Notification 53

7/27/2019 Notification 53

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INDIAN HIGHWAYS, JUNE 2009 91

NOTIFICATION NO. 53 dated the 28 May 2009

Subject: AmendmentstoClauseNo.202.3,208,209.7,218.5and222ofIRC:6-2000StandardSpecications

and Code of Practice for Road Bridges Section : II Loads and Stresses (Fourth Revision)

FourthRevisionofIRC:6-2000StandardSpecicationsandCodeofPracticeforRoadBridgesSection:

II Loads and Stresses was published in December, 2000 and reprinted in April 2006 incorporating uptodate amend-

ments till that time.

The Indian Roads Congress has decided to further amend the above document. Accordingly, the

AmendmentNo.8isherebynotied.

These amendments shall be effective from the 1 June 2009.

(R.P. Indoria)

Secretary General

Encl: As above

Clause No. For Read

202.3 The load combination shown in Table 1

shall be adopted for working out the stress-

es in members. The permissible increases of

stresses in various members due to these com-

binations are also indicated therein. These

combinations of forces are not applicable for

working out base pressure on foundations for

which provisions made in relevant IRC Bridge

Code shall be adopted.

The load combination shown in Table 1 shall be

adopted for working out the stresses in the mem-

bers. The permissible increase of stresses in mem-

bers. The permissible increase of stresses in vari-

ous members due to these combinations are also

indicated therein. These combinations of forces

are not applicable for working out base pressure

on foundations for which provisions made in rel-

evant IRC Bridge Code shall be adopted. For cal-

culating stresses in members using working stressmethod of design the load combination as shown

in Table 1 shall be adopted.

The load combination as shown in Appendix 3

shall be adopted for working out the stresses in

members using limit state design approach.


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INDIAN HIGHWAYS, JUNE 2009

NOTIFICATION NO. 53

92

218.5 Permissible Increase in Stresses and Load

Combination

Tensile stresses resulting from temperature ef-

fects not exceeding in the value of two third

of the modulus of rupture may be permittedin prestressed concrete bridges. Sufcient

amount of non-tensioned steel, shall, howev-

er, be provide to control the thermal cracking.

Increase in stresses shall be allowed for calcu-

lating load effects due to temperature restraint

under load combinations.

Permissible Increase in Stresses and Load

Combinations

Tensile stresses resulting from temperature

effects not exceeding in the value of two third

of the modulus of rupture may be permitted inprestressedconcretebridges.Sufcientamountof

non-tensioned steel, shall, however, be provide to

control the thermal cracking. Increase in stresses

shall be allowed for calculating load effects due to

temperature restraint under load combinations.

Note:

Permissible increase in stresses and load

combinations as stated under Clause 218.5 is not

applicable for limit state design of bridges.

208 Note: However, it should be ensured that the

reduced longitudinal effects are not less sever

than the longitudinal effect, resulting from

simultaneous load on two adjacent lanes.

Note: However, it should be ensured that the re-

duced longitudinal effects are not less severe than

the longitudinal effect, resulting from simulta-

neous load on two adjacent lanes. Longitudinal

effects mentioned above are bending moment,shear force and torsion.

209.7 P-1 Normal

Containment

Bridges carrying

Expressway, or

equivalent

15kN vehicle

at 110 km/h

and 200

angle of

impact

P-1 Normal

Containment

Bridges carrying

Expressway, or

equivalent

150 kN vehicle

at 110 km/h

and 20o angle of

impact

P-2 Low Con-

tainment

All other bridges

except bridge

over railways

15kN vehicle

at 80 km/h

and 20o

angle of

impact

P-2 Low Con-

tainment

All other bridges

except bridge

over railways

150 kN vehicle

at 80 km/h and

20o angle of

impact

P-3 High Con-

tainment

At hazardous

and high risk

locations, over

busy railway

lines, complex

interchanges, etc.

30kN vehicle

at 60 km/h

and 20o

angle of

impact angle

of impact

P-3 High

Containment

At hazardous and

high risk loca-

tions, over busy

railway lines,

complex inter-

changes, etc.

300kN vehicle

at 60 km/h and

20o angle of

impact


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NOTIFICATION NO. 53


Appendix 3

COMBINATION OF LOADS FOR LIMIT STATE DESIGN

1 Loads to be considered while arriving at the appropriate combination for carrying out the necessary checks

for the design of road bridges and culverts are as follows:

1) Dead Load

2) Snow load (See note i)

3) Superimposed dead load such as hand rail, crash barrier, footpath and service loads.

4) Surfacing or wearing coat

5) Back Fill Weight

6) Earth Pressure

7) Primary and Secondary effect of prestress8) Secondary effects such as creep, shrinkage and settlement.

9) Temperature including restraint and bearing forces.

10) Carriageway live load, footpath live load, construction live loads.

11) Associated carriageway live load such as braking, tractive and centrifugal forces.

12) Accidentaleffectssuchasvehiclecollisionload,bargeimpactandimpactduetooatingbodies.

13) Wind

14) Seismic Effect

15) Erection effects

16) Water Current Forces

17) Wave Pressure

18) Buoyancy

Notes:

i) The snow loads may be based on actual observation or past records in the particular area or local

practices, if existing.

ii) The wave forces shall be determined by suitable analysis considering drawing and inertia forces

etc. on single structural members based on rational methods or model studies. In case of group of

piles, piers etc., proximity effects shall also be considered.

2 Combinationofloadsforthevericationofequilibriumandstructuralstrengthunderultimatestate

Loads are required to be combined to check the equilibrium and the structural strength under ultimate limit state.

The equilibrium of the structure shall be checked against overturning, sliding and uplift. It shall be ensured that the

disturbing loads (overturning, sliding and uplifting) shall always be less than the stabilizing or restoring actions.

The structural strength under ultimate limit state shall be estimated in order to avoid internal failure or excessive

deformation. The equilibrium and the structural strength shall be checked under basic, accidental and seismic

combinations of loads.


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NOTIFICATION NO. 53

94

3. Combination Principles

The following principles shall be followed while using these tables for arriving at the combinations:

i) All loads shown under Column 1 of Table 3.1 or Table 3.2 or Table 3.3 or Table 3.4 shall be combined to carry

outtherelevantverication.ii) While working out the combinations, only one variable load shall be considered as the leading loads at a time.

All other variable loads shall be considered as accompanying loads. In case if the variable loads producesfavorable effect (relieving effect) the same shall be ignored.

iii) Foraccidentalcombination,thetrafcloadontheupperdeckofabridge(whencollisionwiththepierduetotrafcunderthebridgeoccurs)shallbetreatedastheleadingload.Inallotheraccidentalsituationsthetrafcload shall be treated as the accompanying load.

iv) During construction the relevant design situation shall be taken into account.

v) These combinations are not valid for verifying the fatigue limit state.

4. Basic Combination

4.1. Forcheckingtheequilibrium

For checking the equilibrium of the structure, the partial safety factor for loads shown in column no. 2 or 3under Table 3.1 shall be adopted.

4.2. Forcheckingthestructuralstrength

For checking the structural strength, the partial safety factor for loads shown in column no. 2 under Table 3.2shall be adopted.

5. Accidental Combination

For checking the equilibrium of the structure, the partial safety factor for loads shown in column no. 4 or 5

under Table 3.1 and for checking the structural strength, the partial safety factor for loads shown in column no. 3under Table 3.2 shall be adopted.

6. Seismic Combination

For checking the equilibrium of the structure, the partial safety factor for loads shown in column no. 6 or 7under Table 3.1 and for checking the structural strength, the partial safety factor for loads shown in column no. 4under Table 3.2 shall be adopted.

7. CombinationofLoadsfortheVericationofServiceabilityLimitState

Loads are required to be combined to satisfy the serviceability requirements. The serviceability limit statecheckshallbecarriedoutinordertohavecontrolonstress,deection,vibration,crackwidth,settlementandtoestimate shrinkage and creep effects. It shall be ensured that the design value obtained by using the appropriate

combination shall be less than the limiting value of serviceability criterion as per the relevant code. The rarecombination of loads shall be used for checking the stress limit. The frequent combination of loads shall be usedforcheckingthedeection,vibrationandcrackwidth.Thequasi-permanentcombinationofloadsshallbeusedforchecking the settlement, shrinkage creep effects and the permanent stress in concrete.

7.1. Rare Combination

For checking the stress limits, the partial safety factor for loads shown in column no. 2 under Table 3.3 shallbe adopted.


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NOTIFICATION NO. 53


7.2. FrequentCombination

Forcheckingthedeection,vibrationandcrackwidth,inprestressedconcretestructures,partialsafetyfactorfor loads shown in column no. 3 under Table 3.3 shall be adopted.

7.3. Quasi-permanentCombinationsFor checking the crack width in RCC structures, settlement, creep effects and to estimate the permanent stress

in the structure, partial safety factor for loads shown in column no. 4 under Table 3.3 shall be adopted.

8. CombinationforDesignofFoundations

For checking the base pressure under foundation and to estimate the structural strength which includes thegeotechnical loads, the partial safety factor for loads for 3 combinations shown in Table 3. 4 shall be used.

The material safety factor for the soil parameters, resistance factor and the allowable bearing pressure forthese combinations shall be as per relevant code.

Note: An Explanatory note will be included in a Special Publication on Limit State Design of Bridges as and when

the Concrete Bridge Codeisnalized.Table 3. 1

PartialSafetyFactorforVericationofEquilibrium

Loads Basic Combination Accidental Combination Seismic Combination

(1) (2) (3) (4) (5) (6) (7)

Overturning

or Sliding or

Uplift Effect

Restoring

or Resisting

Effect

Overturning

or Sliding or

Uplift Effect

Restoring

or Resisting

Effect

Overturning

or Sliding or

Uplift Effect

Restoring

or Resisting

Effect

Permanent Loads:

Dead Load, Snow load

if present, SIDL ex-ceptsurfacing,Backll

weight, settlement, creep

and shrinkage effect

Surfacing

Prestress and Secondary

effect of prestress

(refer note 5)

Earth pressure due toBack Fill

Variable Loads :

Carriageway Live

Load, associated loads

(braking, tractive and

centrifugal forces) and

Pedestrian Live Load

1.05

1.35

1.50

0.95

1.0

-

1.0

1.0

1.0

1.0

1.0

-

1.0

1.0

1.0

1.0

1.0

-


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NOTIFICATION NO. 53

96

(a) As Leading Load

(b) As accompanying Load

(c) Construction Live Load

Thermal Loads(a) As Leading Load


Wind

(a) As Leading Load


Live Load Surcharge

effects (as accompanying

load)

Accidental effects:

i) Vehicle collision (or)

ii) Barge Impact (or)

iii)Impactduetooating

bodies

Seismic Effect

(a) During Service

(b) During Construction

Construction Condition:Counter Weights:

a) When density or self

weightiswelldened

b) When density or

self weight is not well

dened

c) Erection effects

Wind

(a) Leading Load

(b) Accompanying Load

Hydraulic Loads:

(Accompanying Load):

Water current forces

Wave Pressure

Hydro dynamic effect

Buoyancy

1.5

1.15

1.35

1.50

0.9

1.50

0.9

1.20

-

-

-

-

-

1.05

1.50

1.20

1.0

1.0

-

1.0

0

0

0

0

0

0

0

0

-

-

-

0.9

0.8

0.95

0

0

0

0

-

-

0.75

0.2

1.0

-

0.5

-

-

-

1.0

-

-

-

-

-

-

-

1.0

1.0

-

1.0

0

0

0

-

0

-

-

-

-

-

-

1.0

1.0

-

-

-

-

-

-

-

-

0.2

1.0

-

0.5

-

-

-

-

1.0

0.5

-

-

-

-

-

1.0

1.0

1.0

1.0

-

0

0

-

0

-

-

-

-

-

-

1.0

1.0

-

-

-

-

-

-


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NOTIFICATION NO. 53


Notes:

1) During launching the counterweight position shall be allowed a variation of 1m for steel bridges.

2) For Combination principles refer Para 2

3) Thermal load includes restraints associated with expansion/contraction due to type of construction (Portalframe, arch and elastomeric bearings), frictional restraint in metallic bearings and thermal gradients. This

combination, however, is not valid for the design of bearing and expansion joint.

4) Wind load and thermal load need not be taken simultaneously.

5) Partial safety factor for prestress and secondary effect of prestress shall be as recommended in the relevant

codes.

6) Wherever Snow Load is applicable, Clause 224 shall be referred for combination of snow load and live load.

7) Seismic effect during erection stage is reduced to half when construction phase does not exceed 5

years.

8) Forrepair,rehabilitationandretrottingtheloadcombinationshallbeprojectspecic.

Table 3.2

PartialSafetyFactorforVericationofStructuralStrength

Ultimate Limit State

Loads Basic Combination Accidental Combination Seismic Combination

(1) (2) (3) (4)

Permanent Loads:

Dead Load, Snow load

if present, SIDL except

surfacing

(a) Adding to the effect of

variable action

(b) Opposing the effect of

variable action

Surfacing:

Effects adding to the

effect of variable action

Effects opposing the effect

of variable action

Prestress and Secondary

effect of prestress(refer note no. 2)

BackllWeight

Earth pressure due to

Back Fill

(a) Leading Load


1.35

1.0

1.75

1.0

1.50

1.50

1.0

1.0

1.0

1.0

1.0

1.0

-

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0


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NOTIFICATION NO. 53

98

Variable Loads:

Carriageway Live Load

and associated actions

(braking, tractive and

centrifugal forces) andPedestrian Live Load:

(a) Leading Load


(c) Construction Live Load

Wind during service and

construction

(a) Leading Load


Live Load Surcharge (asaccompanying load)

Erection effects

Accidental Effects:

i) Vehicle Collision (or)

ii) Barge Impact (or)

iii)Impactduetooating

bodies

Seismic Effect(a) During Service

(b) During Construction

Hydraulic Loads (Accom-

panying Load):

Water Current Forces

Wave Pressure


Buoyancy

1.5

1.15

1.35

1.50

0.9

1.2

1.0

-

-

-

1.0

1.0

-

0.15

0.75

0.2

1.0

-

-

0.2

1.0

1.0

-

-

1.0

1.0

-

0.15

0

0.2

1.0

-

-

0.2

1.0

-

1.0

0.5

1.0

1.0

1.0

0.15

Notes:

1) For Combination principles, refer Para 2.


codes.

3) Wherever Snow Load is applicable, Clause 224 shall be referred for combination of snow load and live

load.


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NOTIFICATION NO. 53


Table No. 3.3

PartialSafetyFactorforVericationofServiceabilityLimitState

Loads Rare

Combination

Frequent

Combination

Quasi-permanent

Combination

(1) (2) (3) (4)

Permanent Loads:

Dead Load, Snow load if present, SIDL including

surfacing

BackllWeight

Prestress and Secondary effect of prestress

(Refer note no. 4)

Shrinkage and Creep Effects

Earth Pressure due to Back Fill

Settlement Effects

(a) Adding to the permanent effect

(b) Opposing the permanent effect

Variable Loads:

Carriageway Live Load and associated loads(braking,

tractive and centrifugal forces) and Pedestrian Live Load

(a) Leading Load


Thermal Loads

(a) Leading Load


Wind

(a) Leading Load


Live Load Surcharge (Accompanying Load)

Hydraulic Loadss (Accompanying Load):

Water Current Forces

Wave Pressure

Buoyancy

1.0

1.0

1.0

1.0

1.0

0

1.0

0.75

1.0

0.6

1.0

0.60

0.80

1.0

1.0

0.15

1.0

1.0

1.0

1.0

1.0

0

0.75

0.2

0.6

0.5

0.60

0.50

0

1.0

1.0

0.15

1.0

1.0

1.0

1.0

1.0

0

-

0

-

0.5

-

0

0

-

-

0.15


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NOTIFICATION NO. 53

100

Notes:

1) For Combination principles, refer Para 2.

2) Thermal load includes restraints associated with expansion/contraction due to type of construction (Portal

frame, arch and elastomeric bearings), frictional restraint in metallic bearings and thermal gradients. This

combination, however, is not valid for the design of bearing and expansion joint.

3) Wind and thermal loads need not be taken simultaneously.


codes.

5) Where Snow Load is applicable, Clause 224 shall be referred for combination of snow load and live load.

Table 3. 4

CombinationforBasePressureandDesignofFoundation

Loads Combination

(1)

Combination

(2)

Seismic / Accidental

Combination

(1) (2) (3) (4)

Permanent Loads:

Dead Load, Snow load if present, SIDL except

surfacing,BackFillearthlling

SIDL Surfacing

Prestress Effect

(Refer note 4)

Settlement Effect

EarthPressureduetobackll

(a) Leading Load


Variable Loads:

All carriageway loads and associated loads

(braking, tractive and centrifugal) and pedestrian

load

(a) Leading Load


Thermal Loads as accompanying load

Wind

(a) Leading Load


Live Load Surcharge as Accompanying Load (if

applicable)

1.35

1.75

1.0 or 0

1.50

1.0

1.5

1.15

0.90

1.5

0.9

1.2

1.0

1.0

1.0 or 0

1.30

0.85

1.3

1.0

0.80

1.3

0.80

1.0

1.0

1.0

1.0 or 0

-

1.0

(0.75 if applicable) or 0

0.2

0.5

-

0

0.2


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NOTIFICATION NO. 53


Accidental Effect or Seismic Effect

Seismiceffectduringconstruction

Erection effects

Hydraulic Loads:

Water Current

Wave Pressure


Buoyancy:

For Base Pressure

For Structural Design

-

-

1.0

1.0 or 0

1.0 or 0

-

1.0

0.15

-

-

1.0

1.0 or 0

1.0 or 0

-

1.0

0.15

1.0

0.5

1.0

1.0 or 0

1.0 or 0

1.0 or 0

1.0

0.15

Notes:

1) For combination principles, refer para 2.

2) Where two partial factors are indicated for loads, both these factors shall be considered for arriving at the

severe effect.

3) Wind and Thermal effects need not be taken simultaneously.


codes.

5) Wherever Snow Load is applicable, Clause 224 shall be referred for combination of snow load and live

load.

6) Seismic effect during erection stage is reduced to half when construction phase does not exceed 5 years.

7) Forrepair,rehabilitationandretrottingtheloadcombinationshallbeprojectspecic.

222. Seismic Force

222.1 Applicability:

222.1.1

All bridges supported on piers, pier bents, and arches, directly or through bearings, and not exempted below in the

category (a) and (b), are to be designed for horizontal and vertical forces as given in the following clauses.

The following types of bridges need not be checked for seismic effects:

(a) Culverts and minor bridges up to 10 m span in all seismic zones

(b) Bridges in seismic Zones II and III satisfying both limits of total length not exceeding 60 m and spans not

exceeding 15 m

222.1.2

Special investigations should be carried out for the bridges of following description:

(1) Bridges more than 150 m span

(2) Bridges with piers taller than 30 m in Zones IV and V

(3) Cable supported bridges, such as extradosed, cable stayed, and. suspension bridges


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NOTIFICATION NO. 53

102

(4) Arch bridges having more than 50 m span

(5) Bridges having any of the special seismic resistant features such as seismic isolators, dampers etc.

(6) Bridges using innovative structural arrangements and materials.

Notes for special investigations:

1. Inallseismiczones,areascoveredwithin10kmfromtheknownactivefaultsareclassiedasNearField

Regions. For all bridges located within Near Field Regions, except those exempted in clause 222.1.1, special

investigations should be carried out. The information about the active faults should be sought by bridge

authorities for projects situated within 100 km of known epicenters as a part of preliminary investigations at

the project preparation stage.

2. Special investigations should include aspects such as need for site specic spectra, independency of

component motions, spatial variation of excitation, need to include soil-structure interaction, suitable methods

of structural analysis in view of geometrical and structural non-linear effects, characteristics and reliability

of seismic isolation and other special seismic resistant devices, etc.

3. Sitespecicspectrum,whereveritsneedisestablishedinthespecialinvestigation,shallbeused,subjecttotheminimumvaluesspeciedforrelevantseismicZones,giveninFig.13.

222.1.3

Masonry and plain concrete arch bridges with span more than 10m shall be avoided in Zones IV and V and in near

eldregion.

222.2 Seismic Zones

Forthepurposeofdeterminingtheseismicforces,theCountryisclassiedintofourzonesasshowninFig.11.For

eachZoneafactorZisassociated,thevalueofwhichisgiveninTable5.

Fig.11SeismicZonesofIndia(IS:1893(PartI):2002)


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NOTIFICATION NO. 53


Note: Bridge locations and towns falling at the boundary line demarcating two zones shall be considered in the

higher zone.

TABLE 5 ZONE FACTOR (Z)

Zone No. Zone Factor (Z)

V 0.36

IV 0.24

III 0.16

II 0.10

222.3 Components of Seismic Motion

The characteristics of seismic ground motion expected at any location depend upon the magnitude of earthquake,

depth of focus, distance of epicenter and characteristics of the path through which the seismic wave travels. The

random ground motion can be resolved in three mutually perpendicular directions. The components are consid-ered to act simultaneously, but independently and their method of combination is described in section 222.4. Two

horizontal components are taken as of equal magnitude, and vertical component is taken as two third of horizontal

component.

In zones IV and V the effects of vertical components shall be considered for all elements of the bridge.

The effect of vertical component may be omitted for all elements in zone II and III, except for the following cases:

(a) prestressed concrete decks

(b) bearings and linkages

(c) horizontal cantilever structural elements

(d) for stability checks and

(e) bridgeslocatedintheneareldregions

222.4 Combination of Component Motions

1.The seismic forces shall be assumed to come from any horizontal direction. For this purpose two separate analyses

shall be performed for design seismic forces acting along two orthogonal horizontal directions. The design seismic

force resultants (i.e. axial force, bending moments, shear forces, and torsion) at any cross-section of a bridge compo-

nent resulting from the analyses in the two orthogonal horizontal directions shall be combined as below (Fig.12).

a) r10.3r

2

b) 0.3r1r

2

Where

r1= Force resultant due to full design seismic force along x direction.

r2= Force resultant due to full design seismic force along z direction.

2. When vertical seismic forces are also considered, the design seismic force resultants at any cross section of a

bridge component shall be combined as below:

a) r10.3r

20.3r

3

b) 0.3r1r

20.3r

3

c) 0.3r1 0.3r

2r

3


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NOTIFICATION NO. 53

104

where r1

and r2areasdenedaboveandr

3is the force resultant due to full design seismic force along the vertical

direction.

Note: Analysis of bridge as a whole is carried out for global axes X and Z and effects obtained are combined for

design about local axes as shown.

222.5ComputationofSeismicResponse

Following methods are used for computation of seismic response depending upon the complexity of the structure

and the input ground motion.

(1) Formostofthebridges,elasticseismicaccelerationmethodisadequate.Inthismethod,therstfundamental

mode of vibration is calculated and the corresponding acceleration is read from Fig. 13. This acceleration is

applied to all parts of the bridge for calculation of forces as per clause 222.5.1.

Fig.12:CombinationofOrthogonalSeismicForces


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NOTIFICATION NO. 53


(2) Elastic Response Spectrum Method: This is a general method, suitable for more complex structural systems

(e. g. continuous bridges, bridges with large difference in pier heights, bridges which are curved in plan,

etc),inwhichdynamicanalysisofthestructureisperformedtoobtaintherstaswellashighermodesof

vibration and the forces obtained for each mode by use of response spectrum from Fig. 13 and clause 222.5.1.

These modal forces are combined by following appropriate combinational rules to arrive at the design forces.Reference is made to specialist literature for the same.

222.5.1 Horizontal Seismic Force

The horizontal seismic forces acting at the centers of mass, which are to be resisted by the structure as a whole, shall

be computed as follows:

Feq

= Ah

(Dead Load + Appropriate Live Load)

where,

Feq = seismic force to be resistedA

h = horizontalseismiccoefcient=(Z/2)*(I)*(S

a/g)).

Appropriate live load shall be taken as per Clause 222.5.2

Z = Zo ne factor as given in Table 5

I = Importance Factor (see Clause 222.5.1.1)

T = Fundamental period of the bridge (in sec.) for horizontal vibrations.

Fundamental time period of the bridge member is to be calculated by any rational method of analysis adopting the

Modulus of Elasticity of Concrete as per IRC: 21-2000, and taking gross uncracked section for moment of inertia.

The fundamental period of vibration can also be calculated by the method given inAppendix-2.

Sa/g=Averageresponseaccelerationcoefcientfor5percentdampingofloadresistingelementsdependingupon

the fundamental period of vibration T as given in Fig. 13 which is based on the following equations.

For rocky, or hard soil sites, Type I soil with N >30

Formediumsoilsites,TypeIIsoilwith,10


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NOTIFICATION NO. 53

106

Fig.13ResponseSpectra

For damping other than 5% offered by load resisting elements, the multiplying factors as given below shall be

used.

Damping % 2 5 10

Factor 1.4 1.0 0.8

Application Prestressed concrete, Steel

and composite steel

elements

Reinforced Concrete

elements

Retrottingofoldbridges

with RC piers

222.5.1.1SeismicImportanceFactor-I

Bridges are designed to resist design basis earthquake (DBE) level, or other higher or lower magnitude of forces,

depending on the consequences of their partial or complete non-availability, due to damage or failure from seismic

events.Thelevelofdesignforceisobtainedbymultiplying(Z/2)byfactorI,whichrepresentsseismicimportance

of the structure. Combination of factors considered in assessing the consequences of failure,- and hence choice of

factor I,- include inter alia,

(a)Extentofdisturbancetotrafcandpossibilityofprovidingtemporarydiversion,

(b) Availability of alternative routes,

(c) Cost of repairs and time involved, which depend on the extent of damages, - minor or major,

(d) Cost of replacement, and time involved in reconstruction in case of failure,

(e) Indirect economic loss due to its partial or full non-availability,

Importance factors are given in Table 6 for different types of bridges.


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Table 6 Importance Factor

Seismic Class IllustrativeExamples Importance Factor I

Normal bridges All bridges except those mentioned in other

classes

1

Important bridges a)Riverbridgesandyoversinsidecities

b) Bridges on National and State Highways

c)Bridgesservingtrafcnearportsandother

centers of economic activities

d) Bridges crossing railway lines

1.2

Large critical bridges in all Seismic

Zones

a) Long bridges more than 1km length across

perennial rivers and creeks

b) Bridges for which alternative routes are not

available

1.5

Note: While checking for seismic effects during construction, the importance factor of 1 should considered for all

bridges in all zones.

222.5.2 Live Load components

(i) Theseismicforceduetoliveloadshallnotbeconsideredwhenactinginthedirectionoftrafc,butshallbeconsideredinthedirectionperpendiculartothetrafc.

(ii) Thehorizontalseismicforceinthedirectionperpendiculartothetrafcshallbecalculatedusing20%ofliveload (excluding impact factor).

(iii) The vertical seismic force shall be calculated using 20% of live load (excluding impact factor).

Note: The reduced percentages of live loads are applicable only for calculating the magnitude of seismic designforce and are based on the assumption that only 20% of the live load is present over the bridge at the time ofearthquake.

222.5.3WaterCurrent,andDepthofScour

The depth of scour under seismic condition to be considered for design shall be 0.9 times the maximum scourdepth.Theoodlevelforcalculatinghydrodynamicforceandwatercurrentforceistobetakenasaverageofyearlymaximumdesignoods.Forriverbridges,averagemaypreferablybasedonconsecutive7yearsdata,orbasedonlocal enquiry in the absence of such data.

222.5.4HydrodynamicandEarthpressureForcesunderSeismicCondition

In addition to inertial forces arising from the dead load and live load, hydrodynamic forces act on the submergedpart of the structure and are transmitted to the foundations. Also, additional earth pressures due to earthquake act onthe retaining portions of abutments. For values of these loads reference is made to IS:1893-2002. These forces shall

be considered in the design of bridges in zones IV and V.

Additional earth pressure forces described above need not be considered on other components such as wing wallsand return walls since these elements are easily repairable at low cost.

222.5.5DesignForcesforElementsofStructuresandUseofResponseReductionFactor

The forces on various members obtained from the elastic analysis of bridge structure are to be divided by ResponseReduction Factor given in Table 7 before combining with other forces as per load combinations given in Table 1.

The allowable increase in permissible stresses should be as per Table 1.


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108

Table7ResponseReductionFactors

BridgeComponent Rwithductile

detailing

Rwithoutductile

detailing

Superstructure N. A 2.0Substructure

(i) Masonry /PCC piers, abutments

(ii) RCC short plate piers where plastic hinge cannot develop in

direction of length, and RCC abutments

(iii) RCC long piers where hinges can develop

(iv) Column

(v) Beams of RCC portal frames supporting bearings

-

3.0

4.0

4.0

1.0

1.0

2.5

3.3

3.3

1.0

Bearings 2.0 2.0

Connectors and Stoppers (Reaction blocks)

Those restraining dislodgement or drifting away of bridge elements.

These are additional safety measures in the event of failure of

bearings.

1.0 1.0

Notes:

(i) Those parts of the structural elements of foundations which are not in contact with soil and transferring load

to it, are treated as part of sub-structure element.

(ii) Response reduction factor is not to be applied for calculation of displacements of elements of bridge and forbridge as a whole.

222.6FullyEmbeddedPortions

Parts of structure embedded in soil below scour level need not be considered to produce any seismic forces.

222.7Liquefaction

Inloosesandsandpoorlygradedsandswithlittleornones,thevibrationsduetoearthquakemaycauselique-

faction, or excessive total and differential settlements. Founding bridges on such sands should be avoided unless

appropriate methods of compaction or stabilisation are adopted. Alternatively the foundations should be taken

deeper;belowliqueablelayers,tormstrata.Referenceshouldbemadetothespecialistliteratureforanalysisof

liquefaction potential.

222.8FoundationDesign

Foundations subjected to seismic load from all sources (ref.222.5.3 and 222.5.4), and taking combinations and

allowable stresses as given in IRC:78 should be designed as per IRC: 78 to limit the bearing stresses within allow-

able limits and, avoiding overturning, sliding, and deep seated failure with safety factor of 1.5, 1.25 and 1.15 respec-

tively.Forthisverication,theseismicloadsonfoundationsshouldbetakenas1.25timestheforcestransmittedto

itbysubstructure,soastoprovidesufcientmargintocoverthepossiblehigherforcestransmittedbysubstructure

arising out of its over strength.


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NOTIFICATION NO. 53


222.9DuctileDetailing

Mandatory Provisions

(i) In Zones IV and V, to prevent dislodgement of superstructure, reaction blocks (additional safety measures

in the event of failure of bearings) or other types of seismic arresters shall be provided and designed for theseismic force (F

eq/R). Pier and abutment caps shall be generously dimensioned, to prevent dislodgement of

severe groundshaking. The examples of seismic features shown in Fig, 14 to 16 are only indicative and

suitablearrangementswillhavetobeworkedoutinspeciccases.

(ii) To improve the performance of bridges during earthquakes, the bridges in seismic zones IV and V may be

specicallydetailedforductilityforwhichIS:13920oranyotherspecialistliteraturemaybereferredto.

Recommended Provisions

(i) In order to mitigate the effects of earthquake forces described above, special seismic devices such as Shock

Transmission Units, Base Isolation, Seismic Fuse, Lead Plug, etc, may be provided based on specialized

literature, international practices, satisfactory testing etc.

(ii) Continuous superstructure (with fewer number of bearings and expansion joints) or integral bridges (in whichthe substructure or superstructure are made joint less, i.e. monolithic), if not unsuitable otherwise, can possibly

provide high ductility leading to better behavior during earthquake.

(iii) Elastomeric bearings with arrester control in both directions may also be considered.

Note:ABackgroundNoteforSeismicForceClauseisgiveninAppendix-4

Fig.14ExampleofSeismicreactionblocksforcontinuoussuperstructure


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110

Fig.15Exampleofseismicreactionblocksforsimplysupportedbridges

Fig.16Minimumdimensionforsupport


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NOTIFICATION NO. 53


Appendix 4CLAUSE: 222 SEISMIC FORCE

(BackgroundNote)

1 INTRODUCTION

A Sub-Committee comprising Late Dr. T.N. Subba Rao, S/Shri S.G. Joglekar and D.K. Kanhere was formed by B-2

Committee to carry out a detailed review of the interim provision in Clause No. 222 relating to Seismic Forces in the

existing IRC: 6 (post January, 2003 revision of the code) and compare the same with the provision in the pre-January

versionofthecodeandsubmititsndingsandviewstotheB-2Committeefordeliberation.Therstinterimreport

of the Sub-Committee was discussed in B-2 Committee meeting on 8th & 9th December 2006. After a series of

detaileddiscussionsanddeliberationsinB-2committee,thedraftDocumenttitledPROPOSEDREVISIONOF

CLAUSE 222: SEISMIC FORCE in IRC: 6 2000 was approved by the B-2 committee in its 9th Meeting held on

3rd November, 2008 for submission to the BSS committee. It was felt that a background note explaining the rationale

and approach behind the proposed revision of clauses will be useful for appreciating the various provisions in the

Clause. This report is accordingly prepared to provide an informative background to the proposed revision.

The following documents have been referred to in preparation of this Background Note:-

IS:1893(Part-1)-2002Generalprovisionsandbuildings.

DraftofIS:1893(Part3)-2005BridgesandRetainingWalls(underconsiderationofBISCommittee).

ThedraftoflongversionofSeismicDesignGuidelinesundernalizationbyB2Committee.

Eurocode:8Designprovisionsforearthquakeresistanceofstructures.

Part-1: General rules, - Part-2: Bridges

FundamentalsofSeismicProtectionforBridges:byYashinskyandKarshenas.

2 BASIS OF RECOMENDATIONS

2.1 Force Based and Performance Based Approaches

Approach of the present IRC code (post January, 2003 revision), like BIS standard IS: 1893 is based on force

based approach, for achieving safety of bridges in seismic event. In this approach, the effect of earthquake is

represented by a set of forces, which should be considered in the design. Internationally, as well as in India, the

force based approach has worked well in regions of low seismicity. However, it is found to be woefully inadequate

in high seismicity zones (refer Fig 1). Most of the international codes dealing with regions of high seismicity have

nowadoptedanewapproach known as PerformanceBasedPhilosophy,which basicallyattempts to specify

the response viz. the desired performance of the bridge during and after the earthquake and achieve the same by

formulating suitable design rules. A detailed description of this approach and the performance criteria adopted in

this method is outside the scope of this note. It is realized that at this stage, it is not possible to fully adopt this

approach, till the Indian National Standards i.e BIS also change over to this approach, when Limit State Codes areintroducedbyIRCandLongVersionofIS1893(Part3)isnalized.

However, it is possible to adopt the underlying concepts, and some of the methods, while continuing with the force

based approach, in order to target at the desired seismic behavior of bridges during and after the Design Basis

Earthquake. The targeted behavior shall be stated in terms of aims to be archived. The use of force based methods

will have to be supplemented by prescriptive recommendations so that the performance targets (aims) of the design

are at least qualitatively taken into account, if not precisely calculated. This has to be done, using engineering

judgment, on the basis of the observations of damages suffered by bridges in seismic events.


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Fig.1:TotalcollapseofyoverduringtheKobeearthquakeof1995

2.2 CombinationofForceBasedApproachandTargetsofPerformance

From the above point of view the following mixed approach is suggested:-

Desirable functional and structural behaviors of bridges during and after earthquake are stated as aims to be

achieved. These are:

The expected levels of service in terms of the full or partial availability of bridge for use after the

Design Basis Earthquake (DBE).

Nosignicantstructuraldamageandmaybesomenon-structuraldamageintheeventofchosenlevel

ofearthquakefordesign.ThislevelisdescribedintermsofDesignBasisEarthquake(DBE),asdened

by IS: 1893-2002 (Part 1). This DBE level is chosen as 50% of the Maximum Considered Earthquake

(MCE), which is considered as the maximum potential seismic event in the zone.

Targeted structural response is described in terms of permissible stresses, permissible overloadingin plastic region of material strains, residual deformations after the event, types of acceptable but

repairable damages of various components and extent of the same, when combined with other loads in

combinations as per IRC: 6-2000.

Since it is not economical to design bridges to remain fully functional in the event of MCE, an overall

balance is sought between the safety in DBE event, limiting risk of damage to the bridge due to

earthquake and consequential indirect economic loss, on one hand; and the cost of repair, temporary

diversion, or complete reconstruction and time involved in partial or full closure of the bridge at higher

level of earthquake, on the other hand. For the present, this balance is necessarily based on the overall

engineering judgment.

In application of these concepts (in order to keep the cost and time of design effort within practical limits),

simpliedprescriptiverulesaretobegivenbytheCodesofPractice,coveringnormaltypesofstructures.This is achieved by :-

Keeping the analytical and design efforts within the capabilities of normally available design tools,

and

Giving prescriptive detailing recommendations to achieve enhanced behavior of the bridge, based on

the national and international experience of major seismic events.

These simple rules should be reviewed from time to time to keep pace with the experience of using the rules

and the newly developing knowledge and methods.


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NOTIFICATION NO. 53


2.3IdenticationofSeismicHazards

Although acceleration of various parts of the structure and resulting inertial forces is the main concern of force

based seismic design, other hazardous situations associated with seismic events do also arise. These need to be taken

into account. The code dealing with seismic design in an overall way has to cover these aspects. IRC: 6 deals with

LoadsandStressesaloneandhaslimitedcoverage.Itshould,however,includeneareldeffectsalso.Otherhazards

mentioned bellow will have to be covered elsewhere :

1. Liquefaction.

2. Landslide

3. Tsunami

4. Structures which unavoidably cross a seismically active fault zone and those exposed to danger of relative

permanent ground movement.

5. Flooding due to dam failure in upstream of the bridge

3 CHOICE OF DESIGN FORCES3.1 Methods and Some Observations

1) DesignEarthquake.

IS: 1893 (Part1) divides the country in four zones. The basic philosophy and approach for zoning is best described

inForewordofIS:1893(Part1)andreferenceismadetothesame.Briey,themaximumgroundacceleration

anywhereineachzoneisdescribedbytheZoneFactorZ.Thisaccelerationisindependentofthelocationof

structure in relation to the locations of seismically active faults.

Inanyseismiczone,exceptintheneareldregions,themaximumseismichazardofgroundaccelerationisdened

in terms of Maximum Considered Earthquake (MCE) in each zone (described by the Zone factor). Designing

bridges for this level of hazard and keeping them within elastic range is expensive. It is international practice to

allow formation of plastic deformations in the structure without reaching failure limit, and to repair the damaged

portions in case of MCE event. In many countries, more than one level of earthquake are used to achieve different

levelsofperformancesatthespeciedlevelsofearthquakes.Theperformancelevelsareusuallydenedtoavoid

completecollapse,tolimitdamagetospecicrepairableparts,ortohavehighercapacityfordeadloadandliveload

combination, but accept damage for dead load condition only and so on.

In IS: 1893, this is achieved by choosing a lower level of hazard for the design, termed as Design Basis Earthquake

(DBE), which the IS: 1893 has recommended as 50% of MCE. IRC: 6 follow the same method. To account for the

relative economic consequences of non-availability/failure of the structure, a factor termed as seismic Importance

Factor I is recommended, use of which effectively changes the level of seismic design forces (increases, if I >1), or

in other words, the risk of damage/failure in actual seismic event is reduced. The MCE is not used for any design

checks.

Internationally, Maximum Considered Earthquake (MCE) and Operating Basis Earthquake are based on the

statistical probability. As explained in Foreword of IS:1893, due to lack of adequate statistical data, this kind of

rationalization and achieving even a semblance of uniformity of risk levels for different structures in different zones

is not yet possible, and a simple reduction factor of 2 is used.

2)MaximumConsideredEarthquake(MCE)andDesignBasisEarthquake(DBE).

Although the seismic potentials of the zones as described by maximum considered earthquake (MCE) are large,


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NOTIFICATION NO. 53

114

only 50% of MCE acceleration has been recommended by IS: 1893 as design basis earthquake (DBE). This has

been chosen, as stated in IS: 1893, on the basis of accepting certain level of damage in the event of DBE, based on

the past experience of building damages (described by MSK-64 scale) and in view of achieving economy in design.

ForeventslargerthanZ/2,certainriskofsignicantdamageexistsandisaccepted.Thisapparentlyoversimplied

uniform treatment for all structures across the zone (by use of common value of Z/2 ) is corrected by means of amultiplyingfactorIwhichmodiesthedesignlevelofacceleration(andforces)fordifferenttypesofstructuresas

explainedabove.Part1,andotherpublishedpartsofIS:1893,recommendthevaluesofIfactorsvaryingfrom1to

2 for structures being used for different purposes. The choice seems to depend partly on the need of immediate post

earthquake availability and partly upon the consequential economic implications due to loss of service. Apparently,

although not stated explicitly, it seems to have been assumed that the loss of life will be avoided (or minimised) by

engineering design of the structures, as per the codal recommendations.

3) Acceptable Risks as per IRC: 6

The performance targets, or aims, of the seismic provisions of this code are :-

(a) to ensure that the bridge does not collapse under the action of design level of earthquake,

(b) its components may suffer minor or major damages depending upon the extent to which it enters in a plastic

deformation stage,

(c) damages to minor and replaceable elements like expansion joints, hand rails etc., are permitted.

(d) Serviceability of bridge can be restored after repairs,

(e) the increased cost to meet the above targeted performance is reasonable.

For some critical bridges, consequences of structure entering in plastic regions, such as residual deformations, or

damage extending to many members, or to inaccessible foundation elements, etc. will lead to long period of closure,

andvaryhighcostandtimeofrepair.Insuchcases,thelikelydamagesmaybedirectlyveriedbyspecialanalyses.

If found unacceptable, the design forces can be upgraded (by using higher I factor) to control the damage.

4) EstimationofDesignForcesActingonStructure

When subjected to ground motion described by Z/2, the accelerations and the inertial forces experienced by various

parts of the structure depend on their overall dynamic characteristics, which in turn depend on the distribution of

mass and stiffness of various components. Two methods of calculating the response are permitted by the code:

In the analysis, two methods are permitted depending on the complexity of the structure, as described bellow:

(a) The Elastic Seismic Acceleration Methodis more commonly used for bridge structures which are on straight

alignment and which have regular structural arrangement in each direction. The natural period of vibration for

each of the two (or three) directions is calculated for the rst (fundamental) mode of vibration. These periods are

used to calculate the acceleration A seen by the bridge as a whole, with help of the response spectra. The response

spectra expresses acceleration responses of single degree of freedom oscillators having different time periods of

natural vibration as a function of time period. The accelerations are expressed in non dimensional form (Sa/g) and

are normalized to the zero period ground acceleration, taken as 1. In this method the maximum acceleration (Ah)

seen by the mass of oscillator is given by [Z* /2] *[Sa/g], which is further modied by multiplying by the importance

factor I.

(b) The Elastic Response Spectrum Methodwhich is a general method suitable for more complex structural

systems; such as continuous bridges, bridge with large difference in pier heights, and bridges which are curved in

plan.


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NOTIFICATION NO. 53


(i) In the rigorous method, the bridge structure is modeled as a multiple degree of freedom system consisting of

lumped masses connected by mass-less members characterized by their elastic stiffness for bending, shear

and axial forces. The structure is analysed for obtaining its response in different modes of vibration, which are

combined giving appropriate weights to participating modes. This is repeated for other directions of motion

andsuitablecombinationalrulesareusedtoobtaintheintegratedresponsefordesignverications.The response obtained is in elastic domain and is used directly in the design if the structure is designed to remain

within elastic limits, (below yield).

(ii) Asimpliedanalysisispermittedforregularbridgeswithsimplefoundations,moreorlessuniformpiersand

beamtypesuperstructure.ThisiscalledSeismicCoefcientMethodinwhichasinglecoefcientAh

is used

toconvertstructuresmassintohorizontalseismicforce.CoefcientAhis based upon the fundamental natural

period of the structure as a whole, and the response spectrum, as in case of the more rigorous method.

5) ResponseReductionFactor

(a) Designing bridges to remain elastic at MCE level is not economical. It is expensive even at the DBE level, as

compared to the designs based on methods recommended by IS: 1893-1984. IS: 1893 2002 has consideredit adequate to ensure that at chosen design level Z /2, the structure is subjected to minor damage, but can be

allowedtoreachyieldatloadfactorof1.2,yieldbeingdenedbythecodalmethodsofassessment(concrete/

steel codes), using characteristic strengths of materials and the partial material factors as per the relevant

codes. In the working load/allowable stress method, as followed presently by IRC, the basic aim remains

the same as that of IS: 1893, but without the load factor of 1.2. In order to permit plastic deformation of the

structure, and verifying the same without using non linear analysis, a method of Response reduction factor is

used, basis of which is explained below :

(b) Fig. 2 shows actual non linear response of the structure, its

idealized bi-linear response, and the fully elastic response assuming

thatthestructureremainselastictillfailure.maxand

erepresent

the maximum displacements of inelastic and elastic systems, whichare assumed to be about equal. The research shows this assumption

to be reasonable for moderate to long period structures. The elastic

ultimate moment Me(forces in general) is obtained by performing

elastic analysis and is reduced by dividing by response reduction

factorR,toobtainMb. M

bis then used in design of the structure

using linear analysis and combining results with moments (forces

in general) resulting from other loads. In effect, the seismic force

consideredintheanalysisisreducedfrom[Z.I/2]*[Sa/g] arrived in

3(a)toalowervalueof[Z.I/2R]*[Sa/g], which is the familiar codal

expression for Ah.

(c) Choice of response reduction factors

The choice of R factor depends on the performance

objectives of the bridge. If one intends to keep the bridge

within more or less elastic limits, a value of 1 is indicated.

If full plasticity is to be exploited, large value is chosen. If

partial plastic development is preferred, leaving margins for

uncertainties, or for a larger seismic event, some intermediate

Fig.2.Elasticandinelasticforcedeformation

relationships (ATC/MCEER 2001).


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NOTIFICATION NO. 53

116

value of Rischosen.Thechoiceisalsoinuencedbytheextentofbuilt-inindeterminancy,whichleadsto

extra(reserve)capacityofdeformationafterdevelopmentoftherstplastichingebeforecompletecollapse

takes place. The overstrength of the member is another margin to be kept in mind while choosing the value

of R factor. Overstrength is the actual extra built-in strength over and above the design strength, arising out

of the conservative estimation of material strengths and detailing practices. Correct calculation of R factoris a complex issue. Much research has gone in this and more is needed. While choosing R factors for Indian

conditions on the basis of international codes and practices, the fundamental differences between design

philosophies, reliability of data base, and more importantly, the performance targets should be carefully

considered.

6) Considerationofdepthofscourandcombinationwiththeaverageyearlymaximumood.

The present practice of considering 0.9 times maximum scour depth for seismic checks is rational, and is recommended

tobecontinued.Thelogicofnotreducingscourdepthfurtherisbasedonthefactthatthescourholeslledup

duringrecedingoodsarelledwiththeloosedepositsandcannotbereliedupontoprovidelateralsupportagainst

large earthquake forces.

Therecommendationtoconsiderdesignlevelofearthquakewithmaximumaverageyearlyoodistoprovideforarare,butreasonable,combinationandavoidcombiningtwoextremelyrareeventsofhighoodof100ormore

years of return period and earthquake.

7) RecommendationsofDraftofIS:1893(Part3)2005onBridges

Draft of the above code which was under discussion in BIS committee was made available by Prof.Thakkar. This

code generally follows the philosophy of IS:1893 (Part-I) and is similar to the presently proposed IRC:6 clauses,

with deviations in applicability of hydro-dynamic forces and earth pressure forces.. IS recommends the same for all

zones,whereasproposedclausesofIRC:6limittheirapplicationinZonesIVandV,andinneareldregions.This

is based on the more or less satisfactory performance of bridges in Zones II and III in the past.

4 PAST PERFORMANCE OF BRIDGES

On the basis of past experience of last 50 years or so of earthquakes, which has been well documented, it can be seenthat very few bridges have collapsed under action of earthquake. In fact, major damages have been in the region of

bearings, dislodgement of superstructure, damage to expansion joints, handrails etc. In very few cases, foundations

have been damaged in the regions of severe soil disturbance, such as liquefaction, and displacement of soil mass

just below the foundations. By and large, such situations are exceptional and highly localised. These can be avoided

withproperidenticationofseismichazards.Briey,it canbestatedthat,in spiteofhavingbeendesignedfor

the lower seismic forces than those presently proposed in IRC: 6, the structures have generally performed well

requiring attention to mainly bearings, dislodgement of superstructure etc.

Two or three reasons could be behind this satisfactory performance. First, most of the bridges have been designed

usingstaticequivalentforceswithoutconsiderationsofexibilityandlongperiodofvibration.Thisresultedinextra

built-in strength for bridges with tall piers and long spans. For medium to small span bridges, the seismic design

forces had been underestimated, but they have also survived. This could be because of built-in margin obtained bycombiningwatercurrentforcesathighoodlevelswithearthquakeforces,andtheseoodsbeingabsentwhenlarge

earthquakes took place.

However, in spite of the above considerations, a large gap still exists between the recommended higher forces by the

present IS:1893/IRC: 6 and unexplained but satisfactory performance of the existing bridges, especially that of the

foundations. Therefore, while accepting more up-to-date knowledge about seismisity of the Indian sub-continent

andadoptingnewscienticmethods,itisnecessarytogiveduecredencetosatisfactoryperformanceofthebridges

designed earlier for lower forces.

Documents

Notification 53