is.2950.1.1981 raft foundations

8/20/2019 Is.2950.1.1981 Raft Foundations

1/30

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3/30


4/30

Gr 6

IS: 2950 (Part I) -1981(Reaffirmed 2008)

Indian Standard CODE OF PRACTICE FOR

DESIGN AND CONSTRUCTION OF RAFT

FOUNDATIONS

PART I DESIGN

(Second Revision)

Fourth Reprint DECEMBER 2004( Including Amendment No.1)

UDC 624.153.61 : 624.0 : 69.001.3

© Copyright 1982

B UR E AU O F INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

September 1982


5/30

IS: 2950

Part

I • 1981

Indian Standard

CODE

OF

PRACTICE FOR

DESIGN

AND CONSTRUCTION OF RAFT

FOUNDATIONS

P RT

I DESIGN

Second Revision

Foundation Engineering Sectional Committee,

BDC

43

Chairman

PROF DINESII MOHAN

Representing

Central Building Research Institute

CSIR ,

Roorkee

Members

DR

R. K. BHANDARI Central Building Research Institute CSIR ,

Roorkee

SHRI eVEN R SH RM lternate

CHIEF ENGINEER CaJcutta

Port Trust. Calcutta

SHRI S. GUH lternate

SHRJ M. O. N V TE The Concrete Association of India, Bombay

SHRI

N. C.

DUGOAL

lternate

SHR R. K. S GUPT Simplex Concrete Piles India Pvt

Ltd,

Calcutta

SHRt

H.

GUHA

BISWAS

lternate

Smu A. O. DASTIDAR In personal capacity 5 , Hungerford Road 121,

Hungerford Street,

Calcutta

SHRI

V. C. DESHPANDE The Pressure Piling Co I Pvt

Ltd,

Bombay

DIRECTOR CSMRS Central

Water

Commission, New Delhi

DEPUTY

DIRECTOR

CSMRS lternate

SHRI A. H. DIVANJI Asia

Foundations and

Construction Co Pvt Ltd,

Bombay

SHRI A. N. J NGLE

lternate

SHRI

A. GH05HAL Stup Consultants Ltd, Bombay

PROF GOPAl

R NJ N University

of Roorkee, Roorkee

DR JAGDISH NARAIN

Indian

Geotechnic Society, New

DeW

PROF

SW MI S R N

lternate

Continued on 2

o Copyright 1982

BUREAU

OF INDIAN

STANDARDS

Thil

publication is protected under the

Indian Copyright

ct

XIV

of 9S

and

reproduction In whole or in part

by

any means except with written permission of the

publisher shall be deemed to

be

an infrigement of copyright under the said Act.


6/30

IS : 2950 Part

I ) -

1981

Continued from page 1 )

Members

Representing

Ministry

o f

Railways

Public

Works

Department,

Chandigarh

Central Warehousing Corporation,

Ne w

Delhi

Machenzies Limited, Bombay

Engineers

India Limited, New

Delhi

Bokaro Steel Plant

Steel

Authority o f India),

Bokaro

Engineer-in-Chief s

Branch,

A rm y H ea dq ua rt er s,

N ew D el hi

BRJO

OMBIR

SISGH

SHRI G. S. JAIN G. S. Jain Associates, Roorkee

SHRI ASHOK K UM A R J AI N

lternate

JOINT

DIRECTOR 0 )

National Buildings Organisation, New

Delhi

SHRI

SUNIL BERY

lternate

JOINT DIRECTOR RESEARCH SM ),

RDSa

JOINT

D I RE C TO R R ES EA RC H

B S ),

RO Sa

lternate

DR R. K. KATTI Indian Instituteof Technology, Bombay

SHRI S. R. KuLKAR N I M. N. Dastur Co Pvt Ltd,

Calcutta

SHRI

S.

Ro y

lternate

SHRI O. P.

MALHOTRA

SHRI

A. P.

MATHUR

SHIH

V. B. M A T H U R

SHRI T. K. D. M U N S I

SHRI

M.

IYEN ,AR

lternate

SHRI Y. V. NARASIMHA

R A O

T ~ O

K.

P.

A N A N D

lternate )

SHRI

B .K. PANTHAKY Th e

Hindustan

C o ns t ru ct i on C o mp a ny Limited,

Bombay

Gammon

India Limited, Bombay

SHRI

V. M. MADGE

lternate

SHKI R. P l : ~ U A Cemindia Co Ltd,

Bombay

SllRJ S. M U K H E R J E E lternate

SHHI

N. E. V.

RAGHVAN

The

Braithwaite Burn

Jessop

Construction

Co

Ltd,

Calcutta

SBRI

A. A. RAJU Vijayanagar Steel Plant

SA l

),

Ne w

Delhi

DR V. Y. S. RAO Nagadi Consultants Pvt L td , N ew D elh i

SHRI

A R J U N

RlJ HSl:\GHANI

Cement

Corporation

o f

I nd ia , N ew D el hi

SHRIO S.

SR VASTAVA

lternate )

DR A. SARGUNAN College of Engineering,

Guindy

SHRI

S. BooMINATHAN lternate

SHRI K. R. SAXENA Public Works

Department,

Government

o f A nd hra

Pradesh,

Hyderabad

United

Technical Consultants Pvt Ltd, New Delhi

R S. P. SHRIVASTAVA

DR R. KAPUR lternate )

SHRI T. N.

SURBA

RAO

SHRI S.

A.

REDOI

lternate

SHRI N. SIVAGlJRL

Ministry o f

Shipping and Transport, New Delhi

SHRI D. V.

SIKKA

lternate

SUPERINTENDING NG1 N B E R

Central

Public

Works Department,

Ne w

Delhi

DESIGNS)

EXECU11VE

ENGINEER D E S I G N S )

V

lternate

Continued on Pat 24)

2


7/30

DECEMBER 1988

S ub stitu te th e folJowing for th e

AMENDMENT

NO

1

TO

IS

I

2950 (

Par t

I ) -

1981

C O D E O F PR AC T IC E FO R

DESIGN

AND

C O N S T R U C T I O N

OF

RAFT

FOUNDATIONS

PART 1 DESIGN

Second Revision ]

(Pagt 4, clause 3.I g) ] - Substitute cIS: 1901-19lJ7t fo r IS : 1904·

1

978t .

Page

4,Joot-note

marked

with

t

mark)

Substitute

th e

following

for the existing

foot-note:

J

f lC()(I of r rnc tko or (t .lgn And c onvtr

uct

on of fnunnRtiuna in loll . : Genera

r e q u i r n ~ n u

third

v Jllln

).

[Page 9,

l use

5.2.1 a), lint

2 ]

-

Substitute

K < 0 5 for

K > 0-5 .

16.

clause

C·2.1.1

) - Substitute

St 8

5.1.1 fo r se« 5.2.1 .

(

Page 19, clause E-I.4 ) - S ub st it ut e t he fol lowing for the existing

matter:

p

3

P e ,

o c y

Page 19,

claw,

E-2.2 ) - Substitu te

th e

following for

the

value of

c

I '

i f [

Pl I + 4

+ Pl ( , )]

[Page

21 , claus,

£.2.3

(b) ]

eltilting

matter:

( 4 Pe - /1 ) ·

4(, + j J- - 2

Page

21,

l use F-1.1 ) - Substitute


8/30

IS

: 2950 Part I • 1981

Indian Standard

CODE

OF

PRACTICE FOR

DESlGN AND CONSTRUCTION OF RAFT

FOUNDATIONS

P RT I DESIGN

Second Revision

R E W R D

0.1 This Indian Standard Part I was adopted by the Indian Standards

Institution on 5 October 1981, after the draft finalized by the Foundation

Engineering Sectional Committee had been approved by tbe Civil Engineer

ing Division Council.

0.2 Raft foundation is a substructure supporting an arrangement

of

columns

or walls in a row or rows and transmitting the loads to the soil by means

of

a continuous slab with or without depressions or openings. Such types of

foundations are found useful where soil has low bearing capacity. This

standard was first published in 1965 and revised in 1973. In this revision,

besides making its contents up-to-date, guidelines have

been given to choose

particular type of methods in particular situations and giving reference to

finite

difference

method

which will be

covered

at

a later stage.

0.3 For the purpose of deciding whether a particular requirement of this

standard is complied with, the final value, observed or calculated, expressing

the result of a test, shall be rounded off in accordance with IS : 2 1960·.

The number of significant places retained in the rounded off value should be

same as that of the specified value in this standard.

1. SCOPE

1.1 This standard Part I covers the design of raft foundation based on

conventional method for rigid foundation

and simplified methods

flexible

foundation

for residential and industrial buildings, store-houses.

silos, storage tanks, etc, which have mainly vertical and evenly distributed

loads.

·Rules for rounding off numerical values

revised

.

3


9/30

IS : 2950

Part

I ) - 1981

2.

TERMINOLOGY

2.1

For

t he p urp ose

of

this

standard th e

definitions

of

terms

given in

IS : 2809-1972· shall apply.

3. N EC ES SA RY

INFORMATION

3.1

For

satisfactory design and

construction

of a

raft foundation. the

following

information

is necessary:

a)

Site Plan

-

Site

plan

showing

the location o f

th e

proposed

as well

as

neighbouring

structure.

b) Building

piau

an d vertical cross-sections showing different floor

levels,

ducts

an d ope ni ngs, etc,

layout of load

bea ri ng w alls,

columns

sh- \r walls, etc.

c)

Loading conditions

preferably

shown

on

a

schematic plan indicating

design

combination of loads transmitted

to

the foundation.

d)

nvironmental actors

-

Information

relating

to

geologic history

of

the area

seismicity

o f th e

region, hydrological

information

indicat

ing

ground

water conditions

and

its

seasonal

variations

climatic

factors like vulnerability

of th e

site

to sudden

flo odin g by su rf ace

run-off, erosion, etc.

e)

Geotechnical Information -

Giving subsurface profile

with

stratifica

tion details see IS : 1892-1979t ), engineering

properties of th e

founding strata

namely, index

properties

effective

shear parameters

determined under appropriate drainage

conditions, compressibility

characteristics, swelling properties, results

of

field t es ts like

static

and

dynamic

penetration

tests,

pressure

meter

tests, etc.

Modulus

o lasucity

andModulus

o

Subgrade Reaction -

Appen-

dix e nu me ra te s t he m et ho ds of determination of modulus of

elasticity an d

Poisson s

ratio

It . Th e modulus of subgrade

reaction

k )

ma y

be

determined

in

accordance with Appendix

B.

g) L im iti ng valu es of

th e

angular

distortion and

differential settlement,

the superstructure

ca n

withstand

see IS : 1904-1978t ).

b) A review

of t he p er fo rm an ce

of

a similar

structure

if

any

in

th e

locality.

·Olossary of terms

and

symbols relatinJ to soil

enaioeerin

first revision ).

t ode of practice for subsurface

investiptioD.

for foundatioDi u lt r v ston ).

:Code

of

practice for Itruetural aafety of buildinp : Shallow foundations 1«0

r vu on ).

4


10/30

IS : 2950 Part - 1981

j Information necessary to assess the possible effects of the new

structure on

the

existing structures in the neighbourhood.

k Proximity of mines

or

major storage reservoirs to the

site.

3.2 Parameters for the Analysis -

These

a re obta ined

by

averaging the

parameters

s 3.1

which

can

be

determined only fo r

relatively less

number

of points of the foundat ion

soil. The

accuracy with

which

the

average

values represent

the actual conditions

is

of

decisive

importance for

the final results.

4. DESIGN

CONSIDERATIONS

4.1 Choice

Raft

Type

4.1.1

For

fairly

small

and

uniform

column

spacing and

when the

support-

ing soil is

no t

too

compressible,

a flat concrete slab having uniform thick

ness throughout a

true mat

is

most suitable

s Fig.

A .

4.1.2 The

s lab may be

thickened

under heavily

loaded

columns to

provide

adequate strength for shear and negative moment. Pedestals may also be

provided in

such

cases s Fig. 1B .

4.1.3 A slab and beam type

of

raft

is likely to be

more economical

for

large

column

spacing

and

unequal

column loads,

particularly when

the

supporting

soil is very compressible

s

Fig. 1C .

4.1.4

For very heavy

structures, provision

of cellular raft or

rigid frames

consisting of slabs and

basement walls

may

be

considered.

4.2 Allowable Bearing Pressure - The allowable bearing pressure shall be

determined

in

accordance

with IS : 6403·1981*.

4.2.1 In

granular soils, the ultimate bearing capacity of

rafts

is genera lly

very large.

However, fo r

rafts

placed

at considerable

depth

for

example

basement

rafts ,

the

possibility

of punching mode of failure should be

investigated. The influence of soil

compressibility

and related scale effects

should also be assessed.

4.2.2 For rafts

on

cohesive

soils

stability against deep seated

failures shall

eanalysed.

4.2.3

In cohesive soils,

the

effect

of

long

term

settlement

due to considera-

tion

shall

e

taken into

consideration.

4.3 Depth of olI datio - The

depth

of

foundation

shall

generally be

not

less

than

1 m.

·Code

of practice for determination

of bearin.

capacity of

ahaJlow

foundation

fir

r vts o«

.

5


11/30

IS ; 2\ 50

Part I - 19S1

b

0

0 0 0

0

0 0 0

0

0 0

0

0

0

0

0

0 0 0

0

A

B

0

0

0

0

0

B

0

0

0

0

0

0 0

0

0

7 ~ ~ ~ ~ ~ , , , , _ ~ A

SECTION

AA

1A

Flat

Plate

SECTION BS

1B Flat

Plate Thickened

Under Columns

o O 0

r -,

1

r--

L._J L_J :

__

1

o

0

0

0 0

r--

.... ..

r--

L_J

L_

L J J

o

o

o

o

o 0

o

0

o l

o a

o

o

o

o

SE.ctION CC

1C Two·Way

Beam

and

Slab

-T I

~ ; h ~

SE TION

00

10 Frat

Plate

with

Pedestals

FIG.

1

COMMON TYPES OF

RAFf

FOUNDATIONS

6


12/30

IS

: 2950

Part

I - 1981

4.4 Sub-soil Water Pressure - The

uplift

due

to

the

sub-soil water shall be

oonsidered in the design.

4.4.1 All

construction below the ground water

level

shall

e checked fo r

flotation

4.S General

4 5 Dimensional Parameters - The

size and shape

of the

foundation

adopted affect the magnitude of subgrade modulus

and

long term deforma-

tion

of

the support ing soil and this, in

turn

influence the distribution

of

contact

pressure. This

aspect

shan be taken into consideration in

the

analysis

4 5 2 Eccentricity

o

Loading -

raft

generally occupies

the entire

area

of

the building and often it

is not feasible

and rather uneconomical

to

pro-

portion it

coinciding the centroid of the raf t with the

line

of action of the

resultant force. In such cases, the effect of

the

eccentr icity on

contact

pressure distribution

shall be taken into

consideration

4 5 3 Properties o the Supporting

o il -

Distribution of contact pressure

underneath a

raft

is affected by

the

physical

characteristics of the

soil sup

porting it. Considerations must

be

given to

the increased

contact

pressure

developed a long the

edges of the foundat ion

on

cohesive soils

and

the

opposite

effect

on granular

soils.

Long

term

consolidation of deep

soil layers shall be

taken

into

account

in the analysis. This

may

necessitate

evaluation of contact pressure distribution

both

immediately after construc-

tion and

after completion

of the

consolidation

process.

The

design

must

be

based on

the worst condit ions

4 5 4 Rigidity o the Foundation

-

Rigidity

of the foundat ion tends

to

iron

ou t uneven deformations and thereby

modifies the

contact pressure

distribution

High

order

of

rigidity is

characterized by

large

moments and

relatively small, uniform settlements. rigid

foundation may also generate

high

secondary stresses in

structural members

The effects

of

rigidity

shall

be

t aken into account

in

the

analysis.

4 5 5 Rigidity

o

the Superstructure -

Free response

of the

foundations

to

soil deformation

is

res tr icted by the rigidity

of the

superstructure

In

the

extreme

case, a stiff

structure may

force a

l xi

ble

foundation

to

behave

as

rigid. This aspect shall

e

considered to evaluate the validity

of

the contact

pressure

distribution

4.6 Heavy Vibratory Loads

- Foundations subjected to heavy

vibratory

loads should preferably e

isolated.

7


13/30

IS

: 2950

u

I • 1981

4.7

EXpaDSioo Joints -

In case the structure supported

by

the

raft

consists

of several parts with varying heights and loads it is advisable to provide

expansion joints

between these

parts.

Joints m y

also

be

provided

wherever

there is a change in

the

direction of

the

raft.

s

MEmODS

OF ANALYSIS

The

essential task in the analysis of a raft foundation is the determina-

tion of

the

distribution of

contact

pressure

underneath

lAC

raft

which is a

complex function of the rigidity

of

the superstructure raft itself and the

supporting

soil, and cannot except in very simple cases, be determined with

exactitude, This necessitates a number of simplifying

assumptions

to make

the

problem amenable to

analysis,

Once

the

distribution

of contact pressure

is determined design bending

moments

and shears can be computed based

on

statics.

The

following

methods

of

analysis

are

suggested which

are

distinguished by

the

assumptions involved. Choice of a

particular

method

should be governed by the

validity

of the assumptions

in the

particular case.

5.1 Rigid FoundatioD CooveDtiooal Method - This is based on the

assumptions

of

linear

distribution of contact

pressure. The basic assump-

tions of this

method

are:

a The

foundation

is r igid relative to the supporting soil

and the com-

pressible soil layer is relatively shallow.

b The contact pressure variation is assumed as

planar

such that

the

centroid of the contact pressure coincides with the line of

action

of

the resultant

force

of

all

loads acting

on the

foundation.

5.1.1 This method

m y

be used when either of the following conditions is

satisfied:

a The structure behaves as rigid due to the

combined

action of

tbe

superstructure and

the foundation with a relative stiffness factor

05

for evaluation of

Appendix C .

b The column spacing is less

than

1 75/> Appendix C .

5.1.2 The

raft

is analysed as a whole in each

of the

two

perpendicular

directions.

The

contact pressure distribution is determined by the procedure

outlined in Appendix D.

Further

analysis is also based on stat ics.

5.1.3 In cases of uni fo rm condi tions when

the

variations in adjacent

column

loads and column spacings

do not

exceed 20 percent of the higher

value. the ra ft may be divided into perpendicular str ips of widths

equal

to

the dis tance between midspans

and

each s trip

m y

be analysed as an in

dependent beam wi th

known column

loads and

known contact

pressures.

8


14/30

IS : 2950 Part I - 1981

Such beams will not normally satisfy statics

due

to shear transfer between

adjacent strips and the design may be based on suitable moment co

efficients, or on moment distribution.

NOTE

- On

soft

soils. for example

normally consolidated

clays, peat.

muck organic

silts, etc. the assumptions involved in the

conventional

method are commonly justified.

5.2

Flexible Foundation

5 2 Simplified Method -

In this method.

it

is assumed

that

the subgradc

consists

of an

infinite

array of

individual elastic springs each

of

which is not

affected

by

others. The spring constant is equal to the modulus

of

subgrade

reaction k

The

contact pressure at any point under the raft is, there

fore, linearly

proportional to the settlement at the point. This mcthoJ may

be used when the following conditions

are satisfied see

Appendix

E :

a The structure combined action of superstructure and

raft

may be

considered as flexible relative stiffness factor

5,

see

Appendix

C

b Variation in adjacent column load does not exceed 20 percent of the

higher value.

5 2 General method - For

the general case

of

a flexible foundation

not

satisfying the req uirements of 5.2.

J.

the method based on closed form

solution

of

elastic plate theory may he used. This method is based

on

the

theory

of

plates on winkler foundation which takes into account the re

straint on deflection

of

a point provided by continuity

of

the foundation in

orthogonal foundation.

The

distribution

of

deflection

and contact

pressure

on the raft due to a column load is determined by the plate theory. Since

the effect

of

a column load on an elastic foundation is damped

out

rapidly,

it is possible to determine the total effect at a point

of

all column loads

within the zone

of

influence by the method

of

super imposition. The com

putation

of

the effect at any point may be restricted to columns of two

adjoining bays in all directions.

The procedure is outlined in Appendix

F.

NOTE

- One of the recent general

methods

based on the above

mentioned

theory is

numerical analysis

by either

finite difference method

or

finite element method. This

method is used for accurate analysis of the raft foundation. The details of this

method

could

be

cover ed at a

later

stage.

6. STRUCTURAL DESIGN

6.1

The

general design for loads, shrinkage, creep

and

temperature effects

and provision of reinforcement and detailing shall conform ot IS: 456-1978*,

the foundation being considered as an inverted beam or slab.

·Code of practice for plain and reinforced concrete third v s on

.

9


15/30

IS : 2950

Part I -

1981

PPEN IX

A

[ Clause 3.1 f ]

DETERMINATION

OF

MODULUS

OF ELASTICITY

E

AND

POISSON S RATIO

I- )

Arl. DETERMINATION

OF

MODULUS OF ELASTICITY E .

A-I.t

Th e modulus

of

elasticity is a function of the composition of the soil,

its void r atio, stress his tory an d loading rate. I n granular soils it is a func

tion

of

the d ep th

of

the s tr ata, while in cohesive soils it is m ar ke dly influen

ced by the moisture content. Du e to its great sensitivity to sampling

disturbance accurate

evaluation of

the modulus in the laboratory is extremely

difficult. For general cases, therefore, determination of the modulus may

be based

011

field

tests

A-2 .

Where a properly equipped laboratory

and

sampling facility arc available, E may be determined in the laboratory

sec

A-3 .

A-2. FIELD DETEltI\llNATION

A-2.1 Th e value of

E

shall be d eter mined

from

plate loan test

given in

lS :

J

R88 1982:\t

E

:: a

B }

u _ ~ _ i :

· s

where

q

intensity of

contact pressure,

B =

:

least lateral dimension of test plate,

s settlement,

p. Poisson s ratio,

L, Influence factor, and

0 82 for a square plate.

A-2.1.1 The average value

of

E shall be based on a. number

of

plate

load tests carried

ou t

over the area, the nu mb er an d location of the tests,

depending upon the extent

an d

importance

of

the structure.

A-2.1.2 Effect of Size - In granular soils, the value of E, corresponding

to the size of the raft shall be det er mi ned as follows:

E

E _l -

B I

B

p

2

• --

p

Bf

B

• Method

of load

test on soils second revision

).

10


16/30

IS : 2950

Part

- 1981

where BI

B

represent sizes

of

foundation and plate and E; is the

modulus

determined

by

the plate load test.

A-2.2 For

stratified deposits or deposits with lenses of different materials,

results of

plate load

test will

be unreliable

and

static cone penetration

tests

may

be carried ou t

to

determine

E•.

A-2.2.1 Static cone penetration tests shall be carried out in accordance

with IS : 4968

Par t I II

-1976*. Several tests shall be carried out

at

regular

depth

intervals

up

to a depth equal to the width of the raft

and

the results

plotted to obtain

an

average value

of

A-2.2.2 The value of E may be determined from the following relation

ship:

E

=

2

t

where

Ci«

cone resistance in kgf/cm

2

•

A-3.

LABORATORY

DETERMINATION OF E

A 3 t

The

value of E shal l be determined by conducting triaxial test in the

laboratory [ see IS : 2720

Part

XI -1971 t and IS : 2720 Part XII -1981 ]

on

samples collected with least disturbances

A-3.2 In the first phase of the triaxial test, the specimen shall be allowed to

consolidate fully under an all round confining pressure equal to

the

vertical

effective overburden stress for the specimen in

the

field. In the second

phase,

after equil ibrium has been

reached,

further drainage

shall

be

prevent

ed and the deviator stress shall be increased from zero value to

the

magnitude

estimated for the field loading condition

The

deviator stress shall then be

reduced to zero and the cycle of loading shall be

repeated

A 3 3

The

value of E shall be taken as the t angent modulus at the stress

level

equal

to

one half the

maximum deviator stress applied during

the

second

cycle of loading.

Method

for subsurface sounding for soils :

Part II I

Static cone penetrat ion test

first revision

.

tMethods

of test for soils:

Part

XI Determination of shear strength parameters of a

specimen tested in unconsolidated undrained triaxial compression without the measure

ment of pore water pressure.

tMethods

of test for soils : Part

XII

Determination of shear strength parameters of

soils from consol idated undrained triaxial compress ion test with measurement of pore

water pressure

irst

revision .


17/30

IS : 2950 Part I ) - 1981

PPEN IX B

[

lause

3.1

]

DETERMINATION

OF MODULUS OF SUBGRADE REACTION

8-1. GENERAL

B-l.1 The modulus of subgrade reaction

I

k as applicable to the case of

load through

a

plate

of size 30 x 30 em or beams 30 em wide on

the

soil is

given in

Table I for

cohesionless soils and in

Table

2

for

cohesive soils,

Unless more

specific

determination of

k is done see B-2 and B-3 ), these

values may be used for design

of raft

foundation in cases where the depth of

the soil affected by the width of the footing may be considered isotropic

and

the extrapolation

of

plate

load

t est r es ul ts is v al id .

TABLE 1

MODULUS

OF

SUDGRADE

REACTION k ) FOR

COHESIONLESS SOflS

SOIL

CKARACT£RISTIC -MODULUS Of SUBGRADB REACTION

k

)

IN k cm

l

r - - - - - - - - - - A . . - - - - - - - ~ r - - - - - - - - - - . A . . . - - - - - ~

Relative Standard

Penetration For

Dry

or

Moist

For

Submerged

Density

Test

Value ) State

State

(I )

2) 3) 4)

Loose <

10

1 5 0-9

Medium 10 to 30 1-5 to 4-7 0 9 to 2 9

Dense 30

and

Over 4 7 to 18-0 2 9 to 10·8

-The

above value

apply to

a

square plate 30 x 30 em

or

beams 30 em wide.

TABLE 2 i \I OD UL US O F SUBGRADE

REACTION

k

) FO R

COHESIVE SOILS

SOIL CHARACTERISTIC -MODULUS Of SUBGRADB

- - - - - - - - - - - - - - ~ R EA CT IO N

k,

)

IN

kg/em

Cons iste ncy Unconfined Comp res siv e

Strength, ka/cm

1)

2)

3)

Stit. I

to

2 2-7 .

Vel v stiff 2 to 4 2 7

to

5 4

HarJ

4 and

over

5 4

to

10-8

-The values apply to a

square

plate 30 x 3D

em .

The above valuesar e

based

on the

assumption

that the

average loading intensity does no t exceed

half the

ultimate

burin.

capacity.

12


18/30

IS

: 2950 (

u

I ) - 1981

B-2.

FIELD

DETERMINATION

B-2•.1

I ncas es

where th e

depth

o f

th e

soi l affected

by

the w idth

o f

th e

footing

may be considered as isotropic, the value of

k

ma y be determined in

accordance with IS : 9214-1979*.

The

t es t s ha ll be

carried out with

a

plate

of size not less than 30 em,

8 -2 .2 T he

average

value

of k

shall

be

based

on

a

number of p la te l oa d

tests

carried

out

over the

area, the n um be r a n d location of th e tests depending

upon t he e xt en t and importance of the structure.

B-3.

LABORATORY

DETERMINATION

B-3.1

For

stratified

deposits or

d ep os it s w it h lenses

of

different

materials,

evaluation of

k

f ro m p la te load test will be unrealistic an d it s determination

shall be

based o n l ab or at or y

tests s IS :

2720 P a rt XI

)-1971

t

an d

IS : 2720 (

Part

XII

)-1981

t

B-3.2 In

carrying

out the

test

the

continuing

cell

pressure

may

be

so selected

as to

be

representative of t he d ep th of average stress influence zone about

O·SBtoB .

8-3.3

Th e value of k shall

be

determined from-the

following relationship:

__ 0 65 12 r E _

\ \j E I ·

_ I J •

where

E

E

I

Modulus

of

elasticity

of

soil (

sec

Appendix

A ),

Young s modulus of

foundation

material,

Poisson s

ratio

of

soil (

see Appendix

A ),

and

=

Moment of inertia of structure if determined or of the

foundation.

8-3.4 In the absence

of laboratory

test

data, appropriate

values of

E an d

may be determined in accordance with Appendix A an d used in B-3.2 for

evaluation of

k

• Method o f determination of subgrade

reaction

( k v a lu e ) o f soi ls in the field.

tMethods

of

test for

s oi ls : P ar t

XI

Determination

of

s he ar s tr en gt h p ar am et er s

of

specimen

tested in

unconsolidated undrained triaxial compression

without

th e me as u re

ment

pore

water

pressure.

Methods of

test for soils :

Part

XI I

Determinat

ion

of shear strength

parameters

of

soil from consolidated undrained triax ial c o mp r es s io n

test

w it h m ea su re me nt of

pore

water pressure first

revision

).

3


19/30

IS : 2950 (

Pu t

I ) - 1981

8-4.

CALCULATIONS

8-4.1 When the st ructure is rigid ( see Appendix C ), the average modulus

o f

s ub gr ad e r ea ct io n m a y a ls o

be

determined

as follows:

k

l l

A v e r a g ~ contact

pressure

•

Average

settlement of

th e raft

PPEN IX C

(

Clauses

5.1.1, 5.2.1

and

B-4.1 )

RIGIDITY

OF SUPERSTRUcrURE AND FOUNDATION

c r DETERMINATION OF mE

RIGIDITY

OF THE STRUcrURE

C-l.l The flexural rigidity £

of

the

structure

of any section may be estimat

ed

according

to

the relation

given below ( see also Fig.

2):

I b

l

[ b

2

]

E I = - 2 H S - ~ E ~ b 1+ - I , ,+1 , ,+/ / ) / i

where

E

=

modulus of

elasticity

of th e

infilling

material

wall

material) in

kg/emit

I = moment

of

inertia

of th e

infilling in em ,

b = length

or breadth

of

t he s tr uc tu re

in

th e

direction

of

bending,

H total height

of th e

infilling in em,

E =

modulus of

elasticity

of frame material

in kg/eml,

t moment

of

inertia

of

th e

beam

in

em ,

lu

= -,,;;

14


20/30

IS : 2950 Part I - 1981

t

/

spacing

of

the columns in em

h;

length of

the

upper

column

in em

h =

length of the lower

column in em

r I

,

:z s

M

moment

of inertia

of

the upper column

in ems,

I

moment of

inertia

of

the

lower column

in ems,

and

I

moment of

inertia of the

foundation

beam or

raft in ems.

NOTE -

The summation is to

be

done over all the storeys, including the foundation

beam of raft. In the case

of

the foundation, 1 / replaces I

band

becomes zero,

whereas for the

topmost

beam,

l u

becomes zero.

b

FIG.

2

DETERMINATION

OF RIGIDITY OF A

STRUCTURE

C 2 R L TIV STIFFNESS FACTOR

K

C-2.1 Whether a structure behaves as rigid

or

flexible

depends

on the relative

stiffness

of

the structure and the

foundation

soil. This relation is expressed

IS


21/30

IS

: 2950 (

Part

I ) - 1981

by the relative stiffness factor K

given below:

El

a) Fo r the whole structure K

=

E baa

b For rectangular rafts

or

beams K

=

r

E d

c) Fo r circular

rafts

K

==

12

E

2

R

where

EI

==

flexural rigidity of the structure over the length a in

kg/ern ,

E

modulus

of

compressibility of

the

foundation soil in

kg/ern ,

b length of the section in the bending axis in em,

a length perpendicular

to the

section under

investigation in

em,

d

thickness of the raft O f

beam

in ern, and

R radius of the raft in em.

e-2.1.1 Fo r K > 0 5, the foundation may be considered as rigid

see

5.2.1 ).

C-3.

DETERMIN TION

OF

CRITIC L COLUMN SP CING

C-3.1 Evaluation of the characteristics ,\ is made as follows:

A= 4{ k

4E,1

where

k

=

modulus

of

subgrade reaction in kg/em for footing

of

width B in em

see

Appendix B ).

B

=

width

of

raft in em

E

:=:: modulus

of

elasticity

of

concrete in kgf/cm

l

1 moment of inertia of the raft in

em

16


22/30

IS : 2950

Part I -

1981

PPEN IX D

Clause 5.1.2

CALCULATION

OF

PRESSURE DISTRIBUTION BY

CONVENTIONAL METHOD

n i DETERMINATION OF

PRESSURE

DISTRIBUTION

0-1.1

The pressure distr ibution q

under the raft shall

be

determined by

the

following formula:

q =

~

± Y::r T. x

• w

where

Q =

total

vertical

load on the raft,

A ·

=

total area of the raft,

e.

, e , [

I

=

eccentricities

and

moments

of inertia about

the

principal

• • , axes through the centroid of the section,

and

x,

y co-ordinates

of any

given point on the

raft with respect

to the x and y axes passing through the centroid

of

the

area

of

the raft.

I.

I

e e

may be calculated from the following equations:

-

.

1

2

.

= I

-

, In

e. = e.

- T

elf and

I ••

=

e

- -

e.

I

where

I =z

moment

of

inertia

of

the area of the raft respectively about the

x

and

y axes th rouah the centroid,

17


23/30

a

and

b

IS : 2950

Part

I ) · 1981

I e f

xyd

A

for the whole area about x and y axes th rough the

centroid, and

ea e

eccentricities

ill

the x and

y.

directions

of

the

load

from the

centroid.

For a rectangular raft the eq uation simplifies to:

q

Q

J

±

12ezX)

A

± b

2

a

l

where

the dimensions of the raft in the x and y directions

respectively.

NOTE -

I

f

one or

more

of

the values

of

q )

are

negative, as calculated

by

the above

formula. it indicates

that

the

whole

area

of

foundation

is not subject to pressure

an d

only a part of the area is in

contact

with the soil,

an d the

above formula will still hold

-good provided

appropriate

values of z

/ v / 11

e

z

and ell

arc u se d with r es pe ct to the

area

in contact with the soil i ns te ad of the whole a rea .

PPEN IX E

( Clause 5.2.1 )

CON1 ACT PRESSURE

DISTRIBUTION AND

MOl\ lENTS

BELOW FLEXIBLE FOUNIJATION

E l CONTACT

PRESSURE

DISTRIBUTION

E-l.1 The distribution of contact pressure is assumed to be linear with

maximum value attained under the columns

and

minimum at mid span.

E l.2 The contact pressure for the full width of the strip under an interior

c ol um n l oa d loc ate d at p oi nt i

Pi

) can be determined as see Fig. 3B ):

p =

5P, + ± ~ i f i

i

where

i average length of adjacent span m ),

Pi

=

column load at

point

i

t ), and

M

=

moment

under

an

interior

columns

located at i.

18


24/30

IS : 2950 Part I - 1981

E-l.3 The minimum contact pressure for the full width of the s tr ip

at

the

middle

of

the adjacent spans pm and pmr can be determined

as

see

Fig. 3A :

pm, = 2P, J _ - Pi

1

ILl

t.

I, I

pmr

2P.

- -

p, -

/r/ I,

pmr pml

pm

= -----2---

where

l., I,

as

shown

in

Fig.

3A.

E-l.4

If

E-2.3

a

governs

the moment

under

the exterior columns

contact

pressures under the exterior columns and

at

end of the s tr ip p. and p, can

be determined as

see

Fig. 3C :

6M

4P,

- Pmll

p,:.::= C

+--h----·

3Me p

po

-CI- - T

where

P

pm,

M,.

/1

C

as

shown in Fig. 3C.

E-l.S If

E-2.3

b governs

the

moment under

the exterior columns the

contact

pressures p.

and

P» are

determined as

see

Fig. 3C :

4P

-

pml}

v-

=

-4 +

1

1

-

E-2. BENDING

MOMENT

DIAGRAM

E-2.1 The bending moment

under

an

interior column located at i see

Fig. 3A

can

be determined as:

P

-

M 4 I (0 24 \1 0-16)

E-2.2 The bending

moment at midspan

is

obtained

as

s Fig.

3B :

M , =

M

M.

where

M

moment

of

simply

supported

beam

is -

= 48

[

p,

I + 4p ,

pc

r ]

where p. I ,p. r . are as shown in Fig. 3B.

19


25/30

IS

: 2950

Part

I ) - 1981

PI-I P

P

••

1-1

L

~ 1 1 2 = t 1 r / 2 t j i . 1

Wi\V4 +>.iC i --... ... I . - _. i' - ml

N1

= •

t I I I f

I I. , I. .. ,

r . I ~ H I ~

I,

Ilrlltt ll 1 L

~ J .

1 1 ~ r i

IM

, I I I

I ..

.

.

I i :.J

,

i

I 1 I I I

I

I

3A

Moment

and Pressure Distribution at Interior Column

Pt t

PmI

( t)

t Pmr

J-l

38

Pressure Distribution Over an Interlor Span

.

...

; III•

.L . . .

--

i l l l l l n l ~ l l l l l l ~

I.i. ..

3C. Moment and Pr

ure Distribution at

Ext.rior

Column

lO 3

MOMENT

AND

PRESSURB

DlSTRlBtmON AT

COLUMNS

20


26/30

IS

: 2950

Part

I • 1981

E-1.3 The bending moment

M

under exterior columns can be determined

as the least

of

see Fig. 3C :

a

(0 13Ml

+

H)6.\C

-

0 50

4P - p l l

C

b - .

4C }

T

PPEN IX

F

l use

5.2.1.1

FLEXIBLE

FOUNDATION

- GENERAL CONDmON

F- . CLOSED FORM SOLUTION OF ELASTIC PLATE THEORY

F-t.1

For a flexible

raft foundation with nonuniform column spacing

and

load intensity, solution

of

the differential

equation

governing the behaviour

of plates on elastic foundation Winkler Type) gives radial moment u

)

tangential moment Me and deflection w

)

at any point by the following

expressions:

PL I (r

w

4D 1

Y

where

P = column load;

r =

distance

of the point under investigation from column

load along radius;


27/30

IS : 2950

Part

I ) - 1981

L

radius o f effective stiffness;

f

k

modulus

o f s ub gr ad c r ea ct io n f or f oo ti ng

o f

width B;

D flexural rigidity of

th e

foundation;

F; .

i2 J

=

t

= ::

raft thickness;

E modulus

of

elasticity

of

th e foundation material;

po poisson s ratio o f foundation material; an d

2

1

Z4

functions

of

shear

moment an d

deflection

see

Fig.

4 ).

F-I.2 The

radial

and tangential moments ca n be converted to rectangular

co-ord ina tes:

=

cos?

¢ +

sin

M

sinl +

cos

where

J ;

is th e

angle with

x axis to

th e

line

joining o rigin

to th e

point

under

consideration.

F-t.3

Tho

shear

Q

pe r

unit

width

o f

raft can

be

determined

by:

Q ~ ~

where

= f un ction f or

shear

sec

Fig. 4 ).

F-l.4 When edge of th e

raft

is located

within

th e

radius of influence, th e

following corrections are to be a ppl ie d. C al cu la te moments an d shears

perpendicular

to

th e edge o f the raft within the ra dius of influence, assum

ing th e raft

to

be infinitely large. Then apply opposite

an d

e qu al m om en ts

and

shears

on th e

edge

of

the mat.

The method

for bea ms

on

elastic

foundation may be used.

F-l.5

Finally all moments

and

shears

calculated

fo r each ind iv id ual column

and

walls

ar e superimposed to

obtain

th e

t ot al m o me n t

an d

shear values.

22


28/30

IS : 2950 Part I • 98

· 1

---- I

Z, r/l

,

I

.//

I

< I..,

\ Z

3 r/lJ

I

I

: -ZI,

rIll

f

•

o

1

o

- () 2

t )

N

0-4

J

0-5 -

N

o

U

z

z

o G

FIG. 4 FUNCTIONS

FOR

SHEAR

MOMENT

AND DEFLECTION

23


29/30

IS : 9SO Part I ) - 1981

Continued

from

POle

2 )

Members

SHRI

M.

D

TAMBU:AR

DR A. VARADARAJAN

DR R.

KAN1RAJ

Alternate

)

SHRIO. RAMAN

Director

C iv

oa

Represen i

Bombay Port

Trust

Bombay

Indian

Institute otTcchnoJoay. New Delhi

Director General

B1S

Ex officio Member

)

Secretary

SHRJ

K.

M.

MATHUR

Deputy

Director Civ

Eng ).

BIS

Bearing Capacity of

Foundation

Subcommittee,

BDC

43

onvener

SHJU

S.

GUllA

Calcutta

Port Trust,

Calcutta

Membn 8

DEPUTY DIRECTOR STANDAI lDS Desips

StaDdar


30/30

URE U OF INDI N ST ND RDS

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41

229 2175

286 1498

240 22 06

254 11 37

3 10 84

237 38 79

221 82 92

221 56 98

252 51 71

22

1451

226 28 08

426 86 59

237 82 51

232 21 04

271 28 33

22 12 6215

23096528

Printed at Simco Printing Press, Delhi

is.2950.1.1981 raft foundations

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