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Court circular Foster and Partners' Great Court project is taking shape. It gives a new heart to the British Museum by reclaiming the quadrangle surrounding the round reading room at the centre of the

Great institutions and their buildings enjoy a complex relationship. The British Museum was founded by Act of Parliament in 1753 and opened to the public in 1759. Its first home was an existing late seventeenth century house, Montague House, which stood on the site in Great Russell Street which the Museum has occupied ever since. Following the death of George III his library was bequeathed to the nation and into the care of the Museum and, in 1823, Robert Smirke was commissioned to design a building to house the King's Library and to provide a proper home for the Museum's collections. Smirke's neo-classical design, which is the core of the existing building, was completed in 1847. It had four principal wings arranged around a central quadrangle measuring 96m x 72m. This was conceived as a major public space at the heart of the museum and its architecture clearly expressed this.

Almost as soon as the building was finished, however, it was unable to meet the ever-growing demands of the collections. The response was to construct a new reading room, surrounded by book stacks, in the quadrangle, to provide increased reading space and book storage for the library collections. This project, which led to the construction of the great circular Reading Room, was conceived by Antonio Panizzi, who was Keeper of Printed Books at the Museum. It was executed by Sydney Smirke, who had succeeded his brother as the museum's architect. Construction began in 1854 and the building was finished by May 1857.

The significance of this historical outline is that the demands of the growing collections meant that Smirke's quadrangle, a space which was fundamental to his design, was filled-in and made inaccessible to the public almost as soon as it was completed. But now the transfer of the British Library has offered the opportunity to recover it and to bring it into new use as a great public space at the heart of the museum. This is the genesis and intention of Foster's project.

The Great Court project

In essence the project is extremely simple. The quadrangle has been cleared of all structures except for the rotunda of the Reading Room, the internal facades are restored and a glazed roof is thrown over the space, now called the Great Court. This will become the focal point for the entire Museum. At the principal floor level there will be information services, a cafe, and space for a greatly expanded museum shop; there will also be direct access to the restored Reading Room, which will be open to all visitors, and will contain compass, a new multi-media access system to the Museum's collections and a 25,000 volume reference library. At a newly created lower level there will be the education centre and galleries for part of the ethnography collection. The rotunda of the Reading Room is to be partially enlarged to the north by a tiered ellipse which will provide space for temporary exhibitions and a restaurant at the upper level. The Great Court and its facilities will remain open in the evening, after the museum has closed to the public, and will serve as part of the network of public routes running north-south through the city.

The virtue of the design is that it preserves this clarity of conception through its execution. But such apparent simplicity is seldom achieved easily and this is particularly the case when, as here, a complex and technically demanding brief has to be satisfied while preserving, in fact whilst enhancing, the qualities of the existing Grade I listed buildings. In many respects these buildings have, through their intrinsic architectural quality and, in the case of the Reading Room, by association with great figures in the cultural history of the past, themselves become significant exhibits. This demands the greatest care in the new design.

History and technology

Robert Smirke's building was based on the construction techniques of the eighteenth century. It is of load-bearing masonry construction and the dimensions of its principal spaces are determined by both the spanning limitations of that technology and by the rules of proportion of Georgian architecture which related the sizes of rooms to the dimensions of their windows in order to achieve a good amount of daylight. But, in the few years between its completion and the construction of the Reading Room, Victorian engineering offered new possibilities of both construction and environmental control. The structure is a frame of cast and wrought iron, infilled with brickwork. The environment of the room made use of the latest techniques, delivering heat to the reading desks in winter and fresh air in summer. Foul air was extracted through decorated outlets in the soffits of each of the twenty window openings and through openings in the oculus. Initially the room was lit purely through natural light, and was closed to readers when the light failed. The first experiment with electric lighting took place in 1879, and in 1881 an installation of Siemens arc lamps, each of 5000 candle power, allowed the opening hours of the room to be extended to 8pm.

The glazed roof

The key element in Foster's design is the glazed roof. The enclosure of the courtyard not only provides the museum, and the city, with a major public space, it also fundamentally transforms the nature of the entire building. The controlled environment of the Great Court has a significant effect in acting as a buffer to the surrounding spaces and the Reading Room. The design of the roof is a tour de force, both structurally and environmentally. The underlying strategy is to produce a canopy which is delicate and unobtrusive, avoiding the need for columns within the court, and having a sense of great transparency. Geometrically the roof has to negotiate the relationship between the Reading Room and the surrounding facades and is further constricted by planning requirements which limited its height relative to existing structures. This has resulted in a geometrical form, generated by a complex mathematical model, in which, despite its apparent simplicity, every single triangular element is unique.

The resulting roof structure, which is now under construction, is a fine steel lattice made-up from purpose-made steel box beams joined at six- way nodes. At its junction with the Reading Room the roof is supported in a ring of 20 composite steel and concrete columns which align with the structural form of the original cast-iron frame. These columns will be concealed by a new skin of limestone surrounding the entire drum of the Reading Room, the exterior of which was not designed to be seen from within the Museum. This skin also provides space for vertical services. At the perimeter the roof is supported by Smirke's original load-bearing masonry walls. The connection is through a sliding bearing carried by a concrete ring beam on top of the existing wall.

This filigree canopy allows daylight to filter through to illuminate the court, to pass into the Reading Room and, in very controlled quantities, into the surrounding galleries. The roof glazing units combine neutral- tint glass with a fritting pattern, achieving a high-performance shading coefficient to reduce solar heat gain, but transmitting a high proportion of the visible spectrum.

Mechanical services

The mechanical aspects of the environmental controls have demanded as much care in respecting the existing fabric as has the roof design, although here the aim has been to make the result as unobtrusive as possible. The first problem was to find a way to bring fresh air into the new spaces, and to the Reading Room. This has been solved by the construction of four new plant rooms in the basement of the existing buildings to the north- east, south-east, south-west and north-west of the court. These do the initial filtering of the incoming air before it is passed to four further plant rooms beneath the court. At these, full conditioning of the air takes place before it is distributed to the education centre, gallery spaces and the restored Reading Room.

In the Reading Room the new systems follow, in broad principle, the original strategy of Panizzi's and Smirke's design by using the surviving 'spider' of air ducts to carry insulated ductwork beneath the floor to supply air through the reading desks as before. Extract will also use the original routes in the structure of the dome. Extract and smoke venting from the new basement spaces is through the new service void formed around the rotunda.

The court will be mechanically ventilated by extract fans at roof level around the Reading Room. Air will enter through high-level inlets between the glazed roof and the perimeter. This flow will prevent hot air build- up beneath the roof; the off-peak capacity of the chiller plant at night will help cool the floor slab of the court.

The new museum

When the Great Court is completed in 2000, it will be possible to read the distinct stages of the building's development immediately upon entering the Great Court. Robert Smirke's Greek revival facades, restored to their original state, will frame Antonio Panizzi's and Sydney Smirke's Reading Room, whose interior will also be restored. These will be linked and protected by a new canopy of glass and steel and the British Museum will enter a new phase in its relationship with its building. The whole institution, in its greatly increased size and complexity, will thus be accommodated in a new, clear configuration of spaces.

Dean Hawkes is an architect and professor of architectural design at the Welsh School of Architecture, Cardiff University

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