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Orangery blossoms in Prague

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Brian Statham looks at Eva Jiricna Architects' replacement for the original Prague Castle greenhouse, built between the Renaissance wall forming the edge of the royal garden and the moat which divides the site from the castle complex. Main photos by Dunca

To view Prague's medieval castle from within Eva Jiricna Architects' orangery is to view it through an uninterrupted arch of silicone-jointed glass panels suspended on stainless steel 'spiders' from an elegant diagonal web of stainless-steel tubes. The experience is of a fusion of Prague's historical past with its commitment to innovation in late twentieth-century design and design technology. In a book in preparation about Eva Jiricna Architects, Petr Kratovil writes of the practice's concern for the 'special wishes' of the client; it 'always maintains an intensive dialogue with the client'. In the case of the Orangery, the architect was invited to make a proposal for a new enclosure that would 'maintain the historical function of the site yet introduce the architectural language of the twentieth century'.

For Eva Jiricna Architects, design must evolve from the logical solution to specific problems - the aesthetic must spring from the design logic. The practice works in close collaboration with the engineer Matthew Wells - their partnership is long-standing. The method is to achieve the 'maximum intensity by using a minimum of means'. The architecture, Kratovil writes, 'can serve as a work of construction and at the same time as an artistic object', and it can provide 'a functional environment'. The brief for the Orangery required that the practice succeed in all three: the building was to front, include, but not lean upon a sixteenth-century wall; it was to provide a stimulating and pleasant environment for visitors passing along a newly opened route from Klarov to Powder bridge, and it was to function as a sophisticated horticultural greenhouse providing three compartments, each with its own specific climatic control and each providing differing facilities for the cultivation of plants from seed to maturity.

Using the footprint of the earlier greenhouse, the structure runs for 100 metres fronting the retaining wall of the royal gardens. The building, largely of glass and steel construction, rises vertically from a steel- clad concrete sill beam to 1200mm, then arches up and over in a continuous circle - apparent rather than real as the suspended glass shell is composed of faceted glass jointed with silicone - to terminate along a tubular steel prismatic truss beam.

The problem of having to retain the historic wall as the rear of the building, but not allowing the structure to rest on the wall, is resolved by the use of the prismatic truss beam. The beam is supported at the intersecting cross walls by pairs of tubular columns. The columns also house the cross walls - a good example of application of minimal means for maximum achievement. Cross-frames are located at these intersections, which give rigidity to the span of the roof. The results of this are uninterrupted exposure of the sections of wall and uncluttered expanse of horticultural space. Visitors looking in are thus rewarded with an experience of late twentieth-century design against a Renaissance backdrop, while taking in something of the activities of Prague Castle's horticulturalists.

The external stainless steel structure is formed by 60.3mm-diameter tubular steel welded crosses, held together by stainless-steel node assemblies (see Working Detail overleaf). The glass panels are frameless and of clear toughened laminated glass. They are jointed with black silicone and suspended at their corners on diagonally placed stainless-steel 'spiders'. Eight faceted planes run the length of the building and compose the 'circular' curve of the roof. The first of these consists of a single row of vertical stainless-steel panels accommodating horizontal glass louvres. The louvres open to allow cross-ventilation from the base of the front wall to butterfly vents set within panels of glass at the apex. Additional similar ventilation louvre panels are spaced along the section running next to the sixteenth- century rear wall, giving additional ventilation flexibility.

Inside the glass shell are electrically operated automatic roller blinds which provide shade in summertime - the orangery is south-facing - and insulation in winter. Heating runs partly in pipes under the blinds - eliminating winter condensation - and partially in the floor slab. Pipes are also suspended from the structure for introducing vapour mist. Maintenance is to be carried out with the use of a pragmatic and visually exhilarating arch-like gantry which moves along the length of the building on two rails, one at the top and one at the bottom. The gantry further illustrates Jiricna's commitment that her work should provide functional, structural and artistic expression. The final achievement is a functional, structural and aesthetic whole bringing together Prague's historic past and twentieth-century engineering and design.

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