The space steel pipe truss structure system is an important member in the large-span space structures. The new campus gymnasium of Zhengzhou University consists mainly of three groups of circumferential trusses, three groups of radial trusses and three groups of struts. The outer ring, external radial trusses and middle ring form the main load-bearing skeleton of the structure. Through the design of a closed outer ring, it forms a tensile hoop that restricts the radial displacement at the sliding support end of the external radial trusses, thereby reducing the vertical deflection of the entire structure. This satisfies the code requirements while achieving the best economic index for the structural steel usage. The horizontal projection of this roof plane is axisymmetric in the shape of petals. Three closed circumferential trusses are set at radii of approximately 7m, 15m, and the periphery. Along the radial direction, 24 spatial trusses are installed and staggered along the circumference using the circumferential trusses as boundaries. The radial trusses are divided into outer, middle, and inner parts. The overall structure has a simple, light appearance with reasonable force distribution, intuitive force transmission, and good overall performance. Exploring it helps to understand the structural properties, guide design and construction, and provide a basis for the application of similar structures.
1 Overview of Truss Structures
In recent years, steel tube structures have been widely used not only in marine engineering and bridge engineering but also increasingly in industrial and civil buildings. The application of steel tube structures in China's building structures is becoming more and more common. For example, square tube trusses were used in Phase III of Baosteel, scaffold tubes; Jilin skating practice hall, Harbin Ice and Snow Exhibition Hall, Shanghai "Oriental Pearl" TV Tower, and Changchun Nangling ten-thousand-person stadium all use square steel pipes as the main structural components. Guangzhou Stadium uses both square steel pipes and round steel pipes for its roof, Shanghai Hongkou Stadium uses round steel pipes as the roof load-bearing system, and Chengdu Shuangliu Airport uses round steel pipes as the main load-bearing components for its roof. In public architecture, unique structural forms of steel tube structures continue to emerge, such as Sydney Aquatic Center, seamless steel pipe factory, Garden Grove Crystal Cathedral in the United States; in the single-layer large-space building field, besides continuing wide application in supermarkets, warehouses, and storage facilities, some super-large structures have also appeared, such as the hangar at Singapore's Zhangxuan Airport and the waiting hall at Osaka International Airport; additionally, there are lightweight long-span structures like pedestrian overpasses and crane structures; other special-purpose structures include antenna masts and aerospace launch stands, etc. The Beijing Botanical Garden exhibition greenhouse, completed in 2001 with a construction area of 7250 square meters, was the first domestic project to adopt curved steel pipe truss structures with intersecting nodes. The steel structure material is 16Mn, the largest pipe specification is 299mm x 12mm, and the total weight of the steel structure is 720 tons. The membrane structure roof of Shanghai Stadium mainly consists of 32 trusses formed by intersecting steel pipes and ring beams, forming a saddle-shaped structure symmetrical north-south. The maximum span is 288.4m, height 31.74-70.54m, the maximum diameter of the main truss steel pipe is 508mm, adopting directly welded K-type nodes. The longest cantilever beam is 74.162m, using UK Steel 50D material. The second-floor exhibition hall of Nanjing International Exhibition Center is a column-free large space measuring 243m long and 75m wide. The roof is arched, with 15m cantilevers at each main entrance on the north and south ends, and another 14m cantilever on the west side. It adopts a structural plan of steel arches and purlins.
2 Forms and Characteristics of Steel Pipe Truss Structures
2.1 Classification of Trusses: According to their stress characteristics and rod arrangement, they can be divided into planar tube truss structures and spatial tube truss structures. All upper chords, lower chords, and web members of planar tube truss structures lie within the same plane, with poor stiffness outside the structural plane, generally requiring lateral bracing to ensure lateral stability. In existing tube truss projects, Warren trusses and Pratt trusses are mostly adopted. Warren trusses are usually the most economical layout. Compared with Pratt trusses, Warren trusses have only half the number of web members and nodes, and the length of web member materials is uniform, which can greatly save materials and processing time. Vierendeel trusses are mainly used when architectural functions or usage functions do not allow diagonal bracing.
Spatial tube truss structures are typically triangular in cross-section. Compared with planar tube truss structures, they can have larger spans, and triangular trusses offer good stability and high torsional stiffness, with an aesthetically pleasing appearance. In situations where no or cannot place out-of-plane bracing, triangular trusses can provide large-span spaces. A group of triangular trusses is similar to a spatial rigid frame structure and is more economical. It can reduce the need for lateral bracing components, increasing lateral stability and torsional stiffness. For small-span structures, lateral bracing may not be necessary.
2.2 Section Form of Connectors Commonly used rod section forms include circular, rectangular, and square shapes. According to different connection component sections, truss forms can be divided into the following types:
C-C Type Truss: Both chord members and web members are composed of intersecting circular tubes.
R-R Type Truss: Both chord members and web members are composed of intersecting square steel pipes or rectangular pipes.
R-C Type Truss: Rectangular section chord members are directly intersectingly welded with circular section web members.
2.3 Truss Shape
From the perspective of truss shape (i.e., based on the type of chord members), they can be divided into linear and curved tube truss structures. With the continuous improvement of social aesthetic requirements, in order to meet the diversity of spatial shapes, tube truss structures are often made into various curved shapes to enrich the three-dimensional effects of the structure. When designing curved tube truss structures, sometimes to reduce processing costs, the members are still processed into straight rods, approximating curves with broken lines. If higher requirements are needed, pipe benders can be used to bend steel pipes into curved pipes, providing better architectural effects.
2.4 Advantages of Tube Trusses
Steel tube structures, due to their beautiful appearance, reasonable stress characteristics, and superior economy, have been widely used in modern industrial plants, warehouses, stadiums, exhibition halls, conference centers, terminals, stations, office buildings, commercial-residential buildings, hotels, and other constructions. For example, Shanghai Stadium, Shanghai Science City, the new terminal building of Capital Airport, the new Baiyun Terminal Building in Guangzhou, Guangzhou International Convention and Exhibition Center, Shanghai New International Expo Center, Nanjing International Convention and Exhibition Center, Nanjing Olympic Sports Stadium, Nantong Sports Convention and Exhibition Center in Jiangsu Province, and other large-scale projects have all adopted steel tube structures. Engineering practice shows that steel tube structures can not only well meet architectural requirements but also make the structure achieve safety, applicability, economy, and other performance indicators, conforming to the latest design concepts of steel structures.
Steel tube sections have a series of unique superior performances, mainly including the following aspects:
① The walls of round tubes and square tubes are generally thin, with a large radius of gyration of the section, so they have good compressive and torsional properties. Symmetrical section forms make the section moment of inertia the same for all axes, which is beneficial for the stable design of single members. The closure of the section improves torsional stiffness, and for local stability of plates, closed sections are superior to open sections with cantilever plates. In many cases, architects also prefer to use the concise appearance of steel tubes to express their architectural intentions.
② Among profile steels with the same cross-sectional area, steel tubes have the smallest external surface area, thus minimizing the contact area with the atmosphere. Moreover, since steel tubes are often sealed at both ends, the inside does not rust, significantly reducing the material consumption and painting workload of anti-corrosion and fireproof coatings. Furthermore, steel tube structures are easier to clean and paint, making maintenance more convenient.
③ Steel tube sections have good fluid dynamic characteristics. When subjected to wind force or water flow loads, the effect of these loads on steel tube structures is much lower than on other section form structures.
④ Steel tubes are easy to process. With the development of multi-dimensional CNC cutting technology, the intersection line cutting of steel tubes is no longer a problem, and many domestic steel structure processing factories have already mastered this technology.
Although the unit price of steel tubes is higher than that of ordinary open-section profile steels, the combination of the above advantages makes steel tube structures one of the preferred basic structural forms among many structural forms.
Conclusion: Using space steel pipe trusses as roof structures has many advantages, not only meeting the large-span requirements of this gymnasium, but also, as a structural system, it meets the development needs of large-span space structures, creating a perfect combination of aesthetics and mechanics in the design concept. This type of structural form has been widely applied in engineering practice and shows strong development momentum.