|Department of Engineering|
|University of Cambridge > Engineering Department > News & Features|
The Millennium experience follows in the tradition of the Festival of Britain and before that the Great Exhibition of 1851. In common with these events, it has been fraught with political difficulties causing lack of decision about the contents and, for a long while, uncertainties about whether the show itself would go on. Both these previous events had their critics but the most notable was Colonel Charles de Laet Waldo Sibthorp, MP for Lincoln. He carried on a vituperative campaign against the crystal palace for three years. In one of his best speeches he called the exhibition …a purpose prejudicial to the people in a moral religious and social point of view……. An exhibition of the trumpery and trash of all nations. Well it turned out to be very successful even if the design of the exhibits was pretty trashy.
The idea of holding a celebration for the millennium had been talked about since 1993 and even before. The Greenwich site had always been a possibility but other sites were also under consideration. In the latter part of 1995 the millennium commission invited bids with design proposals for several sites. Imagination Ltd joined with NEC, and Birmingham City Council to put forward a proposal for Birmingham. Buro Happold assisted Imagination in that bid. Imagination's proposal for content and design ideas was considered the best and they were asked to consider how they would transfer it to Greenwich. In the first months of 1996 Imagination with ourselves put forward a number of proposals for housing an exhibition in pavilions with a large arena for shows and displays. Richard Rogers Partnership was at that time working with British Gas on the master plan for the whole of the gas works site. Their masterplan had a circular road pattern at the northern end which Imagination had incorporated into their exhibition plan.
The separate pavilions were four generous storeys high and involved a considerable amount of construction work and there was a difference between the costs of the designs produced by Imagination to meet the brief and the Millennium Commission's budget. The site was very exposed to wind and rain coming off the river and there was a worry about the impact of this on visitor experience in the winter months. Imagination was trying to deal with this by covering the spaces between the pavilions which were arranged around a central show arena.
In the summer of 1996, faced with time running out, Gary Withers of Imagination and Mike Davies of the architects Sir Richard Rogers Partnership suggested extending the design of that time and covering the whole site with a giant umbrella. This would create a protected environment in which exhibition structures could be designed specifically for the exhibitions and be rapidly erected without the necessity for weather tight cladding. We in Buro Happold picked up that idea and suggested a fabric clad stressed cable dome supported by 12 masts. This concept was welcomed by the client and engineering work got underway.
Back to top
Our initial aim was to design the structures for raft or pad foundations which would be founded at or just below the surface. A subsoil investigation was carried out and this revealed that the top 8-10m of fill and silty clay was softer than anticipated and the predicted settlements were of the order of 300mm. This amount of settlement could not be tolerated so the site would have to be piled with the piles bearing on the terrace gravels below the silt or into the London clay below that.
A further complication was the southbound carriage way of the Blackwall tunnel which passed under the proposed area for the building. The additional loading over the tunnel was restricted to 20kN/m2 and any construction within 50m had to be approved by the HA. The piling solution was to use driven cast in place piles into the gravel for most of the site with continuous flight auger piles into the clay for the areas adjacent to the tunnel.
The ventilation shaft for this tunnel was also within the proposed area and this was surrounded by a ring of land which was dedicated to the Highways Agency for access to the tunnel. A hole in the roof about 50m in diameter would be required to allow the ventilation system to function correctly and the whole area would have to be surrounded by an enclosing wall. Both this and the foundations around the tunnel were subject to lengthy negotiations with the Highways agency and external checks before they could be agreed.
Back to top
Back to top
In 1968 Ted Happold met Frei Otto at the prize giving for the Riyadh Conference centre competition and started a life long friendship which led to their collaboration on a number of remarkable projects. The most remarkable of these was probably the Mannheim Multihalle. This was a doubly curved compression structure constructed out of 50mm square timber and spanning 60m.
An other line of development in this field was air supported structures originally invented by FW Lanchester around 1910 but not properly developed until 1948 when improved materials became available and Walter Bird started building radomes for the US military. The research work to enable these radomes to be reliably built was carried out at the Cornell Aeronautical Laboratory in Buffalo, NY. Walter Bird's work led to the air inflated tennis halls and then to the American pavilion at Osaka and from there to stadiums with air supported roofs which had an area of up to 40,000 m2.
Buro Happold has been engaged in developing tensioned fabric and cable technology since the inception of the firm in 1976 and even in the case of some individuals before that. Interest in this particular field was initiated by Professor Sir Edmund (Ted) Happold FEng and was stimulated by his relationship with Frei Otto. Their first fabric structure was a temporary canopy for the opening of the Forties oil field in 1975. At this time fabric structures technology was in its infancy. The form of the surface and the patterns for making up the cloth were generated by measurement of physical models. The tent for the British Genius exhibition was designed in this way. A physical model was built and measurements taken from it and used for the construction. In fact there was no patterning, the fabric was flat cotton canvas which was stretched into the curved surface by distorting the angles of the weave.
However there were ideas around for developing computational methods for developing patterns. In 1977 Ted obtained some funding from the Wolfson Foundation for his department at Bath University to study air supported structures as a generic form of flexible structures. This group had a few post graduate students at the university but in addition Ted brought in a number of other interested people from a range of disciplines. The research studies included investigations into material properties and the internal environment as well as structural behaviour and the performance of actual structures in the field. The group held two symposia under the auspices of the Institution of Structural Engineers at which the results of the research were published.
At this time, in Buro Happold we set ourselves a target of developing a computer program which
would handle both the formfinding of these structures and their load analysis. The program also can provide the fabric patterning and the other geometric information required for fabricating the components. We also picked up some commissions for fabric structures where we were able to apply this technology and ideas for structural forms and detailing. In each of these projects we would try out something new so that we were able gain an understanding of what worked and what didn't. The projects stimulated development of the software and the software allowed the projects to move forward. By 1980 we were able to process the patterns for this large canopy in Baltimore.
In 1980 we were appointed to carry out a feasibility study for a covered city in the arctic, actually in northern Alberta where the tar sands were mined and processed into oil. As part of this study we looked at the response of people living in an enclosed climate moderating space and concluded that in was necessary to have a transparent roof which let in the full spectrum of visible light. For this project we studied a 35 acre air supported roof which was to be clad with ETFE foil. This is a transparent fluoropolymer foil which has long life characteristics and reasonable strength and toughness.
Part of the study for 58oN was the behaviour of air supported structures under snow loads. Some years later I was asked to advise the owners of the Minnesota Metrodome on problems they had had with their air supported stadium roof. These were mostly related to ponding during snow storms leading to collapse of the roof. Even though the roofs were designed for this eventuality, the collapses resulted in unacceptable damage and such roofs became unacceptable to the owners. The ponding problems were largely resolved in the design of the last stadium project with an air supported roof, that at Tokyo by using higher internal pressures. However the higher engineering standards required for this made these structures less competitive and stadium roofs are now constructed with cable structures as at Atlanta and St Petersburg and prior to that at this arena at Seoul. The reason that this roof looks so blotchy is that the fabric is silicone coated glassfibre which picks up a lot of dirt.
A part of this development process is testing out the public acceptance of the serviceability large deflection structures, something that is difficult to predict. With the airhouse example above it was the owners who bought the technology because of the construction cost and they rejected it because of maintenance difficulties.
In Buro Happold we are still developing our computer software, now into the third generation of hardware. we also strive to maintain a group of engineers able to work with these structures. This means that we train engineers both in the UK and for our New York office where we are able to offer the same consultancy service. Another effect has been to train several other engineers in tension structure design who now have left us and set up their own firms.
Back to top
The structural system for the Greenwich dome came from our observation that marquee tents did not conform to these types. They were flat fabric that, whether the load was upwards or downwards produced tensions in the fabric which are in the same direction. This system was developed for an arch supported roof over an arena. The arch supported straight cables which were stiff enough to stabilise the arch against buckling. Because the cables were straight the installation of the fabric could be simplified by sliding it into a groove in an aluminium extrusion. The arch supported roof was not built but two others were. These were the tennis halls at Eastleigh and the large audience tent for RSSB. The latter is of particular significance since many of the structural elements are similar to the dome.
Slide of point load leading to ponding
Slides of down and up loading
Back to top
The concept quickly evolved; the diameter was reduced to 320m, the main masts were moved out and the outer ring of masts was dropped. To keep the tiedown cables clear of the planned internal structures the masts were supported on a pyramidal base 10m high and the outer ring of tiedown cables was eliminated by using external flying struts. This concept was analysed by computer and became the base design in Aug 96. At this point the architects RRP and ourselves were given a commission to prepare a scheme for the project for a planning submission to be made in Jan 97.
During this period several further developments were made. We realised that there was no advantage in combining the cable lines towards the centre and so we changed over to continuous radial lines which started from a central hub. The masts were moved further out and made higher to create a larger space in the centre and the external flying struts were eliminated. To improve the access to the dome at ground level the radial cables were collected by catenary cables to 24 anchorage points.
It also became apparent that raking ground anchors would not be acceptable at all points and an arrangement of vertical anchors with a ring beam to take the horizontal component of the forces was adopted. Tender documentation for this scheme was completed and sent to contractors at the end of Dec 96
The straight cable structure is very efficient as far as strength is concerned, but it is necessary to ensure that ponding under snow or rain will not occur. The dome roof shape with tapering segments has an advantage in that respect in that as the span of the fabric panels increases their slope increases. However circumferential cables through the nodes were required to maintain their spacing. If these cables were in the surface of the fabric they would cause a dam at each circumferential line possibly initiating ponding, so an arrangement was required which would take these cables out of the surface. This was achieved by raising the circumferential cables above the surface with rigid members (wishbones) and connecting them to the nodes with criss-cross cables. Lower circumferential cables were also required to control the tiedown cables; these were also spaced off the surface but with out the criss-cross cables.
It was also necessary to control the deflection of the radial cables. Their length is very long, 150m from the perimeter to the centre. Because of this if one 25m span were loaded the remainder of the cable in the line would act as springs so the loaded span would not be as stiff as if it was fixed at each end. The only way to gain the necessary stiffness is to use a high pretension, in fact the planned pretension in each radial line is 400kN, about 2/3 of the peak tension. The last element in preventing ponding is the patterning and prestress in the fabric panels.
During the tender period some development of the design continued. There was a problem with the height of the masts in relation to the nearby city airport and an option for lowered masts was investigated but in the end it was decided to stick with the 100m masts. We were also concerned about the safety of the central node which had a single steel tension ring carrying a load of 7000N. If this ring were to fracture the whole roof would come apart and collapse. There was no way that an alternative load path could be included in the design. We decided to change the central node for a 30m diameter cable ring. This was constructed with 12 - 48mm dia. Cables. Because of the redundancy implicit in the 12 cables, failure of one of these cables would not compromise the overall safety of the roof. These changes were brought in to the contract package before the contract was finally placed.
Back to top
Mast head details and the mast penetration through the fabric.
Mast springing details ; A rubber bearing is placed between the mast bottom and the pyramid.
Back to top
The wind loads were initially derived from published data. Subsequently they were confirmed by wind tunnel testing carried out at the BMT tunnel in Teddington. generally the results from these tests were lower.
Ground snow loading was derived from statistical analysis of snow fall data from the nearest stations. The roof accumulation factors were taken from the snow loading code as well as other published references concerning snow drifting on large roofs.
The results of the analysis gave peak loads on the components which were sized according to normal design rules. We were particularly concerned with the design of the 90m high masts which have to resist wind and icing loads as well as have sufficient resistance to buckling.
Resistance of the whole structure to accidental damage is provided by redundancy, i.e. the structure can tolerate the loss of an individual component without collapse. This principle also applies to the support pyramids which are designed to withstand the removal of a leg.
At the request of the client the structure was subject to a design review by independent engineers equivalent to a category three check for a highway bridge. In doing this the engineers, Flint and Neil carried out their own assessment of the loads and independent calculations to find the internal forces.
Back to top
It is difficult to have a translucent fabric roof with insulation but with out any insulation condensation will occur on the underside which, in certain conditions, will fall as rain. This situation would be totally unacceptable in a building that will have a lot of electrical displays. To reduce this risk a lining can be installed under the main fabric. There has been a considerable amount of experience with fabric roofs with linings where condensation has not been a problem. Checks were run on the risk of condensation as part of the environmental modelling and they demonstrated that with two membranes the risk of condensation on the underside was very low.
The available fabrics for cladding the dome are PTFE coated glass fibre or PVC coated polyester fibre. The material must have properties of durability and flame resistance. These are provided by PTFE/glass without the need for any additives. The glass fibres are not affected by UV light but they are damaged by water. The function of the PTFE coating is to protect the fibres from water and abrasion, the PTFE itself is completely inert and not affected by the weather.
With PVC/polyester the fibres are damaged by UV light and they burn so the function of the coating is to protect the fibres from UV light as well as providing the flame proofing. The PVC itself is light stable and does not burn well but it requires a number of other compounds such as pigments, UV stabilisers, plasticisers, fungicides and flame retardants to meet the functional requirements. Since 1987 several of these compounds especially fungicides and heavy metal stabilisers have been banned and this has led to an increase in problems of fungal growth in the yarns which severely discolours the cloth. This situation has recently been improved by the use of anti-wicking treatments to the yarns. The other big problem with PVC coatings has been dirt retention. The PVC coating is porous and the plasticisers absorb dirt. This has recently been improved by the use of fluoropolymer surface lacquers which give it a durable sealed surface. After a investigating the products of the three best coaters in Europe an outer fabric was selected which gave 15% translucency and a lining lining fabric which gave 75%. The combination gave the highest translucency, about 12%, and a good colour rendering.
Other materials such as cotton canvas silicone coated glass and a polyolefin blend thermoplastic on polyester were investigated but it was found that none of these met the requirements for the project.
During this period the use of PVC for the dome became the focus of a world-wide campaign against PVC being waged by Greenpeace. Their claims concerning the release and effects of dioxins and the effects of pthalates in the plasticisers was investigated carefully by the design team who could find little scientific support for them. The design team also examined the alternative materials suggested by Greenpeace only to find that none of them was suitable for the dome
The decision to use PVC coated polyester was taken in late April 1997 when the Dome was expected to have a short design life. However the new government made it clear in their review of the project that all options should be kept open regarding the long term use of the Dome. This completely changed the basis on which the fabric was chosen and a subsequent review concluded that PTFE coated glass fibre was a more appropriate material. It will have a far longer life and avoids the dirt retention and discolouration problems of PVC so it will stay looking clean.
The change to PTFE/glass does not affect the steel and cable structure but the fabric connection details will have to be changed. The material will give a lower translucency and poorer colour rendering than the selected PVC options.
Back to top
The comfort conditions within the dome were modelled by two methods, thermal analysis on a two dimensional section and computational fluid dynamics analysis (CFD) on a three dimensional model. In the CFD analysis the air flows as well as the heat transfer flows are modelled. The client was very concerned that the visitors would not be too hot both in winter when they would be wearing outdoor clothing and in summer. The results were presented as pictorial diagrams related to the relative warmth index.
Back to top
The first of these was the roof steelwork which included the cables and their erection. At the time that the roof went out to tender the CM had not been appointed. BH with a member of the clients organisation prepared the tender documentation and set the programme dates. The tenders were sent out in December 96 after 3 months design work and were received back in February 97. In this period the selection process for the CM was taking place and McAlpine/Laing Joint Venture had been selected by the time the tenders were received. MLJV were then able to participate in the tender negotiations and appointment of the selected steelwork contractor, Watsons. There followed a period in which the alterations to the tendered design and elements of value engineering were brought into the contract.
Back to top
The cable work was subcontracted by Watsons to Bridons of Doncaster. The cables have to be wound from wires which have been previously drawn and galvanised. For the dome project class A galvanising, the lightest, was specified for cables which were beneath the roof and Galfan, a mixture of aluminium and zinc galvanising which is much more durable, for the external cables. The cable has to be prestretched to eliminate the construction stretch and then marked to the correct lengths under the specified prestress load. This is done with the cable in long lengths in a special prestressing bed. Each cable is then cut and the terminations swaged on. A sample number are tested as part of the quality control procedures. Most of the cables are dead length without any provision for adjustment consequently great care has to be taken to ensure that the cables are made up to exactly the right lengths. The cable lengths are calculated from the stressed lengths from the formfinding model with corrections for the details at each end. These are all scheduled out by Watsons and a procedure for eliminating human error has to be followed.
A critical part of this stage was planning the erection since the provisions for this would affect the details. BH had included a method statement for the erection in the tender documentation which allowed for lifting the masts with the suspension cables attached and lifting the stringer cables to them. Watsons preferred to assemble the cable net on the ground and then hoist it with the hangar cables to the mast tops. This required modifications to the mast tops as well as the details at the connection of the wishbones to the cable nodes.
The lifting of the masts has been planned by Watsons with great care. This involved selecting a suitable crane and devising lifting positions which would not overstress the masts. Each mast will be lifted and guyed with the two permanent backstays and two temporary forestays. There is also an intermediate position while the crane was released when only one forestay could be used and a short term guy will be added from the centre of the mast to the adjacent base. While the mast is guyed with the temporary forestays the central ring is lifted by the permanent forestays.
Following the lifting of the ring, the guy system has to be moved to inside the ring so that the rest of the cable net can be assembled and lifted to its place. The stability of the mast with the two guy positions and the crane derigging position was checked using Tensyl. This took into account the sag and stretch of the cables and therefore predicted the true stability. During derigging of the crane and the operations of changing the guy positions the tensions in the guys have to be carefully controlled to maintain the stability of the mast.
Back to top
Subsequently the client made a decision to use the better quality, longer lasting PTFE/glass fabric. The contractor who had made the best offer for this material was Birdair from Buffalo NY. They have been producing structures in PTFE for over 20 years including some 12 covered stadiums of approximately half the area of the dome.
The fabric patterning and attachment details will all have to be modified to accommodate this alternative material and since time has been lost in the programme this has to be done in an even tighter time scale.
Back to top
An alternative approach was adopted. The exhibits themselves may be in enclosed spaces within the dome. There is also planned to be a central performance arena which is required to be blacked out and to have some degree of acoustical separation. These areas will require to be serviced with electric power, water and drains, chilled water for cooling, communications, and possibly sprinkler water. A number of ways of providing these services were discussed with the architects and the client and the conclusion was that there should be six service cores uniformly distributed around the perimeter. These cores would contain the primary services and would also be the location for the main toilets and restaurants. The services would comprise the following:
Water storage and pumped supply
Chilled water distribution from chillers located in external plant rooms
Air handling plant to provide fresh air to the internal street
Deliveries and rubbish compaction points
Services distribution to the potential exhibit locations will be via shallow trenches in the ground running radially to the drum and circumferentially to the exhibition areas. The construction of these and the ground finish are complicated by the requirement to have a gas impermeable membrane below the surface. To allow for secondary cable routes to minor exhibits it was decided to adopt a paving slab system wherever possible. These would be bedded on sand above the membrane so that they could be lifted and cables run beneath where required. Where there was to be a first floor structure there would have to be a reinforced concrete slab to distribute the column loads on the grid of piles below. Such structures would be programmed to be constructed before the ground finish is completed.
Back to top
The fire engineering approach is required to ensure that the open spaces under the dome can be considered as similar to outside as possible thus enabling escape times to be extended. This requires smoke management systems to be in place to ensure that the conditions in the dome remain satisfactory.
The dome is considered to be similar to a small town. In order to prevent the spread of fire between buildings and exhibition areas a number of design and management constraints will be imposed. These will include physical separation in the same way that buildings in the open would be separated. Fire brigade vehicles will be able to gain access the buildings in the dome. There will also be a pressurised fire main with hydrants sited at suitable locations
The smoke management systems start with controlling the size of fires by the use of sprinklers in areas of high fire load or extra risk. Within the dome the ventilation system is used to clear smoke as well as provide fresh air. This consists of extract fans in the masts and opening louvres at high level. There are also fresh air inlets into the dome via openings in the perimeter wall and mechanical ventilation into the internal street. The effects of this are being modelled using the same CFD model that is used for the environmental modelling.
The means of escape are secured by having clear definition of adequately sized escape routes using signage and lighting. There will also be crowd management systems using voice alarms and the public address as well as sufficient numbers of suitable trained stewards. Crowd flows are also being modelled using a program developed at Greenwich University called "Exodus". My feeling is that it will be difficult to persuade visitors to leave the dome. In the event of a fire I think that they will all stand around and watch.
Back to top
I hope I have managed to give an overview of the engineering design and construction process. Engineering is an exacting profession which demands a wide range of skills. We have to invent structures and systems, often innovatory, to meet particular requirements for a project. We have to Produce calculations, working drawings and specifications. We have to work with large teams of designers, managers contractors, other engineers etc We have to get the designs approved by a checking authority and during construction we have to check that it is built correctly and all this has to be carried out within a very tight time scale. This range of engineering work applies to all large buildings, this one happens to have a higher profile than most
The project is going very well on site but we still have an awful lot to do. However we have a great team and the morale is excellent. I would like to acknowledge a few of the key players.
Client, The New Millennium Experience Co Ltd, Jennie Page, David Trench, Peter English, Andy Smith
Architects, Richard Rogers Architects Ltd, Lord Rogers, Mike Davies, Andrew Morris
Engineers, Buro Happold, Roger Webster, Tony Mclaughlin, Glyn Trippick, Paul Westbury
Construction Managers, McAlpine/Laing Joint Venture, Bernard Ainsworth, Colin Holdsworth
And many others who are all making a terrific contribution.
Watsons Joe Lock Peter Miller
Back to top
The lecture will feature the design and construction process for the cable structure and will cover the development of the technology of these structures over the past 20 years through a combination of research and experience with actual structures. Covering such a large area creates its own internal environment which has to be controlled to avoid uncomfortable temperatures. There are also lighting and acoustic considerations to the selection of the cladding materials. In the lecture he will also describe the strategy for dealing with the foundations and ground treatment and for servicing the exhibition areas themselves which have yet to be designed. There are also unusual fire and crowd safety aspects which have been addressed using fire engineering techniques.
The buildings are being procured through construction management, a process in which separate trade contracts are let with the client and managed by the construction manager. The lecture will be given at the time when the foundations are under construction and the roof steelwork will be being assembled on site in preparation for the masts being lifted by a 1000t crane.
Back to top
|| Search | CUED | Cambridge University ||
© Department of Engineering at the University of Cambridge
Information provided by web-editor