Clause 40.1 defines the nominal shear stress τv as Vu/(b·d) for beams. This is compared against the design shear strength of concrete τc (Table 19), which depends on the tension steel percentage and concrete grade. If τv > τc, shear reinforcement is required. If τv > τc,max (Table 20), the section must be redesigned.
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Key Requirements
•Nominal shear stress τv = Vu / (b × d) — must not exceed τc,max from Table 20
•If τv ≤ τc/2: no shear reinforcement needed (but minimum stirrups still required)
Table 19 — Design Shear Strength of Concrete τc (N/mm²)
100Ast/(b·d)
M20
M25
M30
M35
M40+
≤ 0.15
0.28
0.29
0.29
0.29
0.3
0.25
0.36
0.36
0.37
0.37
0.38
0.50
0.48
0.49
0.5
0.5
0.51
0.75
0.56
0.57
0.59
0.59
0.6
1.00
0.62
0.64
0.66
0.67
0.68
1.25
0.67
0.7
0.71
0.73
0.74
1.50
0.72
0.74
0.76
0.78
0.79
1.75
0.75
0.78
0.8
0.82
0.84
2.00
0.79
0.82
0.84
0.86
0.88
2.25
0.81
0.85
0.88
0.9
0.92
2.50
0.82
0.88
0.91
0.93
0.95
2.75
0.82
0.9
0.94
0.96
0.98
≥ 3.00
0.82
0.92
0.96
0.99
1.01
For pt% between tabulated values, interpolate linearly. For pt > 3%, use τc at 3%.
Table 20 — Maximum Shear Stress τc,max (N/mm²)
Grade
M20
M25
M30
M35
M40
τc,max
2.8
3.1
3.5
3.7
4
Formulas
τv = Vu / (b × d)
Nominal shear stress
τv = Nominal shear stress (N/mm²)Vu = Factored shear force at the section (N)b = Breadth of member (mm)d = Effective depth (mm)
Practical Notes
✓For a typical 230 × 450 beam (d=410) with 2 nos 16 mm bars (Ast=402 mm²): pt = 0.43%. In M25: τc ≈ 0.46 N/mm². Maximum Vu without shear steel = 0.46 × 230 × 410 = 43.4 kN.
✓Critical section for shear is at distance 'd' from the face of support (Clause 22.6.2.1). Shear force at the face of support is not used for design.
✓IS 13920 (seismic) overrides: shear design must account for plastic hinge capacity at beam ends, not just analysis forces.
Common Mistakes
⚠Using τv at the face of support instead of at 'd' from face — the critical section for shear is at distance 'd', reducing the design shear force.
⚠Forgetting minimum shear reinforcement — even when τv < τc, minimum stirrups are always required in beams (Clause 26.5.1.6).
⚠Not interpolating Table 19 for intermediate pt% values — jumping to the next higher row overestimates τc.