A masterpiece in glass - Europe’s largest covered public square
On 6 December 2000, Her Majesty Queen Elizabeth II formally opened The Great Court at London’s British Museum, a spectacular £100 million project that has created the largest covered public square in Europe. Designed by internationally acclaimed firm of architects Foster and Partners, the 6,100 sq. metre area is enclosed by a unique glazed roof, which has transformed the Museum’s inner courtyard.
The Great Court is now a new visitor hub for the Museum and the development has also created a magnificent new civic space for London. At the heart of the newly developed area is the Museum’s famous round Reading Room, now restored to its original glory and home to a modern information centre.
A new dimension for visitors
The newly created area is entered from the Museum’s principal level, through an impressive portico. Once inside, visitors can access information points, a bookshop and a café, and they can enter the museum’s many galleries via a number of different entrances.
Two broad staircases encircle the drum of the old Reading Room, leading to upper-level galleries and two mezzanine levels, elliptical in plan, which provide temporary exhibitions gallery and restaurant. Beneath the floor of the area are further new galleries, an education centre with auditoria for 350 and 150 people respectively, and facilities for school children.
The magnificent interior of the Reading Room has been carefully restored, including the interior of the dome and reinstatement of the original 1857 colour scheme. The information centre it now contains combines the best in modern technology with more traditional sources of information retrieval and is accessible by all Museum visitors.
This new public reference resource contains around 25,000 books, catalogues and other printed material, focusing on the world cultures represented in the Museum.
Eye-catching roof structure
To allow the Great Court to be used throughout the year, it is now totally covered with a stunning double glazed roof spanning 96 by 72 metres. The maximum height from the ground level to the highest point of the roof glazing is approximately 26 metres, and amazingly, the structure appears to have no visible supports to detract from the restoration of the classical façades around it. Instead, it spans the gap between the surrounding museum façades and the central drum of the Reading Room as a self-supporting structure. Despite its apparently delicate lattice form, the roof and its integral glazing system is very strong, and has been designed to be regularly accessed by appropriately trained personnel with all necessary cleaning and maintenance equipment.
The aesthetically pleasing design of the roof resulted partly from some unique local challenges. First, local London planning requirements limited the height of any new roof structure relative to heights of existing Museum façades, and, as an additional complication, the historical Reading Room is not exactly in the centre of the Great Court, but some five metres closer to the northern portico. This has resulted in an asymmetrical geometrical form created by a complex computer-generated mathematical model.
The final structure is a vast triangulated steel lattice shell that acts both as primary supporting structure and framing for the glass. This undulating latticework shell is of pleasing appearance, and is made-up from 4,878 individually fabricated steel box-section beams attached to each other by over 1,500 six-way nodes. Each of these is totally unique in terms of x, y and z co-ordinates and rotation angles. The lattice contains about 11 km of steel beamwork, weighs some 478 tonnes in total, and was fabricated to an astonishing accuracy of just 3 mm overall. It supports about 315 tonnes of glass, resulting in an overall roof weight of nearly 800 tonnes.
Each glass panel is unique
Overall, the roof contains enough glass to glaze 500 average-sized domestic greenhouses. But despite its deceptive visual simplicity, every one of the 3,312 triangular double glazed elements is slightly different in size and shape because of the roof's complex geometric form. Individual panels vary in size between 800 mm wide by 1,500 mm long, up to 2,200 mm wide by 3,300 mm long, with an average panel area of approximately 1.85 square metres.
Each triangular shape varies too, the most acute angle between the sides of one triangular panel being about 15º, while generally, planar angles between glass panels vary from between nearly 0º to 30º. The most inclined slope on the glass roof is around 52º relative to the horizontal edges at the boundaries.
The total thickness of the individual insulating glass unit comprising each panel is 38.76 mm. Each of these units consists of an outer 10 mm toughened Pilkington Optifloat™ Green T which is separated from an inner pane of Pilkington Optilam K Glass™ by a 16 mm air filled cavity.
The inner surface of the outer pane is coated with a 57% frit consisting of 4 mm diameter ceramic white dots, which filters ultraviolet rays and substantially reduces solar gain. The inner, laminated pane consists of two 6 mm sheets of annealed glass laminated with a 0.76 mm PVB interlayer. This pane also features a Low-E coating on the surface facing the cavity. The unit structure results in outstanding strength and solar performance: overall, shading coefficient is about 0.26, energy transmission 0.23, light transmission approximately 30%, and Ug value 1.9 W/m²K.
Pilkington Optifloat™ glass was toughened, laminated and screen-printed in Germany by specialist glass processors Bischoff Glastechnik (BGT). International insulating glass unit manufacturer OKALUX then made up the 3,312 individual glazing panels.
The contracting team
While the conceptual design of the Great Court project was produced by architects Foster & Partners, the final geometry for the roof net and the structural design was developed by structural engineers Buro Happold using computerised form-finding processes.
Detailed element design, fabrication of the steel latticework and installation at the museum in Bloomsbury was undertaken by principal trade contractor Waagner-Biro Stahl-Glas-Technik, through its London office under the control of Project Director Paul Lynch.
Site work began in September 1999, and installation of the final glass panels was completed in July 2000.
Images courtesy of Foster and Partners (architects and designers), London.