Code of practice for construction of floors and roofs using precast doubly-curved shell units

IS 6332: Scope & Key Specifications

1. Scope Overview (Clause 3.1)

For designing doubly-curved shell units (floors/roofs), essential data includes:

• Total area of the shell covering.
• Room sizes or column grid dimensions.
• Details of supporting elements (framed structures or load-bearing walls).
• Finished floor/roof level.
• Treatment of junctions with walls or parapets.
• Roof slope for drainage.
• Provisions for services, ceiling fixing.
• Openings in floor/roof.

2. Surface Equation for Shells (Table 1, Clause 5.1.2)

The shell surface for various plan shapes is defined by:

[ z = a \left(1 - \cosh\left(\frac{n \pi x}{2a}\right) \cos\left(\frac{n \pi y}{2a}\right)\right) ]

• (a) = half-span dimension
• (n = 1, 3, 5, \dots) (mode number)
• (x, y) = coordinates on the shell surface

This equation governs the shape of doubly curved shells for efficient load distribution.

Summary Table: Key Inputs for Shell Design

flowchart TD
    A[Start: Shell Design] --> B[Gather Total Area]
    B --> C[Define Room/Column Grid]
    C --> D[Identify Supports & Dimensions]
    D --> E[Set Finished Floor/Roof Level]
    E --> F[Plan Junction Treatments]
    F --> G[Determine Roof Slope]
    G --> H[Provision for Services & Ceiling]
    H -->

IS 6332: Terminology and Definitions - Key Points

• Clause 2.0: Defines terms specific to shell structures for uniform understanding.

• Key Definitions:

  • Web Thickness (2.16): Thickness of in-situ concrete between two precast shell units.

• Shell Surface Equations (Table 1, Clause 5.1.2):
  Defines mathematical surfaces for various shell plan shapes. Example for a shell surface equation:

  [ \cos nTx = \frac{x^2 + y^2 - a^2}{2a} ]

  (Note: The exact formula varies with shell type; refer to Table 1 for specific shell forms like conical, cylindrical, hyperbolic.)

• Rounding Off (IS 2-1960):
  Values should be rounded to the same number of significant digits as specified in the standard.

Summary Table: Typical Shell Surface Forms

Diagram: Shell Surface Concept

graph TD
    A[Shell Plan Shape] --> B[Cylindrical]
    A --> C[Conical]
    A --> D[Hyperbolic Paraboloid]
    B --> E[Equation: z = f(x)]
    C --> F[Equation: cos nTx = (x² + y² - a²)/2a]
    D --> G[Equation: Hyperbolic functions]

Use IS 6332 Clause 5.1.2 Table 1 for exact shell surface equations and IS 2-1960 for rounding rules.

IS 6332: General Requirements and Coordination

Key Inspection Points (Clause 7.1, Table 2)

Design Coordination (Clause 4.2)

Refer to these IS codes for design and materials:

General Coordination (Clause 3.2)

• Ensure all design and construction info (drawings, instructions) is available to responsible personnel.
• Coordination between design and execution teams is critical for quality and defect avoidance.

flowchart TD
    A[Design Information] --> B[Design Team]
    A --> C[Construction Team]
    B --> D[Drawings & Instructions]
    C --> D
    D --> E[Inspection & Execution]
    E --> F{Defects?}
    F -->|No| G[Completion]
    F -->|Yes| H[Rectification]
    H --> E

Summary:

• Maintain strict inspection during precasting, staging, and bearing stages to avoid defects.
• Follow referenced IS codes for design standards.
• Ensure clear communication and coordination between design and construction teams.

IS 6332 - Design Considerations for Precast Concrete Shells

Key References (Clause 4.2)

Essential Design Inputs (Clause 3.1)

For efficient doubly-curved shell design, gather:

• Total covered area
• Room sizes or column grid spacing
• Supporting elements: type (frame or load-bearing walls), dimensions, restrictions
• Finished floor/roof level
• Junction treatment: floor/roof to walls/parapets
• Roof slope: for drainage
• Provisions: fixing services, ceiling, etc.
• Openings: size and location in floor/roof

Coordination (Clause 3.3)

Ensure continuous information exchange among designer, builder, and user throughout construction.

Summary Table for Design Reference Codes

flowchart TD
    A[Project Planning] --> B[Design Inputs: Area, Grid, Supports]
    B --> C[Select Material Codes]
    C --> D[Concrete: IS 456]
    C --> E[Prestressed: IS 1343]
    C --> F[Steel: IS 800]
    C --> G[Timber: IS 883]
    C --> H[Loading: IS 875]
    D & E & F & G & H --> I[Shell Design & Detailing]
    I --> J[Coordination with Builder & User]

Use these IS codes and design inputs as a

IS 6332: Precasting Methods for Doubly-Curved Shell Units

Key Formulas

For rectangular/square shell plans, vertical ordinate (Z) at point ((x,y)) is:

[ Z = Z_{\text{max}} \left(1 - \frac{x^2}{a^2}\right) \left(1 - \frac{y^2}{b^2}\right) ]

• (Z_{\text{max}} =) max central rise (up to ( \frac{10L}{100} ), where (L) = shell size)
• (a =) shell length/2
• (b =) shell width/2
• (x,y =) coordinates from shell center

Precasting Steps (Clause 5.1.1 & 5.1)

• Set out shell surface ordinates on a level platform (600–1000 mm above ground).
• Fabricate mould (steel/timber/plastic) as monolithic block.
• Place reinforcement in shell and edge beam moulds.
• Cast concrete over shell mould; control thickness by gauges.
• Release edge beam mould after 3 hours.
• Lift shell off mould after 24–48 hours (up to 72 hours in cold climates).
• Stack shells supported at four corners (8–10 shells per pile).

Edge Beam

• Cast edge beam simultaneously with shell.
• Thickness controlled by thickness gauges.
• Reinforcement placed between outer mould and platform.

Summary Table: Shell Rise Limits

flowchart TD
    A[Set out ordinates on platform] --> B[Fabricate mould]
    B --> C[Place reinforcement]
    C --> D[Cast concrete over mould]
    D --> E[Release edge beam mould (3 hrs)]
    E --> F[Demould shell (24-48 hrs)]

IS 6332: Construction Procedures for Doubly-Curved Shell Units

Key Construction Steps (Clause 5.1.1):

• Shell Surface Profile:

  For rectangular/square shells, vertical ordinate ( Z ) at point ((x,y)):

  [ Z = Z_{\text{max}} \times \left(1 - \frac{x^2}{a^2}\right) \times \left(1 - \frac{y^2}{b^2}\right) ]

  Where:

  • ( Z_{\text{max}} ) = max central rise (typically ( \frac{L}{10} ) to ( \frac{L}{20} ))
  • ( L ) = shell size
  • ( a, b ) = shell length and width
  • ( x, y ) = coordinates from shell center

• Casting Procedure:

  • Set surface ordinates on level platform.
  • Coat finished surface with oil/grease (release agent).
  • Set outer edge beam mould; place reinforcement.
  • Pour concrete for edge beam and shell thickness (controlled by gauges).
  • Release edge beam mould after 3 hours.
  • Lift shell after 24-48 hours (up to 72h in cold climates).
  • Stack shells at corners in piles of 8-10 for curing.

References for Design:

Summary Diagram of Construction Flow

flowchart TD
    A[Set shell ordinates on platform] --> B[Coat surface with release agent]
    B --> C[Set edge beam mould and place reinforcement]
    C --> D[Pour concrete for shell and edge beam]
    D --> E[Release edge beam mould after 3 hrs]
    E --> F[Lifting shell after 24-48 hrs]
    F --> G[Stack and cure shells]

IS 6332: Handling and Erection Key Points

1. Lifting and Handling of Shells

• Lifting hooks must be provided at the corners within edge beam thickness.
• Shells can be lifted 24 to 48 hours after casting; extend to 72 hours in cold climates (<25°C) or with pozzolana cement.
• Use levers at four corners, operated on one side at a time; never at diagonals.

2. Stacking and Curing

• Shells should be stacked supported at four corners only.
• Maximum 8 to 10 shells per pile.
• Curing is done in the normal way after stacking.

3. Shell Surface and Edge Beam Casting

• Surface shape given by:

[ Z = Z_{\max} \left(1 - \frac{x^2}{a^2}\right)\left(1 - \frac{y^2}{b^2}\right) ]

Where:

• (Z) = vertical ordinate at point ((x,y))

• (Z_{\max}) = max central rise (up to 10% of shell size (L))

• (a, b) = shell length and width

• (x, y) = coordinates from shell center

• Edge beam mould released 3 hours after casting.

• Concrete thickness controlled by thickness gauges.

• Finished surface coated with oil or grease as releasing agent.

Reference Codes for Design

• Reinforced concrete: IS 456-1978
• Prestressed concrete: IS 1343-1980
• Structural steel: IS 800-1962
• Loading: IS 875-1964

flowchart TD
    A[Casting Shell on Platform] --> B[Lifting Hooks Provided]
    B --> C[Lifting Shell after 24-48 hrs]
    C --> D[Stacking Shells (8-10 per pile)]
    D --> E[Curing]
    A --> F[Edge Beam Casting]
    F --> G[Release Edge Beam Mould after 3 hrs]

This summarizes handling, lifting, stacking, and erection per IS 6332.

IS 6332: Edge Beam Details Summary

• Definition (Clause 2.3):
  Edge beams are beams along the shell periphery to stiffen the shell structure.

• Minimum Thickness (Clause 4.2.6):

  • Normal atmosphere: 25 mm
  • Corrosive atmosphere: 35 mm
  • Refer Fig. 3 (IS 6332) for typical edge beam detail in corrosive conditions.

• Minimum Reinforcement (Clause 4.2.4):

  • At least one 6 mm diameter mild steel bar in edge beams.

• Web Thickness (Clause 2.16):

  • Thickness of concrete web between two precast shell units; important for edge beam integration.

Typical Edge Beam Specifications:

Key Notes:

• Edge beams provide shell perimeter stiffness.
• Reinforcement ensures crack control and durability.
• Thickness increase in corrosive atmospheres protects against deterioration.

flowchart LR
    A[Shell Periphery] --> B[Edge Beam]
    B --> C[Minimum Thickness]
    C -->|Normal| D[25 mm]
    C -->|Corrosive| E[35 mm]
    B --> F[Minimum Reinforcement]
    F --> G[1 bar of 6 mm dia mild steel]

For detailed dimensions and reinforcement layout, consult Fig. 3 of IS 6332.

IS 6332: Provision for Openings in Precast Shell Units

Key Points from Clauses 6.1.4.1 & 6.1.4.2:

• Openings should ideally be avoided in precast shell units.
• If openings are known in advance, they must be incorporated during precasting with:
  • Proper stiffening.
  • Additional reinforcement around openings to maintain structural integrity.
• For openings in intergrid shell patterns:
  • Follow the same reinforcement approach.
  • If many openings exist in one grid, replace that grid with solid reinforced concrete with strengthened openings.

Important Definitions:

• Web Thickness (Clause 2.16): Thickness of in-situ concrete between two precast shells, crucial for load transfer around openings.

Design Considerations (Clause 3.1):

• Size and location of openings must be coordinated with:
  • Room sizes, column grids.
  • Supporting structures.
  • Junction treatments.
  • Drainage slope.
  • Fixing provisions for services.

Reinforcement Guidelines Around Openings (General Engineering Practice):

Typical Reinforcement Detail (Conceptual):

flowchart TB
    A[Shell Unit] --> B[Opening]
    B --> C[Reinforcement Ring]
    C --> D[Increased Web Thickness]
    D --> E[Load Transfer]

Summary: Plan openings during precasting, reinforce edges, and if many openings exist, replace shell grid with solid RC. Ensure web thickness and reinforcement are adequate to maintain strength.

IS 6332: Curing and Storage of Precast Concrete Shells

Key Specifications & Procedures:

• Surface Casting & Shape:
  For rectangular/square shells, vertical ordinate ( Z ) at point ((x,y)) is approximated by:
  [ Z = Z_{max} \left(1 - \frac{x^2}{a^2}\right) \left(1 - \frac{y^2}{b^2}\right) ]
  where:

  • (Z_{max} \leq \frac{10L}{100}) (max rise)
  • (a, b) = shell length and width
  • (x, y) = coordinates from shell center

• Curing Time & Conditions:

  • Edge beam mould release: 3 hours after casting
  • Shell lifting from mould: 24-48 hours (up to 72 hours in cold climates < 25°C or with pozzolana cement)
  • Shell stacking: stacked on 4 corner supports, 8-10 shells per pile
  • Normal curing after stacking (water curing or moist curing recommended)

• Storage:

  • Shells must be kept stacked and cured normally after lifting
  • Lifting hooks to be embedded within edge beam thickness at corners for safe handling

Summary Table:

flowchart TD
    A[Cast shell on mould] --> B[Coat surface with releasing agent]
    B --> C[Place edge beam mould & reinforcement]
    C --> D[Cast edge beam & shell concrete]
    D --> E[Release edge beam mould after 3 hrs]
    E --> F[Lifting shell after 24-48 hrs]
    F --> G[Stack shells on 4 corner supports]
   

IS 6332: Finishing and Joint Treatment for Doubly-Curved Shell Floors/Roofs

Key Specifications & Procedures:

• Joints between precast shell units: Finished neatly with cement paste or mortar (Clause 6.1.7.1).
• Surface air holes: Filled with lime or cement paste before painting.
• No raking of shell surfaces or edge beams to receive plaster internally.
• Waterproofing: As per IS 1346-1976 and IS 4365-1967, apply waterproofing treatments similar to reinforced concrete slabs, especially at wall junctions (Clause 6.1.5.1).
• Finishing sequence (Clause 6.1.1):
  • Finish supporting elements to bearing level.
  • Place battens/joists with mortar pads.
  • Position shell units on battens/joists with mortar pads.
  • Leave space between edge beams equal to web thickness for concreting.
  • Place reinforcement in compression flange.
  • Pour in situ compression flange concrete.
  • Fill haunches with dry filling or lean concrete.
  • Remove props after strength gain.
  • Ready for finishing and waterproofing.

Inspection & Quality Control (Table 2, Clause 7.1):

Summary Table for Joint Treatment:

IS 6332 - Material Specifications and References

Key Indian Standards for Design (Clause 4.2)

Inspection Guidelines (Clause 7.1 & Table 2)

• Precasting: Concrete compaction, reinforcement position, sag tolerance ±3 mm.
• Staging and Props: No sagging allowed to avoid compression-tension flange defects.
• Bearing of Joists and Shell Units: Must be level and watertight to prevent slurry leakage.

Important Notes:

• Use IS 2-1960 for rounding off values with consistent significant figures.
• Refer to IS codes for detailed material properties, mix design, and structural detailing.

flowchart TD
    A[Material Type] --> B[IS Code Reference]
    B --> C{Material}
    C -->|Reinforced Concrete| D[IS 456-1978]
    C -->|Prestressed Concrete| E[IS 1343-1980]
    C -->|Structural Steel| F[IS 800-1962]
    C -->|Timber Design| G[IS 883-1970]
    C -->|Loading| H[IS 875-1964]
    C -->|Composite Construction| I[IS 3935-1966]

This summary ensures you refer to the correct IS codes for material specifications and design considerations in structural elements.

Safety and Quality Control in IS 6332 (Clause 7.1 & 4.2)

Key Inspection Stages and Likely Failures (Table 2):

Referenced IS Codes for Design Considerations (Clause 4.2)

Summary for Quality Control

• Maintain fabric sag tolerance: ±3 mm during precasting.
• Avoid sag in staging/props to prevent structural defects.
• Ensure watertight bearing joints to avoid leakage and poor finishes.
• Use relevant IS codes for design and material specifications.

flowchart TD
    A[Precasting Inspection] --> B{Check}
    B -->|Concrete compaction| C[No Cracking]
    B -->|Reinforcement position| C
    B -->|Fabric sag ±3 mm| C
    D[Staging & Props] --> E{No Sag}
    E --> F[Proper flange interaction]
    G[Joists & Shells Bearing] --> H{Level & Watertight}
    H --> I[Good concrete & finish]

This ensures safe, defect-free floors and roofs per IS 6332.