In reinforced concrete design, a T-beam behaves like a rectangular (flanged) beam of width equal to its flange only when the neutral axis lies where? Choose the condition under which the compression block remains entirely within the slab flange.

Introduction / Context:
T-beams arise when a slab and its supporting beam act monolithically. For flexural design, whether the section behaves as a simple rectangular section or a true T-section depends on the position of the neutral axis (N.A.) relative to the flange thickness.

Given Data / Assumptions:

• Monolithic slab and beam (R.C.C.).
• Flange in compression, web (rib) below.
• Working stress or limit state concepts both recognize compression block extent.

Concept / Approach:
If the N.A. falls within the slab thickness, the entire compression zone is in the flange. The web is then largely unstressed in compression; the section can be analysed as a rectangular beam of width equal to the effective flange width. If the N.A. lies below the flange, compression spreads into the web and T-beam analysis is required.

Step-by-Step Solution:
Identify requirement for rectangular behaviour: compression block contained in the flange.This occurs when the neutral axis depth is less than or equal to the effective flange thickness.Therefore, the N.A. must remain within the flange.

Verification / Alternative check:
Design codes treat two cases: N.A. in flange (rectangular compression block) and N.A. in web (T-section). In the first case, breadth = effective flange width, depth = neutral axis depth within flange.

Why Other Options Are Wrong:

• Remains below the slab: Implies N.A. in the web; T-analysis needed.
• Coincides with geometric centre: Not a criterion for behaviour; depends on reinforcement and loads.
• None of these: Incorrect, because there is a correct condition.

Common Pitfalls:
Assuming T-beam action always; ignoring effective flange width limits; misplacing N.A. without checking strain compatibility.

Final Answer:
remains within the flange