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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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