is.456.2000 - plain & reinforced concrete_part13

8/16/2019 Is.456.2000 - Plain & Reinforced Concrete_Part13

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IS 456 : 2000

27 EXPANSION JOINTS

27 1 Structures in which marked changes in plandimensions take place abruptly shall be providedwithexpansiononjointsatthe section where such changesoccur Expansion joints shall beso provided that the

necessary movement occurs with a minimumresistance at the joint The structures adjacentto thejoint should preferably be supported on separatecolumns or walls but not necessarily on separatefoundations Reinforcement shall not extend acrossan expansionjoint and the break between the sectionsshall becomplete

27 2 The details asto the lenith ofa structure whereexpansionjointshave to be providedcanbe detenninedafter taking intoconsiderationvarious factors such astemperature exposureto weather the time and seasonofthe laying of the concrete etc Normally structuresexceeding 45 m in lenith are designed with one ormore expansion joints However in view ofthe largenumber of factors involved in deciding the locationspacing and nature of expansionjoints the provisionof expansion joint in reinforced cement concretestructures should be left to the discretion of thedesigner IS 3414 gives the design considerationswJtich need to be examined and provided for

All dimensionsIn millimetres All dimensions In mlllimetres

Flo 8 Flo 9

ND VIDU L GROUPS

TR NSVERSE REINFORCEMENT

All dimensionsin millimetres

FIG 10FrG

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IS 6: 2000

SECTION 4 SPECIAL DESIGN REQUIREMENTSFOR STRUCTURALMEMBERSAND SYSTEMS

29.2 Lever Ar m

29 DEEP BEAMS

where / is theeffective span taken as centre to centredistance between supports or 1.15 times the clearspan,whichever is smaller and D is the overall depth.

19.3 Reinforcement

28 2 4 Resistanceto Applied Horizontal Force

Additional reinforcement connected to the supportedmember should be provided to transmit this force inits entirety.

when - < 1

D

Iwhen 1 S - 2.5

D

Iwhen - < 1l

z=0.5 Ior

z = 0.2 I 1.5

or

z= 0.61

b For continuous beams.

29 3 Positive Reinforcement

The tensile reinforcement required to resist positivebendingmoment in any span of a deep beam shall:

a extend without curtailm- t between supports;

b be embedded beyond the face of each support,so that at the faceof the support it shall have adevelopment length not less than0.8 LtJ whereL j is the development length see 26.2.1 . forthe design stress in the reinforcement; and

The lever arm zfor a deep beamshall bedetemined below:

a For simply supported beauts:I

z = 0.2 I 2D when I - 2D

29.1 General

a A beamshallbe deemed to be a deep beamwhen

the ratio of effective span to overall depth.

is less than:

1 2.0 for a simply supported beam; and2 2.5 for a continuous beam.

b A deep beam complying with the requirementsof 29.2 and 29.3 shall be deemed to satisfy theprovisions for shear.

8 3 Shear Reinforcement

Shear reinforcement should be provided in the form

of horizontal links distributed in the upper two-thirdof the effective depth of root of the corbel; thisreinforcement should be not less than one-half of thearea of the main tension reinforcement and should beadequately anchored.

28.2 Design

28 2 1Simplifying Assumptions

The concrete and reinforcement may be assumed toact as elements of a simple strut-and-tie system, withthe following guidelines:

a The magnitude of the resistance provided tohorizontalforceshouldbe not less thanone-halfo f the design vertical load on the corbel see also 28 2 4

b Compatibility of strains between the strut-and tie at the corbel foot should be ensured.

It should be noted that the horizontal link requirementdescribed in 28.2.3 will ensure satisfactory serviceability performance.

28 2 2 Reinforcement Anchorag«

Atthefrontfaceof the corbel, the reinforcement shouldbe anchored either by:

a welding to a transverse bar ofequal strength -in this case the hearing area of the load shouldstopshortof thefaceof the support by a distanceequal to the cover of the tie reinforcement,or

b bending buck the bars toform a loop - in thiscase the bearing area of the load shouldnot p ro ject beyond the straigh t portion ofthebars forming the main tension reinforcement,

28.1 General

28 CONCRETE CORBELS

A corbel is a shortcantileverprojectionwhichsupportsa load bearing member and where:

a . the distance a v between the line of the reactionto the supported load and the root of the corbelis less than d the effective depth of the root ofthe corbel ; and

b the depth at the outer edge of the contact areaof the supported load is not less than one-halfof the depth at the root of the corbel.

The depth o f the corbel at the face of the support isdeterminedin accordance with 40.5.1.

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IS 456: 2000

c) be placed within a zone of depth equal to0 25 0.0 adjacent to the tension face ofthe beam where is the overall depthand isthe effective span.

29.3.2 Negative Reinforc menta) Termination of reinforcement - Por tensile

reinforcement required to resist negativebending momentover a supportofa deep beam:1) It shall be permissible to tenninate not more

thanhalfof the reinforcement at a distanceof O ~ from the face ofthe support where is as defined in 29.2;and

2) The remainder shall extend over the fullspan.

b) istribution When ratio of clear span tooverall depth is in the range 1.0to 2.5. tensilereinforcement over a support of a deep beamshall be placed in two zones comprising: 1) a zone ofdepth 0.2D. adjacent to the tension

face. which shall contain a proportion ofthe• tension steel givenby

O.S ~ - o swhere

= clear span. and = overall depth.

2) a zone measuring 0.3 on eithersideofthe mid-depth of the beam which shallcontain the remainderof the tension steelevenly distributed.Forspantodepth ratios lessthan unity. thesteel shall be evenly distributed over adepthof 0.8 measured from the tensionface.

29.3.3 Vertical Reinforctmt1l1If forces are applied toadeep beam insucha w y thathanging action is required. barsor suspensionstil NPSshallbe provided tocarry all the forces concerned.

19.3.4 Side Face ReinforcementSide face reinforcement shall comply with require ments of minimum reinforcement of walls s t 32 4

30 RIBBED.HOLLOW BLOCK ORVQIDEDSLAB

30.1 General

This coversthe slabsconstructed in one of the waysdescribed below:

a) As a aeries of concrete ribswithtoPPinlcastonforms whichmay be removedafterthe concretohas set;

b) As a series of concrete .ribs between precastblocks which remain part of the completed

ItNcture; the topof the ribs m y be connectedby atoPpinlof concreteofthe same strenath asthatusedin theribs;and

c) With a continuous top and bottom face butcontaining voids of rectangular, oval orother shape.

3 2 Analysis or Stnacture

The moments and forces due to desian loads oncontinuousslabs maybe obtainedby the methods livenin Section 3 for solid slabs. Alternatively the slabamay be designed asa seriesof simply supportedspansprovided theyarenot exposed to weather or corrosiveconditions; wide cracks may develop at the supportsand the engineer shall satisfy himselfthat these willnot impair finishes or lead to corrosion of thereinforcement.

30.3 Shear

Where hollow blocks are used, for the purposeof calculating shear stress. the rib width may beincreased to take account of the wan thickness of theblock ononesideoftherib; with narrow precast units.the width of the jointing mortar or concrete may beincluded.

30.4 Deflection

The recommendationsfor deflectionin respect ofsolid

slabs may be applied to ribbed. hollow blockor voidedconstruction. The span to effective depthratios livenin 23.2 for a flanged beam are applicable but whencalculating the final reduction factor for web width.therib width for hollow blockslabs may be assumedto include the walls of the blocks on bothsidesof therib. For voided slabsandslabsconstructed of boxorI-section units.an effective rib width shallbe calculatedassumins all material below the upper flange of theunitto be concentrated in a reetanaular rib hlvins thesame cross-sectional area and depth.

30.5 Sizeand Poeltlonof Rib • .

In situ ribsshall be not less than65 mm wide. Theyshall be spaced at centres not greater than 1 ~m partand their depth. excluding ny topping shall be notmore thanfour times their width. Generally ribs shallbe formed alonseachedge parallel to the spanof oneway slabs. Whenthe edge is built intoa wallor restsona beam a ribat leastas wideas the bearing shall befonned alons the ed e.

30.6 Hollow Blocks an d Formen

Blocks and fonners m y beof ny suitable material.Hollow clay tiles for the filler type shall conform toIS 9 ~1 Part I). When required to contribute to the

is.456.2000 - plain & reinforced concrete_part13

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