Article Reference: AT108p36  Date: 1/5/00  Architect: Marks Barfield Building: London Eye Title: Jeremy Dixon on the genesis of London's poignant new symbol Full circle: Marks Barfield on the South Bank Jeremy Dixon on the genesis of London's poignant new symbol. Jeremy Dixon is a partner in Dixon Jones, whose landmark London projects include the Royal Opera House (AT106), the National Portrait Gallery and the courtyard of Somerset House. The millennium wheel is set to become a symbol for London to match the Eiffel Tower in Paris, the Sydney Opera House and the Bilbao Guggenheim. Big Ben and Tower Bridge have been used as symbols for London and at one time the BT Tower appeared in the masthead of the Evening Standard. The wheel however is bolder, more unusual and genuinely populist. Paris and London have always looked at each other somewhat guardedly, comparing one another's attributes. The Eiffel Tower sits securely on the ground, a shape engineered for stability. The London wheel, on the other hand, hovers precariously over the Thames on a visually unstable structure. The certainties of the nineteenth century are replaced by the nervous uncertainties of the beginning of the new millennium. The wheel is both obvious and subtle. As a symbol of the millennium it represents time as well as the cycle of the seasons and the wholeness of life. As a piece of sculpture it appears to be static yet one knows it is in motion. This imperceptible movement expresses the very passage of time. The wheel rotates at a rate comparable to that of the minute hand of a clock. How did a pair of young architects come to put up this extraordinarily successful public icon? To start with, there was their student experience at the Architectural Association. David Marks and Julia Barfield were always interested in geometry and collected books on the subject, so it was natural for them to join the unit run by Keith Critchlow, who was an obsessive geometrist. Marks then spent time in Mike Gold's unit, where the projects had a fantastical and romantic tendency - he remembers designing a city floating in a cloud. This was followed by periods working respectively for Norman Foster (Barfield was job architect on the Sackler Gallery) and Richard Rogers (Marks was a project architect for Lloyds). One can see how technology, romance and geometry combine in the wheel. In 1993 the Sunday Times and the Architectural Foundation held an open competition to design a landmark for 2001. Contestants were urged not simply to emulate the Eiffel Tower. Amongst the submissions was one by Marks and Barfield proposing a 500 foot diameter wheel alongside the Thames. The jury did not think any of the entries were good enough. Instead of accepting this rejection gracefully, Marks and Barfield proceeded exactly as if they had won the competition and set up a company to design and build the project. They were helped and encouraged by Marks' father, a natural entrepreneur with a successful career in television production. Using their own money and borrowing against their house, Marks and Barfield began to develop the designs and to approach all possible interested parties. Ove Arup & Partners (and in particular Jane Wernick) gave lots of free help. Obtaining planning permission was a major hurdle which took two years. It is no small task to bring together such bodies as Lambeth, Westminster, English Heritage, the Cross-River Partnership, the City, the Port of London Authority and the Government Office for London and get agreement for such a flamboyant project. In the end the wheel was referred to the then Secretary of State, John Gummer who, rather surprisingly and to his credit, fully endorsed the project. There was still the question of money. The climate in the City was not favourable, the comparisons being the loss-making Channel Tunnel and the Dome with its lack of a clear purpose. Eventually money came from British Airways after a chance encounter with Bob Ayling while delivering Christmas cards. A company was set up with the shares split 50:50 between BA and Marks Barfield. Ownership of shares has enabled the authors to keep control of both the design and running of the project. The subsequent drama of the construction, with its use of the river for transport and the nerve-wracking moments associated with the raising of the structure from the horizontal, is already becoming something of a legend. The fact that the wheel is there at all is entirely due to an almost bone-headed persistence on the part of Marks and Barfield. It is a reminder about never giving up and not always believing in the judgement of your elders and betters. It is worth comparing the original competition drawing with the final outcome. What was a rather heavy structure has been gradually refined to a point where its lightness almost beggars belief. For instance, the radial tension members that hold the outer rim in suspension are much more slender than the spokes of a bicycle wheel. Similarly, the capsules have become exercises in pure glass technology, with an absolute minimum of framing. In pursusing their dream to its conclusion Marks and Barfield have given London something remarkable - huge in scale but light in feeling. Technology has been pursued to an extreme and produced an aesthetic refinement to which the public have responded with extraordinary enthusiasm. The wheel is poignant for a number of reasons. The siting over the river in a confrontational relationship to the principal and symbolic buildings of government makes it much more memorable than if it had been placed more conventionally in a park. This wheel is different from its predecessors. In contrast to the traditional wheel, where gravity is used to keep the gondola horizontal, the capsules on the wheel rotate mechanically, remaining outside the structure throughout. This produces a special drama at the top of the ride, where one seems to be floating unsupported. There is a seriousness and grandeur about this wheel which sets it apart from the simple frivolity associated with the traditional Ferris wheel. The experience is the opposite of the quick adrenalin rush. This is all about slowness, silence and a sense of dislocation. There is time to absorb the emotional experience of looking down on London. David Marks describes the idea of the wheel as both the basis of our civilisation and the shape of time. What better way for London to mark the Millennium? Marks Barfield writes: The Rim The rim is a simple tubular space frame 122m in diameter. It is secured to the hub via a series of cables arranged as spokes which transmit gravity loads and wind loads to the hub and spindle. To prevent slackening in strong winds, all cables are pre-stressed which puts the rim in uniform circumferential compression. The anticipated mode of failure would be lateral-torsional buckling; this was a complex analytical problem to resolve, made more so because the elastic critical load of the rim is below the value normally deemed advisable for steel structures. The analytical problem could be solved only by using advanced computer programmes. But computer analysis alone was insufficient for the design; there are no other examples of pre-stressed steelwork known to the designers, so conventional codes of practice offered no guidance. The designers had therefore to evolve their own concepts which took advantage of the fact that pre-stress force is less destabilising than gravity forces of comparable magnitude. The need for lateral thinking was critical. The dominant rim load was pre-stress and larger rims would attract higher wind forces, which in turn would require yet more pre-stress in a potentially unending cycle. Considerable optimisation was therefore required. Stability and construction The rim and cable system is only self-stable once all the cables have been pre-tensioned. This presented a fundamental problem in that an effective engineering solution must recognise both the finished state of any structure and the potential method of its construction, since this may well have a profound influence on final cost. Because of the wheel's height, from the outset the erection method had to be integrated within the total design, particularly as the rim aligns over the river. A further consideration was that the method of tensioning was likely to add distortion to the rim, which would affect the functionality of the wheel in operation and compound the rim-buckling problem. Although the rim could have been built in its vertical position from the outset, this would have risked instability from high winds and added to the risks to operatives. All these issues were resolved by the decision, in a piece of lateral thinking, to build the rim flat over the river and then hoist it up in a single operation. This allowed all works (including alignment maintenance) to be completed at ground level and achieved the programme of erecting the fourth-tallest building in London in a single day. The decision to lift the wheel as a single entity meant that this was the biggest-ever lift of any object from the horizontal to the vertical. It was praticable only because of the river's width and because river traffic mainly occupies shipping lanes on the opposite bank. River access also facilitated speed of construction, with all major wheel components shop-fabricated in the largest units possible and brought in by barge. Wind Engineering The wheel's slender form is stabilised overall by four cables anchored to the rear. It thus functions mainly as a large mass on a spring (ie the cables), with a dominant frequency of around 0.25Hz. Potentially this renders the whole structure excitable from wind buffeting at the same frequency. Such motion would be potentially very destructive, particularly as the basic structure is lightly damped, and even if not destructive, could induce motion sickness. Rather than the obvious solution of radically stiffening the design, the structural engineers adopted the mechanical engineer's technique of adding damping. This was achieved by positioning 64 mass tuned dampers around the rim to suppress any tendency to resonance. The dampers consist of tubes housing a mass and spring. To ensure rapid response and minimum noise, the tubes have been honed internally and then coated. The steel mass is supported on wheels running radially to the tube. The whole system was laboratory tested before installation. No other structure so large is known to incorporate such an active damping system. A problem that has appeared recently on major suspension bridges is cable vibration in certain conditions of wind and rain - rainwater adhering to the cables modifies their shape and changes their lift characteristics. To avoid this, the locked coil cables have been manufactured with one strand deliberately made smaller than the others so as to introduce a continuous spiral, the first time this technique has been used. The Spindle The whole 1500 tonnes mass of rim and capsules is supported on a single cantilever. This audacious concept posed difficult engineering questions - notably how to fabricate the spindle and how to assess appropriate design standards. Conventional codified approaches were considered inappropriate and so the designers tackled the problem by addressing basic questions of reliability. Two fundamental modes of failure were identified, namely over-stress and fracture (the classic conditions for low-temperature brittle fracture are all present, with thick steel, welding and cold temperature). This posed a fundamental dilemma, as thicker walls increased the safety against over-stress but also increased the probability of fracture. Considering fracture a more catastrophic form of failure than flexure, the engineers wished to minimise wall thicknesses consistent with achieving adequate safety margins. Appropriate load factors were therefore derived from fundamental structural reliability theory as set out in the new Eurocode, which is thought to be the first time this has been done. The spindle is fabricated from seven sections of cast steel in wall thicknesses up to 300mm and one rolled section. All sections were welded together and the whole unit heat-treated to enhance material properties and reduce residual stresses. A key target was to achieve high toughness and this was measured by CTOD testing. For assurance against brittle fracture, the package of measures included control of quality, achievement of toughness, reduction of residual stres and extensive NDE and proof testing. Proof testing was partly to assure strength but mostly application of over stress will leave the tips of any cracks in the tension zone not detected by NDE in a state of residual compression and so protect against service crack propagation. The latter technique is not widely known and rarely applied. Capsules A fundamental difference between this and all other observation wheels is that the capsules are driven, rather than achieving their level under gravity and live loads. Motorising the capsules allows passengers to walk around on a level floor and means that the capsules can be placed on the outside of the rim. The challenge was to devise a reliable stability system that could control floor horizontality to within close tolerances, when subject to rapidly varying load eccentricity from the passengers. The system adpted is deceptively simple, with a motor, variable speed controller, gearbox and clutch all driving a rack-and-pinion track fixed on the inside of the mounting rings. An encoder in the hub determines the speed of rim rotation and transmits this information to ground. After data manipulation, the signal is sent up to each capsule where calculators work out how far the motor should turn to maintain the floor level relative to its position on the wheel perimeter. Rapid correction for changes in passenger load is made by the motor activated through the variable speed controller. To assure reliability there is a complex system of duplication, with twin inclinometers which constantly check the floor state. The floor is thus kept in a very steady state, and passengers notice no corrections. In terms of structural engineering, the capsules are aerodynamically shaped to minimise wind drag. They are framed with slender steel tubes with the majority of wall and roof sections glazed to maximise views. The glass is a laminate sandwich which is strong and stiff enough to contain passengers throughout the trip and also helps to maximise the capsule's structural efficiency. The glass is secured by an adhesive which is strong enough to resist the glass-breaking pressure. To achieve the desired shape, the glass had to be doubly curved - a first for laminated glass. Capsule services The capsules incorporate services installations that create safe, secure and comfortable environments for the passengers, writes Loren Butt. Most of the services equipment is discreetly located below the floor but there is a second zone at ceiling level, between the two main structural support rings, with connecting cables housed within the supporting enclosures. The capsule has two links with the ground. Electrical power from a 400 volt five- conductor bus bar around the main wheel rim (itself fed through sliding contacts on the southern restraint tower) enters the capsule through two 230 volt three-conductor slip-ring bus bars and sliding contacts mounted alongside the two main structural support rings. Both supply systems are maintained 'live' but only one is in use at any one time. The two supplies are protected by circuit-breakers located in weatherproof housings attached to the wheel rim and all internal distribution is similarly protected. The second primary link is by UHF radio. As with all safety-related services on the capsule the system is duplicated, with two transmitters and receivers on the boarding platform and two per capsule. The radio systems are linked to programmable logic computers, handling all communications, monitoring and control functions, again with two at ground level and two per capsule. All computers have uninterruptible electrical supplies, via batteries that can sustain all critical operations for several hours. Capsule doors are motorised and automatically controlled to be open while the capsule passes through the boarding platform area. The opening and closing sequence at the boarding platform is programmed into each capsule computer system: the computers 'know' where each capsule is through interpretation of signals from a remote encoder which detects the position of the wheel's central moving hub relative to the fixed spindle on which it rotates. The capsules are kept level within their support rings by being given one reverse revolution for each full turn of the wheel. Power from an inverter-controlled infinitely variable speed motor located below the floor is transmitted through a clutch and gearbox to a small-diameter toothed pinion wheel, which engages with a similarly toothed circumferential rack at one of the structural support rings. The drive motor speed is adjusted by tilt switches which detect movement caused by passengers moving within the capsule. If the main tilt control should fail, secondary tilt switches stop the main wheel drive. At this point the levelling motor can be de-clutched by radio signal from the central control room. The capsule is then able to maintain a safe degree of stability through gravity, its overall mass being predominantly below the floor. Each capsule is climate-controlled by two reversible cycle air-to-air heat pumps located below the floor. These draw air from below the central bench and adjust its temperature by appropriate operation of the refrigeration compressor to provide heating or cooling, delivering conditioned air into a pressurised floor void . From here it re-enters the capsule through a continuous perimeter slot adjacent to the glazing. The heat pumps draw heat or dispose of it through integral fully-ducted fan-assisted heat exchangers in their outdoor air streams - the inlets and outlets are linear grillages set into the capsule enclosure skin just below floor level. Fresh outdoor air enters inlet grilles in the bottom of the capsule, with two small fans delivering the air into the main re-circulating systems. The fresh air proportion of the total circulation is one quarter, sufficient both for the normal needs of 25 passengers and to control condensation. The heat pumps produce condensate water in both heating and cooling modes, which is discharged to the river from each capsule as it passes through the boarding platform. The operational status and key temperatures of the climate control systems are continuously monitored, and can be adjusted, if necessary, from the central control room using radio communication. The capsule lighting system, used only at night, minimises internal reflections while providing sufficient illumination for security and safe embarkation and disembarkation. When the capsule is at the boarding platform, ceiling-level concealed fluorescent lamps provide white light, which fades as the flight commences to be replaced by a soft blue colour for the main part of the journey. The dimmed blue lights can be switched back to white on one or all capsules by radio signal from the wheel control room. The operation of the lighting sequence is controlled by the same encoder signal and PLC operation as that used for the doors. Safety measures include the provision of a separate underfloor compartment to house the capsule levelling motor system; smoke detectors in the air circulation ductwork and the passenger space; and automatic thermal switches on all the heat pump and ventilation components that could overheat. On receipt of any smoke signal, a central control room alarm warning is activated, all air circulation systems are automatically switched off and a roof hatch opens for smoke dispersal. The capsule fire alarm system is fully integrated with coverage for the boarding platform and the ticket facilities in County Hall. All safety-related service elements have battery back-up supplies. Communication and security provisions include cctv surveillance with digital real time image storage; a high quality public address system, which can transmit pre-recorded information, music and commentaries; and two intercom facilities for emergency use by passengers. Monitoring The structure of the wheel is unusual in being light and flexible and incorporating structural damping. It required advanced computerised structural analysis for proof of adequacy. To monitor the structure, the end pins retaining the spoke cables were calibrated with strain gauges, with the data fed to a collector in the hub. This can be read remotely via telephone, enabling both the cable manufacturers in Italy and the wheel constructors in Holland to check on the changing cable loads. Information on cable load allows almost full description of all the forces throughout the entire structure and thereby the stresses, facilitating correlation with theoretical predictions. During erection, as the rim was lifted from horizontal to vertical, the cable forces were compared on a continuing basis with the analytical output from the computer analysis. The match was exceptionally close and gave high confidence that the structure was not being over stressed in the erection phase. In service, correlation is continued between structural response and wind speeds measured at the site, giving on-going confidence in the design's reliability. Ship protection As the rim is aligned over the Thames, the wheel is potentially at risk from errant shipping. Early studies suggested the impracticality of designing for the maximum conceivable mass travelling at maximum conceivable speed. However, given the traffic arrangements, control via bridges and frequency of passage, it seemed unlikely that such an event would occur. To provide a rational basis for design, a probabilistic study was carried out, culminating in a design criterion that would keep the impact risk comparable with the numerical targets set in other safety-related industries (essentially this was a transfer of technology from the nuclear industry). Impact protection was provided by a system taking advantage of the floating pontoon and linking bridges, all interconnected by strong cables terminating in proprietary shock absorbers to provide for controlled absorption of energy under vessel impact with the pier. Safety The wheel will carry up to 800 passengers at any one time. As well as providing technical solutions to the wheel's physical design, a key task for the engineering team was to ensure safety. This was accomplished by positive action rather than simply compliance with regulations. HAZOPs and Risk Assessment were carried out to identify failure modes and consequences. Passenger safety is assured by engineering high reliability of containment within the capsules; ensuring floor horizontality; and by being able to turn the wheel as required for passenger evacuation. This last imposes engineering constraints on the power, drive and control systems. Sensors provide information on the performance of all mechanical parts, with the data controlled and organised by PLCs. In this way errors can be detected and reported and any serious errors controlled to reduce the wheel to a safe state from which passenger recovery is possible. All safely related systems are duplicated to ensure that the wheel can be turned in all conceivable circumstances and minimise the risk of passengers being marooned at height. A written Safety Case has been produced utilising best practice approach from other safety-conscious projects. Structural concept Ove Arup & Partners was asked by David Marks and Julia Barfield to work on the wheel when it was still an entry for an ideas competition, writes Jane Wernick. The aim was to bring the nineteenth-century observation wheel into the twenty-first century. The bicycle wheel is a very efficient structure; horizontal spokes are used to stop the wheel deforming when the vertical load is applied to the hub, while the shallow angle formed by attaching the spokes to either end of the hub allows lateral forces to be caried without the rim bending. The observation wheel is slightly different because the rim does not sit on the ground, so there is no point load on the rim. On the other hand its size makes it much heavier and the lateral loading caused by the wind can be much greater. One of the main problems to be solved was the tendency of the wheel to buckle. The top cables tend to go slack due to the self weight of the wheel and so must be pre-stressed to make sure that they stay taut. The leeward cables would also go slack under wind loads unless they were pre-stressed. All this pre-stressing puts the rim under enormous compression - and compression is what makes structures buckle, unless they are very stiff. We couldn't just add extra metal to make it stiffer, as that would also add weight, which would require more pre-stressing, which would lead to more buckling... Instead we had to analyse how to change the geometry of the rim and the hub to get the highest stiffness, and therefore resistance to buckling, for the lowest weight. The design of the support of the wheel was to some extent inspired by the image of the Skylon tower erected on the South Bank for the 1951 Festival of Britain. It was also influenced by site constraints: we could not interrupt the Queen's Walk beside the river wall, we wanted the wheel to hang over the river and we wanted to touch the ground in Jubilee Gardens as lightly as possible. Inevitably the spindle had to be a huge cantilever. The support structure is an A-frame that also carries the horizontal loads parallel to the river. To stop the A-frame falling over and to hold down the back span of the spindle, cables were needed which are anchored in the ground. Originally these were supported by a back A-frame which allowed them to come down to the ground vertically but later, to save money, the back frame was eliminated. The weight of the wheel is such that there is no danger of it falling back onto land, so no guy cables are needed in the river. We wanted the capsules to be on the outside of the rim, so that the views would not be obstructed by the rim and when you were at the top you would literally feel on top of the world. The best way to do this was to hold the aerodynamically shaped capsules in two rings with bearings that ran around inside them, to keep the floor horizontal as the wheel rotated. We knew that it would be best to keep the wheel moving, to conserve energy and so that the ride would not take too long. So we did studies to show how the passengers could line up in rows and walk forward as the capsule passed slowly by them. This was simulated first on the computer and later with a full size mock-up of the capsule and a group of willing people. The design was eventually taken over by the design & build contractors Hollandia (wheel) and Poma/Sigma (capsules) but as built it retains most of the original intent. The key differences are that the diameter has been reduced to 135m, the number of capsules has been reduced to 32, the support structure has been simplified and the back A-frame eliminated. The erection method chosen by Hollandia also led to changes to the design of the base of the A-frame and at its connection to the spindle. Project team Architect: Marks Barfield Architects; project team: Frank Anatole, Joanna Bailey, Julia Barfield, Margerita Bodman, Peter Brown, Steven Chilton, Malcolm Cook (project director), Natasha Davis, François Girardin, Leigh Jostins, Anna Landucci, Kirsteen Mackay, David Marks, Matthew Morrish, Mariken Slot, Mark Sparrowhawk, Heather Woofter; architectural specifications: John Aherne & Associates; capsule design: Nic Bailey; environmental design: Loren Butt Consultancies; landscape architect: Edward Hutchison, leisure consultant: Ray King; lighting artist: Yann Kersale AIK; checking engineer: Allott & Lomax; structural, civil, marine and mechanical engineers: Atelier One, Beckett Rankine Partnership, Dewhurst Macfarlane & Ptnrs, Infragroep, Ove Arup & Ptnrs, Sigma Plastique, Tony Gee & Ptnrs; construction manager: Mace; client: The London Eye Company (David Marks Julia Barfield/British Airways/The Tussauds Group jv). Selected subcontractors and suppliers Main steelwork: Hollandia, Mercon, Skoda Steel, Mannesman, Tensotecci; steel (rim): Corus; capsules: Poma, Sigma, Semer, Sunglass; capsule roof glazing: Glaverbel; civil engineering: Tilbury Douglas Construction, Chart Engineering, Houlder Offshore Engineering; services: T Clarke, DAL; boarding platform: Littlehampton Welding; fire shutters: LB Securities; maintenance equipment: Latchways; County Hall fit-out: Alandale Construction; landscape: Waterers. Capsules Electrical power slipring collectors: Wampfler; radio transmitters/ receivers, smoke detectors, cctv cameras, pa system, intercom units: Vecsys; programmable logic computers: Schneider; control panels: Semer; door operators: Portalp; levelling system motors, gearboxes, speed controllers: Leroy Somer; levelling system clutches: Stromag; heat pumps: Carrier; fresh air fans: Vent-Axia; batteries: CEAC; lighting units, dimmers: Philips. Critic's Eye - viewing the wheel from different perspectives Gavin Stamp, Daily Mail 'Earth has not anything to show more fair', wrote William Wordsworth in 1807 about the view of London from Westminster Bridge. What on earth would the poet make of that panorama today? For it is no longer dominated by chimneys, steeples and Wren's great dome over St Paul's Cathedral but by a gigantic, 450ft metal wheel. Many seem to disapprove of the London Eye wheel being placed in the heart of the capital, almost opposite the Houses of Parliament on the south bank of the Thames - if only temporarily - because it is an obtrusive structure, intended only for pleasure. I disagree. It seems to me more worthwhile, and much more enjoyable, than the nonsense our government is spending a fortune on to celebrate the millennium down at Greenwich. And why not have a wheel on the horizon? There was a time when the skylines of our cities were dominated by church towers and steeples, as visible symbols of faith. But that faith has retreated and those symbols have been overwhelmed by much less attactive ones which merely celebrate the victory of Mammon. Better a big wheel than another skyscraper. Nor is the symbolism of the wheel inappropriate for the millennium. Revolving slowly, it will suggest the passage of time - at least to those who can still read old-fashioned clocks and do not depend on digital chronometers. After all, such things are not new in London. Exactly a century ago, the pride of the Earls Court Exhibition was Walter B Bassett's Great Wheel, inspired by the giant Ferris wheel that was the main attraction at the 1893 Chicago Exhibition. Bassett's wheel was 300ft in diameter and could carry 1600 people in 40 cars. It was built in 1894-95, but demolished in 1907. The big wheel of that vintage which is still revolving today is, of course, that on the Prater in Vienna, which was again the creation of Mr Bassett and which starred so memorably in the film The Third Man. But although it is not far from the centre of Vienna, it does not compete with the spire of St Stephen's Cathedral or the pinnacles of the town hall. What is remarkable about the London Eye is that it is in the centre of London: an urban landmark and a civic symbol. Click here to order this issue.

Click here for a print version of this page