Extract from The Arup Journal

This article was written for a special edition of the Arup Journal devoted to the Beijing National Stadium. Click here to see a pdf version of the magazine (issue 1, 2009).

The architectural design concept

Introduction
At the time the architectural competition for the Beijing National Stadium was announced, Herzog & de Meuron and ArupSport (Arup’s multidisciplinary practice specialising in sports architecture) were already working together on the Allianz Arena in Munich. This successful creative partnership was based on a shared desire to innovate: Herzog & de Meuron in creating unique buildings with strong local cultural resonances and Arup in designing stadiums that perform ever better for spectators, athletes and operators. For the Beijing competition, the two practices joined forces with one of the leading Chinese Design Institutes, CADG.

Within this integrated team, the architects at ArupSport were responsible in particular for the bowl, the concourses and the spectator facilities, which together defined the form of the stadium. They also produced an initial optimised structural proposal for the roof and envelope, which Herzog & de Meuron then developed. CADG provided vital local expertise during the competition and scheme design, and then took the baton for the final stages of the project, liaising with the local authorities, producing construction information and monitoring the works on site. Backed by Arup’s engineering expertise, the competition team was able to submit a highly developed, fully realisable architectural concept. As a result, despite some significant changes to the brief, the form of the built stadium is very close to the original winning design.

The brief called for a landmark building that would be the main venue for track and field events during the 2008 Beijing Olympics, with a subsequent working life of 100 years. After the Games, it would become an important venue for both athletics and soccer. The stadium was to have a capacity of 100,000 during the Games and 80,000 seats in legacy mode. (The client subsequently decided to reduce the Olympic capacity to 91,000.) There was no defined legacy business plan, and so the design team tried to make the stadium as flexible and adaptable as possible. There is potential, for example, to add a hotel for box holders within the main envelope.

Originally the stadium was to have a retractable roof. (The client omitted this requirement from the brief late in the programme, as part of the general review of the Olympic venues before work started on site.) This was particularly challenging in structural terms as the building also had to have the resilience to withstand a major earthquake.

The bowl
The architects’ ambition was to create not only an instantly recognisable symbol of China’s cultural, sporting and economic renaissance but also the most exciting stadium in Olympic history. Every Games has its own thrilling “I was there” moments, when athletes perform miracles and new records are set. The team wanted to create a stadium that would harness and amplify this excitement in the way the world’s best-loved soccer venues do.

Like most modern stadiums, the “Bird’s Nest” was designed inside out, beginning with the bowl – the competitive field and the seating stands around it. This is because the form of the bowl and the distribution of seating types largely determine all other aspects of a stadium, including the shape and structure of the roof, the levels and locations of the concourses and premium facilities, and the amount of natural light and ventilation reaching the playing area. The team worked closely with the international Olympic and local organising committees to streamline and rationalise the on-field facilities. The result is a more compact bowl with less distance between the spectators and the track.

Bowl design involves a skilful balancing of a number of key criteria. Most importantly, spectators want to be as close as possible to the action and to have a good view of the field, while the stadium developer needs to accommodate a certain number of seats within a defined budget. These requirements often conflict. For example, more space between rows creates better sightlines but draws spectators further away from the field and results in a larger stadium with increased construction costs. Even a tiny adjustment to the configuration of the seats can have a huge impact on the overall design and cost of the building. To find the optimum solution, it is essential to set priorities.

This complex process has been transformed in recent years by parametric relationship modelling. Using powerful computer software, designers can quickly generate the initial form of a stadium within defined parameters such as geometric constraints, environmental factors and the limitations of construction materials. Having produced the initial concept, the architect can rapidly explore and test options by adjusting variables such as the height of a row of seats. For the National Stadium, ArupSport used its own specialist parametric modelling software to develop a bowl geometry optimised for Olympic athletics that would also work well for soccer in legacy mode. The team produced 33 versions of the design to fine-tune the form of the bowl.

The team decided that this landmark stadium should have the same distinctive external form in both Olympic and legacy modes, and so the temporary additional seating needed to be accommodated within the main envelope. The temporary seats, which are mainly to the rear of the top tier, have the least-favourable views in the stadium and are located in zones that can be converted to other revenue-generating uses.

Creating a stadium that will be both an athletics and a soccer venue is always a challenge. Athletics fields are bigger than football pitches, which means that spectators in the stands are further away from the action. Consequently, people in the upper tiers may not be able to see the ball on the pitch, and the atmosphere – which is so important to a soccer crowd – may be seriously diluted. One solution to this problem is to add a moveable lower seating tier for soccer matches. The brief for the National Stadium, however, did not allow for this. Instead, the team opted for a cantilevered middle tier, with the front 15 rows of seating extending over the lower tier. This brings spectators in the middle and upper levels closer to the action and provides a quality of view equivalent to that in a stadium with a moveable tier. The colour of the seats ranges from red in the lower tier to white at the top, helping to make the stadium look full, even when some places are empty.

The team members had to design a stadium that conformed to rigorous local seismic codes while providing a structure stable enough to support a moving roof. To meet these two key elements of the brief, they decided at an early stage to keep the bowl structurally separate from the stadium façade/roof structure. The bowl consists of six structurally-independent segments with 200mm wide movement joints between them. The continuously-curved form of the seating tiers provides better viewing standards for all spectators with lateral views as well as an enhanced C value (the quality of a spectator’s view over the row in front) for VIP and premium seats.

The elliptical form of the bowl, the depth of its structure, the acoustic reflectivity of its envelope and a special lining below the ethyltetrafluoroethylene (ETFE) roof membranes give the stadium an outstanding acoustic quality. During the Olympics, many visitors were surprised and delighted by the atmosphere of intense excitement and drama.

The façade/roof structure
While Arup was working on the bowl, Herzog & de Meuron began gathering ideas for the external form of the stadium. The team members knew that to win this prestigious architectural competition, they would need to come up with an inimitable design that would reflect both China’s rich cultural heritage and its 21st-century technological prowess. The distinctive roof structure does just that. Its appearance, inspired by local crackle-glazed pottery and veined scholar stones, defies structural logic. It is an amazing display of architectural, engineering and construction innovation. Local people affectionately nicknamed the stadium the “Bird’s Nest” while the initial competition entries were on display in Beijing.

The roof structure spans a 313m by 266m space. The structure closely envelops the bowl and concourses, forming both façade and roof. The façade incorporates the stadium’s main staircases. The result is a compact and sinuous external form uninterrupted by masts, arches or stair cores. While the façade is open, a roof covering protects the spectators from wind and rain. This roof is made up of single-layer ETFE membranes stretched between the steelwork sections.

The bowl and external form of the stadium were developed in parallel, with Herzog & de Meuron working on the façade and roof while Arup defined the size of the bowl and proposed an optimised roof structure. The team agreed at an early stage to work with 24 nodes for the primary roof structure support, and Arup very quickly defined the top and bottom roof planes required for the most efficient structure. This provided Herzog & de Meuron with an envelope form that did not change significantly even in the final construction design stage of the project.

The seemingly accidental arrangement of steel members that form the envelope makes it almost impossible to distinguish between the primary structural elements supporting the roof, the secondary staircase structures and the tertiary elements that add to the random effect. Each of the façade’s steel members retains a 1.2m wide external profile as it twists and bends to follow the saddle-shaped geometry of the stadium. The steel structure is painted light grey, contrasting with the red-painted external concrete wall of the bowl, which is clearly visible through the façade. This creates a variety of impressive effects, particularly when lit at night.

Conclusion
With the lavish opening and closing ceremonies, the thrill of broken records and the tragedy of shattered dreams, an Olympic Games is nothing if not theatrical. The architectural team wanted the audience to feel part of the Olympic spectacle from the moment of arrival. To enhance the sense of drama, the team decided to leave the façade unclad, allowing the staircases that form part of the roof structure to remain open. Weaving past each other and offering clear views into every passing zone, they ensure visitors have an unusual degree of interaction with the building. The result is arguably one of the world’s most exciting architectural experiences.

Importantly, the stadium is also one of the most comfortable, usable and high-performance sports venues in the world. Arup has received an unprecedented number of glowing testimonials from athletes (both Olympic and Paralympic), spectators, the media, the organisers and the operators. Everyone loves the “Bird’s Nest”.