Axial tension – thickness reduction factor: In working stress design for steel, the allowable axial tensile stress is usually reduced (thickness factor applied) when the plate thickness exceeds which value?

Introduction / Context:
Plate thickness influences the ductility and the likelihood of lamellar tearing or through-thickness imperfections affecting performance. Many traditional codes apply a reduction (thickness factor) to the permissible axial tensile stress beyond a certain plate thickness threshold.

Given Data / Assumptions:

• Structural steel plates loaded in direct tension.
• Working stress or allowable stress design context.
• Typical legacy Indian practice and common exam conventions.

Concept / Approach:
For plates thicker than a specified limit, allowable stress is reduced by applying a factor less than unity to account for material through-thickness properties and the statistical likelihood of defects. A commonly cited threshold in classic problems is 20 mm, above which the permissible tensile stress is reduced marginally.

Step-by-Step Solution:
Identify the threshold plate thickness where reduction begins.Recall standard exam value used in traditional references: 20 mm.Select the option corresponding to 20 mm.

Verification / Alternative check:
Older editions of steel design texts list a step-down in allowable stress beginning at about 20 mm plate thickness. Modern limit-state codes handle thickness effects via partial factors and material specifications but retain the principle that very thick plates warrant caution.

Why Other Options Are Wrong:

• 10–15 mm: typically below the traditional threshold for reduction.
• 22 mm: not the standard teaching value used in classic problems.

Common Pitfalls:

• Applying thickness reductions to members where governing limit state is net-section rupture instead of gross yield.
• Ignoring toughness requirements (e.g., through-thickness properties) in thick plates for heavily restrained joints.

Final Answer:
20 mm