Long-Span Structures in Construction with Cables

Constructing long-span roofs is no easy feat; it requires finding the right balance between strength and affordability. Luckily, in recent years, high-strength steel cables have emerged as a fantastic solution. They're incredibly efficient at carrying heavy loads by pulling them straight. In this article, we'll take a closer look at cable structures, exploring their different parts, design considerations, and types, all aimed at making our roofs perform better while staying within budget.

 

The Efficiency of Cable Structures

High-strength steel cables stand out as exceptionally efficient components for long-span roof construction. Unlike traditional structural elements that rely on bending or compression, cables resist loads solely through axial tension. This characteristic makes them particularly well-suited for spanning large column-free areas, offering a remarkable balance between load-carrying capacity and cost-effectiveness. While the upfront cost of cables may seem modest compared to their remarkable performance, it's essential to delve into the broader ecosystem of components that make up a cable structure to fully appreciate its economic benefits.

 

Components of Cable Structures

1. Vertical Supports and Anchorages:

   - Vertical supports play a critical role in providing the necessary vertical clearances within the structure, as cables naturally sag below their supports. These supports may take the form of posts, towers, or walls, depending on the architectural and engineering requirements of the project.

   - Anchorages are vital for effectively transferring the tension in the cables to the surrounding structure. Various anchorage methods, such as heavy foundations, pile foundations, compression rings, or tension rings, are employed based on the specific needs of the project.

 

2. End Fittings and Bearing Plates:

   - Each cable is equipped with end fittings, typically threaded, which facilitate the tensioning and anchorage of the cables. These end fittings allow for precise adjustment and ensure the secure attachment of the cables to the supporting structure.

   - Bearing plates are essential components that help distribute the reaction forces exerted by the cables, thereby enhancing the overall stability and structural integrity of the system.

 

Types of Cable Roofs

1. Cable-Stayed Roofs:

   - Cable-stayed roofs feature a deck supported by girders or trusses, which, in turn, are supported at one or more points by cables. This configuration is particularly advantageous for structures requiring long-span cantilevers, such as aircraft hangars and stadiums.

 

2. Cable-Suspended Roofs:

   - In cable-suspended roofs, the roof deck and associated loads are directly supported by cables, eliminating the need for additional support structures. This design offers unparalleled flexibility and aesthetic appeal, making it a popular choice for modern architectural marvels.

 

Stability and Performance

1. Single-Layer vs. Double-Layer Cable Roofs:

   - Single-layer cable roofs, while lightweight and cost-effective, may be susceptible to wind uplift and oscillations. To mitigate these issues, heavy roof decks, such as precast-concrete panels, can be employed. Alternatively, adopting a double-layer system, where primary and secondary cables are pre-tensioned during erection, offers enhanced stability and resistance to wind-induced forces.

   - Double-layer systems, especially those incorporating diagonal struts between the primary and secondary cables, can leverage truss action to further improve structural stability. By ensuring sufficient pretension in the compression chord, the system can effectively withstand increasing loads without experiencing stress reversals.

 

2. Layout Optimization:

   - Circular or elliptical layouts are preferred for cable structures, as they minimize bending in the perimeter compression rings and offer superior efficiency compared to square or rectangular layouts. By optimizing the layout, designers can achieve greater structural efficiency while minimizing material usage and construction costs.

 

3. Cable-Truss Domes:

   - Cable-truss domes, also known as tensegrity domes, represent a sophisticated variation of cable structures. These domes feature a series of radial cable trusses, concentric cable hoops, tension rings, and compression rings, offering unparalleled stability and aesthetic appeal. Covered with fabric roofing, these structures provide a versatile and visually stunning solution for a wide range of architectural applications.

 

Design Considerations

1. Cable Spacing and Roof Deck Compatibility:

   - The spacing of cables depends on the type of roof deck used, with close spacing typically up to a maximum of 10 feet considered economical. Designers must carefully consider the compatibility between the chosen roof deck and the cable spacing to ensure optimal performance and cost-effectiveness.

   - To maintain watertightness and minimize potential issues associated with roof movements at cable penetration points, it is advisable to position cables either completely below or above the roof surface. If cables must penetrate the roof, proper joint sealing and the use of metal-protected, rubber-like collars are essential to prevent water infiltration and ensure long-term durability.

 

2. Structural Rigidity and Movement Mitigation:

   - Special attention must be given to the structural rigidity of cable roofs, particularly if the roof deck does not contribute significantly to overall rigidity. Designers must carefully evaluate the potential for roof movements and take proactive measures to prevent joint opening or concrete deck cracking, which could compromise the watertightness and structural integrity of the roof.

   - Insulation may be necessary to mitigate large thermal movements, while considerations for fire resistance should also be incorporated into the design process to ensure compliance with safety regulations and standards.

 

3. Corrosion and Environmental Factors:

   - While steel cables are generally resistant to corrosion, especially if galvanized, designers must assess the potential for corrosion in chemically polluted atmospheres or aggressive environmental conditions. Adequate measures, such as protective coatings or periodic inspections, should be implemented to mitigate the risk of corrosion and ensure the long-term durability of the cable structure.

   - Additionally, considerations for environmental factors, such as wind loads and seismic activity, should inform the design process to ensure that cable structures can withstand anticipated forces and maintain structural integrity under varying conditions.

 

Cable structures offer a flexible and budget-friendly way to build large roofs. Architects and engineers can get creative with their designs, considering factors like stability and environmental impact. This means we can create beautiful and eco-friendly structures, whether it's a cable-stayed roof or a cable-truss dome. It's amazing how steel cables can push the boundaries of what's possible in modern architecture!