Explanatory Handbook on Indian Standard Code of Practice for Plain and Reinforced Concrete (IS 456:1978)
1983 Edition

Overview of Scope and Applications

• IS SP Part 24 outlines the design, construction, and durability criteria for concrete structures.
• Covers processes such as concrete transportation, placement, compaction, curing, and supervision.
• Addresses challenging conditions including extreme weather, underwater concreting, and aggressive environments.
• Defines sampling, testing, and acceptance norms for concrete strength.
• Structural design principles encompass load considerations, stability checks (overturning and sliding), durability, fire resistance, and structural analysis.
• Incorporates limit state design for failure modes like compression, shear, and torsion, alongside serviceability aspects such as deflection and cracking.
• Specifies working stress method design procedures.
• Appendices provide detailed information on durability, deflection calculation, slab design, effective column lengths, and moment capacities.

Aggregates

• Aggregates must comply with IS 383:1970 standards for coarse and fine aggregates.
• Testing methods follow IS 2386 Parts I-VIII.
• Materials should be free from harmful substances like iron pyrites, coal, mica, shale, clays, alkalis, soft fragments, sea shells, and organic impurities.
• Limits are set on deleterious components including coal, clay lumps, soft particles, shale, and fines passing a 75-micron sieve.
• Reactive silica aggregates (e.g., chert, chalcedony) are to be avoided.
• Soft and porous aggregates such as limestone and sandstone are not recommended for marine exposure.
• Fine aggregates must be clean, free from dust, silt, and organic matter; clay films that impair cement bonding should be avoided.

Lightweight Aggregates

• Properties vary widely; consult producer data.
• Affect tensile strength, modulus of elasticity, creep, shrinkage, and thermal expansion.
• Design implications include anchorage length, durability, shear and torsion resistance, deflection control, and slender column moments.
• Typically, concrete grades above M40 are not feasible with lightweight aggregates.

Cement

• Use cement conforming to relevant IS standards such as IS 12269 for OPC.
• Fly ash may be used as a pozzolanic admixture following IS specifications.

Compaction Methods

• Mechanical vibration (immersion/internal vibrators) is the preferred compaction method.
• Manual techniques like rodding, spading, or tamping are permitted only with approval and in special cases.

Mechanical Vibration Details

• Immersion vibrators should comply with IS:2505-1980.
• Types include flexible shaft (motor-driven) and motor-in-head (electric or pneumatic).
• Operating frequency ranges between 8,000 and 12,000 vibrations per minute.

Procedure

• Concrete is placed in layers of 30 to 45 cm thickness.
• Vibrator is inserted vertically, penetrating to the base of the layer and at least 15 cm into the preceding layer.
• Vibration duration lasts from 5 to 15 seconds until adequate consolidation is achieved.
• The vibrator is withdrawn slowly at approximately 8 cm per second to prevent voids.

Summary of Compaction Parameters

Moment Redistribution Overview

• Applicable to continuous beams and indeterminate frames; not for columns.
• Allows reduction of support moments with a corresponding increase in mid-span moments while maintaining overall equilibrium.

Maximum Redistribution Limits

Procedure Summary

1. Draw elastic moment diagrams for various loading scenarios.
2. Identify maximum support moments.
3. Reduce support moments by the allowable percentage.
4. Increase adjacent moments to preserve equilibrium and balance.
5. Adjust mid-span moments accordingly.
6. Develop an envelope of redistributed moments for reinforcement design.

Ductility Checks for Limit State Design

• Verify neutral axis depth ratio (xu/d) for plastic hinge formation.
• Ensure rotation capacity is adequate.
• Redistribution is permitted only for under-reinforced sections.

Notes

• Redistribution decreases reinforcement congestion at supports.
• No restriction on moment increase at mid-span.
• Service load cracking may occur; design for both elastic and redistributed moments.
• Moment redistribution is not applicable for flat slabs.

Deflection Control Guidelines

• Deflection limits aim to protect partitions and finishes; maximum deflection should not exceed span divided by 350 or 20 mm, whichever is smaller.
• Applies primarily to rectangular beams and slabs under bending at service load.
• Control is independent of the design approach (WSD or LSD).
• Deflections of columns and projections like chajjas and lintels are excluded.
• Use effective depth (d) rather than overall depth for span-to-depth ratio calculations.
• For spans exceeding 10 meters or special cases, explicit deflection calculations are recommended.

Fundamental Expressions

• Moment capacity: M = f × Z = f × b d²
• Deflection for simply supported beams under uniform load: δ = (5 w l⁴) / (384 E I)
• Span to effective depth ratio l/d must comply with code limits.

Modification Factors

• Tension reinforcement factor = 0.225 + 0.00322 fs − 0.625 log₁₀(As / bd)
• Compression reinforcement percentage Pc = 100 × (As′ / b d)

Typical Span/Depth Ratios

Recommendations

• Increase tension steel to reduce deflection if necessary.
• Use effective depth and modification factors for realistic deflection control.
• For flanged beams, neglect flange thickness for conservative design.
• Refer to Appendix E for detailed deflection calculations.

Effective Width for Solid Slabs

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

Minimum Reinforcement

• Applies primarily to solid slabs to control shrinkage and temperature-induced stresses.
• Empirical minimum reinforcement ratios:

Shear Considerations in Flat Slabs

• Punching shear around columns must be checked.
• Shear stress τv = Vu / (b₀ d) should not exceed permissible shear stress τc.
• Vu = Applied shear force; b₀ = perimeter of critical section (generally d/2 from column face); d = effective depth.

Summary Table

Design Considerations for Compression Members

• Cracking checks apply when ultimate axial load Pu < 0.2 fck Ac, where fck is concrete strength and Ac is cross-sectional area.
• Working stress method permissible stresses: concrete approximately 0.4 fck; steel per code limits.
• Short columns: Axial load capacity P = 0.4 fck Ac + 0.67 fy As.
• Slender columns: Consider effective length Le and apply buckling formulas or IS slenderness factors.
• Combined axial load and bending use interaction criteria: (P / Pmax) + (M / Mmax) ≤ 1.

Important Parameters

Reinforcement Properties

• Mild steel bars as per IS 432 Part I with characteristic strength around 250 N/mm².
• High strength deformed bars per IS 1786 with grades Fe 415 and Fe 500.
• Deformed bars have ribs or lugs improving bond strength by over 40% compared to plain bars.
• Minimum elongation ranges from 12% (Fe 500) to 23% (mild steel).

Concrete Cover

• Cover thickness is governed by exposure conditions to ensure durability and corrosion protection.

Crack Control

• Minimum reinforcement and bar spacing limits regulate crack widths.
• Bar spacing must not exceed specified maxima to prevent wide cracks.

Splicing and Anchorage

• Lap splices and anchorage lengths are specified per IS guidelines.
• Hooks and bends should adhere to standard practices.

Additional Details

• Shear and torsion reinforcement rules are provided.
• Special detailing for deep beams, columns, slabs, and punching shear is included.

Purpose and Placement

• Expansion joints separate structural elements with differing mass or stiffness.
• Joint widths must accommodate maximum anticipated relative movements, including thermal and seismic shifts.

Types of Joints

• Construction joints.
• Movement joints, including contraction and expansion joints.

Design References

• IS 3414-1968 provides general joint design guidance.
• Earthquake resistance requires compliance with IS 4326-1976.
• Liquid storage structures follow IS 3370 (Part I)-1965.

Reinforcement at Bends

• Additional ties are necessary at reinforcement bends to resist transverse forces.
• Links must counteract horizontal force components within eight bar diameters.

Typical Joint Widths

General Design Requirements

• Applicable across design methods including Limit State and Working Stress.
• Common rules for analysis, detailing, and design are detailed in Section 3.

Material and Load Factors

• Utilize prescribed material strengths and safety factors as per IS codes.
• Consider dead loads, live loads, wind loads, and seismic forces based on relevant IS standards.
• Load combinations follow IS 456 and IS 875 directives.

Stability Checks

• Ensure safety factors of ≥1.5 against sliding and ≥2.0 against overturning.

Durability and Fire Resistance

• Follow IS 456 for cover requirements and fire resistance provisions.
• Use suitable concrete grades and curing techniques.

Structural Analysis

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

Compression Member Design

• Account for axial loads combined with bending.
• Apply slenderness and buckling verifications.

Definition and Characteristics

• Deep beams have a span-to-depth ratio (l/d) ≤ 2.5 where plane sections assumption is invalid.
• Design considers nonlinear stress distribution and lateral buckling.
• Conventional effective depth calculations do not apply.

Loading and Reinforcement

• Provisions primarily address uniformly loaded deep beams.
• Bottom face loading requires special detailing including vertical reinforcing bars.
• Reinforcement must be distributed through the depth with vertical stirrups and horizontal ties.

Lever Arm and Design

• Lever arm is derived from strain compatibility, not classical assumptions.
• Special detailing reduces lateral buckling risks.

Summary

Reference

• IS SP Part 24 Clause 28 for detailed guidelines.

Types of Slabs

• Ribbed slabs: Concrete ribs with concrete topping.
• Hollow block slabs: Hollow blocks placed between ribs to reduce weight.
• Voided slabs: Incorporate void formers to decrease self-weight.

Topping

• Minimum topping thickness is 50 mm as per IS 6061 Part II-1971.
• Ensures composite action and surface load distribution.

Design Approach

• Typically designed as one-way spanning slabs.
• Two-way ribbed slabs (waffle slabs) are covered under specific clauses.

Shear Design

• Effective rib width determined based on rib spacing and topping thickness.
• Shear stress calculation: τv = Vu / (beff × d), where Vu is shear force and d is effective depth.

Notes

• IS 456 provisions apply for concrete and reinforcement design.
• Hollow blocks reduce weight but do not carry tension.
• Topping ensures slab and ribs act compositely.

Thickness Requirements

• Minimum thickness guided by average steel percentage across panel width at mid-span.
• Refer IS 456 Clause 22.2.1 and relevant figures for specifics.

Shear Considerations

• Punching shear around columns must be checked using IS 456 criteria.

Equivalent Frame Analysis

• Analyze flat slabs as equivalent frames applying methods such as Hardy Cross or elastic analysis.
• Consider slab fixed at supports two panels away.
• Use center-to-center support distances as spans.
• Critical negative moments occur at face of supports.
• Moment redistribution permitted within code limits.

Stiffness Idealization

• Moment of inertia varies; consider gross concrete section.
• Include stiffening from flared column heads if feasible.
• Use provided tables for fixed end moments and stiffness factors.

Critical Shear Section

• Check shear at a distance equal to effective depth (d) from support face.

Concrete Shear Strength

• Shear capacity without reinforcement depends on concrete grade, effective depth, and beam width.
• Use IS 456:2000 provisions for calculation.

Shear Reinforcement Design

• Required when design shear force exceeds concrete's capacity.
• Shear reinforcement shear resistance: Vs = 0.87 fy Asv (d / s), where Asv is area of stirrups, s is spacing.

Combined Shear and Torsion

• Combined checks ensure (Vu / (Vc + Vs)) + (Tu / (Tc + Ts)) ≤ 1.
• Tu is applied torsion; Tc and Ts are torsion resistances from concrete and steel respectively.

Torsion in Concrete Beams

• Design torsion (Tu) includes torsion resisted by concrete (Ts) and reinforcement (Tw).

Reinforcement for Torsion

• Provide longitudinal bars to resist torsion-induced tension.
• Use stirrups or torsion hoops for torsional shear resistance.
• Minimum torsion reinforcement area is proportional to applied torsion moment.

Key Formulas

Serviceability

• Torsion influences deflections; combined bending, shear, and torsion effects must be checked.
• Deflection limits per IS 456 must be respected.