Understanding Tension Members in Structural Engineering

Tension members are structural elements designed to carry tensile loads, meaning they are pulled rather than pushed. They are common in various structures like trusses, bracing systems, and bridges. The choice of the appropriate cross-section for a tension member depends on several factors, including the magnitude of the tensile force, the length of the member, the connection method, cost, and fabrication considerations.

Selecting Tension Members for Long Spans

The question asks about the suitable type of tension member when its length is "too long." This implies a scenario where the length is significant, potentially impacting factors like self-weight effects, handling, and potentially the efficiency of the cross-section under tension over that length. Let's evaluate the options provided:

• Wire rope: Wire ropes are flexible cables made of multiple strands of wire twisted together. They are excellent for applications requiring flexibility, such as suspension bridges or cranes. However, for typical structural tension members in trusses or bracing, their flexibility might not be desirable, and their end connections can be more complex and costly than those for rigid members like bars or angles. While they carry tension effectively, "too long" in a structural context might point towards a need for a more rigid solution or easier connections.
• Rod: Rods are typically circular in cross-section and solid. They are commonly used as tension members, especially for bracing or lighter loads. For longer lengths and potentially higher loads, the required diameter of a rod might become quite large, or a rod might experience excessive sag under self-weight over a very long span, although this is usually a secondary consideration for pure tension members unless lateral loads are present.
• Bar: The term 'bar' in this context usually refers to a solid, substantial steel section, often with a rectangular or square cross-section, but it can also be a large-diameter round bar. Bars offer a significant cross-sectional area and stiffness. For long tension members carrying substantial loads, a bar can provide the necessary area to resist the tensile force efficiently and offer good rigidity. They are generally easier to connect using pins or bolts compared to wire ropes. Their solid section contributes to stiffness over long lengths.
• Single angle: A single angle is a standard structural shape ('L' shaped). Single angles are frequently used as tension members, particularly in trusses and bracing. However, for very long lengths, the effectiveness of a single angle might be limited by its relatively smaller cross-sectional area compared to a solid bar, or considerations regarding connecting multiple angles together for larger loads over the span.

Analysis of Options for Long Tension Members

When a tension member is "too long," the total self-weight increases, which can cause some bending. While tension members are primarily designed for axial pull, a stiffer section resists bending better. Also, connecting members efficiently over long spans is crucial. Comparing the options:

Considering the need for adequate cross-sectional area to carry potentially large loads over a significant length, coupled with the advantage of stiffness and relative ease of connection compared to wire ropes, a solid bar often becomes a practical and efficient choice for "too long" tension members in many structural applications.

Conclusion

Based on structural engineering practices for long tension members requiring substantial capacity and reasonable rigidity, a bar is typically used. It provides the necessary cross-sectional area and stiffness to efficiently carry tensile loads over extended lengths, offering advantages over rods for heavier loads and over wire ropes for applications requiring rigidity and simpler connections.

Revision Table: Tension Member Selection

Reviewing the factors influencing the choice of tension members helps solidify understanding.

• Load magnitude (required area)
• Length (self-weight, slenderness effects - though less critical in tension than compression)
• Connection type (bolted, pinned, welded)
• Fabrication and erection ease
• Cost efficiency

Additional Information: Types of Tension Members

Beyond the options given, other shapes commonly used as tension members include:

• Double angles (back-to-back or star configuration)
• Channels
• Tees
• Plates
• Built-up sections (combinations of plates and shapes)

The choice depends on the specific requirements of the structural design.