Minimum thickness of structural steel members directly exposed to weather (Member is accessible for cleaning and repainting.)

Introduction:
Corrosion control in steel structures begins with adequate member thickness. Codes prescribe minimum thicknesses depending on exposure and maintainability to ensure durability and reasonable life between maintenance cycles.

Given Data / Assumptions:

• Member is directly exposed to weather.
• Member is accessible for routine cleaning and repainting.
• We seek the minimum permissible thickness.

Concept / Approach:
Thicker plates corrode more slowly to a critical loss point, offering longer protective life. When a member is accessible, slightly lower minimum thickness is acceptable compared with inaccessible members (which typically require thicker sections).

Step-by-Step Solution:
1) Identify exposure: direct weathering with accessibility.2) Refer to minimum thickness table for such conditions.3) The usual minimum for accessible, weather-exposed members is 6 mm.4) Therefore select 6 mm.

Verification / Alternative check:
For members not accessible to maintenance, a larger minimum (often 8 mm) is recommended; this confirms that 6 mm is appropriate here due to accessibility.

Why Other Options Are Wrong:
4 mm is too thin for weather exposure; 7 and 8 mm are conservative and typical for inaccessible members, not required here.

Common Pitfalls:
Ignoring accessibility; specifying too thin plates in aggressive environments without corrosion allowance or protective coatings.

Final Answer:
6 mm