The architect HOK Lobb have balanced a series of factors to achieve the optimum configuration that will ensure that the spectators are close to the pitch and have excellent sight lines, seating comfort and safety. High quality facilities for all the family have been provided, including restaurants, shops, bars and fast food outlets. Behind the scenes, below the entrance concourse level there are changing rooms with state of the art physiotherapy and medical facilities, offices, kitchens, storage and parking. Unique to the project is a fully palletised system of interlocking turf modules which can easily be lifted out and replaced when worn or damaged (fig. 2). The whole system can also be completely removed to create one of the largest covered arenas in Europe, capable of hosting almost any indoor event.
230 THE MILLENNIUM STADIUM, CARDIFF Mike Otlet Director of Engineering Design WS Atkins - Oxford INTRODUCTION The Millennium Stadium is located on the site of the original Cardiff Arms Park stadium in the heart of Cardiff the capital City of Wales. Conceived as a prominent and attractive landmark, it received £46 million of lottery money from the Millennium Commission and became one of the major projects to mark the new Millennium (fig 1). Fig 1 It is the first opening roof stadium in the United Kingdom and took four and a half years from conception to completion. In order to hone the design and refine the details to suit the Arms Park site, budget and programme, many structural forms were considered. The Rugby World Cup was to be hosted by Wales in October 1999 and this event, provided both a catalyst and a completion date for the project. This paper reviews some of the key stages in the work of the design office and fabrication workshops, which led to the final spectacular solution. Nowadays, the design process relies heavily on the use of computers and, in this, the Millennium Stadium was no exception. They were used extensively throughout the design process for analysis purposes and to express the design proposals. BACKGROUND The new stadium, which seats 72,500, was built by John Laing Construction, over a three year period on the restricted inner city site of the original Cardiff Arms Park rugby ground. It has close neighbours on all sides, including the River Taff. In order that the stadium can host significant events besides rugby or football, two sections of the roof can be moved across to completely cover the spectator and pitch areas and form a weather- tight arena. This closing roof is the first of its kind in the United Kingdom and the largest in Europe. The quality of the acoustics ensures that noise breakout is reduced to a minimum, neighbours are disrupted as little as possible and there is, within the stadium, an atmosphere that will attract top performers and large audiences to the venue. The architect HOK Lobb have balanced a series of factors to achieve the optimum configuration that will ensure that the spectators are close to the pitch and have excellent sight lines, seating comfort and safety. High quality facilities for all the family have been provided, including restaurants, shops, bars and fast food outlets. Behind the scenes, below the entrance concourse level there are changing rooms with state of the art physiotherapy and medical facilities, offices, kitchens, storage and parking. Unique to the project is a fully palletised system of interlocking turf modules which can easily be lifted out and replaced when worn or damaged (fig. 2). The whole system can also be completely removed to create one of the largest covered arenas in Europe, capable of hosting almost any indoor event. Fig 2 231 In these respects as an advanced technological building and as a focus of urban activity and renewal, the new Millennium Stadium can be considered to be one of the first of the "Fourth Generation" stadia - a stadium for the new Millennium. STANDS In order to hold the required seating capacity and comply with the space restrictions around the site, the stands rake outwards as they rise. The interesting structural solution needed to achieve this, led in turn, to a dramatic architectural form. The structure above the entrance, which is at concourse level, is constructed from 6,500 tonnes of steelwork in CHS, RHS, open sections and plate girders. It comprises a series of frames at typically 7.3 m centres. The frames are stabilised radially by concrete shear walls and, although there are only two basic frame types with shear walls, either close to the pitch or remote from the pitch, the shape of the stadium means that virtually every one of the 76 frames is different. The steel frames are supported by a reinforced concrete substructure and piled foundation system (fig. 3). Fig 3 Pre-cast concrete stepping units sit on raking steel plate girders around the bowl to form the seating areas. At the back of the stands, these girders carry not only the seats but also some of the roof weight and, by means of tie rod hangers, the extensive level 6 upper concourse. Tubular steel props assist in limiting bending moments and deflections in these girders. Level 5 (Box and Restaurant level) and level 4 below (Club level) are of pre-cast concrete slabs and are supported by deep plate girders on steel columns. Holes are provided in all the horizontal plate girders for services penetrations. A horizontally propped raking plate girder supports the seats for the dramatic middle tier. This cantilevers 14 metres out from the floors at levels 4 and 5. ROOF DESIGN DEVELOPMENT The stadium needed to be about 50 metres larger than the pitch in all directions to accommodate the 72,500 seats and the opening had to be at least the size of the pitch. This gave roof dimensions in the order of 220 metres long and 180 metres wide with an opening of approximately 120 metres x 80 metres. At the outset, following discussions with the various members of the team, a number of design criteria were decided upon; 1. To keep the roof as low as possible to reduce the stadium's impact on adjoining buildings e.g. Westgate Street flats. 2. To keep the edge of the opening as low as possible to reduce the extent of shading on the pitch bearing in mind the requirement for roof falls for rainwater drainage 3. To make any structure around the edge of the opening as small as possible, also to reduce the effects of shadows on the pitch. 4. To make the track for the retractable roof to move along, as near to flat as possible, again bearing in mind the roof falls for water run-off and drainage, and also to assist with making the retractable roof mechanism simple and therefore less problematic. 5. It must be a quality design. THE RETRACTABLE ROOF The direction and form of the moving roof was an initial concern. The drive systems however were not considered to be a significant factor in this decision and have not unduly affected the structural form since. Due to the plan shape of the stadium seating bowl, and the aim to create a roof as flat as possible, dome forms were dropped in favour of linear "sliding door" style systems running on straight rails. Most schemes have involved two sets of 5 similar sections combined in some manner to form a total unit at each end of the opening. Initial ideas centred around methods for concertinaing sections so that they could be stored in a shorter length, than the area to be covered, clear of the pitch. One of the original sketches produced at the time of the studies is shown (fig. 4). The third scheme (fig 5) was pursued in the greatest detail and certainly could have been made to operate successfully but the cost was 232 Fig 4 prohibitively expensive. Instead, the efforts were concentrated on creating two 55 metre x 76 metre "doors" to cover the 110m long opening. Fig 5 FIXED ROOF AND SUPPORTING STRUCTURES Design Evolution There was insufficient space on the site both at the ends and each side to allow any arch forms starting at ground level and it was decided not to follow the tied arch and deep truss route used on the Ajax stadium in Amsterdam, due, again, to the shadows created by such a high structure. Instead the schemes investigated all made use of masts and tension systems in an effort to improve structural efficiency. Scheme 1 Over the first weekend of the project we sketched some ideas and started putting rough numbers to the member sizes and depths, for a two mast solution, picking up 2 large lattice trusses for the retractable roof track to sit-on (fig. 6). From this we started to get a "feel" for the scale of the problem and the magnitude of the various elements involved. • - ho„.«.,__ — . | . -V Ktl T Fig 6 An initial idea produced in the first two weeks of the design process, in April 1995, was eventually to bear a surprising resemblance to the final form. The first scheme was developed over the following weeks, ready for the first submission for Millennium funding, which was made in May 1995. This, unfortunately, was not successful. Scheme 2 Following lengthy discussions and the consideration of alternative sites for the stadium through the summer of 1995, a new location, partly on the existing Arms Park site and partly on the site of an existing BT building and TA centre to the south, looked to be feasible. This had the advantage of improved access from Park Street. Again we opted for two masts to support the main structure and retractable roof track, but this time to the south of the stadium (fig 7). Effectively, it was the same as the first scheme but turned through 180°. To avoid the road, the masts were moved towards the centre line of the Stadium and transfer structures were incorporated. This second scheme was submitted for Millennium funding and following close scrutiny by the Millennium Commission and its representatives, received £46 million of lottery money on 23 February 1996. Fig 7 Scheme 3 Through early 1996 we had been having increasing difficulties with the foundations and buried services that would have been too costly to move elsewhere. When these problems were combined with uncertainty regarding the availability of the Empire Pool site to the south, we started to investigate alternative mast arrangements that did not involve such a large site. By going back to the beginning again and considering the options available it became clear that four masts could be successfully employed, one in each corner at 45°, to lift the corners of the opening. Being symmetrically loaded, the ability to offer a more efficient design also became possible (fig 8). After lengthy discussions with the client, the architect et al, the four mast scheme was eventually adopted by all in the summer of 1996 and developed in conjunction with the contractor John Laing Construction, through to the signing of a Guaranteed Maximum Price, in March 1997. Fig 8 Scheme 4 One or two adjustments in early 1997 lead to the final arrangement we have today. These were: i. The seating bowl that originally varied in its row numbers to the sides of the pitch, and was deeper and therefore higher to the long sides than at the ends of the pitch, was rationalised to a constant level. This deepened the radiused corners and pushed the masts further outboard at this point requiring large diameter columns externally to transfer the loads to ground. ii. A section of the original North Stand was retained, cutting into the roof zone adjacent to the Cardiff Rugby Club. This required structure to spread the loads onto the existing concrete stand and an adaptable solution to allow the roof to be extended at some time in the future if required. iii.The masts to the north were rotated by approximately 22° to ensure they did not encroach on adjoining properties' land. Unfortunately the masts to the south could not be similarly rotated and so a less efficient asymmetric structure was the only solution. THE FINAL SOLUTION The Roof Covering Both the fixed and retractable roofs are clad in a standing seam aluminium sheet with about 120 mm of insulation which is supported by a 128 mm deep profiled aluminium sheet (fig 9). This was all manufactured by Hoogevans and installed by Kelsey Roofing Industries Ltd on-site. This type of make-up and weight is unusual for a stadium, but was necessary to comply with the acoustic criteria noted earlier and allow more concerts to be held annually. Fig 9 The top sheet continues from the roof opening out to a perimeter gutter, which runs practically all the way around the perimeter of the bowl. A syphonic drainage system, made by Fullflow, then takes the water away from the gutters to the ground. Roof Services Because the roof is closed completely for special events which require protection from inclement weather, there are a greater number of services suspended from the roof than would otherwise be necessary. There are two rings of walkways running around the stadium to access these. The first is located back from the edge of the opening and the second in the middle of the fixed roof, 24 metres back from the edge of the opening. Both walkways support heavy pitch lights and speakers weighing up to 165kg each, together with cabling (fig 10). Fig 10 234 MAIN STRUCTURES Purlins The roof cladding is supported by 14 lines of purlins that run circumferencially around the roof at 4.0 metres centres. The surface created is very much like that of an egg with varying radii in both directions. As a consequence, the purlins twist from one bay to the next as they pass over the structure below. The roof deck provides lateral restraint at top boom level and small CHS tubes provide lateral restraint at bottom boom level. These tubes also provide support to the metal ductwork suspended from the roof. Tertiary Trusses The Tertiary trusses support the roof deck purlins and walkways for the fixed area of roof (fig 11). There are 44 in total generally at 14.6m centres around the stadium. With a span up to 50 metres, they are supported at one end by the back of the stands and at the other by the Primary/Secondary trusses which surround the opening. To achieve good sight lines the trusses reduce from 4.3 metres deep at the junction with the Primary/Secondary Trusses next to the opening, to only 400 mm deep at the back of the stands. Here, the trusses sit, via individual sliding bearings, on a perimeter truss (fig 11). The perimeter truss spreads the end weight of the Tertiary Truss uniformly onto two adjacent stand frames. Fig 11 The bearings ensure that differential horizontal movements between the roof and the stands will not have an adverse effect on either element, e.g. under wind loads and thermal expansion/contraction. Primary Trusses Two major pieces of structure, known as the Primary trusses, are located on each side of the pitch in a north/south orientation. Rising 35 metres above the pitch, these are continuous over the full 220 metre length of the stadium (fig 12). Support is provided at two intermediate positions (at the corners of the opening) via cables up to the corner masts which are then tied down to anchors outside the stadium. With a 1067m diameter top and bottom boom, the trusses range in depth from 4 metres at each end to 13 metres in the centre. Fig 12 A 778 dia. middle boom, four metres above the bottom boom, provides a connection point for the tertiary truss top booms and resists high compression loads from the mast structures, (ref. analysis) On one side the trusses provide the support and rigidity for the continuous runway beam which support the moving roof. On the other side they provide support for the fixed roofs on the east and west of the opening. 235 Secondary Trusses The secondary trusses run in an East-West orientation and trim the North and South edges of the opening. They traverse the full 180 metres width of the stadium and are formed from a 915 diameter top boom and 550 diameter bottom boom (fig 13). Support is provided at each end by the stand structures, and at the intersection points with the Primary Trusses, by the corner mast and cable assemblies. They principally provide support to the pitch end of the North and South Tertiary Trusses and also by a lesser extent, to an area of roofing to the corners. Fig 13 Bracing and Lateral Restraints The fixed roof is connected together to perform structurally as one homogeneous unit. The straight, rectangular, roof areas are braced in both directions on plan at top and bottom boom level for stability and lateral restraint purposes (fig 14). FIXED POSITKJNS <RJ. Fig 14 The purlins perform the role of lateral restraint, at top boom level, with CHS tubes at bottom boom level. In addition the bracing holds the track for the moving roof in position laterally. This requires both levels of bracing to resist the torsion effects of the moving roof loads being applied eccentrically to the Primary Truss. The corner tertiaries are restrained back to the adjacent parallel roof section (either east-west or north-south). The total roof is trimmed by a 4060 CHS which supports an eccentrically applied cladding load and holds the shallow Tertiary trusses vertical at the bearing positions on the perimeter trusses. THE CORNER MASTS Four corner mast structures are key to both the vertical support and horizontal stability of the roofs. Each mast structure is made up of a pair of lower columns (concrete filled steel tubes 12190) which sit upon a 16000 fabricated steel tensioning chamber which, in turn, rest on reinforced concrete foundations (fig 15). The tensioning chamber is connected to an 8m deep reinforced concrete shear wall via 10 no. 750 Mac Alloy bars cast into the wall. On top of the pair of lower columns is a complex series of connections commonly known as the elbow and knuckle. The elbows form the link between the roof and the stand structures providing total stability horizontally to the roof via the eight elbows, in 4 pairs, and the cross-bracing between them. Fig 15 The A-frame mast rests on the knuckle and is held down by the high tensile forces in the cables which on one side lift the main roof and, on the other, are tied down to the tensioning chamber at the base of the pair of concrete filled steel columns. The high tensile forces in the cables generate compression in the two horizontal structures. On the pitch side these are known as the Mast Tertiaries. These are fabricated units 2.6m deep and are made of 60mm thick plate to form a Tee-shape section which were then welded to a 6600 CHS tube at the bottom. Pairs of Mast 236 Tertiaries are braced together for stiffness and buckling resistance. On the other side of the knuckle outside of the stadium, is an A-frame outrigger made from 9150 CHS tube. The tubes are restrained by a tensegrity structure to stop the outrigger from bowing under its own weight. This also provides buckling resistance. The A-frame masts rise 40 metres above the edge of the roof and 70 metres above the surrounding ground level (74 metres above the pitch). Each leg is a fabricated oval section 915 x 1415 overall, tapering down to 9150 at the knuckle. Again, as with the outriggers, the mast A-frame is restrained by a tensegrity system of McCalls tie rods and struts. The tension system, although loosely described as cables, is in fact a group of 15 mm diameter high tensile steel strands by PSC Freyssinet inside 6 No. 2730 HDPE sleeves. FTOM iro or mmm rrrrcs (70™. tuxi Fig 17 MOVING ROOF There are two moving roof sections that are generally located one to the north and one to the south of the opening over the fixed roofs. Both sections are 76 metres wide and 55 metres long and made up of 5 individual units, each 11 metres wide (fig 16). The units are linked together, principally at the ends with vertically orientated sliding bearings. Each unit is prismatic in cross section and 8 metres deep at the centre. The truss curved in elevation has a single CHS top boom and two CHS bottom booms. The flat roof deck sits on purlins above the bottom boom with all diagonals and the top boom* exposed to the elements. The units are allowed to move differentially horizontally (fig 17) to accommodate curvature of the track on plan. Sliding bearings are provided above the wheels to cater for variations in the distance between the two retractable roof runway rails. The retractable roof units were assembled at ground level and lifted onto the roof in 76 metre sections. Since positioned and connected, it has functioned well with few problems. ttCTXX THROUOM WfSt SK* THE MECHANISM The moving roof sections have no power connection to them whatsoever. The actuating mechanisms are mounted on the fixed roof between the track and the Primary Truss (fig 18). Fig 18 Fig 16 237 These are on each side of the moving roof located at the four comers of the opening to pull each half of the roof open or closed via a cable loop (fig 19). This operation takes 20 minutes in each direction, a speed of only 2.6 metres a minute. Fig 19 A system of hydraulic motors and gearboxes power the drums back and forth. Substantial brakes are provided to each drum. Hydraulic buffers are located at the centre of the travel and where the units rest at the outer ends of the track, as end stops (fig 20). The speed and actuation are all computer controlled such that both sections move at the same time and at the same rate. Fig 20 ANALYSIS The roof was initially broken down into simplified 2D frames before the assembly of a complete 4349 member 3D roof model (fig 21). The model was developed over three months in order to reduce the highly loaded and high displacement/deflection points to acceptable levels. Fig 21 Wind Tunnel testing was carried out prior to the commencement of the design to ascertain the most suitable wind loads to be applied to the roof analysis model. This also provided an opportunity to check the effects of the new stadium on the surrounding buildings. The 3D model was loaded with self-weight, dead load, live load and wind load cases individually and then with combined load cases with the retractable roof units in open, closed and partly closed positions. The output was then sifted to obtain the worst combination of axial and biaxial bending stresses for each roof member. The model was refined to improve structural efficiency and uniformity in truss boom sizes. The Primary Trusses were pre-tensioned by varying amounts until the optimum location and pretension were determined. This showed that maintaining the theoretical Primary/Secondary truss intersection node "level" following installation of all the dead load and retractable roof trusses was the best solution. This meant that the cable system had to be shortened by approximately 500 mm at ground level to achieve the required position. During the construction period, sections of the model were deleted to reflect the partially constructed state. Further analysis was undertaken to ensure that the partial stability and strength of the roof structure and its various components during the erection period were satisfactory. These figures were then used during the tensioning sequence for comparison purposes with the actual figures measured on site. CONSTRUCTION AND CONNECTIONS All the steelwork for the stadium was manufactured in the UK by British Steel (now Corus) and shipped to Italy for fabrication by Costrusioni Cimolai Armando SpA. Due to the large size of all the trusses it was necessary to subdivide them into transportable sections no bigger that 5 metres by 17 metres. It is interesting to note that seventy five percent of the roof structure is there purely to support its own selfweight. 238 With this in mind it is obviously important to keep the selfweight to a minimum. Given that the steel was being transported from Italy to Wales after fabrication it was important that the connections between each piece were both small and efficient. During the fabrication drawing period the Primary truss which was previously a 3- Dimensional prismatic form was redesigned as a 2- Dimensional element for ease of transportation and in particular shipping. Additional lateral restraints were required as a consequence. From an early stage it was decided to follow the simple principle for the connections of: i. The ends of complete trusses or members would be emphasised architecturally within the practical constraints of tolerance, fit-up and economy. ii. All intermediate (splice) connections would be hidden to give the impression of being a continuous monolithic piece. In the majority of cases such as the tertiary truss ends and lateral restraint/bracing member end connections, a simple tapered tube detail was developed with single plates protruding. A plate each side with multiple bolts in a circular arrangement then linked the pieces together. Tolerances for length and direction were achieved via four interfaces each with 3mm oversize holes for M27 bolts (fig 22). Fig 22 The connections for the mast assembles were considered individually due to their varying requirements of movement, tolerance and adjustments The Primary node which connects the Primary, Secondary and Mast tertiaries together as well as numerous smaller bracing and lateral tubes is formed from a single 100mm thick high grade steel plate cut to the external profile of the overall connection, (fig 23) The plate is orientated vertically in the direction of the cables to enable the tube and cable termination housings to be welded directly to each side. Short stubs for the incoming truss members were then welded at the appropriate angle onto the central plate to give sufficient space for bolted splice connections to be made. Where forces were prohibitively high in-line butt welds were made on site, but these were rare. Fig 23 The mast top cable termination, outrigger end cable termination and base tensioning chamber all followed the same principle of the single central plate cut to the external profile of the connection. All other plates and tubes were then welded to the sides of this (fig 24). Cover plates (bent plates) welded outside these have ensured that the external appearance is as smooth flowing as possible, within' the budget constraints. The use of a central plate has ensured that the forces flow more evenly across the connection and high local bending moments and shear forces were kept to an absolute minimum. Fig 24 239 The knuckles which are located at eaves level on 1219 A tubular columns form the focus and connection point for the A - frame masts, Outriggers and Mast tertiaries (fig 25). This central knuckle (or hub) has to resist approximately 40000 kN axial compression from each incoming member. During the cable tensioning process and when the retractable roof sections close, or when it snows, the forces in the cables (which join the outer ends of those members) increase, causing them to elongate significantly. This in turn causes a rotation at the knuckle necessitating a pivot at the same point. Fig 25 A 2.4m diameter cylinder 2.4 metres long orientated horizontally, like the centre of a bicycle wheel has around it removable steel plates with PTFE and stainless steel contact surfaces to allow the small rotation to occur (fig 26). The drum was sized to accommodate the very large circular end plates from each of the three incoming members with the low working stresses in the PTFE material Figure 26 The splice connections between the Primary and Secondary trusses required the greatest development time. Various options were considered (some of considerable weight and complexity) before the final detail was found. Bolted connections were considered to be essential for speed and ease of construction. Due to the large diameter of the tubes involved (770 diameter and above) it was possible to climb inside to make a hidden bolted connection(fig 27). Fig 27 The majority of tubes are highly stressed and an innovative detail was required to solve the problem. The axial loads are transferred by 4 flat plates bolted to fabricated tees welded to the inside of the tubes (fig 29). The tees are of sufficient depth to allow the splice plates to pass over an internal flange on the ends of each section. The flanges are bolted together to transfer shear and torsion. Where tubes were too small to climb inside, port holes were formed to gain access to the bolts and flange plates. Figure 28 [...]...240 The 355 diameter bottom booms of the tertiary trusses were too small for this detail and instead a steel cruciform was welded into each transportable section (fig 29) Flat splice plates were used to link the pieces together Cover plates flush with the outside of the tube hide the splice Client Welsh Rugby Union in conjunction with South Glamorgan County Council (later formed into Millennium. .. possible by the design of practical, workable site connections and the assembly of large sections (generally 50 metres long) at ground level lifted into the air with an 1800 tonnes crane This avoided the use of scaffolding and minimised work at high level To design the first retractable roof stadium in the UK has been a long and at times difficult process due to the varying interests of the people involved... interests of the people involved Structurally it has been a wonderful challenge and opportunity to design to a scale rarely experienced by most engineers, (fig 30) The deadlines were achieved and I believe the results speak for themselves The next one will be easy! Figure 30 ... Ltd) Funding £46 million Millennium Funding Architect Lobb Sport (now HOK Lobb) Civil and Structural Engineers WS Atkins Main Contractor John Laing Construction Mechanical & Electrical Engineers Hoare Lea & Partners (Detail design by Ove Arup and Partners for Drake and Scull) Steelwork Fabrication Costrusioni Cimolai Armando SpA Roof Covering Kelsey Roofing Ind Ltd Fig 29 SUMMARY The 8000 tonnes of roof