Purpose of secondary (distribution) steel — transverse reinforcement placed at right angles to the main reinforcement in slabs and other members primarily serves to:

Introduction / Context:
Reinforced concrete slabs and walls include two orthogonal reinforcement systems: main (flexural) bars in the primary span direction and secondary (distribution) bars transverse to them. Understanding why we provide transverse steel is fundamental to good crack control and serviceability performance.

Given Data / Assumptions:

• Member type: slab or similar thin R.C.C. element.
• Transverse bars are orthogonal to main bars.
• Serviceability limit states (crack width control) and durability are important objectives.

Concept / Approach:

Transverse (distribution) reinforcement equalizes strain fields and restrains crack widths, particularly those caused by temperature variations, drying shrinkage, and local stress concentrations near loads and supports. It also assists in distributing point or line loads to a wider area, reducing peak stresses in concrete. Hence, its functions span load distribution and restraint against temperature- and shrinkage-induced tensile strains.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Design standards specify minimum transverse reinforcement percentages precisely to address these serviceability concerns and achieve a more uniform crack pattern with smaller widths.

Why Other Options Are Wrong:

Options A, B, and C are each true but incomplete alone. Only “all of the above” captures the complete purpose of distribution steel.

Common Pitfalls (misconceptions, mistakes):

Thinking transverse steel is optional because it contributes little to primary moment capacity; underestimating temperature and shrinkage effects in exposed slabs.

Final Answer:

all of the above