The bold glass facade of Oxford Street’s latest retail addition presented both technical challenges and contractual risk to main contractor Mace. Stephen Cousins reports.
It has been referred to by critics as a huge glittering slug and a misconceived cruise liner of a development, but standing on its 6,000m2 island site on London’s bustling Oxford Street, the audacious curving facade of Park House delivers something many shoppers will love: a bit of West End bling.
Even on this, Europe’s busiest shopping street, where flagship stores run by the likes of Selfridges, HMV and John Lewis vie for attention, the £135m development is hard to miss, its nine storey-high glass facade bulging out beyond the existing line of store fronts and curving up and over the roof line to create a seemingly solid capsule that dominates the streetscape.
The non-uniform geometry of the envelope, based on a rotated doughnut-shaped section, raised the complexity of the building’s design and forced main contractor Mace, cladding supplier Focchi and roofing supplier Seele to work up intricate 3D models needed to break the curved surface down into hundreds of panels suitable for manufacture. In certain areas, such as the tight corners of the facade and areas of the roof, there was no underlying geometry at all, which meant every beam, transom, glazing panel and metal panel had to be designed and manufactured to a different size, shape and orientation.
The logistics of the roof installation proved particularly daunting as each panel had to be delivered and installed in a set sequence with millimetre accuracy, and because no two panels were alike, any damaged or missing pieces would have jeopardised the programme, as Sean Tate, project director at Mace, explains: “It was like the biggest, most complex jigsaw you have ever seen and it took a lot of head scratching just to get the roof to a point where it was suitable for manufacture. We were all holding our breath when the first panels arrived on site because so much had to come together perfectly for it to work, there was always the concern that there might be one detail we had missed or hadn’t paid enough attention to.”
Alternate vision and spandrel panels
Located close to Bond Street Tube station near Marble Arch, Park House is the largest development on Oxford Street for 40 years, comprising 8,300m2 of retail space, on the lower ground, ground and first floors, plus 15,000m2 of office space and 39 apartments with stunning views across Oxford Street and Mayfair.
The building’s elongated shape was informed, in part, by the brash architectural context of Oxford Street, which made it important to command a retail presence as well as match the neo-classical designed Selfridges building opposite with a structure of equivalent size and bold design. The planners also required that, despite the variety of uses inside, the building should retain a singular form.
Park House first went to site in December 2008 with Mace appointed by client, Land Securities, as construction manager and Sir Robert McAlpine acting as main contractor. But legal challenges to the planning consent and financial problems brought on by the recession meant the project was subsequently mothballed for 12 months with only the basement level complete.
In May 2010 Mace took full responsibility for the design and build under a fixed-price contract, won through competitive tender, with Land Securities taking on the role of project delivery manager, having sold the building.
With the weight of extra risk on its shoulders, Mace chose to revisit the initial design for the envelope, developed in its entirety by Focchi, which utilised the same cladding system for both the walls and roof, but had a greater potential for latent defects.
This initial system comprised a curved steel sub-frame, bolted to the building’s primary steel frame, with individual flat metal and glazing panels installed into complex rubber gasketry. “On the steeply curved roof areas it meant that one corner of each glazing panel would always stick out, requiring a very tricky gasket design to prevent daylight or water from entering through the gap created,” says Tate. This would have made the roof look more faceted and armadillo-like than curved, and in terms of latent defects, there were around 700 opportunities for the building to leak, he adds: “It made us really nervous, so we looked at ways to stick with the design intent but improve it.”
Luxury apartments are situated at the top of the building
Although Mace chose to retain Focchi’s design for the walls of the building, which are faceted rather than curved and could be easily unitised, cladding contractor Seele was drafted in to work up a more practical solution for the roof. Its design deals with the building’s geometry in a completely different way: rather than relying on the complexity of the structure below the glazing panels to accommodate with the roof’s curvature, it instead uses a faceted carrier system and transfers the complexity into the glazing itself, which is moulded into different curved profiles. It meant the gasket system could be fitted into a steel sub-frame flat and required no twisting to accommodate the glass.
Both warm and cold bending techniques were used to mould the glass panels into shape. A total of 624 glass panels were bent “cold”, ie manipulated on site under their own weight to fit into the gaskets, and fitted primarily along the elongated central section of the roof where the curvature is less pronounced.
The 214 remaining glass panels on the more acutely curved areas of the roof at the north and south ends were warm-bent in a factory by Italian company Sun Glass, a process that involved softening each panel’s four panes of glass in an oven into a pre-defined shape.
“The specific technique remained a trade secret as Sun Glass refused to reveal it to Seele, which itself has a glass manufacturing arm,” says Will Stevens, director of facade engineering at Ramboll, part of the client monitoring team, who now runs his own consultancy Interface Facades. “Our tests on the as-manufactured panels showed that their technique improved the optical quality of the glass, increased its strength, and the client preferred it to using monolithic toughened glass,” he says.
Although the building’s walls are all glazed, the roof comprises a combination of glazed panels and horizontal spandrels and metal infill panels that wrap across the roof in strips and accommodate opening purge ventilation panels for the apartments. Designed to the same principles as the glass, these were easier to bend into shape and fit.
Pedestrian space
The finger-shaped curve of Park House in plan is a response to the lines of buildings on adjacent streets, as well as the gentle curve of Oxford Street, which leans by 2m along the length of the site. Meanwhile, the curve in section was developed to maximise pedestrian space at street level and elevate the roof height, says Niall Monaghan, project architect at Robin Partington Architects: “Analysing right to light restrictions we saw that Park House could be raised a little higher than adjacent buildings, but it would have to step back from the facade, so we decided to introduce a curved roof above an upper level balcony. Inclining the facades at ground level increased the space available for pedestrians,” he says.
The combination of these plan and section parameters led the architect to develop a geometric system based on a series of torus sections — essentially a section through a doughnut — fused together. “Sophisticated 3D modelling software was used to refine the shape and then work out how the geometry could be translated into a panellised system. We were awarded an innovation award by software developer Bentley for this work on the project,” says Monaghan.
The use of 3D software informed much of both the design and installation work. Tekla and Pro-Engineer models were created to coordinate the primary steel structure with the envelope, as well as map out the locations of services. “With so much glass up there, it was important to locate pipes and ductwork in areas that couldn’t be easily seen,” says Tate. A 3D model was needed when setting out the non-uniform geometry of the roof, he adds.
“Traditionally 2D drawings are used to set things out based on a structured grid, but here the grid changes depending on how far above the floor plate you are, the roof shrinks the higher you get, which means there’s nothing constant to reference the drawings to. Early on we saw that it was going to be very difficult to produce any 2D information that made any sense,” he says.
Park House roof level in plan: the spandrels are blue and the glazing light grey
With so much glass on the building, energy performance was an important consideration, but the project was in gestation for several years, so there was a limit to what Mace could achieve beyond the requirements of Part L 2006 without radically altering the design.
High performance solar-controlled glass was used on the upper storeys, with more robust coatings used on the roof to reduce solar gain. Fixed louvers are fitted across the east and west sides of the roof, and electronically operated solar blinds, designed to move across the underside of the curved glass like a second skin, are fitted in the roof level apartments and offices, all of which helped Park House achieve a “very good” BREEAM rating.
The high level of coordination during design and manufacture, and ultimately installation by Seele, ensured that the team was able to meet the +/-0.5mm tolerances required to make the roof structure work and the finished building was handed over in November ready for throngs of excited Christmas shoppers.
“We’ve pushed the glass technology to the limits here. Using curved glass to create both a roof and elements of the facade is unique and to make it work in both applications to the performance we required was a real challenge. Our supply chain did fantastically well to make this nebulous geometric concept a physical reality,” concludes Tate.
A unique commission
The glazed facades of Park House are divided by vertical 76ft high artworks created by the sculptor Walter Bailey. In an unusual approach to commissioned art, Bailey was asked by architect Robin Partington to work up sculptures that could be incorporated within the fabric of the building.
The artist typically works with wood, carving it into shape using a chainsaw, but at Park House he worked for the first time with glass, or more specifically white frit, a ceramic baked on to glass.
“The original idea was to have real timber sculpted panels inserted between the panes of glass, but this would have been very difficult, if not impossible, to install. So we had idea of taking a digital transfer of the artwork and baking it into the glass,” says Sean Tate, project director at main contractor Mace.
Bailey made design drawings, mock-ups, maquettes, and full size pieces for testing by engineers, and once they were happy with the general direction, he was allowed to improvise on a huge scale.
Bailey’s favoured drawing was eventually sent off to be digitised, a screen print was made, then the design was baked into the ceramic, with his brush strokes still apparent. At first, the architect was worried that in the street environment the frit would get too dirty, but Bailey convinced them that the interaction between the glass and the environment was what made
the works exciting.
