For a steel member exposed to weather and accessible for repainting (excluding webs of IS rolled joists and channels), what is the minimum permissible steel thickness for durability?

Introduction / Context:
Minimum thickness limits mitigate corrosion wastage and ensure a reasonable maintenance interval for exposed steelwork. For members that are repaintable, codes prescribe lower bounds on thickness to avoid rapid section loss and to provide practical robustness during fabrication and service.

Given Data / Assumptions:

• Member is exposed to weather but accessible for repainting.
• Webs of rolled joists/channels are excepted from this specific limit.

Concept / Approach:
Thin plates corrode through faster and are difficult to protect. A 6 mm minimum balances durability with economy for repaintable exposed conditions. Thicker limits may be used for more severe environments, but 6 mm is the standard minimum in many traditional specifications for general exposure.

Step-by-Step Solution:
Identify exposure: external, with maintenance access. Apply the minimum thickness rule: t_min ≈ 6 mm. Select 6 mm from the options.

Verification / Alternative check:
Fabrication practices and historical codes adopt similar minima to limit through-corrosion before scheduled repainting cycles.

Why Other Options Are Wrong:

• 4 mm: Too thin; rapid corrosion loss is likely.
• 8–10 mm: Acceptable but not the minimum requirement; may be uneconomical if not justified.

Common Pitfalls:

• Applying the same limit to inaccessible or immersed conditions, which require more stringent provisions.

Final Answer:
6 mm.