Minimum thickness requirements for structural steel members (Exposure and member category considerations.)

Introduction:
Minimum thickness prescriptions in steel design reduce corrosion risk and ensure a practical maintenance interval. Requirements vary with exposure and whether a member is primary (main) or secondary.

Given Data / Assumptions:

• Exposure: weather-exposed vs not directly exposed.
• Accessibility for repainting.
• Member category: main vs secondary members.

Concept / Approach:
Weather exposure accelerates corrosion; accessibility permits more frequent maintenance and allows slightly thinner minimums. Secondary members may have somewhat lower thickness requirements than primary load-carrying members when not exposed.

Step-by-Step Solution:
1) For exposed but accessible members, minimum thickness is commonly taken as 6 mm.2) For main members not directly exposed, 6 mm minimum ensures durability.3) For secondary members not directly exposed, 4.5 mm minimum is acceptable.4) All three statements are therefore correct.

Verification / Alternative check:
Comparative durability charts show faster section loss in exposure; minimum thicknesses are set accordingly to maintain structural capacity between maintenance cycles.

Why Other Options Are Wrong:
Options A–C are each correct; hence the best choice is “All of the above”.

Common Pitfalls:
Applying exposed-member minima to sheltered interiors; neglecting coating systems and detailing that improve durability (drip edges, sealants).

Final Answer:
All of the above.