Criteria for design of RCC staging for overhead water tanks

IS 11682: Scope - Key Points & References

Scope:

• Applicable to column type staging and circular/polygonal shaft staging for overhead water tanks. (Clause 1.3)
• Covers design, detailing, and ductility requirements for seismic coefficients ≥ 0.05. (Clause 7.2)

Key Specifications & References

Development Length Formula (Working Stress Method)

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

Where:

• (L_d) = Development length
• (\phi) = Bar diameter
• (\sigma_s) = Stress in bar at section face
• (\tau_{bd}) = Design bond stress (depends on concrete grade)

Summary Diagram: Structural Elements in Staging

graph TD
    A[Column] --> B[Junction with Bracing]
    B --> C[Reinforcement Detailing]
    A --> D[Shaft Wall]
    D --> E[Horizontal Ties]
    D --> F[Openings with Additional Reinforcement]
    A --> G[Construction Joint]
    G --> H[Water Bar]

IS 11682: Key Definitions & Formulas Summary

Clause 2.0: Definitions

• Defines terms relevant to design, detailing, and construction of steel staging for water tanks.

Important Formulas & Specifications:

Development Length (Ld) for Reinforcement Bars (Ref: IS 456-1978)
[ L_d = \frac{\sigma_s \times \phi}{4 \times \tau_{bd}} ]

Where:

• (L_d) = Development length
• (\sigma_s) = Stress in bar at section face
• (\phi) = Diameter of bar
• (\tau_{bd}) = Design bond stress (depends on concrete grade)

Reinforcement Detailing (Clause 7.2 & Figures 8-11):

• Reinforcement must extend beyond openings and junctions to ensure adequate anchorage and ductility.
• Horizontal ties at intervals maintain cover and resist hoop compression.
• Minimum internal bend radius for bars is specified to avoid concrete bearing failure (see Fig. 11).
• For seismic zones (design seismic coefficient ≥ 0.05), ductility requirements per IS 4376:1976 must be followed.

Typical Reinforcement Detailing Highlights:

flowchart TD
    A[Reinforcement Bars] --> B[Development Length Ld]
    B --> C{Adequate Anchorage?}
    C -->|Yes| D[Structural Integrity]
    C -->|No| E[Increase Ld or Add Hooks]
    D --> F[Maintain Cover with Ties]
    F --> G[Resist Hoop Compression]

Note: Stresses and bond values are per working stress method (IS 456-1978). For detailed tables and figures, refer to IS 11682 clauses 7.2-7.4 and IS 456.

IS 11682 - Loads and Forces: Key Formulas & Tables

1. Imposed Loads (Clause 3.2)

• Follow IS 875 (1964) for live, snow, and wind loads.
• Water weight:
  • As live load for members containing water.
  • As dead load for staging design.

2. Horizontal Forces & Axial Forces (Clause 7.1.1.3)

• Horizontal forces act on tank and tower.

• Axial force in columns calculated by equating:

  [ \sum M_{horizontal} = M_{axial,restraint} ]

3. Permissible Stresses in Concrete (Clause 8.2.6.1)

• (f_{cu}) = 28-day ultimate cube strength (N/mm²)

4. Permissible Stresses in Steel (Clause 8.2.6.2)

• (f_{sy}) = Yield or proof stress of steel (N/mm²)

Summary

• Use IS 875 for load magnitudes.
• Apply permissible stresses from Tables 8.2.6.1 & 8.2.6.2.
• Calculate axial forces in columns by moment equilibrium of horizontal loads.

If needed, I can provide a schematic of force distribution or moment equilibrium.

IS 11682: General Design Considerations for RCC Staging of Overhead Water Tanks

Key Points from Clauses:

• Clause 4.1 (General):

  • Design must consider all stress conditions per mechanics and sound engineering practice.
  • Monolithic construction effects must be accounted for bending moments and shear.

• Clause 6.2 (Tank Shapes):

  • Besides common shapes, spherical, conical, or multicell tanks may be used based on designer discretion.

• Clause 7.2 (Seismic & Detailing):

  • For seismic coefficient ≥ 0.05, follow IS 4376-1976 clauses 7.2-7.4 for ductility.
  • Reinforcement must have adequate development length into columns and around openings.
  • Shaft walls are thickened locally for bending moments.
  • Horizontal ties maintain cover and resist hoop compression.

Important Formula (Development Length, IS 456:1978 reference):

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

Where:

• (L_d) = Development length
• (\phi) = Bar diameter
• (\sigma_s) = Stress in steel at section face
• (\tau_{bd}) = Design bond stress (depends on concrete grade and bar type)

Minimum Internal Radius of Bend (Fig. 11):

• To avoid concrete bearing failure at bends, radius (r) must satisfy limits per IS 456.

Summary Table: Design Considerations

Basis of Design for Reinforced Concrete Members (IS 11682)

• Design Codes Referenced:

  • IS 456:1978 (Plain and Reinforced Concrete)
  • IS 875:1964 (Loading)
  • IS 4376:1976 (Ductility for seismic design, if seismic coefficient ≥ 0.05)

• Loading Combinations:
  Consider both tank full and empty conditions, including wave action on tank walls (Clause 4.2).

• Design & Detailing:
  Follow IS 456:1978 for design and detailing of RC members (Clause 9.1). Typical reinforcement details for columns, bracings, shafts, and Intze tanks are provided (Figs. 8-11).

• Development Length (Ld):
  Calculated per IS 456:1978 (Clause 25.2.1 & 25.2.2.5):
  [ L_d = \frac{0.87 f_y d_b}{4 \tau_{bd}} ] Where:

  • ( f_y ) = yield stress of steel
  • ( d_b ) = bar diameter
  • ( \tau_{bd} ) = design bond stress (depends on concrete grade and bar type)

• Minimum Internal Radius of Bend:
  Ensures effective anchorage without exceeding bearing stress in concrete (Fig. 11).

• Seismic Detailing:
  For seismic zones (design seismic coefficient ≥ 0.05), refer to IS 4376 clauses 7.2 to 7.4 for ductility detailing and reinforcement anchorage.

Summary Table: Key Specifications

IS 11682: Layout of Overhead Water Tanks — Key Points

1. Staging and Supports

• For staging height > 6 m, columns must be rigidly connected with horizontal bracings spaced at max 6 m vertically (Clause 7.2.1).
• Supports can be arranged with 4, 6, or 12 columns depending on tank size (Fig.4).
• Typical supports include circular beams at tank base or mid-level for load distribution.

2. Effective Length of Columns (Fig.5)

• Unsupported length: ( L )
• Effective length: ( L_{er} \geq 1.2L )
• Calculate ( L_{er} ) per Appendix D of IS 456:1978 (consider no sway).
• Calculate stiffness ( K_c ) (columns) and ( K_b ) (beams) in bending plane.
• Ignore torsional stiffness in other planes.

3. Horizontal Forces (Clause 7.1.1.2)

• Calculate actual horizontal forces/moments in critical directions.
• Use accepted analysis methods, e.g., space frame analysis.

Summary Table: Support & Bracing Requirements

flowchart TD
    A[Overhead Tank Layout] --> B[Supports: 4, 6, or 12 Columns]
    B --> C[Circular Beams at Base/Mid-level]
    A --> D[Staging Height > 6m]
    D --> E[Horizontal Bracing every ≤ 6m]
    A --> F[Column Effective Length]
    F --> G[Calculate \(L_{er} \geq 1.2L\)]
    F --> H[Use IS 456 Appendix D]
    A --> I[Horizontal Forces]
    I --> J[Calculate actual forces & moments]
    J --> K[Use space frame analysis]

For detailed design, refer to **IS

IS 11682: Analysis of Staging (Clauses 7.1, 8.2, 8.2.4)

Key Points from the Code:

• Clause 7.1 (Staging Components):
  Defines staging components such as columns, braces, platforms, ladders, and their design considerations for load transfer and stability.

• Clause 8.2 (Circular Shaft Staging):
  Focuses on the analysis of staging supporting circular tanks/shafts. It requires consideration of:

  • Vertical loads (self-weight + live loads)
  • Lateral loads (wind, seismic)
  • Stability against buckling and overturning

• Clause 8.2.4 (Analysis for Shaft Staging):
  Specifies the method for structural analysis of staging supporting the shaft, including:

  • Load combinations as per IS 875 (Part 3) for wind and seismic forces
  • Moment and shear force calculations in columns and braces
  • Deflection limits and stability checks

Typical Formulas & Specifications:

• Load on staging columns:

  [ P = W_{tank} + W_{water} + W_{staging} + L_{live} + L_{wind/seismic} ]

• Wind load on staging (IS 875 Part 3):

  [ F_w = A \times p \times C_f ]

  Where:

  • ( A ) = projected area
  • ( p ) = design wind pressure
  • ( C_f ) = force coefficient

• Buckling check for columns:

  [ \sigma_{allow} \geq \frac{P}{A} + \frac{M}{S} ]

  Where:

  • ( P ) = axial load
  • ( M ) = bending moment
  • ( A ) = cross-sectional area
  • ( S ) = section modulus

Typical Staging Analysis Steps:

flowchart TD
    A[Determine Loads] --> B[Calculate Axial Loads & Moments]
    B --> C[Check Buckling & Stability]
    C --> D[Deflection & Vibration Analysis]
    D --> E[Design Members & Connections]

Reference Tables (IS 11682):

IS 11682: Shaft Type Staging Key Points

1. Shaft Type Staging (Clause 1.3 & 8)

• Applies to circular and polygonal shaft staging for overhead water tanks.
• Design must satisfy requirements for column type staging as well.

2. Circular Shafts Staging (Clause 8.2)

• Shaft acts as a vertical support column for the tank.
• Must be designed for combined axial load, bending moments, and shear due to wind and seismic forces.

3. Analysis for Shaft Staging (Clause 8.2.4)

• Shaft analyzed as a cantilever column fixed at the base.
• Consider:
  • Axial load (P)
  • Bending moment (M) due to lateral loads
  • Shear force (V)
• Use interaction formulas for combined stresses:

[ \frac{P}{P_{allow}} + \frac{M}{M_{allow}} \leq 1 ]

Where:

• (P_{allow}) = Allowable axial load
• (M_{allow}) = Allowable bending moment

4. Staircase and Ladder Access (Clause 7.4)

• Access via steel ladder or RCC stairs with landings tied to staging.
• For vertical ladders > 6 m, provide safety cages starting at 3 m height.

Summary Table: Shaft Staging Design Parameters

flowchart TD
    A[Tank Load] --> B[Shaft (Cantilever Column)]
    B --> C[Axial Load P]
    B --> D[Bending Moment M]
    B --> E[Shear Force V]
    B --> F[Design Check: P/P_allow + M/M_allow ≤ 1]
    G[Access] --> H[Steel Ladder or RCC Stairs]

IS 11682 Detailing Key Points:

1. Reference Standards:

   • Detailing follows IS 456-1978 clauses.
   • For seismic design (Seismic Coefficient ≥ 0.05), refer to IS 4376-1976 Clauses 7.2 to 7.4 for ductility.

2. Development Length (Ld):
   As per IS 456-1978 (Cl. 25.2.1 & 25.2.2.5),
   [ L_d = \frac{\sigma_s \times \phi}{4 \times \tau_{bd}} ] where:

   • (\sigma_s) = Stress in bar at section face (working stress method)
   • (\phi) = Bar diameter
   • (\tau_{bd}) = Design bond stress
   • Bend radius (r) must meet minimum internal radius for anchorage (Fig. 11).

3. Typical Reinforcement Details:

   • Junction of Column & Bracing: Bars must extend development length beyond face (Fig. 8).
   • Shaft Walls: Thickened locally for bending moments; horizontal ties maintain cover; additional reinforcement around openings (Fig. 9).
   • Intze Tank: Staggered development length; construction joints with water bars; horizontal ties at intervals (Fig. 10).

4. Additional Specifications:

   • All reinforcement must extend beyond openings by development length.
   • Horizontal ties maintain concrete cover and resist hoop compression in shaft walls.
   • Construction joints use water bars to prevent leakage.

Summary Table: Development Length Parameters (IS 456-1978)

flowchart TD
    A[Reinforcement Bar

IS 11682: Safety and Access Provisions - Key Points

1. Development Length & Reinforcement (Clause 7.2)

• Reinforcement bars must have adequate development length (Ld) into columns and bracing junctions.
• Bars must extend beyond openings and construction joints to maintain structural integrity.
• Horizontal ties at intervals maintain concrete cover and resist hoop compression.
• Shaft walls are thickened locally to resist bending moments from tank walls.
• Minimum internal radius of bends for bars follows IS 456:1978 to avoid concrete bearing failure.

Development Length Formula (Working Stress Method):

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

Where:

• ( L_d ) = development length
• ( \sigma_s ) = stress in bar at section face
• ( \phi ) = bar diameter
• ( \tau_{bd} ) = design bond stress (from IS 456)

2. Access (Clause 7.4)

• Access via steel ladders or RCC stairs with landings tied to staging.
• For vertical ladders >6 m, safety cages must be provided starting at 3 m height.

Summary Table: Safety Access

flowchart TD
    A[Tank Access] --> B[Steel Ladder / RCC Stairs]
    B --> C{Ladder Height > 6 m?}
    C -->|Yes| D[Provide Safety Cage starting at 3 m]
    C -->|No| E[No Cage Required]
    A --> F[Reinforcement Detailing]
    F --> G[Development Length per IS 456]
    F --> H[Horizontal Ties for Cover]

References:

• IS 11682 Clause 7.2, 7.4
• IS 456:1978 (Development length, bend radius)
• IS 4376:1976 (Ductility requirements for seismic design)

This ensures safe, durable tank staging and access