Code of practice for design and construction of machine foun dations, Part 3: Foundations for rotary type machines (Medium and high frequency)

Scope & Key Specifications from IS 2974 Part 3 (Clause 6.1 & 314.16)

This part covers design data requirements and foundation specifications for turbo-generators and similar machines.

Machine Data Required (Clause 6.1)

The designer must obtain from the manufacturer:

• Loading diagram: Location, magnitude, direction of all static & dynamic loads.
• Speed: Operating and critical speeds.
• Foundation outline dimensions.
• Mass moment of inertia of machine components.
• Details of inserts & embedments.
• Piping/ducting layout and supports.
• Operating temperatures at different zones.
• Allowable displacements at bearing points.

Normal Unbalance Force on Turbo-Generator Foundation (Clause 314.16)

• Operating speed, ( \omega = 3000 , \text{rpm} = 314.16 , \text{rad/s} )
• Eccentricity, ( e = 0.02 , \text{mm} )
• Unbalance force formula:

[ F = m e \omega^2 \sin(\omega t) ]

where:

• ( m ) = mass of rotor,
• ( e ) = eccentricity,
• ( \omega ) = angular velocity.

Additional Notes

• Foundation must be isolated from adjoining structures with an air gap to prevent vibration transmission.
• Refer to Annex A for related Indian Standards.

flowchart LR
A[Machine Manufacturer] --> B[Provide Machine Data]
B --> C[Designer]
C --> D[Foundation Design]
D --> E[Isolation from Structures]

This ensures vibration control and foundation integrity per IS 2974 Part 3.

Referenced Indian Standards in IS 2974 (Part 3):1992 (Annex A, Clause 2.1)

Key Formula for Normal Unbalance Force (Clause 314.16 & Annex C)

[ F = m e \omega^2 \sin(\omega t) ]

• (m) = rotor mass (kg)
• (e) = eccentricity of rotor mass (m), calculated as:

[ e = \frac{G}{\omega} ]

• (G) = balance quality grade in mm/s (for design use G6.3)
• (\omega = 2 \pi \times \text{rpm}/60) (rad/s)

Notes on Unbalance Force:

• Turbo-generators are typically balanced to grade G2.5, but G6.3 is used for foundation design.
• Example: For 3000 rpm, (\omega = 314.16 , \text{rad/s}), and (e = 0.02 , \text{mm}).

Summary

• IS 2974 (Part 3) references key IS codes for materials, seismic design, and machine foundation practice.
• Unbalance forces are critical for foundation design; use the provided formula when manufacturer data is unavailable.
• For abnormal loads like

IS 2974 Part 3: Terminology - Key Points

• Terminology Reference:
  Clause 3.0 defines common terms in structural dynamics and machine foundation design. For a detailed list, see IS 2974 Parts 1 & 2.

• Nomenclature:
  Clause 4.0 specifies foundation component names (see Fig. 1):

  • Transverse beams
  • Longitudinal beams
  • Top deck (cast in one operation per Clause 12.8.2)

• Typical Models and Diagrams:

  • Fig. 1: Framed foundation for turbo-generator
  • Fig. 2: Loading diagram
  • Fig. 3: Space frame model of foundation

• Abnormal Loading (Annex B):

  • Loss of blade unbalance: Large dynamic forces at bearings during blade failure.
  • Short circuit force: Moment formula:
    [ M(t) = Ae^{-0.4t} \sin \omega t - Be^{-0.4t} \sin 2\omega t + Ce^{-0.15t} ]
    Where:
    • (\omega) = mains angular frequency
    • (A = 10 \times) normal power torque
    • (B = 5 \times) normal power torque
    • (C =) normal power torque

• Normal Unbalance Force (Annex C):
  For balance quality grade (G) (mm/s), eccentricity (e) (mm) is:
  [ e = \frac{G}{\omega} ]
  where (\omega) = machine speed in rad/s. Use grade G6.3 for design (one grade higher than machine’s G2.5).

• Referenced IS Codes (Annex A):

  • IS 2974 Parts 1 & 2 (machine foundations)
  • IS 456 (plain & reinforced concrete)
  • IS 1893 (earthquake design)
  • IS 432 Parts 1 & 2 (steel reinforcement)

Summary Diagram of Foundation Components (Fig. 1)

graph LR
  TD[Top Deck]
  TB[Transverse Beam]
  LB[Longitud

IS 2974 Part 3 – Nomenclature of Foundation Components (Clause 4.0)

This clause defines standard names for foundation parts, ensuring uniformity in design and communication.

Key Components (per Fig. 1 in the code):

• Deck: Top slab or plate supporting the machine.
• Columns: Vertical members transferring loads from deck to mat.
• Mat/Foundation Base: The large base slab distributing loads to soil.
• Isolation Gap: Air gap between foundation and adjoining structures to prevent vibration transfer.

Important Specifications:

• The entire structure includes deck, columns, and mat (Clause 4.6).
• Isolation from adjoining structures must be ensured by an air gap at all levels above the base mat (Clause 5).

Typical Nomenclature Table (simplified):

Vibration Mode (Clause 3.6):

• A mode of vibration is a characteristic harmonic motion pattern with a specific frequency.
• Multiple modes can exist simultaneously in multi-degree systems.

graph TD
    Deck --> Column
    Column --> Mat
    Mat --> Soil
    Foundation -. Isolation Gap .-> Adjacent Structure

This schematic shows load transfer and isolation gap for vibration control.

For detailed terms, refer also to IS 2974 Parts 1 & 2.

Isolation from Adjoining Structures (IS 2974 Part 3: Clause 5)

• Requirement: The foundation (deck, columns, mat) must be isolated from the main building and other plant structures.
• Method: Provide an air gap at all levels above the base mat to prevent vibration transfer.
• Purpose: Avoid dynamic interaction and resonance between the machine foundation and adjoining structures.

Key Specifications:

Additional Notes:

• Foundation should be designed so that the resultant load passes through the center of gravity of the base mat contact area (Clause 8.6).
• Small eccentricities allowed: up to 3% of base dimension.
• Dynamic forces like short-circuit torque or blade loss unbalance should be considered separately (Annex B).

Summary Diagram of Isolation Concept

graph LR
  A[Main Building] ---|Air Gap| B[Foundation Structure]
  B --> C[Deck]
  C --> D[Columns]
  D --> E[Base Mat]
  style A fill:#f9f,stroke:#333,stroke-width:2px
  style B fill:#bbf,stroke:#333,stroke-width:2px
  style C fill:#bbf,stroke:#333,stroke-width:2px
  style D fill:#bbf,stroke:#333,stroke-width:2px
  style E fill:#bbf,stroke:#333,stroke-width:2px

References:

• IS 2974 Part 3, Clause 5
• Clause 12.8.2 (Construction joints)
• Clause 8.6 (Load eccentricity)

IS 2974 Part 3: Necessary Data for Machine Foundation Design

1. Machine Data (Clause 6.1)

The machine manufacturer must provide:

• Loading diagram: Location, magnitude & direction of all loads (including dynamic).
• Speed & critical speeds of the machine.
• Outline dimensions of the foundation.
• Mass moment of inertia of machine components.
• Details of inserts and embedments.
• Layout of piping/ducting and support details.
• Operating temperatures in various zones.
• Allowable displacements at bearing points.

2. Geotechnical Data (Clause 6.2)

Site investigation must provide:

• Allowable bearing pressure or pile capacities.
• Dynamic soil properties as per IS 5249:1992.

3. Loading on Foundation (Clause 7)

Consider the following loads:

4. Isolation (Clause 5)

• Provide air gap isolation between foundation and adjoining structures to avoid vibration transmission.

Summary Table: Key Data for Designer

flowchart TD
    A[Machine Manufacturer Data] --> B[Foundation Designer]
    C[Geotechnical Investigation] --> B
    B --> D[Foundation Design]
    E[Load Types (Dead, Dynamic, Thermal, Seismic)] --> B
    F[

Loading on Foundation (IS 2974 Part 3)

Key Loads to Consider (Clause 7):

• Dead Loads (DL): Self-weight of foundation + machine weight
• Operation Loads (OL): Friction, torque, thermal elongation, vacuum, piping forces
• Unbalance Loads (NUL): During normal operation
• Temperature Loads (TLF): Uniform & gradient temperature effects
• Short Circuit Forces (SCF)
• Loss of Blade / Bearing Failure Loads (LBL/BFL)
• Seismic Loads (EQL)
• Erection Loads

Load Combinations (Clause 9.5.2):

Foundation Sizing (Clause 8.6):

• Resultant force from all weights should pass through the center of gravity of the base area.
• Allowable eccentricity up to 3% of the base dimension along the displacement axis.

Soil-Structure Interaction (Clause 10):

• Ignored for steady dynamic/static loads.
• Considered for seismic zones.

Modeling (Clause 11):

• Base mat modeled as plate bending elements or grillage beams.
• Soil/piles idealized as spring elements.

flowchart TD
    A[Loads on Foundation] --> B[Dead Loads (DL)]
    A --> C[Operation Loads (OL)]
    A --> D[Unbalance Loads (NUL)]
    A --> E[Temperature Loads (TLF)]
    A --> F[Short Circuit Forces (SCF)]
    A --> G[Loss of Blade/Bearing Loads (LBL/BFL)]
    A --> H[Seismic Loads (EQL)]
    A --> I[Erection Loads]

    subgraph Load Combinations
        J[Operating] --> K[DL + OL + NUL + TLF]
        L[Short Circuit] --> M[DL + OL + NUL + TLF + SCF]

IS 2974 Part 3: Sizing of Foundation – Key Points

1. Geotechnical Data (Clause 6.2)

• Determine allowable bearing pressure or pile capacities from site investigation.
• Obtain dynamic soil properties as per IS 5249:1992.

2. Loads to Consider (Clause 7)

• Dead loads (foundation + machine weight)
• Operational loads (friction, torque, thermal forces, vacuum, piping)
• Unbalance forces during operation
• Temperature effects (uniform & gradient)
• Short circuit and failure loads
• Seismic forces
• Erection loads

3. Foundation Sizing (Clause 8.6)

• Dimension foundation so resultant force passes through centre of gravity of base area.
• Allow eccentricity up to 3% of base dimension if unavoidable.
• Include weights of machine, deck, slabs, base mat, columns.

4. Isolation (Clause 5)

• Provide air gap between foundation and adjoining structures to avoid vibration transfer.

Typical Formula for Bearing Pressure:

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

Where:

• (P) = Total vertical load (including machine + foundation)
• (A) = Base area of foundation
• (q_{allow}) = Allowable soil bearing pressure

Summary Table: Foundation Design Parameters

flowchart TD
    A[Site Investigation] --> B[Determine Soil Parameters]
    B --> C[Calculate Allowable Bearing Pressure]
    D[Load Assessment] --> E[Sum of Loads]
    C & E --> F[Foundation Sizing]
    F --> G[Check Eccentricity ≤ 3%]
    F --> H[Design Base Area]
    H --> I[Provide Isolation Gap]

**

IS 2974 Part 3: Structural Analysis Key Points

1. Modeling (Clause 9.1.1)

• Model foundation as a 3D space frame: columns/beams as 3D beam elements (6 DOF/node).
• Slabs/walls: thin shell (plate bending) elements.
• Columns fixed at base (ignore base mat flexibility).
• Nodes at bearing points, beam-column junctions, mid/quarter points.
• Use lumped-mass approach (machine + foundation mass lumped).
• Use uncracked sections for moment of inertia; consider shear rigidity.
• Young's Modulus (E):
  • Static: (E = 5700 \sqrt{f_{ck}}) (IS 456:1978)
  • Dynamic (N/mm²):

• Damping: 2% critical (normal), 5% (emergency loads).

2. Load Combinations (Clause 9.5.2)

• DL: Dead Load, OL: Operating Load, NUL: No-Load, TLF: Thermal Load Factor, SCF: Short Circuit Factor, LBL/BFL: Blade/Bearing Failure Load, EQL: Earthquake Load.

3. Base Mat Analysis (Clause 11)

• Model base mat as plate bending elements or grillage of beams.
• Soil/piles idealized as spring elements.
• Foundation assumed fixed at base raft level for static analysis.

4. Foundation Dimensioning (Clause 8.6)

• Resultant force from machine

IS 2974 Part 3: Frequency Criteria for Machine Foundations

Key Frequency Criteria (Clause 9.2.1 & 9.2)

• Fundamental natural frequency (fn) must be at least 20% away from the machine operating speed (fm):

  [ f_n < 0.8 f_m \quad \text{or} \quad f_n > 1.2 f_m ]

• Preferably, maintain a 50% frequency separation for safety:

  [ |f_n - f_m| \geq 0.5 f_m ]

• The highest natural frequency should be at least 10% higher than the operating frequency:

  [ f_{max} \geq 1.1 f_m ]

Forced Vibration Analysis (Clause 9.3)

• Perform forced vibration analysis at:

  • Operating speed (f_m)
  • Frequencies of selected modes (for transient resonance)

• Check calculated displacements against allowable limits.

Definitions (Clause 1.2)

• (f_n): Fundamental natural frequency of foundation (Hz)
• (f_m): Operating speed of the machine (Hz)

Summary Table

flowchart LR
    A[Machine Operating Frequency \(f_m\)] --> B[Check \(f_n\)]
    B -->|If \(f_n < 0.8 f_m\) or \(f_n > 1.2 f_m\)| C[Frequency Criteria Met]
    B -->|Else| D[Adjust Foundation Design]
    C --> E[Perform Forced Vibration Analysis]
    E --> F[Check Displacements]
    F -->|Within Limits| G[Design Approved]
    F -->|Exceeds Limits| D

This ensures the foundation avoids resonance and excessive vibrations.

IS 2974 Part 3: Key Points on Bearing Pressure & Pile Load

Bearing Pressure (Clause 11.1)

• Maximum bearing pressure on soil ≤ 80% of net allowable bearing pressure.
• For piles, load on the heaviest loaded pile ≤ 80% of safe pile load capacity.

Geotechnical Data (Clause 6.2)

• Site investigation must determine:
  • Allowable bearing pressure.
  • Pile capacities.
  • Dynamic soil properties (per IS 5249:1992).

Load Combinations (Clause 9.5.2)

• Operating: DL + OL + NUL + TLF
• Short circuit: DL + OL + NUL + TLF + SCF
• Bearing failure: DL + OL + TLF + LBL/BFL
• Seismic: DL + OL + NUL + TLF + EQL

Foundation Sizing & Design

• Resultant load should ideally pass through base mat’s centroid (Clause 8.6).
• Allow eccentricity ≤ 3% of base dimension.

Typical Formula for Safe Bearing Pressure:

[ q_{safe} = \frac{Q_{ultimate}}{F.S.} ]

Where:

• ( Q_{ultimate} ) = ultimate bearing capacity from soil tests.
• ( F.S. ) = factor of safety (usually 2.5 to 3).

Pile Load Capacity

[ Q_{safe} = Q_{allowable} \times 0.8 ]

Where:

• ( Q_{allowable} ) = pile load capacity from geotechnical investigation.

Soil-Structure Interaction (Clause 10)

• Ignored for static and steady dynamic loads.
• Considered for seismic zones.

flowchart TD
    A[Site Investigation]
    B[Determine Allowable Bearing Pressure & Pile Capacity]
    C[Calculate Load Combinations]
    D[Check Bearing Pressure ≤ 0.8 × Allowable]
    E[Check Pile Load ≤ 0.8 × Safe Capacity]
    F[Design Foundation Dimensions]
    G[Model Soil-Structure Interaction (Seismic only)]

    A --> B --> C --> D
    D --> E --> F --> G

This ensures safe design respecting IS 2974 Part 3 requirements.

IS 2974 Part 3: Key Formulas & Specifications for Materials and Reinforcement Detailing

1. Reinforcement Detailing (Clause 12.7)

• Clear spacing between bars ≥ (Largest aggregate size + Largest bar diameter) + 5 mm
  Ensures ease of concreting and proper compaction.

2. Minimum Reinforcement (Clause 12.5.3, Table 12.5.3)

3. Concrete Cover (Clause 12.6)

• Top deck and columns: Minimum 50 mm clear cover
• Base mat: Minimum 100 mm clear cover

Summary Table:

flowchart TD
    A[Reinforcement Detailing] --> B[Clear Spacing ≥ Aggregate + Bar Dia + 5mm]
    A --> C[Minimum Reinforcement %]
    C --> D[Beams: 0.25% top/bottom, 0.1% sides]
    C --> E[Columns: 0.8% longitudinal]
    C --> F[Raft: 0.12% top/bottom, 0.06% shrinkage

IS 2974 Part 3: Construction Joints and Practices (Clause 12.8)

• Base Mat Casting:

  • Must be cast in a single uninterrupted operation to ensure monolithic behavior.

• Construction Joints Locations:

  • Between base mat and columns.
  • Between columns and top deck.
  • At mid-height of columns if column length > 8 m.

• Isolation from Adjoining Structures:

  • Provide an air gap at all levels above the base mat to prevent vibration transfer.

• Reinforcement Detailing (Clause 12.7):

  • Clear spacing between bars ≥ (aggregate size + largest bar diameter + 5 mm).

Summary Table: Construction Joint Guidelines

flowchart TD
    A[Base Mat] -->|Construction Joint| B[Columns]
    B -->|Construction Joint| C[Top Deck]
    B -->|Mid-height Joint (if >8m)| D[Column Mid-section]
    A -. Air Gap .-> E[Adjoining Structures]

This ensures structural integrity, vibration isolation, and ease of concreting per IS 2974 Part 3.

List of Referred Indian Standards (IS 2974 Part 3 - Annex A)

Key Formula for Normal Unbalance Force (Annex C)

[ \text{Unbalance force} = m e \omega^2 \sin(\omega t) ]

Where:

• (m) = rotor mass (kg)
• (e) = eccentricity of rotating mass (mm)
• (\omega) = angular speed in rad/s
• (t) = time (s)

Eccentricity (e) from balance quality grade (G):

[ e = \frac{G}{\omega} \quad \text{(in mm)} ]

• (G) = balance quality grade (mm/s), e.g., for foundation design use (G=6.3) (one grade higher than machine grade G2.5)
• (\omega = 2 \pi N / 60), where (N) = rpm

Notes:

• Refer to IS 2974 Parts 1 & 2 for detailed machine foundation design.
• For abnormal loads like blade loss or short circuit, use vendor data or Annex B formulas.
• IS 1893 is essential for seismic design considerations.

flowchart LR
    A[IS

IS 2974 (Part 3): Abnormal Loading Key Points

1. Abnormal Loads Considered (Clause 9.5.1 & Annex B):

• Loss of Blade Unbalance (LBL):

  • Occurs if turbine blades break, causing large dynamic forces at bearings.
  • Forces supplied by manufacturer or checked for strength during coasting down.

• Short Circuit Force (SCF):

  • Huge torque at generator terminals during short circuit.
  • Moment formula:
    [ M(t) = A e^{-V0.4} \sin \omega t - B e^{-V0.4} \sin 2\omega t + C e^{-0.15} ]
  • Where:
    • (\omega) = mains angular frequency
    • (A, B, C) = machine-specific coefficients (if unknown, use:
      (A = 10 \times) normal power torque,
      (B = 5 \times) normal power torque,
      (C =) normal power torque)

• Normal Unbalance Force (Annex C):

  • For balance quality grade (G) (mm/sec), eccentricity (e) (mm), and speed (\omega) (rad/sec):
    [ e = \frac{G}{\omega} ]
  • Use grade (G = 6.3) mm/sec for foundation design (one grade higher than machine G2.5).

2. Load Combinations (Clause 9.5.2):

3. Definitions:

• DL = Dead Load
• OL = Operating Load
• NUL = Normal Unbalance Load
• TLF = Temperature Load on Foundation
• SCF = Short Circuit Force
• LBL/BFL = Loss of Blade Load / Bearing Failure Load

Normal Unbalance Force on Turbo-Generator Foundation (IS 2974 Part 3:1992)

Key Formula (Annex C, Clause 9.3.1)

When manufacturer data is unavailable, unbalance force can be estimated as:

[ F(t) = m e \omega^2 \sin(\omega t) ]

Where:

• ( m ) = mass of rotor (kg)
• ( e ) = eccentricity of rotor mass (mm)
• ( \omega ) = angular speed (rad/s)
• ( F(t) ) = unbalance force at time ( t ) (N)

Eccentricity Calculation from Balance Quality Grade ( G ):

[ G = e \omega ]

• ( G ) = balance quality grade in mm/s (For turbo-generators, use ( G = 6.3 ) mm/s for design, one grade higher than G2.5)
• ( e = \frac{G}{\omega} )

Example for 3000 rpm (Clause 314.16):

• ( \omega = 314.16 , \text{rad/s} )
• ( e = \frac{6.3}{314.16} = 0.02 , \text{mm} )

Summary Table:

Notes:

• Use sinusoidal forcing function for dynamic analysis.
• Always verify with machine manufacturer data if available.
• This force acts at bearing locations and is critical for foundation design.

graph LR
A[Balance Quality Grade G (mm/s)] --> B[Eccentricity e = G/ω (mm)]