seismic isolation a solution to the earthquake problems of ... journal/1988/may/seismic... ·...

Special Report

— Seismic IsolationA Solution to the

Earthquake Problems of thePrecast Concrete Industry

iRonald L. MayesPresidentDynamic Isolation Systems, Inc.Berkeley, California

Ian G. BuckleVice President-EngineeringDynamic Isolation Systems, Inc.Berkeley, California

Lindsay R. JonesVice PresidentDynamic Isolation Systems, Inc.Berkeley, California

An innovative design strategy called

seismic isolation" provides aneconomic practical alternative for thedesign of new structures and the seismicrehabilitation of existing buildings,bridges and industrial equipment.Rather than resisting the large forcesgenerated by earthquakes, seismic iso-lation decouples the structure from theground motion and thus reduces earth-

quake forces by factors of 5 to 10. Thislevel of force reduction is very signifi-cant because it may eliminate the duc-tility demand on a structural system.This fact alone is a very significant ben-efit for the precast concrete industry.

There are now approximately 100civil engineering structures that havebeen constructed using the principles ofseismic isolation, including a 12-story

24

prefabricated building.' Four of thecompleted structures (two in Japan, onein New Zealand and one in the UnitedStates) have been subjected to realearthquakes, with the largest being aRichter Magnitude 6.2 seismic event inNew Zealand in February 1987. Al-though these seismic events have notbeen major earthquakes, all four of theisolated structures have shown the forcereductions expected.

One of the major difficulties facing theprecast and prestressed concrete indus-try in earthquake prone areas of theUnited States is the provision of ade-quate ductility in prefabricated struc-tural systems that are used as the lateralload resisting system. Researchers havefocused on solutions to this problem,and in September 1986, a PCI work-shop a was dedicated to this subject area.A paper presented by Mueller' at theworkshop incorporated the followingconclusion:

"To markedly change the marketposition of precast and prestressedconcrete in areas of high seismicity,a more radical approach is needed:base (seismic) isolation. While partof the research and developmentfunds should be spent on deter-mining the actual earthquake resis-tance of existing systems, a largepart should be spent on developinga pre-engineered, prefabricated,modular base isolation systemrather than on trying to achievemarginal improvements with con-ventional concepts. Rather thantrying to undercut the competitionwith cheaper precast and pre-stressed concrete solutions, themarket strategy should be to offer,for the same price as the competi-tion, a better product: an earth-quake protected rather than anearthquake resistant system."Clearly, Mueller's conclusion indi-

cates significant benefits for the precastindustry by combining the advantages ofseismic isolation with those of precast

SynopsisThe authors present a state of the

art report on the background, basicconcepts, design principles, feasibilityand code issues of seismic isolation— especially in relation to precastconcrete structures.

A summary of the projects com-pleted so far and the durability anddesign issues associated with elas-tomeric bearings are discussed. Inaddition, the design force implicationsfor precast and prestressed construc-tion are presented together with acost-benefit evaluation.

and prestressed construction. The com-pleted, 12-story precast Union House inAuckland, New Zealand, described laterin this paper, attests to the Fact that it isboth economic and practical. There arecurrently five bridges, three buildingsand three major pieces of equipment inthe United States that have been or arein the process of being constructedusing seismic isolation principles.Others are in the design developmentphase, including an eight-story hospital.

This paper discusses the background,basic concepts, design principles, feasi-bility and code issues of seismic isola-tion. A brief summary of the projectscompleted to date and the longevity anddesign issues associated with elas-tomeric bearings are also discussed. Inaddition, the design force implicationsfor precast and prestressed constructionare presented together with a cost-benefit evaluation.

BACKGROUND

In an article in Architecture Magazineby Chris Arnold, 4 the introduction of in-novative solutions to earthquake engi-neering problems is discussed. The

PCI JOURNAL/May-June 1988 25

early part of this background materialhas been extracted from that article.

"Because of today's concern forliability, engineering innovationsmust be exhaustively tested and an-alytically proven to a degree un-known in the past. Early engineerswere respected for their ability todesign from first principles andproduce designs that were concep-tually right even though analyticalor laboratory methods did not existthat would remove all doubt. Forthe most part, the great early engi-neers removed doubt by force oftheir personality and confidence.They took risks that would be un-thinkable today.

The field of seismic design is, asperhaps befits a subject directlyconcerned with both life safety anduncertainty, cautious and slow toinnovate. In practice, improvedseismic design does not represent amarket opportunity because seismicsafety is generally taken for granted.Like other code-dominated issues,and like airplane safety, seismicsafety has never been much of aselling point. Money diverted toimprove seismic resistance is oftenseen as a detraction from more visi-ble and enjoyable attributes.

Improvement in seismic safety,since about the time of the SanFrancisco earthquake of 1906, hasbeen due primarily to acceptance ofever-increasing force levels towhich buildings must be designed.Innovation has been confined to thedevelopment and acceptance ofeconomical structural systems thatperform reasonably well, accom-modate architectural demands suchas open exteriors and the absence ofinterior walls, and enable materialssuch as steel and reinforced con-crete to compete in the marketplaceon near equal terms.

The vocabulary of seismic designis limited. The choices for lateral

resistance lie among shear walls,braced frames and moment-resis-tant frames, Over the years, thesehave been refined and their detailsdeveloped, and methods of analysisand modeling have improved andreduced uncertainty . But the basicstructure has not changed: constructa very strong building and attach itsecurely to the ground. This ap-proach of arm wrestling with natureis neither clever nor subtle, and itinvolves considerable compromise.

Although codes have mandatedsteadily increasing force levels, abuilding in a severe earthquakemay still encounter forces severaltimes above its designed capacity ifit were able to remain elastic. Thisdiscrepancy between seismic de-mand and capacity is traditionallyaccommodated by reserve capacityor ductility of the structural system.The ability of materials and struc-tural members to dissipate energyby permanent deformation -which is called ductility — greatlyreduces the likelihood of total col-lapse."Modern buildings contain extremely

sensitive and costly equipment that hasbecome vital in business, commerce,education, and health care. Electroni-cally kept records are essential to theproper functioning of our society. Thesebuilding contents frequently are morecostly and valuable than the buildingsthemselves. Furthermore, hospitals,communication and emergency centers,and police and fire stations must be op-erational when needed most: immedi-ately after an earthquake.

Conventional construction can causevery high floor accelerations in stiffbuildings and large interstory drifts inflexible structures. These two factorscause difficulties in ensuring the safetyof the building components and con-tents (see Fig. 1).

"In the last decade, an alternativeto the brute force response to nature

26

Fig. 1. The deformation pattern in a conventional structure during an earthquake.Accelerations of the ground are amplified on the higher floors and the loose contents aredamaged. Building deformations and distortions could be permanent (from Ref. 43).

has finally reached a stage, if not offruition, at least of application. Thisapproach is obvious and easily ex-plainable at the cocktail party level:Why not detach the building fromthe ground in such a way that theearthquake motions are not trans-mitted up through the building, orare at least greatly reduced? Thisconceptually simple idea has re-quired much research to make itfeasible, and only with moderncomputerized analysis has it be-come possible. Application has de-pended on very sophisticated mate-rials research into both natural andcomposite materials in order to pro-vide the necessary performance.

This new concept, now generally

termed "seismic isolation," meetsall the criteria for a classic moderntechnological innovation, Imagina-tive advances in conceptual think-ing were necessary, as were mate-rials new to the industry, and ideashave developed simultaneously ona worldwide basis. But the methodthreatens conventional and estab-lished design procedures, so theroad to seismic isolation innovationis paved with argument, head.shaking, and bureaucratic caution.All, to some extent, well-inten-tioned and necessary, given ourlitigious society."Buildings mounted on an isolation

system can prevent most of the hori-zontal movement of the ground from

PCI JOURNAUMay-June 1988 27

wII_--- -Iiirrre--- Ii,,® st

Fig. 2. The deformation pattern in an isolated structure during an earthquake. Movementtakes place at level of isolators. Floor accelerations are low; the building, its occupants,and its loose contents are safe (from Ref. 43}.

being transmitted to the building. Thisresults in a significant reduction in flooraccelerations and interstory drifts,thereby providing protection to thebuilding contents and components (seeFig. 2).

The principle of seismic isolation is tointroduce flexibility at the base of astructure in the horizontal plane, whileat the same time introducing dampingelements to restrict the amplitude orextent of the motion caused by theearthquake. The essential feature is toensure that the period of the structure iswell above that of the predominantearthquake input.

The concept of isolating structuresfrom the damaging effects of earth-quakes is not new. The first U.S. patent

for a seismic isolation scheme was takenout in 1907,° and since that time severalproposals with similar objectives havebeen made. 1- 13 Nevertheless, until thelast decade, few strictures have beendesigned and built using these princi-ples.

New impetus was given to the con-cept of seismic isolation by the suc-cessful development of mechanicalenergy dissipators and elastomers withhigh damping properties, ' 4 '9 Mechan-ical energy dissipators, when used incombination with a flexible isolationdevice, can control the response of thestructure by limiting displacements andforces, thereby significantly improvingseismic performance. The seismicenergy is dissipated in components spe-

28

cifically designed for that purpose, re-lieving structural elements such asbeams and columns from energy dissi-pation roles (and thus damage).

There are now approximately 100civil engineering strictures throughoutthe world that have been constructedusing the principles of seismic isolation,and Kelly" and Buckle 12 "i 3 have bothsummarized this activity in recent pub-lications.

The advantages of seismic isolationinclude:

1. The ability to eliminate ductilitydemand.

2. Significant reduction of structuraland nonstructural damage.

3. Enhancement of the safety of thebuilding contents, occupants, andarchitectural facades.

4. Reduction of seismic design forces.In addition to the safety issues, the

elimination of the ductility demand onthe structural system removes one of themajor problem areas for the use of pre-cast and prestressed construction inearthquake prone areas of the world.With seismic isolation, reduction factorsof 5 to 10 in the elastic forces may beachieved for stiff structures on firm soils,such as low and medium rise buildings,nuclear power plants, bridges and manytypes of industrial equipment. How-ever, these force reductions are not al-ways available and some tectonic andsoil conditions may preclude the use ofseismic isolation altogether.

AN IDEA WHOSETIME HAS COME

As noted above, and discussed inmore detail later, the elastomeric bear-ing and the mechanical damper arefundamental components in most prac-tical seismic isolation schemes. But it isnot just the invention of the elastomerichearing and the energy dissipator whichhas made seismic isolation a practicalreality. Three other parallel, but inde-

pendent, developments have also con-tributed to its recent success.

The first of these was the develop-ment of reliable software for the com-puter analysis of structures so as to pre-dict their performance and determinedesign parameters. Work has been inprogress for more than 15 years on theinelastic analysis of structural systemsfor dynamic loads, and there are manyavailable computer programs. Applica-tion to seismically isolated structures isstraightforward and correlation studieswith model tests show the software to besoundly based.

The second development was the useof shaking tables, which are able tosimulate the effects of real recordedearthquake ground motions on differenttypes of structures. The results of shak-ing table tests over the last 12 years' °-211

have provided another mechanism toenhance confidence in the way build-ings respond during real earthquakes. Inaddition, the results provide an oppor-tunity to validate computer modelingtechniques which are then used on full-sized strictures.

A third important development is theskill of the engineering seismologist inestimating ground motions at a particu-lar site. Recent advances in seismologyhave improved the prediction of site-specific ground motions which take intoaccount fault distances, local and globalgeology, and return period. These designmotions are basic input to the computermodeling of seismically isolated systemsand are a vital step in the estimation ofsystem performance.

In summary then, five recent de-velopments are together responsible forthe claim that isolation is indeed an ideawhose time has come:

1. The design and manufacture ofhigh quality elastomeric (rubber) pads,frequently called bearings, that are usedto support the weight of the structurebut at the same time release it fromearthquake induced forces.

2. The design and manufacture of


mechanical energy dissipators (absorb-ers) and high damping elastomers thatare used to reduce the movement acrossthe bearings to practical and acceptablelevels and to resist wind loads.

3. The development and acceptanceof computer software for the analysis ofseismically isolated structures which in-cludes nonlinear material propertiesand the time varying nature of theearthquake loads.

4. The ability to perform shakingtable tests using real recorded earth-quake ground motions to evaluate theperformance of structures and provideresults to validate computer modelingtechniques.

S. The continuing development ofprocedures for estimating site-specificearthquake ground motions for differentreturn periods.

CONSIDERATIONS FORSEISMIC ISOLATION

Seismic isolation should be con-sidered if any of the following situationsapply:

• Increased building safety, post-earthquake operability and busi-ness survivability are desired.

• Reduced lateral design forces aredesired.

• Alternate forms of constructionwith limited ductility capacity(such as precast and prestressedconcrete) are desired in an earth-quake region.

• An existing structure is not cur-rently safe for earthquake loads.

For new structures, current buildingcodes apply in all seismic zones, andtherefore many owners, architects andengineers may feel that the "need" forseismic isolation does not exist becausethe code requirements can be satisfiedby current designs. However, the com-mentary to the Uniform Building Code(UBC) incorporated in the StructuralEngineers Association of California

(SEAOC) Recommended Lateral ForceRequirements ? ' states that buildings de-signed in accordance with its provisionswill:

• Resist minor earthquakes withoutdamage.

• Resist moderate earthquakes with-out structural damage but withsome nonstructural damage.

• Resist major earthquakes withoutcollapse but with structural andnonstructural damage.

These principles of performance alsoapply to buildings that are rehabilitatedto code Ievel design forces.

It is clear that modern design codes donot protect a structure or its contentsagainst damage in a major earthquake.Only life safety is preserved.

Seismic isolation gives the option ofproviding a building with better per-formance characteristics than our cur-rent code approach. It thus represents amajor step forward in the seismic designof civil engineering structures. In thecase of building retrofit, the need forisolation may be more obvious: thestructure may simply not be safe in itspresent condition. In such cases, seis-mic isolation should be compared withalternative solutions, such as strength-ening, to determine the cost effective-ness.

SOLUTIONS FORNONSTRUCTURAL. DAMAGE

One of the more difficult issues to ad-dress from a conventional design view-point is that of reducing nonstructuraland building content damage. Often ig-nored, this can be very expensive to in-corporate in conventional design. Infact, the additional cost to satisfy themore stringent bracing requirements ofnonstructural elements in a Californiahospital is about $2 to $4 per sq ft morethan that required for a conventionalbuilding.

There are two primary mechanisms

30

that cause nonstructural damage. Thefirst is related to interstory drift betweenfloors, and the second is related to flooraccelerations. Interstory drift is definedas the relative displacement that occursbetween two floors divided by the storyheight. Floor accelerations are theabsolute accelerations that occur as a re-sult of the earthquake, and in conven-tional construction, floor accelerationsgenerally increase up the height of thebuilding. Together, these two compo-nents cause damage to the buildingcontents, architectural facades, parti-tions, piping and ductwork, ceilings,building equipment and elevators (seeFig. 1).

There are two different designphilosophies that are debated within thestructural engineering profession whichdeal with minimizing nonstructuraldamage. One argues that stiff buildingsare the best solution. Stiff buildings re-duce interstory drift, but they producehigh floor accelerations. The otherschool of thought argues that flexiblebuildings are the solution, because theyattract less force and tend to reduce flooraccelerations. Although this is true,flexible buildings have much higherinterstory drifts, and this accentuatesdamage to components that are sensitiveto drift.

Clearly, a design concept that reducesboth interstory drift and floor accelera-tions combines the best aspects of thesetwo current design philosophies. Seis-mic isolation is such a concept (see Fig.2), since it significantly reduces bothfloor accelerations and interstory driftand thus provides a practical solution tothe difficult problem of reducing non-structural earthquake damage.

BASIC ELEMENTSOF SEISMIC

ISOLATION SYSTEMSThere are three basic elements in any

practical seismic isolation system.

These are:I. A flexible mounting (support) so

that the period of vibration of thetotal system is lengthened suffi-ciently to reduce the force re-sponse.

2. A damper or energy dissipator sothat the relative deflections be-tween building and ground can hecontrolled to a practical designlevel.

3. A means of providing rigidityunder low (service) load levelssuch as wind and minor earth-quakes,

FlexibilityBridge structures, for many years,

have been supported on elastomeric

ISOLATED

ELASTOMERiC -STRUCTURE

NF AS NO

(a) Structure at rest

-";.'"iL

(b) Ground motion to the left

I ^

I ^

I I

-^utir-

(c) Ground motion to the right

Fig, 3. Flexible supports for isolatedstructures: Elastomeric bearings.

PCI JOURNAL`May-June 1988 31

1 SOLATED

ROLLERS IT'RUCTURE


/ - t t v t


I I


Fig. 4. Flexible supports for isolatedstructures: Rollers.

ISOLATEDSTRUC TLJR^

GROUNDSL}RFACE_

aSLEEvEO ----PILES

_L RESTRA'.N

^.. DEVICEaEOROCIS s^ r .....


_-^flrr I

I-

d r


^{ a I-,—

Vl1/1/1r


Fig. 5. Flexible supports for isolatedstructures: Sleeved piles.

bearings,29 and as a consequence havealready been designed with flexiblemounts. It is equally possible to supportbuildings on elastomeric bearings, andnumerous examples exist where build-ings have been successfully mounted onpads. To date, this has been done pri-marily for vertical vibration isolationrather than seismic protection. Morethan 100 buildings in Europe and Aus-tralia have been built on rubber bear-ings to isolate them from vertical vibra-tions from subway systems, By increas-ing the thickness of the bearing, addi-tional lateral flexibility and period shiftfor seismic isolation can he attained.

While the introduction of lateral flexi-bility may be highly desirable, addi-

tional vertical flexibility is not. Verticalrigidity is maintained by constructingthe rubber bearings in layers and sand-wiching steel reinforcing plates be-tween each layer. The reinforcingplates, which are bonded to each layerof rubber, constrain lateral deformationof the rubber under vertical load, re-sulting in vertical stiffnesses severalhundred times the lateral stiffness andcomparable to the vertical stiffness of aconventional structural column.

An elastomeric bearing is not the onlymeans of introducing flexibility into astructure, but it certainly appears to bethe most practical method and the onewith the widest range of application.other possible devices include rollers,

32

lSOLATEDSTRUCTURE

NO CONNECTION



A


Fig. 6. Flexible supports for isolatedstructures: Rocking.

SUSPENSIONPOINTS ARE

_CABLESCONNECTED T4THE GROUND

ISOLATEuNO CONNECTION - STRUCTi1RE


I ^


I I

I ^

u - A


Fig. 7. Flexible support for isolatedstructures: Cable suspension.

friction slip plates, cable suspensions,sleeved piles and rocking (stepping)foundations.

Figs. 3 through 7 show the various de-vices and mechanisms for attainingflexibility in isolated structures.

The reduction in force with increasingperiod (flexibility) is shown schemati-cally in the force-response curve of Fig.8. Substantial reductions in base shearare possible if the period of vibration ofthe structure is significantly lengthened.The reduction in force response illus-trated in Fig. 8 is primarily dependenton the characteristics of the earthquakeground motion and the period of thefixed-base structure. Further, the addi-tional flexibility needed to lengthen the

period of the structure will give rise tolarge relative displacements across theflexible mount. Fig. 9 shows anidealized displacement response curvefrom which displacements are seen toincrease with increasing period (flexi-bility).

However, as shown in Fig. 10, if sub-stantial additional damping can be in-troduced into the isolation system, thedisplacement problem can be overcome.It can also be seen that increasing thedamping reduces the forces on thestructure at a given period and removesmuch of the sensitivity to variations inground motion characteristics, as indi-cated by the smoother force-responsecurves at higher damping levels.

PCI JOURNAUMay -June 1988 33

ACCELERATION

PERIOD SHIFT

PERIOD

ACCELERATION RESPONSE SPECTRUM

Fig. 8. Idealized force response spectrum.

DISPLACEMENT

INCREASING DAMPING

---- PERIOC

DISPLACEMENT RESPONSE SPECTRUM

ACC:EL E RATION

SPLAOEMENT

PERIOD SHIFT

—^i

PERtCO

DISPLACEMENT RESPONSE SPECTRUM

Fig. 9. Idealized displacement responsespectrum.

Energy DissipationOne of the most effective means of

providing a substantial level of dampingis through hysteretic energy dissipation.The term "hysteretic" refers to the offsetin the loading and unloading curvesunder cyclic loading. Work done duringloading is not completely recoveredduring unloading and the difference islost as heat. Fig. 11 shows an idealizedforce-displacement loop where the en-closed area is a measure of the energydissipated during one cycle of motion.Mechanical devices which use the plas-tic deformation of either mild steel or

INCREASING DAMPIN;.

c==

ACCELERATION RESPONSE SPECTRUM

Fig. 10. Response spectra for increasingdamping.

POReC/PLASTIC DEFORMA-!T,

DISPLACEMENT

HYSTERESIS LOOP

Fig. 11. Hysteretic force-deflection curve.

lead to achieve this behavior have beendeveloped, 14.1 and several mechanicalenergy dissipation devices developed inNew Zealand' s are shown in Fig. 12-

34

Loading ArmsAnchors Anchor

^ f e

Lead

LoadingPiston

Extrusion OrificeTorsionalBeam

Torsional Beam Device Lead Extrusion Device

in

Fiexural Plate Device

Superstructure Anchor

0

Cylir^JricaISleeve

Pier Anchor / W .-^ sphericalCone

0%STJLai

1 Beam

Flexural Beam Device

)ersiruciurehr

Flexural Beam Device

Superstructure^^^ Anchor

00

ElaslornericBearing

' I '1

Lead • PierAnchor

Lead-Rubber Device

Fig. 12. Various mechanical energy dissipators.


Many engineering materials are hys-teretic by nature, and all elastomers ex-hibit this property to some extent. Bythe addition of special purpose fillers toelastomers, it is possible to increasetheir natural hysteresis without undulyaffecting their mechanical properties.''Such a technique gives a useful sourceof damping, but so far it has not beenpossible to achieve the same level ofenergy dissipation as is possible with alead-rubber elastomeric bearing.

Friction is another source of energydissipation which is used to limit de-flections. However, it can be a difficultsource to quantify, and reliable systemstend to be of a magnitude more expen-sive than either of the above mecha-nisms. A further disadvantage is thatmost frictional devices are not self-cen-tering, and a permanent offset betweenthe sliding parts is a real possibility afteran earthquake.

Hydraulic damping has been usedsuccessfully in some bridges and a fewspecial purpose structures.'s Potentiallyhigh damping forces are possible fromviscous fluid flow, but maintenance re-

Table 1. Alternative sources of flexibilityand energy dissipation.

Flexible Unreinforced rubbermounting blockssystems Elastomeric bearings

(reinforced rubberblocks)

Sliding platesRoller and/or ball

bearingsSleeved pilesRocking systemsSuspended floorsAir cushionsSlinky springs

Damping Plastic deformation ofdevices I a metal=.mechanisms Friction

High damping elastomersViscous fluid dampingTuned mass damping

quirements and high initial cost have re-stricted the use of these particular de-vices.

Rigidity for Low Lateral LoadsWhile lateral flexibility is highly de-

sirable for high seismic loads, it isclearly undesirable to have a structuralsystem which will vibrate perceptiblyunder frequently occurring loads suchas minor earthquakes or wind loads.

Lead-rubber bearings (and other me-chanical energy dissipators) provide thedesired low load rigidity by virtue oftheir high elastic stiffness (see Fig. 11).Some other seismic isolation systemsrequire a wind restraint device for thispurpose—typically a rigid componentdesigned to fail under a given level oflateral load. This can result in a shockloading being transferred to the struc-ture due to the sudden loss of load in therestraint. Nonsymmetrical failure ofsuch devices can also introduce unde-sirable torsional effects in a building.Further, such devices will need to bereplaced after each failure.

Table I summarizes the sources offlexibility and energy dissipation thathave been discussed here. A more de-tailed explanation of these concepts canbe found in the proceedings of a work-shop on base isolation and passiveenergy dissipation that was conductedby Applied Technology Council.3°

PERFORMANCE OFELASTOMERIC BEARINGSAs the application of seismic isolation

continues to increase, the design profes-sion is now focusing its attention on theperformance of the structural compo-nents that make it work, To date, morethan 90 percent of the 100 completedisolation projects utilize an elastomericbearing as the element of flexibility.Furthermore, the more than 100 build-ings that are vertically isolated to avoid

36

subway vibrations all incorporate elas-tomeric bearings. Some of these appli-cations have more than a 40 year history.In addition to these applications, rein-forced elastomeric bearings have beenused as bridge hearings for over 40 yearsand unreinforced elastomeric bearingshave been used for over 95 years." Theuse of elastomeric bearings is, therefore,not new.

What is extremely important from anisolation design perspective is the longterm stability of the shear stiffness prop-erties of the bearings, since this pro-vides the period lengthening compo-nent of an isolation system. Althoughthe amount of long term data is not ex-tensive, what is available shows that themechanical properties of natural rubberhearings are stable with time.

Data are available for two case his-tories of natural rubber elastomericbearings.'-33 One example is of rubbermountings manufactured in 1953 by theAndre Rubber Company for the isola-tion of a gun testing machine at FortHalstead in Kent, England. The indi-vidual units are natural rubber singlelayers, 18 1/2 x 8 in. (470 x 203 mm) inplan by 3'/2 in. (89 mm) thick, bondedbetween steel plates, The machine issupported on 162 of these in triangularstacks.

The machine has been in use fornearly 30 years and the bearings are stillfunctioning perfectly. Some sparebearings were manufactured at the sametime and were stored beside themachine. Two were selected that couldbe identified with the original test data.These bearings were retested in Feb-ruary 1983 at the Rubber and PlasticsResearch Association. One bearing hadincreased in stiffness by 15 1/2 percentand the other by 4'/2 percent. This repre-sents a remarkably small change in stiffness over such a long period, and agingmechanisms are such that the probabil-ity is that most of this stiffening tookplace over the first few years, very littlefurther change would now he observed.

Recently, two natural rubber bridgebearings were removed from a motor-way bridge in Kent for testing tinder aMinistry of Transport project. 32 Testingafter 20 years of service showed that theshear stiffness had increased by only 10percent above the original specifiedvalue.

It is worth noting that a reasonablylarge change in shear stiffness can betolerated in an isolation system beforethe forces transmitted to the structure bythe bearings are significantly affected.For example, a 25 percent change inshear stiffness makes only a 10 percentchange in the period, and since mostearthquake response spectra are rela-tively flat in the period range of an iso-lated structure, there will be littlechange to the seismic response as a con-sequence.

SEISMIC ISOLATIONDESIGN PRINCIPLES

The design principles for seismicisolation are illustrated in Fig. 13. Thetop curve in this figure shows the realis-tic elastic forces based on a 5 percentground response spectrum which willbe imposed on a nonisolated structurefrom the new SEAOC BIue Book. n Thespectrum shown is for a rock site if thestructure has sufficient strength to resistthis level of load. The lowest curveshows the forces for which the UniformBuilding Code 37 requires a structure bedesigned, and the second lowest curveshows the probable strength assumingthe structure is designed for the UBCforces.

The probable strength is 1.5 to 2.0times higher than the design strengthbecause of the design load factors, actualmaterial strengths which are greater inpractice than those assumed for design,conservatism in structural design, andother factors. The difference betweenthe maximum elastic force and the prob-able yield strength is an approximate

PCi JOURNALIMay-June 1988 37

indication of the energy which must beabsorbed by ductility in the structuralelements.

When a building is isolated, themaximum elastic forces are reducedconsiderably due to period shift andenergy dissipation, as shown in Figs. 8and 10. The elastic forces on a seis-mically isolated structure are shown bythe small dashed curve of Fig, 13. Thiscurve corresponds to a system with ap-proximately 30 percent equivalent vis-cous damping.-''' If a relatively stiffbuilding with a fixed base fundamentalperiod of 0.8 seconds or less is isolated,then its fundamental period after isola-tion will be increased into the 1.5 to 2.5second range (Fig. 8).

This results in a reduced UBC designforce (Fig. 13), but more important isthat in the 1.5 to 2.5 second range, theprobable yield strength of the isolated

building is approximately the same asthe maximum force to which it will besubjected. Therefore, there will be littleor no ductility demand on the structuralsystem. Furthermore, the lateral de-sign forces are reduced by approximate-Iy 50 percent when compared to theUBC design forces for a conventionalstructure.

SEISMIC ISOLATIONFEASIBILITY

Structures are generally suitable forseismic isolation if the following condi-tions exist:

• Height of structure is two stories orgreater (unless unusually heavy).

• Site clearance permits horizontaldisplacements at the base of about6 in, (152 min),

___ PDiHerence must be Absorbed by Ductility

\/

j Force on Non-Isolated StruclureII Sufficiently Strong (RTC-6 Spectrum}

Probable Strength of Structure Designed

Force on Isolated StructureSpectrum)

JEjTTJ(Inelaslic

- -

Increased F^exibiiity Isolated Slruciures Period (secs)Range of Flexibility

0.4 08 1.2 1.6

2.0 24 2.8

Fig. 13. Design principles of seismic isolation.

1.00

0.80

060

0.40

0.20

000

38

ISOLATORS

Fig. 14. Optional locations for seismic isolatorsin buildings.

• Geometry of structure is not slen-der in elevation.

• Lateral wind loads or other non-seismic loads are less than ap-proximately 10 percent of theweight of the structure.

• Site ground motion is not domi-nated by long period components.

Each project must be assessed in-dividually and early in the design phaseto determine its suitability for seismicisolation. For this assessment, there aredifferences between new constructionand the retrofit of existing structures.

In new construction there are twopotential locations for the isolators, asshown in Fig. 14. The sub-basementoption has the advantage of minimalarchitectural disruption, but it requiresan additional structural floor. Placingthe isolators at the top of the basementcolumns is more attractive from a firstcost perspective, but these columns mayneed additional strength if elastic per-formance is required.

Another consideration when assessingthe suitability of a structure is the soilcondition and the geology of the site.Generally, the stiffer the soil the moreeffective the isolation. The flexibility ofthe structure determines how it will re-spond to a given earthquake motion.However, the form of the earthquakemotion as it arrives at the base of astructure may be modified by theproperties of the soil through which theearthquake waves travel.

If the soil underlying the structure is

very soft, the high frequency content ofthe motion may be filtered out, and thesoil may produce long period motions.An extreme example of this was seen inthe lake bed area of the 1985 MexicoCity earthquake. Lengthening theperiod of a still structure on these soilconditions will amplify rather than re-duce the ground motions. Hence, for asite on the Mexico City lake bed, seis-mic isolation should not be considered.

Retrofit of structures to improve theirearthquake safety involves additionalconsiderations, compared with newconstruction, because of the constraintsalready present. Buildings with sub-basements or piled foundations aremore suitable for isolation retrofit.Bridge superstructures are also verysuitable since they are generally sup-ported on steel bearings. Replacementof these bearings with elastomeric typebearings is a relatively simple, low costoperation and will lead to a significantreduction in earthquake forces. 35 Inaddition, it permits the option of redis-tributing forces away from weak sub-structures into substructures more ca-pable of sustaining earthquake loads.

Buildings are often more difficult toretrofit than bridges. However, com-pared to conventional strengtheningschemes, seismic isolation is an attrac-tive alternative. This is because few, ifany, new structural elements are re-quired above the isolation level; mostconstruction work is confined to thebasement level. Conventional methods


Table 2. Directory of worldwide base isolation activity excluding the United States.

Country Constricted facilities Active organizations

Belgium D'Appolonia

Canada I coal shiploader, Prince University of British Columbia,Rope rtB.C. Vancouver

Swan Wooster Engineering,Vancouver

Khanna Consultants Intl.

Chile I ore shiploader, Guacolda University of Chile

China 2 houses (1975), Central Research Institute of Building1 weight station (1980), and Construction, Beijing1 4-story dormitory, Beijing

(1981)

England I nuclear fuel reprocessing Malaysian Rubber Producers Researchplant Association

Rubber Consultants, Ltd.Imperial College of Science and

Technology, LondonUniversity of Southhampton

Finland Imatran Voima Company

France 4 houses (1977-1982) Centre National de la RechercheI 3-story school, La.mbesc Scientifique, Marseille

(1978) Centre d'Etudes Nucleaires de SaclayI nuclear waste storage facility Electricite de France

(1982) Spie Batignolies2 nuclear power plants, Cruas

and Le Pellirin

Germany GERB — Gesellschaft fur Isolierunge;Berlin; Kraftwerke Union;Engineering Decision Analysis;Polensky and Zolher. FrankfurtJupp Grote

Greece 2 office buildings, Athens University of Patras

Hungary 'Technical University of Budapest

Iceland 5 bridges Iceland Highway Department

India University of RoorkeeBhabha Atomic Research Center

40

Table 2 (cont.). Directory of worldwide base isolation activity excluding the United States.

Country Constructed facilities Active organizations

I.ranilract 1 nuclear power plant, KanunRiver(1978)

1 12-story building, Teheran(1968)

Israel Israel Institute of Technology, Haifa

Italy` 3 viaducts Autostrade, RomaTESIT, MilanoPolytechnic of Milan

Japan* I airport control tower Taisei Corporation, Tokyo Kenchiku,3 residential houses Okumura Corp., Ohbayashi-gumi,5 research laboratories Ltd., Oiles Industry, SumitomoI museum Construction, Takenaka Komuten Co.,3 office buildings Kajima Corp., Shimizu Construction

Co., Ministry of Construction,University of Tokyo, TohokuUniversity, CRIEPI/Federation ofElectric Power Companies

Mexico 1 4-story school, Mexico City Gonzales Flores, Consulting Engineer(1974)

Middle East Storage tanks for liquidpropane and butanet

I Jew Zealand 2 office buildings, Auckland Physics and Engineering Laboratory,and Wellington (1982 and DSIR1983) University of Auckland

37 bridges Ministry of Works and Development2 industrial structures (chimney Beca, Carter, Ilollings & Ferrer

and boiler) Holmes Consulting Group

Roumania Polytechnic Institute of Jassy

South Africa 1 nuclear power plant, Koeberg

Soviet Union 1 7-story building, Sevastopol

Switzerland Swiss Federal Institute ofTechnology,Zurich

Seisma A.G.

Yugoslavia 1 3-story school (1969), Skopje University of "Kiril and Metodij"

Both Italy aiid J;tpan also have a large number of partially isolated bridges which are not included inabove table.

t These five tanks ire only partially isolated for seismic loads and have therefore not keen counted in abovetable.

PCI JOURNAL'May-June 1988 41

Fig. 15. Union House -- a precastconcrete, isolated building underconstruction in Auckland, New Zealand.See also Fig. 16 for view of elevation ofstructure.

generally require the addition of stnic-tural elements to all levels of the build-ing. This trade-off can be very importantif continued use of the facility duringretrofit is desired.

AIthough seismic isolation reducesearthquake forces, it does not eliminatethem. Consequently, the strength andlimited ductility of an existing structuremust be sufficient to resist the reducedforces that result from isolation. If thestrength capacity of the existing struc-ture is extremely low, then additionalstrengthening of the structure will herequired. In such cases, the economicsof significant additional strengtheningversus limited strengthening and theprovision of isolators would need to bestudied.

APPLICATIONS AROUNDTHE WORLD

Seismically isolated structures havebeen built in at least 17 countriesthroughout the world, and there areanother eight countries with active re-search programs in this area. There areapproximately 100 isolated structuresalready built or under construction. Pre-cise numbers are difficult to obtain, butthis figure is considered to be a con-servative estimate — the actual figure isincreasing rapidly because of the activ-ity in Japan, New Zealand, France andthe United States.

This number includes 31 buildings(houses, schools, research laboratories,office buildings), 49 bridges, 6 nuclearpower-related structures, and 9 miscel-laneous structures (mainly of an indus-trial type). However, it does not include136 bridges known to be partially iso-lated for seismic loads in Japan andItaly.

This figure is also an underestimate.Partial isolation (i.e., isolation in onehorizontal direction only) has been pop-ular for bridges in several countries formany years, and the total number ofsuch structural systems is today prob-ably closer to 200.

Table 2 summarizes this activity bycountry (alphabetic order), giving a listof known constructed facilities as wellas consultants, governmental agenciesand research establishments who areactive in seismic isolation. Examinationof this table will show that significantactivity has taken place in China, Japan,Europe and New Zealand.

Of interest to the precast concrete in-dustry from this list is the 12-story pre-fabricated concrete Union House inNew Zealand.' The completed structureand a section through the building isshown in Figs. 15 and 16. This buildinguses sleeved piles to provide flexibilityand steel cantilever plates as energy dis-sipators.

42

12

11

10

9

8

7

6

5

G

3

2

G

roe

EI^' E^ ^ xE E

E £C"

Fig. 16. Section through Union House showing sleeved piles and steel energy dissipators.


APPLICATIONS IN THEUNITED STATES

Table 3 lists eleven known appli-cations of seismic isolation in the UnitedStates. It can be seen that significant ac-tivity has taken place in this countryduring the past 2 years. A short descrip-tion of each project follows.

BuildingsThe Foothill Communities Law and

Justice Center in San BernardinoCounty, California, is a five-story, con-centrically braced steel frame building,415 x 110 ft (126 x 33.6 m) in plan, andseated on 98 high-damping rubberisolators as shown in Figs. 17 and 18.The structure is clad with glass fiberreinforced concrete panels. Completedin 1986, the building is sited within 14miles (23 km) of the San Andreas faultand within 2 miles (3.2 km) of the SierraMadre fault. It is therefore in a region ofhigh seismicity, and a full scale test ofthis isolated building, by at least a mod-erate earthquake, is expected in the nearfuture. In fact, a minor event (magnitude4.9) occurred at Redlands in October1985, 20 miles (32 km) from the build-ing.

Instruments in the structure showedthat the isolators attenuated the effectsof the ground motion, albeit veryslightly. Other buildings in the vicinityshowed acceleration amplification byfactors of 2 to 5 — which is expected inconventional (nonisolated) structures.Despite the small size of this earth-quake, the performance of this building,as recorded by the California StrongMotion Instrumentation Program, is en-couraging evidence that isolation is in-deed beneficial to the structure.

The Salt Lake City and County Build-ing is a five-story, Romanesque stylestructure measuring 265 x 130 ft (80.8 x39.6 m) in plan (see Fig. 19). Con-structed between 1892 and 1894, it isbuilt of unreinforced brick, masonry,and sandstone. A 12-story clock tower iscentrally located and is also constructedfrom unreinforced masonry.

As part of a complete restoration pack-age for the building, seismic retrofitusing isolation is currently under con-struction. A total of 447 lead-filledisolators are being installed on existingfoundations, and the building weightwill be transferred to the isolators be-ginning the summer of 1988. Whencomplete, the structure will be pro-tected against damage for a 0.2g earth-

Fig. 17. Foothill Communities Law and Justice Center. San Bernardino County,California. Note that glass fiber reinforced concrete panels were used to clad thestructure. See Fig. 18 for view of building elevation.

44

0CMz

LC

Fig. 18. Section through the Foothill Communities Law and Justice Center showing location of isolators in sub-basement.

Table 3. Directory of seismically isolated structures in the United States (completed orunder construction as of February 1988).

Structure Owner Date Description/Engineers

230kV Circuit California 1979 Elastomeric Isolators/Breakers Department of Delfosse

Water Resources

Sierra Point Calitbrnia 1984/5 Lead-Idled elastomericBridge Retrofit Department of bearings/Caltrans, DIS(US101) Transportation

Foothill Communities County of San 1985/6 "High-damping" elastomericLaw and Justice Bernardino bearing (filled rubber)/ ReidCenter and Tarics, J.M. Kelly,

MRPRA

Santa Ana Metropolitan 198617 Lead-filled elastomericRiver Bridge Water District of bearings/Lindvall-Richter,Retrofit Southern DIS

California

Salt Lake City Salt Lake City 1987 Lead-filled elastomericand County Building Corporation hearings/Forell Elsesser,Restoration E. W. Allen & Associates,

D'S

Mark II Detector Stanford Linear 1987 Lead-filled clastomeric(equipment) Accelerator bearings/SLAC, DIS

Center

faun Yard Vehicle Los Angeles 1987 Lead-filled elastomericAccess Bridge County bearings/Southern California

Transportation Rail Consultants, DISCommission

Eel River Bridge California 1987 Lead-filled elastomericRetrofit (US101) Department of bearings/Caltrans, DIS

Transportation

Salt Lake City Evans and 1987 Lead-filled elastomericManufacturing Sutherland bearings(Reaveley, DI8Facility

Liquid Argon Stanford Linear 1987 Lead-filled elastomericCalorimeter Accelerator bearings/SLAC, DIS(Equipment) Center

All American Canal California 1987 Lead-filled elastomericBridge Department of bearings/Caltrans, DIS

Transportation

Note,1. i]IS = Dyttarnic Isolation Systems, Berkeley, Califontia.

MRPRA = Malaysian Rubber Producers Research Associates.Caltrans = California Department ofTrtnsportation.

46

quake event. Further, the isolators aredesigned to he stable up to and includ-ing a 0.4g event, should an earthquakegreater than the design event be experi-enced at the site.

The Evans and Sutherland Building inSalt Lake City is a new, four-story, steelmoment frame manufacturing facility forflight simulators. The building mea-sures 280 x 160 ft (85.4 x 48.8 m) in planand rests on 98 lead-filled elastomerfcbearings.

BridgesThe Sierra Point Overhead is a highly

skewed bridge on ten simply supportedspans which vary in length from 26 to100 ft (7.93 x 30.5 m) as shown in Fig. 20.Supported on 27 nonductile columns,each 3 ft (0.91 m) in diameter, thebridge was particularly vulnerable be-cause it could resist only 25 percent ofthe design event for the site. Retrofit byisolation was feasible because existingsteel bearings could be removed and re-placed by lead-rubber isolation bear-ings. No column strengthening wasnecessary because the factor of6 reduc-tion in force was sufficient to ensureelastic behavior.

The Santa Ana River Bridge carriesthe Upper Feeder water pipeline intothe Los Angeles Basin for the Metropol-itan Water District of Southern Califor-nia as shown in Fig. 21. The existingpiers and steel trusses were under-strength and isolation bearings wereused to reduce the seismic forces tobelow the existing capacities. Nostrengthening of the piers or trusses wasthen necessary.

Other bridge isolation retrofits re-cently completed or under constructioninclude the Eel River Bridge for Cal-trans near Rio Dell, the Vehicle AccessBridge for the Los Angeles CountyTransportation Commission near LongBeach, and the All American CanalBridge for Caltrans in Imperial County,all in California.

Fig. 19. Salt Lake City and CountyBuilding, Salt Lake City, Utah.

Industrial EquipmentThe first isolation system to be in-

stalled in the United States was for theprotection of a 230 kV circuit breaker forthe California Department of Water Re-sources. These circuit breakers havefragile porcelain columns with limitedresistance for lateral loads. In this appli-cation, four conventional elastomericbearings were used to isolate each phaseof the circuit breaker. Since the units arerelatively lightweight, the isolators aremore slender than those used elsewhere.This was necessary to obtain the re-quired period shift and, as a conse-quence, stability issues were a concernin the design.

In contrast, the Mark II Detector atthe Stanford Linear Accelerator Centeris of substantial weight [3200 kips(1420O kN)]. Its isolation by four lead-filled elastomeric bearings is now com-plete. An integral part of the Depart-ment of Energy's new Linear ColIider at


Fig. 20. Sierra Point Overhead, U.S. Highway 101 near San Francisco, California.

Fig. 21. MWDSC Pipeline Bridge over Santa Ana River, Riverside County, California.

48

Stanford University, this fragile item ofindustrial equipment is now protectedagainst damage for a 0.6g NRC responsespectrum, which is representative of theearthquake anticipated on the adjacentSan Andreas fault. A second item for thecollider, the Liquid Argon Calorimeter,is also being isolated in the same way.

CODE CONSIDERATIONS -STRUCTURES AND

ISOLATORSRecognizing the growing importance

of seismic isolation as an approach to re-ducing structural response due to earth-quakes, the Structural Engineers As-sociations of Northern and SouthernCalifornia created committees to de-velop necessary guidelines for the de-sign of seismically isolated buildings.The Northern group (SEAONC) com-pleted tentative design guidelines forconventional buildings" in September1986, and these are now being reviewedby the State Seismology Committee forpossible adoption in the UniformBuilding Code. In January 1987, theBuilding Safety Board of the CaliforniaOffice of Statewide Health Planning andDevelopment adopted "An AcceptableMethod for Design and Review of Hos-pital Buildings Utilizing Base Isola-tion."

The background on the approach andobjectives of the SEAONC document forthe isolation of conventional buildingsis provided in the document's introduc-tion, an extract of which is given below:

"The advantages of seismic isola-tion and the recent advancements inisolation system products have al-ready led to the design and con-struction of a number of seismicallyisolated buildings and bridges inCalifornia and Salt Lake City. Thisactivity has, in turn, identified aneed to supplement existing codeswith design requirements de-veloped specifically for seismically

isolated buildings. This need isshared by the public which requiresassurance that seismically isolatedbuildings are ` safe' and by the en-gineering profession which re-quires a minimum standard uponwhich design and construction canbe based. Accordingly, the BaseIsolation Subcommittee of theSeismology Committee of theStructural Engineers Association ofNorthern California has developedthe following seismic isolation de-sign requirements to supplementthe `Recommended Lateral ForceRequirements and Commentary'(Blue Book) document."

"Rather than addressing a spe-cific method of seismic isolation,the document provides general de-sign requirements applicable to awide range of possible seismic iso-lation schemes. In remaining gen-eral, the design requirements relyon mandatory testing of isolationsystem hardware to confirm the en-gineering parameters used in thedesign and to verify the overalladequacy of the isolation system.Some systems may not be capable ofdemonstrating acceptability by testand, consequently, would not bepermitted.

In general, acceptable seismicisolation systems will:• Remain stable for required de-

sign displacements.• Provide increasing resistance

with increasing displacement.• Not degrade under repeated

cyclic load.• Have quantifiable engineering

parameters (e.g., force-deflectioncharacteristics and damping)."The design requirements are

based on a severe level of earth-quake ground motion which corre-sponds, approximately, to a 500-year return period event as de-scribed by the recommendedground motion spectra of the Blue


Book. 34 The isolation system, in-cluding all connections and sup-porting structural elements, is re-quired to be designed (and tested)for the full response effects of thislevel of ground motion."There are several codes throughout

the world that provide criteria for thedesign of elastomeric hearings. Theseprovisions vary in their specific re-quirements but generally address thefollowing issues:

• Shear displacement• Vertical displacement• Rotational capacity• Vertical loads• StabilityIn essence, these criteria cover the

design aspects of elastomeric bearingssubject to vertical loads and inducedshear (thermal, creep) displacements.However, these two conditions are notsufficient to cover the additional effectsimposed by earthquakes. Consequently,the existing criteria must be modified toinclude earthquake loads. Elastoznericbearings which are used for seismicisolation may be subjected to large,earthquake induced displacements.Earthquakes are infrequently occurringevents and the factors of safety requiredunder these circumstances will be dif-ferent than those required for more fre-quently occurring loads.

Since the primary design parameterfor earthquake loading is the displace-ment of the bearing, the design guide-lines must be capable of incorporatingthis displacement in a logical and con-sistent manner. Current AASHTO de-sign requirements for elastomericbearings in the United States limit verti-cal loads by use of a limiting compres-sive stress and therefore do not have amechanism for including the simultane-ous effects of seismic lateral displace-ments.

The British Specification BE 1/76 andits more recent successor, BS 5400, rec-ognize that shear strains are induced inreinforced hearings by both compres-

sion and shear deformation. In thesecodes, the sum of these shear strains islimited to a proportion of the elonga-tion-at-break of the rubber, The propor-tion (Y2 or 1/3 ) is a function of the loadingtype.

These British codes are probably themost widely used elastomeric bearingspecifications in the world. They haveserved as the basis of hearing design inGreat Britain, Australia, New Zealand,and parts of Europe and Asia. Since theapproach used in BE 1/76 and BS 5400incorporates shear deformation as part ofthe criteria, they can be readily modi-fied for seismic isolation bearings.

DESIGN FORCES FORPRECAST CONSTRUCTIONOne of the benefits of seismic isola-

tion is its ability to eliminate the ductil-ity demand on a structural system. Thishelps alleviate one of the current con-straints on the design of precast momentframes, s since they must qualify as"ductile moment-resisting frames" ifthey are intended to resist earthquakeinduced lateral loads. In addition toeliminating the ductility demand, theforce reduction feature of seismic isola-tion may also reduce the design forcesfor some structural systems.

Englekirk and Selna3B give a summaryof the design forces required by theUniform Building Code (UBC) for pre-fabricated concrete systems. Their de-velopment of design forces is sum-marized herein to provide a comparisonwith the forces that would be requiredfor the seismic isolation design of a pre-fabricated system.

The design base shear for buildings isdeveloped in Section 2312 of the 1985UBC as:

V – ZIKCSW

whereV = total lateral force or base shear

force

Z = numerical coefficient dependenton seismic zone

K = numerical coefficient dependenton structural resisting system

C = numerical coefficient dependenton period of building

S = numerical coefficient for sitestructural resonance

W = total dead load of buildingThe base shear for prefabricated sys-

tems can be reduced to;

V = 0.14 KW

if the value of CS is taken as its maxi-mum and Z and I are assumed to beequal to one.

For comparative purposes, Englekirkand Selna included load factors (U) andreliability factors (0) such that:

V=0.14(KU

IW

For the purpose of comparing theUBC design forces with those requiredFor seismic isolation, the 0 factor will betaken as unity. In this case, the baseshear V,N, is defined as follows:

V.=0.14(KU)W

Moment FramesAs stated above, the Uniform Building

Code in high seismic zones does notpermit the use of any frame which doesnot qualify as a "ductile moment-re-sisting space frame" (if it is intended toresist earthquake induced loads). How-ever, if we can assume this requirementcan be met, and if we restrict the discus-sion to low rise, ductile, moment-re-sisting space frames with a period lessthan 0.8 seconds, a K coefficient of 0.67and load factor of 1.4, the design baseshear (V, ) required by the UBC is:

VV = 0.14 (0.67x1.4)W =0.13W

Wall PanelsWall panels which provide lateral

earthquake bracing are classified as

shear walls. When these panels are theprimary vertical load carrying system,the building is classified as a box systemwith K = 1.33. Load factors for this typeof system are 1.4. Thus, for low risebuildings with a period less than 0,8seconds, the design base shear V, is:

VN =0.14(1,33x1.4)W =0.26W

DiaphragmsLoading criteria for diaphragms are a

function of the loading criteria for thestructure. it also depends on the loca-tion of the diaphragm in the structureand the force levels anticipated at thefloor in question. Maximum valueswould govern in most cases for prefabri-cated concrete buildings. Thus:

V5- 0.3(U)W=0.3(1.4)W= 0.42 W (diaphragm shear)

Seismic IsolationSeismic isolation involves a trade-off

between lower forces and increaseddisplacements across the isolator. Asforces decrease, displacements increase.In addition, the forces on an isolatedstructure are a function of the soil type.For comparative purposes, a paper byKelly et al. 39 provides a preliminary de-sign procedure for a lead-rubber isola-tion system. For the highest seismiczone (Z = 1) and for isolation periods of2.0 and 2.5 seconds, the maximum elas-tic base shear V,5 , and relative isolatordisplacement obtained from the proce-dure in Ref. 39 are shown in Table 4.

The soil types SI , S2 , and Ss are de-fined in Ref. 34. In essence, they arerock, medium to stiff clays, and softsoils, respectively. Table 4 illustratesthe force-displacement trade-off as theperiod of the isolated structure in-creases, and it also illustrates the in-crease in both force and displacement asthe soil condition softens.

If an isolated structure is designed to

PCI JOURNALJMay-June 1988 51

Table 4. Seismic isolation base shear forces anddisplacements.

Soil typeIsolationperiod

Elasticbase shear

Isolatordisplacement

S, 2.0 sees 0.11W 4.5 in.S, 2.5 sees 0.10W 5.9 in.Ss 2.0 sees 0.15W 6.0 in.S , 2,5 sees 0.12W 7.5 in.Ss 2.0 secs 0.20W 8.0 in,Ss 2.5 sees 0.16W 10.0 in.

Note: I in. = 25.4 mm.

resist the elastic base shear forces VS,given in Table 4, then the structure willrespond elastically without ductilitydemand on the structural system.

Design Force ComparisonsIn order to facilitate a comparison of

the preceding design forces, the fol-lowing assumptions are made. Thenonisolated structure has a period of lessthan 0.8 seconds. When isolated, thissame building has a period of 2 seconds.For the diaphragm comparison, it is as-sumed that the distribution of the baseshear force for an isolated structure isuniform up the height of the building.The ratios given in Table 5 are the de-sign base shear forces V ,ti, for a conven-tional, prefahricated structure dividedby the elastic base shear Vsr of an iso-Iated structure given in Table 4.

It is clear from the ratios in Table 5that seismic isolation can provide re-duction factors ranging from 1.30 to 2.40in the design base shear forces for a wall

Table 5, Comparison of V 1Vs, ratio

Soil Moment Walltype fume panel Diaphragm

S, 1.18 2.36 3.82S 2 0.87 1.73 2.80Ss 0.65 1.30 2.10

panel type building. For moment-frametype structures, isolation design forceswould be similar to current code levelsfor soil types S, and S,, and there wouldhe a 50 percent increase for soil type S3.

COST-BENEFITEVALUATION

The cost-benefit issue is an importantone for prefabricated construction sinceit is very difficult, if not impossible, tosatisfy the current building code ductil-ity requirements in high seismic zones.A solution to this problem is thereforerequired before the economics of isola-tion becomes an issue for the precast in-dustry.

Seismic isolation clearly offers an in-novative and practical solution to theductility problem because it avoids theneed for ductility altogether. In addi-tion, isolation also decreases the designforces for wall panel buildings anddiaphragms. For moment frames, thedesign forces required in using seismicisolation are similar to those currentlyrequired for conventional ductile con-crete construction.

The economic feasibility of seismicisolation depends on an objective eval-uation of the costs and benefits in finan-cial terms. Typically, the costs of seis-mic isolation will be identified duringthe design phase of a project; these are

52

first costs. Benefits front seismic isola-tion, such as reduced design forces, mayreduce first costs but most likely will re-sult in comparable initial costs to thoseof conventional construction. Somestudies have shown that even under ad-verse conditions the difference in firstcosts rarely exceeds 2 percent. On theother hand, the designer of the prefabri-cated Union House' reports a 3 percentfirst cost savings in addition to a reduc-tion in construction time by 3 months.

Long term benefits result from a re-duction in earthquake damage costs. Fora true comparison, the various costs andbenefits must he reduced to the samemonetary base or equivalent dollarvalue. Seismic isolation will be justifiedeconomically if the benefits, or cost re-ductions, exceed the additional cost, ifany, of isolation.

Cost effectiveness of seismic isolationis assessed by assigning dollar values tothe costs and benefits obtained. Factorsto he considered include:

• Cost of isolation system• Cost of changes to accommodate

isolation• Savings in structural system• Reduced repair costs• Reduced earthquake losses• Continuing operability and surviv-

ability of a businessCost comparisons should include both

seismic isolation and any alternativescheme, such as the provision of astronger structural frame and equipmentbracing, that will achieve the same levelof seismic protection. For the precastindustry, it may simply mean the abilityto compete in a new market.

If mechanical system performance isimportant, then cost savings in equip-ment bracing alone may dominate thedecision to select isolation. The cost ofbracing mechanical systems to meet theCalifornia Hospital Code ranges from $2to $4 per sq ft more than that required bythe Uniform Building Code. However,life cycle cost reductions are potentiallythe most dramatic, and when assessed,

overwhelm any first cost considerations.It should be noted that, implicit in cur-rent codes is the acceptance of structuraland nonstructural damage during anearthquake; the cost of this plus the lossof function of the building should be in-cluded when assessing life cycle cost.

Another important and perhaps over-whelming benefit is the ability of seis-mic isolation to significantly improvethe chances of business survival after amajor earthquake. The National FireProtection Association has shown that70 percent of the companies that suffer amajor fire are out of business within 3years, regardless of whether or not theyare insured. The economics of businesssurvivability following an earthquakewould thus overwhelm a 1 to 2 percentfirst cost premium for the incorporationof seismic isolation.

First cost studies should incorporateconsideration of the factors given inTable 6.

Life cycle cost studies are difficult toperform because the data base of infor-mation is so small. There are two ap-proaches to addressing the life cyclecost issue. Thiel 40 has developed a pres-ent value formulation which incorpo-rates an experience based earthquake

Table 6. Cost factors for isolation design.

Cost increases Cost reductions

Seismic isolationsystems

Architectural Savings indetails to structural systemaccommodateisolation

Special mechanical Savings in curtainand electrical walls (reduceddetails drift

requirements)

Additional design Savings incosts nonstructural

componentbracing

PC JOURNAL'May-June 1988 53

damage matrix. 4 Thiel's overall conclu-sions are:

• That seismic isolation is a very im-portant means of controlling thelife cycle cost of seismic exposure.

• That when disruption costs and thevalue of building contents are im-portant, seismic isolation has a sub-stantial economic advantage overother systems regardless of initialconstruction costs, under almost allconditions.

An alternative method of evaluatingearthquake damage costs is to utilize thework of Ferritto.42 His study of a typicalNavy building recognized the two pri-mary components of interstory drift andfloor acceleration that cause earthquakedamage. Ferritto developed damagematrices in an attempt to quantify thedamage resulting from these two com-ponents. For a specific project, hismethodology enables a comparative as-sessment of the costs of damage thatwould result from a conventional designand from an isolation design.

CONCLUSIONSeveral practical systems of seismic

isolation have been developed and im-plemented in recent years, and interestin the application of this technique con-tinues to grow, as evidenced by the ap-proximately 100 structures constructedto date. Although seismic isolation offers

significant benefits, it is by no means a"panacea." Feasibility studies are re-quired early in the design phase of aproject to evaluate both the technicaland economic issues. If its inclusion isappropriate after a technical and firstcost evaluation, then significant lifecycle cost advantages can be achieved.Thus, seismic isolation represents a veryimportant step forward in the continuingsearch for improved earthquake safety.

From the perspective of the precastand prestressed concrete industry,seismic isolation offers an immediatesolution to the current problem of duc-tility demand on structures in highseismic zones. Isolation is particularlyattractive because it avoids the need forductility in structural members. Thealternative strategy is to engage in along, difficult and expensive researchprogram to make precast and pre-stressed members more ductile.

It is recognized that there are diffi-culties to be overcome if seismic isola-tion is to be implemented by the precastindustry, but at least there is light at theend of the tunnel, Seismic isolationmeets all the criteria for a classic mod-ern technological innovation, and it pro-vides the potential for the precast con-crete industry to compete in the highersiesmic areas of the United States. Thisis a significant step forward for an in-dustry wishing to expand its market op-portunities.

54

REFERENCES

1. Boardman, P. R., Wood, B. J., and Can, A.J., "Union House — A Cross-BracedStructure With Energy Dissipators,"Applied Technology Council, Report No.17, Palo Alto, California, 1986.

2. Englekirk, Robert E,, "Overview of PCIWorkshop on Effective Use of PrecastConcrete for Seismic Resistance," PCIJOURNAL, V. 31, No. 6, November-De-cember 1986, pp. 48-58,

3. Mueller, P., "Tackling Seismic Resis-tance of Precast Concrete by Design In-novation,° PCI Workshop on EffectiveUse of Precast Concrete for Seismic Re-sistance, October 1986.

4. Arnold C. "Seismic Design: Now ComesBase Isolation," Architecture Magazine,March 1987.

5. Englekirk, Robert E., "Concepts for theDevelopment of Earthquake ResistantDuctile Frames of Precast Concrete,"PCI JOURNAL, V. 32, No. 1, January-February 1987, pp. 30-47.

6. Bechtold, Jacob, "Earthquake-ProofBuilding," U.S. Patent No. 845,046,1907.

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56

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Note: Discussion of this paper is invited. Please submityour comments to PCI Headquarters by February 1, 1989.

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