Price & Myers 3D Engineering
Thomas Vale Construction
The brief for the Spiral Café was to design a small café for St Martin’s Square, the main public space in the £500m Bullring development in Birmingham’s city centre. The purpose of this café was to create a landmark structure, which would be part sculpture and part revenue generator, while at the same time helping to animate the terraces of the hard landscape in which it sits.
The form of the café is derived from sweeping a Fibonacci spiral to create a shell-like canopy. The structure is contained between the inner and outer curved surfaces of the canopy; it both supports the cantilevering roof and provides accurate formwork from which the rest of the construction can take its shape.
The eight structural ribs are arranged radially in plan and each tilts up relative to its neighbour to create the shell-like form. A series of CHSs are set diagonally between the ribs; together they act as a cantilevering shell structure. The ribs are supported at points under the roof of the servery and at the roof level of the rear annexe. Further stiffness is generated by the CHS braces between the ribs, which together act as a cantilevered truss supported at the outer tips of the first three ribs.
To simplify fabrication as much as possible, the structure was made from mild-steel plate ribs cut on a computercontrolled plasma cutting machine. This meant that the form of the building could be manufactured easily, with a number of curved ribs defining its shape. Obviously threedimensional modelling was critical to the design. A product design program was used to develop a parametric threedimensional model. The program was used to model all structural elements and many architectural elements, such as cladding profiles and glazing interfaces. As part of the design process, the architect and the structural engineer worked together on the same three-dimensional model, taking sections and profiles from it to develop further details that were not modelled three-dimensionally.
Drawings generated from the three-dimensional model included a set of true plans of each component. This allowed the steelwork contractor to take the profiles and add additional information, such as bolt holes and splice locations, before cutting the metal. A very high degree of fabrication accuracy was achieved in this way. The level of accuracy in the steelwork allowed us to use the frame as a building-wide template for production for other information later in the programme. Elements such as capping pieces could be designed on the threedimensional model with full confidence that they could fit to what was on site.
The constantly varying curved envelope of this small building provides a fascinating sculptural landmark, which sits well in this major city-centre retail area.
The cantilevered structural form, based on rigorous geometrical parameters, is ingenious. Its execution and reflects a high degree of cooperation between the team. The finished structure was achieved by the skill of the steelwork contractor who made a full trial assembly in his shop, and incorporated sufficient bolted connections to enable the dismantling, transport and final re-assembly on site.
This small “gem” is a testament to craftsmanship in steel.