Amid the retail frenzy of Birmingham's Bullring Shopping Centre, it is refreshing to come across the elegant, shell-like spiral of copper that is the Spiral Café. Designed by Marks Barfield Architects, it is only 60m 2, small enough to be viewed as a piece of sculpture in its own right, yet large enough to allow shoppers to sit comfortably and enjoy the view. The café flanks the curved walkway that runs along the western side of St Martin's Square, a new space that links the new shops and central boulevard, including the Selfridges store by Future Systems, to Edgbaston Street, stepping down in a series of wide terraces to St Martin's Church.
If you find the shape of the café particularly pleasing, it's because it is based on the Fibonacci sequence - a mathematical sequence related to the Golden Section and identified by Leonardo Fibonacci in the 13th century, in which each term is the sum of the previous two, thus:
0, 1, 1, 2, 3, 5, 8 ,13, etc. The sequence has been found to have an almost mystical relationship with the natural elements, from the shape of shells, the pattern of seed heads and pine cones, to complex fractals in galaxies.
'We looked for a natural mathematical form as an appropriate concept to bridge the space between the commercial and the spiritual, ' explains project designer Ralph Parker.
The concept is simple: take the Fibonacci number sequence and lay it out as a series of squares; draw quarter circles in each square to form a spiral. Based on the spiral, make a series of steel ribs from curved steel plate; set them 9° apart on plan and tilt each one higher than the one before. Cover them with a copper roof and fill the end openings with frameless glass. This elegant yet dramatic statement is the essence of the new café. The spiral tilts upwards towards the church, its large end opening filled with a Planar bolted-glass wall, which frames the church perfectly.
Although the café floor spans across the shell, the integrity of the structure is maintained by making the ribs continuous; they curve under the floor and emerge in the main interior of the café as a low curved wall that acts as the servery. The floor spans across the ribs, partly in the form of laminated glass so that the continuity of the structure is visible. This continuity maintains the structural integrity of the shape and works well in practice, as the terrace had to be reinforced substantially below ground level anyway to take the weight of the café. A small store to the café is slotted against the rear side of the ribs, with a curved wall to match their profile. Its side walls are made of steel plate and brace the main structure.
The curved ribs are clad on the outside with post-patinated copper sheets and on the inside with smooth bronze plates finished with clear lacquer. 'We saw it as something organic, ' explains Parker. 'The outside is weathered, but you look inside to a smooth interior - like opening an oyster shell.' To maintain the logic, the front face of the servery, where it emerges on the inside, is also clad in copper.
The Bullring regeneration project has commissioned art and sculpture to enhance its public spaces. The shell-like form of the new café could well be taken for one of these pieces, but one that also serves a good cup of cappuccino.