Integrating the new with the old
Imperial College’s new landmark building for the Tanaka Business School, designed by a team including architect Foster and Partners and consulting engineers Buro Happold, is a striking example of how a new building can successfully integrate with existing structures and services to achieve a balance between user comfort and architectural aspirations.The £26 million building for Tanaka Business School at Imperial College in London, which was officially opened on 24 June by Her Majesty The Queen, is a mixture of new build and refurbishment. The new-build element comprises a striking stainless-steel drum housing a vertical stack of interactive lecture theatres and a multi-purpose Forum area, enclosed study spaces, café and breakout area. The tower and the business school’s new entrance are enclosed in a protective ETFE foil cushion and glass atrium, behind a grand colonnade announcing its presence on Exhibition Road. The atrium is the most striking aspect of the new building. It creates a year-round usable space, which adjoins several existing buildings, including the school of mechanical engineering. Peter Roberts, group manager with Buro Happold, and building-services team leader on the project, describes the atrium as ‘a lively, open space which facilitates state-of-the-art, shared learning’. The new atrium encloses existing offices, which were once able to open windows to the outside. This presented the design team with the challenge of achieving a balance between the architectural aspirations for the atrium and the need to create a comfortable environment within the atrium and also within the previously naturally ventilated offices adjoining it. The environmental strategy was key to achieving this — and the engineers employed the use of displacement ventilation, underfloor heating and cooling, natural ventilation and shared peak lopping techniques as the means to do it.
Environmental Strategy A key challenge was to optimise the environment within the atrium and keep temperatures comfortable at lower levels whilst also ensuring that temperatures outside the office windows higher up do not exceed peak summertime temperatures. Unchecked, the temperature at these higher levels could reach a sweltering 40°C. A balance was needed to counteract the heat gain within the atrium, whilst sensitively integrating building services into the design. As the atrium’s thermal environment behaves similarly to a greenhouse, environmental measures are essential to achieve a cooler environment within the occupied zone and reduce the temperature outside the highest office window. In winter, displacement ventilation is combined with underfloor heating to provide heating to the occupied zone and fabric protection to the space. Tempered fresh air is supplied to the basement plant room from an air-handling unit mounted on the roof. This air is heated to the required temperature and supplied to the lower ground and entrance area through integrated displacement grilles. Stale, vitiated air is mechanically extracted at high level. In summer, the atrium conditioning uses a displacement-ventilation strategy combined with underfloor cooling. The fresh air is supplied to the basement plant and cooled to 18°C before being supplied to the displacement grilles. The cooled slab assists the cooled air from the grilles to remain at a low level for a longer period, enabling it to be thrown further and optimising the environment within the occupied zone. The cooling strategy for the atrium is tied to that of the lecture theatres within the tower. These each require a dedicated air-handling unit to maintain room temperature at 23°C. To minimise the building’s dependence on risers, the atrium is treated as a large fresh-air plenum serving the occupants of the atrium, drum lecture theatres and the adjacent mechanical-engineering offices. As a consequence, fresh air is drawn from the atrium to serve the lecture theatres. This air is supplied to the space by grilles below seats to maintain comfort conditions. The ‘discharge air’ from these lecture theatres is returned to the atrium at 27°C, which peak lops temperatures within the atrium, resulting in a maximum temperature of 29°C outside the highest window. In spring and autumn the space is naturally ventilated to reduce energy consumption. Automatically controlled louvres in the atrium provide large periods of passive mid-season ventilation assistance. Before the new business school was constructed, the existing adjoining offices adjoining had enjoyed very good levels of daylight and natural ventilation. To understand the impact of the construction of the new atrium, a Computational Fluid Dynamics (CFD) model and computer-generated daylighting modelling were developed to better predict the conditions at every point within the atrium. As a consequence Buro Happold was able to balance the opposing requirements to admit as much daylight as possible (to ensure acceptable daylight levels within existing offices) without too much heat gain. The lessons learnt from the modelling were fed back into the design, helping to optimise the solar performance of the atrium’s ETFE foil cushion, and glazed façade. Further modelling was used to explore ways in which the existing office ‘box’, dubbed the ‘Black Tower’ by the users, could be brought up to an appropriate standard of appearance commensurate with the new entrance atrium facility. Full recladding was prohibitively expensive in terms of cost and time. Instead an approach of ‘overcladding’ was developed with Buro Happold’s specialist lighting-analysis department LiT (Lighting Technologies). LiT simulated the original existing base condition and compared several different solutions in terms of appearance, daylight quantity, daylight distribution, glare and view. All these parameters were modelled as photorealistic renderings using the Radiance program with the addition of Buro Happold glare calculations. Black, mirrored, gloss and matt surfaces were assessed. The proposal reached was a rather unorthodox louvre arrangement where the lamellas are pitched in an ‘upside down’ configuration. The final solution simultaneously successfully achieves several goals — visual screening of the old elevation as seen from the street, scooping of the zenith skylight into the offices and a degree of solar-glare control. The restriction of views to street level from the offices has been minimised by the offset and spacing of the lamellas. The overall result is a very cost efficient and elegant solution which unifies a previously incongruous cube of office space within the contemporary composition of the new entrance and business school facility. Throughout the building are examples of how Buro Happold has been able to integrate the building services seamlessly into the design. As part of the environmental strategy, natural ventilation is supplied through the glass louvres that cover the Black Tower, above the reception desk. As well as being aesthetic, the louvres create thermal mass to minimise the use of chillers; in peak conditions they remain closed The thermally active slab provides thermal mass to minimise the use of chillers, and can be ‘charged’ using off-peak capacity from the chillers, smoothing out the load profile The reception desk conceals a displacement grille that supplies air to the atrium The fully glazed façade structure uses exposed heating pipe to minimise condensation, but Buro Happold was able to integrate this to give a clean design.
Energy strategy At the start of the project, electrical demand for the campus was near full capacity, with about 2 MW of waste heat in the form of cooling water from the college’s combined heat and power plant being emitted at roof level. This conflicted with the aims of the business school, which had placed best practice and greater energy efficiency as the heart of the project brief — a philosophy which Buro Happold shared. Buro Happold viewed the electrical energy shortage as an opportunity to create an energy-efficient, absorption cooling strategy that would reuse the waste heat from the CHP process to cool the business school. This strategy involved optimising the operation of a steam-absorption chiller to reduce Imperial College’s electrical consumption and thus shorten the chiller’s ‘payback’ time. However, feedback from a lifecycle analysis made it clear that 100% absorption cooling was not appropriate to meet the client’s budget constraints. However, a balance between cost and the environment could be achieved using 50% absorption ‘topped up’ by a conventional electric chiller during peak conditions throughout the year. The use of the electric chiller is minimised, and the system is therefore flexible and adaptable with lower capital and maintenance costs. Peter Roberts concludes: ‘I believe we delivered on every promise we gave the client and architect regarding integration of services and truly maximising the useable floor area. Buro Happold has serviced a very challenging space without compromising the architecture. I believe that we have understood the architectural vision for this space and created opportunities to help the architect realise it.’ John Walsh, assistant director of estates — projects, said of the project: ‘We are very proud of this building. It not only provides a magnificent new home for our business school but an impressive entrance to one of the country’s finest universities. The design-led development of this building is fully in line with the college’s masterplan to provide a quality environment suitable for world class teaching, learning and research.’ www.burohappold.com
Photos: Buro Happold/Mandy Reynolds
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