economical design of earhquake resistant structure

7/31/2019 Economical Design of Earhquake Resistant Structure

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AMARDEEP KAUR

M.TECH(STRUCTURES)


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INTRODUCTION

With the occurrence of every major earthquake, there has been in thepast, almost a world-wide tendency to increase the capacity demand ofthe structure to counteract such events.

It is only in the last decade that new strategies have been successfully

developed to handle this problem economically. There is an increasing realization that apart from techniques for

improving ductility, the structural engineers tool-box should includeenergy-dissipating and energy-sharing devices and those that cancontrol the response of the system.

There have also been further advances on appropriate methods anddevices of preventing dislodgement or unseating of thesuperstructure in the event of severe ground shaking. How these ideascan be used in economical earthquake resistant design of bridges is thesubject of this paper


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PLASTIC HINGING AND DURABILITY

There is a marked difference in seismic design aspects of bridgesand buildings.

The reduced degree of indeterminacy of bridge structures leadsto reduced potential of dissipating energy and load

redistribution. In bridges, the superstructures (piers and abutments) are the

main structural elements provide resistance to seismic action.

This essentially means that the formation of plastic hinges orflexural yielding is allowed to occur in these elements during

severe shaking to bring down the lateral design forces toacceptable levels.

Since yielding would lead to damage, plastic hinging arelocalized by design at points accessible for inspection and repair,no plastic hinges are, of course, allowed to occur in the

foundations or in the bridge deck.


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Well-designed structures dissipate seismic energy by inelastic deformations inlocalized zones of selected members

PLASTIC HINGING AND DURABILITY


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Cont.. Ductile behavior is ensured by confinement of the concrete

compression zone lying in the plastic hinge region of thesub-structures.

Closely spaced horizontal hoops, restraining the main

vertical reinforcement bars of the substructure, areeffectively used for this purpose.

The main functions of the hoops and ties in thesubstructure can be summarized as follows:

Confining of concrete core so as to enhance concrete strengthsand to sustain higher compressive strains,

Restrain longitudinal reinforcement against buckling,

Provide shear resistance during flexure


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Confinement of column sections by

transverse and longitudinal reinforcement

A. Circular hoops orspiral

B. Rectangular hoops withcross ties


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D. Overlappingrectangular hoops

C. Rectangularoctagonal hoops


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E. Confinement bytransverse bars

F. Confinement bylongitudinal bars


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SUPERSTRUCTURE DISLODGEMENT PREVENTION

AND INTEGRAL BRIDGES

Bearings, in general, are comparatively fragile andbrittle elements

Usual bearings of various types (metallic, elastomeric,pot, etc.) can be designed to have the capacity ofsustaining lateral forces of about 25-30% of theirvertical load carrying capacity (IRC, 1999).

For larger lateral forces, as in the cases of Zones IV andV (IRC, 2000), it is more suitable and economical toprovide


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Resistance to these forces separately by some otherstructural element

In many earthquakes it was noticed that thesuperstructure was dislodged and had fallen onto theground or was damaged due to loss of support causedby large displacements of elastomeric bearings or dueto out-of-phase displacement of piers.




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Provide reaction blocks or other types of seismicrestrainers for preventing dislodgement ofsuperstructure at pier/abutment cap level,

Provide adequate support lengths for superstructure

on pier/abutment cap

Design and construct integral bridges whereby thesubstructure and superstructure can be mademonolithic


AND INTEGRAL BRIDGESTo counteract such failures the following counter

measures are suggested:


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Surajbari Old Bridge: Metallicbearings destroyed duringearthquake

Girder shifted in the longitudinal

direction with loss of seatingduring shaking


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Example of longitudinal seismicrestrainer for continuous bridges

Britannia Chowk Flyover:Elevation of restrained pier

Longitudinal seismic restrainer (vertical

elastomeric pad introduces damping tolongitudinal forces)


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Prevention of dislodgement

Longitudinal Tie Bars Holding-Down Bars


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Integral bridges need wide exposure because bearingsand expansion joints are elements that are of seriousconcern in earthquake-prone areas

As already mentioned, they also happen to be theweak points in bridge structures from the point of viewof strength, durability and maintenance

Their elimination in many types of bridges has now

become a distinct possibility. Whereas theemployment of advanced design techniques isessential, the construction could possibly becomesimpler and safer.




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cont... Integral bridges can be made earthquake-resistant

more conveniently than bridges with bearings.

Apart form obviating the necessity of providingseismic restrainers and/or wide support lengths for the

superstructure, the number of potential locations ofplastic hinges can be increased, and ductility of a highorder introduced into the system.

The associated problems of a deck with significant

skew (700) were overcome


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Flyover using Integral Bridge concept for Delhi Metro (the curved flyover

has 70 skew and has no bearings or expansion joints on piers/abutments;length: 115 m)

Kalkaiji Flyover: Integral construction, high durability, low

maintenance, increased safety during earthquakes


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BASE ISOLATION, ENERGY DISSIPATION AND

ELASTOMERIC BEARINGS The use of special devices that reduce the seismic forces

can be effectively utilized in the structure. By decouplingthe structure from seismic ground motions it is possible to

reduce the earthquake-induced forces in it. This can bedone in two ways:

Increase natural period of the structure by base isolation,

Increase damping of the system by energy-dissipatingdevices.

The central issues are to limit the seismic energy enteringinto the structure from the ground in the first place andthen to dissipate as much of it as possible by dampingdevices.


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Cont.. By incorporating a layer of discontinuity (such as a properly designed

elastomeric bearing) which has a low lateral stiffness as compared tothe structural elements above and below it, the natural period of thestructure with a fixed-base can be elongated substantially.

Increasing the natural period of the structure invariably results inincreased deformations. Such deformations need to be controlled sothat the resulting stiffness of the structure is appropriate to itsserviceability requirements.

Some devices incorporate features of both base isolation as well asenergy dissipation. Examples of such devices include high dampingrubber bearings (HDR) and lead rubber dampings (LRB) . It ishighlighted that usual elastomeric bearings designed in accordance

with IRC (1999) may not be suitable in this regard. Only standard devices having detailed experimental data of their

performance should be used


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Base isolation and energy dissipation (two in one)


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Energy sharing Sometimes it is advantageous that the seismic energyentering from the ground into the structure does not get

localized. Special devices exist which can avoid significantenergy accumulation and ensure its distribution to variousstructural elements.

Here, the idea is not to reduce the total seismic energyentering into the structure but to judiciously distribute itamongst all the designated resisting elements. Suchdevices go by the name of Shock Transmission Units(STUs).

As Structure A and Structure B move slowly relative to eachother, the fluid is able to migrate through narrow orificesfrom one side of the piston to the other. For rapidmovements (e.g., earthquakes) the transfer of fluid is notpossible thereby locking the piston to its cylinder. In suchcircumstances the device acts as a rigid link between

Structure A and Structure B.


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Cont In bridge structures the inertial force from the

superstructure can be transmitted to designated sub-structures.

In bridge structures the inertial force from thesuperstructure can be transmitted to designated sub-structures.

Application of STUs to a 1.0 km long bridge with

expansion joints only at the abutments and centralpier is shown in Figure wherein the seismic forces aretransmitted to three piers in each of the two halves ofthe structure.


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Shock Transmission Unit The principle


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NHAIs Ganga Bridge at Allahabad showingapplication of STUs


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CONCLUSION

There is scope after both passive control by prescribeddetailing procedures as well as active control byspecific devices for earthquake-resistant bridges. The

judicious use of these ideas can lead to economical andsafe bridge structures.


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REFERENCES

ISET Journal of Earthquake Technology, Paper No. 453,Vol. 42, No. 1, March 2005, pp. 13-20 by MaheshTandon

AASHTO (1999). AASHTO LRFD Bridge DesignSpecifications, 1999 Interim, American Association ofState Highway and Transportation Officials,Washington, D.C.

BIS (1993). IS 13920-1993: Indian Standard Code ofPractice for Ductile Detailing of Reinforced ConcreteStructures Subjected to Seismic Forces, Bureau ofIndian Standards, New Delhi

economical design of earhquake resistant structure

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