ENGINEERING MARVEL

The National Stadium - A Public Space for Beijing

Ms. Sudeshna Mukherjee, Assistant Editor, Civil Engineering and Construction Review


The National Stadium is situated on a gentle rise in the centre of the Olympic complex to the north of Beijing. Its location is predefined by the master plan. All other urban and architectural decisions were initiated by a competition project and subsequently implemented in the Herzog & de Meuron, 2007 construction project. The most important principle throughout had been to develop an architecture that will continue to be functional following the Games in 2008, in other words, to create a new kind of urban site that will attract and generate public life in this part of Beijing. Significantly, the Chinese themselves nicknamed the stadium “Bird’s Nest” in the very early stages of the project, thereby essentially assimilating it as their own, before it had even left the drawing board. For 2008 Olympics games a vision of international teams of architects and engineers were called for an adventure of steel and concrete, made with modern techniques. The Stadium’s steel roof is 320 m long and 297 m wide. It had accommodated 91,000 spectators to see 200 countries competing.

The stadium has a gross volume
of three million cubic metres and
is the world’s largest enclosed space.
It is also the world’s largest steel structure with 26 km of unwrapped steel used

From a distance, the stadium looks like a gigantic collective shape, like a vessel whose undulating rim echoes the rising and falling ramps for spectators inside the stadium. From this distant perspective, one can clearly distinguish not only the rounded shape of the building but also the grid of the load-bearing structure, which encases the building, but also appears to penetrate it. What is seen from afar as a geometrically clear-cut and rational overall configuration of lines, evaporates the closer one comes, finally separating into huge separate components. The components look like a chaotic thicket of supports, beams and stairs, almost like an artificial forest.

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This space, surrounding the interior of the stadium, is façade, structure, decoration and public space all in one. It is the link between the city outside and the interior of the stadium and is, at the same time, an autonomous, urban site. Herein lays the real potential of the project; it aims to be more than an Olympic sports arena for one single, admittedly unique occasion. This area between inside and outside affords the opportunity to create a new kind of urban and public place – even more so in view of the fact that people in Beijing love public life and are experienced users of public space. Sports, games, later rock concerts and other activities will, of course, dominate the use of the interior, while the new park on the plinth will invite people to stop a while and relax; the really novel feature of the project is clearly the transitional space between interior and exterior.

The Plinth

The geometries of the plinth and stadium merge into one element, like a tree and its roots. Pedestrians flow on a lattice of smooth slate walkways that extend from the structure of the stadium. The spaces between walkways provide amenities for the stadium visitor: sunken gardens, stone squares, bamboo groves, mineral hill landscapes, and openings into the plinth itself. Gently, almost imperceptibly, the ground of the city rises and forms a plinth for the stadium. The entrance to the stadium is therefore slightly raised, providing a panorama of the entire Olympic complex.

Structure - Façade - Roof - Space

The spatial effect of the stadium is novel and radical, and yet simple and of an almost archaic immediacy. Its appearance is pure structure. Façade and structure are identical. The structural elements mutually support each other and converge into a spatial grid-like formation, in which façades, stairs, bowl structure and roof are integrated. To make the roof weatherproof, the spaces in the structure of the stadium are filled with a translucent membrane, just as birds stuff the spaces between the woven twigs of their nests with soft filler. Since all of the facilities – restaurants, suites, shops and restrooms – are self-contained units, it is largely possible to do without a solid, enclosed façade. This allows natural ventilation of the stadium, which is the most important aspect of the stadium’s sustainable design.

The Bowl

The stadium is conceived as a large collective vessel, which makes a distinctive and unmistakable impression both when it is seen from a distance and from close. Inside the stadium, an evenly constructed bowl-like shape serves to generate crowd excitement and drive athletes to outstanding performances. To create a smooth and homogeneous appearance, the stands have minimal interruption and the acoustic ceiling hides the structure in order to focus attention on the spectators and the events on the field. The human crowd forms the architecture.

Earthquake Resistance of Beijing National Stadium

As China is surrounded by some of the deadliest fault lines, the stadium had to be earthquake proof. The building structure of Beijing Olympics stadium had to pass vigorous seismic tests. Mr. J. Parrish was given the responsibility of making the Bird’s nest earthquake proof. The immediate problem was that the stadium was so large that it would not shake uniformly altogether with the same frequency. As a solution, he had decided to construct concrete bow in parts, then surround it with metal bow.

Dividing the concrete building structure of Beijing stadium into 6 parts was the key to safety. Being isolated, each part could move flexibly and independently in earthquake so that it could absorb the energy by its free movement, instead of being broken into pieces by this energy. Then it was surrounded by metal bow. So that, if in the worst case concrete fails, the steel would be unaffected and would support the structure frame. The structural guidelines of the Olympic committee were also met accordingly i.e. the stadium had to have giant screens, shops, restaurants, cafes, bars and above all spectacular views of the competitions. Every seat was designed to be as close to the action as possible. No seat was more than 142 m from the center of the stadium.

Structural Design of Bird Nest Stadium

With the design phase completed, work on the concrete bow structure of the stadium began. By May 2003, construction was on the way. But the project faced new problem when the newly constructed airport terminal in France, having the same un-supported roof structure as in Beijing stadium, fell to ground without any warning. The roof of the terminal was designed without any interior supports, same as the design of the bird’s nest.

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The work on the project came to a halt as Chinese officials reviewed the safety of the stadium. The officials then reduced the number of seats, canceled the roof design and recommended a new cheaper and safer design.

After 5 months the project resumed. The architects had to redesign the stadium as an open air, which made the stadium cheaper and safer. Another problem was of the time, too much time had been wasted in the reviewable process, and now more workers were required. Also winter was approaching in which water in concrete could freeze, causing strength reduction problems. 7,000 workers had to be brought in, who worked in 3 shifts. With increased speed of about 1 floor a week, the workers finished concreting in 5 months. On 28th October, 2005, the 1st steel pillar was hoisted and for the rest of the structural design, the architects’ plans called for massive steel beams that would twist and turn around the 300 m wide stadium. The pattern needed to look random to create the illusion of the Bird’s nest. But at the same time, the beams had to support each other and carry the heavy loads safely so that this artistic vision could stand on its own.

Use of CAD in Beijing Olympics Stadium

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To organize the beams in the required patter, engineers built a 3D model of the Beijing stadium using advanced CAD technology. For this purpose, Aerospace software was used. The structure consisted of 24 pillars around the stadium. The pillars were in the form of trusses, encircling the concrete bow. Unlike other concrete structures where skeleton is inside, in this stadium the skeleton was designed to be on the outside of the reinforced concrete structure. A second set of beams filled the spaces between earlier ones, and the linked beams hold it together to form a braided structure. A third set of beams supports the stairways and provides a frame for the roof membrane cover. Its main purpose is to fill all the gaps left by the second system.

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Before construction could begin, the team had to make sure that the steel had enough strength to bear the loads especially around the curved edges where the steel beams had the twists. The beams start from the floor to the top of the building, bend perpendicularly over the roof and then cross the stadium to bend down again to reach the floor. The length of these beams comes out to be more than 300 metres from one end to the other.

For these highly ambitious beams, the team had engineered a new, special kind of steel to handle the loads and yet flexible enough to withstand the earthquake shaking. Computer models of the stadium were developed and shaking load was applied to see the dynamics of the structure. This enabled the engineers to determine the amount of shaking and distress caused in the stadium due to earthquake loads. The flexibility of steel required to withstand the earthquake was determined by CAD models of the Beijing Stadium. The existing brands of steel did not have the required flexibility so Q460 - a new brand of steel was invented specially for this Project.

This brand was to be manufactured in Shanghai. Many tests were performed on this steel to confirm its strength. But the more complex work of really constructing this steel structure was still left undone. These steel sections had to be curved, lifted, placed and connected properly to each other. Making curving beams also required to be engineered. To bear the loads and create the right aesthetics, engineers came up with the box design.

Four steel plates were welded together and bent into the required shape to form a twisted bar section.

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On site the workers welded the beams together to form even larger sections, forming a truss; some of the truss formed was up to 12 m in length. Each weighed up to 350 tons and was hoisted by 800-ton cranes to the top of the roof. Positioning these heavy structures also required great skill because the pattern was random, and the job was just as to fit pieces of jigsaw puzzle together. It was difficult to fit all the parts without any error because in positioning one end the other would go out. This took 2-3 days to weld the joints and secure them in place. As the strength of the structure also depended now on welds, so for welding of the special steel, over 1,000 skilled workers were trained. Welding the Beijing stadium was a hard, dangerous and slow job. Q460 proved to be hard steel and required temperatures higher than common steel for welding.

Some ends were left open and un-welded to allow for temperature changes during construction. Temperature in Beijing ranges from 20°C - 30°C. When the temperature raises, so does the Bird’s nest and when it drops, the Bird’s nest also settles down.

Removal of Formwork

Now the form work and other supporting structure were removed which held the beams in place. Until then the building was standing on 78 supporting columns. These columns were evenly spaced below the structure and were subjected to 42,000 tons of steel load. The unloading was to start in September 2006.

To remove the columns from beneath the roof, hydraulic jacks were used. These jacks would first support the roof, the columns would be removed, and the load would transfer to the jacks which would eventually lower, allowing the structure to support its own weight.

According to calculations, the roof had to settle about 30 cm and it did so after the supports were removed. Still they had to wait to see any further sinking or cracking. If so, it would mean that serious damage would occur. In full 1 week all the supports were removed. The frame was finally standing on its own on

Sept 17, 2007. Still the structure was just a hollow steel structure. It had to be furnished and fabricated to be an Olympic stadium. Flouro-Carbon polymer transparent sheets were used to fill up the spaces between the steel structural members.

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Finance

The China International Trust and Investment Corporation (CITIC) consortium, which raised 42% of the finance for the project in return for a 35-year tender after the Olympics were finished, comprises the CITIC Group, the Beijing Urban Construction Group, the Golden State Holding Group of the United States, and the CITIC Group affiliate Guoan Elstrong (a public, private partnership arrangement).

The remaining 58% was funded by the Beijing Municipal Government and this had been entrusted to the Beijing State-owned Assets Management Co Ltd as the city government’s representative.

Conclusion

The stadium has a gross volume of three million cubic metres and is the world’s largest enclosed space. It is also the world’s largest steel structure with 26 km of unwrapped steel used. The innovative structure was designed by Herzog & De Meuron Architekten, Arup Sport and the China Architecture Design and Research Group and has been nicknamed the “bird’s nest” due to the web of twisting steel sections that form the roof. As well as designing a modern stadium, the team was challenged with creating a venue that was part of the culture of China and would put Beijing on the map, which they fulfilled, marvelously.

References

1. https://www.arup.com/projects/chinese-national-stadium
2. https://www.herzogdemeuron.com/index/projects/complete-works/226250/226-national-stadium.html
3. https://www.aboutcivil.org/beijing-birds-nest-national-olympics-stadium-construction.html
4. https://en.wikiarquitectura.com/building/beijing-olympic-stadium/
5. https://www.designbuild-network.com/projects/national_stadium/
6. https://en.wikipedia.org/wiki/Beijing_National_Stadium