When it comes to architectural forms, especially those represented by steel structures, people might think of the "Bird's Nest", the new site of CCTV, and the Yintai Center... Looking at urban construction, all large and unique buildings have chosen steel structures without exception in terms of architectural form. Is it because modern designers have developed a preference for it, or because builders are fond of it? Recently, I visited the new site of CCTV and had a conversation with Ouyang Chao, a steel structure expert from China State Construction Engineering Corporation.

China's steel structure architecture started late but has a high starting point.

Before the formal conversation, Ouyang Chao first introduced a story to the reporter. A century ago, a fire that lasted for three days and nights nearly turned the entire Chicago to ashes. However, Americans took this as an opportunity to ignite a new torch of architectural art. Buildings made of glass and steel then spread across the major states of the United States, even ordinary chain stores became steel structures...

It is reported that until the mid-1980s, China's steel structure architecture was still at "zero". At this time, China's steel structure construction, which lagged decades behind the West, started at a very high point, with high standards and strict requirements, and began to develop rapidly. The starting point of China's steel structure construction can be traced back to 1984, when China State Construction Engineering Corporation won the bid for China's first steel structure building - the Shenzhen Development Center. At that time, in the domestic construction arena dominated by concrete structures, steel structure construction was an undeveloped virgin land. Quickly mastering the world's advanced carbon dioxide gas shielded welding technology became the "golden drill" in the hands of builders to complete this project. Thus, ten outstanding welders from the corporation went to Japan to learn. Experts at the Kawaguchi Iron Training Center, upon learning that most of them only had a junior high school education, asserted, "Even those with a solid cultural foundation need to study for 3 to 6 months. Your contract study period is only one month, and it would be good if one-third of you pass!" Faced with this humiliation, the ten Chinese welders concentrated on studying for 7 hours a day, discussing and honing their skills after meals. After 30 days, these ten young Chinese men graduated successfully. In the subsequent Shenzhen Development Center project, they played an important role, from welding a thickness of 130 mm, breaking the previous record of 110 mm, to ensuring that 350,000 linear meters of welds passed ultrasonic and magnetic particle testing, and achieving 98% of the welds meeting the highest (D-level) standards in the United States... From then on, China's steel structure construction could not be stopped!

Large public buildings using steel structures each have their own characteristics.

Ouyang Chao said: "Steel structures are now quite popular, and their shapes and construction difficulties are increasing. For example, the representative public buildings in China now include the new CCTV site project, the Shanghai International Trade Center, the Guangzhou TV Tower, and the National Stadium project. The new CCTV site project has both height and difficulty, posing a challenge for the construction of irregular building structures. The Shanghai International Trade Center is a super high-rise building, and the tower shape of the Guangzhou TV Tower is very complex, making construction with steel structures even more complicated. The National Stadium can be called a representative work of unusual shape." Taking the "CCTV" as an example, it has an inclination (overall double inclination of 6 degrees) and double cantilever (70 meters and 60 meters respectively) structure, and the cantilever structure will have some sag under the force of gravity; the steel used in this project is of a higher grade, while Q345D or Q390D is commonly used in general steel structure projects, the "CCTV" project used Q420D and Q460D, which is rare in general public buildings, thus bringing many technical difficulties and requirements for the production and welding of steel; the main building of the new CCTV site consists of two towers connected by a 14-story large cantilever structure, forming a huge "gate" shape. This shape poses a daunting challenge to traditional construction techniques. The cantilever sags under the force of gravity, requiring reverse deformation during construction, and the pre-adjustment value must also be calculated during the welding of the steel structure, a value that is rarely used in previous public buildings; additionally, the measurement of the project is also a challenge, as the structure is inclined, requiring each column to be positioned in the air, and if the pre-adjustment value is added, the installation difficulty will increase; as the grade of steel increases, its thickness has also increased to 135 mm, with up to 11 super long and super thick steel structure welds in the CCTV project, of which 7 have been completed.

Ouyang Chao also told the author that the steel structure of the "Bird's Nest" project features a spatial curve structure, which imposes higher requirements on the production of steel structures, because once an error occurs, it becomes very difficult to align the welding between steel structures. Additionally, the welding process is also relatively strict; the Yintai Center project is more conventional and not very complex, with the biggest difficulty in construction being the need to assemble two 39-meter-long trusses on the ground and then hoist them with a tower crane. The project is divided into an inner and outer tube, forming a frame in the shape of a "回" character, with the inner tube consisting of 35 columns, mainly for installing high-speed elevators. These columns require very precise installation and verticality during construction, otherwise, it will affect the elevator installation. He said that the three projects of China World Trade Center Phase III, Yintai, and North Electric Center are all super high-rise and adopt construction techniques such as large spans and super long welding. It can be said that the large public buildings in Beijing using steel structure construction each have their own characteristics and difficulties.

Super high-rise, large span, and special shapes are the development direction.

Ouyang Chao pointed out that the future development direction of steel structure buildings should focus on three aspects: super high-rise, large span, and special shapes. Special shapes are represented by the Guangzhou TV Tower, "CCTV", and the "Bird's Nest", and the common point of these buildings is that concrete construction is not feasible. Currently, buildings such as the capital, Wuhan, Baiyun Airport, and some modern train stations are also beginning to develop towards spatial curves and large spans, which can only be achieved through steel structures. Additionally, the grade of steel is getting higher and the amount of steel used in steel structure buildings is also increasing. The North Electric Center uses about 40,000 tons, China World Trade Center Phase III about 60,000 tons, and "CCTV" about 120,000 tons.