Chapter 14-Design of Structures With Seismic Isolation (1)

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Chapter 14

Design of Structures with Seismic Isolation

Ronald L Mayes, Ph.D.Consulting Engineer, Berkeley, California

Farzad Naeim, Ph.D., S.E.Vice President and Director of Research and Development, John A. Martin and Associates, Inc., Los Angeles, California

¨

Key words: Base Isolation, Damage Control, Design Examples, Damping, Earthquake Engineering, Energy Dissipation,Feasibility of Isolation, Friction Devices, High-Damping rubber bearings, IBC-2000, Lead-Rubber Bearings,New Construction, Preliminary Design, Response Spectrum Analysis, Seismic Isolation, SeismicRehabilitation, Static Analysis, Time-History Analysis.

Abstract: This chapter surveys the principles, benefits, and the feasibility of seismic isolation. The basic principles ofseismic isolation are introduced first. Contrary to a perception held by many engineers, neither the conceptof seismic isolation is new nor its application is necessarily complex. What is new is the availability ofrelatively new materials and devices worked to perfection over the last two decades and advances incomputational techniques now commonly in use by practicing engineers. Force-deflection characteristics otcommonly used isolation devices are introduced next followed by guidelines for evaluation of the feasibilityof seismic isolation as an alternative for a given project. The differences in approach to new constructionand rehabilitation of existing structures are highlighted. The building code provisions for seismic isolationare covered next. The very recently released year 2000 edition of the International Building Code (IBC-2000) takes a much more simple approach to seismic isolation than did its direct predecessor, the 1997edition of the Uniform Building Code (UBC-97). This is true even though the theory and objectivesimplemented in both of these codes are the same. The simplification is largely due to incorporation ofspectral hazard maps in IBC-2000. A very practical side-effect of this incorporation is elimination of near-fault factors from the design process simply because now they are explicitly contained in the map. In manycases, design according to the new IBC-200 requirements will result in smaller displacement and forcedemands on the isolation system and the structure above the isolation plane. This in terms mean that seismicisolation can be implemented much more economically than it was possible under UBC-97. The IBC-2000design provisions for seismic isolation are discussed in detail. A simple preliminary design procedure isprovided to aid engineers in initial sizing of the isolation devices. Several examples are provided to illustratethe practical application of the material covered in this chapter.

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14. Design of Structures with Seismic Isolation 725

14.1 INTRODUCTION

Because of today’s concern for liability,engineering innovations must be exhaustivelytested and analytically proven to a degreeunknown in the past. Early engineers wererespected for their ability to design from firstprinciples and produce designs that wereconceptually right even though analytical orlaboratory methods did not exist that wouldremove all doubt. For the most part, the greatearly engineers removed doubt by force of theirpersonality and confidence. They took risks thatwould be unthinkable today.

The field of seismic design is, as perhapsbenefits a subject directly concerned with bothlife safety and uncertainty, cautious and slow toinnovate. In practice, improved seismic designdoes not represent a market opportunitybecause seismic safety is generally taken forgranted. Like other code-dominated issues, andlike airplane safety, seismic safety has neverbeen much of a selling point. Money diverted toimprove seismic resistance is often seen as adetraction from more visible and enjoyableattributes.

Improvements in seismic safety, since aboutthe time of the San Francisco earthquake of1906, have been due primarily to acceptance ofever-increasing force levels to which buildingsmust be designed. Innovation has beenconfirmed to the development and acceptanceof economical structural systems that performreasonably well, accommodate architecturaldemands such as open exteriors and the absenceof interior walls, and enable materials such assteel and reinforced concrete to compete in themarketplace on near-equal terms.

The vocabulary of seismic design is limited.The choices for lateral resistance lie amongshear walls, braced frames, and moment-resistant frames. Over the years, these havebeen refined and their details developed, andmethods of analysis and modeling haveimproved and reduced uncertainty. But thebasic approach has not changed: construct aductile and/or strong building and attach itsecurely to the ground. This approach of arm

wrestling with nature is neither clever norsubtle, and it involves considerablecompromise.

Although codes have mandated steadilyincreasing force levels, in a severe earthquake abuilding, if it were to remain elastic, would stillencounter forces several times above itsdesigned capacity. This situation is quitedifferent from that for vertical forces, in whichsafety factors insure that actual forces will notexceed 50% of designed capacity unless aserious mistake has been made. For verticalforces, this is easy to do. But to achieve similarperformance for seismic forces, the structurewould be unacceptably expensive and itsarchitectural impact would be extreme. Thisdiscrepancy between seismic demand andcapacity is traditionally accommodated byreserve capacity, which includes uncalculatedadditional strength in the structure and often thecontribution of portions and exterior cladding tothe strength and stiffness of the building. Inaddition, the ability of materials such as steel todissipate energy by permanent deformation—which is called ductility—greatly reduces thelikelihood of total collapse.

Modern buildings contain extremelysensitive and costly equipment that havebecome vital in business, commerce, educationand health care. Electronically kept records areessential to the proper functioning of oursociety. These building contents frequently aremore costly and valuable than the buildingsthemselves. Furthermore, hospitals,communication and emergency centres, andpolice and fire stations must be operationalwhen needed most: immediately after anearthquake.

Conventional construction can cause veryhigh floor accelerations in stiff buildings andlarge interstory drifts in flexible structures.These two factors cause difficulties in insuringthe safety of the building components andcontents (Figure 14-1).

In the past decade, an alternative to thebrute-force to nature has finally reached thestage of more widespread application. Thisapproach is obvious and easily explainable at

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the cocktail-party level: why not detach thebuilding from the ground in such a way that theearthquake motions are not transmitted upthrough the building, or are at least greatlyreduced? This conceptually simple idea hasrequired much research to make it feasible, andonly with modern computerized analysis hasbecome possible. Application has depended onvery sophisticated materials research into bothnatural and composite materials in order toprovide the necessary performance.

Figure 14-1. Conventional Structure

This new concept, now generally termedseismic isolation, meets all the criteria for aclassic modern technological innovation.Imaginative advances in conceptual thinkingwere necessary, as were materials new to theindustry, and ideas have developedsimultaneously on a worldwide basis. But themethod threatens conventional and establisheddesign procedures, so the road to seismic-isolation innovation is paved with argument,head shaking, and bureaucratic caution—all, tosome extent, well-intentioned and necessary,given our litigious society.

Mounting buildings on an isolation systemwill prevent most of the horizontal movementof the ground from being transmitted to thebuildings. This results in a significant reductionin floor accelerations and interstory drifts,

thereby providing protection to the buildingcontents and components (Figure 14-2).

Figure 14-2. Base Isolated Structure

The principle of seismic isolation is tointroduce flexibility at the base of a structure inthe horizontal plane, while at the same timeintroducing damping elements to restrict theamplitude of the motion caused by theearthquake. The concept of isolating structuresfrom the damaging effects of earthquakes is notnew. The first patent for a seismic isolationscheme was taken out in 1909(14-1) and sincethat time several proposals with similarobjectives have been made (see References 14-2 to 14-8). Nevertheless, until the last twodecades, few structures have been designed andbuilt using these principles.

However, new impetus was given to theconcept of seismic isolation by the successfuldevelopment of mechanical-energy dissipatersand elastomers with high damping properties(see References 14-8 to 14-15). Mechanical-energy dissipaters, when used in combinationwith a flexible isolation device, can control theresponse of the structure by limitingdisplacements and forces, thereby significantlyimproving seismic performance. The seismicenergy is dissipated in components specificallydesigned for that purpose, relieving structuralelements, such as beams and columns, fromenergy-dissipation roles (and thus damage).There are over two hundred civil engineeringstructures that have now been constructed using


the principles of seismic isolation. Kelly(14-6),Buckle and Mayes(14-7) and Naeim and Kelly(14-8)

provide an excellent history of world overview.Other references containing overview materialare given in references 14-25 and 14-41.

The advantages of seismic isolation includethe ability to eliminate or very significantlyreduce structural and nonstructural damage, toenhance the safety of the building contents andarchitectural facades, and to reduce seismicdesign forces. These potential benefits aregreatest for stiff structures fixed rigidly to theground, such as low- and medium-risebuildings, nuclear power plants, bridges, andmany types of equipment. Some tectonic andsoil-foundation conditions may, however,preclude the use of seismic isolation.

14.1.1 An Idea Whose Time Has Come

The elastomeric bearing and the mechanicaldamper are fundamental components in manyseismic isolation schemes. But it is not just theinvention of the elastomeric bearing and theenergy dissipater which has made seismicisolation a practical reality. Three other parallel,but independent, developments have alsocontributed to its success.

The first of these was the development ofreliable software for the computer analysis ofstructures so as to predict their performance anddetermine design parameters. Work has been inprogress for more than 25 years on the softwarefor inelastic analysis of structural systems, andthere are many available programs. Applicationto seismically isolated structures isstraightforward, and correlation studies withmodel tests show many software systems to besoundly based.

The second development was the use ofshaking tables which are able to simulate theeffects of real recorded earthquake groundmotions on different types of structures. Theresults of shaking-table tests over the last 20years (see Reference 14-16 to 14-22 and 14-31to 14-40) have provided another mechanism toenhance confidence in the way buildingsrespond during real earthquakes. In addition,

the results provide an opportunity to validatecomputer modeling techniques which are thenused on full-size structures.

A third important development is in the skillof the engineering seismologist in estimatingground motions at a particular site. Recentadvances in seismology have given moreconfidence in site-specific ground motionswhich take into account fault distances, localand global geology, and return periods. Thesedesign motions are basic input to the computermodeling of seismically isolated systems andare a vital step in the estimation of systemperformance.

In summary then, five recent developmentsare together responsible for elevating seismicisolation from fantasy to practical reality:

The design and manufacture of high-qualityelastomeric (rubber) pads, frequently calledbearings, that are used to support the weight ofthe structure but at the same time protect it fromearthquake-induced forces.

The design and manufacture of mechanical-energy dissipaters (absorbers) and high-damping elastomers that are used to reduce themovement across the bearings to practical andacceptable levels and to resist wind loads.

The development and acceptance ofcomputer software for the analysis ofseismically isolated structures which includesnonlinear material properties and the time-varying nature of the earthquake loads.

The ability to perform shaking-table testsusing real recorded earthquake ground motionsto evaluate the performance of structures andprovide results to validate computer modelingtechniques.

The development and acceptance ofprocedures for estimating site-specificearthquake ground motions for different returnperiods.

14.2 CONSIDERATIONS FORSEISMIC ISOLATION

The need for seismic isolation of a structuremay arise if any of the following situationsapply:

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– Increased building safety and post-earthquake operability are desired.

– Reduced lateral design forces are desired.– Alternate forms of construction with limited

ductility capacity (such as precast concrete)are desired in an earthquake region.

– An existing structure is not currently safefor earthquake loads.For new structures current building codes

apply in all seismic zones, and therefore manydesigners may feel that the need for seismicisolation does not exist because the coderequirements can be satisfied by currentdesigns. Code designs, however, are generallycontrolled by a design philosophy whichproduces structures which are much more proneto damage than their seismic isolatedcounterparts. A typical building code statementof philosophy(14-23) states that buildingsdesigned in accordance with its provisions will– resist minor earthquakes without damage,– resist moderate earthquakes without

structural damage but with somenonstructural damage,

– resist major earthquakes without collapsebut with structural and nonstructuraldamage.These principles of performance also apply

to conventional buildings that are rehabilitatedto code-level design forces.

Seismic isolation promises the capability ofproviding a building with better performancecharacteristics than our current code approachtowards conventional buildings and thusrepresents a major step forward in the seismicdesign of civil engineering structures. In thecase of a building retrofit, the need for isolationmay be obvious: the structure may simply notbe safe in its present condition should anearthquake occur. In such cases, if seismicisolation is suitable, its effectiveness comparedwith alternative solutions such as strengtheningshould be examined.

14.2.1 Solutions for NonstructuralDamage

One of the more difficult issues to addressfrom a conventional design viewpoint is that ofreducing nonstructural and building-contentdamage. This is very often ignored, and whenaddressed, can be very expensive to incorporatein conventional design. In fact, the cost ofsatisfying the more stringent bracingrequirements of nonstructural elements in aCalifornia hospital is on the order of $2 to $4per square foot more than for ordinarycommercial buildings.

There are two primary mechanisms thatcause nonstructural damage. The first is relatedto interstory drift between floors, and thesecond to floor accelerations. Interstory drift isdefined as the relative displacement that occursbetween two floors divided by the story height.Floor accelerations are the absoluteaccelerations that occur as a result of theearthquake, and in conventional constructionthey generally increase up the height of thebuilding. Together, these two components causedamage to the building contents, architecturalfacades, partitions, piping and ductwork,ceilings, building equipment, and elevators(Figure 14-1).

Clearly, a design concept that reduces bothinterstory drifts and floor accelerationscombines the best aspects of these two currentdesign philosophies. Seismic isolation is such aconcept (Figure 14-2), since it can significantlyreduce both floor accelerations and interstorydrift and thus provide a viable economicsolution to the difficult problem of reducingnonstructural earthquake damage.

14.3 BASIC ELEMENTS OFSEISMIC ISOLATIONSYSTEMS

There are three basic elements in anypractical seismic isolation system. These are:


1. a flexible mounting so that the period ofvibration of the total system is lengthenedsufficiently to reduce the force response;

2. a damper or energy dissipater so that therelative deflections between building andground can be controlled to a practicaldesign level; and

3. a means of providing rigidity under low(service) load levels such as wind and minorearthquakes.Bridge structures have for a number of years

been supported on elastomeric bearings(14-24),and as a consequence have already beendesigned with a flexible mount. It is equallypossible to support buildings on elastomericbearings, and numerous examples exist wherebuildings have been successfully mounted onpads. To date this has been done more forvertical-vibration isolation rather than seismicprotection. Over 100 buildings in Europe andAustralia have been built on rubber bearings toisolate them from vertical vibrations fromsubway systems below, and are performing wellmore than 40 years after construction. Byincreasing the thickness of the bearing,additional flexibility and period shift can beattained.

While the introduction of lateral flexibilitymay be highly desirable, additional verticalflexibility is not. Vertical rigidity is maintainedby constructing the rubber bearing in layers andsandwiching steel shims between layers. Thesteel shims, which are bonded to each layer ofrubber, constrain lateral deformation of therubber under vertical load. This results invertical stiffness and of a similar order ofmagnitude to conventional building columns.

An elastomeric bearing is not the only meansof introducing flexibility into a structure, but itappears to be one of the most practicalapproaches. Other possible devices includerollers, friction slip plates, capable suspension,sleeved piles, and rocking (stepping)foundations (Figures 14-3 to 14-7). The mostpopular devices for seismic isolation ofbuildings in the United States are the lead-rubber bearings, high-damping rubber bearingsand the friction pendulum system (Figure 14-8).

Figure 14-3.Elastomeric bearings

Figure 14-4. Rollers

Figure 14-5. Sleeved Piles

Figure 14-6. Rocking

Figure 14-7. Cable Suspension

The reduction in force with increasing period(flexibility) is shown schematically in the force-response curve of Figure 14-9. Substantialreductions in base shear are possible if theperiod of vibration of the structure issignificantly lengthened.

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Figure 14-8. Most popular building isolation devices(Top: the high damping rubber device; Middle: the lead-rubber device; Bottom: the friction pendulum device).

The reduction in force response illustrated inFigure 14-9 is primarily dependent on thenature of the earthquake ground motion and theperiod of the fixed-base structure. Further, theadditional flexibility needed to lengthen theperiod of the structure will give rise to largerelative displacements across the flexiblemount. Figure 14-10 shows an idealizeddisplacement response curve from whichdisplacements are seen to increase withincreasing period (flexibility). However, asshown in Figure 14-11, if substantial additionaldamping can be introduced into the structure,the displacement problem can be controlled. Itis also seen that increasing the damping reducesthe forces at a given period and removes much

of the sensitivity to variations in ground motioncharacteristics, as indicated by the smootherforce response curves at higher damping levels.Care must be taken, however, not to induceexcessive damping into the system because thatcould produce story accelerations difficult topin down in an ordinary dynamic analysis.

Figure 14-9. Idealized force response spectrum

Figure 14-10. Idealized displacement response spectrum

Energy Dissipation One of the mosteffective means of providing a substantial levelof damping is through hysteretic energydissipation. The term “hysteric” refers to theoffset in the loading and unloading curvesunder cyclic loading. Work done during loadingis not completely recovered during unloading,and the difference is lost (dissipated) as heat.Figure 14-12 shows an idealized force-displacement loop, where the enclosed area is a


measure of the energy dissipated during onecycle of motion. Mechanical devices which usefriction or the plastic deformation of either mildsteel or lead to achieve this behavior have beendeveloped (14-9 to 14-14), and several mechanical-energy dissipation devices developed in NewZealand are shown in Figure 14-13.

Figure 14-11. Hysteretic force-deflection curve

Many engineering materials are hysteretic bynature, and all elastomers exhibit this propertyto some extent. By the addition of special-purpose fillers to elastomers, it is possible toincrease their natural hysteresis without undulyaffecting their mechanical properties(14-10). Sucha technique gives a useful source of damping,but so far it has not been possible to achieve thesame level of energy dissipation as is possible

with, say, a lead-rubber elastomeric bearing orsupplemental viscous dampers.

Friction is another source of energydissipation which is used to limit deflections.However, with the exception of the frictionpendulum system, it can be a difficult source toquantify. A further disadvantage is that mostfrictional devices are not self-centering, and apermanent offset between the sliding parts mayresult after an earthquake. The frictionpendulum system overcomes this problem byusing a curved rather than flat surface on whichthe friction occurs. In proportioning a lead-rubber system or a friction pendulum systemcare must be exercised in design to ensure thatthe restoring force during expected seismicevents would overcome the resistance of thedevice to self-centering. In practice it iscommon to compliment lead-rubber bearingswith ones without a lead core and this approachhas proved to be very successful.

Hydraulic damping has been usedsuccessfully in some bridges and a few special-purpose structures(14-7). Potentially highdamping forces are possible from viscous fluidflow, but maintenance requirements and highinitial cost have restricted the use of suchdevices.

Rigidity for low lateral loads and flexibilityfor high seismic loads is very desirable. It isclearly undesirable to have a structural system

Figure 14-12. Response spectra for increasing damping

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which will vibrate perceptibly under frequentlyoccurring loads such as minor earthquakes orwind loads.

Lead-rubber bearings, well designed highdamping rubber bearings, as well as othermechanical-energy dissipaters provide thedesired low load rigidity by virtue of their highelastic stiffness (Figure 14-14). Some other

seismic isolation systems require a windrestraint device for this purpose—typically arigid component designed to fail under a givenlevel of lateral load. This can result in a shockloading being transferred to the structure due tothe sudden loss of load in the restraint.Nonsymmetrical failure of such devices canalso introduce undesirable torsional effects in a

Figure 14-13. Various mechanical energy dissipaters


building. Further, such devices will need to bereplaced after each failure.

Table 14-1 summarizes the sources offlexibility that have been discussed above. Amore detailed explanation of these concepts canbe found in the proceedings of two workshopson base Isolation and Passive EnergyDissipation that have been conducted byApplied Technology Council(14-25 and 14-41) aswell as a recent textbook by Naeim andKelly(14-8).

Figure 14-14. Idealized force-displacement relationshipsfor isolation systems

Table 14-1. Alternative Sources of Flexibility and EnergyDissipation

Flexible Mounting SystemsUnreinforced rubber blocksElastomeric bearings(reinforced rubber blocks)Sliding platesRoller and / or ball bearingsSleeved pilesRocking systemsSuspended floorsAir cushionsSlinky springs

Damping Devices/ MechanismsPlastic deformation of a metalFrictionHigh-damping elastomersViscous fluid dampingTuned mass damping

14.4 FORCE-DEFLECTIONCHARACTERISTICS

Conceptually, there are four basic types offorce-deflection relationships for isolationsystems. These idealized relationships areshown in Figure 14-15, with each idealizedcurve having the same design displacement Dfor the design-level earthquake.

A linear isolation system is represented bycurve A and has the same isolated period for allearthquake load levels. In addition, the forcegenerated in the superstructure is directlyproportional to the displacement across theisolation system. A linear isolation system willrequire some form of wind-restrainingmechanism to be added to the system.

A hardening isolation system is representedby curve B. This system is soft initially (longeffective period) and then stiffness (effectiveperiod shortens) as the earthquake load levelincreases. When the earthquake load levelinduces displacements in excess of the designdisplacement in a hardening system, thesuperstructure is subjected to higher forces andthe isolation system to lower displacementsthan in a comparable linear system. Like alinear system, a hardening system will alsorequire some form of additional wind-restraining mechanism.

A softening isolation system is representedby curve C. This system is stiff initially (shorteffective period) and softens (effective periodlengthens) as the earthquake load level inducesdisplacements in excess of the designdisplacement in a softening system, thesuperstructure is subjected to lower forces andthe isolation system to higher displacementsthan in a comparable linear system. The highinitial stiffness of a softening system is thewind-restraining mechanism.

A flat sliding isolation system is representedby curve D. This system is governed by thefriction force of the isolation system. As in thesoftening system, the effective period lengthensas the earthquake load level increases, and theloads of the superstructure remain constant. Thedisplacement of the sliding isolation system

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after repeated earthquake cycles is highlydependent on the vibratory characteristics of theground motion and may exceed the designdisplacement. Consequently, minimum designrequirements do not adequately define the peakseismic displacement for seismic isolationsystems governed solely by friction forces. Thevalue of the coefficient must be high enough toresist the wind forces.

14.5 SEISMIC-ISOLATIONDESIGN PRINCIPLES

The design principles for seismic isolationare illustrated in Figure 14-16. The top curve ofthis figure shows the realistic forces based on a5% ground response spectrum which will beimposed on a non-isolated structure fromtypical code forces(14-28). The spectrum shown is

for a rock site if the structure has sufficientelastic strength to resist this level of load. Thelowest curve shows the forces which a typicalcode(14-28) requires a structure to be designedfor, and the second-lowest curve shows theprobable strength assuming the structure isdesigned for the corresponding code forces. Theprobable strength is typically about 1.5 to 2.0times higher than the design strength because ofthe design load factors, actual material strengthswhich are greater in practice than thoseassumed for design, conservatism in structuraldesign, and other factors. The differencebetween the maximum elastic force and theprobable yield strength is an approximateindication of the energy which must beabsorbed by ductility in the structural elements.

When a building is isolated, the maximumelastic forces are reduced considerably due toperiod shift and energy dissipation, as shown in

Figure 14-15. Design principles of seismic isolation


Figures 14-10 and 14-12. The elastic forces ona seismically isolated structure are shown bythe dashed curve in Figure 14-16. This curvecorresponds to a system with as high as 30%equivalent viscous damping.(14-29)

If a stiff building, with a fixed-basefundamental period of 1.0 sec or less, isisolated, then its fundamental period will beincreased into the 1.5- to 2.5-sec range (Figure14-10). This results in a reduced code designforce (Figure 14-16), but more importantly inthe 1.5- to 2.5-sec range the probable yieldstrength of the isolated building isapproximately the same as the maximum forcesto which it will be subjected. Therefore, therewill be little or no ductility demand on thestructural system, and the lateral design forcescan be theoretically reduced by approximately50%, if the building code permits such areduction.

14.6 FEASIBILITY OF SEISMICISOLATION

Structures are generally suitable for seismicisolation if the following conditions exist:– The subsoil does not produce a

predominance of long period ground motionsuch as that obtained in Mexico City.

– The structure has two stories or more (or isunusually heavy).

– The site permits horizontal displacements atthe base of the order of 8 in. or more.

– The structure is fairly squat.– Wind lateral loads and other non-earthquake

load are less than approximately 10% of theweight of the structure.Each project must be assessed individually

and early in the design phase to determine itssuitability for seismic isolation. For thisassessment, there are differences between newconstruction and the retrofit of the existingstructures. The following sections provide someguidelines for each of the situations.

14.6.1 New Construction

Structure The first consideration inassessing the suitability of a new project is thestructure itself. Seismic isolation achieves areduction in earthquake forces by lengtheningthe period of vibration at which the structureresponds to the earthquake motions. The mostsignificant benefits obtained from isolation arein structures for which the fundamental periodof vibration without base isolation is short—less than 1 sec. The natural period of a buildinggenerally increases with increasing height.Taller buildings reach a limit at which thenatural period is long enough to attract lowearthquake forces without isolation.

Therefore seismic isolation is mostapplicable to low-rise and medium-risebuildings and becomes less effective for high-rise ones. The cut-off depends mainly on thetype of framing system. Shear-wall structuresand braced-frame structures are generally stifferthan moment frames of equivalent height, andso, for shear walls and braced frames isolationmay be effective up to 12 to 15 stories, whereaswith moment frames the cut-off is generallyabout 8 to 10 stories. These numbers are onlygeneralizations and there are, of course,exceptions, as discussed to the retrofits of the19-story Oakland City Hall and the 28-storyLos Angeles City Hall. The isolation systemmust also resist maximum lateral loads fromother sources without yielding in order to avoidunacceptable displacements and vibrationsunder service loads, such as wind. Therefore, ifthese service lateral loads exceed about 10% ofthe structure’s weight, the building should notbe isolated.

Soil Conditions The second considerationwhen assessing the suitability of a structure forseismic isolation is the soil condition and thegeology of the site. Generally, the stiffer thesoil, the more effective the isolation.

The flexibility of the structure determineshow it will respond to a given earthquakemotion. However, the form of the earthquakemotion as it arrives at the base of a structuremay be modified by the properties of the soil

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through which the earthquake waves travel. Ifthe soil underlying the structure is very soft, thehigh frequency content of the motion may befiltered out, and the soil may produce long-period motions. An extreme example of thiswas seen in the 1985 Mexico City earthquake.Lengthening the period of a stiff structure inthese lake-bed soil conditions will amplifyrather than reduce the ground motions, andhence for sites such as Mexico City seismicisolation should not be considered.

Another geologic consideration is thedistance from a major fault. For near-faultsituations, generally the design forces anddisplacements are amplified to allow for therecently observed fling or pulse effect of near-fault ground motions.

Adjacent Structures A third considerationin assessing suitability is any constraintsimposed by adjacent structures at the proposedsite. As discussed earlier, the basic concept ofseismic isolation systems minimize thesedisplacements, but nevertheless basedisplacements of the order of 8 to 20 in.generally occur. If the site is very confined dueto neighbouring buildings built on theboundary, it may not be possible toaccommodate these displacements.

14.6.2 Retrofit of Existing Structures

Retrofit of existing structures to improvetheir earthquake safety involves additionalconsiderations, compared with newconstruction, because of the constraints alreadypresent. Some structures are inherently moresuitable for retrofit using seismic isolation thanothers. For example, bridge superstructures aregenerally supported on steel bearings.Replacement of these bearings with elastomericones is a fairly simple, low-cost operation thatwill lead to a reduction in earthquake forces andallow the option of redistributing forces awayfrom the weak substructures into abutmentsmore capable of sustaining them(14-30).

Buildings are often more difficult to retrofitthan bridges. However, seismic isolation mayoften be an effective solution for increasing the

earthquake safety of existing buildings withoutthe addition of new structural elements whichdetract from the features which originally makethe building worth preserving. Althoughseismic isolation reduces earthquake forces, itdoes not eliminate them. Consequently, thestrength and ductility of an existing structuremust at least be sufficient to resist the reducedforces that result from isolation. If the strengthof the existing structure is extremely low (lessthan 0.05 of the weight of the building), thenadditional strengthening versus somestrengthening and the provision of isolation willneed to be studied.

In addition to the conditions discussedabove from new buildings, the issues to beaddressed in the seismic isolation retrofit of anexisting structure are:– Is there sufficient clearance with adjacent

buildings to permit a movement of 6 to 24inches?

– Do the building and its existing foundationshave sufficient strength and ductility toresist the reduced seismic forces?

– What is the appropriate level for the plane ofisolation—foundation level, basement level,ground level, or the top, bottom, or mid-height of the columns?

– The pros and cons with regard to the planeof isolation are:

– Any structure with a full subbasement orbasement that can be temporarily disruptedis a good isolation candidate, since the workcan be confined to that area.

– A structure with piled foundations can bemore easily retrofitted at the foundationlevel than one with spread footings.

– Provisions for the zone of isolation at thetop, bottom, or mid-height of the basement-,first-, or second-level columns requires adetailed evaluation of the column capacities.If the strength of the column is not sufficientto resist the reduced isolation forces, threepotential options exist. First, the columnmay be strengthened and act as a cantilever.Second, a new framing system with stiffbeams may be developed at the plane ofisolation to reduce the column forces. Third,


the mid-height column solution may beconsidered, since it reduces the columnmoments significantly.In summary, seismic rehabilitation of an

existing structure provides the ability to confinemost of the construction work to the levelwhere the plane of isolation is to be provided,whereas conventional methods generallyrequire the addition of structural elements to alllevels of the building. This trade-off can bevery important if continued use of the facility isdesired, as in hospitals or command and controlcenters.

14.6.3 Uplift and Overturning

In many types of structural systemsincreasing lateral forces will induce net tensionsin elements once the axial loads caused by theoverturning moment exceeds the gravity loads.This may occur for example at the edges ofshear walls or the columns in braced ormoment-resisting frames.

In conventional design this tension isresisted in the base connections andfoundations, although only if it occurs under thecode levels of the earthquake lateral loads. Themore severe loading occurring under actualmaximum earthquakes will produce overturningmoments much greater than the design value,and therefore tension forces will be inducedeven where none are indicated under codeloading. In this case, it is assumed that thestructural detailing and redundancies aresufficient to prevent failure due to the uplift.

More recent studies(14-16) have indicated thatuplift may in fact be beneficial in reducingearthquake forces in conventional structures. InFact, at least two actual structures in NewZealand have been explicitly designed for upliftas a form of seismic isolation: a stepping bridgeand a chimney stack.

For a structure isolated on elastomericbearings, the effects of uplift must be examinedmore carefully, since the elastomeric bearing isnot suitable for resisting large tensile loads. Fora fully bolted connection, an elastomericbearing can resist 250 to 300 psi in tension

before significant softening of the bearingoccurs.

Therefore, if uplift is indicated in an isolatedstructure, detailed analysis must be performedto quantify the vertical displacements forconnection design. This involves a nonlinearanalysis with realistic maximum credibleearthquake records and requires significantanalytical effort.

To avoid this, the optimum strategy is toavoid or minimize uplift. This is done bycareful configuration of the lateral load-resisting elements. The important parametersare the height-to-width ratio of the lateral load-resisting system and the amount of gravity loadcarried by these elements. Another alternative isto utilize the “loose-bolt” connections whichpermit certain amount of isolator uplift withoutsubjecting the bearing to net tension. Suchconnections have been successfullyimplemented in several major buildings insouthern California such as the Los AngelesCity Hall seismic retrofit and the LakeArrowhead and Saint John new hospitalbuildings.

14.7 DESIGN CODEREQUIREMENTS

By the time this book reaches the market thedesign of new seismically isolated buildings inUnited States will be probably governed by theInternational Building Code 2000 (IBC-2000)(14-42). It is likely, however, that design insome jurisdictions will be still controlled by theprovisions of the IBC-2000 predecessor, (UBC-97)(14-43). As documented by Naeim andKelly)(14-8) UBC-97 is an unnecessarilycomplicated and conservative as far as seismicisolation design is concerned. Therefore, in thissection we limit our discussion to the provisionsof IBC-2000. Readers who are interested inlearning more about UBC-97 and itspredecessors are referred to the referencedtextbook by Naeim and Kelly.

Primarily intended to regulate the design ofnew buildings, the IBC-2000 does not reallycover the retrofit of existing buildings using

738 Chapter 14

isolation, although most retrofit projects dofollow either the IBC or UBC regulationsclosely. IBC-2000 regulations are written insuch a way as to be nonspecific with respect toisolation systems. No particular isolationsystems are identified as being acceptable, butthe regulations require that any isolation systemshould be stable for the required displacement,provide increasing resistance with increasingdisplacement, and have properties that do notdegrade under repeated cyclic loading.

The underlying philosophy is that anisolated building designed using IBC-2000 willout-perform fixed-base construction inmoderate and large earthquakes. It is not theintent of the code to reduce the constructioncost but to minimize damage to isolatedstructures and their contents.

Increasingly, the seismic upgrade design ofexisting structures is influenced by the NEHRPGuidelines for the Seismic Rehabilitation ofBuildings (FEMA-273) and its commentary(FEMA- 274), which are published by theFederal Emergency Management Agency(14-44,

14-45). FEMA-273 provisions are very similar tothose of the IBC-2000 with one exception:FEMA-273 permits a new analysis approachcalled Static Nonlinear Analysis or the“Pushover” method (see Chapter 15).

A 1986 document published by asubcommittee of the Structural EngineersAssociation of Northern California (SEAONC)and generally referred to as the Yellow Book(14-

26) has served as the backbone of all new codeprovisions.

The seismic criteria adopted by currentmodel codes involve a two-level approach toseismic hazard, which are as follows:– The Design Basis Earthquake (DBE): That

level of ground shaking that has a 10%probability of being exceeded in 50 years(475 year-return period earthquake)

– The Maximum Considered Earthquake(MCE): The maximum level of groundshaking that may ever be expected at thebuilding site. MCE is taken as 2%probability of being exceeded in 50 years(2500-year return period earthquake).

Notice that this is different from UBC-97definition of MCE which was 10%probability of being exceeded in 100 years(1000-year return period earthquake)

14.7.1 Design Methods

Static Analysis: For all seismic isolationdesigns it is necessary to perform a staticanalysis. This establishes a minimum level fordesign displacements and forces. The staticanalysis is also useful both for preliminarydesign of the isolation system and the structurewhen dynamic analysis is required and fordesign review; under certain circumstances itmay be the only design method used.

Static analysis alone will suffice if:1. The structure is located at a site with S1 <

0.60g. S1 is determined using the spectralacceleration maps published as a part ofIBC-2000.

2. The site soil is classified as Class A, B, C,or D (see Chapter 3).

3. The structure above the isolation plane isnot more than four stories or 65 feet inheight.

4. The effective period at maximumdisplacement of the isolated system, TM,does not exceed 3.0 seconds.

5. The effective period at design displacement,TD, is greater than three times the elastic,fixed-base period of the structure.

6. The structural system above the isolationplane is regular.

7. The effective stiffness of the isolationsystem at design displacement is greaterthan one third of the effective stiffness at20% of design displacement.

8. The isolation system can produce therestoring force requirements mandated bythe code (IBC-2000 Sec. 1623.5.1.4).

9. The force deflection characteristics ofisolation system are independent of rate ofloading, vertical load, and bilateral load.

10. The isolation system does not limit MCEdisplacements to less than SM1/SD1 times thetotal design displacements.


Dynamic Analysis: Dynamic analysis maybe used in all cases and must be used if therequirements mentioned for adequacy of staticanalysis are not satisfied. Dynamic analysismay take the form of response spectrumanalysis or time-history analysis.

Response spectrum analysis would suffice ifrequirements number 2 and 7-10 as mentionedfor static analysis, are satisfied. Otherwise, atime-history analysis will be required. Use ofmore than 30% critical damping is notpermitted in response spectrum analysis even ifthe system is designed to provide for more.

Regardless of the type of dynamic analysisto be performed a site-specific design spectracorresponding to DBE and MCE events must bedeveloped and used (instead of the codepublished default spectra) if:– The structure is located on a Class E or F

site, or– The structure is located at a site with S1 <

0.60g.If time history analysis is to be performed,

then a suite of representative earthquake groundmotions must be selected that satisfy thefollowing requirements:1. At least three pairs of recorded horizontal

ground motion time-history componentsshould be selected and used.

2. The time histories should be consistent withthe magnitude, fault distance, and sourcemechanisms that control the DBE and/orMCE events.

3. If appropriate recorded time-histories arenot available, appropriate simulated time-histories may be used to make up the thetotal number of required records.

4. For each pair of horizontal ground motioncomponents, the square root sum of thesquares (SRSS) of the 5 percent-dampedspectrum of the scaled horizontalcomponents is to be constructed.

5. The time-histories are to be scaled such thatthe average value of the SRSS spectra doesnot fall below 1.3 times the 5 percent-damped design spectrum (DBE or MCE) bymore than 10 percent over a range of 0.5TD

to 1.25TM where TD and TM are effective

isolated periods at design displacement andmaximum displacement, respectively.

6. Each pair of time histories is to be appliedsimultaneously to the model considering themost disadvantageous location of masseccentricity. The maximum displacement ofthe isolation system is to be calculated fromthe vectorial sum of the two orthogonalcomponents at each time step.

7. The parameters of interest are calculated foreach time-history analysis. If three timehistory analyses are performed, then themaximum response of the parameter ofinterest is to be used for design. If seven ormore time histories are used, then theaverage value of the response parameter ofinterest may be used.As Naeim and Kelly have pointed out (14-8),

this formulations contains implicit recognitionof the crucially important fact that designspectra are definitions of a criteria for structuralanalysis and design and are not meant torepresent characteristics of a single event.

14.7.2 Minimum Design Displacements

Four distinct displacements calculated usingsimple formulas and used for static analysis,also serve as the code permitted lower boundvalues (subject to some qualification) fordynamic analysis results. These are:– DD: the design displacement, being the

displacement at the center of rigidity of theisolation system at the DBE;

– DM: the displacement,at the center of rigidityof the isolation system at the MCE;

– DTD: the total design displacement, being thedisplacement of a bearing at a corner of thebuilding and includes the component of thetorsional displacement in the direction of DD

– DTM: same as DTD but calculated for MCE.DD and DM are simply spectral displacementvalues calculated assuming constant spectralvelocity from code published spectral maps andadjusted for damping.

740 Chapter 14

D

DDD B

TSgD 1

24

=

π(14-1)

M

MMM B

TSgD 1

24

=

π(14-2)

where g is the gravitational acceleration, SD1

and SM1 are spectral coefficients, TD and TM areisolated periods, and BD and BM are dampingcoefficients corresponding to the DBE andMCE level responses, respectively.

SD1 and SM1 are functions of two parameters:– S1, the MCE 5% damped spectral

acceleration for the site available from themaps accompanying the IBC-2000 and alsoavailable on Internet via the USGS andCDMG web sites, and

– Fv, the site coefficient defined for varioussite classes and acceleration levels (seeChapter 3).

Such that

11 SFS vM = (14-3)

11 3

2MD SS = (14-4)

The effective damping in the system, β , atthe DBE and MCE response levels (referred toas βD and βM are computed from

=

2max,

loop hysteresis of area total

2

1

DD

DDKπ

β (14-5)

=

2max,

loop hysteresis of area total

2

1

MM

MDKπ

β (14-6)

KDmax and KMmax are effective stiffness termsdefined in Section 14.7.3. The dampingreduction factors BD for the DBE and BM for theMCE are given in a tabular form (IBC-2000,

Table 1623.2.2.1), with linear interpolation tobe used for intermediate values. A very closeapproximation to the table values is given byNaeim and Kelly(14-8) as

( )βln125.01 −=B

(14-7)

where β is given as the fraction of criticaldamping (not as a percentage).

14.7.3 Effective Isolated System Periods

The effective isolated periods TD and TM

corresponding to the DBE and MCE responseare computed from

gK

WT

DD

min

2π= (14-8)

gK

WT

MM

min

2π= (14-9)

whereW = the weight of the buildingg = gravityKDmin = minimum effective horizontal stiffnessof the isolation system at the designdisplacement (DBE).KMmin= minimum effective horizontal stiffnessof the isolation system at the maximumdisplacement (MCE).

The values of KDmin, and KMmin are notknown to the engineer during the preliminarydesign phase. The design procedure will beginwith an assumed value which is obtained fromprevious tests on similar components or byusing the material characteristics and aschematic of the proposed isolator. After thepreliminary design is satisfactorily completed,prototype isolators will be ordered and tested,and the values of KDmin, KDmax, KMmin, and KMmax

will be obtained from the results of theprescribed program of tests on the prototypes.


The total design displacements, DTD and DTM

(which include torsion), are

++=

22

121

db

eyDD DTD (14-10)

++=

22

121

db

eyDD MTM (14-11)

where b and d are plan dimensions at theisolation plane, e is the actual eccentricity plus5% accidental eccentricity, and y is the distanceto a corner perpendicular to the direction ofseismic loading.

14.7.4 Design Forces

The superstructure and the elements belowthe isolation interface are designed for forcesbased on the DBE design displacement, DD.The isolation system, the foundation andstructural elements below the isolation systemmust be designed to withstand the followingminimum lateral seismic force

DDb DKV max= (14-12)

If other displacements rather than DD

generate larger forces, then those forces shouldbe used in design rather than the force obtainedfrom Equation 14-12.

The structure above the isolation planeshould withstand a minimum shear force, Vs, asif it was fixed base where:

I

DDs R

DKV max= (14-13)

In above equations KDmax is the maximumeffective stiffness of the isolation system at thedesign displacement (DBE) in the horizontaldirection and RI is a reduction factor analogousto the R factor that would have been used forthe superstructure if it was not isolated (seeChapter 5). IBC-2000 defines RI as

0.28

30.1 ≤=≤ RRI (14-14)

If dynamic analysis is performed, it ispossible to have design displacements anddesign forces that are less than those given byEquations 14-12 and 14-13. In such cases, Thetotal design displacement, DTD , for the isolationsystem can be reduced to not less than 90% ofthat given by the static formula, and the totalmaximum displacement, DTM , can be reducedto not less than 80% of the static formula result.Furthermore, the code permits a furtherreduction by replacing DD and DM in the staticformulas by D’D and D’M , where

2

'

1

+

=

D

DD

TT

DD (4-14)

2

'

1

+

=

M

MM

TT

DD (4-15)

In all cases the value of Vs should not be lessthan– the seismic force required by the code

provisions for a fixed-base structure;– the base shear corresponding to the factored

design wind load– one and a half times the lateral force

required to fully activate the isolationsystem, i.e., the yield load of a lead-plugrubber bearing or slip threshold of a slidingbearing system

14.7.5 Vertical Distribution of DesignForce

In order to conservatively considerparticipation of higher modes in response, thevertical distribution of the force on thesuperstructure of an isolated building is similarto that prescribed for fixed-base construction.

742 Chapter 14

This is so, although the seismic isolation theorysuggests a uniform distribution of forces overthe height of the superstructure. Therefore, thelateral force at level x, denoted by Fx, iscomputed from the base shear, VS, by

∑ =

=N

i ii

xxsx

hw

whVF

1

(14-15)

where wx and wi are the weights at level i or xand hx and hi are the respective heights ofstructure above isolation level.

14.7.6 Drift Limitations

The maximum interstory drift (relativedisplacement of adjacent floors) permitted bythe IBC-2000 is a function of method ofanalysis in that more drift is permitted whenmore sophisticated analyses are performed.

Static Analysis: The drift at any level x iscalculated from Equation 14-16 and should notexceed 0.015hsx (hsx is the story height belowlevel x).

E

seIx I

R δδ = (14-16)

where δse is the drift determined by an elasticanalysis and IE is the occupancy importancefactor for the building as defined in Chapter 5.

Response Spectrum Analysis: The drift atany level x calculated from response spectrumanalysis should not exceed 0.015hsx.

Time-History Analysis: The drift at anylevel x calculated from a time-history analysisconsidering the nonlinear behavior of theisolators should not exceed 0.020hsx. The codehas an additional paragraph stating that thisdrift should be calculated using Equation 14-16.However, the relevance of such a provision tononlinear time-history analysis is not clear andthis may be just a printing error in the very firstedition of the IBC that has just been released atthe time of this writing. P-∆ effects must be

considered whenever the interstory drift rationexceeds 0.010/RI.

14.7.7 Peer Review

IBC-2000 similar to its predecessorsrequires the design of the isolation system andthe related test programs to be reviewed by anindependent team of registered designprofessionals and others experienced in seismicanalysis methods and theory and application ofseismic isolation. The scope of this reviewincludes, but is not limited to the followingitems:1. Review of site-specific design ground

motion criteria such as design spectrum andtime-histories as well as other project-specific information.

2. Review of the design criteria and thepreliminary design procedures and results.

3. Overview and observation of the prototypetesting program.

4. Review of the final design of the entirestructural system and supporting analysesand calculations.

5. Review of the isolation system qualitycontrol and production testing program.

14.7.8 Testing Requirements forIsolators

Code testing requirements of the isolatorunits before they can be accepted are containedin Section 16.23.8 of IBC-2000. The coderequires that at least two full-sized specimens ofeach type of isolator be tested. The sequenceand the necessary number of cycles of testingvary with the amount of deformation theisolators are subjected to. For example, twentyfully reversed cycles of loading is to beperformed at a displacement corresponding tothe wind design force.

The tests required are a specified sequenceof horizontal cycles under D + 0.5L from smallhorizontal displacements up to DTM. Themaximum vertical load used during testing is1.2DL + 0.5LL + Emax, and the minimum is0.8DL - Emin where Emax and Emin are the


maximum downward and upward load on theisolator that can be generated by an earthquake.

14.7.9 Design Example

Consider a small building with a plandimension of 150 feet by 70 feet. The totalweight of the structure is estimated at 4200kips. The lateral load resisting system consistsof ordinary steel concentrically braced frames(R=5). The building is regular in both the planand the elevation. The actual distance betweenthe center of mass and the center of rigidity ofeach floor is 80 inches.

The project site is located in downtown LosAngeles on a site with soil Class C. Evaluationof IBC-2000 seismic hazard maps (see Chapter3) has produced values of SS=1.5g andS1=0.60g. The fixed base period of the buildingis 0.40 secs. The isolation system shouldprovide effective isolated periods in the vicinityof TD = 2.0 and TM = 2.3 seconds, respectively.The anticipated damping is about 15% critical.A margin of +10% variation in stiffness fromthe mean stiffness values of the isolators isconsidered acceptable. Estimate the minimumdesign displacements, minimum lateral forces,and maximum permitted interstory drift ratiosaccording to the IBC-2000 requirements.SOLUTION:TD and TM are given. Therefore, fromEquations 14-8 and 14-9:

kips/in. 81

4.386

420023.2

kips/in. 107

4.386

420020.2

min

min

min

min

=⇒

=

=⇒

=

M

M

D

D

K

K

K

K

π

π

As specified in the problem, we assume a +10%variation about the mean stiffness values.Therefore,

( )

( ) k/in. 9990.0

8110.1

k/in. 13190.0

10710.1

max

max

==

==

M

D

K

K

A Linear interpolation of values of 1.2 and 1.5given in IBC-2000 Table 1623.2.2.1 for 10%and 20% damping results in B = 1.35.Alternatively, From Equation 14-7:

( )38.1

7243.0)15.0ln1(25.0ln125.01

=

=−=−=

BB

β

The same level of damping is assigned to bothDBE and MCE events for preliminary designpurposes. The value of Fv = 1.3 is obtainedfrom IBC-2000 Table 1615.1.2 (see Chapter 3)for site Class C and S1 = 0.60 > 0.50. TheSpectral coefficients needed for calculation ofminimum displacements are obtained fromEquations 14-3 and 14-4:

( )( )

( ) gSS

gSFS

MD

vM

52.078.03

2

3

2

78.060.03.1

11

11

===

===

The minimum design displacements now maybe obtained from Equations 14-1 and 14-2 as:

( )( )

( )( )in. 02.13

35.1

3.278.0

4

4.386

in. 55.735.1

0.252.0

4

4.386

2

2

=

=

=

=

π

π

M

D

D

D

The eccentricity needed to calculate totaldisplacements is

( )( )( ) in. 1701215005.080 =+=e

and from Equations 14-10 and 14-11 noting thatthe same multiplier applies to both equations

744 Chapter 14

( )

( )( )( )( ) in. 1.1947.102.13

in. 1.1147.155.7

and 47.170150

170

2

1501

121

22

22

====

=

++

=

++

TM

TD

D

D

db

ey

The minimum design shear force for theisolation system and structural elements belowthe isolation plane is obtained from Equation14-12:

( )( ) kips 98955.7131max === DDb DKV

which corresponds to a seismic base shearcoefficient of 0.24. The reduction factor fromEquation 14-14 is:

( ) 0.2875.158

3

8

3 ≤=== RRI

The design base shear for design of thesuperstructure (Equation 14-13) is:

kips 527875.1

989max ====I

b

I

DDs R

V

R

DKV

which in turn translates to a seismic base shearcoefficient of 0.126. Remember that this forcehas to be larger than the base shear obtained fora similarly situated fixed-base building with aperiod of 2.0 sec. The procedure forcalculating base shear force for conventionalbuildings is explained in Chapter 5 andtherefore not repeated here.

14.8 SEISMIC-ISOLATIONCONFIGURATIONS

The seismic-isolation configuration,including the layout and the installation detailsfor the isolation system, depends on the siteconstraints, type of structure, construction, andother related factors. The following details are

provided as an aid in determining appropriatelayouts for particular projects and are notintended to restrict, the designer in individualcases.

14.8.1 Bearing Location

Figures 14-16 to 14-19 provide typicalplanes of isolation for elastomeric bearings inbuildings both with and without separatebasement levels. Some of the advantages anddisadvantages associated with each layout arelisted in the figures. The following generalguidelines are considerations for determining asuitable layout:

• The bearing location should permitaccess for inspection and replacement,should this become necessary.

A full diaphragm above or below the isolatorsto distribute lateral loads uniformly to eachbearing is preferable. If distribution is by tiebeams only, the bearings should be arranged inproportion to the lateral load taken by eachelement, i.e., larger bearings under stifferelements.

– Free movement for the maximum predictedhorizontal displacement must be available.

– A layout which allows stub walls orcolumns as a backup system for verticalloads should be used wherever possible.

– Consideration must be given to thecontinuity of services, stairways, andelevators at he plane of isolation.

– Consideration must be given to details forcladding if it will extend below the plane ofisolation.

14.8.2 Connection Details

Although connection details vary from eachproject, the design principles remain the same:1. The bearing must be free to deform in shear

between the outer shims; i.e., the uppersurface of the bearing must be able to movefreely horizontally.


Figure 14-16. Bearings located in sub-basement

Figure 14-17. Bearings located at top of basementcolumns

Figure 14-18. Bearings located at bottom of first storycolumns

Figure 14-19. Bearings located at top of first storycolumns

746 Chapter 14

2. The connections must have the capacity fortransferring maximum seismic forcesbetween the substructure and thesuperstructure.

3. Ease of construction must be kept in mind toinsure access for installation and, in the caseof a retrofit, temporary support for thesuperstructure.

The most common bearing construction hasouter load plates of ¾ - 1½ in. steel covered by1/8 in. rubber layers. During the manufacture,holes for bolts or dowels are formed throughthe outer rubber layers and load plates. Exteriorcover plates with bolts or dowels are then addedto the bearing prior to installation. Theseexterior plates may be either welded or boltedto the structure. It is important to insure that thebolts or dowels do not intrude into the internalrubber layers. Figure 14-20 is an example of aconnection detail using dowels. The morecommon trend is to use fully bolted rather thandowelled connections.

14.8.3 Provision for Bearing Removal

Where practical, provision should be madeto ease removal and replacement of the bearingsshould this ever be necessary. This requires twothings: (i) a means of supporting the building

weight while the bearing is removed, and (ii) ameans of removing the bearing without unduedamage to the connections.

The ease of meeting this first requirementwill depend on the location of the bearings andtype of backup safety system used. In asubbasement, jacks can generally be usedbetween the foundation and basement floor tosupport the bearing load. If a backup safetysystem is used (as described in the followingsection), provision for jacking may beincorporated into the design. Bearing locationsat the top of columns will require shoring to beerected around columns to provide a jackingplatform if a backup system has not beenprovided.

The removal of the bearing once the load isremoved will be simplified if boltedconnections are used to connect to the structure.For example, the connection detail shown inFigure 14-20 could be modified to simplifybearing removal. In this modification, doubleplates would be added at the bottom of thebearing as shown in Figure 14-21. The bearingcomplete with dowel plates could then beremoved. For a welded connection, removalwould entail cutting the welds.

A combination of a removal and backupsafety-system detail is shown in Figure 14-22.

Figure 14-20. Installation using dowels


Figure 14-21. Details for replacement bearings

Figure 14-22. Backup and removal detail

14.8.4 Backup Safety System

Depending on the importance of thebuilding, it may be considered desirable toincorporate such a system depends on thebearing location and configuration. For bearinglocations at the top of columns a layout isshown schematically in Figure 14-23. Thisprovides a means of supporting the verticalload, and a lateral displacement limiter. Analternate to the scheme of location bearings atthe top of columns is to locate them at the baseof the columns as shown in Figure 14-24.

Figure 14-23. Bearings at top of columns

Figure 14-24. Bearings at base of columns

14.9 ISOLATOR DESIGNPROCEDURES

Basic procedures for design of the highdamping and low damping rubber isolators(HDR, LDR), lead-rubber isolators (LRB), andthe friction pendulum isolators (FPS) arepresented in this section. The primary purposeof this information is to aid design engineer inpreliminary sizing of the isolators needed for a

748 Chapter 14

given project. For is information The reader isencouraged to read the recent textbook byKelly(14-46) for a very detailed coverage ofmechanical characteristics and modeling ofHDR and LRB isolators. A less exhaustive butmore practical coverage of the same topics maybe found in a recent textbook by Naeim andKelly(14-8). Further instructions and details fordesign of FPS isolators may be obtained fromthe patent-holder, Earthquake ProtectionSystems of Berkeley, California and fromReference 14-40.

The need for an isolation system which isstiff under low levels of lateral load (e.g. wind)but flexible under higher levels (i.e.earthquakes) necessarily leads to a nonlinearsystem. The properties of most isolator systemsare characterized as bilinear. Although a tri-linear model with stiffening at large horizontaldisplacements better represents the performanceof HDR isolators.

Any complete design procedure shouldinsure that (i) the bearings will safely supportthe maximum gravity service loads throughoutthe life of the structure and (ii) the bearings willprovide a period shift and hysteric dampingduring one or more design earthquakes. Thesteps to achieve these aims are:1. The minimum required plan size is

determined for the maximum gravity loadsat each bearing location.

2. The total rubber thickness or dimensions ofthe FPS isolator is computed to give theperiod shift during earthquake loadings.

3. The damping characteristics of the isolatorsystem is calculated to ensure proper valueof the hysteric damping and wind resistancerequired.

4. The performance of the bearings as designedis checked under gravity, wind, thermal,earthquake, and any other load conditions.

14.9.1 Elastomeric Isolators

One of the most important parameters indesign of elastomeric bearings is the shapefactor, S, defined as

area free-forcearea loaded=S

For a circular pad with a diameter of Φ anda single layer rubber thickness, t

tS

4

Φ= (14-17)

Generally a good design tries to keep thevalue of S to somewhere between 10 and 20.

The horizontal stiffness of a single isolatoris given by

rH t

GAK = (14-18)

where G is the shear modulus of the rubber, A isthe full cross-sectional area of the pad, and tr isthe total thickness of rubber. The maximumshear strain, γ, experienced by the isolator is themaximum horizontal displacement, D, dividedby the total rubber thickness, tr.

rt

D=γ (14-19)

The vertical stiffness of a rubber bearing isgiven by

r

scV t

AEK = (14-20)

where Ec is the compression modulus of therubber-steel composite and As is the area of asteel shim plate. For a circular pad without anyholes in the center

26GSEc = (14-21)

For bearings with very large shape factors thecompressibility of rubber affects the value ofEc. In such cases a more accurate estimate of Ec

may be obtained from


KGS

KGSEc +

= 2

2

66

(14-22)

where K is the bulk modulus of rubber andgenerally varies from 145,000 psi to 360,000psi depending on the type of rubber being used.The value of 290,000 psi is most commonlyused.

14.9.2 Lead-Rubber Isolators (LRB)

The lead-rubber bearings is a nonlinearsystem which may be very effectively idealizedin terms of a bilinear force—deflection curvewith constant values throughout many cycles ofloading (Figure 14-25). Formulas developed inthe previous section are also applicable herewith some additional equations that model thelead core properties.

Figure14-25. Typical bilinear hysteresis loop

The characteristic strength, Qd, can beaccurately estimated as being equal to the yieldforce of the lead plug. The yield stress of lead isabout 1,500 psi. The effective stiffness of thelead-plug bearing, Keff, at a horizontal

displacement D larger than the yielddisplacement Dy, may be defined in terms of thepost-elastic stiffness, Kd, and characteristicstrength, Qd, as

yd

deff DDD

QKK ≥+= (14-23)

The natural period is given as

gK

WT

eff

π2= (14-24)

As a rule of thumb for lead-rubber isolatorsKu is taken as 10Kd. Kelly(14-46) has shown thatwith this assumption, the effective percentageof critical damping provided by the isolator,βeff, can be obtained from

( )( )DQDK

KQDQ

du

uddeff +

−=π

β2

94(14-25)

14.9.3 Friction Pendulum System

If the load on an FPS isolator is W, and theradius of curvature of the FPS dish is R, thenthe horizontal stiffness of the isolator may bedefined for design purposes as

R

WKH = (14-26)

The natural period of and FPS isolatedsystem is only a function of R

g

RT π2= (14-27)

The effective (peak-to-peak) stiffness of theisolator is given by

D

W

R

WKeff

µ+= (14-27)

750 Chapter 14

where µ is the friction coefficient and all otherterms are defined previously. The frictioncoefficient has been shown to be independent ofvelocity for pressures of 20 ksi or more on thearticulated slider(14-8). The damping provided bythe system, β, is a function of horizontaldisplacement and may be obtained from

RD+=

µµ

πβ 2

(14-28)

An estimate of the rise of the structure(vertical displacement) as a result of movementalong the curved surface of the isolator may beobtained from

R

DV

2

21≅δ (14-29)

14.9.4 Design Example

Assume you are in charge of designing afour story isolated building. The owner, apublic entity, requires that the designaccommodate competing isolation systems tobid on the job. The architect needs to know themaximum dimensions of the isolators so thatshe can complete her schematic design. Yourengineering team needs to know the design baseshears for proportioning the structural systemabove and the elements below the isolationsurface. You would like to estimate thesevalues for three alternative isolation systems:a) a high damping rubber systemb) a lead-rubber system which may or may not

be complimented by ordinary low-dampingisolators, and

c) a friction pendulum system.The following information is also available

to you at this time.– The structural system above the isolation

plane is a shear wall system with R = 6.– The total weight of the building is 14,120

kips.– There are a total of 60 support points (i.e.,

60 isolators).

– The average sustained load on an interiorisolator is 500 kips.

– The fixed-base period of the super-structureis estimated to be about 0.70 seconds.

– From IBC-2000 for this site, SD1=0.56Estimate the size of isolators needed for each ofthe three alternatives and the correspondingseismic design base shears so that the architectand engineers could make substantial progresswhile you are performing your final design ofthe isolators and preparing for procurement andprototype testing process.

SOLUTION

sec. 1.2)7.0(33 ==≥ −basefixedD TT

T be on the safe side, take TD=2.5 sec forpreliminary design. The reduction fact, RI forthe superstructure is calculated from Eq. 14-14as

0.20.225.2)6(83

0.1 =⇒≤==≤ II RR

a) High-Damping Rubber IsolatorsTo be conservative we size the isolator

under largest sustained load. That is an interiorisolator under 500 kips of load. We takedamping to be 10% subject to verification.Therefore, from Eq. 14-17 or from Table1623.2.2.1 of IBC-2000, BD=1.20.

We take a typical high damping rubbercompound with G=145 psi and K=300 ksi.Therefore, our first estimate for the horizontalstiffness of the isolator is obtained from Eq. 14-8 as

k/in. 35.75.2

23865002

22

=

=

= ππ

Tg

WKH

The design displacement is obtained fromEq. 14-1

( )( )in. 43.11

20.15.256.0

4 2 =

=

πg

DD


Usually we want to achieve thisdisplacement at about 150% shear strain. FromEq. 14-19 , we can estimate the total rubberthickness required

in. 6.750.143.11 ==⇒= r

r

tt

Dγ

Now we calculate the cross-sectional areaand the required diameter of the bearing fromEq. 14-18

( )

in. 24 Use

in 12.224(384)4A

in 384145.0

6.733.7 2

=Φ

===Φ

===

ππ

G

tKA rH

Now we re-calculate A, KH and TD based onthis bearing diameter:

( )

( )( ) sec 2.1 sec 3.265.833.750.2

k/in 65.838445235.7

in 452424

42

22

f==

==

==Φ=

D

H

T

K

Aππ

Selecting a shape factor of S=10, from Eq. 14-17 we can calculate the thickness of individualrubber layers, t

( )

( ) in 5.78512

12say ,1.12856.7

layers ofnumber

"85say in, 6.010424

4

==

==

==Φ=

rt

St

Using 0.1in thick steel shim plates and one inchtop and bottom end plates, the total height ofthe bearing is

( ) ( ) in 6.101.0110.125.7 =++=h

Let us now estimate the base shear coefficientfor design of the superstructure, Cs, and thecorresponding value for the base, Cb.

( )

10.0

20.0500

43.1165.8

≅=

==≅=

I

bs

Hbb

R

CC

W

DK

W

VC

b) Lead-Rubber IsolatorsIt is usually more beneficial to begin

designing isolation systems using LRB isolatorsas a system and then assign individual isolatorproperties. The reason is that often the bestsolution is a combination of LRB isolators andlow damping rubber isolators (i.e., isolatorswithout the lead plug).

In LRB isolators since damping comes fromthe lead core, usually there is no need to usehigh damping rubber and therefore ordinaryrubber is generally used. Given the solution inPart (a) of this problem, it is obvious that we donot need a large amount of damping here.Therefore, we use 15% critical damping subjectto verification and a rubber compound with ashear modulus of G=60 psi.

The same target period of 2.5 seconds ismaintained. Either from Eq. 14-17 or fromTable 1623.2.2.1 of IBC-2000, for β=15%,BD=1.35 and from Eq. 14-1

( )( )in. 16.10

35.15.256.0

4 2=

=

πg

DD

Treating the entire isolation system as a unit,the required stiffness corresponding to thisperiod is

k/in. 2315.2

2

386

120,14222

=

=

= ππ

Tg

WKH

The energy dissipated per cycle is

752 Chapter 14

( )( ) ( )in-k 462,22

15.016.1023122 22

=

== πβπ effeffD DKW

The area of the hysteresis loop, however, isalso given by

( )ydD DDQW −= 4

and if ignore Dy because of its relatively smallsize

( ) kips 55216.104

462,22

4==≅

D

WQ D

d

Now, we can estimate Kd from Eq. 14-23:

kips/in 17616.10

552231

=

−=−=D

QKK d

effd

and since

( ) in. 35.01769552

9

then, 10 and

==≅

≈−

=

d

dy

dudu

dy

K

QD

KKKK

QD

The total cross sectional area of the leadplug area needed for the entire isolation systemis

2in 3685.1

552 === pby

dtotalpb F

QA

For the sake of simplicity, we keep thediameter of all isolators the same at Φ=24 in.Using 3.5 inch diameter lead cores in 40 of the60 isolators provides a lead cross sectional areaof slightly more than 385 square inches. Nowwe have to recalculate Qd based on this newarea of lead

( ) kips 5785.1385 ==dQ

The stiffness provided by lead plugs is

in-k 5716.10

578 ===D

QK d

pb

and the remainder of required stiffness has to beprovided by rubber. Therefore,

in-k 17616.10

552231 =−=−=

D

QKK d

Hrubber

The total cross sectional area of the rubber is

( ) 22

in 744,26385424

60 =−= πrubberA

and from Eq. 14-18, we can now establish therequired total rubber thickness, tr, as

( )( )

in 1.9

176744,261060 3

=

×==−

rubberr K

GAt

Therefore, assuming 1.0 inch thick top andbottom end plates and steel shims, our isolatorswill have a height of less than 12 inches.

The seismic shear coefficients are calculatedas in Part (a):

( )

083.0217.0

167.0120,14

16.10231

==

==

s

b

C

C

c) Friction Pendulum SystemUsing the same target period of 2.5 seconds,

from Eq. 14-27

in 23.61386

25.2 =⇒= RRπ

Eq. 14-28 indicates that effective dampingand maximum displacement are inter-related.For example, assuming a coefficient of friction


of µ=0.06 and a design displacement of D=12inches, we get

%1523.610.1206.0

06.02 =+

=π

βeff

The selected value of D=12 inches satisfiesthe minimum code prescribed displacement of10.16 inches which was calculated for the samebasic parameters (T=2.5 sec., β=15%, B=1.35)in Part (b).

From Eq. 14-27 the effective total stiffnessof the FPS isolation system consisting of 60identical isolators will be

( )k/in 301

0.12

120,1406.0

23.61

120,14 =+=effK

and the seismic base shear coefficients arecalculated as before:

( )

125.0225.0

25.0120,14

0.12301

===

===

I

bs

effb

R

CC

W

DKC

14.10 CONCLUSIONS

Several practical systems of seismicisolation have been developed and implementedin recent years, and interest in the application ofthis technique continues to grow. Althoughseismic isolation offers significant benefits, it isby no means a panacea. Feasibility studies arerequired early in the design phase of a project toevaluate both the technical and the economicissues. If its inclusion is appropriate from atechnical and first-cost perspective, thensignificant life-cycle cost advantages can beachieved. Thus, seismic isolation represents animportant step forward in the continuity searchfor improved seismic safety.

The construction costs of incorporatingseismic isolation in new buildings in the United

States indicates that it depends on two primaryvariables: the design force level of theconventional building and the location of theplane of isolation. The theory of seismicisolation permits substantial cost savings forisolated buildings compared to conventionconstruction. However, given the current coderegulations, the initial cost for seismic isolatedstructures can be equal to or exceed the cost fora similarly situated fixed base building by asmuch as 5%. However, one should keep inmind that this is a very minor price to pay forachieving a structures which will have asubstantially better seismic performance duringmajor earthquakes. Simply stated, achieving thelevel of performance provided by seismicisolation is virtually impossible throughconventional construction.

For the retrofit of existing buildings, seismicisolation may only be technically applicable inone out of approximately eight buildings. Whenit is technically feasible it has the attractivefeature that most of the construction work isconfined to the basement area. Retrofitconstruction costs, when compared to aconventional code force level upgrade, havebeen shown to be comparable. In addition,disruption to the operation of the facility maybe avoided during construction with the use ofseismic isolation.

One of the major difficulties in comparingthe costs and benefits of a conventional and anisolated structure is the significant difference intheir performance characteristics. In the onlysuch design performed to date, a critical FireCommand and Control Facility for Los AngelesCounty required both a conventional and anisolated two story structure to meet the samestringent performance criteria. In this case theisolated design was shown to be 6% lessexpensive.

If equivalent performance designs are notperformed then the costs and benefits ofdifferent structural design schemes can only beassessed by calculating and comparing the fourprincipal cost impact factors: 1) constructioncost: 2) earthquake insurance premium: 3)physical damage that must be repaired and 4)

754 Chapter 14

disruption costs, loss of market share andpotential liability to occupants for their losses.Earthquake damage studies have shown thatseismic isolation can reduce the cost ofearthquake damage factors of 4 to 7.Furthermore, the estimated dollar value ofearthquake damage in an isolated building hasbeen shown to be less than the currentlyavailable 10% earthquake insurance deductible.

REFERENCES

14-1 Calantariants, J. A., “improvements in andConnected with Building and Other Works andAppurtenances to Resist the Action of Earthquakes andthe Like,” Paper No. 325371, Engineering Library,Stanford University, CA, 1909.

14-2 deMontalk, Robert Wladislas, Shock Absorbing orMinimizing Means for Buildings,” U.S. Patent No.1,847,820, 1932.

14-3 Bechtold, Jacob, “Earthquake-Proof Building,” USPatent No. 845,046, 1907.

14-4 Wright, F.L., An Autobiography: Frank LloydWright, Horizon Press, New York, 1977.

14-5 Green, N.B., “Flexible First Story Construction forEarthquake Resistance, “Trans. Amer. Soc. Civil Eng.100, 645, 1935.

14-6 Kelly, J.M. “Aseismic Base Isolation: Its Historyand Prospects,” Joint Sealing and Bearing Systems forConcrete Structures, Publication SP-70, AmericanConcrete Institute, 1982.

14-7 Buckle, I.G. and Mayes, R.L., “Seismic Isolation:History, Application and Performance - A WorldView, “Earthquake Spectra Journal, Theme Issue:Seismic Isolation, EERI, Vol. 6, No. 2, May 1990; andBuckle, I.G., “Development and Application of BaseIsolation and Passive Energy Dissipation: A WorldOverview,” Applied Technology Council Report 17,Palo Alto, CA, Mar. 1986.

14-8 Naeim, F. and Kelly, J.M., Design of SeismicIsolated Structures: From Theory to Practice, JohnWiley and Sons, Inc., New York, 1999.

14-9 Skinner, R.E., Tyler, R.G., Heine, A.J., andRobinson, W.J., “Hysteretic Dampers for theProtection of Structures from Earthquakes,” Bull. NewZealand Nat. Soc. Earthquake Eng. 13, No.1, Mar.1980.

14-10 Way, D. and Lew, M., “Design and Analysis of aHigh Damping Rubber Isolation System,” AppliedTechnology Council Report No. 17, Palo Alto, CA,1986.

14-11 Jolivet, F. and Richli, M., “Aseismic FoundationSystem for Nuclear Power Stations,” Transactions of

the Fourth Conference on Structural Mechanics inReactor Teachnology, San Francisco, Vol. K, No. 9/2,1977

14-12 Castiglinoni, A., Urbano, C., and Stupazzini, B.,“Seismic Design of Bridges in High Activity Region,”Proceedings of the Seventh European Conference onEarthquake Engineering, Athens, Vol. 6, 186-203,1982.

14-13 Ikonomou, A.S., “Seismic Isolation of Bridges withthe Alexisismon,” Proceedings of the Conference onShort an Medium Span Bridges, Toronto, 141-153,1982.

14-14 Robinson, W.H., “Lead-Rubber HystereticBearings Suitable for Protecting Structures DuringEarthquakes,” J. Earthquake Eng. And StructuralDynamics 10, 593-604, 1982.

14-15 Blakeley, R. W. G., et al., “Recommendations forthe Design and Construction of Base IsolatedStructures,” Bull. New Zealand Nat. Soc. EarthquakeEng. 12, No. 2, 1979.

14-16 Kelly, J. M. and Tsztoo, D., “EarthquakeSimulation Testing of a Stepping Fram with Energy-Absorbing Devices,” Report No. UCB/EERC-77/17,Earthquake Engineering Research Center, Univ. ofCalifornia, Berkeley, 1977.

14-17 “Earthquake Simulator Tests of a Nine-Story SteelFrame with Columns Allowed to Uplift,” report No.UCB/EERC-77/23, Earthquake Engineering ResearchCenter, Univ. of California, Berkeley, 1977.

14-18 Kelly, J. M., Eidinger, J. M., and Derham, C. J., “APractical Soft Story System,” Report No. UCB/EERC-77/27, Earthquake Engineering Research Center, Univ.of California, Berkeley, 1977.

14-19 Kelly, J. M., Beucke, K. E., and Skinner, M. S.,“Experimental Testing of a Friction Damped AseismicBase Isolation System with Fail-Safe Characeristics,”Report No. UCB/EERC-80/18, Earthquake EngineerigResearch Center, Univ. of California, Berkeley, 1980.

14-20 Kelly, J. M., Beucke, K. E., and Skinner, M. S.,“Experimental Testing of an Energy-Absorbing BaseIsolation System,” Report No. UCB/EERC-80/35,Earthquake Engineering Research Center, Universityof California, Berkeley, 1980.

14-21 Kelly, J. M., and Hodder, S. B., “ExperimentalStudy of Lead and Elastomeric Dampers for BaseIsolation Systems,” Report No. UCB/EERC-81/16,Earthquake Engineering Research Center, Univ. ofCalifornia, Berkeley, 1981.

14-22 Kelly, J. M., Buckle, I. G., and Tsai, H. C.,“Earthquake Simulator Testing of a Base IsolatedBridge Deck,” Report No. UCB/EERC-85/09,Earthquake Engineering Research Center, Univ. ofCalifornia, Berkeley, 1985.

14-23 Structural Engineers Association of California,Recommended Lateral Force Requirements andCommentary, San Francisco, 1983.


14-24 Stanton, J. F. and Roeder, C. W., “ElastomericBearings: Design, Construction and Materials,”NCHRP Report 248, Transportation Research Board,Washington, 1982.

14-25 Applied Technology Council, “Proceedings of aSeminar and Workshop on Base Isolation and PassiveEnergy Dissipation,” ATC Report No. 17, Palo Alto,CA, 1986.

14-26 Structural Engineers Association of NorthernCalifornia (1986), Tentative Seismic Isolation DesignRequirements, San Francisco, 1986.

14-27 Structural Engineers Association of NorthernCalifornia, Tentative Lateral Force Requirements, SanFrancisco, 1985.

14-28 International Conference of Building Officials,Uniform Building Code, Whittier, CA 1994.

14-29 Kelly, T. E., Mayes, R. L., and Jones, L. R.,“Preliminary Design Procedures for SeismicallyIsolated Structures,” Proceedings of a Seminar on BaseIsolation and Passive Energy Dissipation, Report No.17, Applied Technology Council, Palo Alto, CA, 1986.

14-30 Buckle, I.G. and mayes, R.L. (1990), “TheApplication of Seismic Isolation to Bridges,”Proceedings ASCE Structures Congress: SeismicEngineering - Research and Practice, pp 633-642, May,1990.

14-31 Chalhoub, M.S., and Kelly, J.M., (1989)“Earthquake Simulator Evaluation of a CombinedSliding Bearing and Tension Controlled RubberBearing Isolation System.” Proceeding, 1989 ASMEPressure Vessels and Piping Conference, AmericanSociety of Mechanical Engineers, Hawaii, Vol. 181, pp59-64.

14-32 Constantinou, M.C., Mokha, A., and Reinhorn,A.M., (1990) “Teflon Bearings in Base Isolation II:Modeling, “Journal of Structural Engineering, ASCE,Vol. 116, No. 2 pp. 455-474.

14-33 Griffith, M.C., Aiken, T.D., and Kelly, J.M. (1988)“Experimental Evaluation of Seismic Isolation of aNine-Story Braced Steel Frame Subject to Uplift.”Report No. UCB/EERC-88/05, EarthquakeEngineering Research Center, University of California,Berkeley.

14-34 Kelly, J.M.., Eidenger, J.M. and Derham, C.J.(1977) “A Practical Soft Story System,” Report No.UCB/EERC-77/27, Earthquake Engineering ResearchCenter, University of California, Berkeley.

14-35 Kelly, J.M., Beucke, K.E. and Skinner, M.S.(1980), “Experimental Testing of an Energy-AbsorbingBase Isolation System,” Report No. UCB/EERC-80/35, Earthquake Engineering Research Center,University of California, Berkley.

14-36 Kelly, J.M. and Hodder, S.B. (1981),“Experimental Study of Elastomeric Dampers for BaseIsolation Systems,” Report No. UCB.EERC-81/16,Earthquake Engineering Research Center, Universityof California, Berkeley.

14-37 Kell, J.M., Buckle, I.G. and Tsai, H.C. (1985),“Earthquake Simulator Testing of Base Isolated BridgeDeck,” Report Bo UCB/EERC-85/09, EarthquakeEngineering Research Center, University of California,Berkley.

14-38 Mokha, A., Constantinou, M.C., and Reinhorn,A.M., (1990), “Teflon Bearings in Base Isolation I:Testing, “Journal of Structural Engineering, ASCE,Vol. 116, No. 2, pp. 438-454.

14-39 Mohka, A., Constantinou, M.C., and Reinhorn,A.M., (1990), “Teflon Bearings in a Seismic BaseIsolation. Experimental Studies and mathematicalModeling.” Report No. NCEER-88-0038, NationalCenter for Earthquake Engineering Research, StateUniversity of New York, Buffalo.

14-40 Zayas, V., Low, S.S., and Mahin, S.A., (1987) “TheFPS Earthquake resisting System, ExperimentalReport.” Report No. UCB/EERC-87/01, EarthEngineering Research Center, University of California,Berkeley.

14-41 Applied Technology Council, “proceedings of aWorkshop on Seismic Isolation, Passive Energy

14-42 International Code Council (2000), InternationalBuilding Code, March.

14-43 International Conference of Building Officials,Uniform Building Code, Whittier, CA 1997.

14-44 Federal Emergency Management Agency (1997),NEHRP Guidelines for the Seismic Rehabilitation ofBuildings, FEMA-273, Washington, D.C., October.

14-45 Federal Emergency Management Agency (1997),NEHRP Commentary on the Guidelines for theSeismic Rehabilitation of Buildings, FEMA-274,Washington, D.C., October.

14-46 Kelly, J.M. (1996), Earthquake-Resistant Designwith Rubber, 2nd Edition, Springer-Verlag, London.

756 Chapter 14

Chapter 14-Design of Structures With Seismic Isolation (1)

Documents

applied technology

earthquake

friction pendulum

passive energy

probable yield

entire isolation

response spectrum

bottom end