Seismic Irs

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Emerging Seismic Design Criteria for Bridges and IR, A K Gupta

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Emerging Seismic Design Criteria for Bridges and Indian Railways

Anil Kumar Gupta1987 Batch IRSE

Director/ B&S/ CB-II, RDSO

SYNOPSIS

Seismic design of bridges has undergone major advancement in the last two decades. Our present IRScodal provisions rely on the IS: 1893:1984 provisions for seismic design and is based on seismiccoefficient method. Modal analysis is required for all important bridges but no guideline exist as tohow this would be carried out. The 2002 edition of IS: 1893 (Part-1) and proposed Drafts of IS: 1893(Part 3) and IRC 6 have brought new design approach based on international experience. But they too

fail to lay down adequate design criteria for carrying out seismic design of important bridges includingmodal analysis. There is an urgent need for developing seismic design criteria for bridges on IndianRailways based on all these developments. This paper deals with these emerging concepts with a viewto developing design criteria for Indian Railways. The paper would help railway engineers inunderstanding these concepts in better way.

1. INTRODUCTION

Existing seismic design practice on Indian Railways is based on considering horizontal seismic force as

a fixed 10-15% of the vibrating weight of the bridge structure and designing the structure on working

stress or limit state of serviceability method. An additional vertical seismic force is also considered

which is 50% of the horizontal seismic force. Provisions of seismic design in IRS Bridge Rules and

Substructures & Foundations Code are based on provisions of IS-1893: 1984. In keeping abreast of the

growth of knowledge in this field, the Earthquake Engineering Sectional Committee, CED 39 of

Bureau of Indian Standards decided to split 1893 in five parts, of which Part 1 covers general

provisions and buildings, Part 3 covers Bridges & retaining walls. Part 1 was published in 2002 and

Part 3 is still under deliberations of the Sub-committee, with the draft circulated in July 2003. The

issue has recently assumed such importance that even before Part 3 of IS: 1893 is finalized, Indian

Road Congress went ahead in drafting their own new IRC 6: November 2003, which is also under

deliberations of a committee.

Internationally there has been massive up gradation of knowledge and experience in Analysis, Design

and Retrofitting of existing bridges, based on the understanding of behavior of each of the bridge

components in recent earthquakes, for achieving a earthquake resistant structure. US Department of

Transportation came out with a Workshop Manual on Seismic Design of Highway Bridges as early as




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in 1970s, which covers the design approach now being proposed by the BIS and IRC. IIT Roorkee has

designed a short-term one-week course on seismic design of bridges and provides bound lecture notes

covering new seismic design approach. California Department of Transportation (CALTRANS) has

carried out extensive research and field studies in designing, constructing and retrofitting of highway

bridges. They came out with elaborate seismic design criteria in Feb. 2004.

These latest developments in earthquake engineering were noted by Railway Board and RDSO has

initiated a research project in association with IIT Kanpur. The project is aimed at creating research and

testing facilities at IIT, upgrading our IRS Bridge Codes, developing our own seismic design criteria

for bridges, making design commentaries and design examples, and training railway engineers. This

paper presents the emerging seismic design criteria for bridges based on above referred literature with

aim on creating awareness among railway engineers.

2.0 EXISTING DESIGN PHILOSOPHY ON INDIAN RAILWAYS

In India IS 1893 and IRC Section-6 provides specifications for earthquake resistant design. The

existing seismic design philosophy of IRS Codes is based on 1984 edition of IS code. It involves

computing seismic forces, that each component of a bridge would face during an earthquake at its

center of mass, and carry out elastic design of the structures based on working stress or limit states of

serviceability method. This method is known as seismic coefficient method. Basic Seismic

coefficients for each of the five seismic zones (I to V) are specified (0.01 to 0.08) and this is multiplied

with an importance factor (1 for ordinary bridge & 1.5 for important bridge) and a soil foundation

system factor (1 to 1.5) to get the horizontal seismic coefficient for a particular bridge. Ground motion

due to earthquake can be revolved in any three mutually perpendicular directions, but the predominant

direction of vibration is considered horizontal. The design vertical seismic coefficient is taken as half

of the design horizontal seismic coefficient discussed above. Seismic force on each component of bridge is the product of its mass and the horizontal/ vertical seismic coefficient. The horizontal force

could come from any direction but each of the two perpendicular horizontal forces is considered

separately with the vertical force. Consideration of seismic forces in design is restricted to bridges with

overall length more than 60m or spans more than 15m for Zone I to III whereas all bridges in Zone IV

and V. Slab box and pipe culverts are not needed to be designed for seismic forces.




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Above design philosophy is based on a generalization and attempts to add some extra forces on the

already available Dead Load and Live Load combinations. Hence, it does not consider extreme

earthquakes. Further, the allowable stresses in material and soil is correspondingly increased in

varying degree from 25-50%. Further, it maybe pointed out that during the expected maximum

intensity of earthquake in the various seismic zones, structures will be subjected to a bigger force.

Capacity of the structure in plastic range is relied upon for absorbing the kinetic energy imparted by the

earthquake. But its capacity is unquantified and unanalyzed. IS:1893-1984 leaves it to just ductile

detailing as per IS:4326-1976, ‘Code of Practice for Earthquake Resistant Design and Construction of

Buildings’. In 1993 a separate code IS 13920 was developed specifically dealing with ductile detailing

of reinforced concrete structures. IRS Substructure and Foundation Code does not talk of any ductility

detailing in construction.

3.0 RESPONSE SPECTRUM METHOD AS IN IS: 1893-1984

IRS Codes specifically say that Response Spectrum Method need not be used for computation of

seismic forces in railway bridges (Para 2.12.3.2 of Bridge Rules). But IS: 1893-1984 laid down

process for this method. Before proceeding on to explain new concepts of seismic design it is important

to know this method.

Spectrum of an earthquake is the representation of the maximum dynamic response of idealized single

degree freedom spring during an earthquake. The dynamic response could be any of absolute

acceleration, maximum relative velocity or maximum relative displacement. Such response depends on

the natural period of vibration and degree of damping available. For the purpose of design, IS Code

considers acceleration spectra as the most useful as they give the seismic force on a structure directly.

Ideally, the response spectrum of a structure would depend on the intensity of the earthquake, but for

use in design, average spectra as developed by Prof. G.W. Housner of USA, was used in IS Code asreproduced in figure 1.

This is a plot of average acceleration coefficient

versus natural period of vibration (in seconds) for 0-

20% damping. For calculating the design value of

horizontal seismic coefficient of a particular bridge

in a particular zone, the average acceleration

Figure 1: Response Spectrum as per IS




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coefficient is multiplied by coefficient of soil foundation system, importance factor and seismic zone

factor. The only difference between seismic coefficient method and Response Spectrum Method is that

later considers the natural period of vibration of a specific structure in finding out the dynamic

response. As no formula was provided in IS Code for determining this period for bridge components,

this might be one of the reasons of not adopting this method in IRS Codes. In any case, the difference

between outputs of horizontal seismic coefficients from the above-described two methods is hardly

significant for most bridges components.

4.0 NEED FOR NEW SEISMIC DESIGN CRITERIA

Bridges are the key to the communication link to the earthquake-affected areas in case of severe

earthquake. In the absence of this communication link the rescue, relief and rehabilitation measures

could be severely affected. The availability of communication link immediately after the earthquake

should be the design philosophy for earthquake resistant design. But this should not result in usual

designing of bridges for the severest of severe earthquakes likely to occur at a site, as this would be

unduly costly and may result in smaller infrastructure in the first place. Hence engineers around the

globe have tried to evolve design criteria, which meets the functional requirements yet it doesn’t result

in uneconomical design. New criteria take into account acceleration, frequency, displacement, inelastic

design, ductility, reinforcement detailing and collapse mechanism. Before this new criteria is explained,

let us first review nature of damage bridges have suffered in India and other countries.

4.1 Nature of Damages to Bridges Due to Earthquake

(a) Failure of Piers due to brittleness of materials like brick & plain concrete- Shimantogawa

Bridge in Nankai Earthquake Dec 21, 1946.

(b) Settlement or movement (horizontally) of soil at the base of foundation- Liquefaction of

soils. (Banyu Highway Bridge- Kanto Earthquake Sept 1923; Nehru Setu, Ahmedabad

2001).

(c) Complete collapse of spans of Masonry Arch Bridge- Koyna Earthquake 1967.

(d) Failure of girder sheets/bed blocks due to transmission of seismic forces through bolts of

bearings, severe stress concentration in the material around (Kaliabhomra Road Bridge in

Assam, 1988).

(e) Tilting of piers due to unequal settlement of foundation (Sonai Bridge, Cachar Earthquake

1984; Kaliabhomra Road Bridge in Assam, 1988). I cases observed, abutments pushed






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code recognizes that the response of a structure to ground vibrations is a combined function of the

nature of foundation soil, materials, form, size, mode of construction and the duration/ characteristics

of ground motion. Vertical seismic forces are considered significant in bridges with large spans, and

those elements in which stability is considered significant parameter. They require special attention in

prestressed or cantilevered beams, girders and slabs. Horizontal seismic forces in both the orthogonal

directions have been considered together in various load combinations, in certain types of bridges.

When responses from the three earthquakes components are to be considered, the response due to each

component is combined using the assumption that when maximum response from one component

occurs, the response from the other two components are 30% of their maximum, including variation in

signs.

Recent proposals in IS 1893 and IRC 6 provide for

computation of seismic forces based on zone factors

provided in table 2 of IS 1893 (Part 1): 2002. These

factors vary from 0.36 for zone V to 0.1 for zone II. These

reflect the Peak Ground Acceleration (PGA) for MCE in

the respective zones. It is multiplied by a factor 0.5 for

obtaining PGA for DBE. Structural response factors,

which is nothing but response spectra for 1g PGA, is

given figure 2 of the code for three types of soil; rock or hard rock, medium soil, soft soil. This is

reproduced in figure 2 here. This coefficient is about 12.5 times the average acceleration coefficient

given in 1984 edition of the code for the same damping. The coefficient shown here actually represents

the factor by which the acceleration of the structure would increase beyond the peak ground

acceleration. At a very low natural period of oscillation the seismic acceleration in the structure is same

as the peak ground acceleration, whereas for very large natural period of oscillation the acceleration

tends to become zero and the structure oscillates with the displacements equal to the ground

displacements. Different graphs are given for three types of soil (Rock/ hard soil, medium soil and soft

soil). These three graphs differ only for structural elements having natural period greater than 0.5

seconds. Such higher (12.5 times) acceleration coefficient is due to consideration of Maximum

Considered Earthquake in the design.

Figure 2: Design Response Spectrum (IS

1893: 2002)




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For obtaining the response factor for a particular bridge, first its natural oscillation period is determined

from following formula,

T = 2√δ ----- (5.1)

Where δ= horizontal displacement at the acting position of the inertial force of the superstructures

when the force corresponding to 80% of the weight of the substructure above ground surface for

seismic design and all weight of the superstructure portion supported by it is assumed to act in the

acting direction of inertial force. The elasticity of substructure and foundation should be accounted for

in evaluating the displacement. Above expression is for simply supported bridges where the vibration

unit of substructure can be idealized as a single cantilever pier carrying the superstructure mass, resting

on well, pile or open foundation. Alternatively, the fundamental natural period T1 (in seconds) of pier/

abutment of the bridge along a horizontal direction may be estimated by the following expression:

T1 = 2√W/1000F ----- (5.2)

W= Appropriate dead load of the superstructures, and live load in kN. F= Horizontal force in kN

required to be applied at the center of mass of superstructure for one mm horizontal deflection

at the pier/abutment along the considered direction of horizontal force.

Design horizontal seismic coefficient is calculated by multiplying the DBE PGA with the response

factor derived above and importance factor of the bridge, which is same as existing in IRS codes. This

when multiplied with the seismic mass of the structural component it gives the inertia force. Maximum

elastic force resultant at the chosen cross-section of that bridge component due to earthquake shaking

along the considered direction is then divided by the appropriate response reduction factor to get the

design seismic force resultant at that cross section. Table 1 gives the response reduction factor R for

various bridge components. R is a new concept introduced called ‘Response Reduction Factor’. The

definition provided in the code for this is, a factor by which the actual base shear force, which would be

generated if the structure were to remain elastic during its response to the Design Basis Earthquake,

shall be reduced to obtain the design lateral force. The factor for soil foundation system no longer

exists, as different graphs have been used for different types of soil. This wide variation represents the

acknowledgement of varying degree of plastic strength redundancy and over strength in bridge

components. The design acceleration vertical coefficient is taken as 2/3 of the horizontal coefficient.

Annexure A provides a flow chart for Seismic Coefficient (Equivalent Static Force) Method of design.

The code doesn’t elaborate on the relation between the response reduction factor and the ductility of a

member, and the method of achieving a higher ductility. It also doesn’t talk about deformations, the




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relation between deformation and seismic force, and demand and capacity of deformation of bridge

members.

The proposed draft part (III) is lacking in detail on; site-specific spectra, site-specific peak ground

acceleration, modal analysis, foundation structure interaction and principles of ductile designing.

Without addressing these aspects, seismic design of multi span, important and complex bridges cannot

be carried out. It is true that codes cannot lay down criteria for all such bridges but even for carrying

out special seismic design through consultants there is a need for criteria to be followed.

Table 1: Response Reduction Factors (IS 1893: 2002)

Some such concepts are explained in brief

while dealing with dynamic modal analysis.

6.0 DYNAMIC MODAL ANALYSIS

IR Bridge rules require modal analysis of

types of bridges laid down in para 2.12.8 (a)

& (b) for seismic zone IV and V. They cover

special types as; cable stayed, horizontally

curved, RCC arch, steel arch, bridges.

Further when the height of substructure from

the base of foundation to the top of the pier is

more than 30m or when the bridge span is

more than 120m, modal analysis should be

carried out. Further important bridges where

there is a possibility of amplification of

vertical seismic coefficient, modal analysis

should be done.

The dynamic response of bridge to

earthquake motion depends upon the

foundation-structure interaction, dynamic

characteristics of structure (frequencies,

Sl.

No.

Structure, Component or connection R

(i) Superstructure, RCC 3.0

(ii) Superstructure, steel, PSC 2.5

(iii) Substructure

(a) RCC piers with ductile

detailing, cantilever and walltype

2.5

(b) RCC piers w.o. ductiledetailing, cantilever and wall

type

2.0

(c) Masonry piers 1.5

(d) RCC framed construction in piers with ductile detailing

3.0

(e) Steel Framed piers 2.5

(f) Steel cantilever piers 2.0

(g) Steel trussed arch 2.5

(h) RCC arch 2.0

(i) Abutment of mass concrete,

masonry

2.0

(iv) Bearings 0.8

(v) Expansion joints and connections 0.8

(vi) Structure hinge 3.0

(vii) Stoppers in bearings and all types of foundations

1.0




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modes and damping) and characteristics of ground motion. Such analysis enables following

information, which is not possible by the equivalent static (seismic coefficient) approach:

(a) Forces developed in the various parts of structure considering dynamic effect of earthquake

(b) Displacements in various parts of structure, particularly at the level of bearings. Which are

susceptible to damage

(c) Forces developed in the foundation due to dynamic effect

Following steps are involved in dynamic analysis of simply supported girder type bridge on rocker and

roller bearings;

• Mathematical model of substructure

• Determination of dynamic characteristics of bridge substructure

• Design spectra for site

• Modal Analysis for dynamic response

While performing dynamic model analysis of bridges, the engineer is forced to make modeling

assumptions at the abutment supports and hinges, which lead directly to forces and deformations in

theses areas. Response spectra without using any reduction factor should be used to get a more realistic

picture of the actual deformations the system. Reduction to the design level can then be made

depending on the component under consideration. For example, an abutment key is more brittle than aductile column. This component then would require a much lower reduction factor (ie higher design

force). However, if it is determined that failure of the key would not contribute to a collapse condition,

it could be designed to fail before excessive forces reached the abutment. This refinement in the

arrangement of the criteria puts examination of collapse mechanisms, relative component importance,

system deformations and energy absorbing characteristics of each structural element in the hands of the

engineer.

7.1 Mathematical Model of Substructure:

The mathematical model consists of a lumped mass system of the structure. In the lumped mass system,

the distributed mass of the structure is lumped at discreet points and these masses are connected with

each other by mass less elastic segments. There should be enough number of lumped masses in the

model in order to represent the dominating frequencies (lower two or three) of the real structure. A

typical mathematical model of structure is shown in figure 6 below.




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Any lumped mass should consist of self weight, in filled

water, and virtual mass of water. The top mass should

include dead weight of superstructure if it is rocker

bearing. The mass should also include live load as per

IS:1893.

7.2 Design Spectra for Site:

Intensity of ground shaking during earthquakes and the

associated damage to structures are greatly influenced by local geologic and soil conditions. Two

methods have been adopted toward characterizing the influence of soil conditions on ground motions;

statistical procedures involving records from past earthquakes and Analytical Procedures involving

evaluation of the non-linear soil effect of on the upward propagation of shear waves from an underlying

rock formation.

Average acceleration spectra for different site

condition are shown in figure-4.

Analytical procedure of developing site specific

spectra involves; determination of the peak rock

acceleration at site, determination of dynamic

properties of the soil deposit like- shear wave

velocity layer thickness, unit weight of each

layer, ground water table, material type, shear

modulus and damping relationships for

increasing strain levels. Following relationship

is the most important,

Es = DS2

Where, Es = Dynamic elastic shear modulus, D = In situ density, S = Shear wave

velocity

The determination of the shear velocity is best performed in an undisturbed in-situ location and is

accomplished through a variety of geophysical exploration methods and equipments. The basic idea is

to generate, identify, isolate, and measure the time rate of travel of a shear wave from a given source to

Period,

Figure 4- Average Design Spectra by Moharz for 2%dam in

Figure 3: Substructure and its Mathematical Model




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various monitoring positions. Shear waves could be produced by hitting horizontally (for S wave) or

vertically ( for P waves) with a 15lb hammer to the ends of a 8” x 12” x 8’ wooden plank pressed by a

wheel of a vehicle to apply 1 to 2 psi pressure. Wave is captured, in a plastic tube drilled in ground,

through sensors at various depths. There are wave propagation programs, like ‘SHAKE’ developed in

1972 by seed and others, which is based on a one-dimensional model, which could be used efficiently

for computing the response of soil profile to vertically traveling shear waves.

An investigation of seismicity of the site is necessary to determine the epicenters of past earthquakes,

depth of focus, distance of most probable epicenter from the site and frequency characteristics of the

ground. The expected ground motion and design spectra is obtained as explained in para 2.3 and 2.4.

7.3 Modal Analysis

For structures that cannot be idealized as single degree of freedom systems, it is generally necessary to

perform a computer analysis of the dynamic response. When the modes and frequencies of the system

have been obtained, the modal responses for a design earthquake loading are determined for each mode

considering the participation for that mode. These responses are maximums and generally will not

occur simultaneous to the woodmen responses of other modes. It is, therefore necessary to combine the

various modal responses in some statistical manner in order to obtain a realistic value of the actualmaximum response of the total structure at a given location.

In the modal analysis of an important multi span river bridge (like Sone, Ganga, Brahmaputra etc.), the

main objectives from seismic view point should be (i) to determine the forces, moments and

displacements at various sections of the bridge, (ii) to determine the base and soil reactions under

seismic conditions, (iii) to estimate the displacements in the bearing of suspended span due to possible

out of phase motion of spans and (iv) to suggest how to achieve needed dynamic displacements at the

articulations and to restrain them from further displacements to avoid the falling of suspended spans.

7.4 Important Points to be Taken Care of in Getting Modal Analysis from

Design Consultants:

1) A brief introduction about the software being used along with its qualities,

should be provided.




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2) The software used should allow the user to define the structural problem in

familiar structural design terms.

3) Steps involved in the actual modal analysis should be clearly spelt out in the

report preferably this should be shown in a flow chart, highlighting the iterative

processes.

A typical flow chart is given in Annexure-B.

8.0 DUCTILE DESIGN AND REINFORCEMENT DETAILING

Elastic seismic design is carried out for a much lower seismic force than that caused by the DBE, and

the factor by which DBE seismic force is reduced to get such design force is known as response

reduction factor. Hence it is imperative that with DBE and lower seismic forces under moderate

earthquakes the structure would undergo deformations beyond its yield limit. This would be plastic

deformation. The ratio of Δmax, maximum displacement beyond the yield limit up to which the structure

retains the seismic loads and Δy, maximum displacement at yield limit is known as ductility of the

structure. This is required to be used in seismic design to avoid uneconomic structural sizes. If a

structure undergoes occasional deformation in this range, there would be small damages to it. Whereas

under an earthquake higher than DBE but lower than MCE, the structure might undergo severe damage

but total collapse could be avoided with a judicious design of the bridge system. This ductile approach

of seismic design of Indian codes require careful ductile detailing to account for equivalent response

reduction factor considered in the design.

Ideally bridge structures should be designed so that the earthquake energy will be dissipated by the

individual members acting in a ductile manner, avoiding brittle shear failures. This is, however, not

possible in all cases of bridge design, since some of the components may be have in a non-ductile

fashion. Since the ductility levels may vary for the individual components of a bridge, reduction of theelastic response spectrum for design may be somewhat misleading and may result in some members

being under designed. Hence, elastic design response spectrum should be used to predict the overall

structure response and then the ductile components should be designed to absorb the required energy.

The important aspect in designing is to predict how a bridge would actually behave during an

earthquake. Further, one must qualify ductility as either being available ductility or required ductility.

Another distinction must also be made between ductility of the section of an individual component of a

structure and the overall ductility of a structure. The seismic design involves matching of the available




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ductility with the required ductility for a particular R used in the design. Ductility also explains the

damping in the structure. A structure undergoing a cyclic loading with significant loss of energy in

plastic deformation would have a higher damping.

The criteria used for determining the required structure ductility factor is dependent on the period of the

structures. The following three period ranges and corresponding response reduction factors, R, are

generally used for design.

loadDesign

loadResponseElastic= R

Table 2: Relation Between R and Ductility

Period Range R Criteria

Short 1 Force

Long µ∆ Displacement

Intermediate 12 −∆µ Energy

For the short period structures, the response factor is 1 in the response spectrum, i.e. the force level

must be maintained, conserving force (acceleration); thus there is no reduction using an elastic analysis.

For the long period structures, the elasto-plastic displacements of a structure are assumed to be equal to

the elastic displacement, i.e. R = µ∆

For the intermediate period range, energy is conserved and the reduction is based on an equal energy

concept. This implies that the potential energy stored in the elastic system at maximum deflection is

equal to the energy in the elasto-plastic system at maximum deflection. This condition gives

12 −∆µ

From the above formulations, it is possible to determine the ductilities required for the intermediate and

long period structures for various desired response reduction factors.




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9.0 MAKING CONCRETE STRUCTURE DUCTILE

Concrete is known to be brittle material, i.e. it fails suddenly when subjected to load. But concrete can

be made ductile when confined by reinforcement. Figure 7 shows the behavior of unconfined and

confined concrete.

It can be seen that confinement not only increases the strength of concrete, but it tremendously

increases the ductility of concrete. The confinement of concrete is obtained by providing stirrups as

shown in figure 8. The stirrups should be hooked at 135o into core concrete; otherwise these stirrups

open up under force due to earthquake and the confining action is not available. Further, even with

confinement, RC members are sufficiently ductile in bending action only, but not in axial and shear

action. Therefore, we have to ensure that RC members should yield only in flexure and not in axial or

shear action. This can be ensured by designing the RC members in such a way that their shear and axial

load capacity is higher than their capacity in flexure. This is called ‘Capacity Design’. By suitable

selection of flexure, shear and bending capacity, a structure could be designed to behave in a particular

way. At the junction of pile cap and pier, a pier could be designed to intentional yield to ensure that

excessive shear is not generated to damage the foundation or cause collapse. Creation of such intention

locations is known as creating hinges at which structural member rotate plastically without losing

structural integrity. Figure 7 shows the

possible locations of such plastic hinges in

bridge piers

Such hinge locations should be specially

designed with additional stirrups for making

the concrete ductile. Further the stirrups,

circular and rectangular, should be specially

shaped to provided better anchorage in

concrete and prevent them from opening up.

Fig 5: Behavior of Confined/ Unconfined Concrete Fig6: Confining Concrete by hoops/ stirrups

Figure 7: Location of Plastic Hinges in Piers




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Some of these details are shown in Annexure C.

10.0 CONCLUSION

Seismic Design of bridges has undergone a major change in philosophy as well as detail. IS 1893 (Part

1) :2002, draft IS 1893 Part(3) and draft IRC 6 provides several such requirements which should be

followed in all new bridge designs on Indian Railways. But these requirements are for standard and

ordinary bridges, and seismic design of important and special bridges should be carried out on the basis

of good engineering practices explained briefly in this paper. There is a need for developing

comprehensive design criteria for Indian Railways, but till this is done, zonal railways could use the

design consultancy contracts for important bridges for developing a sound design practice taking help

of criteria explained in this paper. The flow charts at annexure A & B, and ductile detailing given at

Annexure C would help them in achieving this.



,

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Annexure-A

Flow Chart For Equivalent Static Force Method

Process

COMPONENTSADEQUATE

Yes

Assume A Structure

Select site dependent elastic

design response spectrum from fig.4

Idealize structure andfind natural period of first mode of vibration

Obtain response acceleration coefficient

Find resulting displacement and member forces

Adjust For Ductility And Risk

Adjust

componentforces

Load Factor And Group Loading

Complete details including ductiledetailing

Revise thestructure

No

Find equivalent static force in each component

Check ductility and displacement

Complete Design



,

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Inadequate

Yes

Assume a structure

Develop site-specific design response spectrum

Create dynamic model of structureSubstructure & Foundation

Model analysis for MCE acceleration to

find frequency, displacement & forces invarious modes

Adjustcomponentforces

Load Factor And Group Loading

Complete details including ductiledetailing

Revise thestructure

No

Geotectonic & geophysical site exploration, site specific MCE

Design Complete

Annexure-B

Flow Chart For dynamic Model Analysis

Adequate

Adjust design forces for MDE, R & I factors

Check ductility and displacement



,

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Annexure C

Typical Ductile Detailing

Figure a: End Zone Confinement in Wall Pier

Figure c: Rectangular Column Ties

Figure d: Confinement in Hollow Section

Figure e: Ties in Foundation Column Joint

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