Singly reinforced RCC beam — fundamental assumption: Which of the following statements correctly describes a singly reinforced beam in flexure under service loads?

Introduction / Context:
A singly reinforced beam has tensile reinforcement only; no compression steel is provided. Understanding the basic assumptions and load sharing between concrete and steel is essential in RCC flexural design.

Given Data / Assumptions:

• Working stress/limit state fundamental behavior under service levels.
• Beam has tension steel only.

Concept / Approach:
In a singly reinforced section, concrete in the compression zone resists compression, while tension is carried by steel (concrete in tension is ignored for strength). Other classical assumptions include plane sections remain plane and perfect bond exists.

Step-by-Step Solution:
Identify the attribute unique to singly reinforced sections: no compression steel, so compression is resisted by concrete alone.Check competing statements: plane sections remain plane is true but not unique to singly reinforced; steel does not have initial stresses prior to loading; elastic moduli of steel and concrete are different.Therefore, the most appropriate answer is (a).

Verification / Alternative check:
Design textbooks state that for singly reinforced beams: compression → concrete; tension → steel; assumptions of compatibility and linear strain distribution apply.

Why Other Options Are Wrong:

• (b) Incorrect: there are no “initial stresses”.
• (c) True assumption but not specific to “singly reinforced”.
• (d) False: steel and concrete moduli differ significantly.

Common Pitfalls:
Assuming concrete contributes to tension capacity; thinking plane-sections-only is the defining feature rather than a general flexure assumption.

Final Answer:
Compression is resisted entirely by concrete in the compression zone