Code of Practice Concrete structures for the storage of liquids, Part 3: Prestressed concrete structures

IS 3370 Part 3: Scope & Basis of Design (Summary)

Scope:

• Applies to design of prestressed concrete structures for liquid storage.
• Covers all stress conditions per mechanics principles and sound engineering practice.
• Special attention to monolithic construction effects on bending moments and shear.

Key Specifications & Tables

1. Basis of Design (Clause 3.2)

• Design must consider all possible stress states.
• Use recognized design methods and engineering judgment.
• Account for monolithic action in bending and shear assessments.

2. Limiting Tensile Strength of Concrete (Clause 3.3.2, Table 1)

Used for estimating cracking resistance in prestressed concrete.

References for Workmanship & Testing (Clause 9.1)

• Follow IS 3370 Part I and IS 1343-1960 for workmanship and inspection.
• IS 1343 covers prestressed concrete practices.

Visual Summary: Design Process Flow

flowchart TD
    A[Start: Define Liquid Storage Structure] --> B[Assess Load & Stress Conditions]
    B --> C[Apply Mechanics & Design Principles]
    C --> D[Consider Monolithic Construction Effects]
    D --> E[Check Tensile Strength Limits (Table 1)]
    E --> F[Detail Prestressing & Reinforcement]
    F --> G[Workmanship & Testing per IS 3370 Part I & IS 1343]
    G --> H[Final Design & Approval]

Summary: IS 3370 Part 3 guides prestressed concrete liquid storage design, emphasizing stress analysis, tensile strength limits, monolithic action, and compliance with workmanship standards.

IS 3370 Part 3: General Requirements - Key Points

1. Basis of Design (Clause 3.2 & 3.2.1)

• Design must consider all stress conditions per mechanics principles and sound engineering practice.
• Effects of monolithic construction on bending moments and shear must be included.
• Design generally follows IS 1343-1960 (Prestressed Concrete Code), except where IS 3370 Part 3 specifies otherwise.

2. Design Principles

• Members are designed according to IS 1343-1960.
• Structural behavior under liquid storage loads, temperature, shrinkage, and prestressing forces are considered.

3. Workmanship, Inspection, and Testing (Clause 9.1)

• Follow IS 3370 Part I (General Requirements) along with IS 1343-1960 for prestressed concrete workmanship and testing.

Typical Design Considerations Include:

Summary Diagram of Design Process:

flowchart TD
    A[Load & Stress Analysis] --> B[Consider Monolithic Effects]
    B --> C[Design per IS 1343-1960]
    C --> D[Check Bending & Shear]
    D --> E[Workmanship & Testing per IS 3370 Part I & IS 1343]
    E --> F[Final Structural Design]

Note: For detailed formulas and tables, refer directly to IS 1343-1960 and IS 3370 Part 1 & 3 for liquid storage structures.

IS 3370 Part 3 - Basis of Design Summary

• General Basis: Design shall follow IS 1343-1960 (Prestressed Concrete Code) unless otherwise specified (Clause 3.2.1).
• Design Principles:
  • Consider all stress conditions per mechanics and sound engineering practice (Clause 3.1).
  • Account for monolithic construction effects on bending moments and shear.
• Workmanship & Testing: Must comply with IS 3370 Part 1 and IS 1343-1960 (Clause 9.1).

Key Design References from IS 1343 (as per IS 3370 Part 3):

Important Notes:

• Design must ensure liquid tightness and durability.
• Follow sound engineering practice and recognized design methods.
• Refer to IS 1343 for detailed prestressed concrete design formulas, tables, and loss calculations.

flowchart TD
    A[Start: Design of RCC Liquid Storage Structure] --> B[Refer IS 3370 Part 3]
    B --> C{Is member specified in 3.2.2?}
    C -- No --> D[Design as per IS 1343-1960]
    C -- Yes --> E[Follow specific clauses in IS 3370 Part 3]
    D --> F[Consider stresses, monolithic action, losses]
    E --> F
    F --> G[Check workmanship & testing per IS 3370 Part 1 & IS 1343]
    G -->

IS 3370 Part 3: Floors of Tanks Resting on Ground – Key Points

1. Floor Construction (Clause 4.2)

• Concrete floor with minimum 0.15% reinforcement of gross concrete cross-section.
• Ground must carry load without appreciable subsidence.
• Floor cast in panels ≤ 4.5 m × 4.5 m with contraction/expansion joints.
• Place a screed layer ≥ 75 mm thick on ground first.
• Cover screed with bitumen paper or similar sliding layer to break bond with floor concrete.

2. Loading Considerations (Clause 6.2)

• Design covers for:
  • Gravity loads (roof slab, earth cover, live loads, equipment).
  • Upward loads if internal gas pressure exists.

3. Base of Wall Conditions (Clause 7.3)

• Ignore restraining effects if hinged/sliding at base.
• Include prestressing moment effects parallel to tank axis.

Summary Table: Floor Reinforcement and Panel Size

flowchart TD
    A[Ground] --> B[Screed Layer (≥75 mm)]
    B --> C[Sliding Layer (Bitumen Paper)]
    C --> D[Concrete Floor (0.15% Reinforcement)]
    D --> E[Expansion/Contraction Joints ≤ 4.5 m apart]

This ensures controlled cracking and load transfer without subsidence risk.

IS 3370 Part 3: Provision of Joints - Key Points

• Movement Joints must be provided as per Clause 8 of IS 3370 Part 1 (1965), which governs spacing and type.

• Types of Joints:

  • Contraction Joints: Partial or complete to accommodate shrinkage and temperature changes.
  • Expansion Joints: To accommodate expansion and avoid excessive stresses.

• Spacing Considerations (Clause 5.1.1.1):

  • Governed by factors like temperature variation, structural dimensions, and material properties.
  • Refer to Clause 8 of IS 3370 Part 1 for detailed spacing guidelines.

• Coordination (Clause 6.1):

  • Movement joints in roofs should align with those in walls if monolithic construction is used.
  • If sliding joints are used between roof and walls, exact alignment is less critical.

Typical Joint Spacing (from IS 3370 Part 1, Clause 8)

Summary Formula for Joint Spacing (approximate):

[ L = \frac{\Delta T \times \alpha \times E}{f_s} ]

Where:

• (L) = Joint spacing (m)
• (\Delta T) = Temperature variation (°C)
• (\alpha) = Coefficient of thermal expansion (≈ 10^-5 /°C)
• (E) = Modulus of elasticity of concrete (N/mm²)
• (f_s) = Allowable stress in concrete (N/mm²)

flowchart LR
    A[Concrete Structure] --> B{Movement Joints?}
    B -->|Yes| C[Contraction Joints]
    B -->|Yes| D[Expansion Joints]
    C --> E[Spacing per IS 3370 Part 1]
    D --> E
    E --> F[Align Roof & Wall Joints if Monolithic]
    E --> G[Sliding Joints allow Non-alignment]

**Refer IS 3370 Part

IS 3370 Part 3: Loading and Watertightness Key Points

1. Loading on Fixed Covers (Clause 6.2 & 6.2.1)

• Design for gravity loads:
  • Self-weight of roof slab
  • Earth cover (if any)
  • Live loads (e.g., maintenance personnel)
  • Mechanical equipment loads
• Consider upward loads if tank has internal gas pressure.
• Account for unequal loading during earth cover placement:
  • Temporary uneven loads on spans
  • Specify safe temporary load limits for construction phase

2. Watertightness (Clause 6.3)

• Roofs of tanks for domestic water storage must be watertight.
• Achieve watertightness by:
  • Limiting tensile stresses within permissible limits
  • Using waterproof membranes or equivalent coverings

3. Stress Considerations (Clause 7.9)

• Consider combined effects of:
  • Internal liquid pressure
  • External pressures (soil, etc.)
  • Temperature variations
  • Shrinkage and restraint effects from roof

Typical Design Load Formula for Roof Slab:

[ \text{Total Load} = W_{roof} + W_{earth} + W_{live} + W_{equipment} \pm W_{upward} ]

Where:

• (W_{roof}) = Self-weight of roof slab
• (W_{earth}) = Earth cover load (if any)
• (W_{live}) = Live load (maintenance, etc.)
• (W_{equipment}) = Load of mechanical equipment
• (W_{upward}) = Upward load due to internal gas pressure (if applicable)

Summary Table: Loading Considerations

flowchart TD
    A[Fixed Cover Design] --> B[Gravity Loads]

Key Formulas & Specifications for Cylindrical Tanks (IS 3370 Part 3)

1. Stresses in Prestressed Concrete Cylindrical Tanks (Clause 7.1)

• Hoop & Longitudinal Steel Tensile Stress: ≤ limits in Clause 3.4 (usually max tensile stress allowable in steel)
• Concrete Compressive Stress:
  [ \sigma_c \leq \frac{f_{cu}}{3} ]
  where ( f_{cu} ) = specified cube strength of concrete.
• Average Shear Stress in Concrete:
  [ \tau \leq 0.6 \times f_{cu} ]
• Minimum Concrete Compression (Tank Full):
  [ \sigma_c \geq 7 \text{ kg/cm}^2 ]
• Maximum Concrete Tensile Stress (Tank Empty):
  [ \sigma_t \leq 10 \text{ kg/cm}^2 ]

2. Prestressing (Clause 7.10)

• Investigate necessity of prestressing in the vertical (axial) direction of the cylinder wall to control tensile stresses.

3. Corrosion Protection (Clause 6.4)

• Provide protective measures under the roof to prevent condensation corrosion.
• Alternatively, design the underside of the roof as a liquid retaining face with minimum cover to reinforcement.

Summary Table of Stress Limits

flowchart TD
    A[Cylindrical Tank Design] --> B[Check Hoop & Longitudinal Steel Stress]
    A --> C[Check Concrete Compressive Stress ≤ fcu/3]
    A --> D[Check Average Shear Stress ≤ 0.6 fcu]
    A --> E[

IS 3370 Part 3: Spacing and Cover of Prestressing Steel

Key Specifications:

• Minimum Concrete Cover (Clause 8.1):

  • 35 mm minimum cover to prestressing rods, wires, cables, sheathings, and spacers on the liquid face.

• Spacing of Prestressing Steel (Clause 8.2):

  • Follow IS 1343:1960 for spacing requirements.
  • Typical spacing depends on diameter and arrangement to ensure proper concrete compaction and corrosion protection.

• Additional Protection (Clause 7.7):

  • Prestressing wires placed outside walls must have 40 mm cover with pneumatic mortar protection.
  • In aggressive environments (marine, industrial), cables must be placed inside walls and fully grouted.

Reference from IS 1343:1960 (Spacing Guidelines)

Summary:

• Cover: 35 mm (liquid face), 40 mm (external wires with protection)
• Spacing: As per IS 1343, generally ≥ 2× diameter or minimum 25-40 mm
• Corrosion Protection: Grouting inside walls in aggressive environments

flowchart LR
    A[Prestressing Steel] --> B{Location}
    B -->|Liquid Face| C[Min Cover 35 mm]
    B -->|Outside Walls| D[Min Cover 40 mm + Pneumatic Mortar]
    B -->|Aggressive Atmosphere| E[Inside Walls + Grouting]

This ensures durability and structural integrity per IS 3370 Part 3.

IS 3370 Part 3: Workmanship, Inspection and Testing

Key Points (Clause 9):

• Compliance: Follow IS 3370 Part I (General Requirements) and IS 1343-1960 (Prestressed Concrete Code).
• Workmanship: Ensure high-quality concrete placement, compaction, curing, and prestressing steel handling per IS 1343.
• Inspection: Regular checks on materials, reinforcement, prestressing steel spacing (per IS 1343), and concrete quality.
• Testing: Concrete tests (slump, cube strength at 28 days), prestressing steel tensile tests, and leak tests for liquid-tightness.

Relevant Table: Limiting Tensile Strength of Concrete (for Crack Resistance) [Clause 3.3.2]

Prestressing Steel Spacing

• Follow IS 1343-1960 requirements for minimum spacing to ensure proper concrete consolidation and bond.

Summary Diagram of Inspection & Testing Flow

flowchart TD
    A[Material Approval] --> B[Reinforcement & Prestressing Steel Inspection]
    B --> C[Concrete Mixing & Workmanship Check]
    C --> D[Concrete Testing (Slump, Cube)]
    D --> E[Prestressing Steel Tensile Test]
    E --> F[Leakage & Structural Integrity Testing]
    F --> G[Final Approval]

Note: For detailed workmanship and testing procedures, refer directly to IS 3370 Part I and IS 1343.

Protection Against Corrosion (IS 3370 Part 3)

• Clause 6.4:

  • Provide protective measures on the underside of the roof to prevent corrosion from condensation.
  • Alternatively, design the underside as a liquid-retaining face with strict adherence to minimum concrete cover to reinforcement.

• Minimum Cover Requirements:
  For parts not in contact with liquid, refer to IS 1343-1960 for cover thickness. Typically:

  Exposure Condition	Minimum Cover (mm)
  Mild exposure	20-25
  Moderate exposure	30-40
  Severe exposure (corrosive)	40-50

• General Notes:

  • Use corrosion-resistant materials or coatings if exposure to moisture or aggressive agents is expected.
  • Ensure proper compaction and curing to reduce permeability.
  • Avoid cracks by controlling shrinkage and thermal stresses.

flowchart TD
    A[Roof Underside] --> B{Condensation Risk?}
    B -- Yes --> C[Provide Protective Measures]
    B -- No --> D[Design as Liquid Retaining Face]
    C --> E[Ensure Minimum Cover to Reinforcement]
    D --> E
    E --> F[Use IS 1343 for Cover Thickness]

Summary: Protect reinforcement by adequate cover and protective measures on roof undersides; follow IS 1343 for cover thickness to prevent corrosion.

Losses in Prestress (IS 3370 Part 3: Clause 3.6 & Related Clauses)

Key Points:

• Losses in prestress arise due to:

  • Creep of concrete
  • Shrinkage of concrete
  • Relaxation of steel
  • Shortening of concrete at transfer
  • Friction and slip at anchorage

• All losses must be accounted for when assessing stresses during tensioning and service.

• IS 1343-1960 provides detailed guidelines on permissible stresses and loss calculations.

Typical Losses in Prestress (approximate values):

Permissible Stresses in Concrete (from Table 1, Clause 3.3.2)

Summary of Stress Limits (Clause 7.1)

• Max tensile stress in steel: As per IS 1343.
• Max compressive stress in concrete: ≤ 1/3 of specified cube strength.
• Max shear stress in concrete: ≤ 0.6 × specified cube strength.
• Min compression in concrete when full: ≥ 7 kg/cm².
• Max tensile stress in concrete when empty: ≤ 10 kg/cm² (prefer residual compression if frequent emptying).

Formula for Total Prestress Loss

Shrinkage and Creep of Concrete (IS 3370 Part 3)

• Clause 3.5 & 3.6 refer to IS 1343-1960 for detailed provisions on shrinkage and creep.
• These affect losses in prestress and stress calculations in concrete and steel.
• Key factors causing prestress losses:
  • Creep of concrete and steel
  • Shrinkage of concrete
  • Shortening at transfer
  • Friction and slip of anchorage

Relevant IS 3370 Table (3.3.2) — Limiting Tensile Strength for Crack Estimation

Additional Notes:

• Use IS 1343-1960 for formulas and coefficients of creep and shrinkage.
• Typical creep coefficient, φ(t,t₀), and shrinkage strain, ε_sh, depend on concrete age, mix, and environment.
• Losses in prestress due to creep and shrinkage can be estimated by:

[ \Delta \sigma_{creep} = \sigma_{initial} \times \phi(t,t_0) ] [ \Delta \sigma_{shrinkage} = E_c \times \varepsilon_{sh} ]

where:

• (\sigma_{initial}) = initial stress in concrete
• (E_c) = modulus of elasticity of concrete

flowchart LR
    A[Initial Prestress] --> B[Creep of Concrete]
    A --> C[Shrinkage of Concrete]
    A --> D[Shortening at Transfer]
    A --> E[Friction & Anchorage Slip]
    B --> F[Prestress Loss]
    C --> F
    D --> F
    E --> F

**

IS 3370 Part 3: Permissible Stresses in Concrete and Steel

1. Permissible Stresses in Concrete (Clause 3.3)

• As per IS 1343-1960, permissible stresses depend on prestressing and working loads.
• For resistance to cracking, use limiting tensile strengths from Table 1 (Clause 3.3.2):

2. Permissible Stresses in Steel (Clause 3.4)

• Permissible stresses for steel used in prestressing are as per IS 1343-1960.
• Typically, permissible stress in prestressing steel = 0.8 × Ultimate tensile strength (fpu).
• For mild steel reinforcement, permissible stresses follow IS 456.

Summary:

• Concrete tensile strength limits from Table 1 guide crack control.
• Steel permissible stress = 80% of ultimate tensile strength for prestressing steel.
• Refer IS 1343 for detailed modulus of elasticity and stress limits during prestressing.

flowchart TD
    A[Concrete Grade] --> B[Permissible Tensile Strength]
    B --> C[Direct Tensile Strength]
    B --> D[Bending Tensile Strength]
    E[Steel Type] --> F[Permissible Stress = 0.8 × fpu]
    subgraph Concrete
        A
        B
        C
        D
    end
    subgraph Steel
        E
        F
    end

Design Considerations for Movement and Sliding Joints (IS 3370 Part 3)

• Sliding Joints at Base of Wall (Clause 5.1.1):
  Allow wall expansion/contraction independent of floor; prevent base moments from fixity by using sliding joints.

• Spacing of Vertical Movement Joints (Clause 5.1.1.1):
  Refer to Clause 8 of IS 3370 Part 1 (1965) for spacing rules.

  • Use mostly partial or complete contraction joints.
  • Provide adequate expansion joints to accommodate thermal and shrinkage movements.

• Provision of Movement Joints (Clauses 4.1 & 6.1):

  • Follow Clause 8 of IS 3370 Part 1 for joint design and placement.
  • Ensure movement joints in roofs align with walls if monolithic to prevent sympathetic cracking.
  • If sliding joints separate roof and wall, exact alignment is less critical.

Key Reference: IS 3370 Part 1, Clause 8 (Movement Joints)

Summary Diagram of Sliding Joint Concept:

flowchart LR
    Wall -->|Expansion/Contraction| SlidingJoint[Sliding Joint at Base]
    SlidingJoint --> Floor
    Roof -.->|Movement joint alignment| Wall

Note: Consult IS 3370 Part 1 Clause 8 for detailed formulas and joint spacing tables.

IS 3370 Part 3: Special Design Considerations - Key Points

1. Basis of Design (Clauses 3.1 & 3.2)

• Design must consider all stress conditions per mechanics principles and sound engineering practice.
• Monolithic construction effects on bending moments and shear must be accounted.
• Design follows IS 1343-1960 for prestressed concrete unless otherwise specified.

2. Stress Conditions to Consider (Clause 7.9)

• Internal liquid pressure
• Surrounding soil or water pressure
• Temperature effects
• Shrinkage and creep
• Restraint from roof or adjoining structures

3. Typical Stress Formula for Pressure Vessels (Cylindrical Tanks)

• Hoop stress, σ_h = (p × r) / t
• Longitudinal stress, σ_l = (p × r) / (2 × t)

Where:

• p = internal liquid pressure
• r = radius of tank
• t = wall thickness

4. Design Recommendations

• Use IS 1343 for prestressing details.
• Consider temperature gradients and restraint forces in moment calculations.
• Account for shrinkage strains and their restraint to avoid cracking.

flowchart TD
    A[Start Design] --> B[Identify Stress Conditions]
    B --> C{Stress Types}
    C -->|Internal Pressure| D[Calculate Hoop & Longitudinal Stress]
    C -->|External Pressure| E[Consider Soil/Water Loads]
    C -->|Temperature| F[Assess Thermal Stresses]
    C -->|Shrinkage| G[Evaluate Restraint Effects]
    D & E & F & G --> H[Apply IS 1343 for Prestress]
    H --> I[Check Against Allowable Stresses]
    I --> J[Finalize Design]

For detailed prestressing and reinforcement requirements, refer directly to IS 1343-1960 as mandated by IS 3370 Part 3.