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One New Change: St Paul's Grid

The St Paul’s grid generates the geometry of Jean Nouvel’s One New Change

A revered historic monument can engender a form of planning blight. An example is the zone of mediocre office buildings around the Vatican in Rome.

The area around London’s St Paul’s Cathedral has fared little better. Creativity has been stifled by committees, recalling architectural historian James Stevens Curl’s phrase ‘a hotbed of cold feet’, describing the procurement of college buildings in Oxford.

But Atelier Jean Nouvel’s faceted, monolithic design for One New Change, a mixed development for Land Securities adjacent to Christopher Wren’s masterpiece, could not be described as cautious. And although not a homage to its illustrious neighbour, it nevertheless seems fitting that it shares Wren’s preoccupation with geometry.

The One New Change brief called for a mixed-use retail and office development. Views of the cathedral were an import­ant consideration, as was the provision of public space. Views of One New Change, including its roof, were another factor, as were maximised floor plates. But the principal driver behind what is undoubtedly a pragmatic response was a set of restrictions on building heights, known as the St Paul’s grid. Development within each grid square is not permitted to exceed a certain height. There is also a set of contour lines, which limits building heights beyond the grid. Viewing corridors that protect the skyline presence of St Paul’s were another factor.

From its early stages, Nouvel’s proposed massing responded to these constraints and sought to optimise floor area and volume. But in the Watling Street viewing corridor, the line of the proposed facade was set back further than that of its predecessor. Also, views of St Paul’s from the heart of the development were provided by a large chasm, which serves as a public space and allows natural light to penetrate the centre of the building. More public space is provided by a large rooftop terrace, designed to exploit views maintained by the St Paul’s grid, so Nouvel’s scheme provides a greater public realm than the development it replaces.

Nouvel’s team produced physical models and used Autodesk Revit to develop the building’s faceted form. The envelope was conceived as a crystalline formation of glass cladding, providing spectacular views.

Sidell Gibson Architects (SGA) and specialist subcontractor Gartner both joined the project at the planning stage. SGA conducted the setting out of the building and the communication and understanding of its form, while Gartner refined the design and established three-dimensional setting-out coordinates.

Developing a suitable cladding system was a major project in itself. The geometry of the building generated 4,300 unique glass panel sizes, with 6,000 panels in all. These were grouped into units, each with up to three facets, which optimised the amount of assembly work that could take place under controlled factory conditions for units that were small enough to be transported and installed on site.

Gartner silicone-bonded the glass panels to a carrier system, which it extruded in its factory, and then fixed this system to aluminium or, where the spans were larger, steel mullions. The project team agreed to use a universal joint width of 28mm for the junctions between all panels and units. This was large enough to accommodate all tolerances and deflections.

The design team specified grey gaskets and increased the density of the frit pattern at the edges of the panels in order to reduce the apparent size of the joints. The largest panels are 7.5 x 2.5m.

The design team realised that the support steelwork would be highly visible and could obstruct views. SGA offset the external envelope of the CAD model by 350mm (including 50mm for tolerance) to establish the structural fixing zone, and Arup was responsible for designing all cladding support steelwork, with guidance by SGA. Software was used to detect clashes.

The minute elliptical frits applied to the glass are more dense in areas where it conceals insulation and where solar protection is required. The colours of this fritting are brick red, grey and Portland stone to match the materials of the neighbouring buildings.

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