IS :456-2000: PLAIN AND REINFORCED CONCRETE

Section 3 : General Design considerations

1

2

3 The vertical deflection limits :CantileverSimply supportedcontinuous

4 For spans above 10metres

Cantilever

Simply supported

continuous

5

6

7 For flanged beams the values in (3) and (4) be modified as per

and reinforcment percentage for use in fig 4and 5 should be based on

8 Max crack width 9 Formulae for Crack width =

h =overall depth of memberx =depth of neutral axis

10 Calculation of Average steel strain

As =area of reinforcementb =width of section at the centriod of the tensile steel

Final deflection due to all loads including effect of temperature, creep and shrinkage and measured from as cast level of the supports of floors, roofs and all other horizontal members should not exceed

The deflection including the effects of temperature, creep and shrinkage occurring after erection of partitions and the application of finishes

Depending on area and stress of steel for tension reinforcement the values in above shall be modified by multiplying with

Depending on area of compression reinforcement the span to depth further modified by multiplying with

Design surface crack width = Wcr =

acr = distance from the point considered to the surface of the nearest longitudinal bar

Cmin = minimum cover to the longitudinal bar

Ɛm =average steel strain at the level considered

Ɛm =average steel strain at the level considered

d =effective depthModulus of elasticity of steel = Es = may be taken as

11 Vertical Deflectionl = effective span in metresD = overall depth of section

12 Shrinkage

For design purposes design Shrinkage strain taken asFor more information IS:1343

13 Creep of concrete

Creep of concrete should not more than

Creep may be assumed proportional toCreep coefficientAge at loading7 days28 days1 year

14 Thermal Expansion

Quartzitesand stoneGraniteBasaltLime stone

Ɛ1 = strain at the level considered, calculated ignoring the stiffening of the concrete in the tension zone,

a =distance from the campression face to the point at which the crack width is being calculated, and

Modulus of elasticity of concrete = Ec = 5000 √(fck)

Vertical deflection = (40 * l2) / D

Note : If abve vertical deflection is greater than the deflection happened during 24hours loading then it is not necessary to measure recovery deflection.

The total shrinkage of concrete depends upon the constituents of concrete, size of the member and environmental conditions. For a given humidity and temperature, the total shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing and, to a lesser extent, by the cement content.

The value of coefficient of thermal expansion for concrete withdifferent aggregates may be taken as below:

15 Other forces and Effects

16 Combination of loads

17 Design load

18 Stability of Structure

19 Sliding

In this case 0.9 times charecterstic dead load shall be taken in account.

20 Moment connection

21 Lateral SwayUnder transient wind load the lateral sway at top

Note : In In ordinary buildings, such as low rise dwellings whose lateral dimension do not exceed 45 m, the effects due to temperature fluctuations and shrinkage and creep can be ignored in &sign calculations.

Design load is the load to be taken for use in theappropriate method of design; it is the characteristicload in case of working stress method and characteristicload with appropriate partial safety factors for limitstate design.

Shall not be less than The stability of a structure as a whole againstoverturning shall be ensured so that the restoring moment shall be not less than the sum of 1.2 times the maximum overturning moment due to the charac&stic dead load and 1.4 times the maximum overturning moment due to the characteristic imposed loads. Incases where dead load provides the restoring moment, only 0.9 times the characteristic dead load shall be considered. Restoring moment ilue to imposed loads shall be ignored.

Factor against sliding should be greater than 1.4 under the most adverse combination of applied charecterstic forces.

In designing the framework of a building provisions shall be made-by adequate moment connections or by a system of bracings to effectively transmit all the horizontal forces to the foundations

For seismic loading ref IS :189322 Fire Resistence

The fire Resistence of structural element is expressed in terms ofGeneral requirements of fire resistence

23 Analysis

Effective Span isClear span + effective depth of slabClear span + effective depth of beamCentre to Centre of supports whichever is less

24 Continuous beam or slab

Min requirements of concrete cover and member dimensions for normal weight aggregate concrete members

nominal cover required exceeds 40 mm for beams and 35 mm for slabs, to give protection against~spalling.

All strucures may be analysed by the Linera elastic theory tocalculate internal actions produced by design loads.

For end span with one end fixed and the other continuous or for intermediate spans, the effective span shall Abe the clear spanbetween supports;

For end span with one end free and the other continuous, the effective span shall be equal to the clear span plus half the effective depth of the beam or slab or the clear span plus half the width of the discontinuous support, whichever is less;

In the case of spans with roller or rocket bearings, the effective span shall always be the distance between the centres of bearings.

Span/250

72026

Fig 6

0.3mm

h-x

b (h-x) (a-x)

should not exceed span/350 or 20mm whichever is less

deflection calculations should be made separately

above value multiplied by 10/span in metres

above value multiplied by 10/span in metres

modification factor obtained as per Fig 4 in IS

modification factor obtained as per Fig 5 in IS

area of section equal to br d

3 acr Ɛm

1+ 2(a cr -C min)

Ɛ1 - 3 Es As (d-x)

200

1 m1 m

40 m

0.0003

stress

Coefficient2.21.61.1

KN/mm2

KN/mm2

1/3rd of charecterstic compressive strength

coefficient of Thermal expansion/0C

1.2 to 1.3 x 10-5

0.9 to 1.2 x 10-5

0.7 to 0.95 x 10-5

0.8 to 0.95 x 10-5

0.6 to 0.9 x 10-5

Foundation Movement (IS 1904)

Elastic axial shorteningSoil and fluid pressuresVibrationFatigueImpact (IS 875 (Part-5) )

Erection loads (IS 875 (Part-2) )

As per IS 875 (Part-5)

should not exceed H/500H = Total height of building

Stress concentration effect due to point load and the like

hours in accordance of IS:1641Ref IS :1642

Ref Page 47,34 IS 456-2000

Additional measures such as application of tire resistant finishes, provision of fire resistant falseceilings and sacrificial steel in tensile zone

If width of supprot is less than 1/12 of clear span

If width of supprot is more than 1/12 of clear span or 600mm whichever is less.