Cladding the Cube – the ‘city within a city’ that forms the final stage in Birmingham’s Mailbox development – has involved no fewer than seven innovative and complex systems

Birmingham’s 25-storey mixed-use Cube building, now two-thirds complete, certainly deserves its nickname. Standing on a square 52.5m x 52.5m footprint, next to the start of the Worcester & Birmingham canal, the building’s four monolithic metallic walls are each clad with a matrix of projecting anodised aluminium boxes and double-glazed windows that create a pixelated pattern reminiscent of 1980s computer graphics.

The cubist exterior is only one part of the complex cladding challenge for architect Make, German specialist cladding contractor Haga, and contractor BuildAbility, a subsidiary of the Birmingham Development Company, as there are actually a total of seven different cladding systems being installed at the building.

Twisting through the Cube’s centre is a glass-clad open-roofed atrium that doesn’t incorporate a single 90-degree angle. On the canal-side elevation, a triangular wedge the width of the facade has been cut away, to be filled by a free-standing aluminium-clad fretwork screen. A two-storey angular glass-clad structure housing a restaurant will crown the scheme and project out over the street below, providing impressive views of the city.

‘This has been more like seven projects than one, the level of co-ordination between different cladding systems has been incredible,’ says Frances Gannon, project architect at Make Architects. ‘The result will be a building people either love or hate, but we set out to make a statement. If people were ambivalent, we would have failed.’

The £100m Cube is the final stage of the Mailbox retail development that is intended to regenerate the Ladywood area of Birmingham. On a four-storey podium of restaurants and retail space above a basement car park, the concrete-framed building will support five floors of offices, nine floors of flats, a two-storey hotel and two-storey restaurant.

Make’s concept for the building was a jewellery box: a solid exterior protecting the delicate architectural treasures within. Anodised aluminium was chosen for the glistening exterior cladding to reflect the city’s industrial heritage and because its colour changes dramatically according to the angle from which it is viewed. ‘The anodising process means you never get one perfect colour. We chose light and dark bronze but, in reality, the panels will cover a spectrum of colours,’ says Gannon.

The panels each consist of two insulated spandrel panels, an aluminium box section and a central window. Panels were manufactured in lengths of 3m and 3.75m depending on the floor-to-floor heights of the flats or offices behind, but are all based on a 750mm module to preserve the uniformity of the geometric pattern on the outside and to maintain the illusion that each floor is of an equal height.

The result will be a building people love or hate, but we set out to make a statement

Frances Gannon, Make

A total of 2,100 panels in 12 different types have been installed in various configurations across the elevations to create the complex pattern. These allowed the design team to tailor the facade to increase the number of windows to boost natural daylight, or insulation where necessary, while ensuring that the building is not viewed as four separate elevations.

‘One of Make’s mission statements is not to do glass boxes, but ensure we get as much insulation into the facade as possible. We had ambitious insulation targets for the cladding – the whole scheme has been rated BREEAM “excellent” – and by making a feature of the metal boxes we had an opportunity to boost this by adding insulation behind them,’ explains Gannon.

To reduce tolerances between adjacent panels Haga was forced to replace the conventional sawing equipment at its factory with a high-precision robotic water-cutting machine. ‘The solutions used on this project are unique and specifically engineered,’ says Michael Karlein, managing director of Haga.

One of these ‘firsts’ is the complex twisting geometrical form of the Cube’s central atrium, which begins two floors above street level and is open to the sky to allow light to flood down to the lower levels. ‘The cladding here is completely different and exciting,’ enthuses Gannon. ‘The public have got used to the outside, but they won’t be expecting this.’

The atrium’s geometry is based around a six-sided hole cast into the centre of each floor slab. These voids rotate and shift in position on successive floors. Unitised cladding panels fitted with opaque and translucent glass in a sub-frame are fixed to the six sides of concrete. Individual panes of glass are bonded to the subframe using high-strength adhesive tape, instead of the more common structural silicone glazing system. This new technology had to be presented to Birmingham City Council’s Building Control department to verify its safety and performance.

The geometry of the ‘twist’, where adjacent cladding panels are offset at acute angles, creating dart-shaped soffits and ledges between panes on different storeys, was a major challenge. Clever gymnastics were required to achieve precise intersections.

Higher up in the building, the atrium opens out on the canal-side elevation to bring light into the upper level flats. With a large chunk of the building missing, the Cube’s distinctive form will be preserved by reinstating the absent elevation using a ‘transparent’ fretwork screen clad in anodised aluminium.

People are going to be amazed at the scale of this thing. We have not found any precedents in the UK

Neil Edginton, BuildAbility

‘People are going to be amazed at the scale of this thing, we searched and couldn’t find any precedents in the UK or Europe,’ says Neil Edginton, director of BuildAbility. ‘The architect wanted the screen to look as random as possible with no continuous horizontal or vertical members, which meant creating a structural net of steel columns bolted together to form interlocking shapes.’

Like a pyramid, the structure doesn’t get its strength and become self-supporting until all the box sections are bolted together, so temporary works were required to keep it stable during erection. The screen has yet to be started, but gazing into the 53m-wide void you get an idea of how impressive and precarious it will look.

Equally impressive will be the rooftop restaurant, a steel-framed structure built on top of the concrete roof, whose curving glass curtain walls are intended as a manifestation of the inner atrium ‘exploding’ out of the top of the building. Haga’s challenge here has been compounded by the fact that the restaurant is 70m up and some walls cantilever out over the side of the building.

The architect also wanted the curtain walling erected without any visible steel members. ‘Normally a curtain walling system uses steel profiles, glazing and then a pressure suction cap in between, but here we are planning to use a stepped type of insulated glazing [where one pane is larger than the other], which is a first in technical terms,’ says Haga’s Karlein. The team developed a system using two ‘stepped’ panels of slightly different sizes, one of which would be toughened. These would be held in place by a T-junction to create a flush joint between the panes.

Most toughened glass breaks into tiny pieces when damaged, but the panes used here are heated during manufacture so they won’t shatter and can be quickly repaired using the building’s maintenance systems. ‘We’ve used it on smaller projects, but not on 3.5m-long panes like these,’ adds Karlein. ‘I can imagine people sitting there eating their meals, totally oblivious to the troubles poor Haga has been through to ensure they enjoy the view!’

There’s no exterior scaffolding on this project, so the glazing will be installed from cleaning cradles attached to the building’s maintenance cranes. The same system will also be used to replace any damaged panes.

As the 12 month cladding programme progresses, BuildAbility is confident the external aluminium envelope will be completed by the end of September, at which point work on the restaurant’s steel frame will begin. The building should be completed by June next year.

‘The concept of the Cube has always been very strong. The challenge has been sticking to the details we wanted and I’m proud to say that throughout the design very little has changed,’ says Gannon. cm

Up-front fee stabilises cladding supplier

The cladding package for the Cube has a tortuous history.

A Polish company initially won the contract, but it had to be retendered when the company went bust. Swiss company Schmidlin was then earmarked, but it too went bust, leaving the project team very nervous.

‘With so many cladding contractors in a fragile state we saw Germany’s Haga as the best option. They are a large, well-established family firm and comparatively stable,’ says Frances Gannon, project architect at Make. ‘Even though they have never worked in the UK and had to set up here from scratch we were confident they could deliver.’

To allow Haga to seriously invest in the project, the client, Birmingham Development Company, took the unusual step of paying the company a £1m fee up front, as main contractor BuildAbility’s director Neil Edginton explains: ‘There’s a big problem with cladding contractors going bust because they do so much design work up front, which is hugely expensive and they rarely receive any payment until the first panels are delivered to site maybe 18 months later. We overcame this obstacle by paying £1m to allow Haga to invest in its design team.’

This investment allowed Haga to dedicate 12 engineers exclusively to the Cube, as well as invest in the development of a state-of-the-art IT logistics system to monitor every stage of the project, from manufacturing to delivery and installation.

The system tracks each engineer, fabricator, installer and driver in Haga’s team, and the information is monitored at a central server by three IT specialists. Using Palm Pilot devices, staff make data entries every day, recording everything from materials costs to truck capacity and whether or not deliveries have been made on time.

‘The building programme is so well co-ordinated that if just one cladding box is missing there could be a real knock on effect. And any delays are amplified when you’re working with a company based thousands of miles away in Germany,’ says Edginton.

Michael Karlein, managing director of Haga adds: ‘When installing curtain walling it has to be done panel by panel, floor by floor. You can’t work on different areas of the building simultaneously, so using this system we can see immediately see if there’s a break in the logistics chain.’