Is.456.2000 - Plain & Reinforced Concrete_Part11

8/16/2019 Is.456.2000 - Plain & Reinforced Concrete_Part11

http://slidepdf.com/reader/full/is4562000-plain-reinforced-concretepart11 1/5

BEAM

IS456:

2000

I I

I

I

I

I

T-+----+-+----+---

Xl

I I

~

SECTION

XX

FIG.

3 TRANSVERSE

REINFORCEMENT IN

FLANGB

OFT-BEAM WHEN MAIN REINFORCEMENT Of

SLABIS

PARAIJ.EL

TO

THE

BEAM

e) For flanged beams, the values of a) or b) be

modified

as

per Fig. 6 and the reinforcement

percentage foruse inFig.4 and

5

should

be

based

on area of sectionequal

to

b d

NOTE-When

deflections

are

required

to

be calculated,

the

method given

inAnnex

C may be used.

2·0

,6

\

\ \

\ \

\

\

r-,

\

\

r

I

<

r-

\

<

r

-..

I -...

r ....

f

5

-

-,

<

I --

I-...

10. ,.

r---

f 90

r

I

f--...

fu240

-- -

h·290

,

I

Note: f . IS STEEL STRESS OF SERVICE

LOADS IN mm

2

I

o

0 4 0, ,2 1 6 2·0

PERCENTAGE TENSION

REINFORCEMENT

I .0.58 f ArMof

CI OII _tion

ohteel

required

•

.

Areaof

Cmll •

aection

of steel

provided

Flo. 4 MODIACATJON FAcroR

FOR

TENSION REINFORCEMENT

38

2 8 )0



18456:

2000

100

. . .

L

V

/

V

/

0-50

00

1-50 2-00 2-50

PERCENT GE COMPRESSION REINFORCEMENT

F IG. 5

MOOIHCATION FACTOR FORCOMPRESSIONREtNPoRCBMENT

1/

/

/

/

0095

a:

e

e sc

L

z 0.85

Q

t

0·80

a:

1 1

0·75

0.70

o

0 2 0 4 0·8 0 8 1·0

RATIO

OF WEB

WIDTH

TO FLANGE WIDTH

FIG. 6 REDUCTION

FACTORS

FORRA 110S

OF

SPAN TO EJ rEcnVE DEPTH FOR FLANGED BEAMS

23.3 Slenderness Limit for Beams to Ensure

Lateral

Stability

A simply supported or continuous beam shall be so

proportioned that thecleardistance between the lateral

250b

z

restraints does not exceed 60

b

Of

whichever

d

is less, where d is the effective depth of the beamand

thebreadth of thecompression facemidwaybetwecn

the lateral restraints.

For a cantilever, the clear distance from the free end

of the cantilever to the lateral restraint shall not

100b

z

exceed 25

b

or _ - whichever is less.

24 SOLID SLABS

24.1 General

Th e

provisions of 13 .2 for beams apply to slabs

also.

NOTBS

1 Forslabsspanninl

in

twodirections,the shorterof the two

spans should

be

used for calculatingthe span to

etTective

depth ratios

2 Fortwo-wavslabsof shorterspans(up to m)withmild

stoelreinforcement, the span to overalldepth ratiosgiven

below may generally be assumed

10

satisfy vertical

deflectionlimits forloadingclassup to 3

kN m

J

•

Simplysupportedslabs

Continuous slabs 40

Forhighstn:ngthdefonnedbarsof

smde Fe

thevalues

givenaboveshouldbemultiplied by0.8.

24.2 Slabs Continuous Over Supports

Slabs spanning in one direction and continuous over

supports shallbedesigned according to the provisions

applicable to continuous beams.

24.3

S ~ ~ b s

Monolithic with Supports

Bendingmomentsin slabs(except

flat

slabs)constructed

monolithicallywith the supports shall becalculated by

takingsuch slabseitheras continuousover supports and

39



d For cantilever solid slabs, the effective width

shall he calculated in accordance with the

followingequation:

b

er

=

2a

l

c For two or more loads not in a line in the

direction

of thespan if theeffective width of

slabforone load does notoverlaptheeffective

width

ofslab

foranotherload,

both

calculated

as in a above,thentheslabforeach loadcan

be designedseparately If theeffectivewidth

ofslabforoneload overlaps theeffective width

of

slabfor an

adjacent load

the

overlapping

portion

of the slab

shall

be

designed

for the

combined effectof the two

loads

Table14

Values

01k

for

Sbnply

Supported and

Continuous Slabs

Clause 24.3.2.1

where

ber = effective width,

=distanceof theconcentrated load from the

face of the cantilever support, and

=

widt h of contact area of the concentrated

load measured parallel to the supporting

edge.

Provided thattheeffective widthof thecantilever

slab shall not exceed one-third the lengthof the

cantilever slabmeasured

parallel

tothefixededge.

Andprovidedfurtherthatwhentheconcentrated

loadis

placed

nearthe

extreme

endsof

the length

of cantileverslab in the directionparallel to the

fixed edge. the effectivewidth shall not exceed

the above value, nor shall it exceed half the

abovevalueplusthedistanceof theconcentrated

load from the extreme end measured in the

direction parallel to the fixededge. .

24.3.2.2For slabs other than solid slabs, the effective

width shall depend on the ratio of the transverse and

longitudinalflexuralrigidities of the slab. Where this

ratio is one, that

is,

where the transverse and

longitudinal

flexural

rigidities arc approximately

equal, the value of effective width as found for solid

slabs

may

be used But as the ratio decreases,

proportionatelysmaller valueshall

he

taken.

IS 456: 2000

capableof freerotation.or as membersof a continuous

framework with the supports, taking into account the

stiffnessof suchsupports If suchsupports are

formed

dueto beamswhichjustify fixityat thesupportof slabs,

then the effects on the supportingbeam, such as, the

bending of the web in the transverse direction of the

hewn and

thetorsionin thelongitudinal directionofthe

beam.whereverapplicable, shallalso be consideredin

the designof the beam.

24.3.1 For the purpose of calculation of moments in

slabs in

a

monolithic structure,

it

will

generally

be

sufficiently accurateto assumethatmembersconnected

to the ends of such slabs are fixed in position and

direction at the ends remote from their connections

with the slabs.

24 3 1 Slabs CarryingConcentratedLoad

24 3 2 1

Ifa solidslabsupportedon twooppositeedges,

carries concentrated loads the maximum bending

moment caused y the concentrated loads shall be

assumed to be resisted by an effectivewidth of slab

measuredparallel to thesupportingedges as follows:

a For a single concentrated load. the effective

width shallbe calculatedin accordancewith the

following equation provided that it shall not

exceed the actual width of the slab:

b

r

kx

1

.. .. +

a

lef

where

d

= effective width of slab,

k constanthavingthevaluesgiveninTable

14dependingupon theratioof thewidth

of the

slab to the effective span {

x

=

distance

of the

centroid of

the

concentrated load from nearersupport,

I

r f

= effective span, and

a

width of the

contact area

of

the

concentrated load from nearer support

measuredparallel to thesupportededge.

And provided further that incase of a loadnear

the unsupported edge of a slab. the effective

width shall not exceed the above value nor half

theabovevalueplusthe

distance

ofthe

load from

the unsupported edge.

b For two or more concentrated loads placed in a

line in the direction of the span, the bending

moment per metre width of slab shall be

calculated separatelyfor each loadaccordingto

its appropriateeffectivewidthof slabcalculated

as in a above and added together for design

calculations.

40

0 1

0.2

0.3

0.4

S

0.6

0.7

0.8

0.9

1.0and above

i

lorSlmpl,

Supported S

0.4

0.8

1 16

1.48

1.72

1.96

2.12

2.24

2.36

2.48

i rCoatlDUQUI

0.4

0.8

1 16

1.44

1.68

1 84

1.96

2.08

2.16

2.24



24.3.2.3

Any

other recognizedmethodof analysisfor

Casel

of Ilaba

covered

by

24.3. .1

and 24.3.2.2 and

fot all othet ~ s e of slabs may be used with the

approvai OfaM engineer*in-charge.

24.3.2.4 The critidilMedon for checking shearshall

be

as

Jiven

in

34.2.4.1.

24.4 Slabs SpaDllinllD two Directions at Rllbt

ADII_

The slabs spanning

in two directions at right angles

and carryinS uniformly distributed load may be

desilned by

any

acceptable theory or by usina

coefficients liven in

Annex

D. For determining

bending

moments in slabs

spanningin two directions

at right anales and carrying coaeentrered load, any

acceptedmethod approved

by

the engineer-In-charge

maybeadopted.

NOTS The IDOII

commonly UJed elasticmethods are based

onApaud , or

Wester-pard ,

theory

and

themost commonly

uled

limit

ItaJeof

coUapsemethod

is

based

onJohansen s

yield-

linetheory.

4 4

Restrained Slabwith

Unequal Conditions

at

Adjacent Pan

In some cases the support moments calculatedfrom

Table

26 for adjacent panelsmay differ significantly.

The following procedure may

be

adopted to adjust

them:

a) Calculate

the

sum of

moments at midspan

and

IUpports

(nea1ecting

signs).

b)

treat the values

from

Table 26 as fixed end

m<mieDts.

c) According t the relative stiffness of adjacent

spans, distribute the fixed end moments across

the supports,living new supportmoments.

d) Adjult midspanmomentsuch that, whenadded

to the support moments from c) neglecting

IS 56:2000

signs),the totalshouldbe equal to that from a).

If the resulting support moments are

signifi

cantlygreaterthan the value from Table26, the

tension steel over the supports will need to be

extended further. The procedure should be as

follows:

1) Takethe spanmomentas parabolic between

supports: its maximum value is as found

from d).

2)

Determine

thepoints of contraflexure of the

new support moments [from c)] with the

span moment[from 1)].

3) Extendhalfthesupporttensionsteelateach

end to at least an effective depth or 12bar

diameters beyond the nearest point of

contraflexure.

4)

Extend

the

full area of the support tension

steel at each end to half the distance from

3).

24.5 Loads on SuPportiDI Beams

The loads on beams supporting solid slabs spanning

in two directions at right angles and supporting

uniformly

distributed loads, may be assumed to bein

accordance withFig. 7.

25 COMPRESSION MEMBERS

25.1 Definitions

25.1.1Columnor strut is a compressionmember, the

effecti

velengthofwhichexceeds threetimes the least

lateral dimension.

25 2 ShortandSlenderCompression Members

A compression member may

be

considered as short

· I

h

when boththe slenderncb; ~

D

and b are less

than

12:

A

6 818 07 1

LO IN THIS SHADED

AREA T BE CARRIED

V BEAM e

LOAD IN THIS SH E AREA

TO BE

CARRIED

Y BEAN

FlO.

7 LO CARRJIID BY

SUPPORTING

BEAMS

41

B



IS 456 : 2000

where

1

M

= effective length in respect of the major

axis,

== depth in respect of the major

axis,

I -

effective length in respect of the minor

ry

axis, and

b

=

width of the member,

It shall otherwise be considered as a slender

compression member,

25 1 3 Unsupported Length

The unsupported length, I of a compression member .

shall he taken as the clear

distance

between end

restraints except that:

a in flat slab construction, it shall be cleardistance

between the floor and the lower extremity of

the capital, the drop

panel

or

slab

whichever is

the least.

h

in

beam and

slab

construction,

it

shall be the

c lear d is tance between

the

floor and the

underside of the shallower beam framing into

the columns in each direction at the next higher

floor level.

c in columns

restrained

laterally

by

struts,

it

shall

be the

clear

distance between consecutive

struts in each vertical plane, provided that to be

an adequate support, two

such

struts shall

meet the columns at approximately the same

level and the

angle between vertical

planes

through the struts shall not vary more than

3Qt

from a right angle. Such struts shall be of

adequate dimensions and shall have sufficient

anchorage to restrain themember against lateral

deflection.

d in columns restrained laterally by struts or

beams,

with

bracketsused at thejunction, it shall

be

the

clear distance between the

floor and

the

lower edge of the bracket, provided that the

bracket width equals that of the beam strut and

is at least half that of the colurnn.

25.2 Effective Length of Compression Members

In the absence of more exact analysis, the effective

length ler of columns may

be

obtained as described in

Annex E.

25.3 Slenderness Limits for

Columns

25.3.1 The

unsupported

length between endrestraints

shall not exceed 60 times the least lateral dimension

of a column.

25.3.2 If,

in any given

plane, one end of

a column

is

unrestrained, its unsupportedlength,

it

shall notexceed

JOOb

2

[

where

b

= width-of that cross-section, and

= depthofthe

cross-section

measuredinthe

plane underconsideration.

25.4 Minimum Eccentricity

All columns shall be designed for mmrmum

eccentricity,

equalto theunsupported

length

of

column

500plus lateraldimensions/30, subject to a minimum

of20

rom.

Wherebi-axialbending

is

considered,it is

sufficient to ensure that eccentricity exceeds the

minimum aboutone axis at a time.

26 REQUIREMENTS GOVERNING

REINFORCEMENT

AND

DETAILING

26.1 General

Reinforcing steelof sametypeand

grade

shallbe used

as main

reinforcement

in a

structural

member

However.

simultaneous

use of

twodifferent

types or

grades

of steel for mainand

secondary

reinforcement

respectively is permissible.

26.1.1 Bars may be arranged singly. or in pairs in

contact,

or

in groups of three or four bars

bundled

in

contact. Bundledbarsshall

be

enclosedwithin

stirrups

or ties. Bundled bars shall be tied together to ensure

the bars remaining together. Bars larger than32 mm

diametershall notbe bundled.

except

in columns.

26 1 2

The

recommendations

for

detailing

for

earthquake-resistant construction given in IS 13920

should be taken into consideration, where applicable

seealso

IS 4326 .

26.2 Development

of

Stress in Reinforcement

The calculated tension or compressionin

any

bar at

any

section shall be developed on each side of the

section

by

an appropriate

developmentlength or end

anchorage or

by

a combination thereof.

26 2 1 Development Length

Bars

The development

length

L

d

is given by

L

_ a.

1

f d

where

, =

nominaldiameterof

the

bar,

a =stressinbarat the sectionconsideredat design

load,

and

t

hd

=design bond stress given in 26 Z

NOTES

1

he

development

IcnJlh

includca

anchomae

vDlues

of

hooks

in

tension reinforcement.

2 For barsof

sections

other th n

cireular.

the

development

lenathshould be sufficient to

develop

the stras in thebar

by bone

0°

...

:

..... l

42

I,

:i

c

.;.

Is.456.2000 - Plain & Reinforced Concrete_Part11

Documents