Explanatory Handbook on Indian Standard Code of Practice for Plain and Reinforced Concrete (IS 456:1978)

Scope of IS SP Part 24: Key Points & Specifications

Scope Summary:

• IS SP Part 24 covers design, construction, and durability requirements for concrete structures.
• It includes transporting, placing, compaction, curing, and supervision of concrete.
• Special conditions like extreme weather, underwater concreting, and aggressive environments are addressed.
• Sampling, testing, and acceptance criteria for concrete strength are detailed.
• Structural design covers loads, forces, stability (overturning/sliding), durability, fire resistance, and analysis.
• Limit state design for collapse (compression, shear, torsion) and serviceability (deflection, cracking) is included.
• Working stress method design is also specified.
• Appendices provide durability requirements, deflection calculation, slab design, effective length of columns, and moments of resistance.

Key Specifications & Tables (Examples)

Important Formula Examples

• Nominal Shear Stress, τv:

[ \tau_v = \frac{V}{b \times d} ]

Where:

• (V) = Shear force

• (b) = Width of section

• (d) = Effective depth

• Minimum Cement Content (Table 19):
  Typically ranges from 300 to 450 kg/m³ depending on exposure conditions.

• Water-Cement Ratio:
  Should not exceed limits to ensure durability, e.g., 0.45 for severe exposure.

Durability Requirements (Appendix A)

• Cement content and max water-cement ratio

IS SP Part 24 - Materials: Aggregates and Cement

Aggregates (Clause 4.2)

• Aggregates must conform to IS 383:1970 (Coarse & Fine Aggregates from natural sources).
• Testing methods per IS 2386 (Parts I-VIII):1963.
• Aggregates must be free from deleterious materials: iron pyrites, coal, mica, shale, clay, alkali, soft fragments, sea shells, organic impurities.
• Limits on deleterious materials (coal, clay lumps, soft fragments, shale, material passing 75-micron sieve) as per IS 383.
• Avoid reactive silica aggregates (chert, chalcedony).
• Soft/porous aggregates (limestone, sandstone) not recommended for sea water exposure.
• Fine aggregates must be free from dust, silt, organic impurities; clay films reduce cement adhesion.
• Clay & silt increase water demand; dust from crushing is detrimental.

Lightweight Aggregates (Clause 4.2.2)

• Properties vary; specific data from producer recommended.
• Affect tensile strength, modulus of elasticity, creep, shrinkage, thermal expansion.
• Design considerations: development length, durability, shear/torsion resistance, deflection, slender column moments.
• Concrete grades above M40 unlikely with lightweight aggregates.

Cement

• Use cement conforming to relevant IS standards (e.g., IS 12269 for OPC).
• Fly ash as pozzolana/admixture per IS specifications.

Key Formulas (from related clauses)

Summary Table: Aggregate Quality Requirements (per IS 383)

IS SP Part 24: Key Points on Compaction of Concrete

Compaction Methods (Clause 12.3.1)

• Preferred: Mechanical vibration (immersion/internal vibrators).
• Allowed (if approved): Manual methods (roding, spading, tamping).
• Other methods like spinning or mechanical tamping are for special cases only.

Mechanical Vibration Details

• Type: Immersion vibrators (IS: 2505-1980).
• Types:
  • Flexible shaft (motor-driven).
  • Motor-in-head (electric/pneumatic).
• Frequency: 8,000 to 12,000 vibrations/minute.

Procedure for Internal Vibrators

• Deposit concrete in 30 to 45 cm thick layers.
• Insert vibrator vertically at uniform spacing.
• Penetrate rapidly to the bottom of the layer and at least 15 cm into previous layer.
• Hold vibrator for 5 to 15 seconds until compaction is adequate.
• Withdraw vibrator slowly at about 8 cm/sec.

Summary Table: Compaction Parameters

flowchart TD
    A[Deposit Concrete Layer (30-45 cm)] --> B[Insert Vibrator Vertically]
    B --> C[Penetrate Bottom & 15 cm into Prev. Layer]
    C --> D[Hold Vibrator (5-15 sec)]
    D --> E[Withdraw Vibrator Slowly (~8 cm/s)]
    E --> F[Compaction Adequate?]
    F -- No --> B
    F -- Yes --> G[Place Next Layer or Finish]

This ensures proper consolidation, minimizes voids, and improves concrete strength and durability.

Moment Redistribution - Key Points from IS SP Part 24

Applicability:

• Continuous beams and indeterminate frames (not columns).
• Permits reducing support moments and increasing mid-span moments while maintaining equilibrium.

Permissible Redistribution Limits:

Procedure Summary (Clause 43.2):

1. Plot elastic moment diagrams for various load cases (adjacent spans loaded alternately).
2. Identify maximum moments at supports (e.g., MB1).
3. Reduce max support moment by allowable % (e.g., 15% for WSM) to get M'B1.
4. Increase adjacent support moments to maintain equilibrium and equalize moments.
5. Adjust mid-span moments accordingly.
6. Draw envelope of redistributed moments for design.

Important Checks (Limit State Method):

• Depth of neutral axis ( x_u/d ) to ensure ductility.
• Rotation capacity of sections.
• Redistribution only if section is under-reinforced (to allow plastic hinges).

Key Formula for Neutral Axis Check (Approximate):

[ \frac{x_u}{d} \leq \text{limiting value depending on steel type and reinforcement ratio} ]

• Redistribution allowed only if this condition is satisfied.

Notes:

• Redistribution reduces congestion of reinforcement at supports.
• No limit on moment increase at mid-span.
• Redistribution causes cracks at service loads; design for both elastic and redistributed moments.
• Redistribution not applicable for flat slabs.

Visualization of Moment Redistribution:

graph LR
A[Elastic Moment Diagram] --> B[Identify Max Support Moment]
B --> C[Reduce Support Moment by 15% (WSM)]
C --> D[Increase Adjacent Support Moments]
D --> E[Adjust Mid-span Moments]
E --> F[Draw Redistributed Moment Diagram]
F --> G[Design Reinforcement]

References: Clauses 21.7, 36.1.1, 43.2, and Fig. E-58 to E-59 in IS SP Part 24.

Control of Deflection per IS SP Part 24 (Clause 22.2)

Key Points:

• Deflection limits are set to protect partitions/finishes:
  • Max deflection ≤ Span / 350 or 20 mm (whichever is less).
• Applies to rectangular beams/slabs under bending at service loads.
• Deflection control is independent of design method (WSD or LSD).
• Column deflections and projections (chajjas, lintels) are excluded.
• Use effective depth (d), not overall depth, for span/depth ratios.
• For spans > 10 m or special cases, explicit deflection calculation is recommended.

Basic Formulas

• Moment capacity:
  [ M = f \times Z = f \times b d^2 ]

• Deflection of simply supported beam under uniform load ( w ):
  [ \delta = \frac{5 w l^4}{384 E I} ]

• Span to effective depth ratio controls deflection:
  [ \frac{l}{d} \leq \text{limit (from Code)} ]

Multiplication Factors for Deflection Control

• Tension reinforcement factor:
  [ \text{Factor} = 0.225 + 0.00322 f_s - 0.625 \log_{10} \left(\frac{A_s}{b d}\right) ]
  where ( f_s ) = service stress in steel, ( A_s ) = area of tension steel.

• Compression reinforcement factor:
  [ P_c = 100 \times \frac{A'_s}{b d} ]
  Increased compression steel reduces long-term deflection (creep/shrinkage).

Span/Effective Depth Ratios (Typical Limits)

Recommendations

• Increase tension steel to reduce deflection if needed.
• Use effective depth and modification factors for realistic control.
• For flanged beams, ignore flange for conservative design.
• Explicit deflection calculations per Appendix E for

IS SP Part 24: Key Points for Solid Slabs

1. Effective Width (Clause 23.3.2.1)

• For solid slabs supported on two opposite sides, the effective width (b_eff) is calculated using elastic theory.
• Typically, the effective width extends beyond the support width to account for slab continuity and load distribution.

2. Minimum Reinforcement (Clause 25.5.2.1)

• Applies mainly to solid slabs.
• Minimum steel is provided to control shrinkage and temperature stresses.
• Empirical minimum reinforcement ratios (as per IS 456 or IS 13920):

3. Shear in Flat Slabs (Clause 30.6)

• Check for punching shear around columns.
• Use IS 456 provisions for punching shear:

[ \tau_v = \frac{V_u}{b_0 d} \leq \tau_{c} ]

Where:

• (V_u) = shear force
• (b_0) = perimeter of critical section (usually (d/2) from column face)
• (d) = effective depth
• (\tau_c) = permissible shear stress from IS 456 Table 19

Summary Table for Solid Slab Design

flowchart TD
    A[Load on Solid Slab] --> B[Calculate Effective Width (b_eff)]
    B --> C[Determine Bending Moments]
    C --> D[Provide Minimum Reinforcement]
    D --> E[Check Shear (Punching) around Columns]
    E --> F[Design Complete]

IS SP Part 24: Compression Members (Clause 42.2 & Related)

Key Points:

• Cracking Check (Clause 42.2):
  For members under combined axial load and bending, check cracking if ultimate axial load ( P_u < 0.2 f_{ck} A_c ).

  • ( f_{ck} ) = characteristic compressive strength of concrete
  • ( A_c ) = cross-sectional area of concrete
    This corresponds approximately to the balanced failure condition.

• Compression Member Design (Working Stress Method):

  • Permissible stresses:
    • Concrete: ( \sigma_c \leq 0.4 f_{ck} ) (approx.)
    • Steel: as per Clause 44.2 (usually ( \sigma_s \leq 0.6 f_y ))
  • Short columns:
    Use direct axial load capacity ( P = 0.4 f_{ck} A_c + 0.67 f_y A_s )
  • Long/slender columns:
    Include slenderness effects using effective length ( L_e ) and Euler buckling formulas or IS code effective length factors (Appendix D).

• Combined Axial Load and Bending (Clause 46):

  • Use interaction formula:
    [ \frac{P}{P_{max}} + \frac{M}{M_{max}} \leq 1 ]
    where ( P_{max} ) and ( M_{max} ) are permissible axial load and moment capacities respectively.

Important Tables & Formulas Summary

Key Requirements for Reinforcement and Detailing (IS SP Part 24)

1. Reinforcement Types & Properties (Table E-1):

   • Mild steel bars (IS:432 Part I) with characteristic strength ~250 N/mm².
   • Cold-worked high strength deformed bars (IS:1786) with strengths 415 and 500 N/mm².
   • Deformed bars must have ribs/lugs to improve bond strength by ≥40% compared to plain bars.
   • Minimum elongation varies from 12% (Fe 500) to 23% (mild steel).

2. Cover to Reinforcement (Clause 25.4.2):

   • Concrete cover depends on exposure conditions for durability.
   • Minimum cover ensures protection against corrosion and fire.

3. Crack Control (Clauses 25.5.1.1 (a) & 25.3.2):

   • Minimum reinforcement and spacing limits control crack widths.
   • Bar spacing should not exceed limits to avoid wide cracks.

4. Splicing & Anchorage:

   • Lap splices and anchorage lengths must follow IS guidelines (see pages 64-66).
   • Hooks and bends should be standard (page 58).

5. Additional Details:

   • Shear and torsion reinforcement rules (page 69).
   • Detailing for deep beams, columns, slabs, and punching shear provided in appendices.

Typical Reinforcement Detailing Parameters

Simplified Formula for Development Length (L_d)

[ L_d = \frac{\phi \times \sigma_{sd}}{4 \times \tau_{bd}} ]

Where:

• (\phi) = bar diameter
• (\sigma_{sd}) = design

IS SP Part 24: Expansion Joints Key Points

1. Purpose & Location (Clause 26.1)

• Expansion joints separate adjacent structures or parts with different mass/stiffness.
• Joint width must accommodate maximum expected relative movement (thermal, seismic).

2. Classification of Joints (Clause 26.3)

• Construction joints (see IS 456:12.4)
• Movement joints:
  • Contraction joints
  • Expansion joints

3. Design Guidance

• Refer to IS 3414-1968 for joint design in buildings.
• For earthquake-resistant design, separation and joint width must comply with IS 4326-1976.
• For liquid storage structures, see IS 3370 (Part I)-1965.

4. Additional Notes

• At bends in reinforcement (offset cranks), provide additional ties to resist transverse forces (Clause 25.5.3.3).
• Links must resist horizontal components of forces within 8 bar diameters (8d).

Typical Expansion Joint Width Estimation (Simplified):

Diagram: Force Transfer at Offset Bend (Fig. E-26)

graph LR
A[Longitudinal Bar] -->|Stress induces| B[Horizontal Force Component]
B --> C[Additional Ties (Links)]
C --> D[Resist Outward Transverse Force]

Summary:
Expansion joints must be designed to accommodate maximum relative movements due to thermal, seismic, and settlement effects. Use IS 3414 for general joint design, IS 4326 for seismic considerations, and provide adequate reinforcement ties at bends per Clause 25.5.3.3.

IS SP Part 24: General Special Design Requirements - Key Points

1. General Design Requirements (Clause 43.1 & Section 3)

• Applicable to all structures regardless of design method (Limit State or Working Stress).
• Refer Section 3 for common analysis, detailing, and design rules.

2. Design Values (Clause 35.3)

• Use prescribed material strengths and load factors.
• Incorporate safety factors as per IS standards.

3. Loads and Forces (Section 3, Clauses 17.1 - 17.9)

• Dead Load (DL): Self-weight + fixed components.
• Live Load (LL): As per IS 875 (Part 2).
• Wind Load: IS 875 (Part 3).
• Earthquake Forces: IS 1893.
• Load Combinations: Use IS 456 & IS 875 guidelines.

4. Stability (Clause 19)

• Check for Overturning and Sliding.
• Ensure factor of safety ≥ 1.5 for sliding, ≥ 2.0 for overturning.

5. Durability & Fire Resistance (Clause 20)

• Follow IS 456 for concrete cover and fire resistance.
• Use appropriate concrete grade and curing.

6. Analysis (Clauses 21.1 - 21.7)

• Use effective span and stiffness concepts.
• Moment redistribution allowed within limits (up to 30%).

7. Compression Members (Clause 42.2)

• Design for axial load + bending.
• Use slenderness ratio limits and buckling checks.

Important Formulae

Simplified Load Combination Diagram

graph TD
    A[Dead Load (DL)] --> C[Structure]
    B[Live Load (LL)] --> C
    D

Key Points on Deep Beams (IS SP Part 24, Clause 28)

• Definition: Deep beams have an effective span to overall depth ratio (l/d) ≤ 2.5, where plane sections do not remain plane after bending.
• Design Approach: Non-linear stress distribution and lateral buckling must be considered; classical flexural theory is not applicable.
• Effective Depth: Clause 22.0 states normal effective depth definitions do not apply to deep beams.
• Loading: Provisions mainly apply to deep beams uniformly loaded from the top; special detailing required for bottom-face loading (28.3.3).
• Reinforcement: Must be distributed over considerable depth; special detailing and disposition rules apply (Clause 28.3).
• Lever Arm: Clause 28.2 discusses lever arm calculation specific to deep beams, accounting for non-linear strain distribution.

Design Essentials

Reinforcement Detailing (Clause 28.3)

• Vertical web reinforcement (stirrups) must be closely spaced.
• Horizontal ties to prevent lateral buckling.
• Distribution of tension steel over depth to resist diagonal tension.

Summary Diagram of Deep Beam Stress Flow

flowchart TD
    A[Top Compression Zone] --> B[Diagonal Compression Strut]
    B --> C[Bottom Tension Zone with Distributed Reinforcement]
    C --> D[Supports]
    B -. Non-linear Stress Distribution .-> C
    C -. Vertical & Horizontal Reinforcement .-> B

References for Further Detailing

• IS SP 24 Clause 28 (Deep Beams)
• Specialist literature for non-uniform loading (Refs 7, 20, 32 in the code)
• IS 456 for general reinforcement detailing principles with modifications for deep beams

Note: Deep beam design requires careful consideration beyond classical beam theory,

IS SP Part 24: Ribbed, Hollow Block & Voided Slabs Key Points

1. Slab Types (Clause 29.8)

• (a) Ribbed slabs: Concrete ribs with topping.
• (b) Hollow block slabs: Hollow clay or concrete blocks placed between ribs.
• (c) Voided slabs: Use of void formers to reduce weight.

2. Topping Thickness

• Minimum topping thickness = 50 mm (IS:6061 Part II-1971).
• Provides composite action and surface for load distribution.

3. Design Approach (Clause 23.2)

• Designed as one-way spanning slabs.
• For two-way spanning ribbed slabs (waffle slabs), see Clause 23.4.

4. Effective Width of Rib for Shear (Fig. E-30)

• Effective width depends on rib spacing and topping thickness.
• Used to calculate shear stress in ribs.

5. Typical Formulae

6. Design Notes

• Use IS 456 for concrete and reinforcement design.
• Hollow blocks reduce self-weight but do not carry tension.
• Topping ensures slab acts compositely with ribs.
• Follow Clause 29 for detailed reinforcement and load calculations.

flowchart TD
    A[Slab Types] --> B[Ribbed Slabs]
    A --> C[Hollow Block Slabs]
    A --> D[Voided Slabs]
    B --> E[Topping Thickness ≥ 50 mm]
    C --> E
    D --> E
    E --> F[Effective Rib Width for Shear]
    F --> G[Calculate Shear Stress]
    G --> H[Design Reinforcement per IS 456]

Key IS Code (IS SP Part 24) Specifications for Flat Slabs:

1. Thickness of Flat Slab (Clause 30.2.1)

• Use average percentage of steel across the panel width at mid-span to compute modification factor for tension reinforcement.
• Refer IS 456:22.2.1 and Fig.3 for minimum thickness guidelines.

2. Shear in Flat Slab (Clause 30.6)

• Check for punching shear around columns.
• Use equivalent frame method for slab-column system analysis.

3. Equivalent Frame Analysis (Clause 30.5.2.2)

• Analyze slab as an equivalent frame using Hardy Cross or elastic methods.
• Consider slab fixed at supports two panels away.
• Use distance between center lines of supports as span.
• Moments at face of supports are critical for negative moment design.
• Redistribution of moments is allowed per 30.5.2.2 and 30.5.2.3.

4. Stiffness Idealization (Figures E-36 & E-37)

• Slab moment of inertia varies along axis; consider gross concrete section.
• Include stiffening effect of flared column heads if possible.
• Use Tables E-4 to E-7 for:
  • Fixed End Moments (FEM)
  • Stiffness (K)
  • Carry-Over Factors (COF)

Typical Formulas & Parameters

Simplified Punching Shear Check (per IS 456: Clause 39)

[ \tau_v = \frac{V_u}{b_0 d} \leq \tau_{c} ]

• ( V_u ) = Shear force around column
• ( b_0 ) = Perimeter of critical section (usually ( d/2 ) away from column face)
• ( d ) = Effective depth
• ( \tau_c ) = Perm

IS SP 24: Design for Shear – Key Points

1. Critical Section for Shear (Clause 21.6.2)

• Shear is checked at a distance d (effective depth) from the face of the support.
• Critical section = distance d from the support face.

2. Design Shear Strength of Concrete (Clause 39.2)

• Shear strength of concrete without shear reinforcement, ( V_c ), depends on:
  • Concrete grade
  • Effective depth ( d )
  • Width of the beam ( b )
• Use IS 456:2000 guidelines for ( V_c ).

3. Design of Shear Reinforcement (Clause 47.4)

• Shear reinforcement is provided when design shear force ( V_u ) exceeds ( V_c ).
• Shear reinforcement design formula:

[ V_s = 0.87 f_y A_{sv} \frac{d}{s} ]

Where:

• ( V_s ) = shear resisted by stirrups
• ( f_y ) = yield strength of stirrup steel
• ( A_{sv} ) = area of shear reinforcement within spacing ( s )
• ( d ) = effective depth
• ( s ) = spacing of stirrups

4. Shear and Torsion (Clause 48.3)

• Combined shear and torsion design requires checking: [ \frac{V_u}{V_c + V_s} + \frac{T_u}{T_c + T_s} \leq 1 ]
• Where ( T_u ) is applied torsion and ( T_c, T_s ) are torsion resistances of concrete and steel.

Summary Table: Shear Reinforcement Design

IS SP 24 - Key Points on Torsion (Clauses 40.3, 48.3, 40.4)

1. Torsion in Beams (Clause 40.3 & 48.3)

• Design Torsion (T_u) must be checked along with shear.
• Total torsion = T_u = T_s + T_w
  • T_s: Torsion resisted by concrete.
  • T_w: Torsion resisted by reinforcement.

2. Reinforcement for Torsion (Clause 40.4)

• Provide longitudinal reinforcement to resist torsion-induced tension.
• Provide stirrups or torsion hoops for torsional shear.
• Minimum torsion reinforcement area, ( A_{t,min} ), is proportional to the applied torsion moment.

3. Key Formulas

4. Serviceability (Deflection)

• Torsion increases deflection; consider combined bending, shear, and torsion effects.
• Limit deflection as per IS 456.

flowchart TD
    Torsion -->|Resisted by| Concrete(T_s)
    Torsion -->|Resisted by| Reinforcement(T_w)
    Reinforcement -->|Longitudinal| Longitudinal Bars
    Reinforcement -->|Transverse| Stirrups/Hoops

Summary: Design torsion considering combined shear, provide adequate reinforcement (longitudinal + stirrups), and check serviceability deflection limits.