In the realm of structural engineering, tension members play a crucial role in ensuring the stability and strength of structures under tensile forces. A tension member is a structural component that is specifically designed to bear tensile (pulling) forces, typically aligned along its longitudinal axis. These members are sometimes referred to as tie members or simply ties. The type of tension member used in a construction project depends on factors such as the structure’s design and the method of end connections. Broadly, tension members can be categorized into four main groups: wires and cables, rods and bars, single structural shapes and plates, and built-up sections. Each type of tension member has its own set of characteristics, applications, and limitations.
Classification of Tension Members
The selection of a tension member is influenced by the structure’s design and the method used to connect the member’s ends. These members are generally classified into four categories:
- Wires and Cables
- Rods and Bars
- Single Structural Shapes and Plates
- Built-up Sections
Each type has its own use cases and technical advantages, as well as limitations that engineers need to consider in their designs.
Types of Tension Members
1. Wires and Cables
Wires and cables are one of the most common types of tension members used in construction, especially in applications that require flexibility and the ability to withstand significant loads. These are typically used in situations such as hoists, derricks, rigging slings, guy wires, and the hangers for suspension bridges. The primary advantage of using wires and cables is their flexibility and the ability to withstand tensile forces over long distances, making them ideal for use in suspension systems and for lifting purposes. However, they are not as rigid as other tension members, which limits their use in applications where stiffness is required.
2. Rods and Bars
Rods and bars are frequently used for smaller tension members and are commonly made from round or square steel. These types of tension members are particularly useful in applications where high strength-to-weight ratios are needed. Round bars, for example, are often used with threaded ends, allowing for pin connections that are straightforward and easy to install. Square bars and rods are sometimes enlarged at their ends by forging to create eye bars, which can be used with pin connections.
Despite their versatility, rods and bars have one significant disadvantage: they lack sufficient stiffness, which can lead to noticeable sag under their own weight, especially over longer spans. This sagging may require additional support or the use of stiffer materials to maintain structural integrity.
3. Single Structural Shapes and Plates
Single structural shapes, such as angle sections and tee-sections, are commonly used as tension members in structural steel construction. These sections provide higher rigidity compared to wires and rods, making them suitable for more demanding applications. However, when the length of a tension member increases significantly, single structural shapes can become flexible and may lose their effectiveness under certain load conditions.
Angle sections, for example, are more rigid than rods and bars but can experience eccentricity in both planes when used in riveted connections. Channel sections, on the other hand, have eccentricity in only one direction, offering better rigidity in that axis. In some cases, I-sections may be used as tension members because of their higher rigidity and cost-effectiveness, especially in large structures like bridges. Single I-sections are more economical than built-up sections and are used where the required cross-sectional area is within the limits of a single section.
4. Built-up Sections
Built-up sections are made by combining two or more individual structural elements to form a larger, more robust member capable of bearing higher tensile forces. When a single rolled section does not provide the necessary cross-sectional area, built-up sections offer a solution by assembling multiple members into one unit. These sections are commonly used in applications like roof trusses and bridge girders, where large, load-bearing members are required.
For example, double angle sections of unequal legs are commonly used in roof trusses. These angle sections are often arranged back-to-back on either side of a gusset plate. When both angle sections are placed on the same side of the gusset, they experience eccentricity in one plane and are subjected to both tension and bending forces simultaneously. Another configuration involves arranging angle sections in a star shape, with angles placed diagonally opposite to each other, often connected by batten plates. This star shape allows for a symmetrical and concentric connection, improving stability.
Built-up sections can also consist of two channels placed back-to-back with a gusset in between. These sections are typically used for medium loads in single-plane trusses. For larger, more complex structures such as long-span girders or bridge trusses, built-up sections may consist of plates and angles, allowing them to resist both tension and compression. These sections are also capable of handling reversal of stress, which can occur during extreme loading conditions.
Conclusion
The selection of the appropriate tension member for a given structure depends on multiple factors, including the type of load, the length of the span, the required rigidity, and the methods of connection. Engineers must consider all these factors when choosing between wires, rods, bars, single structural shapes, or built-up sections. Each type of tension member comes with distinct advantages and limitations that must be balanced to ensure the structure’s overall stability and efficiency. From the flexibility of wires and cables to the high rigidity of built-up sections, the choice of tension member plays a crucial role in the design and safety of structural systems.