Code of Practice Concrete structures for the storage of liquids, Part 2: Reinforced concrete structures

IS 3370 Part 2 – Scope Summary

• Scope Exclusions (Clause 1.2):

  • Does not cover:
    • Storage of hot liquids.
    • Liquids with low viscosity and high penetrating power (e.g., petrol, diesel).
    • Dams, pipes, pipelines, lined structures, and damp-proofing of basements.
    • Special shrinkage problems in non-aqueous liquid storage and chemical attack protection.

• Applicable Liquids:

  • Normal temperature storage of aqueous liquids and solutions without harmful effects on concrete/steel.
  • Sewage storage with adequate protection measures.

• Referenced Standards:

• Design Basis (Clause 4.4.2):
  • Follow IS 3370 Part 1 and IS 456 for design and construction unless otherwise specified.

Key Notes:

• Use IS 3370 Part 2 only for cold aqueous liquid storage tanks.
• For design tables and detailed structural parameters, refer to IS 3370 Part 4 (1967).
• Reinforcement and concrete quality must comply with IS 456 and IS 1786.

flowchart LR
  A[IS 3370 Part 2 Scope] --> B[Excludes hot liquids, petrol, diesel]
  A --> C[Excludes dams, pipes, pipelines]
  A --> D[Applies to aqueous liquids at normal temp]
  A --> E[Design per IS 3370 Part 1 & IS 456]
  E --> F[Use IS 3370 Part 4 for design tables]

This concise scope ensures correct application and compliance with structural and durability requirements for liquid retaining concrete structures.

IS 3370 Part 2: Limit State Requirements Summary

• Limit States to consider (Clause 4.4.1):

  • Ensure safety and serviceability by checking all relevant limit states.

• Limit State of Collapse (Clause 4.4.1.1):

  • Follow IS 456 guidelines for ultimate load design.
  • Use limit state design principles for strength and stability.

• Limit States of Serviceability (Clause 4.4.1.2):

  • Deflection: Limits as per IS 456 (typically span/250 to span/500 depending on structure).
  • Cracking: Maximum crack width ≤ 0.2 mm on surface for direct tension, flexure, and restrained temperature/moisture effects.
  • Ensure specified concrete cover to control crack width.

Key References from IS 456 (for IS 3370 design):

Formula for Crack Width (approximate):

[ w = \beta \times \frac{f_{ct}}{E_s} \times \frac{d_{eff}}{A_s} ]

Where:

• ( w ) = crack width
• ( f_{ct} ) = tensile stress in concrete
• ( E_s ) = modulus of steel
• ( d_{eff} ) = effective depth
• ( A_s ) = area of steel reinforcement
• ( \beta ) = coefficient depending on bond and strain distribution

flowchart LR
    A[Design] --> B{Check Limit States}
    B --> C[Limit State of Collapse]
    B --> D[Limit State of Serviceability]
    C --> E[Use IS 456 Ultimate Strength]
    D --> F[Deflection Limits (IS 456)]
    D --> G[Crack Width ≤ 0.2 mm]
    G --> H[Ensure Specified Cover]

Summary:
Use IS 456 for collapse and serviceability checks. Control deflection within span/250–span/500

IS 3370 Part 2: Methods of Design — Key Points

1. Methods of Design (Clause 4.3)

• Design of liquid retaining structures is generally based on:
  • Working Stress Method (WSM)
  • Limit State Method (LSM)
• The structure must safely withstand:
  • Hydrostatic pressure from liquid
  • Temperature stresses
  • Soil pressure (if applicable)
  • Other imposed loads (wind, seismic, etc.)

2. Basis of Design (Clause 4.4.2)

• Design shall ensure:
  • Safety against failure
  • Serviceability (crack control, deflections)
  • Durability and watertightness
• Design loads include:
  • Hydrostatic pressure = ( p = \rho g h )
  • Temperature stresses due to expansion/contraction
  • Earth pressure if tank is underground or partially buried

3. Design Tables (Part 4, 1967)

• Provide pre-calculated values for:
  • Thickness of walls and base slabs
  • Reinforcement details for common tank sizes and shapes
• Useful for quick reference and preliminary design

Typical Hydrostatic Pressure Formula:

[ p = \rho \times g \times h ]

• (p) = pressure at depth (h) (N/m²)
• (\rho) = density of liquid (kg/m³)
• (g) = acceleration due to gravity (9.81 m/s²)
• (h) = depth of liquid (m)

Summary Diagram: Design Considerations

graph TD
    A[Liquid Retaining Structure] --> B[Hydrostatic Pressure]
    A --> C[Temperature Stresses]
    A --> D[Soil/Earth Pressure]
    A --> E[Other Loads (Wind, Seismic)]
    B --> F[Calculate p = ρgh]
    C --> G[Allow for expansion/contraction]
    D --> H[If underground or partially buried]
    E --> I[As per relevant IS codes]

Note: For special forms or unusual circumstances, refer to specialized literature or perform detailed analysis/testing for safety verification.

IS 3370 Part 2: Limit State Design (LSD) - Key Points

1. Limit State Requirements (Clause 4.4.1)

• Design must satisfy all relevant limit states ensuring safety and serviceability.
• Two design methods allowed: Limit State Design (4.4) and Working Stress Design (4.5).
• Structural elements not exposed to water/moisture follow IS 456.

2. Basis of Limit State Design (Clause 4.5.1)

• Plane sections remain plane after bending.
• Steel and concrete are perfectly elastic; modular ratio as per IS 456.
• For crack resistance, tensile stress in concrete is limited (refer to Table 1 in IS 3370 Part 2).
• For strength, concrete tensile strength is neglected.

3. Key Formulas (from IS 456 & IS 3370 principles)

• Modular ratio, m: [ m = \frac{E_s}{E_c} ] where (E_s) = modulus of steel, (E_c) = modulus of concrete.

• Stress in concrete (tension limited): [ \sigma_{ct} \leq \text{Value from IS 3370 Table 1} ]

• Ultimate moment capacity (flexure): [ M_u = 0.87 f_y A_s (d - \frac{A_s f_y}{f_{ck} b}) ] where:

  • (f_y) = yield strength of steel,
  • (A_s) = area of steel,
  • (d) = effective depth,
  • (f_{ck}) = characteristic compressive strength of concrete,
  • (b) = width of section.

4. Crack Control (Tensile Stress Limits)

(Refer to IS 3370 Table 1 for exact values)

IS 3370 Part 2: Permissible Stresses & Crack Control

1. Permissible Concrete Stresses (Clause 4.5.2.1, Table 1)

• Note: Shear stresses per IS 456.

2. Crack Spacing (Clause 1.4, Eqns 2 & 3)

[ S_{max} = \frac{f_{ct}}{k \cdot f_b \cdot P \cdot \phi} ]

Where:

• (f_{ct}) = tensile strength of concrete
• (f_b) = average bond strength (1 for plain bars, 2/3 for deformed bars)
• (P) = steel ratio
• (\phi) = bar diameter

3. Maximum Crack Width Estimation (Eqns 4 & 5)

[ W_{max} = S_{max} \times (\varepsilon_{sh} + \varepsilon_{th}) ]

Where:

• (\varepsilon_{sh}) = shrinkage strain (~100×10⁻⁶)
• (\varepsilon_{th} = \alpha \times \Delta T) (thermal strain)
• (\alpha) = coefficient of thermal expansion of concrete
• (\Delta T) = temperature fall (e.g., 30°C summer, 20°C winter)

4. Steel Stress Limits for Crack Control (Clause 4.4.3.1)

• Plain Bars: ≤ 115 N/mm²
• Deformed Bars: ≤ 130 N/mm²

Summary Diagram of Crack Control Parameters

IS 3370 Part 2: Design of Floors in Water Retaining Structures

Key Specifications & Design Principles

• Design Methods: Use either Clause 4.4 (Limit State Design for thin slabs) or Clause 4.5 (alternative method).
• Material Exposure: Floors exposed to water/moisture follow IS 3370; otherwise, IS 456 applies.
• Floor-Wall Connection: Clause 5.3.1 requires considering bending moments at floor-wall junctions and direct forces due to floor suspension or support by walls.

Important Formulas (Limit State Design - Thin Slabs)

• Bending Moment (M) calculation depends on slab span, loading, and support conditions.
• For continuous slabs (typical for floors):

[ M_{max} = \frac{wL^2}{8} \quad \text{(simply supported)} \quad \text{or} \quad M = \text{from design tables for continuous slabs} ]

Where:

• ( w ) = uniformly distributed load (kN/m²)

• ( L ) = span length (m)

• Shear Force (V):

[ V = \frac{wL}{2} ]

Design Tables (from IS 3370 Part 4, 1967)

• Provide bending moment and shear values for various slab thicknesses and spans.
• Use these for preliminary design and reinforcement calculations.

Reinforcement Detailing

• Consider moments at junctions (floor-wall).
• Provide adequate anchorage and development length.
• Check for direct forces transferred between floor and walls.

flowchart TD
    A[Water Retaining Floor] --> B[Check Exposure to Water]
    B -->|Exposed| C[Design per IS 3370]
    B -->|Not Exposed| D[Design per IS 456]
    C --> E[Choose Design Method: Clause 4.4 or 4.5]
    E --> F[Calculate Bending Moments & Shear]
    F --> G[Consider Floor-Wall Junction Moments (5.3.1)]
    G --> H[Detail Reinforcement]

Summary: Use limit state design (Clause 4.4) for thin floors, consider moments at floor-wall junctions (Clause 5.3.1

Design of Walls as per IS 3370 Part 2

1. Pressure on Walls (Clause 6.2)

• Walls resist liquid pressure via vertical and horizontal bending moments.
• Horizontal tension from direct water pressure on end walls is combined with bending tension.

2. Rectangular/Polygonal Tank Walls (Clause 6.3)

• Walls act as two-way slabs with boundary conditions (fixed/hinged/free).
• Stress condition on liquid-retaining faces:

[ \sigma_{cr} + \sigma_{chr} \leq \sigma_{c0} + \sigma_{cbr} ]

Where:

• (\sigma_{cr}) = direct tensile stress in concrete

• (\sigma_{chr}) = tensile stress due to bending

• (\sigma_{c0}), (\sigma_{cbr}) = permissible tensile stresses (see Table 1 of IS 3370 Part 2)

• Provide horizontal reinforcement and haunch bars at vertical edges for horizontal bending moments.

• Moment coefficients for walls are given in IS 3370 Part 4.

3. Cylindrical Tank Walls (Clause 6.4)

• Assume fully fixed base unless detailed analysis proves otherwise.
• Wall deformation restricted at base.
• Use ring tension and vertical moment coefficients from IS 3370 Part 4.

Summary Table for Tensile Stress Limits (Typical from IS 3370 Part 2 Table 1)

Refer IS 3370 Part 2 Table 1 for exact values based on concrete grade.

flowchart TD
    A[Water Pressure] --> B[Vertical Bending Moment]
    A --> C[Horizontal Bending Moment]
    C --> D[Direct Horizontal Tension]
    B & D --> E[Combined Tensile Stress in Wall]
    E --> F[Check against Permissible Tensile Stress]
    F -->|Safe| G[Design OK]
    F -->|Exceeds| H[

IS 3370 Part 2: Design of Roofs (Water Retaining Structures)

Key Points:

• Roofs of water tanks are designed as water retaining structural elements.
• Design follows either Limit State Design (Clause 4.4) or Working Stress Design (Clause 4.5).
• Additional provisions for roofs are detailed in Clause 7.
• Structural elements not exposed to water or moisture follow IS 456.

Design Considerations for Roofs (Clause 7):

• Roof slabs are generally reinforced concrete slabs designed to resist:
  • Hydrostatic pressure from water.
  • Self-weight and superimposed loads.
• Roofs may be flat slabs or domes depending on tank type.

Important Design Parameters:

• Water pressure on roof = 0 (if roof is above water level).
• Roof must resist dead load, live load, and wind load.
• Minimum concrete grade: M20.
• Minimum cover as per IS 456 (typically 20-25 mm for exposure conditions).

Typical Formula for Roof Slab Design (Limit State):

[ M_u = \frac{w \times l^2}{8} ]

Where:

• (M_u) = Ultimate bending moment (kNm)
• (w) = Uniform load (kN/m²) (including self-weight + superimposed loads)
• (l) = Span length (m)

Reinforcement Calculation:

[ A_s = \frac{M_u}{0.87 f_y z} ]

Where:

• (A_s) = Area of steel (mm²)
• (f_y) = Yield strength of steel (N/mm²)
• (z) = Lever arm (approx. 0.95d, d = effective depth)

Reference Table: Minimum Thickness of Roof Slabs (Typical)

flowchart TD
    A[Water Tank Roof Design] --> B{Design Method

IS 3370 Part 2: Reinforcement Detailing Key Points

1. Size of Bars, Spacing, Laps, Bends

• Follow IS 456 for bar size, spacing, laps, and bends.
• For member depth D < 500 mm, each reinforcement face controls D/2 depth of concrete.
• For D > 500 mm, each face controls 250 mm depth (ignoring central core).

2. Minimum Reinforcement to Control Cracking
[ P_{enit} = \frac{L_a f_{ct}}{f_y} ]

• (P_{enit}): Critical steel ratio (minimum steel ratio to distribute cracks)
• (L_a): Effective length
• (f_{ct}): Direct tensile strength of immature concrete (varies with grade)
• (f_y): Yield strength of steel

3. Maximum Crack Spacing (S_{max})
[ S_{max} = \frac{\phi \cdot f_{ct}}{4 \cdot f_b \cdot P} ]

• (\phi): Bar diameter
• (f_b): Average bond strength (unity for plain bars, 2/3 for deformed bars)
• (P): Steel ratio by gross concrete area

4. Maximum Crack Width (W_{max})
[ W_{max} = S_{max} \times (\epsilon_{sh} + \epsilon_{th}) ]

• (\epsilon_{sh}): Shrinkage strain
• (\epsilon_{th}): Thermal contraction strain (adjusted for immature/mature concrete)

For thermal contraction:
[ \epsilon_{th} = \alpha \times \Delta T ]

• (\alpha): Coefficient of thermal expansion (~10^-5 /°C)
• (\Delta T): Temperature drop from hydration peak to ambient (e.g., 20-30°C)

5. Surface Zones for Thick Sections (D > 500 mm)

• Surface zone thickness = 250 mm (see Fig. 1 & 2 in IS 3370)
• No bottom reinforcement for walls/slabs with thickness 300-500 mm.

Summary Table: Crack Width Control Parameters

| Parameter

IS 3370 Part 2: Annex A - Crack Width Calculations Due to Temperature and Moisture

Key Formulas:

1. Maximum Crack Spacing (SMax):

[ S_{max} = \frac{20 \times f_{ct}}{f_b} \times \frac{\phi}{P} ]

• (f_{ct}): Tensile strength of concrete
• (f_b): Average bond strength between steel and concrete (take as 1 for plain bars, 2/3 for deformed bars in immature concrete)
• (\phi): Diameter of reinforcing bar
• (P): Steel ratio (steel area / gross concrete area)

2. Maximum Crack Width (WMax):

[ W_{max} = S_{max} \times \epsilon ]

• (\epsilon = \epsilon_s + \epsilon_t)
• (\epsilon_s): Shrinkage strain
• (\epsilon_t): Total thermal contraction strain (after peak hydration temperature)

3. Thermal Contraction Strain for Cooling:

[ W_{max} = S_{max} \times \alpha \times (T_1 + T_2) ]

• (\alpha): Coefficient of thermal expansion of mature concrete
• (T_1): Temperature fall from peak hydration to ambient (typically 20-30°C)
• (T_2): Seasonal temperature variation (can be ignored if movement joints ≤ 15 m)

Notes:

• For immature concrete, thermal contraction coefficient is about half that of mature concrete.
• For thick sections (D > 500 mm), consider a surface zone with higher temperature gradients; core temperature rise is at least 10°C higher.
• Movement joints spaced ≤ 15 m reduce the need to consider seasonal temperature fall (T_2).

Table: Permissible Concrete Tensile Stresses (Annex B, Table 1)

IS 3370 Part 2 - Annex B: Crack Width in Mature Concrete

Key Formulas for Crack Width (WMax) in Mature Concrete:

• Maximum Crack Spacing (SMax):

[ S_{Max} = \frac{20 \times f_{ct}}{f_b} ]

Where:

• ( f_{ct} ) = tensile strength of concrete
• ( f_b ) = average bond strength between concrete and steel
• For design, considering bar diameter ( \phi ), steel ratio ( P ), width ( b ), depth ( D ), and number of bars ( n_g ):

[ S_{Max} = \frac{20 \times f_{ct} \times b \times D}{f_b \times n_g \times \phi} ]

• Maximum Crack Width:

[ W_{Max} = S_{Max} \times (\varepsilon_{sh} + \varepsilon_{th}) ]

Where:

• ( \varepsilon_{sh} ) = shrinkage strain

• ( \varepsilon_{th} ) = total thermal contraction strain after peak hydration temperature

• For mature concrete cooling from peak hydration:

[ W_{Max} = S_{Max} \times \alpha \times (T_1 + T_2) ]

Where:

• ( \alpha ) = coefficient of thermal expansion of mature concrete
• ( T_1 ) = temperature fall from peak hydration to ambient (typically 30°C summer, 20°C winter)
• ( T_2 ) = additional seasonal temperature fall (considered zero if movement joints ≤ 15 m)

Table: Permissible Concrete Stresses for Crack Resistance (N/mm²)

Notes:

Composition of Committee (IS 3370 Part 2: 2009 - Annex C)

The committee responsible for formulating IS 3370 Part 2 consists of experts from diverse organizations:

• Chairman: Shri Jose Kuman
• Member Secretary: Dr. B. K. Rao
• Representatives from:
  • Cement and Concrete Sectional Committee, CED 2
  • Central Water Commission, Central Public Works Department, Central Building Research Institute (CSIR)
  • Cement Corporation of India, Cement Manufacturers' Association
  • Atomic Energy Regulatory Board, Indian Institute of Technology Roorkee
  • National Council for Cement and Building Materials, Structural Engineering Research Centre (CSIR)
  • Various cement companies (ACC Ltd, Ultra Tech Cement, Madras Cements, etc.)
  • Government departments (PWD Maharashtra, Tamil Nadu, Ministry of Road Transport & Highways)
  • Research organizations and industry bodies (Indian Concrete Institute, Indian Roads Congress)
  • Military Engineer Services, Nuclear Power Corporation of India Ltd
  • Other industry experts and alternate members

Key Notes:

• The committee includes technical experts, government representatives, industry leaders, and research institutions.
• This multidisciplinary composition ensures comprehensive coverage of cement and concrete standards.

Design Tables & Crack Width Assessment (Clause 4.4.1.2 and Annex B)

Crack Width in Flexure:

[ w = 2 (d - C_{min}) \frac{1 + 3 \alpha \epsilon}{D - x} ]

• (w) = design surface crack width
• (d) = distance to nearest longitudinal bar
• (C_{min}) = minimum cover to tension steel
• (D) = overall member depth
• (x) = neutral axis depth
• (\epsilon) = average strain at crack level (see B-2)
• (\alpha) = factor related to stiffening effect (see B-3)

Stiffening Effect (Flexure):

For crack width 0.2 mm:

[ \epsilon_2 = \frac{1.5 b_1 (D - x)(d' - x)}{3 E_s A_s (d - x)} ]

Where:

• (b_1) = width at tension steel centroid
• (d') = distance from compression face

IS 3370 Part 2: Design Tables & Additional Guidance

• Clause 4.4.2 (Basis of Design):
  Design is based on working stress method considering durability, watertightness, and safety against cracking.

• Clause 4.5 (Working Stress Design):

  • Stresses limited to permissible working stresses in concrete and steel.
  • Use modular ratio and stress-strain relationships per IS 456.

• Design Tables (Part 4, 1967):
  Though not detailed in Part 2, Part 4 provides tables for:

  • Minimum reinforcement ratios
  • Thickness requirements
  • Crack width limits
  • Load factors for hydrostatic pressure

• Additional Guidance:
  For special forms or unusual cases, alternate design methods allowed if proven safe by analysis/tests.

Key Formulas (Working Stress Design)

• Permissible Tensile Stress in Steel:
  ( f_{st} = 0.6 f_y ) (where ( f_y ) is yield strength)

• Permissible Compressive Stress in Concrete:
  ( f_c = 0.33 f_{ck} ) (where ( f_{ck} ) is characteristic compressive strength)

• Modular Ratio:
  ( m = \frac{E_s}{E_c} )

Summary Table Example (Typical Values)

flowchart TD
    A[Design Requirements] --> B[Working Stress Design]
    B --> C[Permissible Stresses]
    B --> D[Modular Ratio]
    A --> E[Design Tables (Part 4)]
    A --> F[Special Cases]
    F --> G[Analysis/Test