| Very tall buildings are few in most UK cities, and the ones there are dominate their local city skylines. In the UK, the urban context is the most important criteria to consider, therefore, whether an architect is designing a new tall building or significantly altering an existing one. The urban/city fabric is built up over centuries of “organic” additions and subtractions, so has many layers of functional, commercial, political and social context.
A Tale of Two Towers
Two recent PB projects illustrate how design and the technical application of CAD modelling helped to create high impact city centrepieces and contributed toward rejuvenating a part of the city centres of two of the UK’s premier cities Cardiff and Birmingham. Both projects are owned and occupied by British Telecom (BT), both are towers, both original buildings date back to the early 1970s, and both are icons of BT in their local environments.
BT Stadium House in Cardiff, the capital of Wales, and the BT Radio Tower in the centre of Birmingham, England’s second largest city, are high-profile design projects. In both cases, the attitudes of the local planners and council planning committees significantly shaped the character of the buildings and their development. The Cardiff planners tended to be more conservative, whilst the Birmingham council were recognised throughout the UK for promoting exciting and future facing designs as a mechanism for urban regeneration.
Figure 1: Current Schema CAD city model analysis
before (top) and after(bottom)
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Figure 2: BT Stadium House "exoskeleton" scheme, which was superseded by current design |
Figure 3: BT Stadium House and the
Millennium and the Millennium Stadium
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Figure 4 : BT Radio Tower, Birmingham
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Cardiff. The original BT Stadium House tower architecture was a dull brown, 18-storey, squat and expressionless building clad with exposed aggregate precast concrete (Figure 1). It was and still is a principle telecommunications network exchange. With the changes in technology and introduction of more compact communications systems, however, this strong and massive structure was no longer needed to cater for large amounts of heavy exchange eq uipment. The building’s archit ecture wa s representative of a historic age of the telecommuni- cations industry. In the present, BT wanted a new, 21st century iconic building that would show it to be a blue chip, progressive company.
The current completed design for the new Cardiff tower (Figure 1) superseded our original design (Figure 2) partly because of cost, but more so because of the planning authority’s resistance to allow any commercial signage or display devices on a tall building. Their view was that doing so would have created a planning precedent for other tall buildings within Cardiff. They considered Cardiff city centre to be a historic conservation area, as it contains predominately Victorian city architecture with a mix of conservatively designed 1950-80s developments. It is interesting to note that recent developments in the Cardiff Bay area outside the city centre historic confines have been allowed to be more adventurous in their architectural form and expression.
Our concept for the new architecture was to create a “sugar cube” in the city and to use re-cladding materials to alter the proportions of the existing squat building into a tall, sparkling centrepiece punctuating the skyline. Because of the tower’s near proximity to the new Millennium Stadium, it needed to create an architectural dialogue with the modern white structure of the stadium’s corner structural “pylons” (Figure 3).
The new cladding was designed not only for aesthetic and contextual purposes, but also to resolve inherent structural faults in the existing cladding. The building’s original pre-cast concrete cladding was failing, with small chunks of concrete falling from great heights causing a health and safety hazard. Vibrations from construction of the neighbouring Millennium Stadium during 1997-98 had exacerbated this problem.
Our initial survey revealed the presence of longitudinal cracks in the vertical protruding mullions of the pre-cast concrete panels. We found that in those areas where the mullions had cracked, packers had been used during the construction phase to set the depth of the horizontal soft joint. These packers had not been removed when the panel was positioned, thus rendering the joint ineffective.
A calculation of the expected creep and shrinkage in the building during the 25+ years since its construction indicated that the designed joints would have been adequate for the expected dimensional change if they had been constructed properly. Because the joint space was filled, however, there was a tendency for load being shed from the building into the cladding panels a load for which these panels were not designed.
Following a further survey of the building to ascertain the possible effects of this load, which had been shed into the pre-cast cladding, a number of further anticipated failure modes together with some further construction defects were discovered, including horizontal cracks running through the panels at quarter and three-quarter points, and water penetrating the building through vertical joints.
We evaluated several options for repair based on technical merit, health and safety (least risk), disturbance caused by the works to the occupants of the building, aesthetic value, and the client’s commercial sensitivity. Ultimately, our structural solution was to sheath the existing building shell in a new, sealed over-cladding consisting of composite aluminium cassette panels and large white glass curtain walling to the edge portions of each facade.
With the addition of a roof mounted “skylon” and carousel structure that hides all of the telecommunications equipment, the 120-m (400-foot) BT tower is now the tallest in Wales and is a positive addition to the image of BT for the city of Cardiff.
Birmingham. Birmingham is a large city conurbation with pockets of significant buildings of varying scales. The BT Radio Tower is the principle structure and building icon for the city (Figure 4). At 160 m (520 feet), it is the tallest structure in the Midlands, which is the central heartland of the UK. The planners encouraged us to be bold and to light the building up at night, intending it to act as a catalyst for other owners to illuminate their tall buildings. Birmingham is in the process of creating a modern city with a more architecturally progressive skyline.
To satisfy the requirements of the Birmingham City Planning Authority, the tower was recoated in an anti-carbonation paint. We chose to colour the building to help express more clearly its features and form, and to refresh the original colour scheme of dull ochre and dark brown. The main body of the building is now a grey-white, whilst the recessed portions and the balconies are multi-coloured and vary from facade to facade.
Figure 5:
BT Piper Logo (top),
BT Globe Logo (bottom) |
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Figure 6: LED Lights
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Figure 7: Current Scheme CAD model
street view |
Figure 8: Exoskeleton CAD
city model analysis.
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Figure 9: BT Stadium House, Cardiff,
from the River Taff |
The facade colours were chosen using Feng Shui colour principles dependent on the compass directions of the facade. Colours that relate to earth, fire, air and water were selected and balanced to give each facade a coherence and hidden significance. As the daylight changes, so do the colours, and their balance alters between adjacent colour elements. At night the colours help to make the tower act as a beacon for the city centre in the large sprawl of the city.
The company logo coincidently changed from the BT “Piper” to the “globe” colours during the course of the project (Figure 5). By happy coincidence, the colours chosen for the tower and the those of the new logo matched exactly.
A key architectural feature is the lighting, which uses a
new technology. On top of the tower a coloured light emitting diode (LED) display system acts as a citywide barometer display. It uses Philips Lighting’s latest “LEDLine” light fittings (Figure 6). Each light fitting incorporates a series of LED lamps, each with a lens and reflector. The resultant coloured light beam is very intense and well defined, perfect for shining brightly horizontally across the entire city day and night.
The Design Process
With both projects, we had the significant responsibility of imposing new or altered buildings on locally loved and familiar cityscapes, so it was important for us to consider carefully the balance of contextual imperatives with the client’s requirements and commercial drivers. For any project that requires presentation to a client or public interest group, our approach is to fully model the scheme. In the case of the two towers, we constructed CAD models (Figures 2, 7 and 8), many of which were applied to a photo montage of the
surrounding context for presentation purposes (Figures 1 and 9). For Cardiff, the surrounding city was roughly modelled so that fly-through animations could be used to assess the impact of the tower’s form from various vantage points.
When creating a CAD model, the rendition of the materials requires particular attention. Often part of the architectural concept is conceived by the consideration of how a building’s form is perceived in varying lighting conditions (lighting directions and colourations). Both towers’ design concepts played on the notion that a building’s form can alter subtly depending on the quality and nature of the ambient light. This concept was particularly successful in the case of BT’s Cardiff tower, enabling the tower to alter its proportions and size. With its tower in Birmingham, the colour helps to define the various features both from near and far vantage points, making it easier for viewers to see and appreciate the building.
BT Stadium House in Cardiff was completed in January 2004 and it is now known to the local people of Cardiff as “The Doughnut”a term of affection
in contrast to the city’s previous ambivalence to the tower.
The BT Tower in
Birmingham was completed in June 2004. A “Switching On Ceremony,” which was attended by a well-known UK celebrity and included local television and radio coverage, boosted public interest in the tower.
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