Is 2974 Part 3 Design and Construction of Machine Foundations Code of Practice Part 3 Foundations for Rotary Type Machines Medium and Hih Frequency

8/12/2019 Is 2974 Part 3 Design and Construction of Machine Foundations Code of Practice Part 3 Foundations for Rotary Ty

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IS 2974 ( Part 3 ) : 1992

hdian StandardDESIGN AND CONSTRUCTION OF MACHINE

FOUNDATIONS CODE OF PRACTICEPART 3 FOUNDATIONS FOR ROTARY TYPE MACHINES(MEDIUM AND HIGH FREQUENCY)

( Second Revision )First Reprint NOVEMBER 1993

UDC 6241591 : 621313-2182

0 BIS 1992BURE U OF INDI N ST ND RDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002August 1992 Price Group 4

( Reaffirmed 2006 )


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Foundation Engineering Sectional Committee, CED 43

FOREWORDThis Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized bythe Foundation Engineering Sectional Committee had been approved by the Civil EngineeringDivision Council.The installation of heavy rotary machines, namely. steam turbo-generators, turbo-compressors andblowers involves design of their foundations taking into considerations the vibration characteristicsof the foundarions system. While many of the special features relating to the design andconstruction of such machine foundations are guided by the manufacturers, slill a large part of thedetails shall have to be according to certain general principles of design covering machinefoundations. This code of practice for design and construction of machine foundations ( IS 2974 )is being published in parts. This part lays down the general principles for frame foundations forrotary machines of medium to high frequencies. The other parts of this code are:

IS 2974 Code of practice for design and construction of machine foundatians:Part 1 : 1982 Foundations for reciprocating type machinesPart 2 : 1980 Foundations for impact type machines ( hammer foundations )Part 4 : 1979 Foundations for rotary type machines of low frequencyPart 5 : 1987 Foundations for impact type of machines other than hammers ( forging andstamping press, pig breakers, drop crusher and jolter )

In the design and construction of foundations for rotary machines, a proper coordinations betweenthe different branches of engineering, including those dealing with erection and commissioning isessential.Coordinated efforts by the different branches would result in satisfactory performance, convenienceof operation, economy and a good general appearance of the complete unit. The main unit withall its auxiliaries and adjacent piping must be provided for, when making the foundation plans andal1 the details should be well worked out, before going ahead with the design.This standard first published in the year 1967 and subsequently revised in 1975. This revision hasbeen prepared, based on a numbers of comments received on this standard, keeping in view thecurrent design practices followed in India and abroad. The sizes and capacities of turbo-generatorshave increased ( up to 500 MW ) since the last revision of the code. There have been fundamentalchanges in the design philosophy of turbogenerator foundations, for example use of slender columns,long and flexible top decks, etc. With the advent of powerful computers and finite element analysiscomputer programmes the use of three-dimensional space frame models for static and dynamicanalysis has become common in design offices. The code has been made more relevant to designoffice use. Aspects such as preliminary sizing of the foundations and loading combinations areexpected to be useful to the less experienced designers.For large sized foundations with complex structural arrangement, it has been observed that two-dimensional plane frame models are nof possible to use. For such foundations three-dimensionalspace frame model is recommended for analysis.For the purpose of deciding whether a particular require,ment of this standard is complied with, thefinal value, observed or calculated, expressing the result of a test, shall be rounded o t in accordancewith IS 2 : 1960 Rules for rounding off numerical values ( revised ). The number of significantplaces retained in the rounded off value should be the same as that of the specified value in thisstandard.


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IS2974 Part3) : 1992ndian Standard

DESIGNANDCONSTRUCTIONOFMACHINEFOUNDATIONS CODEOFPRACTICE

PART 3 FOUNDATIONS FOR ROTARY TYPE MACHINES(MEDIUM AND HIGH FREQUENCY)

Second Revision )1 SCOPE1.1 This code is primarily meant fordesigningframedtype foundations for turbo-generators machinery.However, the provisions of this code may be usedsuitably for other machine foundations of similar types,for example, foundations of turbo-compressors, boilerfeed pumps, etc.1.2 The following classification shall apply tomachines based on their operating speeds:

Medium frequency 25 Hz


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IS2974 Part3) :19926 NECESSARY DATA6.1 Machine DataThe following data shall be made available to thedesigner by the machine manufacturer (see Fig. 2):

4b)c)44f-ls)h)8

Loading diagram of the machine showing thelocation, magnitude and direction of all loadsincluding dynamic loads;Speed of the machine;Critical speeds of the machine;Outline dimensions of the foundation;Mass moment of inertia of the machine com-ponents;Details of inserts and embedments;Layout of piping, ducting, etc, and their sup-porting details;Temperatures in various zones during op-eration; andAllowable displacements at the machine bear-ing points during normal operation.

6.2 Geotechnical DataInvestigation of the site where the foundation is tobe located shall be. done to evaluate the followingparameters:

a) Allowable bearing pressure/pile capacities.b) In-site dynamic soil properties as per

IS 5249 : 1992.7 LOADING ON THE FOUNDATIONThe following loads shall be considered for thefoundation design (see Annex B):

a> ead loads which include the self weight ofthe foundation and dead weight of the ma-chine;

b)

44e)fl

Operation loads supplied by the machinemanufacturer which include friction forces,power torque, thermal elongation forces,vacuum in the condenser, piping forces,etc;Unbalance forces during normal operation;Temperature forces caused by uniform tem-perature change and gradient temperature;Short circuit breaker;Loss of blade unbalance forces/bearing failureload;

g) Seismic forces; andErection loads.

8 SIZING OF THE FOUNDATION8.1 The preliminary sizing of the various elements ofthe TG foundation are to be done to arrive at afoundation configuration which will need least changesafter detailed analysis and design.It is convenient and preferable to provide the samesoffit level for all the girders from the point of view ofdesign and detailing.8.2 The geometric layout ofthe fouftdation, the shapeof the girder cross sections and columns shall bearranged, as fast as possible, symmetrically withrespect to the vertical plane passing through the longi-tudinal axis of the machine.8.3 Sizing of the Top DeckThe proportioning of the deck is basically governedby the machine manufacturers drawing giving thesole plate locations and opening details for the variousparts of the machine.While fixing the depth of the girders the followingguidelines may generally be used:Gir ders Supporting the Tur bine:Clear span-to-depth ratio = ranging from 2 to 3Depth to width ratio = ranging from 1 to 3Gir ders Supporti ng the Generator:Clear span-to-depth ratio = ranging from 2.5 to 3.5Depth to width ratio = ranging from 1 to 1.58.4 Sizing of ColumnsThe following guidelines may be followed for columnsizing:

a)

b)

c)

As far as possible pairs of columns should beprovided under each transverse girder;Compressive stresses and elastic shorteningshould be kept uniform in all the columns asfar as possible; andThe first two natural frequencies of columnwith its top and bottom ends fixed shall beaway from the operating frequency of theturbo-generator by at least 20 percent.

8.5 Sizing of Base MatThe base,mat shall be sufficiently rigid to preserve theshaft alignment. Following are some guidelines forthebase mat sizing:

a) The mass of the machine plus top deck,b) The ratio of the bending stiffness of the base

raft and largest columns in the transversedirection should be at least two, and

c) The thickness of the base raft should not beless than 007 L4j in which L is the average of


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the two adjacent clear spans. This is appli-cable to rafts supported directly on soil. Thisshall not be used for piled foundations.8.5.1 The guidelines given in 8.5 shall be used for theinitial sizing of the raft. The final raft thickness,however, would depend on the design forces.8.6 As far as possible, the foundation shall be sodimensioned that the resultant force due to the weightof the machine, the deck, intermediate slabs (if any)and the base mat together with the weight of thecolumns passes through the centre of gravity of thebase area in contact with the base mat. In cases, wheresmall eccentricities are unavoidable, an eccentricityof up to 3 percent of the base dimension along whichthe centre of gravity gets displaced may be allowed.9 STRUCTURAL ANALYSIS9.1 ModellingThe analysis shall be done using a simulated mathe-matical model of linear-elastic properties. For turbo-generator foundations of more than 100 MW capacity,a three-dimensional space frame model is recom-mended. The modelling should take into account thebasic characteristics of the system, that is, mass, stiff-ness and damping. Special attention is requiredwhile idealing the points of excitation. The modelshould simulate the vibration characteristics of themachine foundation system to a sufficient degree ofaccuracy (see Fig. 3).For smaller foundations (for example, turbo-genera-tor foundations of less than 100 MW capacity) witha regular framing arrangement, plane frame modelsmay be used in the transverse and longitudinaldirection.9.1.1 The following points shall be considered whileconstructing the model for dynamic analysis:

a)

b)

cl

The foundation shall be modelled as a three-dimensional space frame in which the col-umns and beams are idealised as 3-D beamelements with six degrees of freedom at eachnode. Slabs and walls, if present, may bemodelled using thin shell (plate bending) ele-ments. The columns shall be assumed to befixed at the base, disregarding the base mat.Nodes shall be specified to all bearing points,beam-columnjunctions, mid-points and quar-ter points ofbeams and columns and whereverthe member cross-sections change significantly.Generally, the number of modes specified onany member should be sufficient to calculateall the modes having frequencies less than orequal to the operating speed.Lumped-mass approachshall be used forcom-puting modal masses of the foundation. Themachine shall be modelled to lump its masstogether with the mass of the foundation. Thestiffness and damping of the shaft and casingshall generally be disregarded.

4

e)

f)

IS2974(Part3) :1992Untracked sections may be used for calculat-ing moments of inertia of the members. Therotational inertia may be disregarded. Shearrigidity shall be considered.Youngs modulus shall be coIS 456 : 1978. (E = 5 700 ck) for static?Ip

uted as per

analysis. For dynamic analysis the followingrange of elastic modulus may be used:Grade of Dynamic Elastic h4adulusConcrete Nbd

M20 25590 - 30000M25 28500 - 34000M 30 31200 - 37000

Damping shall be assumed to be 2 percent ofcritical damping under normal operating loads.A higher damping of 5 percent may be usedunder emergency loads like blade failure, short-circuit, bearing failure, etc.

9.2 Free Vibration AnalysisFree vibration analysis shall be carried out to calculatethe natural frequenciesand modeshapesofthe founda-tion. The highest natural frequency calculated shouldbe at least 10 percent higher than the operating fre-quencyofthe machine. Damping may be neglected forthe purpose of free vibration analysis.9.2.1 Frequency CriteriaThe following frequency criteria shall be checked:

The fundamental natural frequency shall be atleast 20perccnt away from the machine operatingspeed.that is, fi < 0.8 fm

orfi > 1.2fin

wherefn = fundamental natural frequency of thefoundation, andfm = operating speed of the machine.

However, it is preferable to maintain a frequencyseparation of 50 percent.9.3 Forced Vibration AnalysisForced vibration analysis shall be performed at theoperating speed and also at irequencies correspondingto certain selected modes for transient resonance. Thecalculated displacement shall be checked against thespecified criteria.9.3.1 Forcing FunctionGenerally, the unbalance forces are furnished by themachine manufacturer at each bearing location underdifferent operating conditions.


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IS2974 Part3) :1992A sinusoidal forcing function of the form

F(t) = F, sin (tot t $)shall be used for analysis. In the absence of data fromthe manufacturer the unbalance forces may be derivedfrom the balance quality grade of the machine (seeAnnex C).9.4 Seismic AnalysisResponse spectrum analysis shall be carried out as perIS 1893 : 1984. At least the first five modes shall beconsidered for mode superposition.9.5 Static AnalysisA detailed static analysis of the foundation shall beperformed to ensure that the foundation carries all theloads safely. The same model which has been used fordynamic analysis may be used for static analysis.9.5.1 Load Casesa>

b)cld)

Dead loads (DL)Operating loads (OL)Normal machine unbalance load (NUL)Temperature loads in the foundation (ILF)1) Uniform temperature change2) Temperature gradients across membersShort circuit forces (SCF)Loss of blade unbalance (LBL) or bearingfailure load (BFL)Seismic loads (SL)

9.5.2 Loads Combinationsa)b)c)

4

Operating conditionDLtOL+NUL+TLFShort circuit conditionDLtOL+NULtTLFtSCFLoss of blade condition/Bearing failureconditionDL + OL + TLF t LBL/BFLSeismic conditionDL+OL+NUL+TLFtEQL

10 SOIL-STRUCTURE INTERACTIONThe effects of soil-structure interaction on the dy-namic response of the TG foundation may be ignoredunder steady state dynamic loading. However, if theTG foundation is located in zones of high seismicily,soil-structure interaction shall be considered for seis-mic analysis. No soil-structure interaction need beconsidered for static analysis .and it is sufficient tomodel the foundation fixed at the base raft level.

11 BASE MAT ANALYSISThe base mat may generally be modelled with platebending elements or as a grillage of beams. The soil orpiles beneath the base raft shall be idealised as springelements.11.1 The bearing pressure on soil or the load on theheaviest loaded pile shall not exceed 80 percent ofthe net allowable bearing pressure or the safe loadcapacity of piles respectively.12 DESIGN12.1 Working stress method as per IS 456 : 1978 shallbe used.12.2 Increase in Permissible StressesWhere stresses due to either earthquake, short-circuitor loss of blade unbalance forces/bearing failure loadsare combined with those due to dead and permanentload, the permissible stresses as specified in IS 456 :1978 may be increased by 25 percent.12.3 Fatique FactorA fatique factor of 2 shall be used for the dynamicforces caused by normal unbalance.12.4 Grade of ConcreteThe following grades of concrete shall be used:

Top deck : M 20 or higher gradeColumns : M 20 or higher gradeBase mat : M 20 or higher grade

For turbo-generator foundations of capacities higherthan 100 MW, the minimum grade of concrete for theend columns shall be M 25.12.5 Reinforcement Steel

125.1 Mild steel bars conforming to IS 432 (Parts 1and 2) : 1982 or high yield strength deformed barsconforming to IS 1786 : 1985 may be used.12.5.2 Minimum dia of reinforcement bars used asmain reinforcement shall be 12 mm.125.3 Minimum ReinforcementBeams of top deckTop and bottom :

Sides

ColumnsLongitudinal :reinforcement

0.25 percent (each) of gross sec-tional area0.1 percent gross sectional area ontach side

0.8 percent of gross sectional area


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Base m tTop and bottom :Intermediate :layer

0.12 percent (each) of gross sec-tional area in each directionShrinkage reinforcement of 0.06percent ineach direction if the raftthickness is more than two metres

12.5.4 The maximum spacing of the reinforcementbars shall not exceed 300 mm and the minimumspacing shall not be less than 150 mm.125.5 Splices in the reinforcement bars shall be stag-gered and shall be given in the compression zone as faras possible.12.6 Concrete CoverMinimum clear cover to reinforcement shall be 5Ommfortopdeckandcolumnsand 100mmforthebasemat.

IS2974 Part3) :199212.7 Reinforcement etailingCare should betaken while detailing to facilitate easeof concreting. The clear spacing between bars shouldbe at least 5 mm more than the sum of aggregate sizeand the largest bar diameter used.12.8 Construction J oints128.1 The base mat shall be cast in a single uninter-rupted operation. Properly designed constructionjoints shall be provided between the base mat andcolumns and between columns and the top deck.Construction joint may also be provided approxi-mately at the mid-height of columns if the length of thecolumn exceeds 8 me&es.128.2 The top deck shall be cast in a single uninter-rupted operation.

-COLUMNS

EASE MAT

FIG. 1 TYPICAL RAMED FOUNLIA-IIONFOR A ~IIRESO-GENERATOR


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IS2974 Part3) : 1992

I

D


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IS2974(Part3) :1992

IS No432(Part 1) : 1982

432(Part 2) : 1982

456 : 1978

1786 : 1985

ANNEX AChse2.1)

LIST OF REFERRED INDIAN STANDARDS

TitleSpecification for mild steel andmedium tensile steel bats andhard-drawn steel wire forconcrete reinforcement: Part 1Mild steel and medium tensile steelbars ( third revision )Specification for mild steel andmedium tensile steel bars and hard-drawn steel wire for concretereinforcement : Part 2 Hard-drawnsteel wire ( third revision)Code of practice for plain andreinforced concrete ( secondrevision )Specification for high strengthdeformed steel bars and wires for

ISNo

1893 : 1984

2974(Part 1) : 1982

2974(Part 2) : 1980

ANNEX BClause 7)ABNORMAL LOADING

B-l LOSS OF BLADE UNBALANCEThe turbine rotor is balanced dynamically toenable smooth operation of the machine. An emer-gency condition can occur when one or more turbineblades break loose from the rotor, which would im-pose a large dynamic force on the foundation at thebearing locations. The forces corresponding to amassing last-row blade for each turbine section aresupplied by the machine manufacturer in the formof unbalance forces or equivalent static forces.Since the turbo-generator is tripped in such a conditionthese forces occur for a short time required for thecoasting down time of the machine. It is sufficient tocheck the foundation for strength under these forces.B-2 SHORT CIRCUIT FORCEWhen a line-to-line or line-to-ground short circuitoccurs at the generator terminal, it imposes a hugetorque on the TG foundation. The short-circuit mo-ment has the form (see Fig. 4).

Titleconcrete reinforcement ( thirdrevision )Criteria for earthquake resistantdesign of structures (fourthrevision )Code of practice for designand construction of machinefoundations: Part 1 Foundationsfor reciprocating type machine( second revision )Code of practice for design andconstruction of machinefoundations: Part 2 Foundationsfor impact type machine ( hammerfoundations ) first revision )

M t) = Ae -vo.4in cot - Be +m4 in 2mt + Ce MJJ~where

w = angular frequency of the mains.A, B, C = coefficients specific to generatordesign.

The forcing function is generally supplied by theMachine Manufacturer. It is advisable to perform adynamic analysis of the foundation. Sometimes, onlythe equivalent static force is supplied which assumesan infinitely rigid foundation and can make the foun-dation design highly conservative.In the absence of vendor supplied data, the followinginformation may be used.In the short circuit equation, the coefficient A, B, andC may be assumed as:

A = 10 times normal power torqueB = 5 times normal power torquec = normal power torque


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IS2974 Part3) : 1992

iizipis

- 50 Hr. OSCILLATINGh -100th OSCILLATINGrLg ----- UNIFOLAR TOROUEa+-a14a I TIME(SEC) -

FIG. 4 SHORT CIRCUITOMENTDIACJRAM

ANNEX CClause 9.3.1)

NORMAL UNBALANCE FORCE ON TURBO-GENERATOR FOUNDATION

The unbalance forces caused by the machine duringtbe normal operating condition are supplied by themachine manufacturer. However, in the absence ofsuch information, the following method may be usedto calculate the unbalance forces.Turbo-generator and other similar machines are clas-sified undertbe balance quality grade of G2.5. Consid-ering one grade higher for the foundation design, thatis, G6.3, the eccentricity of the rotor mass can beobtained from

G = eowhere

G = balance quality grade in mm/set,

e = eccentricity of rotating mass in mm, and0 = operating speed of the machine in rad/sec.Example :

G = 6 3 mm/set0 = 3 000 pm or 314.16 rad/secconsequently,

e = 0.02 mmunbalance force = meor sin otwhere

nr = mass of the rotor


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Is 2974 Part 3 Design and Construction of Machine Foundations Code of Practice Part 3 Foundations for Rotary Type Machines Medium and Hih Frequency