irs br cs with comments

8/2/2019 IRS BR CS With Comments

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PROPOSED AMENDMENTS IN SEISMIC PROVISIONS OF IRS BRIDGE RULES

(This paragraph on Seismic Provisions has been drafted to keep it in line with the existing

paragraph 2.12 of IRS Bridge Rules. It contains only those seismic provisions which are

directly be associated to bridge loading. For inclusion of other aspects of seismic analysis and

design, necessary amendments in the Substructure Code, Concrete and Steel Bridge Codes etc

should be incorporated simultaneously. Short explanatory notes are shown in RED)

2.12 Forces and Effects due to Earthquakes

2.12.1 Definitions : The following definitions shall apply in the context of seismic forces andseismic design of bridges :

(a) Critical Damping : The minimum damping above which free vibratory motion of astructural system ceases to be oscillatory.

(b) Damping : The energy dissipating property of the structural system manifested bymaterial and structural characteristics like internal friction, imperfect elasticity,

slipping and sliding, etc., which reduces the amplitude of vibration. The damping in a

structure is expressed as a percentage of the critical damping.

(c) Design Basis Earthquake : The earthquake which a structure can reasonably beexpected to encounter at least once during its life time.

(d) Design Seismic Force : It is the force which the bridge component is expected toresist during the seismic motion of the bridge structure within its ductile capacity forinelastic deformations. Apart from the level of seismic vibration, the design seismic

force also depends on the structural configuration, ductility detailing, member

flexibility and the resultant structural capacity for inelastic deformations under

seismic vibrations.

(e) Elastic Seismic Force : It is the member force generated in a perfectly elasticstructural member representing the bridge component during the actual seismic

motion of the perfectly elastic structure.(f) Horizontal Acceleration Coefficient (h) : It is the seismic coefficient that is used to

obtain the horizontal seismic inertial forces which act on different nodal points of the

structure during seismic motion. It is expressed in terms of spectral acceleration

(Sa/g), which is a function of natural period of vibration of the structure, and the

seismic zone factor.

(g) Importance Factor (I) : This is a factor used to scale up the design seismicacceleration depending upon the hazardous consequences to the life and property dueto failure of structure during seismic activity.

(h) Maximum Credible Earthquake (MCE) : Maximum credible earthquake is the largestreasonably conceivable earthquake that appears possible along a recognized fault or

within a tectonic province.

(i) Natural Period : Natural period of a structure is its time period for the undamped freevibration in a normal mode due to dynamic excitation. The free vibration modes (and

therefore natural periods) represent the inherent vibration properties of thestructural system.

Fundamental Period (T) of a structural system is the natural period corresponding

to the first or fundamental mode of vibration.

(j) Normal Mode of Vibration : A system is said to be vibrating in normal or principalmode of vibration when all its nodal masses attain their maximum amplitudes

(displacements) as well as equilibrium position (zero amplitude) simultaneously

during the free vibration.(k) Response Spectrum : Response spectrum is the graphical representation of the peak

response (spectral displacement, spectral velocity and spectral acceleration) of a

series of idealized single degree of freedom (SDOF) systems having different natural

periods (T) and damping during the seismic motion. The maximum response of these


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SDOF systems for various damping values is plotted against the undamped naturalperiod of these systems. In these provisions, the response spectrum represents the

spectral acceleration plotted against fundamental period of the structure.

(l) Response Modification Factor (R) : The response modification factor for a structuralmember represents the ratio of the elastic seismic force generated in a perfectly elastic

structural member under seismic motion and the design seismic force for which the

structural member is actually designed.

(m) Spectral Acceleration (Sa/g) : It is the average response acceleration which isderived from the Response Spectrum curve and depends on the natural period anddamping of the structure. Spectral acceleration along with the Zone factor determines

the basic horizontal seismic coefficient (o) exerted by Design Basis Earthquake.Design Basis Earthquake is the earthquake which can reasonably be expected to occur

at least once during the service life of structure.

(n) Seismic Weight : The seismic weight at a nodal point is that part of the dead loadcombined with the appropriate portion of the live load which is lumped at the nodalpoint and participates in seismic motion. Seismic mass is the seismic weight divided

by acceleration due to gravity.

(o) Zone Factor (Z) : The zone factors are the scaling factors assigned to different seismiczones of the country to account for the different levels of relative seismicity andperceived seismic risk to the structures located in a particular seismic zone. In effect,

the zone factor facilitates in adjusting the level of seismic activity in different zones of

the country while specifying a common acceleration response spectrum for the entirecountry.

2.12.2 Seismic Design Philosophy :

2.12.2.1 Unlike the common loads acting on a structure, which can be determined to a large

degree of certainty, the seismic loads originate from release of vast amounts of lockedup energy beneath the earth surface. Apart from the uncertainty in the input ground

motion resulting from the complexity of the seismic phenomenon at source and the

variations in the soil medium from source to the earth surface, the dynamic

characteristics of the structural system also contribute in amplification or de-amplification of the structural response. The response of the structure under such loadcan be improved only by increase in the strength and deformability of the structure.

2.12.2.2 The seismic design for the maximum credible earthquake (MCE) resulting from thestrongest seismic shocks at the site of the structure is considered unreasonable and

economically unacceptable since such shocks, though probable, are quite rare. Theseismic design, therefore, aims at catering to the design basis earthquakes (DBE) which

is smaller than MCE but can be expected to occur at least once during the life time of thestructure.

2.12.2.3 The seismic design philosophy outlined in these provisions accepts the probability of

occurrence of local and significant damages to some structural members during themedium and large seismic events but aims at preventing the collapse of the structure.

The main objectives of the seismic design of railway bridges are threefold :

(a) bridges should be able to perform its function of maintaining communications

without any significant structural damage during small earthquakes,

(b) bridges should be able to perform its function of maintaining communications

without significant disruptions to traffic during medium earthquakes. The bridge maysuffer limited structural damage but permanent repair should be feasible to restore the

design capacity, and

(c) bridges shouldpreserve the structural integrity, keep the local structural damages

to the minimum possible level and prevent collapse of the structure during largeearthquakes.


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2.12.3 Seismic Ground Motion :

2.12.3.1 The hazardous effectsof the uncontrolled release of vast amount of locked up energy

during the earthquakes are transmitted to the ground through seismic waves in theform of random and complex ground motion. The structures respond to the ground

motion in the form of structural vibration and consequent cyclic deformations. Theresponse of the structures often exceeds the input ground motion amplified by the

duration and frequency content of the seismic ground motion, soil properties at siteand dynamic characteristics of the structure.

2.12.3.2 The seismic ground motion can be resolved in three mutually perpendicularcomponents. The predominant direction of seismic vibrations is usually in the

horizontal plane and may be oriented in any direction. Both horizontal and verticalseismic forces due to seismic motion have to be taken into account for design of

different components of the bridge structure. Horizontal force in each of the two

principal directions shall be considered separately with the vertical force in accordance

with the combination rules given in clause 2.12.6.

2.12.3.3 The vertical seismic motion is particularly important in large span bridges, bridges withcantilever construction, prestressed concrete bridges and the structures where stability

is a criterion for design. In all such cases, the effect of increase/decrease in the gravityforce should particularly be investigated.

(Clauses 2.12.2 and 2.12.2.1 have been adopted from IRS Substructure Code with minor

editorial changes only. Provision of Clause 2.12.2.2 has been adopted from clause 6.1.1 of

IS 1893 Pt 1 : 2002)

2.12.4 Methods of Seismic Analysis : The following methods are generally employed for

seismic analysis of structures :

(a) Seismic Coefficient Method

(b) Response Spectrum Method

(c) Time History Method(d) Pushover Analysis

The seismic coefficient method is the simplest method of seismic analysis. It uses the

fundamental period of vibration of the bridge structure to derive the design

acceleration from the response spectrum curve. The seismic coefficient method willgenerally be sufficient for computing seismic forces in regular railway bridges.

Response spectrum method based on modal analysis shall be necessary forcomputation of seismic forces for bridges covered in clause 2.12.8. The time history

analysis may be used to determine the response of structure if the site specificearthquake records have been developed for this purpose. The pushover analysis can

be used to determine inelastic deformation capacity of the structure under lateral loads

with the use of a standard software package.

(Regular simply supported railway bridges have simplest structural configuration where

seismic coefficient method will give sufficiently accurate seismic forces, hence this

provision)

2.12.4 Seismic Zones : Based on the known magnitudes and known epicentres of the past

damaging earthquakes and other geological parameters, the country has been dividedin four seismic zones (zone II, III, IV & V) as shown in Appendix XV. For the purposes of

design, the zone factors,Z, for the different seismic zones shall be taken as under :

Zone Factors for Horizontal Seismic Coefficient

Seismic Zone II III IV V

Z 0.10 0.16 0.24 0.36

Even though the peak ground accelerations in different seismic zones cannot bepredicted with sufficient accuracy either on deterministic or probabilistic basis at


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present, the seismic zone factors tabulated above are a reasonable estimate of theeffective peak ground acceleration generated by severest seismic activity (MCE) in

different seismic zones.

(Taken from Table 2 of IS 1893 Pt 1 : 2002. Since a common response spectrum has been

adopted for the entire country, zone factors have been used to differentiate between the

relative seismicity of different seismic zones.)

2.12.5 The Elastic Response Spectra : The mostcommon way of describing seismic activityis to specify the acceleration response spectrum of the ground motion. The seismic

response of structures depends on the magnitude, duration and harmonic content ofthe ground motion, the nature of the soil deposits at site and the fundamental period

and damping of the structure. For design purposes, the acceleration response spectrumfor three different types of soil strata is specified as shown in Figure 1 below. The effect

of the local soil conditions is also included in the acceleration spectrum for three types

of soil sites, i.e., hard soils, medium soils and soft soils. The spectral acceleration

coefficient obtained from figure 1 is the elastic response of the structure and shall besuitably factored by Zone factor, Importance factor and response modification factor.

2.12.6 Design Seismic Coefficients :

2.12.6.1 Horizontal Seismic Coefficient : The horizontal seismic coefficient to be used in the

seismic analysis in seismic coefficient method shall be computed as below :

h=

where, Z = seismic zone factor and

= Spectral acceleration coefficient for 5% damping to be calculatedfrom clause 2.4.12.2.

The value of h shall not be take less than Z/4 irrespective of the fundamental

period of the structure.

(IS 1893 (Pt I) : 2002, in clause 6.4.2, has specifiedh = and the R values for

building structural systems have been tabulated with a rider that I/R shall not be greater

than 1. In the draft of Part 3, however, the situation is rather ambiguous. Even though R

values for different structural components have been specified, they are of no help in

deriving R for the structural model of the bridge since it does not specify how R is to be

chosen for finding h. IITK Guidelines has specified h =

and R has been

subsequently used to derive the member design forces. In the clause proposed above,I/R

factor has been completely dissociated from h and is associated with derivation of design

force in the structural members in clause 2.12.5.2. Since the importance factor, I, and

Fig.1 : Elastic Response Spectra for Various Soil Types for 5% DampingSpectralAcceleration,

Sa/g


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response reduction factor, R, are somewhat compensatory in nature, it is prudent to use

them as I/R factor and not individual parameters to be used at different places.

The seismic design is not just about flexibility. Whereas the brittle and rigid structures are

generally undesirable, the seismic design philosophy now revolves around a judicious

blend of limited flexibility and greater lateral stiffness with a liberal overdose of ductility

in the structural members which enhances their capacity for inelastic deformations in the

extreme seismic events. Limiting condition of hZ/4 is aimed towards this objective. Thecoefficient Z/4 is roughly equivalent to the basic seismic coefficient provided in the

present IRS Bridge Rules which has withstood the test of time.)

2.12.6.2 For embedded portions of foundations at depths of 30m or more below the bed level,

horizontal seismic coefficient may be assumed to be h. Between the bed level and thedepth of 30m, horizontal seismic coefficient may be obtained by linear interpolation. In

scourable river beds, the seismic scour level corresponding to Mean Annual Flood may

be taken as bed level.

(The above provision is based on clause 6.4.4 of IS 1893 Pt 1 and notes appended to clause

2.12.3.3 of IRS Bridge Rules. Whereas the provision of IS 1893 appears to be relevant since

that is applicable to underground structures included in other parts of that Code, this

provision in IRS Bridge Rules seems tricky since the embedded parts of the bridge arepresumed to vibrate with same frequency (and hence same acceleration) as the parts

above ground to be consistent with free vibration analysis. The fundamental assumption

in free vibration analysis of a structure is that in any normal mode, all the nodal masses

vibrate with same frequency and hence same acceleration. Instead, the clause below in

blue is proposed)

In scourable river beds, where the seismic scour level is substantially below the normalbed level, the horizontal seismic coefficient may be assumed to be h at depths of 30m

or more below the normal bed level. Between the normal bed level and the depth of30m, horizontal seismic coefficient may be obtained by linear interpolation. The

seismic scour level corresponding to Mean Annual Flood may be taken as seismic scour

level. The normal bed level for this purpose may be assumed to be average bed levelcorresponding to Low Water Level.

[The motivation for this clause comes from The Seismic Design and Retrofit of Bridges byMJN Priestley & others (Page 73-74) and off course the IS clause mentioned above

extended in a different but probably more appropriate context. The issue of normal bed

level is open for discussion. ]

2.12.6.3 The Spectral Acceleration Coefficient for 5% damping for different soil types shall beobtained from the following expressions :

For rocky, or hard soil sites (Type I)

{

For medium soil sites (Type II)

{

For soft soil sites (Type III)

{

In the above expressions, T is the fundamental period of the structure. The Spectral

Acceleration Coefficient can also be directly obtained from the acceleration responsespectrum given in clause 2.12.4.

[The above provisions, along with acceleration spectra in clause 2.12.4, are directly

adopted from IS 1893 Pt 1. IITK guidelines suggest that between time period of 0 and 0.1,


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Spectral Acceleration Coefficient should be taken as 2.5 for fundamental mode and

variable for other modes. This appears to be a bit inconsistent. Moreover, it is to be kept in

view that the proposed spectrum is already flattened and widened at the peak from 0.1 to

0.40/0.55/0.67 second period to account for uncertainties. IITK guidelines also suggest a

constant coefficient of 0.33/0.45/0.56 for periods 3 sec. However, accelerationcoefficient reduces faster (proportional to 1/T2 instead of 1/T) beyond periods of 3

seconds. Instead, the minimum values of h have been specified in clause 2.12.5.2.]

2.12.6.4 Subsoil Types :

For the purposes of clause 2.12.5.4 above, the subsoil shall be treated as Type I(rockyor hard soil sites) when the subsoil profile consists of bed rock with or without the

overburden of less than 20 metres of very stiff cohesive soil (undrained shear strength

100 kPa) or very dense sand (average N value 30).

The subsoil shall be treated as Type III(soft soil sites) when the subsoil profile consistsof various soil deposits exceeding the depth tabulated below :

Soil TypeRepresentative Undrained Shear

Strength (USS)/N values

Minimum Depth of

Soil Deposit (m)

Cohesive Soil USS (kPa) N ValueSoft 12.5 - 25 - 20

Firm 25 - 50 - 25

Stiff 50 - 100 - 40

Very Stiff 100 - 200 - 60

Cohessionless Soil

Loose - 4 - 10 40

Medium - 10 - 30 45

Dense - 30 - 50 55

Very Dense - > 50 60

The soil deposits not falling under any of the above two soil sites shall be treated as

Type II(medium soil sites).

(The above provision has been adopted from Transit New Zealand Bridge Manual, clause

5.2.1 in slightly modified form.)

2.12.6.5 Generally, the damping of 5% shall be assumed in the design of bridges. The Elastic

Response Spectrum in Fig. 1 above has been plotted for 5% damping. In case of

damping lower/higher than 5%, the spectral acceleration coefficients obtained above

shall be multiplied by factors given in the Table below.

Multiplying Factors for Other Damping Percentages

Damping % 0 2 5 7 10 15 20Multiplying

Factor3.20 1.40 1.0 0.90 0.80 0.70 0.60

(There is no point specifying multiplying factors beyond damping values which we cannot

adopt in design)

2.12.6.6 Fundamental Period of Vibration (T) : The fundamental period of vibration, T, for the

substructure shall be calculated as under:

(a) Where the vibrating unit of substructure can be modelled as a uniform cantilever

resting on rigid foundations, the time period may be calculated from the following

formulae :

(i) Single column or wall type

T = 1.16 (Suitable figure can also be inserted here)


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W = Seismic weight from superstructure (including appropriate Live Load and noImpact Load as per clause 2.12.8.2) and pier cap plus one fourth the seismic

weight of cantilever column.

h = Height of cantilever column from foundation top to bottom of bearing.

E= Modulus of elasticity of substructure.

I = Net moment of inertia of cantilever column along the appropriate axis.

(ii) Multiple Column bents/portals connected by common Cap

T = 0.58 (Suitable figure can also be inserted here)W = Seismic weight from superstructure (including appropriate Live Load and no

Impact Load as per clause 2.12.8.2) and pier cap plus 0.40 time the seismic weightof columns.

h = Height of portal frame from foundation top to bottom of bearing.

E= Modulus of elasticity of substructure.

I= Net moment of inertia of one column along the appropriate axis.

n = Number of columns

(The above two formulae are derived from standard literature and are given here to

simplify calculations. The first is taken from Structural Dynamics by Mario Paz, Pages

188-190 and the second is derived from Fixed-fixed uniform beam with mass at centregiven in Harris Shock and Vibration Handbook, Page 1.12 annexed at the end of thisdocument. It may be noted that only 25% mass of cantilever column is to be lumped at

top instead of 80% recommended in IITK guidelines)

(b) Where the vibrating unit of substructure can be modelled as a single non-uniform

cantilever carrying the superstructure mass and resting on well/pile foundations, thefundamental period may be calculated from the following formula :

T =2 = Horizontal Displacement at the top of pier due to horizontal forces equivalent to

gravity loads of lumped masses at appropriate nodes in the structural model. Theelasticity of the structure and the foundation should be accounted for while evaluating

the displacement.

(There is some departure from IS 1893 and IITK guidelines as far as the value of is

concerned; but this in conformity with the standard literature.)

(c) In absence of detailed modelling of the soil foundation system of the embedded

portion of foundation by suitable lateral and vertical spring supports, the point offixity for the purpose of calculating the fundamental period of structure the point of

fixity of the cantilever may be assumed as under :

(i) Open/raft foundation : the top of open foundation.(ii) Pile foundation : two (four?) times pile diameter below the seismic scour level.

In case of buried pile caps in unscourable beds, top of pile cap may be assumedto be point of fixity.

(iii) Well foundation : Seismic scour level.(There are no such provisions in IS 1893 or IITK guidelines. Item (i) is implied in the

Explanatory Examples in the Guidelines. Embedded portions of well foundations are

generally massive and sufficiently rigid. So, fixity at bed level is reasonable. In case of

embedded pile caps also, the assumption is reasonable. In case of piles cantilevering fromthe scoured bed, it is presumed that soil will be disturbed in upper portion up to 2

diametres. In any case, these assumptions will provide conservative estimates of Time

Period.)

(d) Where the substructure cannot be idealised as a single cantilever pier model, the


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fundamental period may be calculated by free vibration analysis of an appropriatemodel of the bridge structure.

2.12.6.7 Vertical Seismic Coefficient : In seismic zone II and III, the effect of vertical seismicmotion may be omitted except for the special structural forms mentioned in clause

2.12.2.2. When effects due to vertical seismic motion are to be considered in the design,the vertical seismic coefficient may be taken as two third of the values of the horizontal

seismic motion derived from Clause 2.12.5.

2.12.6.8 The vertical seismic response of the bridge superstructure to seismic motion shall be

considered to actnon-concurrentlyto horizontal seismic response. The superstructureshall be designed to remain elastic under both positive and negative vertical

accelerations.

[The provisions in this clause have been adopted from clause 5.2.6 of Transit New Zealand

Bridge Manual (available on Web), and there is marked departure from IITK Guidelines

and draft IS 1893 Pt 3.]

2.12.6.9 For vertical seismic response of bridge superstructure, the fundamental period may beestimated by free vibration analysis of appropriate model of the superstructure. For

simply supported spans with fairly uniform flexural rigidity, the fundamental period for

vertical seismic response, Tv, may be estimated by using the expression = 0.20 where L is the effective span, W is the total seismic weight of the superstructure

including 50% live load without impact and EI is the gross flexural rigidity of the

superstructure.

2.12.6 Design Seismic Forces

2.12.6.1 The inertial forces acting on the structure due to mass of each component or portion

thereof shall be obtained as under :

Fi= Wmh(orv)

where, Fi = Seismic inertial force due to seismic mass to be applied at centre of massof the component for seismic analysis.

Wm = Weight of the seismic mass under consideration ignoring reduction due

to buoyancy.

h or v= Horizontal or vertical seismic coefficients as specified in clauses 2.12.4.1and 2.12.4.5 respectively.

2.12.6.2 The elastic seismic forces in the structural members may be calculated by applyingequivalent lateral forces calculated above at appropriate nodal mass points. Static

analysis may be done to calculate elastic member forces, Ve, in different components of

bridge structure.

2.12.6.3 Before combiningwith other member forces for different load combinations, the design

seismic forces Vin different components shall be computed by the following expression:

V= The values of Importance factor (I) and Response Modification Factor (R) shall be takenin accordance with clause 2.12.6.4 clause 2.12.6.5 respectively.

[The I/R factor has been used here instead of clause 2.12.4.1 to obtain design forces. The

partial load factors for these design loads should taken as 1.0 in both the serviceability as

well as ultimate limit states with appropriate load factors for other loads in the seismic

load combinations. New Zealand Bridge Manual, Page 3-18 & 3-19 (Combination 3A) and

AASHTO LRFD Bridge Design Specification, page 3-13 (Extreme Event-I). These

documents are available on Web.]

2.12.6.4 The Importance factor (I) shall be taken as 1.50 for important bridges, 1.20 for major

bridges and 1.0 for all other bridges. A bridge is classified as Important if it has a linearwaterway of 300 m or total waterway of 1000 sq. m. or more. A bridge is classified as


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Major if it has total waterway of 18.30m or more or an individual span of 12.20m ormore.

(There is departure from IITK guidelines as well as IS 1893. No differentiation is proposed

on the basis of route category since almost all stretches of Gr A&B routes have local

alternative routes but many of Gr D&E routes do not have.)

2.12.6.5 The Response Modification Factor (R) for various structural components of the bridge

shall be obtained from the values given in Table below.Response Modification Factor R for Bridge Components and Connections

STRUCTURAL COMPONENTS R

Superstructure 1.0

Substructure

RCC Piers with ductile detailing - Single Column, Wall Type

- Multiple Columns, Frame Type

2.5

3.25

RCC Piers without ductile detailing - Single Column, Wall Type

- Frame Type

2.0

2.5

Steel Framed Construction 2.5

Steel Framed Piers (with properly designed cross bracings) 3.5Masonry/PCC piers and abutments (unreinforced ) 1.5

RCC Abutment 2.0

Connections (including bearings and expansion joints)

Column or Piers to Capping Beam or Superstructure 1.0

Columns or Piers to Foundations (including with Well/Pile Caps) 1.0

Wells/Piles to Capping Beams 1.0

Bearings and Expansion joints 0.8

Foundations including Well/Pile Foundations 1.5

Notes:1. Masonry/PCC piers should be avoided in Seismic zone IV and V.2. For stability analysis of well foundations by conventional method, seismic forces

can be further reduced by a factor of 2.0 to take advantage of the higher damping.

3. Response reduction factor is not to be applied for the calculation of seismicdisplacements.

4. Simply supported superstructure shall be designed to function elastically during theseismic motion.

(This table has been compiled from various sources and there is lot to be discussed as yet.

However, R value of 3 for superstructure in draft IS 1893 Pt 3 and the value of 2 in IRC 6

are rather unconvincing.)

2.12.7 Combination of Seismic Forces :

2.12.7.1 The bridge as a whole and every part of it shall be designed and constructed to resistforces produced by horizontal and vertical seismic motion. For horizontal accelerationthe forces shall be calculated as the effect of seismic loads applied horizontally at the

centre of mass of the elements of the bridge structure into which it is conveniently

divided for the purpose of analysis. The forces shall be assumed to come from any

horizontal direction.

2.12.7.2 For the bridges in straight alignment, the two orthogonal horizontal directions are

usually the longitudinal and transverse directions of the bridge. For such bridgesanalysis shall be done for seismic forces in longitudinal and transverse direction

separately. The seismic force resultants (Bending Moment, Shear Force and Axial

Force) in the bridge components as obtained from the analysis in longitudinal and

transverse directions shall be considered separately.

2.12.7.3 When the lateral load resisting members of the bridge are not oriented along theprincipal horizontal directions of the bridge structure (for example, skew bridge piers

and abutments), the seismic analysis of the bridge can be carried out along the two


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principal directions separately for such members. The seismic force resultants(Bending Moment, Shear Force and Axial Force) in such components as obtained from

the analysis in two principal directions shall be considered separately.

The seismic force resultants along the two principal horizontal axes (x and y-axes) of

the cross section of such components can also be obtained from the followingprocedure :

(a) For principal x-direction of the member, the maximum of the following twocombinations : + 0.30 + 0.30

(b) For principal y-direction of the member, the maximum of the following two

combinations : + 0.30 + 0.30 where, and are the absolute values ofx- and y-components of seismicforce obtained from seismic analysis of the bridge in longitudinal direction;and and are the absolute values ofx- andy-components of seismic force obtained from seismic analysis of the bridge in transverse direction. Thexandyare the principal directions of the member cross section under consideration.Alternatively, the seismic force resultants can also be obtained by adopting the

squire root of sum of squires (SRSS) procedure, i.e., () () in r-direction.

2.12.7.4 When the vertical seismic component is also to be combined along with the horizontal

seismic components in a bridge member (not the superstructure), the resultant seismic

forces along the principal direction of the bridge component can be obtained bycombining the absolute values of the seismic force components resulting from the

seismic analysis in three principal directions in the manner given below. The seismic

force in member direction rshall be the maximum of the following values :

(i) + 0.30 + 0.30 (ii) + + 0.30 (iii) + 0.30 + where, and are seismic force components in principal direction r ofbridge member obtained from the seismic analysis of the bridge structure inlongitudinal, transverse and vertical directions respectively.

SRSS procedure may also be used to obtain the seismic force resultants, i.e.,

() () () in r-direction.(The combination rules proposed in this paragraph are essentially the same as given in

IITK Guidelines and draft IS 1893 Pt 3. However, this clause has been drafted with a view

to eliminate the degree of ambiguity present in the corresponding clauses of abovedocuments. The 100%-30% or SRSS procedure of combination is of the seismic force

components along a specific member direction and is intended to ensure that the bridge

component has adequate strength along any direction in which EQ may strike. It is not

intended to combine seismic effects striking simultaneously from two/three different

directions.)

2.12.8 Slab, Box/Pipe culverts and buried structures need not be designed for seismic forces.

Detailed seismic analysis is not needed for single span bridges up to 35m span

irrespective of the seismic zone. In lieu of rigorous seismic analysis, the connection

between superstructure and abutment should be designed in each horizontallyrestrained direction for zero period seismic force transmitted from superstructure.

Minimum support length shall also be ensured on each abutment for such single spanbridges.

(The buried structures have been added here on the basis of similar provisions in AASHTO

LRFD Specifications, Page 3-52 and other American codes. Many American codes -


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AASHTO Division 1-A, Seismic Design, clause 3.11 and SCDOT Seismic Design

Specifications, 2001 clause 3.1 do not specify any detailed seismic analysis for ALL single

span conventional bridges in all seismic zones except for connection design and minimum

seating width as stipulated above. In the provisions proposed above, on single span

bridges up to 35m span only have been excluded from detailed seismic analysis. The

reason appears to be the fact that abutments are locked in the adjacent approaches and

cannot vibrate independently to amplify the response.)

2.12.8.1 Subject to the provisions of clause 2.12.8 above, the design of super and sub-structures

of bridges in different seismic zones shall include the seismic forces as specified below:

Zones II & III Seismic forces shall be considered in case of bridges of overall-

length more than 60m or spans more than 15m.

Zone IV & V Seismic forces shall be considered for all spans.

2.12.8.2 Horizontal seismic force due to live load on the bridge shall be ignored when acting in

the direction of traffic but when acting in the direction perpendicular to traffic, this is to

be considered for 50 per cent of the design live load without any additional impactfactor.

(This clause does not imply that there will be no or half LL on the bridge during theseismic activity. Because of no fixity with superstructure and its free moving nature, the LL

will not participate in the structural vibration in longitudinal direction. In transverse

direction, the participation of LL in structural vibration has been assumed to be 50% due

to the vehicle suspension system acting as coupler between superstructure and the LL.)l

2.12.9 Modal analysis shall be necessary for the following cases in Zones IV and V:

(a) In the design of bridges of types such as cable stayed bridge, horizontally curved

girder bridge, reinforced concrete arch or steel arch bridge, and

(b) When the height of sub-structure from base of foundation to the top of the pier is

more than 30m or when the bridge span is more than 120m.

(c) In case of important bridges where there is possibility of amplification of verticalseismic coefficient.

2.12.10 If the bridgesreferred to in clause 2.12.9 above are located within 10 kms of a known

active fault, it would be preferable to develop minimum three site specific

accelerograms (in two orthogonal horizontal and the vertical directions) and carry outinelastic time history analysis of the structure.


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STRUCTURAL DYNAMICS MARIO PAZ 4th Edition (See Example 6.5)


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HARRIS SHOCK AND VIBRATION HANDBOOK 5th Edition1.12 CHAPTER ONE


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SEISMIC DESIGN AND RETROFIT OF BRIDGES Priestley & Others, 1996


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