Steel design (minimum thickness): For steel members not exposed to weather, the minimum thickness that should be specified to ensure durability and ease of fabrication is ________.

Introduction / Context:
Minimum plate or component thickness limits in steel design prevent premature corrosion, facilitate welding, and ensure practical fabrication. Codes prescribe larger minimum thicknesses for members exposed to weather and slightly lower values for protected, interior members.

Given Data / Assumptions:

• Steel members located in internal, non-weather-exposed conditions.
• Standard building occupancy with normal humidity control.
• Objective is to choose a recognized minimum that balances durability and constructability.

Concept / Approach:

Design practice and common code guidance adopt 6 mm as a practical minimum for non-weather-exposed steel plates and similar elements. Thicker minimums (often 8 mm or more) are required for weather-exposed components or where corrosion risk is elevated. Thinner plates can be difficult to weld and are more susceptible to local buckling or damage in handling.

Step-by-Step Solution:

Verification / Alternative check:

Project specifications and many national standards reflect 6 mm as a lower bound for protected plates and flats, with welding procedure qualifications tailored accordingly.

Why Other Options Are Wrong:

• 4.5 mm: generally too thin for structural members; fabrication and durability concerns.
• 8 mm or 10–12 mm: acceptable but exceed the typical minimum for non-exposed situations; these are more aligned with exposed or robust applications.

Common Pitfalls:

• Copying exterior minimums (≥8 mm) for interior members without need.
• Overlooking paint/coating systems and service environment in final selection.

Final Answer:

6 mm.