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Technical & Practice: Rainscreen cladding

Llewelyn Davies Yeang’s cladding for the Great Ormond Street Hospital redevelopment project is bespoke but easy to build, says Felix Mara

It’s no secret that the ragbag of orthodoxies once known as ‘rational’ cladding design has been consigned to history. Llewelyn Davies Yeang’s cladding to the Morgan Stanley Clinical Building at Great Ormond Street Hospital for Children demonstrates this shift.

The stone and render-clad rainscreen to this London hospital extension, due to be completed in December, relies heavily on off-site fabrication and unitisation to tackle programme constraints and the restricted site footprint. The hospital chose an architect with longstanding experience in hospital design and with the integrated design expertise to address these constraints. Nevertheless, the rainscreen facades are bespoke, with few repetitive units, as required by the building’s geometry.

Llewelyn Davies Yeang associate Mark Gage explains that this project, part of the Mittal Children’s Medical Centre, uses unitised rainscreen back-up walling that can be installed quickly, minimising storage requirements and enabling the facade to be sealed at an early stage. This reduces the construction time significantly, using the Nordicon prefabricated element system, made by the Finnish manufacturer Ruukki. Galvanised thermal purlins provide the units’ frame structure, and oval slots in the purlins’ webs reduce their thermal conductivity. Gage explains that the geometry of these slits restricts the flow of heat by increasing its travelling distance.

Before novation, Llewelyn Davies Yeang developed initial proposals and a performance specification for the rainscreen facades, working to a 7.8m planning grid, ideal for hospital design because it can accommodate two bedrooms. Floor-to-floor heights are 3.8m and 300mm-deep post-tensioned concrete slabs maximise floor -to-ceiling heights. Feature channels on the facades express these heights. At this stage Szerelmey joined the project team as rainscreen-cladding contractor in the first part of a two-stage tender. Llewelyn Davies Yeang and Szerelmey wanted to rationalise the cladding without compromising its performance or the design concept – in particular the unusual geometry of the west elevation, where ribbons of solid wall appear to hang from the attic storey.

Options included structural insulated panels, but Nordicon was chosen because it was sufficiently versatile for the demands of the concept and site. Nordicon’s thermal purlins, for which Ruukki holds the patent, enabled the facade to achieve a U-value 15 per cent lower than required by Approved Document L. The construction details for the stone-clad and rendered portions are similar in principle and both are vertically spanning. However, in response to the envelope’s geometry, the stone cladding to the west facade is fixed to the face of the structure, whereas the rendered cladding to the east facade – a Sto system – is supported off the slab edges.

Szerelmey construction manager Martyn Swash-Wallbank compares Ruukki’s thermal purlins to perforated steel lintels. He explains that Nordicon can be used to produce panels of any size, subject to transportation restrictions. Referring to Ruukki’s integrated use of 3D CAD software and CNC machinery, crucial in the fabrication of the bespoke units, he says: ‘We’re building an envelope the same way services contractors build M&E systems.’

The use of offsite fabrication has never precluded bespoke design. It is an enabler in some ways, because it is possible to carry out more complex operations in a factory than on site. But factory production is traditionally geared towards standardised modules, which are more economical to produce, with less margin for error. But with the sophisticated machinery, software and quality-control systems now available, and in a more competitive market, there is a lower premium on repetitive design.

However, many designers still favour repetitive cladding units as part of their design philosophy, or as a regulating device in facade composition.

Credits

Start on site – November 2008

Contract duration – 37 months

Gross internal floor area – 18,529m²

Form of contract – Design and build

Total cost – £88.5 million

Cost per m2 – £4,776

Client – Great Ormond Street Hospital for Children NHS Trust

Architect – Llewelyn Davies Yeang

Structural engineer – WSP Buildings

Cost consultant – Gardiner & Theobald

Cladding consultant – Szerelmey

Project manager – Gardiner & Theobald Management Services

Main contractor – BAM Construction

Annual CO2 emissions – Not supplied

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