Concrete intervention: radical treatment of a nineteenth-century building creates dramatic new spaces.

When Aparicio+Fernandez-Elorza were asked to create an architectural documentation centre and lecture theatre in the arcades of the Nuevos Ministerios on the Paseo de la Castellana in Madrid, they adopted two principles: first, the existing should be modified only when absolutely necessary and, second, resulting spaces should be created to offer maximum flexibility.

Though apparently quite simple, the project was complicated by the existing buildings and by the presence of train platforms under the Paseo de la Castellana. A direct connection to the lecture theatre was required from both street level and that of the underground railway platforms. Further complications were caused by the fact that platform level was above that of the floor of the vault under the arcades (which are already adapted to being an exhibition gallery for architecture as part of the Ministry of Public Works).

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The architects decided on a bold move. They chose to make the floor level of the lecture theatre the same as that of the platforms. To achieve a workable space, the floor of the arcades that covered the footprint of the theatre was demolished. Additional difficulties were encountered as a consequence. The cut-away slab of the arcades had braced the side walls, which are of masonry and partly underground, so subject to lateral pressure from surrounding earth. To prevent the walls buckling and to create a suitable space for lectures and conferences, a huge and stiff U-shaped concrete beam was created under the new hole. The device provides lateral stability for the old masonry walls and allows thin services ducts to be created between concrete and the old stone and brick. (One of the architects' design aims has been to suppress signs of services throughout the scheme.)

Coming into the new intervention from Paseo level, much of the strategy is quickly apparent. Usually, you can look down to the right into the lecture theatre in its concrete cradle. But when a lecture or conference is in progress, the lower space is obscured by black velvet curtains. Down at theatre level, these curtains (which can easily be adjusted to achieve all sorts of different effects) create enough blackout to allow projections to be easily seen, yet at the same time they allow gentle washes of light to flow down the grey concrete walls. The tall space acquires surprising subtlety and complexity.

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At the southern end of the theatre, the 1.65m difference in level between it and the archive centre had to be resolved, for they will often be used together. To make the link, there is a platform that can rise or fall on scissor jacks; it can act as a goods lift or a rostrum for speakers. Stairs that can be folded away when the link is not needed flank the platform.

The archive centre is under the 107m long old vault of the arcades. Few obvious changes have been made but there is a double wall--a long duct--in which services are concealed, and a staircase connection to the gallery above has been introduced. The stair is elegantly made, a light filigree of steel rod and plates that contrasts with the heavy mass concrete solidity of the vault. It was such sensitivity to materials, light and space, that convinced the jury that the scheme should get an award.