Choice of tension members for long lengths: In steelwork, when a tension member is required over a very long length (e.g., roof bracing or long ties), which form is most commonly adopted for practicality, weight saving, and ease of tensioning?

Introduction / Context:
Long tension members are frequently required in roofs, trusses, temporary works, and cable-supported structures. The choice of form affects dead weight, erection convenience, and the ability to adjust tension on site. Cables and rods are typical solutions, with cables (wire ropes) preferred where very long, flexible ties are needed.

Given Data / Assumptions:

• Member primarily carries tension over a long clear length.
• Serviceability (sag, vibration) can be controlled by adequate prestress/tensioning.
• Connections and end fittings are available for the selected member type.

Concept / Approach:

Wire ropes (cables) provide high strength-to-weight ratio, ease of coiling/transport, and straightforward on-site tensioning with turnbuckles or jacks. Round rods can also be used but become heavy and unwieldy at long lengths; bars and single angles are stiffer and heavier, better suited to shorter bracing lengths or where compression might occur (then they risk buckling). For very long pure tension ties, cables are standard practice in both building and bridge applications.

Step-by-Step Solution:

Verification / Alternative check:

Architectural trusses, large-span roofs, and guy systems frequently use cables as the long tension element, confirming industry practice.

Why Other Options Are Wrong:

• Solid round rod: usable, but heavy and less convenient for very long runs.
• Flat bar/single angle: heavier; angles suffer from handling/twist and offer no advantage in pure tension over very long spans.
• Plate girder flange strip: inappropriate for a discrete long tie.

Common Pitfalls:

• Ignoring end anchorage and turnbuckle detailing for adjustability.
• Allowing excessive sag by under-tensioning long cables.

Final Answer:

A wire rope (cable).