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Page 1: 6403 bc 16-oct09

© BIS 2006

B U R E A U O F I N D I A N S T A N D A R D SMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

IS 6403 : 1981(Reaffirmed 2002)

Edition 2.1(1984-05)

Price Group 4

Indian StandardCODE OF PRACTICE FOR

DETERMINATION OF BEARING CAPACITYOF SHALLOW FOUNDATIONS

( First Revision )(Incorporating Amendment No. 1)

UDC 624.151.5 : 624.131.52 : 006.76

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IS 6403 : 1981

© BIS 2006

BUREAU OF INDIAN STANDARDS

This publication is protected under the Indian Copyright Act (XIV of 1957) andreproduction in whole or in part by any means except with written permission of thepublisher shall be deemed to be an infringement of copyright under the said Act.

Indian StandardCODE OF PRACTICE FOR

DETERMINATION OF BEARING CAPACITYOF SHALLOW FOUNDATIONS

( First Revision )

Foundation Engineering Sectional Committee, BDC 43

Chairman Representing

PROF DINESH MOHAN Central Building Research Institute (CSIR), Roorkee

Members

DR R. K. BHANDARI Central Building Research Institute (CSIR), RoorkeeCHIEF ENGINEER Calcutta Port Trust, Calcutta

SHRI S. GUHA ( Alternate )SHRI M. G. DANDAVATE The Concrete Association of India, Bombay

SHRI N. C. DUGGAL ( Alternate )DR R. K. DAS GUPTA Simplex Concrete Piles (India) Pvt, Ltd, Calcutta

ADDITIONAL CHIEF ENGINEER ( Alternate )SHRI A. G. DASTIDAR In personal capacity ( 5 Hungerford Road, 121

Hungerford Street, Calcutta )SHRI V. C. DESHPANDE The Pressure Piling Co (I) Pvt Ltd, BombayDIRECTOR (CSMRS) Central Water Commission, New Delhi

DEPUTY DIRECTOR (CSMRS) ( Alternate )SHRI A. H. DIVANJI Asia Foundations and Construction Co Pvt Ltd,

BombaySHRI A. N. JANGLE ( Alternate )

DR JAGDISH NARAIN Indian Geotechnic Society, New DelhiPROF SWAMI SARAN ( Alternate )

SHRI G. S. JAIN G. S. Jain & Associates, RoorkeeSHRI ASHOK KUMAR JAIN ( Alternate )

JOINT DIRECTOR (D) National Buildings Organisation, New DelhiSHRI SUNIL BERY ( Alternate )

JOINT DIRECTOR RESEARCH (SM) Ministry of Railways (RDSO)JOINT DIRECTOR RESEARCH (B & S) ( Alternate )

( Continued on page 2 )

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IS 6403 : 1981

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( Continued from page 1 )

Members RepresentingDR R. K. KATTI Indian Institute of Technology, BombaySHRI S. R. KULKARNI M. N. Dastur & Co Pvt Ltd, Calcutta

SHRI S. ROY ( Alternate )SHRI O. P. MALHOTRA Public Works Department, Chandigarh Administra-

tion, ChandigarhSHRI A. P. MATHUR Central Warehousing Corporation, New DelhiSHRI V. B. MATHUR Machenzies Limited, BombaySHRI T. K. D. MUNSI Engineers India Limited, New Delhi

SHRI M. IYENGAR ( Alternate )BRIG OMBIR SINGH Engineer-in-Chief’s Branch, Army Headquarters,

New DelhiMAJ H. K. BHUTANI ( Alternate )

SHRI B. K. PANTHAKY The Hindustan Construction Company Limited,Bombay

SHRI V. M. MADGE ( Alternate )SHRI M. R. PUNJA Cemindia Co Ltd, Bombay

SHRI S. MUKHERJEE ( Alternate )SHRI N. E. V. RAGHVAN The Braithwaite Burn & Jessop Construction Co Ltd,

CalcuttaSHRI A. A. RAJU Vijayanagar Steel Plant (Steel Authority of India),

New DelhiPROF GOPAL RANJAN University of Roorkee, RoorkeeSHRI T. N. SUBBA RAO Gammon India Limited, Bombay

SHRI S. A. REDDI ( Alternate )SHRI Y. V. NARASIMHA RAO Bokaro Steel Plant (Steel Authority of India), BokaroDR V. V. S. RAO Nagadi Consultants Pvt Ltd, New DelhiSHRI ARJUN RIJHSINGHANI Cement Corporation of India, New Delhi

SHRI O. S. SRIVASTAVA ( Alternate )DR A. SARGUNAN College of Engineering, Guindy

SHRI S. BOOMINATHAN ( Alternate )SHRI K. R. SAXENA Public Works Department, Government of Andhra

Pradesh, HyderabadDR S. P. SHRIVASTAVA United Technical Consultants Pvt Ltd, New Delhi

DR R. KAPUR ( Alternate )SHRI N. SIVAGURU Ministry of Shipping and Transport, New Delhi

SHRI D. V. SIKKA ( Alternate )SUPERINTENDING ENGINEER

(DESIGNS)Central Public Works Department, New Delhi

EXECUTIVE ENGINEER(DESIGNS) V ( Alternate )

SHRI M. D. TAMBEKAR Bombay Port Trust, BombayDR A. VARADARAJAN Indian Institute of Technology, New Delhi

DR R. KANIRAJ ( Alternate )SHRI G. RAMAN,

Director (Civ Engg)Director General, ISI ( Ex-officio Member )

SecretarySHRI K. M. MATHUR

Deputy Director (Civ Engg), ISI

( Continued on page 16 )

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IS 6403 : 1981

3

Indian StandardCODE OF PRACTICE FOR

DETERMINATION OF BEARING CAPACITYOF SHALLOW FOUNDATIONS

( First Revision )0. F O R E W O R D

0.1 This Indian Standard (First Revision) was adopted by the IndianStandards Institution on 30 January 1981, after the draft finalized bythe Foundation Engineering Sectional Committee had been approvedby the Civil Engineering Division Council.

0.2 The bearing capacity of foundations is needed for dimensioning thefoundation for any structure. Several methods are available for thedetermination of bearing capacity of shallow foundations and thisstandard gives some of the methods which are commonly used for thepurpose. Comparison of the results shows that when each of thevarious methods is applied to different problems no one methodconsistently gives higher or lower values of allowable bearingpressure. The designer must therefore regard the methods as aids todesign which cannot replace the critical role of engineering judgement.

0.2.1 This standard was first published in 1971. The principal modi-fications made in this revision are: (a) keeping its terminology in linewith that of other related Indian Standards, (b) giving generalizedequations for calculation of ultimate bearing capacity, (c) including conepenetration methods and (d) deleting adjustment for fine sand and silt.

0.3 This edition 2.1 incorporates Amendment No. 1 (May 1984). Sidebar indicates modification of the text as the result of incorporation ofthe amendment.

0.4 For the purpose of deciding whether a particular requirement ofthis standard is complied with, the final value, observed or calculated,expressing the result of a test or analysis, shall be rounded off inaccordance with IS : 2-1960*. The number of significant placesretained in the rounded off value should be the same as that of thespecified value in this standard.

*Rules for rounding off numerical values ( revised ).

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IS 6403 : 1981

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1. SCOPE

1.1 This standard covers the procedure for determining the ultimatebearing capacity and allowable bearing pressure of shallowfoundations based on shear and allowable settlement criteria.

2. TERMINOLOGY

2.0 For the purpose of this standard, the following definitions shallapply.

2.1 Terms Relating to Bearing Capacity

2.1.1 Net Loading Intensity — The net loading intensity on thefoundation is the gross intensity of loading minus the weight ofdisplaced soil above the foundation base.

2.1.2 Ultimate Bearing Capacity — The intensity of loading at the baseof the foundation which would cause shear failure of the soil support.

2.1.3 Safe Bearing Capacity — Maximum intensity of loading that thefoundation will safely carry without the risk of shear failure of soilirrespective of any settlement that may occur.

2.1.4 Safe Bearing Pressure or Net Soil Pressure for SpecifiedSettlement — The intensity of loading that will cause a permissiblesettlement or specified settlement of the structure.

2.1.5 Allowable Bearing Capacity — The net intensity of loading whichthe foundation will carry without undergoing settlement in excess ofthe permissible value for the structure under consideration but notexceeding net safe bearing capacity.

2.2 General Terms

2.2.1 Density Index ( Relative Density ) — The ratio of the differencebetween the void ratios of cohesionless soil in the loosest state and anygiven state to the difference between its void ratios at the loosest anddensest states.

2.2.2 Effective Surcharge at the Base Level of Foundation — Theintensity of vertical pressure at the base level of foundation, computedassuming total unit weight for the portion of soil above the water tableand submerged unit weight for the portion below the water table.

2.2.3 Footing — A structure constructed in brickwork, masonry orconcrete under the base of a wall or column for the purpose ofdistributing the load over a larger area.

2.2.4 Foundation — That part of a structure which is in direct contactwith soil and transmits loads into it.

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IS 6403 : 1981

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2.2.5 Shallow Foundation — A foundation whose width is greater thanits depth. The shearing resistance of the soil in the sides of the found-ation is generally neglected.

3. SYMBOLS

3.1 For the purpose of this code and unless otherwise defined in thetext, the following letter symbols shall have the meaning indicatedagainst each:

A = Area of footing in cm2

A' = Effective area of footing in cm2

B = Width of strip footing, width of footing,side of square footing, diameter of circularfooting in cm

B' = Effective width of footing in cmb = Half of Bc = Cohesion in kgf/cm2

c1 = Undrained cohesion of the top layer inkgf/cm2

c2 = Undrained cohesion of the lower clay layerin kgf/cm2

Df = Depth of foundation in cmDw = Depth to water table in cm

d = Depth of top clay layer with undrainedcohesion c1

dc, dq, dγ = Depth factorse = Eccentricity of loading in cm

eB = Eccentricity of loading along the width incm

eL = Eccentricity of loading along the length incm

H = Horizontal component of loading in kgfic, iq, iγ = Inclination factors

Kd = Depth factor (varies linearly from 1 fordepth Df = 0 to 1.33 for depth Df = B )

L = Length of footing in cmL' = Effective length of footing in cmN = Corrected standard penetration value

Nc, N'c, Nq, N'q, Nγ, N'γ = Bearing capacity factors

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IS 6403 : 1981

6

4. GENERAL

4.1 Sufficient number of undisturbed samples, about 40 to 100 mm indiameter or more or block samples should be obtained, where possible.These samples are for the determination of field density of soil andconducting tests for determining the relevant shear and consolidationparameters of the soil. Tests on soils should be conducted inaccordance with relevant parts of IS : 2720*.

4.2 Position and fluctuation of water table should be ascertained.Reference may be made to IS : 1892-1979† and IS : 2132-1972‡ forguidance regarding investigations and collection of data.

5. ULTIMATE NET BEARING CAPACITY

5.0 General — Three types of failure of soil support beneath thefoundations have been recognized, depending upon the deformationsassociated with the load and the extent of development of failure surface.

N = tan2 ( /4 + /2)q = Effective surcharge at the base level of

foundation in kgf/cm2

qa = Net soil pressure for a specifiedsettlement of 25 mm in kgf/cm2

qc = Static cone penetration resistance in kgf/cm2

qd = Net ultimate bearing capacity based ongeneral shear failure in kgf/cm2

q'd = Net ultimate bearing capacity based onlocal shear failure in kgf/cm2

R = Relative density of soilW' = Correction factor for location of water

tablesc, sq, sγ = Shape factors

= Inclination of the load to the vertical indegrees

= Angle of shearing resistance of soil indegrees

γ = Bulk unit weight of foundation soil kgf/cm3

*Methods of test for soils.†Code of practice for subsurface investigations for foundations ( first revision ).‡Code of practice for thin-walled tube sampling of soils ( first revision ).

π

α

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IS 6403 : 1981

7

They are: a) general shear failure, b) local shear failure andc) punching shear. The choice of which method of analysis is best suitedin a given situation is difficult to make, because only limited test dataare available on full sized foundations to verify the reliability of thecomputed bearing capacity. However, guidelines given in relevantclauses may be used for guidance. Wherever possible bearing capacitycalculations shall be made on the basis of shear strength parameters and c obtained from appropriate shear tests [ see IS : 2720 (Parts XIand XIII)* ] or from plate load test results as given in IS : 1888-1981†or from static cone penetration resistance qc obtained from static conepenetration test as given in IS : 4968 (Part III)-1976‡.5.0.1 Effect of Eccentricity

a) Single Eccentricity — If the load has an eccentricity e, withrespect to the centroid of the foundation in only one direction,then the dimension of the footing in the direction of eccentricityshall be reduced by a length equal to 2 e. The modified dimensionshall be used in the bearing capacity equation and in determiningthe effective area of the footing in resisting the load.

b) Double Eccentricity — If the load has double eccentricity ( eL andeB ) with respect to the centroid of the footing then the effectivedimensions of the footing to be used in determining the bearingcapacity as well as in computing the effective area of the footingin resisting the load shall be determined as given below:

L' = L – 2 eL

B' = B – 2 eB

A' = L' × B'5.1 Soils with Cohesion and Angle of Shearing Resistance5.1.1 The following formulæ shall be used for calculating ultimate netbearing capacity in the case of strip footings:

The values of Nc, N'c, Nq, N'q, Nγ and N'γ may be obtained fromTable 1.

*Methods of test for soils: Part XI Determination of shear strength parameters of aspecimen tested in unconsolidated undrained triaxial compression without themeasurement of pore water pressure, and Part XIII Direct shear test ( first revision ).

†Method of load test on soils ( second revision ).‡Method for subsurface sounding for soils: Part III Static cone penetration test

( first revision ).

a) In case of general shear failure qd = cNc + q ( Nq – 1 ) +

Bγ Nγ

b) In case of local shear failure q' d = cN' c + q ( N'q – 1 ) +

BγN'γ

12---

23---

12---

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IS 6403 : 1981

8

5.1.2 The ultimate net bearing capacity obtained in 5.1.1 for stripfooting shall be modified to take into account, the shape of the footing,inclination of loading, depth of embedment and effect of water table.The modified bearing capacity formulæ are given as under:

5.1.2.1 The shape factors shall be as given in Table 2.

TABLE 1 BEARING CAPACITY FACTORS( Clause 5.1.1 )

BEARING CAPACITY FACTORS

(Degrees)Nc Nq Nγ

0 5.14 1.00 0.00

5 6.49 1.57 0.45

10 8.35 2.47 1.22

15 10.98 3.94 2.65

20 14.83 6.40 5.39

25 20.72 10.66 10.88

30 30.14 18.40 22.40

35 46.12 33.30 48.03

40 75.31 64.20 109.41

45 138.88 134.88 271.76

50 266.89 319.07 762.89

NOTE — For obtaining values of N'c, N'q and N'γ, calculate ' = tan–1 (0 67 tan ).Read Nc, Nq, and Nγ , from the Table corresponding to the value of ' instead of which are values of N'c, N'q, N'γ respectively.

a) In case of general shear=

cNc scdcic + q( Nq – 1 ) sqdqiqfailure qd + Bγ Nγsγdγiγ W'

b) In case of local shear=

cN'c scdcic + q( N'q – 1 ) sqdqiq

failure q'd + BγN'γsγdγiγ W'

TABLE 2 SHAPE FACTORS

SL NO.

SHAPE OF BASE SHAPE FACTOR

sc sq sγi) Continuous strip 1.00 1.00 1.00

ii) Rectangle 1 + 0.2 B/L 1 + 0.2 B/L 1 – 0.4 B/L

iii) Square 1.3 1.2 0.8

iv) Circle 1.3 1.2 0.6

Use B as the diameter in the bearing capacity formula.

12---

2

3---

12---

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IS 6403 : 1981

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5.1.2.2 The depth factors shall be as under:

NOTE — The correction is to be applied only when back filling is done with propercompaction.

5.1.2.3 The inclination factor shall be as under:

ic = iq =

iγ =

5.1.2.4 Effect of water table

5.2 Cohesionless Soil ( c = 0 ) — The ultimate net bearing capacityshall be calculated as given in 5.2.1 and 5.2.2.5.2.1 Based on Relative Density — The formulæ given in 5.1.1 and 5.1.2shall be used, together with relevant shear strength parameter.5.2.1.1 The relative density as given in Table 3 shall be used as a guideto determine the method of analysis.

dc = 1 + 0.2 Df/B

dq = dγ = 1 for < 10°dq = dγ = 1 + 0.1 Df/B for > 10°

a) If the water table is likely to permanently remain at or below adepth of ( Df + B ) beneath the ground level surrounding thefooting then W' = 1.

b) If the water table is located at a depth Df or likely to rise to thebase of the footing or above then the value of W' shall be takenas 0.5.

c) If the water table is likely to permanently got located at depthDf < Dw < ( Df + B ), then the value of W' be obtained by linearinterpolation.

TABLE 3 METHOD OF ANALYSIS BASED ON RELATIVE DENSITY

SL NO.

RELATIVE DENSITY (DENSITY INDEX)

VOID RATIO

CONDITION METHOD OF ANALYSIS

i) Greater than 70 percent Lessthan0.55

Dense General shear

ii) Less than 20 percent Greaterthan0.75

Loose Local shear (as well aspunching shear)

iii) 20 to 70 percent 0.55 to0.75

Medium Interpolate b e t w e e ni) and ii)

1α90-------

2

1 αφ---

– 2

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IS 6403 : 1981

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5.2.2 Based on Standard Penetration Resistance Value — The standardpenetration resistance shall be determined as per IS : 2131-1981* at anumber of selected points at intervals of 75 cm in the vertical directionor change of strata if it takes place earlier and the average valuebeneath each point shall be determined between the level of the base ofthe footing and the depth equal to 1.5 to 2 times the width offoundation. In computing the value, any individual value more than50 percent of the average calculated shall be neglected and averagere-calculated (the values for all loose seams shall however be included).5.2.2.1 The ultimate net bearing capacity shall be calculated fromfollowing formula (covering effect of other factors as mentionedin 5.1.2):

qd = q( Nq – 1 ) sqdqiq + BγNγsγdγiγ W'

Where may be read from Fig. 1, Nγ, Nq may be read from Table 1,Sq, dq, iq, sγ, dγ, iγ, and W ' may be obtained as in 5.1.

5.2.3 Method Based on Static Cone Penetration Test — The static conepoint resistance ‘qc’ shall be determined as per IS : 4968(Part III)-1976† at number of selected points at intervals of 10 to15 cm. The observed values shall be corrected for the dead weight ofsounding rods. Then the average value at each one of the location shallbe determined between the level of the base of the footing and thedepth equal to 1 to 2 times the width of the footing. The average ofthe static cone point resistance values shall be determined for each oneof the location and minimum of the average values shall be used in thedesign. The ultimate bearing capacity of shallow strip footings oncohesionless soil deposits shall be determined from Fig. 2.5.3 Cohesive Soil (when = 0)

5.3.1 Homogeneous Layer5.3.1.1 The net ultimate bearing capacity immediately after cons-truction on fairly saturated homogeneous cohesive soils shall becalculated from following formula:

qd = cNc sc dc icwhere

Nc = 5.14.

*Method for standard penetration test for soils ( first revision ).†Method for subsurface sounding for soils: Part III Static cone penetration test

( first revision ).

12---

12---

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IS 6403 : 1981

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FIG. 1 RELATIONSHIP BETWEEN AND N

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IS 6403 : 1981

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The value of c shall be obtained from unconfined compressivestrength test. Alternatively, it can also be derived from static cone test( see 5.3.1.2 ). The values of sc, dc and ic may be obtained as in 5.1. If

the shear strength for a depth of B beneath the foundation does notdepart from the average by more than 50 percent, the average may beused in the calculation.

5.3.1.2 Alternately, cohesion c shall be determined from the static conepoint resistance qc using the empirical relationship shown below:

5.3.2 Two Layered System — In the case of two layered cohesive soilsystem which do not exhibit marked anisotropy the ultimate netbearing capacity of a strip footing can be calculated by using theformula given below:

qd = c1 Nc

where Nc may be obtained from Fig. 3.

FIG. 2 CHART FOR STATIC CONE TEST

Soil Type Point Resistance Values ( qc ) kgf/cm2

Range of Undrained Cohesion (kgf/cm2)

Normally consoli-dated clays

qc < 20 qc/18 to qc/15

Over consolidatedclays

qc > 20to

23---

qc26-------

qc

22------

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IS 6403 : 1981

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5.3.3 Desiccated Soil — In the case of desiccated cohesive soils, the un-drained cohesion is likely to decrease along with depth and is likely toget stabilized at some depth below ground level, around 3.5 m, if otherfactors do not influence. If a plot of undrained cohesion, values as shownin Fig. 4 is obtained, and where the pressure bulb falls within thedesiccated top soil the ultimate net bearing capacity shall be obtainedwith the assumption of cylindrical failure surface from Table 4.

FIG. 3 BEARING CAPACITY FACTORS FOR LAYEREDCOHESIVE SOIL DEPOSITS

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IS 6403 : 1981

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, c1 shall be obtained from the borehole data, and for a knownvalue of the width of the strip footing ‘B’, by trial and error qd can be

estimated by matching with .

6. ALLOWABLE BEARING CAPACITY

6.1 The allowable bearing capacity shall be taken as either of thefollowing, whichever is less:

a) Net ultimate bearing capacity as obtained in 5 divided by suitablefactor of safety, that is, net safe bearing capacity.

FIG. 4 DECREASING COHESION WITH DEPTH IN CASE OF DESICCATED COHESIVE SOIL

TABLE 4 DATA FOR DETERMINING ULTIMATE NETBEARING CAPACITY

0.0 5.70.2 5.00.4 4.50.6 4.00.8 3.61.0 3.2

4λ Bqd

----------- qdc1-------

λ

4λ Bqd

------------qdc1------

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IS 6403 : 1981

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b) The net soil pressure ( see 6.1.1 ) that can be imposed on the basewithout the settlement exceeding the permissible values as givenin IS : 1904-1978* to be determined for each structure and type ofsoil, that is, safe bearing pressure.

6.1.1 Safe Bearing Pressure — The permissible settlements for differenttypes of soil formations are specified in IS : 1904-1978*. The methods forcalculations of settlements for assumed pressure from standardpenetration resistance are specified in IS : 8009 (Part I)-1976†; bycalculating the settlements for two or three probable soil pressures andinterpolating, the net soil pressure for permissible settlement may beestimated. This safe bearing pressure can also be calculated based onplate load test ( see IS : 1888-1982‡ ).

*Code of practice for structural safety of buildings: Shallow foundation ( secondrevision ).

†Code of practice for calculation of settlement of foundations: Part I Shallowfoundations subjected to symmetrical static vertical loads.

‡Method of load test on soils ( second revision ).

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( Continued from page 2 )

Bearing Capacity of Foundation Subcommittee, BDC 43 : 4

Convener Representing

SHRI S. GUHA Calcutta Port Trust, Calcutta

Members

DEPUTY DIRECTOR STANDARDS (B&S), CB-I, RDSO

Ministry of Railways

EXECUTIVE E N G I N E E R (DESIGNS) V

Central Public Works Department, New Delhi

SHRI T. N. MUKHERJEE Martin & Burns Co Ltd, Calcutta

SHRI B. G. RAO Central Building Research Institute (CSIR), Roorkee

SHRI AMAR SINGH ( Alternate )

DR SWAMI SARAN University of Roorkee, Roorkee

SHRI K. R. SAXENA Engineering Research Laboratory, Hyderabad

SHRI O. S. SRIVASTAVA Cement Corporation of India, New Delhi

SHRI S. K. CHATTERJEE ( Alternate )

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Bureau of Indian StandardsBIS is a statutory institution established under the Bureau of Indian Standards Act, 1986 to promoteharmonious development of the activities of standardization, marking and quality certification ofgoods and attending to connected matters in the country.

CopyrightBIS has the copyright of all its publications. No part of these publications may be reproduced in anyform without the prior permission in writing of BIS. This does not preclude the free use, in the courseof implementing the standard, of necessary details, such as symbols and sizes, type or gradedesignations. Enquiries relating to copyright be addressed to the Director (Publications), BIS.

Review of Indian StandardsAmendments are issued to standards as the need arises on the basis of comments. Standards are alsoreviewed periodically; a standard along with amendments is reaffirmed when such review indicatesthat no changes are needed; if the review indicates that changes are needed, it is taken up forrevision. Users of Indian Standards should ascertain that they are in possession of the latestamendments or edition by referring to the latest issue of ‘BIS Catalogue’ and ‘Standards : MonthlyAdditions’.This Indian Standard has been developed by Technical Committee : BDC 43

Amendments Issued Since Publication

Amend No. Date of IssueAmd. No. 1 May 1984

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