In flexural design, “steel beam theory” refers to an approach applied to which situation among the following?

Introduction / Context:
When transforming reinforced concrete sections for analysis, simplified theories are sometimes used for special cases. “Steel beam theory” is one such simplification historically used for doubly reinforced beams where concrete in compression is neglected and compression is assumed to be carried entirely by compression steel.

Given Data / Assumptions:

• Doubly reinforced RC beam with both tension and compression steel.
• Concrete compressive contribution ignored for the idealization.
• Linear elastic assumptions for transformed steel areas within service range.

Concept / Approach:
By neglecting the compressive resistance of concrete, the section is treated as a “steel-like” flanged member where a couple is formed by tension and compression steel. This simplifies calculations of stress–strain compatibility and moment capacity for conceptual understanding or preliminary checks, though modern code design prefers more accurate methods acknowledging concrete compression.

Step-by-Step Solution:
Identify the context demanding compression steel (e.g., limited depth, high bending).Apply “steel beam theory”: assume compression is resisted by compression steel only.Compute internal couple from equal and opposite steel forces times lever arm.

Verification / Alternative check:
Comparing with conventional RC theory (concrete in compression + tension steel), steel beam theory gives conservative/approximate insights but should be cross-checked against full rectangular stress block approaches per current codes.

Why Other Options Are Wrong:

• (a) Concerns steel-only members, not the specific RC simplification.
• (b) Composite behavior is different and includes concrete compression explicitly.
• (d) Relates to shear design criteria, unrelated to the flexural idealization named here.

Common Pitfalls:
Using steel beam theory as a final design method without verifying code-compliant stress blocks, ductility, and detailing; confusing the concept with composite beam theory.

Final Answer:
Doubly reinforced concrete beams while neglecting compressive stress in concrete (compression carried by steel).