Clause 43.1 (read with Clause 23.2.1) provides the deemed-to-satisfy span-to-depth ratio method for controlling deflection. Instead of computing actual deflection, the code allows checking that the actual span/depth ratio does not exceed the permissible value (basic ratio × modification factors). This is the most commonly used deflection check in Indian practice.
Modification Factor kt for Tension Reinforcement (Fig. 4 — approximate)
pt%
fs=145 MPa (Fe250)
fs=240 MPa (Fe415)
fs=290 MPa (Fe500)
0.2
2
2
1.68
0.4
1.95
1.6
1.35
0.6
1.75
1.35
1.16
0.8
1.56
1.18
1.05
1.0
1.4
1.06
0.98
1.2
1.28
0.98
0.92
1.5
1.14
0.89
0.85
2.0
1
0.8
0.76
3.0
0.9
0.72
0.68
fs = 0.58 fy × (Ast,req/Ast,prov). Interpolate for intermediate values.
Formulas
L/d (allowable) = Basic ratio × kt × kc
Permissible span-to-depth ratio
Basic ratio = 7 (cantilever), 20 (simply supported), 26 (continuous)kt = Modification factor for tension reinforcement (Fig. 4) — depends on pt% and fskc = Modification factor for compression reinforcement (Fig. 5) — depends on pc%
fs = 0.58 fy (Ast,req / Ast,prov)
Service stress in tension steel for Fig. 4 entry
fs = Estimated service stress in steel (MPa)fy = Characteristic yield strength (MPa)Ast,req = Area of steel required by calculation (mm²)Ast,prov = Area of steel actually provided (mm²)
Practical Notes
✓For a simply supported slab with Fe500 and pt ≈ 0.4%: kt ≈ 1.35. Allowable L/d = 20 × 1.35 = 27. For a 4 m span: minimum d = 4000/27 = 148 mm → use 150 mm slab (overall 175 mm with 25 mm cover).
✓Providing more steel than required (Ast,prov > Ast,req) reduces fs and increases kt — this is a practical way to pass a tight deflection check without increasing depth.
✓For continuous slabs, the L/d ratio of 26 applies to the shorter span. Don't use 26 for end spans — use 20 (simply supported) for the end bay.
Common Mistakes
⚠Using basic L/d = 20 for a continuous slab — it's 26 for intermediate spans, but only 20 for end spans (simply supported edge).
⚠Not computing fs correctly — fs depends on ratio of Ast required to Ast provided, not just on fy.
⚠Ignoring the 10/span reduction for spans exceeding 10 m — a 12 m continuous beam: basic L/d = 26 × 10/12 = 21.7.