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A grand stand for Wales

Cardiff 's 72,500-seat Millennium Stadium, a major new landmark in the city, was constructed from 14,000 tonnes of British steel

Rugby is the national game of Wales and the Millennium Stadium in Cardiff is its new shrine. The magnificent 72,500 seat arena, built on the site of the famous Cardiff Arms Park, is situated adjacent to the city centre and is easily accessible from local bus and rail stations.

Its 80m high steel masts dominate the Cardiff skyline, visible throughout the city and the surrounding areas. They support a retractable roof - the largest in Europe and the first for a stadium in the UK - which can completely cover the pitch.Not only can spectators view matches sheltered from the elements but the stadium can also host other events - such as concerts or religious festivals - irrespective of weather conditions.

A new walkway along the River Taff, which runs alongside the stadium, and a series of new urban squares are being created to lock this huge structure into the urban fabric of Cardiff.

The stadium has four main levels of activity: the basement is devoted to car parking, services and players' changing rooms.A low level of tiered seating - with 25,000 seats - flanks the pitch, with bars, shops and restaurants situated behind. An intermediate tier contains 15,000 VIP seats, with hospitality boxes and restaurants located behind these, while the uppermost tier contains a further 32,000 seats.

The tiers of seating are formed of pre-cast concrete units. The lowest level sits on a pre-cast frame over the basement. The upper levels of precast concrete seating step outwards as they rise, creating a bowl-like enclosure, curved at the corners, to the pitch.They are supported by a series of steel frames which surround the arena at 7.3m centres. Stability and stiffness are provided by concrete stair cores and shear walls connected to the substructure, together with tubular diagonal bracing between the frames in isolated locations.

The topmost level of seating rises to the eaves where the upper parts of the support girders are propped by canted 508mm diameter CHS columns.A series of tie-rods are pinned to the top ends of the columns to give support to the main concourse which projects from upper levels of the steel frames, clad with Celestia red and plum panels.

The mast and outrigger structures are set at the outer corners of the stadium to support the steel roof structure while providing a column-free enclosure. Two primary trusses 220m long flank the main axis of the pitch.

They are massive, with top and bottom booms of 1,067mm diameter and a maximum depth of more than 12m at mid-point.

Secondary trusses run along the short axis of the roof and support the ends of intermediate lattice trusses spanning 50m. Pre-tensioned cables transfer loads from the crucial points of connection between the trusses to the mast structures. The retractable roof trusses span 76m and roll along tracks supported on the primary trusses.

The roof itself is certainly the stadium's most impressive feature and when closed it only heightens the cathedral-like atmosphere at the home of Welsh rugby.

Detail which accommodates movement between a steel frame and roof structure

The topmost level of precast seating rises to the eaves supported by canted steel plate-girder beams; the upper parts are propped by canted 508mm diameter CHS columns.

The soffit of the seating and its support structure of beams and columns are exposed on the elevations trimmed at eaves level by a band of curved steel cladding, with a ribbed gun-metal-grey reflective finish.

Both stand and roof structures are designed to move independently of one another. The roof structure is supported between the stand frames by a perimeter truss fabricated from 457 x 191mm universal beam chords and uprights, with 219mm diameter CHS lattice braces.

Between the top chord of the perimeter truss and the ends of the intermediate lattice roof trusses there are specially designed elastomeric bearings. An inverted Tplate welded to the truss ends rests on neoprene 'pots' contained within the thick steel base plates, allowing rotation of up to 2degrees. Machined steel 'claws', faced with neoprene and stainless steel and welded to the base plate, clamp the upper surface to enable wind uplift forces to be resisted. This arrangement allows horizontal movement in all directions with a maximum movement value of up to 100mm parallel to the roof trusses.

The structural joint is reflected in the eaves cladding with a beakshaped movement joint which comprises a weathering system, a flexible panel of neoprene sheet and pairs of nylon brush seals which intermesh. The eaves are clad with pewter-coloured profiled steel cladding panels fixed to curved steel T-sections, with pre-curved aluminium panels above the joint.

The steelwork

The roof and substructures were built with 14,000 tonnes of steel manufactured in the UK by Corus (formerly British Steel), and shipped to Italy for fabrication by Construzioni Cimolai Armando SpA. The structure was designed with bolted connections between transportable lengths of trusses for speed and ease of construction. The diameters of the tubes used in the construction of the major roof elements were relatively large (the smallest are 48mm diameter while the majority are over 400mm and up to 1,600mm diameter). Hidden bolted connections were used extensively between the chords of the trusses. For tubes of more than 914mm diameter it was possible to climb inside and make a hidden bolted joint; other connections were made by hand using a combination of portholes and cover plates to give access.

CREDITS

ARCHITECT The Lobb Partnership (now HOK Sports): Sean Jones, David Elder, Damon Lavelle

STRUCTURAL ENGINEER W S Atkins: Mike Otlet, David McLaughlan

MECHANICAL & ELECTRICAL ENGINEER Hoare Lea & Partners

MAIN CONTRACTOR John Laing Construction

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