John Stephen Parker

Proceedings of ICECivil Engineering 156 May 2003

Pages 70–77 Paper 13048

Keywordsbridges, cables & tendons; concrete

structures; dynamics; foundations; piles &piling; risk & probability analysis; river

engineering; safety & hazards

is a technical director of WSP Group

Gareth Hardwick

is project director for WestminsterCity Council

Martin Carroll

is project manager for CostainNorwest Holst joint venture

Nigel Peter Nicholls

is an associate with Gifford andPartners

David Sandercock

is a senior engineer with Halcrow

In recent years pedestrians found crossingthe Thames between Charing Cross sta-tion and the South Bank Centre neithereasy nor pleasant. The walkway attachedto the downstream side of Sir JohnHawkshaw’s 1864 Hungerford railwaybridge was narrow, noisy, frequently wetand somewhat unsafe underfoot.

It was not always so. The first bridgeon the site, which opened in 1846, was asubstantial pedestrian suspension bridgebuilt by Isambard Kingdom Brunel (Figs1 and 2). It was a toll bridge, providingaccess to Hungerford market and to boatlandings used by people as diverse asAustralia-bound convicts and QueenVictoria. Unfortunately both the marketand the bridge were in the path of theCharing Cross railway. The market wassold to make way for the station, themasonary piers of the footbridge becamesupports for the railway bridge and thesuspension chains were re-used inBrunel’s 1864 Clifton bridge near Bristol2

(completed after his death).

The new rail bridge had cantileveredwalkways on each side, though theupstream walkway was demolished whenthe structure was widened in the 1870s.3

For the Festival of Britain in 1951, theRoyal Engineers temporarily recreated anupstream walkway when they erected aBailey bridge to provide access from thenorth bank to the festival site on thesouth.4 This was supported on timberpiles driven into the river bed a fewmetres upstream of the rail crossing, andprovided a wide walkway and a view ofthe Houses of Parliament.

Over 40 designs submitted

In the mid-1990s the Cross RiverPartnership—comprising public and pri-vate bodies with a shared interest in pro-moting links across the Thames—identifiedHungerford Bridge as being in need ofimprovement. Not only was the railwaybridge one of the most ‘aesthetically notori-ous’ bridges in the world,5 the unpleasant

The £50 million Hungerford Bridge millennium project inLondon provides two stunning new cable-stayed footbridgesacross the Thames, one on each side of Charing Cross railwaybridge. Effectively it recreates Brunel’s 1846 suspensionfootbridge to Hungerford market, the piers of which remain inthe railway bridge structure. Heightened fears overunexploded bombs in the river bed led to a major redesignjust after work started but a switch to the NEC contracthelped ensure a smooth completion.

C I V I L E N G I N E E R I N G

Hungerford Bridgemillennium project—London

pedestrian route discouraged people northof the river from visiting the South Bankarts complex. With lottery funding, thepossibility of improving the crossingbecame more realistic and a design compe-tition was organised.

From over 40 entrants, the judges invit-ed six finalists (which were paid a smallfee) to submit more detailed proposals.The competition was judged following apublic consultation exercise and exhibi-tions around London.

A team comprising engineers WSPGroup, architects Lifschutz Davidson andcost consultants Davis Langdon andEverest submitted the winning design(Fig. 3). Twin cable-stayed bridges wereproposed, one each side of the railwaybridge, with other features including boatlandings and low-level link bridges fromthe South Bank area to Brunel’s south(‘Surrey’) pier. In particular the judgesliked the respect shown for the historicalcontext: new spans matched the old andthe design emphasised the importantparts of the existing structure.

Wide range of funding sources

Westminster City Council took the leadin the project from the start, organisingthe design competition and committing a

substantial share of the funding. Othermembers of the Cross River Partnershipalso promised to share the cost, either byproviding money or by waiving chargesfor services.

An application was made for lotteryfunding from the MillenniumCommission, and a grant was made in1996. The Millennium Commission wereparticularly concerned that the designquality should remain high and they main-tained close involvement throughout thedesign period.

Other funding came from Railtrack, theLondon Borough of Lambeth and theDepartment of the Environment,Transport and the Regions’ single regener-ation budget. Although Westminster wasrelatively prosperous, the same was nottrue of Lambeth. It was agreed that thebridge would encourage economic devel-opment south of the Thames and soregeneration funding was made available.For the same reason the Mayor of Londonprovided a substantial grant when thefuture of the project was later threatenedby heightened fears over rail tunnel safety.

Pre-tender design

The footbridges were to be built acrossa busy waterway, next to a main line rail-

Fig. 1. The original Hungerford pedestrian suspension bridge com-pleted by Isambard Kingdom Brunel in 18461

Fig. 2. John Hawkshaw converted Brunel’s bridge into a rail crossing in1864. The suspension chains were used to build the Clifton suspensionbridge in Bristol

HUNGERFORD BRIDGE MILLENNIUM PROJECT, LONDON

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Fig. 3. Winning concept included low-levellink bridges from the south (‘Surrey’) pier

the Mayor ofLondonprovided asubstantial grantwhen the futureof the projectwas laterthreatened

PARKER ET AL.

way bridge and above a number of tubetunnels (Figs 4, 5 and 6). They were onestablished commuter routes to and fromWaterloo and Charing Cross and theirprominent location required integratedengineering and architecture to achieve ahigh-quality result.

The pre-tender design was for a multi-span cable-stayed arrangement with rela-tively short (approximately 50 m) spanssupported by Macalloy bar stays. At thetime it was felt that the visual qualities ofthe available end fittings for bars were bet-ter than those for strand cables. A largenumber of stays—28 per pylon—wereprovided, both for visual interest and tominimise the size of each bar. Pylons weretapered steel cylinders, held at an angle tothe vertical by more Macalloy bars andsupported on concrete foundations.

At the Surrey pier each deck was sus-pended from a pair of pylons that were sup-ported on a new island, one upstream andone downstream. The arrangement wouldhelp to focus attention on the historic struc-ture and allow a staircase to descend to the

72 C I V I L E N G I N E E R I N G

0 m 50

Northern Line tube tunnels

Bakerloo Line tube tunnelsLink bridge(not constructed)Link bridge(not constructed)

Deep telecomtunnel

RoyalFestival

Hall

District Line tube tunnel

Embankmenttube

station

Charing Crossrailwaystation

Charing Crossrailwaystation Charing Cross railway bridge

Downstream footbridge

Upstream footbridge‘Hispaniola’restaurant ship

Fig. 4. Site plan, showing proximity of underground rail and utility tunnels

0 m 10

Downstreamfootbridge

Upstreamfootbridge

Widenedrailway bridge

1864 railwaybridge

Backstays

Pylon

Boredpiles

Deckstays

Railwaybridgecaissons

Beam

Pilecap

Fig. 5. Cross-section at a typical mid-river support, showing ship impact beam

0 m 50

VictoriaEmbankment

Queen’sWalk

Middlesex pier Surrey pier Link bridge

Bored piles

Main navigation channel

Fig. 6. Bridge elevation at contract award, with both north and south piers coinciding with Brunel’s original ‘Middlesex’ and ‘Surrey’ piers

HUNGERFORD BRIDGE MILLENNIUM PROJECT, LONDON

island. From here a walkway would passthrough Brunel’s structure, and steel foot-bridges just above high water level wouldjoin the island to the south bank.

Accounting for ship impact

One of the most complex problemsaddressed at the initial design stage wasship impact. The Port of LondonAuthority advised that ships up to 3000 tdisplacement could use the river, travellingat 6.2 m/s (12 knots). Impact loads of30 MN were calculated following theMinorsky method6 with an angle of impactup to 15° from the line of the river.Producing a substructure that was capableof resisting these forces without beingvisually obtrusive required substantialdesign effort.

Other design features include liftswhich were provided at the ends of thebridges to allow access by mobility-impaired people. Safety considerations ledto straight flights of stairs: pedestrianscould see all the way up or down, elimi-nating fears of unseen attackers. On thebridge deck, handrails were of stainlesssteel, with horizontal infill bars to afford abetter view of the river, but made difficultto climb by inclining the posts inwards by15°. Stair handrails were glass with plas-tic coating that could be replaced ifscratched by vandals. Lighting wasdesigned to make the bridge a feature ofthe London skyline and to provide safelevels of illumination on the deck.However, excessive light was avoided to

enhance night-time views from the bridge.The design team also considered intro-

ducing canopies and vertical screens toprotect pedestrians from wind and rain.These were rejected principally on mainte-nance grounds as it would have been diffi-cult to design a safe, economical andunobtrusive external access system. Dirtycladding would have spoiled the view fromthe bridge when the weather was good andcovered bridges were generally felt to beunpleasant inside. There was also a viewthat covering the bridge was unnecessarywhen the roads and footpaths leading to itwere all exposed to the elements.

Bomb fears trigger redesign

In October 1999, only a few weeksafter the construction contract wasawarded to a joint venture of Costain andNorwest Holst, it became apparent thatunderground railway operator LondonUnderground was in the process of re-evaluating its criteria for building neartunnels beneath the Thames. The projectbecame embroiled in the development ofthis new guidance.

The new approach took more accountof extremely low risks with serious conse-quences. In this case the risk was that anunrecorded, unexploded World War IIbomb could be present, buried deep inthe river bed and unaffected by driven pil-ing for the Festival of Britain Baileybridge. Piling operations could reactivatethe timer in such a bomb and, if detona-tion occurred, it was possible that a path

could be opened between the river andthe tunnel, causing inundation of theunderground railway. Clearly this combi-nation of circumstances was extremelyunlikely but the potential human and eco-nomic losses were unacceptable.

In order to rule out the risk it wouldhave been necessary to close the railwaytunnel floodgates for the duration of pilingwork and until detonation by time-delayfuses was no longer a possibility. Allowinga suitable margin of error, the tunnelswould have to be closed for four days afterpiling work had ceased, so weekend-onlytunnel closures were impossible. It wassimply not practical to consider such dis-ruption to London Underground so theclient commissioned consultant Halcrowto review whether the project should con-tinue and if so, in what form.

The consultant recommended continu-ing with a revised design that would elim-inate the risk to London Underground.The foundation near the north(‘Middlesex’) bank was moved out of theriver. The asymmetric pylon arrangementwas not possible in this location and so anA-frame support was introduced (Figs 7and 8). The next foundation out from thenorth bank was further from the BakerlooLine but to eliminate the risk its pileswere replaced with a 5 m diameter hand-dug shaft. The two link bridges wereomitted from the southern end of thescheme because one had foundationsclose to the Northern Line and the cost ofrailway possessions was not justifiable forwhat were non-essential elements.

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0 m 50

VictoriaEmbankment

A-framesupport Queen’s

Walk

Middlesex pier Surrey pier

Bored pilesHand dug shaftfoundations

Main navigation channel

Fig. 7. Revised bridge elevation, with north pier moved onto river bank, north pier superstructure changed to A-frame, hand-dug foundations and southern low-level link bridges removed

it was possible that a path could be opened betweenthe river and the tunnel, causing inundation of theunderground railway

PARKER ET AL.

Switch to NEC contract

The consultant’s review also recom-mended a major re-evaluation of the con-tract. It had been based on the ICE’sDesign and Build form that had beenamended to incorporate more designdetail than was normal for a design-and-build project, the result being called‘adopt and build’.

The client and the contractor bothagreed that the ‘adopt and build’ con-tract was no longer suitable because thechanges to the design would have to becarried out by the client’s engineerbefore the contractor’s team could starton them. This would cause additional

delay and unnecessary extra cost.Furthermore, it was agreed that a moreequitable allocation of cost and riskwould be to everyone’s benefit.

A new contract was thus negotiated,based on the NEC Engineering andConstruction Contract with main optionC (target contract with activity sched-ule). This allocated all design responsi-bility (except for dynamics, whichremained with the client’s engineer) tothe contractor and its designer, and setout a system of open-book payments. Avalue-engineering exercise was also car-ried out at this stage to reduce additionalcosts. This involved close cooperation ofthe whole project team and identifiedsignificant cost savings.

Introducing a new contract part waythrough the project was an unusual stepbut it worked well, avoiding what couldhave been a serious dispute over the allo-cation and level of extra costs.

Post-tender design development

The contractor’s designer, Gifford andPartners, had been charged with devel-oping the design after tender and pro-ducing a set of construction drawings.This was a complex task: the multi-spancable-stay arrangement produced a struc-ture with multiple redundancies in whichalmost every element had a differentloaded length. In addition the changesaround the north bank required thewhole bridge to be redesigned.

The firm introduced a number ofinnovations to the original design. Forexample, unobtrusive spherical bearingswere incorporated in the bases of thepylons and in the pylon top fabrications(known as ‘angel wings’ because of theirshape—Fig. 9) to relieve global bendingeffects resulting from temperature, creepand shrinkage. The bearings made itpossible to design the pylons as taperedpin-ended struts; they also made erec-tion simpler.

Sections of flat plate, rolled to therequired cylindrical or conical shape, werewelded together to form the pylons. Theplate—generally grade S355J2—wasthicker near the ends, where the radiuswas smaller. The 150 mm thick billetsforming the pylon tip were gradeS355NL, with enhanced Charpy energy

74 C I V I L E N G I N E E R I N G

Fig. 9. ‘Angel wings’ at top of pylons incorporate spherical bearings to relieve bending stresses

Fig. 8. A-frame on north pier replaced origi-nal asymmetric pylons (photo: Ian Lambot)

HUNGERFORD BRIDGE MILLENNIUM PROJECT, LONDON

absorption requirements and improvedthrough-thickness properties. The platessupporting the top spherical bearingswere grade S460NL, again with enhancedCharpy energy absorption requirements.

Sophisticated three dimensional finite-element modelling was also employed tooptimise the steel section and minimisecomplex welding requirements. At thelower end of the stays, fabricated steeldeck connections were replaced by cast-ings stressed onto the concrete.

Originally the deck was to have beenstressed with tendons at the neutral axis.However when the span adjacent to thenew A-frame was lengthened, axial pre-stress was no longer sufficient and thecompleted bridge required draped ten-dons in order to deal with the greatersagging moments at mid-span. Also, dur-ing the launch this led to undesirablestress reversals and so temporary exter-nal stressing was required. The processbecame very complicated and, because itwas possible to design a reinforced con-crete deck with the same cross-section,prestressing was not employed.

A new articulation layout was intro-duced because of the A-frame support atthe northern end. The difference in spanlengths on each side of the A-frame ledto large uplift forces at the northernabutment and it was not feasible to tiethe deck down while allowing longitudi-nal movement. Instead, a simple, mainte-nance-free stainless steel dowelledexpansion joint was introduced to thesouth of the A-frame to allow longitudi-nal movement and rotation but resist lat-eral and vertical movement, with anintegral fixed support at the north end.The remainder of the bridge was fixed atthe Surrey pier by bracing the pylons anda second expansion joint was provided atthe south bank. Lateral restraint wasprovided by steel struts between thedeck and the backstay nodes.

Gaps were provided between the rail-way bridge and all parts of the newbridge so that under normal circum-stances no additional load was applied toolder structure, which could also articu-late independently of the new. In theevent of a major ship impact, however,plastic deformation would occur and thefoundations of both bridges would acttogether to resist the load.

Innovations in construction

The contractor and its designer pro-posed a number of innovative methodsfor constructing the bridge, the mostoriginal of which was for the founda-tions. In order to resist ship-impactforces the foundations on each side ofthe rail bridge were to be joined togetherwith concrete beams.

Tender drawings envisaged the impactbeams being constructed in situ but thecontractor proposed precast constructioninstead. This avoided the need to con-struct sheet-piled caissons beneath therailway bridge, which would haverequired large amounts of time-consum-ing and difficult low headroom working.

The beams were precast in the con-tractor’s yard at Erith and delivered tosite in pairs by barge (Fig. 10). Theywere then transferred to temporaryworks, joined together and lowered intoplace. Reinforcement comprised a largeuniversal column section cast into thebottom of the beam. The connections tothe pilecaps were made initially usinglarge steel pins that passed through care-fully set-out holes and the final connec-tion was made with in situ concrete.

The main bridge decks were constructedusing the incremental-launch technique. Acasting bed was built on a steel frameworkin the non-navigable span adjacent to thesouth bank and a section of deck approxi-mately 50 m long was cast. A temporarystiffening truss was bolted to the top ofthe deck section and hydraulic strandjacks were used to pull it northwards(Fig. 11). The whole process was repeateduntil the full length of deck was complete.A temporary support, comprising rectan-gular concrete ‘biscuits’ stressed to thepilecap beneath, was provided at each pierposition. Steel supports would have beeneasier to construct, but they would havebeen vulnerable to a major failure in theevent of ship or floating plant impact.

The TakLift 5/400 t floating crane wasused to lift the pylons into place (Fig. 12).Each pylon was erected with its backstaysattached and the stays were kept straightduring lifting by attaching them to steelstrong-backs. On completion of the launch,the deck was jacked up. Deck stays werefabricated to preset lengths, temporarilysupported by strong-backs and installed

75C I V I L E N G I N E E R I N G

Fig. 12. A 400 t floating crane was used to liftthe pylons into place, shown here with thenorth pier A-frame

Fig. 11. A blue temporary stiffening truss wasused to support the incrementally launchedbridge decks, which were cast in 50 m sec-tions and jacked northwards

Fig. 10. Ship-impact beams were precast anddelivered in pairs by barge rather than beingcast in situ in sheet-piled caissons

PARKER ET AL.

with a preset sag to help connection of thefork-and-spade pin anchorages at each end.Following installation of all deck stays thedeck was slowly lowered in a controlledsequence until it was supported entirely bythe stays. The temporary truss and towersupports were then removed (Fig. 13).

Stay tensions were monitored simplyand effectively by measuring their natur-al frequency, this procedure beingchecked and calibrated by a limited num-

ber of strain gauges and hydraulic stress-ing jacks. A small number of the staysrequired adjustment in order to rectifyeither high or low tensions resultingfrom fabrication tolerances or to opti-mise the final deck profile.

Ensuring a wobble-free design

The dynamic behaviour of the bridge7

was studied in detail, using computer

models, wind tunnel tests and measure-ments on site. Because the deck was rela-tively wide, restrained at reasonablyclose centres and made of concrete, theanalysis results predicted a lateral natur-al frequency of 1.6 Hz, above the normalrange of concern for pedestrian footfalls.In the vertical direction, the fundamentalfrequency was around 0.7 Hz—too lowfor pedestrians to excite. There wassome concern that higher frequencies

76 C I V I L E N G I N E E R I N G

Parameter Quantity

Project cost £50 million approximatelyDeck length 316 mDeck width 4.7 mDeck thickness 650 mmPylon length A-frame 35 m,standard 25 mPylon diameter 450 mm to 813 mmPiles 1.5 m dia., 9 m to 42 m longFoundation shafts near 4 m dia. (land),Bakerloo Line 5m dia. (river) 20 m deep

Structural steel• pylons and struts 471 t• pylon backstays 66 t• deck stays 96 t• castings connecting

stays to deck 77 t• stair structure 84 t

Total 794 t

Concrete• piles and shafts 4630 m3

• substructure—precast 730 m3

• substructure—in situ 5360 m3

• superstructure 4750 m3

Total 15 470 m3

Balustrades—total length 1500 mPaving slabs 10 500

Loads• Pedestrian live load 5 kN/m2 (adjusted in

accordance with BD37/88)• Lateral wind load 0.95 kN/m• Ship impact load

parallel to river 30 MN• Ship impact load

perpendicular to river 8 MNMean tidal range 5.47 m

Table 1. Principal dimensions and quantities

Backstays stiffened with steel strongbacks and erected as a frame with the pylon

Deck stays stiffenedwith steel strongbacksand installed witha preset sag

Deck stiffening trussand temporary launchsupports removed

Permanentsupport

Temporarysupport

Deck jackedabove finallevel after launch

Deck lowered tocorrect level until

all load taken bydeck stays

Fig. 13. Sequence for attaching deck stays

Fig. 14. After satisfactory dynamic testing,the project was fully opened to the public inSeptember 2002 (view from south bank)

could be excited deliberately, so socketswere cast into the deck soffit to allowdampers to be attached at a later date.

The physical tests involved a machinewith a rotating eccentric weight, sup-plied by the Building ResearchEstablishment, and 160 pedestrians, sup-plied by Westminster City Council. Thetests showed that the calculated frequen-cies were accurate and there was no dan-ger that the bridge would bounce orwobble under normal pedestrian loads.Damping measurements gave a lowerthan expected value (0.4% of critical).

Conclusion

The upstream bridge opened in May2002 (Fig. 14), with its downstreamcounterpart following in September 2002.It has proved popular—pedestrian flowshave increased compared to the old walk-way and it has already become a favouritespot for tourist photographs (Fig. 15).

A summary of the principal dimensionsand quantities are shown in Table 1.

Acknowledgements

The project team was as follows: client—City of Westminster; funding bodies—

Cross River Partnership as recipients ofgovernment single regeneration budget,Department of the Environment, Transportand the Regions, London Borough ofLambeth, Mayor of London, MillenniumCommission, Railtrack, Railway HeritageTrust, Transport for London andWestminster City Council; concept engi-neers and dynamics—WSP; concept archi-

tect—Lifschutz Davidson; lighting archi-tect—Speirs and Major; client’s advisor—Halcrow; cost consultant —Davis Langdonand Everest; contractor —Costain NorwestHolst joint venture; contractor’s designer—Gifford and Partners.

The authors also thank the variousmembers of the project team for supplyingthe photographs in this paper.

Fig. 15: London Eye viewed from the upstream footbridge at night (Photo: Cross River Partnership)

HUNGERFORD BRIDGE MILLENNIUM PROJECT, LONDON

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References1. GOAD S. London’s Bridges. ICE Library, 1920.2. BARLOW,W. H. Description of the Clifton suspension bridge. Proceedings of the Institution of Civil

Engineers, 1866–1867, 26, 243–257.3. HAYTER, H. The Charing Cross Bridge. Proceedings of the Institution of Civil Engineers, 1862–1863,

22, 512–527.4. Engineering in the Festival of Britain IV: the temporary bridges at the South Bank exhibition.

Engineering, May 11 1951, 555–558.5. BURKE, M. P. Aesthetically notorious bridges. Proceedings of the Institution of Civil Engineers, Civil

Engineering, 126, 39–47.6. Ship collision with bridges and offshore structures. Proceedings of a Colloquium for the

International Association for Bridge and Structural Engineering, [Copenhagen] 1983.7. FLETCHER, M. S. and Parker, J. S. Will it wobble? The dynamics of the Hungerford Millennium

Footbridges. Proceedings of the Institution of Civil Engineers, Bridge Engineering, 156, No. BE2.

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