Shear capacity check in beams: If the computed nominal shear stress in a reinforced concrete beam exceeds what value, must the beam dimensions be revised (rather than relying only on stirrups)?

Introduction / Context:

Beam design checks nominal shear stress (tau_v) against limits for concrete. Beyond certain thresholds, increasing only shear reinforcement is insufficient; section dimensions must be revised to keep stresses within code bounds.

Given Data / Assumptions:

• Nominal shear stress in an RC beam computed from V / (b * d).
• Old-style units (kg/cm2) often used in competitive exams.
• Typical code limits imply a practical upper bound near 2 N/mm2 (~20 kg/cm2) for concrete under shear before section revision is mandatory.

Concept / Approach:

If tau_v exceeds permissible concrete shear stress (including max with shear reinforcement), codes direct either increasing section size (b or d) or reducing shear demand. A common exam convention flags 20 kg/cm2 as a change-of-dimensions trigger.

Step-by-Step Solution:

Verification / Alternative check:

Converting 20 kg/cm2 ≈ 2 N/mm2 aligns with familiar upper bounds in many design charts for concrete shear capacity.

Why Other Options Are Wrong:

• 10–15 kg/cm2: Conservative but not the typical exam threshold.
• 25–30 kg/cm2: Too high; risks crushing of web and brittle shear failure.

Common Pitfalls:

• Mistaking allowable shear with design shear carried by stirrups alone; both concrete and steel contributions are bounded.

Final Answer:

20 kg/cm2