Posted June 2012
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| CCTV Headquarters was originally featured in the 2008 CTBUH Journal Issue III and OMA recently held the official construction completion ceremony, as featured in the CTBUH Global News, after more than eight years of work. |
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| Other Featured Tall Buildings |
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“It’s mainly the end of our work, but it’s actually the beginning of its life. From here on, the building finally will be what it’s made for.”
- Ole Scheeren, Co-Designer while at OMA
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The new headquarters of China Central Television contains the entire television-making process within a single building. The tower redefines the form of the skyscraper, with the primary system comprised of a continuous structural tube of columns, beams and braces around the entire skin of the building. The unusual brief, in television terms, was that all the functions for production, management, and administration would be contained on the chosen site in the new Beijing Central Business District (CBD), but not necessarily in one building. In their architectural response, however, OMA decided that by doing just this, it should be possible to break down the ‘ghettoes’ that tend to form in a complex and compartmentalized process like making TV programs, and create a building whose layout in three dimensions would force all those involved to mix and produce a better end-product more efficiently.
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Competition
China Central Television (CCTV), the country’s state broadcaster, plans to expand from 22 to 200 channels and compete globally in the coming years. To accommodate this expansion, they organized an international design competition early in 2002 to design a new headquarters building. This was won by OMA (Office of Metropolitan Architecture) and Arup, which subsequently allied with the East China Design Institute (ECADI) to act as the essential local design institute (LDI) for both architecture and engineering.
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| Figure 1. Architect’s impression of the building |
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The winning design for the 473,000m², 234m tall, CCTV building combines administration and offices, news and broadcasting, program production and services – the entire TV-making process – in a single loop of interconnected activities around the four elements of the building: the nine-storey ‘Base’, the two leaning Towers that slope at 6° in each direction, and the nine to 13-storey ‘Overhang’, suspended 36 stories in the air.
The public facilities are in a second building, the Television Cultural Centre (TVCC), and both are serviced from a third Service Building. The whole development will provide 599,000m² gross floor area and covers 187,000m², including a landscaped media park with external features.
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Architecture
The CCTV Headquarters combines the entire process of TV-making into a loop of interconnected activities. Two structures rise from a common production platform that is partly underground. Each has its own character: one is dedicated to broadcasting, the second to services, research and education; they join at the top to create a cantilevered penthouse for the television management.
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The ‘traditional’ approach to such a project would be to fragment all these programmatic functions into their individual parts: production studios in the inexpensive, industrial parts of town; administration and management in the financial district; the creative departments located in the ‘hip’ areas of the city. However, the consolidation of all these activities within a single building allows each employee to be permanently aware of the activities of his co-workers - a chain of interdependence that promotes solidarity rather than isolation, collaboration instead of opposition. This way the architecture itself contributes to the coherence of the organization. |
Figure 2. Diagram showing interconnected activities
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While CCTV is a secured building for staff and technology, public visitors will also be admitted to the 'loop' – a dedicated path circulating through the building and connecting to all elements of the program. Here the CCTV can present itself as a media organization to the public, with the looped nature of the circulation becoming a circuit of continuous dialogue between staff and visitors, allowing them to observe each other, to meet and to congregate.
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Figure 3. Looking up at the Overhang
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| This looped circulation not only promotes social interaction, but also offers multiple routes of egress in emergency scenarios. Compared to a single tower comprising the same floor area, the total evacuation time of the CCTV loop is less than half that of a purely vertical structure; in addition, it offers an alternative escape path through the other tower connected by the high-level overhang. |
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The design of the CCTV then aims to reinvent the tall building by creating a truly three-dimensional experience; a canopy that symbolically embraces the entire population, as oppose to a predictable two-dimensional tower that points only skywards. The creation of a continuous series of spaces and activities will promote the building as a giant social catalyst – a city in itself – where 10,000 employees will work and thousands of visitors will gain an insight into the functioning of all aspects of a television station.
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Figure 4. The circulation ‘loop’ of the building |
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Structural Design
From the outset, it was determined that the only way to deliver the desired architectural form of the CCTV building was to engage the entire façade structure, creating in essence an external continuous tube system. This would give the structure the largest available dimensions to resist the huge bending forces generated by the cranked, leaning form – as well as loads from wind and extreme earthquakes.
The ‘tube’ is formed by fully bracing all sides of the façade. The planes of bracing are continuous through the building volume in order to reinforce and stiffen the corners. The system is ideally suited to deal with the nature and intensity of permanent and temporary loading on the building, and is a versatile, efficient structure which can bridge in bending and torsion between the Towers, provide enough strength and stiffness in the Towers to deliver loads to the ground, and stiffen up the Base to reinforce the lower Tower levels and deliver loads to the foundations in the most favorable possible distribution.
The tube was originally envisaged as a regular pattern of perimeter columns, beams, and diagonal braces set out on a typical two-storey module. However, results of the preliminary analysis showed that the forces in the braces varied considerably around the structure, with particular concentrations near the roof of the Overhang and at the connection to the Base. This led to an optimization process in which the brace pattern was modified by adding or removing diagonals (i.e. ‘doubling’ or ‘halving’ the pattern), depending on the strength and stiffness requirements of the design, based on a Level 1 earthquake analysis. This was an extremely iterative process due to the high indeterminacy of the structure, with each changing of the pattern altering the dynamic behavior of the structure. It was carried out in close collaboration with the architect, since the pattern of visually expressed diagonals was a key aesthetic aspect of the cladding system.
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| Figrue 5. Unfolded’ view of final bracing pattern |
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Testing
Although the 234m height of the CCTV Headquarters is within the code’s height limit of 260m for steel tubular structural systems in Beijing, its geometry is noncompliant. The Seismic Administration Office of the Beijing Municipal Government appointed an expert panel of 12 eminent Chinese engineers and academics to closely examine the structural design, focusing on its seismic resistance, seismic structural damage control, and life safety aspects.
As part of the expert panel approval process, there was a requirement for three physical tests to be carried out, in order to verify the analytical calculations:
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1. Joint Test (‘butterfly plate’): Beijing’s Tsinghua University tested a 1:5 scale model of the column-brace joint to confirm its performance under cyclical loading, in particular the requirement that failure takes place by yielding of the element rather than at the connection.
2. Composite column: Tongji University in Shanghai tested 1:5 scale models of the project’s non-standard steel reinforced columns. These tests resulted from concerns that the high structural steel ratio might lead to reduced ductility.
3. Shaking table test: A 7m tall 1:35 scale model of the entire building was constructed to test the structural performance under several seismic events including a severe Level 3 earthquake. The tests were undertaken by the China Academy of Building Research (CABR) in Beijing, using the largest shaking table outside America or Japan. |
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Figure 6. Shake table test model |
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Construction
The first column element was placed on 13 February 2006. In total, 41 882 steel elements with a combined weight of 125 000 tonnes, including connections, were erected over the next 26 months, at a peak rate of 8000 tonnes per month.
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Construction of the Overhang began after the steelwork for the two Towers was completed to roof level. Tower 2 Overhang began first, in August 2007, and the structure was cantilevered out piece-by-piece from each Tower over the course of the next five months. This was the most critical construction stage, not only in terms of temporary stability but also because its presence and the way it was built would change the behavior of those parts of the Tower already constructed.
The contractor chose to connect seven link elements at the inside corner of the Overhang during the initial connection phase. These were lifted into place – to less than 10mm tolerance – and temporarily fixed with pins in the space of a few minutes at 9:00am on 8 December 2007, before the Towers started to move relative to each other. The pins allowed them to carry the thermal loads while the joints were fully welded over the following 48 hours.
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Figure 7. Overhang construction
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| A topping-out ceremony on 27 March 2008, on a specially-constructed platform at the corner of the Overhang, marked the completion of the steelwork installation. |
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| Figure 8. CCTV at Night |
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Conclusion
The skyscraper was born over 100 years ago, when the elevator made it possible to have access to previously unimaginable levels of a building. Also steel made it possible to build higher and faster and electricity to illuminate deeper spaces and to inject conditioned air. Over the past 100 years all these technologies have improved, but nothing has essentially changed.
Instead of competing in the hopeless race for ultimate height - dominance of the skyline can only be achieved for a short period of time, and soon another, even taller building will emerge - CCTV Headquarters is an icon that actively engages the city space and has challenged the form of the traditional skyscraper.
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