Deep beam criterion — simply supported RC beam (When is a simply supported beam classified as a deep beam?)

Introduction:Deep beams are members with small span-to-depth ratios where plane sections do not remain plane and the usual beam theory is not applicable. Recognizing a deep beam triggers special analysis and detailing provisions.

Given Data / Assumptions:

• Member is simply supported.
• We compare effective span l_eff to overall depth D.

Concept / Approach:For simply supported beams, the limit for being treated as a deep beam is commonly based on l_eff / D. Below a threshold, strut-and-tie or deep-beam methods are recommended rather than standard flexural theory.

Step-by-Step Solution:1) Compute ratio R = l_eff / D.2) If R is less than 2.0, the beam is a deep beam.3) For continuous beams, the corresponding threshold is typically higher (about 2.5).

Verification / Alternative check:Design manuals show deep-beam regions with arching action developing between supports and load points when spans are short relative to depth.

Why Other Options Are Wrong:Option A is an equality, not the criterion; Option B and D use the continuous-beam threshold; the correct limit for simply supported members is R less than 2.0.

Common Pitfalls:Using overall depth versus effective depth inconsistently; applying continuous-beam limits to simply supported cases.

Final Answer:Effective span to overall depth ratio less than 2.0.