The diagrid structural system is an innovative and efficient framework used in the construction of buildings, particularly in high-rise and large-span structures. Characterized by a network of diagonal members, the system provides both strength and stability, making it an ideal choice for buildings with complex geometries or unique curved shapes. In this article, we will explore the features of the diagrid structural system, how it functions, its advantages and disadvantages, and the materials commonly used in its construction.
What is a Diagrid Structural System?
A diagrid is a diagonal grid structure typically used to provide support and stiffness in a building. It is composed of intersecting diagonal members made from materials such as steel, concrete, or wood, forming a triangular framework. Unlike traditional vertical and horizontal frameworks, the diagrid system distributes forces more efficiently across the structure, making it particularly beneficial for high-rise buildings and complex geometric designs.
The diagrid system has gained popularity in the construction of large, tall buildings due to its ability to provide both structural support and aesthetic value. Notably, it is used in some of the most iconic modern skyscrapers, thanks to its clean, geometric appearance and its ability to reduce the need for internal columns, thus creating open, flexible floor plans.
Module Geometry of the Diagrid Structural System
The diagrid system is designed using modular geometry, with key parameters such as the optimal angle of the diagonal members and the module dimensions playing critical roles in its performance.
1. Diagrid Optimal Angle
The diagonal members in a diagrid structure are responsible for both shear and moment transfer, which means their placement angle is crucial to the system’s effectiveness. The optimal angle for the diagonals depends largely on the height of the building.
For typical buildings, the most effective angle for bending rigidity is 90 degrees, while the ideal angle for shear rigidity is around 35 degrees. In practice, the optimal angle for most diagrid systems falls between 60-70 degrees. As the height of the building increases, the optimal angle also tends to increase to maintain structural efficiency.
2. Diagrid Module Dimensions
The module dimensions of the diagrid system typically include two primary considerations:
- Height of the Diagrid Module: This depends on the number of floors stacked in a single diagrid module. Commonly, 2 to 6 floors are grouped together in each module, with larger buildings sometimes employing more floors per module.
- Base of the Module: The base width or size of each diagrid module is influenced by the height of the building and the optimal angle of the diagonals. These factors ensure that the system provides sufficient stiffness and strength while minimizing material use.
Diagrid Structural System Node Design
The nodes of a diagrid system are where the diagonal members intersect. These nodes play an essential role in transferring loads from one diagonal member to another. There are two primary types of loads that the nodes are designed to handle:
- Vertical Load: The vertical load is transferred axially through the diagrid members, from the top members above the node to the gusset plates and stiffeners, and finally to the members below the node.
- Horizontal Shear: The horizontal shear is also transferred axially but is divided into compression and tension forces on opposite sides of the node. These forces travel through the gusset plate and stiffeners before being transferred to the members below.
Designing these nodes requires careful consideration, as shear forces generated under lateral loads (such as wind or seismic activity) can be very high. In particular, bolt connections at the nodes must be carefully designed to prevent failure, especially during events like earthquakes, where lateral forces are significantly amplified.
Types of Diagrid Structural Systems and Materials of Construction
The materials chosen for a diagrid system depend on various factors such as availability, erection time, durability, and cost. The choice of material also influences the construction method and the final performance of the system.
1. Steel Diagrid Structural System
Steel is the most commonly used material in diagrid construction due to its high strength-to-weight ratio, ease of fabrication, and fast erection time. Steel sections commonly used in diagrid construction include rectangular hollow structural sections (HSS), rounded HSS, and wide flanges. These materials are designed to resist high bending loads, which are crucial for the stability of tall buildings. Steel also enables fast construction, reducing labor costs and minimizing on-site time.
2. Concrete Diagrid Structural System
Concrete is another material used in diagrid construction, particularly in buildings requiring higher fire resistance. Concrete diagrids can be precast or cast in-situ. Precast concrete elements are particularly advantageous for their flexibility, as they can be easily tailored to fit the specific geometry of the building. While concrete diagrids increase the dead load of the structure, they also offer benefits in terms of fire protection and thermal mass.
3. Timber Diagrid Structural System
Timber is the least commonly used material in diagrid systems but may be used in certain contexts, particularly for low-rise or smaller-scale buildings. The primary advantages of timber include its easy availability and low installation cost. However, timber is less durable than steel or concrete, and its susceptibility to weathering and decay limits its use in high-rise or long-lasting applications.
Advantages and Disadvantages of the Diagrid Structural System
Advantages
The diagrid system offers a range of benefits, both from a structural and aesthetic perspective:
- Aesthetic Appeal: The geometric design of the diagrid structure enhances the visual appeal of the building, offering a modern and sleek appearance.
- Material Efficiency: The diagrid system can reduce the amount of steel used by up to 20% when compared to traditional brace-frame systems, making it a more material-efficient choice for high-rise buildings.
- Simplicity of Construction: The diagrid system requires less specialized labor than other advanced systems, as the construction technology is simpler to implement.
- Maximized Floor Space: The diagrid allows for column-free spaces, creating wide-open floor plans and more flexible layouts. This is particularly useful in office buildings, hotels, and other commercial properties.
- Natural Light: When combined with glass facades, diagrids allow for significant natural light to penetrate the interior of the building.
Disadvantages
Despite its advantages, the diagrid system also has some limitations:
- Limited Exploration: The system is still not fully explored and understood, and as such, its use is not yet as widespread as traditional framing systems.
- Labor Intensive: While the system itself is relatively simple to construct, it still requires skilled labor for installation. A shortage of skilled workers familiar with the specifics of diagrid design and construction may pose challenges.
- Design Limitations: The design of a building with a diagrid system is constrained by the framework itself, limiting flexibility in certain architectural styles.
- High-Rise Use Only: The diagrid system is most effective for high-rise buildings, making it less suitable for low-rise structures.
- Risks with Poor Design or Installation: If not designed or installed correctly, diagrid structures can pose risks to the safety and economy of the building. Inadequate bolt connections, for example, could lead to structural failure under lateral loads.
Conclusion
The diagrid structural system represents a significant advancement in modern architecture, providing both aesthetic appeal and structural efficiency. While it is primarily used in high-rise buildings with complex geometries, it is also being explored for a wider range of applications. The system’s ability to reduce material use, create column-free spaces, and enhance natural light makes it a preferred choice for architects and engineers working on modern skyscrapers.