-65), Brunel’s eldest son Isambard Brunel (1837-1902), and leading engineers of the day, including James Simpson (1799-1869), George Parker Bidder (1806-78) and John Fowler (1817-98).
A new authorising Act received royal assent on 28th June 1861. Under its provisions, ownership of the existing masonry structures was transferred to the new bridge company for £2,000 in shares. Sir John Hawkshaw (1811-91) and William Henry Barlow (1812-1902) were appointed engineers and designed a new superstructure. As a further tribute to Brunel, the wrought iron chains of his Hungerford Suspension Bridge (1845) across the Thames in London, his only other bridge of this type, were purchased for £5,000 and re-used. Hungerford Bridge had been demolished in 1860 to make way for the new Charing Cross Railway Bridge.
Hawkshaw and Barlow followed the spirit of Brunel’s scheme, though their design differs in several respects. First, there are three suspension chains on each side of the bridge instead of two. Brunel had hangers at 3.65m centres, each attached to the joint between two links and to the midpoint of the link above or below. With three chains, the hangers are at 2.45m centres, each attached to the joint between two links, avoiding excessive bending moments. The chains act as inverted arches, stiffened by the deck structure to act as one structure, so theoretically rotation only occurs where the chains are pinned at the saddles.
Second, the inverted chain catenaries envisaged by Brunel for under-deck stiffening were not used. Instead, land saddles hold the chains, guiding them into new anchor tunnels some 23m long, inclined at around 45 degrees. The ends of the chains are fixed to cast iron anchor plates embedded in the rock and surrounded by Staffordshire blue brickwork with cement mortar jointing. Brunel’s anchorage tunnels were filled in.
Third, the use of timber is reduced. During the 1840s and 50s, Brunel had developed various options for timber stiffening girders, either as trusses or as solid laminated beams, with an all-timber deck. As constructed, the bridge is stiffened by longitudinal wrought iron plate girders with cross girders beneath. Only the deck is timber.
Fourth, the Egyptian sphinxes for the towers were omitted.
By July 1863, nine cables, weighing 2 tonnes each, had been hung between the towers. A temporary footbridge was made by binding timber planks to a row of six cables. It "oscillated … at every breeze" and had to be tethered with guy ropes. Two cables 1.1m above the planking served as handrails and tracks for the grooved wheels of the cradles used to support the suspension chains. One overhead cable carried a travelling 'car' to transport materials. Chain construction commenced from the anchorage plates upwards and from the towers towards midspan. Individual links were laid out on the temporary bridge and assembled into chains in situ.
The pair of main suspension chains are set 6.1m apart, each composed of three sub-chains. They are made up of wrought iron eyebar links, 7.3m long and 178mm by 25mm in section, stacked one above the other, and contain a total of 4,200 links connected by bolts 117mm in diameter and 635mm long.
The midspan deflection of the chains is 21.3m. They are carried over the towers, 22.25m above the roadway, passing under the cast iron tower caps on wrought iron saddles resting on cast steel rollers in cast iron roller frames. The saddles also came from Hungerford Bridge, modified to accommodate the extra chain. The chains are anchored about 73m behind the towers in the new tunnels. The bridge measures 412m, anchorage to anchorage.
Chain installation was completed on 8th May 1864, and construction of the 9.4m wide deck structure began from the abutments. A 914mm high plate girder, assembled from 5m sections, is suspended below each chain by 81 vertical wrought iron hanger rods, ranging in length from 19.8m at the ends of the span to 914mm in the centre. The plate girders are braced by 81 lattice cross girders below the hanger positions. The wrought iron was lifted using a purpose-built crane running on rails laid over temporary planks, and advanced in stages.
The cross girders include flanges of angle iron, 83mm x 83mm x 16mm, and diagonals of bar iron, 63mm x 16mm. The rounded outer ends support the vertical cast iron lattice girder railings. Footways, 1.7m wide flank the 6.1m wide roadway. The upper flanges of the cross girders curve slightly in elevation to give the roadway a camber and the footways a slope.
Girder work finished on 2nd July. The deck structure was completed by the installation of 127mm thick longitudinal beams of Baltic pine topped by 51mm transverse timber planking on the roadway, and 63mm planking on the footways. Construction of four single-storey flat-roofed masonry toll houses, two at each end of the bridge, began on 18th August.
On 5th November, the deck was load tested using 508 tonnes of stone. On the 7th, government representative Sir Charles Manby (1804-84) examined the bridge with its engineers, and reported that "the strength ... is ample and the quality of workmanship throughout the structure is very good”. The ironwork was painted brown and the nuts on the chains gilded. The structure weighs about 1,524 tonnes including its chains, rods, girders and deck, and contains 3,500 loadbearing bolts and thousands of rivets.
On 8th December 1864, the bridge was opened by the Lords Lieutenant of Gloucestershire and Somerset in a day-long celebration. The bridge was illuminated for the occasion with variable success by electricity, limelight and magnesium lamps. The following day, 21-year-old Mary Griffiths of Hanham paid the one penny toll and raced over the bridge from Clifton to Leigh Woods, narrowly beating a young man to become the first member of the public to cross.
Wrought iron is relatively brittle, and after gales some hanger rods near midspan failed between the turnbuckle and the eye connection to the plate girder. Two failed in 1877 and three in 1887. The chains were unaffected. The ironwork was originally coated in tar, later replaced with bituminous compounds and ultimately by specially prepared paint. The transverse timber deck planks were replaced in 1884 and again in 1897, when the deck was surfaced with mastic asphalt.
In 1910, Howard Humphreys & Sons (later Howard Humphreys & Partners Ltd) was appointed consulting engineer to the bridge trust. As part of a 1918 structural survey, bolts from every tenth hanger rod were removed for checking. A few had cracks from forging defects but the majority were acceptable. In 1923, inspection of the rods resulted in the replacement of the most highly stressed. The rods removed were load tested, revealing an average tensile strength of 35,118 tonnes per sq m.
Damp in the anchor tunnels caused some corrosion at the chain ends, which were strengthened in 1925-6 (Leigh Woods) and 1939 (Clifton) by the addition of an extra open-link chain in each tunnel. The tunnel ends were then filled with a 2.7m plug of concrete. The land saddles were strengthened in 1932. In 1948, the deck planks were replaced and in January 1950, the tower masonry repointed.
In 1952, the present Clifton Suspension Bridge Trust was formed by Act of Parliament to replace the old company. The organisation is headed by a panel of 12 unpaid trustees and is responsible for the bridge’s operation and maintenance, funded by toll revenues.
By 1953, it was evident that the bridge deck was in poor condition and the original 6.1 tonnes (6 tons) vehicle weight limit was reduced to an axle load of 2.54 tonnes, or a gross vehicle weight of 4.06 tonnes. This weight restriction remains in force today.
The behaviour of the bridge structure was investigated in 1953-4, using deflection and strain measurements. The following year, two wrought iron deck cross girders from near the abutments were removed and load tested to destruction. They showed little loss of strength resulting from corrosion — they were affected by corrosion more than other girders because their proximity to the abutments made them difficult to paint.
The two girders were replaced with rolled mild steel joists as structural quality wrought iron was no longer available. The rest of the wrought iron girders were grit blasted in place and sprayed with zinc. The junctions were strengthened by bolt replacement with high yield steel bolts, and loose rivets were fixed.
In 1958, the entire timber deck was replaced. This revealed that some 30 percent of the beams had been replaced previously. Most of the deterioration found had resulted from the asphalt/tar surfacing clogging the gaps between planks, stopping drainage. New Douglas fir timbers, incised and pressure creosoted before installation, were laid with large gaps in between. The tollhouses at the east end of the bridge, which had deteriorated owing to water ingress, were demolished and rebuilt, and in operation by 27th November.
The bridge has been illuminated on a number of occasions, such as coronations, jubilees and anniversaries, including the centenary of Brunel’s death in 1959 — also commemorated with a plaque unveiled by Brunel's granddaughter Lady Celia Noble (1872-1962). In January 1959, the bridge was Grade I listed. Further illuminations followed in 1964, when 6,000 lamps were used to mark the centenary of the bridge’s opening, and in 1967 permanent lighting was installed.
In 1969, the west abutment foundations were inspected — both abutments are founded on hard carboniferous limestone with some well-compacted Triassic breccia, though the gorge has been subject to landslips.
In the 1970s, bolts on the shorter hanger rods at the centre of the bridge were replaced (33 on each side). A monitoring programme began in 1972 to investigate the risk of fatigue failure. The study ran until 1984, combining controlled loading and measurement with laboratory testing of full-size replica components. It found the most fatigue-sensitive element to be the eye of the chain links at the tower saddles. These are monitored regularly using a fibre optic probe.
The pedestrian toll was abolished in 1973, and an automated barrier toll collection system installed in 1975-6. In March 1984, the two Leigh Woods toll houses and the 1.2m high flanking walls, approximately 46m long to the north and 91m to the south, were Grade I listed. It was reported that 3.15 million vehicles crossed the bridge in 1985.
On 4th September 1986, a plaque commemorating the 150th anniversary of the laying of the Leigh Woods foundation stone was unveiled on the east pylon by Sir George Porter (1920-2002, later Baron Porter of Luddenham). In 1988, some of the longitudinal beams and cross planks of the timber deck were replaced at the west end of the bridge.
In 1998, the Trust opened a temporary visitor centre and display.
Up until 2002, the Leigh Woods abutment was believed to be solid sandstone masonry. However, an inspection revealed that behind walls almost 2m thick, the abutment contains 12 vaulted chambers in two tiers linked by shafts and tunnels. The largest chambers measure 17m by 5.6m and are up to 10.7m high.
Meanwhile the bridge’s lighting systems were upgraded. Festoon lighting had been installed in 1979 but proved vulnerable to corrosion and vandalism. It was replaced in 1991 by the Guide Lite system with a power consumption of 23.7kWh. This in turn suffered from lamp and connection failures, weather damage and bird attack, and had been removed by October 2004.
The present lighting system (2017) was installed in 2005-6 for the double centenary of Brunel’s birth. It was designed to reduce energy consumption, and minimise light pollution and bulb replacement. The chains are lit by 2,796 bulbs with more on the hangers. The scheme uses 3,072 LED bulbs, plus fluorescent tubes on the latticework railings, and tower and abutment lighting. Most switch off at midnight, with some left dimmed for pedestrian safety.
The Trust's engineer since 2006 has been Flint & Neill (est. 1958, part of COWI from 2008, rebranded as COWI in 2017). Extensive ground investigation has been undertaken along with numerical assessment to produce a geological model for each abutment foundation (from 2007). Flint & Neill has also reviewed risks generally and developed maintenance strategies (from 2009). The bridge retains around 99 percent of its original elements and is monitored continuously, plus full annual inspections are carried out.
Recent computer analysis shows that Brunel had calculated almost perfectly the minimal weight required to maintain maximum strength in the design of the joints between links in the suspension chains. Examining the chains thoroughly, especially the central one, remains a challenge and ultrasonic inspection for fatigue cracking has proved useful.
In 2007-8, new asphalt was laid on the carriageway, and the drainage system renewed using asphalt kerb units 190mm wide by 100mm high (not bonded to the road surface) backed by fibreglass drainage channels. The movement joints in the carriageway were replaced.
In a decade-long refurbishment project (2007-17), the Leigh Woods abutment vaults were made accessible by creating a path down the gorge, installing metal ladders, drilling a doorway in the masonry at the foot of the abutment and removing rubble from the interior. The stonework was restored in 2010-2. The public can now take tours through two of the upper vaults.
In 2009, a stress fracture was found in a hanger (now on display in the visitor centre) close to midspan. Repairs entailed refurbishing 7 percent of the wrought iron hangers, and replacing 10 percent of them in steel. Ongoing hanger refurbishment includes restoring the articulation of hanger eyes and repairing enlarged bolt holes. Some 1,300 bolts under the bridge deck have been replaced.
New offices for the bridge master and a new visitor information centre were constructed in 2013-4 at the west end of the bridge. The visitor centre opened in 2014, in time for Clifton Suspension Bridge's 150th birthday. The event was celebrated with a recreation of the opening cross-bridge parade.
Restoration of the pylons was carried out in 2014-16, including repointing works, the installation of cathodic protection, interior waterproofing, and saddle and guttering refurbishment. The suspension chains were repainted. The under-deck ironwork was repainted the following year. All the metal work is white.
A toll system still operates on the bridge, though pedestrians, cyclists and horses can cross free. The toll barriers restrict vehicles to a width of 2.4m.
Chains (1840-43): Sandys, Carne & Vivian, Copperhouse Foundry, Hayle
Saddles (ex Hungerford Bridge): Barrett, Exall & Andrewes, Reading
Consulting engineer (1910-c2005): Howard Humphreys & Sons
Consulting engineer (2006-2016): Flint & Neill
Consulting engineer (2017 onwards): COWI
Contractor (1860-64): Cochrane, Grove & Co, Woodside Foundry, Dudley
Contractor's agent (1860-64): Thomas Airey
Architect (Clifton toll houses, 1958): J Ralph Edwards
Contractor (2008-c2017): Dean & Dyball
Kerb drainage (2008): Extrudakerb
Abutment waterproofing (2014-16): Rateavon Ltd
Research: ECPK
FOOTNOTE
In 1811, philanthropist and inventor Sarah Guppy (1770-1852), wife of Bristol metal founder and merchant Samuel Guppy (1755-1830), patented a hanging bridge, with a planked deck laid over chains. Claims that this patent was the basis for the design of Clifton Suspension Bridge are incorrect — the patent was intended for a low-level bridge over the Avon at Hotwells, and differs in structural concept (a hanging deck versus a suspended one).
bibliography
"A Decade in the Life of the Clifton Suspension Bridge”, James Sutherland History Lecture, speakers E.J. Rees, A.S. Mulcahy and R.A.N. Mackean, Institution of Structural Engineers, London, 16th February 2017, webinar available at https://istructe.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=cea7ed04-4f7d-4afa-b7e0-8818ce4e8c49
"The A-Z of Curious Bristol" by Maurice Fells, The History Press, June 2014
"A Critical Analysis of the Clifton Suspension Bridge" by Daniel Richards, Proceedings of Bridge Engineering 2 Conference 2010, University of Bath, Bath, April 2010
"I.K. Brunel — Exploding the Myth" by Michael R. Bailey, Transactions of the Newcomen Society, Vol.78, Issue 1, pp.1-10, London, 2008, read at the Brunel Bicentenary Conference, Bristol, 6th July 2006
"Bridge Management: inspection, maintenance, assessment and repair", eds J.E. Harding, G.A.R. Parke and M.J. Ryall, E & FN Spon, Chapman & Hall, London, 1990
"The Life of Isambard Kingdom Brunel, Civil Engineer" by Isambard Brunel B.C.L., Longmans, Green, & Co., London, 1870
"Description of the Clifton Suspension Bridge" by W.H. Barlow, Minutes of the Proceedings of the ICE, Vol.26, pp.243-257, London, February 1867
"Discussion. Suspension Bridges of Great Span" by W.H. Barlow et al, Minutes of the Proceedings of the ICE, Vol.26, pp.265-309, London, February 1867
"The Clifton and Other Remarkable Suspension Bridges of the World" by Lewis Wright, 2nd edition, John Weale, London, 1866
http://extrudakerb.com
http://openplaques.org
www.brunel200.com
www.cliftonbridge.org.uk
www.cowiuk.com
www.gracesguide.co.uk
www.historicengland.org.uk
www.istructe.org
www.newcivilengineer.com
reference sources   IKBcatBDCE2CEH W&W
Location

Clifton Suspension Bridge