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Retrofit: HLM Architects reclads Sheffield Arts Tower

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HLM Architects’ recladding of the University of Sheffield’s Arts Tower refuses to sacrifice performance to conservation

Like it or not, a significant proportion of British architects’ work in the coming years is likely to be retrofit, largely driven by the need to meet rising standards in building performance, especially thermal performance.

The good news is that work to the outside of the envelope is likely to be sufficient for most existing buildings. This will help architects secure work that calls for creative design skills as well as technical aptitude.

In the case of buildings with facades that are cherished by owners, users and the public or protected by legislation, practices can offer a different range of skills, including sensitivity to the design qualities of existing construction and the judgement needed to find the right balance between appearance and performance. This balance can be especially complex in the case of 20th-century buildings, whose architects subscribed to a functionalist design philosophy.

HLM Architects’ retrofit of the Arts Tower at Sheffield University is an interesting case in point. The current phase of this work, which began in August 2009 and will be completed in September, will replace the existing facade with a new envelope complying with modern performance standards.

The Arts Tower, completed in 1966, was designed by GMW Architects. At a height of 80m, it is the tallest university building in the UK and it was listed as a Grade II* structure in 1993. Like the practice’s Commercial Union offices in the City of London, completed in 1969, it is sometimes seen as a refinement of the pioneering work in the USA by Mies van der Rohe. His projects, an outstanding example of which is the 1958 Seagram Building in New York, are distinguished by sophisticated glass and metal envelopes.

The Arts Tower is similar in some respects, with the exception of the opening sashes that were essential for natural ventilation. Mies designed out this feature for greater visual control. In January 2007, the university appointed HLM to make proposals for the tower. Working to a wide brief to identify problems and suggest tangible improvements, the practice proposed new entrances, break-out space, upgraded IT facilities, stripping out the upper floors and reinstating the eroded building fabric, without replacing the existing facade.

The proposals obtained planning and listed-building consent. HLM then reapplied for consent for extensive changes to the external envelope, with Gifford as facade consultant. Clive Everett, an associate at Gifford, explains that the original facade comprised anodised aluminium sash window units, which were installed in sequence ‘from the top left to the bottom right corner of the facade’.

The corners of the tower, which is a simple rectangle on plan, had fixed glazing. There were anodised aluminium panels in the vertical zone between the windows, which extended the full height of the building, running between the spandrels and the louvres at the top of the tower. These vertical strips align
with the concrete perimeter columns, set out at 1.2m centres on plan.

Like Mies at the Seagram Building, GMW emphasised the structure on the facade. The spandrels had 10mm castglass panels, framed by a grid of aluminium cover strips, like the windows and aluminium panels. These spandrels had a distinctive, aqueous quality.

English Heritage had to be convinced that it was not feasible to retain the existing facade. HLM and Gifford presented an overwhelming case. ‘The system had corroded together so you couldn’t take it out,’ says HLM associate Alan Robshaw, adding that re-anodising, which was long overdue, ‘would only be possible if the entire facade was removed’.

Many of the window sashes were jammed - a major concern in a building with high solar gain that was designed to be naturally ventilated and had no space for ducts, particularly with growing IT heat loads. English Heritage was less concerned with another major flaw: the heat loss through the facade. However, it was easily persuaded by the high safety risk in a facade that could only be reglazed by absailers.

The design team demonstrated that it wanted to retain the facade’s appearance and substantiated all modifications, so English Heritage agreed to wholesale replacement of the facade. As Robshaw explains, the design of the replacement facade was ‘a balancing act’ between performance requirements and appearance.

Working with facade contractor Parry Bowen, the design team developed a partially drained facade with modified Sapa sash windows and clear, high-performance glass. This glass, and the spandrels, can be replaced from within the building, as the before and after drawings on these pages show.
Although the replacement window profiles are more chunky than the originals, most are concealed behind the perimeter columns.

Click on the images below to see full-size pdfs of the pages from AJ 08.04.10

Replacement curtain walling specification

Appearance To match existing
System guarantee 30 years
Ventilation Improved control
Glazing and framing Current safety standards
Natural ventilation Resolve operational conflicts between opening windows and internal blinds
Thermal efficiency of facade Current standards
Solar gain To be reduced
Resistance to water permeability Current standards
Air leakage Current standards
Maintenance Resolve unsafe procedures. Integrate facade with existing cradle system
Sash system Modified and formed into semi-unitised module
Compliance standard CWCT (Centre for Window and Cladding Technology) standard for systemised building envelopes
Air permeability to windows 600/600Pa
Air permeability exfiltration 10m/hour/m at 50Pa
Water penetration BS EN 12208: 9A at 600Pa
Vision glass Centre pane U-value: 1.1W/mK. Average area U-value: 2.2W/mK
Spandrel units Average area U-values: 1.1W/mK
Column cladding Average area U-values: 1.1W/mK
Solar transmission through double-glazed vision units Total light transmittance: 0.65 per cent. Total solar radiant heat transmittance: 0.36 per cent.
Total shading coefficient 0.42 per cent


Project data

Start on site August 2009
Completion September 2010
Form of contract JCT design and build 2005, incorporating revision 1
External area of facade 8,960m Architect HLM Architects
Facade engineer/structural engineer Gifford
Fire engineer Arup
Services engineer Jacobs
Specialist M&E contractor Airedale
CDM coordination Faber Maunsell
Conservation plan Avanti Architects
Main contractor BAM Construct UK
Facade contractor Parry Bowen
Energy performance Airedale reported that the new system had the potential to reduce heating demand by 40 per cent

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