Plain and Reinforced Concrete —
Code of Practice
The foundational Indian Standard governing the design of all reinforced concrete structures. Adopted nationwide since 1953, this code defines how engineers proportion, detail, and specify concrete — from M15 pedestrian paths to M60 high-rise columns.
📋 Amendments Applied: This summary incorporates Amendment Nos. 1–4 up to the reaffirmation date (2021). Always verify against the latest BIS amendment sheet before design.
IS 456:2000 applies to the design of plain and reinforced concrete structures used in buildings and civil engineering works. It covers both the Limit State Method (primary) and the Working Stress Method (permitted only for specific cases).
The code does not cover:
- › Prestressed concrete structures (see IS 1343)
- › Liquid-retaining structures (see IS 3370)
- › Concrete roads and pavements
- › Mass concrete (e.g., dams)
Key Change from 1978 Edition The 2000 revision shifted emphasis entirely to the Limit State Method. Working Stress is retained in Annex B only for legacy/replication work, not recommended for new design.
IS 456 adopts the Limit State Design (LSD) approach, which ensures that a structure remains fit for use throughout its intended life by checking two primary limit states:
Concrete is designated by its characteristic compressive strength (fck) at 28 days, expressed in N/mm². The prefix 'M' denotes the mix grade.
| Grade | fck (N/mm²) | Typical Use | Min. Cement (kg/m³) |
|---|---|---|---|
| M15 | 15 | Plain concrete, levelling courses | 250 |
| M20 | 20 | Minimum grade for RCC (Cl. 6.1.3) | 300 |
| M25 | 25 | Residential beams, slabs | 300 |
| M30 | 30 | Commercial building columns | 320 |
| M35 | 35 | Heavy industrial structures | 340 |
| M40 | 40 | High-rise column lower floors | 360 |
| M45–M60 | 45–60 | Special structures, precast | As per mix design |
Minimum Grade for RCC = M20 IS 456:2000 (Amendment 3) raised the minimum grade for RCC from M15 to M20. For plain concrete, M15 remains acceptable. For severe exposure, minimum M25 applies.
Nominal cover is the design depth of concrete cover to all steel reinforcements, measured from the exposed surface to the outermost bar. It protects against corrosion and fire.
| Exposure | Description | Nominal Cover (mm) |
|---|---|---|
| Mild | Protected interior, dry environment | 20 |
| Moderate | Sheltered, humid, occasional wetting | 30 |
| Severe | External in coastal/industrial zone | 45 |
| Very Severe | Direct contact with aggressive soil/water | 50 |
| Extreme | Tidal/splash zone, corrosive fumes | 75 |
Cover is Not Specified by Architect Nominal cover is a structural requirement, not an architectural choice. Reducing cover below Table 16 values compromises durability regardless of aesthetic preferences.
| Grade | fck | Ec (N/mm²) | Ec (GPa) |
|---|---|---|---|
| M20 | 20 | 22,360 | 22.4 |
| M25 | 25 | 25,000 | 25.0 |
| M30 | 30 | 27,386 | 27.4 |
| M35 | 35 | 29,580 | 29.6 |
| M40 | 40 | 31,623 | 31.6 |
For long-term loading, the effective modulus accounts for creep: Ec,eff = Ec / (1 + θ), where θ = creep coefficient (typically 1.6 to 2.0).
| Load Combination | DL | LL | WL / EL |
|---|---|---|---|
| DL + LL | 1.5 | 1.5 | — |
| DL + WL | 1.5 | — | 1.5 |
| DL + LL + WL | 1.2 | 1.2 | 1.2 |
| DL + EQ | 1.5 | — | 1.5 |
| DL + LL + EQ | 1.2 | 1.2 | 1.2 |
| Material | Limit State of Collapse (γm) | Serviceability |
|---|---|---|
| Concrete (fc) | 1.5 | 1.0 |
| Steel (fy) | 1.15 | 1.0 |
Therefore, the design strength of concrete = fck / 1.5 = 0.67 fck, and the design yield stress of steel = 0.87 fy.
IS 456 uses a rectangular stress block for the compression zone in limit state flexure design. The stress block parameters are defined in Cl. 38.1.
| Parameter | Fe 250 | Fe 415 | Fe 500 |
|---|---|---|---|
| xu,max / d | 0.53 | 0.48 | 0.46 |
| Mu,lim factor | 0.148 | 0.138 | 0.133 |
| Ru,max factor | 0.219 | 0.138 | 0.111 |
| Condition | Requirement | Clause |
|---|---|---|
| Min. tension steel (beam) | Ast,min = (0.85 bd)/fy | Cl. 26.5.1.1 |
| Max. tension steel (beam) | 0.04 × b × D (gross area) | Cl. 26.5.1.2 |
| Side face reinforcement | 0.1% of web area if depth > 750 mm | Cl. 26.5.1.3 |
Maximum Shear Stress The nominal shear stress τv = Vu/(b × d) must not exceed τc,max given in Table 20. If it does, increase the section size or concrete grade — stirrups alone cannot compensate.
Columns are classified as short or long based on the slenderness ratio (le/least dimension).
| Requirement | Value | Clause |
|---|---|---|
| Min. longitudinal steel | 0.8% of gross area | Cl. 26.5.3.1 |
| Max. longitudinal steel | 4% (6% at laps) | Cl. 26.5.3.2 |
| Min. bars (rectangular) | 4 | Cl. 26.5.3.1 |
| Min. bars (circular) | 6 | Cl. 26.5.3.1 |
| Min. bar diameter | 12 mm | Cl. 26.5.3.1 |
| Min. eccentricity | Max (L/500 + D/30, 20 mm) | Cl. 25.4 |
| Tie spacing | Least of: least lateral dim, 16d, 300 mm | Cl. 26.5.3.2(c) |
| Type | Span Ratio (Ly/Lx) | Bending Behaviour |
|---|---|---|
| One-Way Slab | ≥ 2 | Sags along shorter span; main steel along Lx |
| Two-Way Slab | < 2 | Sags in both directions; steel in both Lx and Ly |
| Support Condition | Simply Supported | One End Cont. | Both Ends Cont. | Cantilever |
|---|---|---|---|---|
| Span/Effective Depth | 20 | 23 | 26 | 7 |
Values for Fe 415 steel. Multiply by 0.8 for Fe 500. Apply modification factors for actual steel percentage per Fig. 4 of IS 456.
| Bar Type | M20 | M25 | M30 | M35+ |
|---|---|---|---|---|
| Plain bars (τbd) | 1.2 | 1.4 | 1.5 | 1.6 |
| Deformed bars (τbd) | 1.8 | 2.1 | 2.3 | 2.4 |
| Condition | Lap Length | Clause |
|---|---|---|
| Flexural tension (general) | Ld (full development length) | Cl. 26.2.5.1 |
| Compression (lap) | Ld | Cl. 26.2.5.1 |
| Rule-of-thumb Fe 415 | 47ϕ (tension), 37ϕ (compression) | — |
| Rule-of-thumb Fe 500 | 57ϕ (tension), 45ϕ (compression) | — |
| Min. lap for bars ≤ 12 mm | 300 mm | Cl. 26.2.5.1 |
Lap Splice Location Laps should be staggered — not more than 50% of bars lapped at one section in tension. Laps should not be located within L/4 from the face of supports in beams (where plastic hinges form during seismic events).
| Element | Requirement | Value | Clause |
|---|---|---|---|
| Beam | Min. main bar diameter | 12 mm | Cl. 26.5.1.1(a) |
| Beam | Max. spacing of stirrups | Min (0.75d, 300 mm) | Cl. 26.5.1.5 |
| Column | Min. tie diameter | Max (d/4, 6 mm) | Cl. 26.5.3.2 |
| Slab | Max. spacing of main steel | Min (3d, 300 mm) | Cl. 26.3.3(b)(1) |
| Slab | Max. spacing of distribution steel | Min (5d, 450 mm) | Cl. 26.3.3(b)(2) |
| Footing | Min. main steel diameter | 10 mm | Cl. 26.5.2.2 |
| Wall | Min. vertical steel (RCC) | 0.12% of gross area | Cl. 26.5.2.1 |
| Amendment | Year | Key Changes |
|---|---|---|
| Amd. No. 1 | 2002 | Corrections to Table 19 shear strength values; editorial fixes |
| Amd. No. 2 | 2005 | Clarification on epoxy-coated rebars; revised fire resistance tables |
| Amd. No. 3 | 2007 | Minimum RCC grade raised from M15 to M20; enhanced durability provisions |
| Amd. No. 4 | 2014 | Updated reference to IS 10262:2009 (mix design); minor corrections |
Reaffirmed: 2021 The code was reaffirmed in December 2021 (no technical changes). A full revision is expected in the coming years — likely to adopt fib Model Code provisions.