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FORMING THE STRUCTURE

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buildings

The form of the cafe is derived from sweeping a Fibonacci spiral to create a shell-like canopy, writes Tim Lucas. 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 floor 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 computer-controlled 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 three-dimensional modelling was critical to the design. A product design programme was used to develop a parametric three-dimensional model. The programme 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 fabricator 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 of other information later in the programme. Elements such as capping pieces could be designed on the three-dimensional model with full confidence that they would fit to what was on site.

Tim Lucas is a structural engineer with Price & Myers 3D Engineering

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