Code of practice for design and construction of radar antenna, microwave and TV tower foundations

IS 11233 - Scope & Key Specifications

Scope (Clause 2.1)

• Applies to design and construction of foundations for TV and microwave towers.
• Definitions as per IS 2809-1972 (Soil Engineering Glossary).

Design Data Required (Clause 3.1.6)

• Wind data including cyclones/tornadoes.
• Frost depth and penetration.
• Earthquake data.

Types of Foundations & Loading (Clause 4.1.1, Table 1)

Construction

• Concreting as per IS 456-1978.

Units (SI system)

• Length: meter (m)
• Force: newton (N = kg·m/s²)
• Pressure/stress: pascal (Pa = N/m²)

flowchart TD
    A[Load Type] --> B[Class A: Uplift + Shear]
    A --> C[Class B: Overturning + Shear]
    A --> D[Class C: Heavy Downward Load]

    B --> E[Wide base towers]
    C --> F[Poles/Columns]
    D --> G[Heavy Equipments]

    E --> H[Enlarged/Under-reamed Footing]
    F --> I[Base or Piles]
    G --> J[Base/Under-reamed/Piles]

    H --> K[Soil Reaction: Weight of earth/Pull-out]
    I --> L[Soil Reaction: Lateral resistance + Soil pressure]
    J --> M[Soil Reaction: Allowable soil pressure + Shaft resistance]

Summary: IS 112

IS 11233 Key References, Formulas & Tables Summary

1. References:

• IS 2809-1972: Definitions of terms.
• IS 456-1978: Concrete design and construction practices.
• Other relevant codes: Safety codes for excavation and blasting.

2. Types of Foundations (Table 1 - Clause 4.1.1):

3. Design Data Required (Clause 3.0):

• Soil properties
• Load details (vertical, lateral, moments)
• Structural requirements

4. Units (SI system):

Quick Concept Diagram

flowchart TD
    A[Load Types] --> B[Footing Classification]
    B --> C[Footing Design]
    C --> D[Soil Reaction]
    D --> E[Foundation Stability]

Summary: IS 11233 bases foundation design on load types, structural needs, and soil reactions, referencing IS 2809 and IS 456 for definitions and

IS 11233 - Information Required for Design

Key Information & Data (Clauses 3.0 to 3.3)

For foundation design, the following are essential:

• Soil Data: Type, bearing capacity, settlement characteristics (refer IS 2809-1972).
• Load Data: Magnitude, type (dead, live, wind, seismic), and combinations.
• Design Forces (Clause 3.2):
  • Axial loads (P)
  • Moments (M)
  • Shear forces (V)
• Environmental Conditions: Groundwater level, corrosive environment.

Design Criteria (Clause 5.1)

• Loads must be factored as per IS 875.
• Load combinations for ultimate and serviceability limit states.
• Safety factors as per IS 456 and IS 1893 (for seismic).

Typical Load Combination (IS 11233 referencing IS 875):

Summary Diagram of Design Inputs:

graph TD
    A[Foundation Design] --> B[Soil Data]
    A --> C[Load Data]
    A --> D[Environmental Conditions]
    C --> E[Axial Load (P)]
    C --> F[Moments (M)]
    C --> G[Shear Forces (V)]

This ensures all necessary parameters are considered for safe and efficient foundation design per IS 11233.

IS 11233 - Types of Foundations: Key Formulas, Tables & Specifications

1. Classification of Footings (Table 1, Clause 4.1.1)

2. Types of Foundations (Clause 4.1)

• Isolated footings under each leg
• Combined raft foundation (with/without beams)
• Annular/ring foundation (for circular RCC towers)
• Pile foundations
• Rock anchors (for towers on rock)
• Combination of isolated + piles/rock anchors
• Shell foundations (for circular RCC towers)

3. Design Criteria Highlights (Clause 5.2)

• Select foundation type based on load magnitude, soil bearing capacity, lateral loads, and overturning moments.
• Ensure soil reaction matches footing type (e.g., pull-out resistance for uplift loads).
• Use allowable soil pressure and shaft resistance values from soil investigation (Clause 3.1-3.3).

4. Basic Formula for Soil Bearing Pressure

[ q_{allow} = \frac{P}{A} \leq q_{safe} ]

• (P) = Total load on footing
• (A) = Area of footing base
• (q_{safe}) = Allowable soil bearing pressure from soil report

flowchart TD
    A[Load Type] --> B{Footing Class}
    B -->|Heavy uplift| C[Class A: Enlarged Base]
    B -->|Overturning + light shear| D[Class

IS 11233: Design Loads and Forces for Tower Foundations

Key Design Loads (Clause 5.0)

• Downward Load (D): Vertical load due to self-weight and superimposed loads.
• Uplift Load (U): Vertical upward force mainly from wind or tension.
• Horizontal Thrust (H): Base shear caused by lateral forces like wind.
• Overturning Moment (M): Moment causing rotation about foundation base.

Design Forces (Clause 3.2)

• Combine vertical and lateral loads to find resultant forces.
• Calculate moments considering eccentricities of loads.

Downward Loads & Overturning Moments (Clause 5.1.2)

• Downward load includes dead load + live load.
• Overturning moment is calculated as:

[ M = H \times h ]

where
(H) = horizontal thrust (base shear)
(h) = height of force application above foundation base

Typical Load Combination (IS 11233 Recommendation)

Summary Table of Forces

graph LR
    D[Downward Load (D)]
    U[Uplift Load (U)]
    H[Horizontal Thrust (H)]
    M[Overturning Moment (M)]

    H --> M
    D --> Foundation
    U --> Foundation
    H --> Foundation
    M --> Foundation

Note: Always refer to IS 11233 Clause 5 for detailed load factors and combinations.

IS 11233: Criteria for Selection of Foundation Type and Design

Key Points from Clause 5.2 & Table 1 (Clause 4.1.1):

Design Criteria (Clause 5.2.1):

• Safe bearing capacity and settlement characteristics of soil govern foundation type selection.
• Cost economics should be decisive among alternatives.
• Consider vertical loads, lateral loads, and overturning moments.

Additional Notes:

• Use safe bearing capacity (q_s) to size footing area:
  [ A = \frac{P}{q_s} ]
  where (P) = total load on footing.
• Check settlement limits per IS 11233 and IS 456.
• For uplift or lateral loads, design for pull-out resistance and lateral soil pressure.

flowchart TD
    A[Load Characteristics] --> B{Type of Load}
    B -->|Uplift with shear| C[Class A Footing]
    B -->|Overturning + Light Shear| D[Class B Footing]
    B -->|Heavy Downward Load| E[Class C Footing]
    C --> F[Enlarged Base / Under-reamed]
    D --> G[With/Without Enlarged Base or Piles]
    E --> H[Base / Under-reamed / Pile

IS 11233: Structural Requirements - Key Points

1. General Structural Requirements

• Refer IS 1905 - 1985 for masonry and general structural design principles (Clause 5.6).

2. Design Loads (Clause 5.1)

• Consider various load types: vertical, lateral, uplift, shear, and overturning moments.
• Load combinations must follow IS code guidelines for safety and serviceability.

3. Foundation Classification (Clause 4.1.1, Table 1)

4. Design Data and Criteria

• Use allowable soil pressure and footing dimensions per soil test results.
• Check footing stability against sliding, overturning, and bearing capacity failure.
• Use IS 2809-1972 for definitions and terminology (Clause 2.1).

Summary Formulae for Footing Design

• Bearing Capacity Check:

[ q_{allow} = \frac{Q_{ultimate}}{FS} ]

Where:

• ( q_{allow} ) = allowable soil pressure

• ( Q_{ultimate} ) = ultimate bearing capacity from soil tests

• ( FS ) = factor of safety (typically 3)

• Overturning Stability:

[ \sum M_{resisting} \geq \sum M_{overturning} ]

• Sliding Stability:

[ F_s = \frac{R}{H} \geq 1.5 ]

Where:

• ( R ) = resisting forces (friction + passive earth pressure)
• ( H ) = horizontal forces

IS 11233: Foundation Construction - Key Points

1. Foundation Classification (Clause 4.1.1, Table 1)

2. Design Criteria (Clause 5.2)

• Select foundation type based on load nature (uplift, overturning, downward)
• Consider soil bearing capacity and lateral resistance
• Ensure safety against pull-out, sliding, and overturning

3. Additional Notes

• Applies to reinforced concrete foundations for self-supporting radar, microwave, TV towers
• Grillage, brick, massive footings, prestressed concrete not covered

Typical Design Formulae (General Engineering Knowledge)

• Allowable Bearing Pressure, ( q_{all} ):

[ q_{all} = \frac{P}{A} \leq q_{safe} ]

where ( P ) = load on footing, ( A ) = footing area, ( q_{safe} ) = safe soil bearing capacity.

• Overturning Stability:

[ \sum M_{resisting} \geq \sum M_{overturning} ]

• Sliding Stability:

[ F_s = \frac{R}{H} \geq 1.5 ]

where ( R ) = resisting force, ( H ) = horizontal force.

flowchart TD
    A[Load Type] --> B{Class of Footing}
    B -->|Heavy uplift + shear| C[Class A: Enlarged/Under-reamed]
    B -->|Overturning + light

IS 11233: Safety Factors and Permissible Stresses Summary

1. Permissible Stresses (Clause 5.4.3)

• Follow IS 456-1978 for concrete and reinforcement permissible stresses.
• For other materials, refer to their respective Indian Standards.
• Under earthquake forces (per IS 1893-1975), permissible stresses can be exceeded by up to 33%.
• Consider fatigue effects due to vibrations from wind and earthquakes when selecting permissible stresses.

2. Factor of Safety (Clause 5.4.1)

3. References

• IS 456-1978: Concrete code for permissible stresses.
• IS 1893-1975: Earthquake loads and stress limits.
• IS 2809-1972: Definitions relevant to foundations.

Quick Formula for Permissible Stress under Earthquake:

[ f_{perm, earthquake} = 1.33 \times f_{perm, normal} ]

flowchart TD
    A[Start: Design Load] --> B{Check Material}
    B -->|Concrete/Reinforcement| C[Use IS 456-1978 permissible stresses]
    B -->|Other Materials| D[Refer respective IS codes]
    C --> E{Earthquake Load?}
    D --> E
    E -->|Yes| F[Increase permissible stress by 33%]
    E -->|No| G[Use normal permissible stress]
    F --> H[Apply Factor of Safety]
    G --> H
    H --> I{Type of Foundation}
    I -->|General Stability| J[FoS = 2.0]
    I -->|Under-cut Uplift| K[FoS = 1.25]
    I -->|Piles/Rock Anchors| L[FoS = 3.0]
    J & K & L --> M[Design Safe Foundation]

This concise guide ensures safety compliance per IS 11233 and related IS codes

IS 11233: Special Considerations for Uplift and Lateral Loads

Key Formulas & Specifications:

• Uplift Resistance (Shallow Foundations)
  Resistance is provided by the weight of footing + weight of inverted frustum of earth with sides inclined up to 30° to vertical (Clause 5.1.1.2).

• Resultant Force Location in Enlarged Footings (Clause 5.1.1.6a):
  The resultant of vertical + lateral forces must lie within 1/6th of footing width from the toe to ensure stability.

• Moment Resistance (Clause 5.1.1.6c):
  Stabilizing moment is mainly due to the earth cone over the heel, taken as half the total weight of the earth cone, acting at the heel tip.

• Piles in Uplift (Clause 5.2.9):
  Design using friction on stem + bearing on annular projections with a Factor of Safety (FOS) = 3.

Summary Table:

Conceptual Diagram (Soil Resistance to Uplift):

graph TD
  A[Footing Base] --> B[Inverted Frustum of Earth]
  B --> C[Earth Cone (Heel Side)]
  C --> D[Stabilizing Moment]
  A --> E[Lateral + Vertical Forces]
  E --> F[Resultant ≤ 1/6 width from Toe]

Note: Use these criteria to check footing stability under combined uplift and lateral loads, considering soil weight, friction, and geometry for safe design.

IS 11233: Foundations on Rock & Rock Anchors – Key Points

1. Strength Development for Footings on Rock (Clauses 5.1.1.10 & 5.3)

• Uplift and horizontal load resistance is from:
  • Dead weight of concrete footing.
  • Anchorage strength of bars embedded in rock or concrete.
• Anchorage strength is the lesser of:
  • (a) Pull-out bond resistance of all anchor bars.
  • (b) Pull-out friction resistance of rock anchors in footing area.
• Apply factor of safety as per Clause 5.4.1.

2. Rock Anchors (Clause 5.5.4.1)

• For uplift counteraction in towers, refer IS 10270:1982 (Code for prestressed rock anchors).
• Rock anchors transfer uplift forces via friction and mechanical interlock in drilled holes.

3. Design Considerations

• Use dead load and anchorage strength combination.

• Ensure proper grouting and embedment length for anchors.

• Verify pull-out resistance using:

  [ R_{anchor} = \tau \times A_{surface} ]

  where
  (\tau) = bond/frictional shear strength,
  (A_{surface}) = surface area of anchor in rock.

Summary Table: Anchorage Strength Components

flowchart LR
    A[Uplift & Horizontal Loads] --> B[Dead Load of Concrete]
    A --> C[Anchorage Strength]
    C --> D[Pull-out Bond Resistance]
    C --> E[Pull-out Friction Resistance]
    D & E --> F[Minimum Value Used]
    F --> G[Apply Factor of Safety]

References:

• IS 11233: Foundations on Rock
• IS 10270:1982: Prestressed Rock Anchors
• Clause 5.4.1: Safety factors for uplift and horizontal loads.

Key Formulas & Specifications for Settlement and Soil Behavior (IS 11233)

1. Safe Bearing Capacity (Clause 5.5.1)

• Determined as per IS 6403:1981.

• Safe bearing capacity, ( q_{safe} ), is the maximum pressure soil can bear without shear failure.

• Formula (Terzaghi’s bearing capacity for shallow footing): [ q_{ult} = cN_c + qN_q + 0.5 \gamma BN_\gamma ] where:

  • ( c ) = cohesion,
  • ( q ) = effective overburden pressure,
  • ( \gamma ) = unit weight of soil,
  • ( B ) = footing width,
  • ( N_c, N_q, N_\gamma ) = bearing capacity factors.

• Safe bearing capacity: [ q_{safe} = \frac{q_{ult}}{FS} ] where FS = Factor of Safety (typically 3).

2. Permissible Settlements (Ref: Clause 3.3)

• Total settlement and differential settlement limits depend on structure type.
• Typical permissible total settlement: 25 mm for rigid structures.
• Differential settlement should be less than 1/500 to 1/300 of the span.

3. Footing Classification & Soil Reaction (Table 1, Clause 4.1.1)

Summary Diagram: Footing Types vs Soil Reaction

graph TD
    A[Heavy uplift] -->|Wide base towers| B[Enlarged/Under-reamed base]
    B -->|Soil Reaction

IS 11233 - Load Testing and Verification: Key Points

1. Design Loads (Clause 5.0 & 5.1)

• Loads to consider for tower foundations:
  • Downward load
  • Uplift load
  • Horizontal thrust (base shear)
  • Overturning moments

2. Load Carrying Capacity of Under-reamed Piles (Clause 5.2.10)

• Determined by load tests as per IS 2911 (Part 4) - 1985.
• In absence of tests, safe loads per IS 2911 (Part 3) - 1980 can be used.

3. Bearing Capacity & Subsoil Parameters (Clause 5.5)

• Must be verified to ensure foundation safety against:
  • Shear failure
  • Excessive settlement

Relevant Formulas & References:

Load Test Summary (IS 2911 Part 4):

• Stepwise loading: Incremental load applied (typically 1.5 times design load)
• Observation: Settlement measured at each load step
• Safe load: Load at which settlement is within permissible limits (usually 10mm or less)

flowchart TD
    A[Start Load Test] --> B[Apply Incremental Load]
    B --> C[Measure Settlement]
    C --> D{Settlement < Permissible?}
    D -- Yes --> E[Increase Load]
    D -- No --> F[Determine Safe Load]
    E --> B
    F --> G[End Test]

Summary: Use IS 2911 (Part 4) for pile load tests to verify load capacity. In absence, adopt safe loads per IS 2911 (Part 3).

IS 11233 Key References & Specifications

• Definitions: Use IS 2809-1972 for foundational terms.
• Concrete Work: Follow IS 456-1978 for plain and reinforced concrete practices.
• General Structural Requirements: Refer IS 1905-1985 for masonry structures.
• Safety Codes: Excavation and blasting must comply with respective safety codes (noted but not specified).
• Units: SI units are standard (e.g., length in meters, force in newtons).

Important Related IS Codes Summary

SI Units & Derived Units (examples)

For detailed design data, always cross-check with these referenced IS codes for compliance and safety.

graph LR
A[IS 11233] --> B[IS 2809 - Foundations Definitions]
A --> C[IS 456 - Concrete Practice]
A --> D[IS 1905 - Masonry Requirements]

IS 11233 Annexures & Illustrations: Key Points

• Concreting (Clause 5.7.2): Follow IS 456:1978 for concrete mix, placing, curing, and quality control.

• Design Data (Clause 3.1.6): Include wind (cyclones/tornadoes), frost depth, earthquake data for foundation design.

• Rock Anchors (Clause 5.5.4.1): Refer IS 10270:1982 for uplift counteracting anchors in towers.

• Units & Symbols: Use SI units as per IS 11233, e.g., length (m), force (N = kg·m/s²), pressure (Pa = N/m²).

Essential Formulae & Tables (Referenced IS Codes)

Typical Concrete Mix Proportions (IS 456:1978)

flowchart TD
    A[Foundation Design] --> B[Gather Data]
    B --> C{Data Types}
    C --> D[Wind Data (IS 875)]
    C --> E[Earthquake Data (IS 1893)]
    C --> F[Frost Depth]
    A --> G[Concrete Design (IS 456)]
    A --> H[Rock Anchors (IS 10270)]

Summary: IS 11233 refers to other IS codes for detailed annexures and illustrations. Use IS 456 for concrete, IS 6403 for bearing capacity, IS 875 for wind, IS 1893 for earthquake, and IS 10270 for rock anchors to