Replacing the facade on a grade II-listed residential block required John Robertson Architects to find an approach of a different stripe
Great Arthur House was completed in 1957 and forms the centre piece of the Golden Lane Estate near London’s Barbican, one of the most important residential redevelopments after the Second World War.
The grade II-listed, 16 storey residential tower block was the first of its type to breach the 100ft height limit in the City of London, providing 120 one-bed apartments for key workers including policemen, teachers, and ambulance workers. The building was the first designed by the newly-formed practice Chamberlin Powell & Bon, which would go on to design the adjacent Barbican Centre, considered by some to be the UK’s greatest example of integrated urban planning.
But unlike the Barbican’s heavy concrete structure, Great Arthur House relies on a light and economical style of architecture made popular by the modernist, post-Bauhaus movement, its east and west elevations split elegantly into six vertical bays and 16 horizontal strips of glass with bright yellow spandrel panels. On the roof, two huge concrete wings containing the block’s water tanks extend out in a dramatic curve reminiscent of Le Corbusier, which were described by architectural historian Nikolaus Pevsner as ‘the first time that such arbitrary, purely decorative or purely expressionist motifs appeared in London’.
Fast forward to today, and Great Arthur House still stands strong, but its distinctive elevations are in a state of severe disrepair, suffering from extreme rainwater penetration, condensation and jammed windows.
As a result, the building’s owner, the City of London Corporation, decided to replace the curtain wall in its entirely, working with John Robertson Architects (JRA), consultant engineer Mott MacDonald and English Heritage to devise a double-glazed facade system to visually match the original but also meet the latest building standards and provide an average 31% saving on residents’ heating bills. The £6m scheme (around £50,000 per flat) was recently granted planning permission by the City of London and a cladding contractor is expected to be appointed soon.
Great Arthur House’s curtain wall framing system, originally manufactured by Wandsworth firm Quicktho, was crudely assembled compared to today’s standards, incorporating hundreds of single leaf sliding windows more commonly used in caravans and London Routemaster buses.
‘An original advertisement for Quicktho promised “rapid fitting, faultless action, perfect ventilation and reduced body weight”,’ recounts John Robertson, founding director of JRA. ‘Well, today the windows certainly have perfect ventilation, they leak like a sieve, and as for faultless action, the window sections slide on felt runners, rather than metal tracks, and have nearly all jammed up.’
The thin 4mm-wide window sections are not thermally broken, so the temperature gradient between the outside and inside is acute, causing severe condensation. ‘In several flats the residents have had to cut away the plaster on the internal walls as it had become saturated and rotten,’ he adds.
An investigation into the existing facade structure, by JRA and Mott MacDonald, revealed several cost-saving construction short cuts. The Quicktho extrusions had been applied using wood screws in horizontal teak boards that were fixed to the concrete slabs. Also, upstands behind the cladding panels were constructed from cheap hollow pot and around half the distinctive dimpled yellow glass used to cover the spandrel panels had been replaced with poor quality replicas, compromising the uniformity of the original design.
Devising an appropriate replacement curtain wall raised a number of challenges. The team’s structural investigations revealed that the scheme’s original builder, Wimpey, had installed just one thin rod of rebar in the slab edges, rather than the two larger rods specified in drawings produced by the scheme’s original engineer Ove Arup.
A computer model of surveyed stress levels in the building, developed by Mott MacDonald, confirmed that the slab edges had almost no capacity to bear any extra weight created by a new double glazed curtain wall system, which would be roughly double that of the existing one. The only option was for the panels to transfer facade loads directly to the building’s main concrete shear walls, which run laterally across the building and are in good structural condition.
The truss acts like a bridge transferring loads to the shear walls, and avoids the need to fix each unit along the slab
‘We had the idea of incorporating a Vierendeel steel truss within the thickness of each 6m wide cladding unit’s insulated spandrel panel. Once hooked into position this will span the shear walls,’ says Robertson. ‘The truss acts like a bridge transferring loads to the shear walls, and avoids the need to fix each unit along the slab, which might have caused the slab and cladding to deflect.’
JRA’s final cladding panel design features a thermally-broken double glazed window with sliding sections, and an insulated spandrel panel below, which together will keep the flats warmer and virtually eliminate the risk of condensation. Although the upgrade to performance increases the facade’s depth considerably, the transoms and mullions will only be only around 10% wider so the overall appearance should be very close to Chamberlin Powell & Bon’s vision. Spectrographic analysis is also being carried out to ensure the correct shade of yellow is used for the covering on the spandrel panels.
Perhaps the most demanding challenge was to design a system that could be installed with minimal disturbance to the block’s residents. Panellisation and offsite manufacture will help speed the process of installation, but JRA also proposes to install a temporary wall of structural insulated panels (SIPs) inside each apartment along the perimeter edge. Once in place this will allow the residents to go about their normal business, while contractors dismantle and replace the facade from a mast climber outside.
‘We will remove and replace the cladding in sections of three floors at a time, reusing the SIPs as we move up,’ says Robertson. ‘The total programme is around 76 weeks and we expect each floor to take at least a week. The residents we’ve spoken to are pretty enthusiastic about the changes, although the curtains in some apartments are permanently drawn – they’re the ones that worry me.’
Perhaps the promise of a 31% saving on heating bills and an end to sodden walls will be enough to reassure owners having to shell out after the slow death of Quicktho.