This guideline details the design and construction of framed foundations tailored for rotary machines functioning at medium to high frequencies, including turbo-generators and turbo-compressors. It encompasses vibration considerations, structural analyses, reinforcement specifications, and loading scenarios such as operational, abnormal, and seismic events. The document is vital for engineers engaged in heavy machinery foundation design to ensure stability, vibration mitigation, and machine operational safety.
Overview
This guideline details the design and construction of framed foundations tailored for rotary machines functioning at medium to high frequencies, including turbo-generators and turbo-compressors. It encompasses vibration considerations, structural analyses, reinforcement specifications, and loading scenarios such as operational, abnormal, and seismic events. The document is vital for engineers engaged in heavy machinery foundation design to ensure stability, vibration mitigation, and machine operational safety.
Audience
Contents
Structure
This section outlines the design and foundational requirements for turbo-generators and comparable rotary machines. It specifies the machine data necessary from manufacturers, such as load diagrams, operating and critical speeds, foundation dimensions, mass moments of inertia, embedment details, piping layouts, operational temperature zones, and allowable bearing displacements. Additionally, it explains the calculation of normal unbalance forces and emphasizes foundation isolation from adjoining structures to mitigate vibration transmission.
This part lists the Indian Standards referenced within the code, including those related to steel reinforcement (IS 432 Parts 1 & 2), concrete practice (IS 456), high strength steel bars (IS 1786), seismic design (IS 1893), and other parts of IS 2974 covering different machine types. It also reiterates the key formula for calculating normal unbalance forces.
Defines terminology used in machine foundation design and structural dynamics, including nomenclature for foundation components such as transverse and longitudinal beams and top deck. Illustrates typical foundation models and diagrams. Addresses abnormal loading scenarios like blade loss unbalance and short circuit forces, with relevant formulas.
Specifies standard names for the various parts of the machine foundation to maintain consistency in design communication. Key elements include the deck, columns, base mat, and isolation gaps. Explains vibration modes relevant to foundation behavior and illustrates the load transfer and isolation schematics.
Details the necessity of isolating the foundation structure—comprising deck, columns, and base mat—from nearby buildings and plant structures by providing continuous air gaps at all levels above the base mat. This prevents vibration transmission and dynamic interaction, ensuring structural and operational integrity.
Lists all essential data types needed for foundation design, including machine data supplied by manufacturers, geotechnical information from site investigations, and comprehensive load types. Describes the importance of air gap isolation and summarizes the key data in a tabulated form for ease of reference.
Enumerates the various loads to be accounted for in foundation design, such as dead, operational, normal unbalance, temperature-induced, short circuit, blade loss, seismic, and erection loads. Discusses load combinations for different operating scenarios and guidelines for ensuring resultant loads pass through the foundation’s center of gravity with allowable eccentricity limits.
Explains the procedure to size foundations based on geotechnical data and load assessments. Emphasizes the importance of ensuring the resultant force aligns with the base area centroid and maintaining eccentricities within allowable limits. Includes formulae for bearing pressure and summarizes design parameters.
Describes modeling approaches for structural analysis, recommending 3D space frame models for large turbo-generator foundations and plane frame models for smaller ones. Details assumptions, element types, material properties including dynamic elastic modulus, damping ratios, and load combinations to be considered.
Defines the frequency requirements to avoid resonance between the machine and foundation, specifying minimum separations between the natural frequency of the foundation and operating frequency of the machine. Details forced vibration analysis protocols and acceptable frequency gaps.
Specifies limits for maximum soil bearing pressure and pile loading, including factors of safety. Outlines geotechnical requirements, load combinations affecting bearing pressures, and foundation sizing considerations to ensure safe design.
Provides standards for materials used, including types of steel reinforcement and concrete grades. Details minimum reinforcement percentages for beams, columns, and raft foundations, spacing requirements, and minimum concrete cover thickness to ensure durability and structural performance.
Recommends construction joint locations and procedures to maintain structural integrity, including continuous casting of base mats and appropriate joint placements in columns and decks. Stresses the importance of isolation air gaps and specifies reinforcement detailing rules to facilitate proper concreting.
Provides a comprehensive list of Indian Standards referenced within the document, covering steel reinforcement, concrete practice, seismic design, and related machine foundation parts. Also includes formulas for normal unbalance forces and notes on abnormal load handling.
Covers abnormal load scenarios like blade loss unbalance and short circuit forces. Describes load combinations, allowable stress increases during such conditions, and provides formulas and coefficients for calculating moments and forces. Emphasizes manufacturer data usage and dynamic analysis recommendations.
Presents the calculation method for normal unbalance forces when manufacturer data is unavailable. Defines the relationship between balance quality grade, eccentricity, rotor mass, angular velocity, and resulting force. Includes example calculations and notes on application.
Frequently Asked
IS 2974 Part 3 specifically applies to foundations designed for rotary machines operating at medium to high frequencies, including steam turbo-generators, turbo-compressors, and blowers. The code addresses frame foundation design, vibration characteristics, and operational considerations for large machines, often up to 500 MW capacity. It is part of the broader IS 2974 series covering various machine types and frequencies.
The standard handles vibration and resonance primarily through dynamic analysis and free vibration studies. It mandates calculating natural frequencies and mode shapes of foundations, ensuring the highest natural frequency exceeds the operating frequency by at least 10% to prevent resonance. Modeling techniques include 3D space frame models for large foundations and plane frame models for smaller ones. Damping is considered at 2% for normal and 5% for emergency conditions, and dynamic elastic moduli are used for accurate stiffness representation.
Materials should include mild steel bars per IS 432 (Parts 1 & 2) or high yield strength deformed bars conforming to IS 1786. Reinforcement detailing requires clear spacing between bars to be at least the sum of 5 mm, aggregate size, and the largest bar diameter. Minimum reinforcement percentages are specified as 0.25% (top and bottom) and 0.1% (sides) for beams, 0.8% longitudinal for columns, and 0.12% for raft foundations with an additional 0.06% shrinkage reinforcement if thickness exceeds 2 meters. Concrete cover minimums are 50 mm for decks and columns and 100 mm for base mats.
Abnormal loads such as blade loss and short circuit forces must be considered using specified load combinations, including dead loads, operational loads, temperature effects, and the abnormal load itself. Permissible stresses may be increased by 25% during these events. Blade loss forces are typically provided by manufacturers and checked for foundation strength during rapid machine deceleration. Short circuit forces are modeled dynamically using exponential decay sine functions with machine-specific coefficients; if unavailable, conservative multiple factors of normal power torque are used.
For turbo-generator foundations above 100 MW, IS 2974 Part 3 recommends using 3D space frame models with beam elements possessing six degrees of freedom per node, and shell elements for slabs and walls. Columns are fixed at the base, with mass lumped to include both machine and foundation. Material properties incorporate dynamic elastic modulus values, and damping is set at 2% for normal operations and 5% for emergencies. For smaller foundations under 100 MW with regular framing, plane frame models in transverse and longitudinal directions are acceptable. Soil-structure interaction is generally ignored except in seismic analyses.
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