Completed in 1981, the former Angel Centre was leased to a telecommunications concern until 2006. After the company moved out of the building, it became apparent that it would not be possible to find new tenants without a considerable investment in the object. The mechanical services were outdated, the layout was inefficient, the facades and internal finishings were no longer acceptable. In short, the building had not aged well and was also disliked by the neighbours. Initial analyses of the actual fabric revealed its potential, however. The concrete structure was sound and pro-vided a viable storey height of 3.7 metres. A rehabilitation of the development seemed appropriate, therefore. The large courtyard and the open spaces within the site boundaries provided scope for extending the floor area, making it financially feasible to design a modern facade and to reorganize the interior.
Further analyses of the grey energy embodied in the building led to the insight that a reuse was essential. Avoiding the demolition and disposal of the reinforced concrete structure and a complete new construction process would substantially reduce CO2 emissions. The volume of the existing concrete also provided a great thermal storage mass. In conjunction with a new fresh-air intake system, considerable savings could be made in cooling the building. Avoiding demolition and redevelopment would also result in large cost savings as well as a significant reduction of the construction time. Ultimately, therefore, the advantages of reuse prevailed, although it was clear that this would lead to complex problems, particularly in coordinating new services with the existing structure.
The extensions – constructed in steel on the street face and in concrete in the former courtyard – were placed in front of the existing concrete structure. At the junction between the two, pairs of old and new columns were created. On most floors, these were clad with a common lining, although structurally the elements function independently of each other. For that reason, expansion joints were created that extend through the entire building. In addition, a fifth storey in steel construction was erected on top of the existing development. Roof terraces around the periphery, together with white-fritted, laminated glazed facades mean that this additional volume is set back physically and visually. As a result of these measures, the functional area of the new Angel Building is roughly 40 per cent greater. What is more, the individual storeys can be divided both horizontally and vertically.
The new facade is articulated at approximately six-metre centres by precisely jointed black aluminium sections. Motor-operated opening lights nearly three metres wide allow users to ventilate the internal spaces by means of the windows. The large areas of glazing are in highly transparent low-iron glass, which helps to establish a direct relationship with the stock of mature trees around the building. Screen printing of the glazing above eye level together with sunshading coatings minimize solar heat gains in the interior. The architects developed the facade system in close collaboration with the manufacturers, using full-size mock-ups. The individual facade elements were prefabricated at works complete with glazing, insulation and surface finishings. Using three cranes and without scaffolding, it was possible to install up to 16 panels a day in preassembled fixings at the edge of the floor slabs.
The newly designed atrium in what was once the courtyard forms the heart of the Angel Building. The centrally located main entrance provides access for both visitors and users to a common reception area. A lounge and cafe as well as a brake-out space at first floor level encourage communication between people, as do the balconies directly linked with the offices on the second and fourth floors.
The atrium is characterized by its strict, exposed concrete structural grid. Two-storey-high window elements on the upper floors accentuate the verticality of this space and ensure good daylighting in the offices. Shadow gaps between the concrete structure and the fenestration provide tolerances. The geometry and form of the atrium facades terminate at the top in the roof construction: a grille of deep concrete beams screens the technical structures on the roof so that they are not visible from inside. The covering, an ETFE cushion construction that can be opened in part, allows the ingress of large amounts of daylight.
A major consideration in replanning the building was to keep emissions low and thus reduce operating costs through the use of passive components, efficient systems and regenerable forms of energy. One example of this is the fritted, coated double glazing in the facade, which serves to reduce solar heat gains. Fly ash, a waste product from power stations, was mixed in the concrete of the atrium. This not only reduces the amount of energy embodied in the structure; it also improves the surface quality. When the windows are closed, the offices are served by fresh-air inlets, which can supply cooled air when required. For roughly 80 per cent of the year, the air supply is drawn in from outside via the roof without cooling. For the remaining time, two water-cooled chillers are available. Two biomass boilers fuelled by wood pellets provide hot water for the heating and sanitary facilities. Other measures include energy-saving lifts and lighting systems, water pumps that can be operated at various speeds, rainwater collection for use as grey water, efficient sanitary installations, waterless urinals and the use of timber from certified sources. In addition, the open spaces, with a stock of new and old trees, as well as the planted areas between the building and the road, help to create a cooler microclimate.
As well as saving grey energy through the reuse of the concrete structure and creating a good link to local public transport, these measures helped to gain the building a certification of “excellent” in accordance with the BREEAM standard of sustainability.
