These structures operate under three-dimensional load conditions, enabling them to withstand loads from all directions. They are particularly effective in seismic-resistant buildings with large spans. There are three types: planar trusses, curved spherical truss frames, and flat-roofed spherical truss frames. Their advantages include lightweight construction and the ability to be shaped into various forms. Planar trusses have all members and nodes lying in a plane, while the members and nodes of spherical truss frames can extend into three-dimensional planes.
When the axes of the truss members and the external forces they are subjected to are not in the same plane, the structure formed by connecting straight rods at their ends to form a primarily bending-resistant structure is called a space truss structure, also known as a three-dimensional truss structure. It is generally composed of two plane trusses connected at a certain distance.
Plane trusses have good in-plane load-bearing performance, but their stiffness is relatively low out of plane. To ensure the integrity of the structure, various supports must be installed. The arrangement of the support structure often consumes a lot of material and must be controlled by the slenderness ratio, which prevents the material strength from being fully utilized. Space trusses can effectively avoid these shortcomings and can be divided into regular triangles, inverted triangles, and rectangles according to their cross-sectional shape.
Components and Materials Used in Space Trusses
Compared with rectangular cross-sections, triangular cross-section space trusses can reduce the number of connecting rods. When the span is large, the upper chord has a large pressure and a large cross-section, so the upper chord can be divided into two to form an inverted triangular space truss; when the span is small, the upper chord has a small cross-section, so it is better to divide the lower chord into two to form a regular triangular space truss. When the two lower chords intersect at a point at the support node, a shuttle shape with pointed ends is formed, also known as a shuttle truss.
Space trusses have the advantages of large out-of-plane stiffness, easy lifting and use, and saving supporting steel. However, calculations are often cumbersome, the spatial angles of the rods are usually non-integer, the node structure is complex, the weld requirements are high, and the production is complex. Regardless of the type of truss, it can withstand loads from all directions and is more functional for buildings with large earthquake-resistant collapse distances. Commonly used basic units include quadrangular pyramid units and triangular pyramid units, and these basic units form a complete ball joint frame system.
Types of space truss structures
Classification by Structural Composition
Plane trusses: All members and nodes lie in the same plane, often used for simplified structures or localized support (such as building roof trusses). All members and nodes are coplanar and can only support loads within that plane, making them suitable for applications with concentrated loads within the plane and small spans. They offer a simple structure, easy manufacturing, and low cost; the forces acting on the members are clearly defined (only axial forces), resulting in high material efficiency.
Space trusses: Members and nodes are combined in three dimensions to form a stable spatial structure, suitable for applications with large spans and complex loads (such as stadiums and spacecraft supports). The arrangement of members and nodes in three dimensions allows them to withstand multi-directional loads, such as wind and seismic effects, making them suitable for applications with large spans and complex loads. They offer strong overall stability and can effectively resist out-of-plane deformation. The spatial combination of members reduces the number of supporting components and increases usable space.
Classification by Geometric Shape
Triangular Pyramid Systems: Composed of triangular pyramid units, they offer high stability (the inherent stability of triangles) and are commonly used in heavy-load structures (such as industrial plants).
- Heavy-load structures: industrial plants, heavy equipment supports (such as blast furnace workshops in steel plants, which need to bear loads of more than 50kN/m²)
- Large span requirements: main trusses of stadiums (span ≥ 60m), roofs of airport terminals
- High stability: The triangular geometry makes the nodes evenly stressed, and the anti-lateral stiffness reaches 1/150~1/200 of the span ratio
- Load distribution: Cone elements are used to convert vertical loads into axial forces, reducing bending moments in members (for example, the central exhibition hall of the National Exhibition and Convention Center uses an inverted triangular cone system, reducing steel consumption by 25%)
Quadrangular pyramid system: It is composed of quadrangular pyramid units and can be combined by upright placement, oblique placement, evacuation, etc., suitable for different span requirements (such as exhibition halls, airports).
- Bidirectional stiffness optimization: By adjusting the cone inclination angle (30°~60°), the in-plane/out-of-plane stiffness is balanced. For example, the roof of Chengdu Tianfu International Airport uses a 45° inclined square pyramid, which reduces the wind vibration coefficient by 30%.
- Improved economy: 15%~20% less steel used than traditional grid structures (e.g., Guangzhou Baiyun International Convention Center)
- Free-form surface system: irregular geometric shape, with curved rods forming curved surface supports, used for special-shaped buildings (such as museums).
- Parametric modeling: Based on BIM technology, complex shapes can be achieved (such as the C-shaped column surface accuracy of Beijing Daxing Airport reaching ±2mm).
- Load adaptability: The wind load shape coefficient (μs≤0.8) is optimized through curvature adjustment. For example, the wind pressure design value of the curved truss of the Shanghai Tower Observation Hall reaches 1.2kPa.
Classification by expansion dimension
One-dimensional expansion: Used for linear supports (such as bridges and towers). This creates a beam-like structure that extends in a single direction and is suitable for linear supports and small spans. It offers a simple structure and easy construction, allowing for quick erection of temporary support structures. Examples include the supporting trusses of pedestrian overpasses (such as the steel truss beams of urban pedestrian bridges) and the roof supports of temporary buildings.
Two-dimensional expansion: Used for flat or curved supports (such as floors and domes). This creates a plate-like structure that expands within a plane and is suitable for flat or curved supports that require large coverage areas. It can create a continuous support surface, reducing the number of nodes, and is suitable for flat structures such as floors and domes. For example, column-free rooftop exhibition halls in large shopping malls use two-dimensional trusses for continuous support, as do platform canopies at train stations.
Three-dimensional expansion: Used for connections in complex configurations (such as space station trusses and large antenna arrays). This creates a space-filling structure that can connect in three dimensions and is suitable for complex configurations and large spans. It offers high spatial rigidity and can withstand multi-directional loads. It can be assembled in modular form for easy expansion. Examples include space station trusses and large antenna arrays: parabolic antenna support trusses for satellite ground receiving stations (using a three-dimensional truss to form a stable parabolic shape). Expandable trusses
Categorized by special functions
Modular nodes (such as ball nodes) and quick-connect designs support on-orbit assembly or on-site expansion (such as spacecraft trusses and emergency buildings). Modular nodes and quick-connect designs support on-orbit assembly or on-site expansion, making them suitable for temporary buildings, emergency projects, or scenarios requiring rapid construction. Modular design shortens construction cycles, allows for reuse, reduces costs, and adapts to complex terrain.
Tensioned trusses: Combined with tensioned cables, prestressing enhances stability, making them suitable for large-span, lightweight structures (such as stadium canopies and large-span roofs). Prestressing offsets deformation caused by loads, reducing material usage; the structure is lightweight and aesthetically pleasing.
Load distribution and structural performance:
As an efficient spatial structural form, large-span spatial steel truss structures possess unique load-bearing characteristics. Under load, its members primarily bear axial tension or compression, resulting in a relatively uniform stress distribution across the cross-section. Taking a common triangular steel truss as an example, when subjected to a uniformly distributed vertical load, the upper chord is in compression, the lower chord is in tension, and the webs, depending on their arrangement, bear either tension or compression. This load distribution allows the material to fully utilize its strength. Compared to a solid beam subjected to bending, a steel truss can span a larger space using less material under the same load conditions. For example, in some large stadiums, steel trusses can achieve spans exceeding hundreds of meters while maintaining a relatively low weight.
In terms of material properties, steel offers the advantages of high strength, plasticity, and toughness. This high strength enables steel trusses to withstand large loads, meeting the load-bearing requirements of large-span buildings. These plasticity and toughness ensure that when subjected to dynamic loads (such as earthquakes and wind vibrations), the structure can absorb energy through plastic deformation, avoiding sudden brittle failure and improving its earthquake and wind resistance. For example, in long-span buildings in earthquake-prone areas, steel frames, with their excellent plasticity and toughness, can maintain structural integrity and minimize damage during earthquakes. Furthermore, steel's excellent workability facilitates factory prefabrication, improving construction efficiency and precision. Space trusses, which are trusses within a single plane, offer greater spans and more flexible spatial arrangements, making them widely used in large buildings such as stadiums, exhibition halls, and airport terminals.
Design, manufacturing, and installation process
Based on the external dimensions, node coordinates, and member tables provided by the design institute, steel structure detailed design software and drawing programs were used to reasonably divide the truss and design the nodes. The entire main structure was broken down into individual members and components, which were then drawn separately. The construction detailed design mainly includes the following content: node assembly general layout diagram, node assembly sequence diagram, member diagram, node diagram, and material list.
Cutting
The main structure was cut from a steel pipe, and the members at the component nodes were all intersecting joints. The steel pipes were cut into standard small pieces along the intersection lines, facilitating smoother welding of the truss assembly later and minimizing dimensional issues.
A wire cutting machine is an industrial device used to automatically cut the ends, holes, and elbows of intersecting lines on round, square, or shaped metal tubes. It is primarily used in industries such as construction, shipbuilding, and mechanical engineering. Plasma models are suitable for a variety of metal materials, while laser models can achieve precise bevel cutting.
Component Assembly
Components are connected and secured using connecting plates. Inspection items include axis deviation, gap, and misalignment at interfaces, member length, lateral deflection and warping, length, and angle.
Manufacturing must strictly adhere to material inspection, processing accuracy, and corrosion protection requirements. Cutting is performed using CNC plasma cutting, and bending and forming must match the mold curves, such as the cold bending process for round tube elbows. Welding joints require pre-assembly on a jig, using submerged arc welding or CO₂ gas shielded welding, followed by post-weld non-destructive testing.
Welding sequence: For upper and lower chord members, weld in the order of vertical welding -> horizontal welding -> overhead welding; for interfaces between upper and lower chord members and web members, arrange the welding sequence based on measurement results. Typically, weld the web member to the lower chord first, then weld the web member to the upper chord, while continuously measuring to adjust the welding sequence and control deformation.
On-site truss assembly
First, use a crane to place the components on the formwork and secure the position of the members according to the truss construction details. If the processing deviation of the members exceeds the allowable range, heating correction of the members is permitted, but the heating temperature must not exceed the normalizing temperature. After heating correction, the members must be cooled slowly. Then weld the joints of the upper and lower chords, and finally assemble the web members and the supports between the upper and lower chords. During assembly, dimensions must be strictly controlled to ensure precision for the subsequent installation process.
Typical applications in various industries
Today, they are widely used in large and uniquely shaped structures such as bridges, stadiums, airports, and train stations. Space trusses are an increasingly common construction technique, particularly for large roof coverings, such as commercial and industrial entrance eaves. Today's larger portable stages and lighting fixtures often utilize spherical and eight-way trusses. Tubular spherical trusses are also widely used in the production of modern motorcycles and automobiles, though monocoque bodies are more common.
Notable examples of spherical trusses in buildings include:
- Bridge: Forth Rail Bridge
One of the world's earliest large-scale applications of space truss structures was a double-track railway bridge. The main span consisted of three massive cantilevered space trusses, extending from the towers on either side and connected by a shorter truss in the middle. The structure featured double chords (upper and lower tubular chords) connected by tubular webs, forming a remarkably rigid three-dimensional framework rather than a flat truss. This design effectively resisted the enormous wind loads acting on the structure, meeting stringent safety requirements.
As the world's first large-scale, air-conditioned indoor domed stadium and the first multi-purpose domed stadium, it was groundbreaking. Its roof structure is a circular, double-layered geodesic dome with a diameter of approximately 218 meters (even larger after later expansions). Composed of a series of grid elements connected by bolted-ball or welded joints, this spatial grid shell structure spans the entire stadium, free of any internal columns, providing a vast, column-free space for on-site activities.
- Airport: Paris Charles de Gaulle Airport Terminal 1
At its core is a main building composed of several inverted reinforced concrete pyramids. The roof and sidewall structures of these pyramids make extensive use of a three-dimensional space truss system (grid). This space truss system not only achieves a large span, meeting the significant airside clearance requirements, but also creates a highly recognizable architectural appearance. The "inner truss + outer skin" design also provides mezzanine space for equipment and piping.
Conclusion
With over two decades of expertise, XTD Steel Structure has been deeply engaged in the field of prefabricated steel structures. From skyscrapers to cross-river bridges, from vibrant sports stadiums to modern exhibition centers, and standardized factories to various large-scale projects, we have successfully delivered thousands of benchmark projects. Building strength with craftsmanship and defining quality with professionalism, we aspire to extend the beauty of prefabricated steel structures to every corner of the world.
