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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 the NEC and Birmingham City Council to put forward a proposal for Birmingham. Buro Happold assisted Imagination in that bid. Imaginations proposal for content and design ideas was judged the best and they were subsequently asked to consider how they would transfer it to Greenwich. In the first months of 1996, Imagination, with assistance from Buro Happold, 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 and English Partnerships on the master plan for the whole of the gas works site. Their master plan had a circular road pattern at the northern end, which Imagination had incorporated into their exhibition plan.
137 SERVICING THE DOME ENVIRONMENT Tony McLaughlin BSc. C Eng. MCIBSE. MASHRAE. M Inst E. Partner, Buro Happold Consulting Engineers SYNOPSIS The Millennium Dome is a fabric clad structure covering some 80,000m2 which is to house a spectacular exhibition for the duration of the Millennium Year. This paper describes some of the constraints on the environmental engineering design, the servicing strategy adopted, the cooling and electrical loads determined, and how the environmental design evolved to meet the changing development of the exhibit designs. INTRODUCTION - THE MILLENNIUM EXPERIENCE 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 the 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 judged the best and they were subsequently asked to consider how they would transfer it to Greenwich. In the first months of 1996, Imagination, with assistance from Buro Happold, 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 and English Partnerships on the master plan for the whole of the gas works site. Their master plan 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 leading to 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 May 1996, faced with time running out, Gary Withers of Imagination and Mike Davies of the architects Richard Rogers Partnership suggested 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-net structure supported by 12 masts. This concept was welcomed by the client and engineering work got underway. THE SITE CONDITIONS Greenwich peninsula is an exposed site with the river Thames on "three sides "of the Dome site leaving it vulnerable to the winter winds from the east. In the depth of winter it can be an inhospitable place when the wind is in the wrong direction. Mike Davies reminded us many times that to the east there is no ground over 100m between Greenwich and the steppes of Moscow. But like most southerly UK sites the met office offers the following synopsis, the prevailing wind is south westerly, the coldest month is January with a mean monthly temperature of 4oC and July is the warmest month with a mean monthly temperature of 17.5oC. As a matter of interest a temperature of 37.8oC was recorded at Greenwich in 1911. Greenwich is 7m above sea level. ENVIRONMENTAL ENGINEERING THE 'UMBRELLA' CONCEPT The concept of developing an "umbrella" environment is nothing new, as many of the mainline rail stations demonstrate. What makes the Millennium Dome different is its physical scale and its intended purpose. The dimensions of the Dome are huge: 320m in diameter, 50m high in the centre, with an enclosing wall structure 10m high and 1km long. The contained volume is approximately 2.1 million cubic meters, which leads to a many well published and interesting statistic eg the weight of air inside the Dome is actually greater than that of the structure that encloses it, not to mention the fact that it could contain 3.8billion pints of beer It is twice the size of Atlanta's Georgia Dome, previously the largest tensile-roofed structure in the world. 138 •mm U Fig 1A The Environmental Concept 4 320m w> Fig IB The Problem of Scale Solar Reduction The Creation of a Meso Environment under the Dome roof from which other structures (core & exhibition buildings) can spawn. FiglC design and passive control systems: mechanbal control systems: • protection from solar radiation in hot weather • fresh air ventilation • protection from precipitation in wet weather • air movement • natural ventilation in hot weather • heating in winter • wind protection in cold weather • comfort cooling in the core • a smoothing of temperature or humidity accommodation and exhibits Fig ID When we first started work on the building services and environmental systems, it is fair to say that the Clients brief was somewhat lacking, both in terms of what was likely to happen inside the Dome, and even more so, what its form and operation was likely to be. The driving issue was time. The only design guidance we had, was to draw on our own previous experience and look at precedents. The idea was to provide a services back bone which would give the desired flexibility for an exhibition theme which was still very much in the melting pot. Naturally this flexibility would also have to have the capacity for the likely energy demands of the future exhibitions, all of which would be unique. At the time, Imagination were leading the team, and it was with their extensive knowledge of past and present exhibitions, as well as a lot of research into utility loads for existing exhibitions that we established the following energy demands: Power supply 35MW Cooling demand 18MW Heating demand 2.5MW for the Dome air intake systems. How these were delivered is addressed later in the paper. ENERGY BALANCE Initially, one of the design team's primary concerns was the environmental implication of putting such high heat loads together with 35,000 visitors under a transparent roof. What were the environmental conditions likely to be experienced by the visitors? What would visitors expect? Would conditions be acceptable? The roof is a double skin fabric, which allows 12% light transmission and has a shading coefficient of 0.08. Externally, the fabric is highly reflective (white), whilst internally the fabric is a white "matt" finish. Initially, the architects and cost consultants preference was for a single skin structure but this had to be rejected environmentally, due to the need for increased solar protection of the double layer and, just as important, the need to provide some thermal insulative properties for the winter conditions. The convincing argument was the much-reduced risk of condensation. The inner fabric liner - which is not structurally taut as the outer layer - also assists in "softening" the enclosure's acoustic characteristics. The following energy balance diagram was first used to illustrate the problem and was the first simple step in our environmental analysis of the Domes environment. One of the underlying design objectives for the "umbrella" environment was that it should use the niinimum amount of energy to provide the transient environment within which the exhibition would operate. Another advantage of the "umbrella" is that it allowed the construction of the exhibition and core buildings under cover, free from the extremes of the British climate. The downside of this latter point was that construction dust etc was trapped which eventually stained the inner liner. Fig 2 Energy balance diagram CONDENSATION The thermally lightweight, low insulation, high occupancy characteristics of the Dome did give the design team some concerns regarding the risk of condensation. The arguments for the inner liner were almost entirely based upon environmental issues of which condensation was one. The others being solar protection, heat insulation in winter and acoustic absorption. Buro Happold's TAS analysis of the volume indicted that with the installed mechanical ventilation systems operating it was possible to limit the build up of condensation. We looked at a number of operating scenarios which indicated to us that the greatest risk of condensation on the internal skin was late in the evening on a cold winters day with a reasonable attendance. We calculated that the build up of moisture to be in the order of 30g/m2, this equates to a very thin wetted surface less than 1mm thick. Condensation dropping is therefor unlikely. To validate our work we commissioned "The Centre for Research in the Built Environment" at Cardiff University to carry out an independent check. This study made the following observations; • Under the design conditions assumed for ventilation, occupancy and internal gains, there is a low risk of surface condensation on the inner surface of the roof. Whilst some condensation is likely it should not be sufficient to cause drips and will quickly evaporate as conditions improve. • Condensation risk in the Dome is however very sensitive to the ventilation of the space. If ventilation rates do not reach those assumed, particularly in winter, there is a very high risk of severe condensation on the inner skin. As occupancy moisture builds up. The simulations described in this report indicate that ventilation rates should be greater than 0. 3 air changes per hour. Thus natural ventilation alone may not be sufficient. • Condensation risk may be significantly reduced by continuous overnight ventilation, even in winter. • Condensation risk may be reduced by reduced by increasing the inner surface temperature. • Condensation risk will be increased by introducing internal water features, planting and other water vapour producing processes. • There is also the risk of condensation on the inner skin of the outer surface this could lead to staining or mould growth. VENTILATION STRATEGIES Initial thoughts were for a totally naturally ventilated building, similar to the railway station environment referred to above, but this alone would never be sufficient because of the scale and usage of the enclosure. MM This picture was taken from Building Services Journal April 1899 Fig 3 Ventilalion systems cross section. 140 Natural ventilation could not penetrate the depth of the dome (particularly so in summer) and entering fresh air would tend to rise a short distance in from the perimeter as it picked up the internal heat gains. Secondly the flow restrictions imposed by the large scale perimeter exhibition buildings would prevent air reaching the centre. Next, the team considered the use of underground air ducts supplying a large displacement system. However, the desire to reduce the amount of ground excavation to the absolute minimum due to the costs of excavation in contaminated ground, made this uneconomic. Further, such a major displacement system imposed on the plan at such an early stage of the design process could impede the future placement of the exhibition structures, so this solution was also rejected. The adopted ventilation strategy relies upon the perimeter zone being naturally ventilated via open doors, a permanently open strip at the top of the perimeter wall and the natural leakage of the structure itself. To move air into the centre of the Dome, two 25m 3 /s air handling units are located in each of the six core buildings providing 300m 3 /s in total. These air systems have a modicum of heating, equivalent to the Building Regulations uninsulated structure, which allows 25W/m 2 of heating input. This input is just sufficient to take the chill off the incoming air. The same applies in summer when again, a modicum of cooling is added primarily to assist the air in dropping into the occupied central zones. There is no attempt made to control the Dome environment as a whole. It will be a few degrees warmer than the external environment in both winter and summer. The same large air handling units have variable speed drives and can operate in full re-circulation mode. The full re-circulation option is used during shut down" and rehearsal hours during the winter months. The following diagrams illustrate the applied layers of ventilation. DESIGN VERIFICATION TWO DIMENSIONAL MODEL To verify the team's proposal, a three dimensional 360° CFD model was developed with AEA Technology in Didcot, Oxfordshire, acting as a sub-consultant to Buro Happold. The model went through a number of refinements as the information on the exhibition structures began to filter through from the exhibition design teams. As it stands today, the model has been generated by 700,000 cells, takes 450 MB of memory, and to run one scenario on AEA's most powerful machine takes approximately four days. Outputs are air speed, air temperature, resultant temperature and a "Comfort Index". As stated earlier, the question posed by our client was what would the internal environment be like and to what is it comparable. Buro Happold set about trying to establish comfort criteria for the space. Fangers or Bedfords comfort criteria were not considered appropriate as they related primarily to "static" office environments. Instead criteria established by United States Department of Transportation called the "Relative Warmth Index" (RWI) was adopted. This was developed for subways and train stations, so it was felt to be the most relevant and appropriate for the Dome environment. RWI = M(Icw +Ia) + 1.13(t - 95) +RIa 74.2 where: M = metabolic rate lew = insulation effect of clothing, clo. Ia = insulation effect of air boundary, clo. t = dry bulb temperature t-95 = difference between dry bulb and average skin temperature. R = mean incident radiant temperature from surrounding surfaces. The above are all imperial units. Explaining our results and "comfort" to our lay client was not an easy task, so the following diagrams were used to illustrate our results. 141 Fig 4 CFD plots with comfort criteria. Its worth noting, that as the model became more accurate, and our Client became more knowledgeable, we reverted back to simply stating Resultant temperatures as our measure of the Domes environment. The following diagrams illustrate some of the CFD model outputs. Hot Summer Day Fig 5a,b & c CFD plots Ventilation tests to date on the installed systems (Andrew Cripps paper) 142 3010 302.5 304.0 305.5 307.0 Temperature (K) ENERGY DEMANDS PREDICTING THE LOAD Formulating the Dome's energy demand twelve months in advance of knowing what was going to happen within the building, came to down to guesswork, albeit educated guesswork. We talked to a number of organisations who had done "something" like it before, even if not on the same scale, we did a lot of reading and research into major exhibitions throughout the world, including asking the Disney Corporation, whose advise was particularly comforting - "Get your exhibition designs first before establishing your energy loads". The result is we have an all-electric building, this decision taken against a brief for a temporary exhibition (and at the time of the decision, also a temporary ^ building). Other energy supply methods were reviewed, including Combined Heat and Power (CHP). The CHP scheme was to be part of the total Greenwich Peninsula development, initially be used to serve the Dome (this being first load on-line) before commercial and domestic loads came on line in the future. Time, cost and lack of funding saw this proposal stranded. Gas heating was excluded because of programme and the cost of reinforcing the mains for the given demand. At the Dome, gas is only used for catering. At first sight the use of electricity as the Dome's sole energy source is questionable, but when viewed against the Clients programme, costs, the temporary exhibition brief and the post exhibition legacy (an electrical infrastructure in place for the future development of the Peninsula), electricity proved the most attractive option. The following figure gives the current break down of areas: SPATIAL BREAKDOWN OF AREAS WITHIN THE DOME m? Exhibition Area 35,000 Central Show Arena 14,750 Catering 3,900 Circulation 11,350 Retail 1,350 Toilets, plant, support 10,300 COOLING LOAD The 18MW of cooling is split between the following functions as follows: Exhibition Structures 5.5MW Central Arena 3.0MW Baby Dome 1.5MW Core Buildings 2.5MW Dome air supply 1.5MW External Buildings 1.5MW Spare (April 1999) 2.5MW Total 18.0MW ELECTRICAL LOAD The installed capacity of electrical power for the landlords' supplies, at 57.5MVA, is only slightly higher than our original estimate. In providing this demand, the team set about standardising the size of transformers, the set up being: • 2x 1.25MVA transformers for each of the six internal core buildings. • lxl.25MVA transformer per exhibition • 4x 1.25MVA for the central area buildings • 4x 2.50MVA for the central show • 9x 1.25MVA for the external buildings and landscape including "Baby Dome" PLANT SELECTION This standardisation of the primary M&E equipment was an early design objective set by the Client and the design team, based on the directive of an exhibition lifetime of two years, and the short design and build times available. This directive was fundamental in the selection of services plant. Tried and tested technology was used, albeit on a very large scale. Standard off-the-shelf plant was selected and positioned around the site. 143 The London International Financial Futures and Options Exchange (LIFFE) was consulted by Buro Happold to evaluate resale values of plant on the futures exchange, and items were selected on this basis. The type of transformer specified, for example, was changed as a result of this input as was the rated output of the packaged air-cooled chillers. Equipment is standardised throughout in order to increase the likelihood of resale and minimise downtime if repairs are required. SERVICING STRATEGY FUTURE FLEXIBILITY At the beginning, it seemed that we were faced with a seemingly impossible task. The site was a barren and formless landscape, with little, if any, infrastructure, and there was increasing public debate as to what to put in it and if it should be built at all. But with an immovable completion date looming, design and construction work on the Millennium Experience had to start. Faced with the uncertainty on the exhibition form and content, the design team had to make some fundamental decisions on how the services should be planned so that the design and construction could progress well in advance of any exhibition designers been appointed. There was much discussion and debate on the servicing strategy and its intended flexibility as it was soon realised this would have a major influence on the exhibition layout and size. A number of scenarios were tested, including services within raised floors or above ground service beams. The adopted solution takes a very pragmatic approach. The dome is split into six equal 'pie' segments, each a mirror image of the other, the core building being the 'heart' of each segment, with plant in a pair of cylinders feeding into each segment. External plant such as the air cooled chillers, HV switchgear, standby generators and water tanks are contained in the twelve prominent service pods, or cylinders, around the perimeter, these operate in pairs to service each of the six segments. All segments have equal capacities although each pair of pods holds slightly different plant. From the cores, the services are distributed into a series of six radial trenches, each six meters wide and 900mm deep, and three circumferential trenches which run under the Dome's ground slab and carry all cable and piped services, including drainage. The radial trenches are generally arranged so an exhibition lies on either side with an access route directly overhead. Two major exhibits are serviced from each, with any excess continuing around the circumferential trenches to pick up any secondary loads. The radial trenches continue into the centre of the Dome to supply the demands of the central arena. As the siting of exhibits and public services was devised during construction, it was necessary to ensure that services would be available throughout the site when required. Despite the uncertainties, the entire M&E services were designed in only nine months and their installation is now complete and commissioning started in March 1999 Fig 6 Services Strategy Fig 7 Early concept diagram for the service pods THE EXTERNAL SERVICE PODS Included in all views of the exhibition since they first entered the public domain, the cylindrical pods surrounding the Dome are now entrenched in the minds of everyone who has seen any photographs or models of the structure. Housed within these aluminium-finned cylinders are all the primary services for the Millennium Experience. Originally intended to be spherical, creating a futuristic space-station look to the structure, they were to hold part of the exhibition. However, space constraints meant that the plant had to be moved outside of the Dome. The team liked the idea of using the now defunct exhibition spheres. However, it became increasingly difficult to fit square plant into a round space, so the spheres are now cylindrical. 144 Fig 8 Architectural image Fig 9 Each pod is split into three levels, plant not requiring weather protection was left open to the elements, the remainder was enclosed in packaged plantrooms assembled off site by GEL and AC Engineering and installed complete. The contents of each pod varies due to the way the required capacities have been apportioned. Where plant is not required, the space is left empty for future expansion of exhibition demands. Two 750kW air cooled chillers are located on the top level of each pod. These are connected in parallel and grouped onto a common header. A chiller system (i.e. a 'pie' segment) consists of four units, the pods working as a pair, giving six systems in total to service the Dome and site. All systems run totally autonomous from each other. On floor two of the pods, one of each pair holds a prefabricated packaged plantroom, which contain close- coupled end-suction chilled water pumps to give a flow of 132 1/s at a head of 200kPa to a common pipe system which distributes around the Dome to null headers. A pressurisation unit and a controls system is also included in this plantroom. In eight pods London Electricity packaged HV switchrooms are sited at this level. On the bottom level, a 92.5m 3 sprinkler water tank is sited in two pods, the volume required for the Category 3 Special system being too large for a single pod. Separate pump rooms have been installed as the tanks are more than 30m apart. A 500kW standby generator has been installed in three pods, each generator servicing two sectors. ACKNOWLEDGEMENTS Client: The New Millennium Experience Company Jennie Page, David Trench, Richard Coffey, Peter English Architect: Richard Rogers Partnership Richard Rogers, Mike Davies, Andrew Morris, Stuart Forbes, Steve Martin, Adrian Williams, Mike Elkan, Laurie Abbott. Construction Manager: MacAlpine Laing Joint Venture Bernard Ainsworth, Gary Nash. Our sub-consultants: Central Area: Cundall Johnston and Partners Ric Carr, Peter O'Halloran, Mike Golding. Lighting Designers: Speirs and Major Jonathan Spiers, Mark Major Lift Consultants: Dunbar + Boardman Peter Boardman, Chris Meering and most of all, to all my numerous colleagues at Buro Happold who have contributed to this project. REFERENCES 1 Constructing the Millennium Dome. Ian Liddell, Lecture to the RA, September 1997 2 The Design and construction of the Millennium Dome. Ian Liddell and Peter Miller The Structural Engineer, 6 April999 3 Servicing the Dome Various, The Building Services Journal, April 1999 4 Subway Environmental Design Handbook Vol 1. U.S Dept. of Transportation