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The configuration of the new Stadium was primarily developed through the brief complied for the competition, and expanded to suit the requirements for expected use. The new Stadium is to be capable of hosting events similar to that of the existing. Together with the primary sporting events of football and rugby, major athletics events were to be capable of being held. Recent national publicity on the topic of athletics at Wembley has shown this to be a very emotive subject, one that thankfully is not for this paper. Other events that complete the portfolio are concerts, pageants and exhibitions. The brief called for a world class Stadium which would continue to uphold the traditions of the existing Stadium. A full bowl arrangement for the terracing is utilised as the whole of the stadium was being reconstructed. This provides the opportunity to ensure all the are spectators are wrapped around the action creating an intimate environment, full of atmosphere. With this arrangement, the Wembley Roar would continue A spectator capacity of 90,000 was eventually chosen. During the development of the business case for the operation of the stadium a 200 bed hotel, interactive museum, 100,000 ft2 of offices a 5,000 seat (the largest in London) banqueting suite and full spectator hospitality and corporate facilities are to be provided.
241 DEVELOPMENT OF THE NEW WEMBLEY STADIUM ROOF Michael J Barker Director Mott MacDonald Ltd ABSTRACT This paper describes the current development of the new Wembley Stadium Roof. The final design has not yet been completed at the time of writing, however the principles of the scheme are established. Once constructed, this roof will be one of the largest in the world. The paper describes the background to the project and the reasons behind the evolution of the current scheme for this large area, long span roof. BACKGROUND The New Wembley Stadium was conceived from the sense that whilst the current stadium could boast a glorious past it had little, if any future. As the centrepiece of the 1924 British Empire Exhibition, it has staged many celebrated football matches, since the famous 1923 "White Horse" FA Cup Final. The most notable football match ever staged however, was the 1966 World Cup Final. The stadium was also the venue for the track and field events during the 1948 Olympics. It has also become the traditional home of the Rugby League Challenge Cup Final. By default therefore it has become the National Stadium. All agree that the current stadium has clearly passed the point at which it can usefully continue to serve the as the National Stadium. Faced with this position and the prospect of a number of applications for major new lottery funded stadia, Sport England established with the Football Association, (FA) the Football League, The FA Premier league, the Rugby Football Union and the British Athletic Federation, a competition for the development of a New National Stadium. In July 1995 bids were received with Wembley emerging as the preferred location. It was intended that the owners of the Stadium, Wembley pic. would take this forward with a body especially established for the purpose, The English National Stadium Trust. The Trust would lease the site from Wembley pic and be responsible for the design, construction and finance for the new stadium. This arrangement ultimately could not be made to work, and in the face of bids from private competitors such as Arsenal FC, the lottery grant assigned to the project would be used to purchase the existing stadium and it's business. The finance would be raised by a development company, Wembley National Stadium Ltd (WNSL) a wholly owned subsidiary of the FA. This finance would be secured on the basis that the FA would continue to hold it's flagship events at the new stadium. After negotiations with Wembley pic the existing Stadium and business was purchased on 15 March 1999. THE NEW STADIUM The configuration of the new Stadium was primarily ^developed through the brief complied for the competition, and expanded to suit the requirements for expected use. The new Stadium is to be capable of hosting events similar to that of the existing. Together with the primary sporting events of football and rugby, major athletics events were to be capable of being held. Recent national publicity on the topic of athletics at Wembley has shown this to be a very emotive subject, one that thankfully is not for this paper. Other events that complete the portfolio are concerts, pageants and exhibitions. The brief called for a "world class" Stadium which would continue to uphold the traditions of the existing Stadium. A full "bowl" arrangement for the terracing is utilised as the whole of the stadium was being reconstructed. This provides the opportunity to ensure all the are spectators are wrapped around the action creating an intimate environment, full of atmosphere. With this arrangement, the Wembley "Roar" would continue! A spectator capacity of 90,000 was eventually chosen. During the development of the business case for the operation of the stadium a 200 bed hotel, interactive museum, 100,000 ft2 of offices a 5,000 seat (the largest in London) banqueting suite and full spectator hospitality and corporate facilities are to be provided. These facilities could not all be fitted into the concourse areas under the spectator bowl structure and are housed mainly on the North side of the building, facing Olympic Way. The roof of the Stadium is designed to cover in one sweep the both the spectators and the other facilities housed in the building. This has resulted in a very large roof, one of the largest in the world, containing long clear internal spans. 242 The roof of a stadium is a very highly visible structure and which dominates all views of the building, both internally and externally. It therefore demands special consideration from the design team. The existing stadium's roof boasted the "Twin Towers". This very emotive and powerful, national icon was the subject of much debate. To keep the Towers or not? It was not a viable option to retain the Towers in their original positions. The new Stadium needed to be moved to the North to create a piazza around the whole of the building. It is possible to relocate the Towers, however it was felt that a new image should be provided for the Stadium. The roof for the new Stadium was the subject of a very intense and detailed option study. It was necessary to provide a column free space within the spectator bowl and to cover the additional facilities housed in the North of the building. The brief required all the spectators to be covered. There was not however a requirement to have a completely retractable roof, as the primary sporting events for the Stadium had to be held in the open air. However, could the capability for a closing roof be excluded and the roof not "future proofed"? It would be very difficult and prohibitively expensive to try and retro fit a fully closing roof once the Stadium was constructed. Future proofing is discussed later in this paper. The configuration of the spectator bowl adopted with it's much shorter sight distances and a roof covering all the spectators also generates problems of shadow lines on the pitch and lack of air movement over the playing surface. Both of the above cause problems with grass growth. Healthy grass needs both direct sunlight and fresh air. The current Stadium configuration with it's low set back roof, running track between the spectators and the pitch, and the shallow seating tiers allows easy air movement over the pitch. Even at 4.45pm in mid May (FA Cup final) there is only a small portion of the pitch covering the South Western corner flag in shadow. Apart from pitch health, this is very important to the television companies as their cameras cannot cope very easily with moving in and out of shadow. Given this existing condition it is imperative that the new configuration gives a similar performance for both pitch health and shadow lines. The solution for the roof needs to be able to address these issues satisfactorily. SCHEME DEVELOPMENT The starting position for the scheme was that there should be no columns in the spectator bowl. The perimeter of the roof therefore needed to be supported at the high back edge of the bowl. The saddle shape of the bowl edge reflects the capacity requirements together with maintaining the required standards for the pitch and scoreboard sightlines. The edge of the roof will be supported on the perimeter truss. This element of structure is formed from the extension of the bowl primary raking beams triangulated with V props to form a continuous perimeter truss on which the roof edge elegantly rests. It was immediately apparent that the North side of the building would be the dominant area and would need to contain the major supporting elements of the roof. The roof structure could then align primarily North South. If a line of support could be gained at the internal leading edge of the North Roof, main trusses could be utilised to span from this edge to the Southern edge of the bowl. Further advantages to the overall solution could be gained by following this concept. As discussed, the problems of shadow lines and air movement over the pitch needed to be addressed. These problems could be largely eliminated if the roof along the Southern side of the Stadium could be retracted back to allow sunlight onto the pitch. They could be closed before or during a match if the weather deteriorated. The main North South trusses could support retractable roof panels running along the truss top chord. The requirement to cover all spectators would still be achievable, albeit only during inclement weather. Fig 1 Schematic Section 243 To enable this option to work, a major supporting element needed to be introduced which could provide support to the internal leading edge of the North roof and could be supported outside the footprint of the building, ensuring still a column free spectator bowl. The initial public scheme utilised 4 masts at the front (North) of the stadium with twinned forestay cables attached to internal edge of the roof. Twin backspans cables were anchored back to foundation blocks to the North of the masts. This scheme worked well as a structure, however it was felt that the intrusion of the masts directly in front of the building, together with their cables, and the same mast type solution with other similar structures would not be special or unique enough for the new National Stadium. Subsequently, a solution that found favour with all was produced. This utilised a massive arch, which was positioned over the Northern roof and spanned the whole building East to West. This solution provided an efficient and elegant solution to both problems of roof support whilst giving the necessary icon to the Stadium. A series of forestay cables is attached to the arch supporting the internal leading edge of the Northern roof, and backstay cables anchor to the edge of the bowl, neatly eliminating all the externally anchored cables of the mast solution. Refer to Figure 1. The arch quickly became the accepted image of the new Stadium, replacing the existing Twin Towers as the icon for the new National Stadium. Once basic agreement on the arch solution was reached working models of the Stadium were constructed, Figure 2. A full wind tunnel test model was prepared tested to confirm existing design data, determine accurate wind loads and highlight and quantify any special effects on the roof and arch for this configuration. THE ARCH The arch takes the form of a 7m dia. 138m high, 315m span open "basket weave" unclad lattice structure. It is formed of 457 dia CHS longitudinal chords with diaphragms at approximately 20m centres. Alternate diaphragms are primary and support the stays. Steel grades are S355 JO or J2 to BS EN 10025. Rolled Hollow Sections S355 J2H to BS EN 10210. Protection is 400 dft micron epxoy primer / buildcoat and a 75dft micron finish coat, over a blast clean surface to Sa 2.5 of BS 7079, giving the period to first maintenance of 30 years. Access to the arch will need to be undertaken for the following reasons: • Structural Inspection • Lighting maintenance / replacement • Repainting (30 year interval) • Festivity / celebration (eg pyrotechnics) • Dressing the arch with flags or banners. It is anticipated that maintenance will be through the centre of the arch. For the more thorough maintenance tasks a pre fabricated platform that is launched from the North Roof would be winched up under the section requiring attention, providing a safe working platform for the maintenance crew. The position of the arch and it's inclination have been the subject of an intensive iterative analysis process. This to try and ensure that under all load combinations the arch acts as far as possible in it's most efficient state, direct compression. The treatment of the springing points of the arch will be important as it is expected that these points will be used by the visiting spectators as favourite spots for Fig 2 Architectural Model 244 photographs. In the design of the knuckle springing points, there is also the security and safety issues to be considered from people potentially being able to scale the arch. Figure 3 below. Fig 3 Arch Support Detail The stays are spiral strand galvanised wires grade 1570. Cables. Cable interior and exterior corrosion protected. First inspection within 5 years of initial coating and a major inspection after 15 years from initial coating. Touch up will be required at the time of the initial inspection, with complete re coating a likely option at the time of the major inspection. Under the approved' maintenance regime the cables have a guaranteed life of 60 years. Generally the forestays cables range between 110m and 135 mm dia. The backstays between 55 and 95 mm dia. There are 8 support points provided along the North roof leading edge. The first 2 supports at the East and West ends are primary picking up the main North South T3 and T4 trusses. These trusses span to the South edge of the bowl and are the main elements which support the whole of the Southern roof area. THE ROOF PLATE The roof plate main structure runs North South. Refer to figure 4 for the member references. The surface is profiled to fall from the North South centreline away to the East and West. The roofing material is to be a mixture of standing seam aluminium (eg Kalzip) and 30% translucent polycarbonate sheeting (eg Lexan). The polycarbonate sheeting is introduced to allow diffused light through the roof towards the leading edges. This provides a light gradient of open to fully solid roof which improves hard shadow lines. Soffit treatment (lining) will be provided in certain areas to hide walkways and services. This lining will also have acoustic benefits during concerts. Further to this there will also be areas treated with specific acoustic lining panels to both absorb sound and modify the general acoustic properties of the spectator bowl. 350 mm cold rolled nested purlins at 3m centres running East West, support the roofing spanning between the main roof trusses. The centre 10 bays are at 13.5m reducing to 10.5m for the end 6 bays. On the North roof, the primary fink type trusses are 6m deep spanning up to 75m. These are supported from the arch stays and the Northern edge of the bowl. The top chord is a box section generally 750 x 500 mm deep with a cable bottom chords and CHS V struts at third points. A longitudinal truss Til is provided to support the alternate secondary North South trusses and to provide in plane rigidity to the roof plate when considering assymetric and dynamic loading. The main T3 and T4 trusses span to the Southern edge of the bowl. T3 spans 155m and T4 129m. These trusses, together with the edge T5 trusses support the main Southern roof and carry the rails for the moving roof panels. Lateral stability of these main trusses is provided by a series of horizontal cable ties. At the leading edge of the Southern roof truss T13 is located. This spans 135 m between the T3 trusses to support the central section of the Southern roof. As the truss is curved in plan, diagonal cable ties are introduced back into the roof plate to counteract the bottom chord kick out. The main North South 6 m deep fink trusses are utilised as runway beams for the sliding roof panels. Fabricated box sections top chords are provided, as in the North roof. Refer to Figure 5 for a complete isometric view of the roof structure. SLIDING ROOF In order to meet the requirement to provide maximum covered seating whilst still allowing daylight for pitch health a moving roof over the whole of the Southern side of the stadium is required. The roof panels are nested over the static section of the roof and at each end double stacked The permanent roof structure running North South provides the runway beams supporting the track for the panels. The area of roof that moves is split into 5 bays, one middle section extending the length of the pitch (135m between trusses T3), and two bays at each end, covering the end stands. These panels are supported of the T3,4 and 5 trusses. The end bay panels are subdivided to allow them to double stack on top of the fixed roof without projecting over the Southern edge of the building. 245 ARCH OVER ARCH OVER Fig 4 Roof Plan During operation, in order to maintain a positive wheel friction to the rails, it is proposed that a wind speed limit of 20 m/s (approximately 50 mph) is imposed. This is in line with common practice at other operable roof stadia. This limitation to operation pertains to operational reasons only. The panels themselves are designed to withstand the design wind loading at any location, fully closed, part open or fully open. The panels are generally framed by fabricated box sections (up to 3m deep for the central large cantilever panel) which are connected to the running bogies. Secondary framing UB sections are utilised with full diagonal rod bracing for each panel to ensure racking of the panel does not occur. A full cycle for the roof to open or close will take 20 minutes. FUTURE PROOFING Whilst there is no current requirement for a fully closing roof for the Stadium, there may be, in the future cause to want this facility. The existing design is able to be adapted to cater for this by the strengthening of certain key elements. the T3 trusses. To cope with this additional load the T3 trusses, the supporting arch stays, the arch and the arch foundations would all need to be strengthened. It is considered that if this strengthening was not carried out at the time of construction, any retro fit would be virtually impossible and expensive to carry out. The technical problems could be overcome however any future fully independent structure over the top of the roof would be visually intrusive and would be unlikely to gain planning permission. This would effectively deny the retro fit route for this option. SUMMARY The roof, especially the arch of the New National Stadium provides the icon that will take over from the Twin Towers and continue the tradition of Wembley for the next 50 years. The stadium will be a truly magnificent stage for the major events that will be held there. It will elevate England into a country capable of hosting almost any international sporting event in style. As the kingpin for bids for the World Cup, and the Olympic Games it is considered that any such bid would be very strong built around the New Wembley Stadium. The fully closing roof would take the form of two additional central overlapping panels clear spanning the length of the pitch (135m). These would be supported on Fig 5 Roof Isometric NOTE The structural engineering for the New National Stadium is being undertaken by the Mott Stadium Consortium.* This consortium is lead by Mott MacDonald and contains Connell Wagner Pty, Modus Consulting Engineers and Weidlinger Associates Inc. REFERENCES 1 Wembley National Stadium, House of Commons - Culture, Media and Sport - Fourth Report 2 March 2000 2 The New English National Stadium, World Stadium Team February 2000 3 Key Development Criteria, Technical Development Criteria, English National Stadium Development Company Limited March 1999 247 KEEPING THE DOORS OPEN: THE OLYMPIC STADIUM, SYDNEY S Morley Principal Modus Sinclair Knight Merz Fig 1 Aerial view of Stadium Australia Sports facilities, where shelter is provided, invariably fall into the category of widespan enclosures. Stadia and arenas tie up vast amounts of a client's capital and the planet's resources and therefore should be designed to be suitable for many uses as far as practicable. This may mean providing a range of environments through altering their widespan enclosures. Where field sports played on natural turf form some of those uses the enclosure needs to be open centred to promote grass growth and comply with current regulations for 'outdoor' sports. Having the ability to close this central opening can greatly increase the possible uses and also provides event surety at least in the face of a temperamental climate. It is, to a degree, possible to quantify the benefits of this flexibility of extent of enclosure by assessing the revenue from additional usage and avoidance of lost revenue from cancelled events (although the latter is difficult to assess as past events at other facilities statistically represent such a small sample of total available event 248 days that little guidance can be drawn from "I've never known an event cancelled in thirty years"). However, this judgement will necessarily be made based on current knowledge of the forthcoming developments in the event markets in which the new facility is intended to compete and the only certain thing about such markets is that they change, often unpredictably. Hence it can be of benefit to provide for flexibility of enclosure or at least for adaptability to be able to introduce such flexibility at a later date. Two of Australia's most recent multipurpose sports and entertainment facilities - Stadium Australia at the Olympic site in Homebush Sydney, and Colonial Stadium, Docklands, Melbourne, are good examples of the application of this strategy. Stadium Australia, the host venue for the 2000 Olympics, is designed to operate in two distinct modes. In Olympic mode the North and South sections of the bowl are unroofed to allow space for large temporary grandstands, each accommodating 15,000 people. After the Olympics these end tiers are due to be removed an the perimeter enclosure completed with the addition of North and South Roofs. At the same time the lower tier of seating will be moved inwards by nearly 16m on the sides and 20m on the ends to greatly improve their proximity to field sports. This strategy was part of an innovative financial package which helped secure the project for the Multiplex led team in a B.O.O.T (build, own, operate and transfer) competition. Under the auspices of such a procurement method the development team was acutely aware of the need for long term financial viability. Whilst, based on 'current' understanding of how the stadium might be used, there as insufficient justification for providing the flexibility of full as well as partial enclosure, it was considered important to 'keep this door open' by allowing adaptability for this flexibility in the future. The roof of Stadium Australia follows , and in fact was borne out of the geometry of the seating bowl. In the longer term post Olympic mode, from the lowest point on the end stands to the highest point on the side stands there was a height differential of perhaps 40 metres and the natural shape to fit this saddle perimeter was the hyperbolic paraboloid. With this geometry the roof over the side stands curves gently downward maximising weather protection for every dollar spent on the roof whilst hugging the higher sightlines. There was also potentially a construction advantage in that this iconic doubly curved form is generated from two sets of straight lines parallel to but progressively rotated from two principal generators at 45 degrees to the main axes of the stadium. Structurally such a surface can be very materially efficient as loads can be transferred by in plane or membrane forces. However this geometric and structural purity is rudely interrupted by the roof plan form in Olympic mode when just crescents of roof over the side stands are required. In plane action across the stadium is not possible in this Fig 2 Architectural Image of the Stadium with End Roofs in Place mode and instead the surface would have to act primarily in bending to some form of edge stiffening along the front edge of each crescent. Nonetheless the other advantages of the hyperbolic parabaloid remained and it was decided to pursue this geometry. True to this surface, the front edge of the crescents were parabolic which suggested a form of arch as edge stiffening. Once provided the arch stiffening would attract a large proportion of the load in both Olympic and post Olympic modes and to increase the efficiency of this system still further the main arch was lifted above the HP surface by up to 12 meters at its apex. The concentrated lateral load components generated by arch forms are best taken directly to foundations on a continuation of the line of thrust (curve of the arch). However, the arch line had to remain above the HP surface to the edge of the roofed area which left it more than 20 metres above precinct level. There was simply not enough site area to continue on this line and therefore some form of cantilever thrust block was required. Interestingly a degree of mitigation was provided by allowing the arch line to change in response to areas of concentrated gravity loads. Because of the geometry of the opening such a concentration occurs where the roof ends meet the arch (here there was also a requirement to support 50 tonne video screens) and the arch line was therefore deliberately diverted inward and downward at the ends like the profile of a crab. This lowered the thrust blocks slightly such that the arches could spring from a point 17 metres above the precinct. The resulting roof geometry therefore was the hyberbolic paraboloid surface curtailed to a vertical cylinder defining the back of the seating and with two crabbed arches stiffening the edges of the central rectangular opening in post Olympic mode and providing full edge support in Olympic mode. Early on in the development of this design the client instructed the designers (Architect Bligh Lobb Sports Architect and Engineers Modus with Sinclair Knight Merz) to consider how a fully closing roof could be provided and what steps might sensibly need to be taken Fig 4 Architectural Image of fully roofed arena now to ensure this remained a possibility for the future. It was found that geometrically the hyberbolic parabaloid could readily accommodate a fully closing roof. The arched edge reinforcement running East West provided natural trackways for a simple sliding system albeit on a constant radius curved track. Furthermore, the fact that the top surface of the end roofs dishes down following the HP surface between these trackways provided additional space for the depth needed for the moving roof panels to clear span the 100 metres plus between the rails. This property was particularly useful as it proved necessary to stack a pair of moving leaves above arch and fixed roof within the curtilage of the stadium. Inclusion of a closing roof would therefore increase loads on the arches and their supporting thrust blocks and foundations requiring commensurate strengthening. Also as this additional loading is to one side of the arch centreline, the roof diagrid supported off the arch which restrains the arch horizontally also would require strengthening locally. The design work instructed allowed informed decisions to be taken on which aspect of their strengthening could and should be carried out during initial construction to 'keep the door open' whilst minimising the amount of capital tied up in steel and concrete for a potentially lengthy period before any revenue is gained from this benefit. The strategy for Fig 5 End segment of arch truss Fig 6 Computer model of west stand strengthening the arches was to add a new central compression element within the arch trusses. This could conceivably be carried out at a later stage by unloading the arches using the same temporary towers employed for their construction and feeding in the new compression member in pieces small enough for assembly by site welding (used extensively for initial construction of the arches). However, strengthening of the arch supports would be altogether more involved to the point of being impractical. Fortunately it was found that the cost of strengthening the pins at the ends of the arches, the pin plates, thrust blocks and foundations was reasonable and therefore these measures were included in the initial development. Also it was found that the diagrid strengthening local to the arches could be achieved simply and with minimal cost by deepening the diagrid in this region. By this means the option of installing the future closing roof has been kept open in a logical way and with the minimum of additional expenditure and construction beforehand. With Colonial Stadium, in Melbourne's Docklands, the decision was taken that the roof should be closeable, subject to the additional costs involved being supported by the business plan. Thereby a stadium designed as an open stadium complete with an AFL sized natural turf pitch could become fully enclosed to operate as a multipurpose venue to attract a greater number or events. Although having a moderate seating capacity of 52,000, the infield is exceptionally large by European or US standards, to accommodate the AFL pitch and closing the 1.7 hectare retractable roof creates an enclosed arena of vast proportions. Several forms of retractions were explored and a simple sliding mechanism with two 50 x 165 m span doors selected for cost efficiency and design efficiency. Such an arrangement naturally concentrates loads on two distinct lines at the ends of the doors and recognising this the Architectural team of Daryl Jackson Architects Pty Ltd and Bligh Lobb Sports Architects, proposed that the upper tier be divided into quadrants allowing four corner supports to be 'pulled in' under these loads. This arrangement could readily be accommodated as the moderate capacity compared with the long infield perimeter meant seats would be redistributed without detriment to viewing quality or efficiency. The Engineering Team of Connell Wagner with Modus used this opportunity to the full to produce a very efficient structure which, even with the 165m span doors [...]... Colonial Stadium main roof support arch and supporting tracks weighed less than 100kg/m2 overall In detail the doors comprise shallow I sections suspended off pairs of asymmetric tied arch prismatic trusses This asymmetry was borne out of a desire to minimise shadows on the pitch and to reduce the visual impact of the roof by chamfering the volume of the inner truss to the pitch side and outer truss to the. .. making the I section roof beams deliberately flexible the doors are better able to accommodate differential movements and slopes of their supporting tracks The tracks themselves comprise 4m deep prismatic trusses which are carried over the 120m span between corner supports again by tied arch trusses with chamfers to the pitch side Great care was taken in developing active positioning systems both of the. .. to the tracks, to control racking of the bogie sets relative to the door trusses, to minimise the forces induced in the structure should racking occur This was particularly important here as racking forces would have the benefit of a 165 metre lever with which to annoy the fixed supporting structure Constructors, Bauderstone Hornibrook have now completed the stadium and it is hoped its flexibility of. .. enclosure will contribute to the long term success of the venue In conclusion, the sports and entertainment market appears increasingly competitive In this market it can be an advantage to have flexibility of enclosure or at least the ability to add this flexibility later Both approaches can be integrated into Structural and Architectural designs seamlessly if included into the design scope early enough