base isolation for increased earthquake resistance of buildings

8/13/2019 Base Isolation for Increased Earthquake Resistance of Buildings

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B A S E I S O L A T I O N F OR I N C R E A S E D E A R T H Q U A K ER E S I S T A N C E O F B U I L D I N G S

R. I . S k i n n e r a n d G . H . M c V e r r yABSTRACT

Inelastic deformation and hysteretic damping increase theearthq uake resist ance of structures beyond that provided by theirelastic strength. For many structures the reserv e flexibil ityand the damping could be supplied effic ientl y and reliably, by th euse of special components.Special compon ents are most effe ctive when they are locatedat the interface between the lowest part of the building and thefoundations. Recently developed hysteretic dampers, utilizingthe plastic de forma tion of solid steel bars, may be combined withone of the many methods suggested for achievi ng base flexib ilityto give a practi cal and efficie nt base-i sola tion system.In addit ion to reducing the gener al level of attack a bas e-isolati on system greatly reduces the vari atio n in severity ofattack resulting from differences in character between eart hquakes .In view of the range of earthq uake types to which a structure may besubjected this standard ization of the earth quake attack isimportant, and is found to be parti cular ly important for stru ctureswith a fundamenta l period of less than 0.4 seco nds.A base- isola tion system reduces ductilit y demands on a building,and minim izes its def orma tion s. These change s improve buildingperf orma nce and allow much greater arc hitec tural f reedom in thechoice of the structural type and in its layout and detailing.Econ omie s are increased and perf orma nce improved by using hig h-strength low-ductility structural configur ations.

1. INTRODUCTIONDuring an earthq uake the princ ipal attackon a structure is by transient hori zonta lforces. The earth quake resistance of thestructure dep ends on a combina tion of elas ticstrength, inelastic deformability and dampingcapacit y. The relative effec tiven ess ofthese three fa ctors depen ds in part on thecharact er of the attacki ng ear thqu ake.Typic al earth quake s have one of fourcharacteristics ^:Type 1 earth quak es are impulsive witha single domi nant lurch in one dire ction .Structures may resist these earthquakes bya combi natio n of elastic strengt h and

inelastic deformability; damping addslittle to their re sist ance .Type 2 eart hquak es are of long dura tionwith irregular noise-like ground motion s.Struc tures resis t them by elastic strengt h,inelastic defor mabi lity , and by dampingwhich reduc es the cyclic buildu p of deform ationsType 3 eart hqua kes are of long durati onand have regular motions with one or moredominan t pe rio ds. They are a conseq uent ofthe part ial reso nanc e of flexi ble ground andare therefore a microzon e effect. Dampingmakes an important contribution to the

earthquake resistance of any structure which* Enginee ring Seismology Section, Physicsand Engine ering Lab oratory, Department ofScientific and Industrial Research, NewZealand.

has a fundament al period si milar to a dominan tperiod of the eart hqua ke. Avo idan ce of suchnear- coinci dence of peri ods may be impededby architectural require ments, by difficultyin predict ing earth quake pe rio ds, and byincrea ses in ground period with increa singground strains.

Type 4 earth quake s are thos e whic hinclude severe ground damage in addition tothe inertia attack, and this pos es specialdesig n problems which are not cons ider ed inthis paper.Economy of design is achieved by allow ing a stru ctur e to defo rm well i nto theinelastic range during severe ear thqua kes.Effective earthquake resista nce may be

obtained provided the structure has adequatecapacity for inelastic deformation and forassociated hysteretic damping.The capacity of a structure to deforminelast ically is expres sed as a ductil ityfacto r, defined as the ratio of 'themaxi mum deflect ion which can be s ustainedfor several cycles' to 'the def lec tio n atinitial yield'. The ducti lity demand ona structure with a funda mental p eriod ofmor e than 0.4 seconds is given appro ximat elyby the ratio of 'the load which would ariseif the structure remained ela sti c' (givenfor example by the dotted spectr a of Fig.l)to 'the load at whic h initial yiel d occ urs '.

Howeve r for short- period stru ctur es theductil ity demand may greatly exceed thisload ratio, as shown in Fig. 4, where P &and Pp give amplitu de and period scalingof the accelerations recorded at El Centro,

B U L L E T I N O F T H E N EW Z E A L A N D S O C IE T Y F OR E A R T H Q U A K E E N G I N E E R I N G V O L . 8 N 0 . 2 . J U N E 19 7 5


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9 41940, N S comp onen t, a typic al typ e 2earthquake.

Increasingly sophisticated techniqueshave been adopted to increase the capaci tyof structu ral member s for inelastic defo rmation and hysteret ic dampin g. The ductili tyof reinforce d co ncrete can be increased byappropriate detailing of the reinforcingsteel but it still suffers progressivedeterioration under successive cycles ofsevere inelastic deformation, whilestructura l steel membe rs may suffer progr essive damage and loss of strength due to localbuckling. Moreover the post-elastic deformations of flexible building frames may re sultin expens ive secondary dama ge. Thus attemptsto provide resis tance to severe earth quakesby provi ding a high ducti lity factor haveintroduced considerable uncertainty to theassessment of structural performance.

Furthermore recent calculations showthat ducti lity deman ds on sh ort-periodbuil dings , designed for mode rate yield levels,may be parti cular ly severe. Hence specialdevices which prov ide high ductili ty in areliable manner will reduce the uncertaintyin earthquake-r esistant design.2. SPECI AL COMPO NENTS TO INCREASEEARTHQUAKE RESISTANCE

Other groups have developed specialcomponents to increase the earthquakeresistance of buildings with flexible frames.The compo nents act as stiffening bracesduring mode rate defo rmat ions and as hystereticdampers during large deforma tions. Slit-wall reinforced-concrete panels have beeninst alled over the height of a number oftall buil ding s in Japan . Anoth er systemconsidered employs a set of steel structuralbeams arranged diagonally to concentratemost of the interstorey deflections in shortlengths of structural steel beam, an exampleof whic h is the Y-bra ce . Since the specialcompon ents for bracing and hystere tic d ampinghave been relieved of the normal buildingloads they may be optim ized for their bracingand damping fun ctio ns. They reduce theducti lity dema nds on the main structu ralframe and reduce the interstorey deformations.However bo th the slit wal ls and the steelbeam brace systems are liable to deterioration under severe cyclic deform ations .

Braci ng and damping component s cons titut ea cons ider able part of a frame since theyextend thro ugho ut the build ing and since theymust provide forces comparable with thelateral resi stan ce of the frame. They areessentially large-force small-deformationsystems, and are appropriate for use withflexible building frames.Base support syst ems whic h provid e majorstructures with substantial isolation fromearthquake attack are a practical possibilityfollowing development work on low-costhysteretic dampers at the EngineeringSeismo logy Sect ion, PEL, DSIR, New Zealand

2 ' 3 . These dampers are based on the hyst eretic def orm ati on of solid steel bar s. Theyhave the requi red damper f orce capacity andopera ting stroke ; typi cally 5 to 50 tonsand 8 inches respect ivel y. A damper foroper ati ons alo ng a single line of actio n isshown in Fig. 6. The outer arms aresupported and the inner arms are loaded to

cause severe inelastic deformations,primar ily tor siona l, in the solid steel bar.A damper suitable for operat ion in anyhoriz ontal direct ion is shown in Fig. 7. Itutilizes a solid steel bar in the form of ashort verti cal canti lever. When the baseof the cantilever is fixed to the foundationshorizontal loads are applied to the top ofthe cantilever via a tube and spheric alsurfac e. An altern ative damper employs twoshort cantil ever bars in a stalag mite -stalact ite confi gurat ion, the adjac ent endsbeing connecte d by a shear pin whi ch allowsaxial exten sion. The base of one cantile veris fixed to the foundations, and the upperend of th e othe r is fixed, to the lowe stfloor of the structure.Hyster etic dampers based on the extr usio nof lead have been developed recently ^,5.These dampers can be designed to have a verylarge ratio of inelastic to elastic defo rma

tion, and theref ore provi de special isolat ordesign possibilities.Hysteretic dampers of the typesdescri bed are charac terize d by a moderateopera ting force and a large strok e. Alsothey can be design ed to operate for hundr edsof cycles at well controll ed force l evel s.Dampers with this set of characteristicsare appro priat e for use in a base -iso lati ngsystem.A further component require d for a bas e-isolating system is one whic h supports thestruct ure, allows relati vely free horiz onta lmoti on, and provi des a mode rate centri ngforce. For many structures the mostconvenient mount which has these characteristics is a lamin ated r ubber bea rin g of thetype frequently used to support bridge decks

3. BASE ISOLATION OF STRUC TURESIt has often been suggested that baseisolation of buildings may be achieved byintroduc ing base supports with la rgeelastic flexibility for horizontal mot ions .While such isolators may opera te sa tisfac torily during type 1 (impulsive) e arthq uakesthey would a llow the cyclic bui ld-up ofintolerable base translations, and ofcons idera ble loads on the buil ding, during

the longer type 2 and type 3 eart hqua kes .Moreov er the buildings would be liable tofrequent unacceptable movement s duringwind storms.The recently developed hystereticdampers 2,3 m a v fc e connected between thebase and the foundations of a flexibly-mounte d struc ture to act as stiff bra cesduring wind stor ms, and to prov ide dampingand softening which limits the build-upof struc tural movemen ts and forces du ring

s e v e r e earthquakes 7 . The increase d dampingand increased effective period, due to thebase isolato r, are found -to be part icul arl yeffec tive in reduci ng the earth quake attackon structures with a fundamental period ofless than 1.0 secon d.A base- isola ted s tructu re wit h afundamental period of less than 1.0 secondmay be represented approximately by a singlemass with a flexible support, for the purposeof computi ng its dynamic re spon se to


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9 5earthquake attack. This model is quiteaccura te for build ings with periods of lessthan 0.5 se cond s. Since all the mas ses of abase-isolated building have comparableacce lera tion s the deform ed shape of thebuildin g is almos t the same as for uniformhorizontal l oads , that is loads proport ionalto building weig hts. The total mass may thenbe taken at the centre of gravity of thebuil ding , and its support should allo w itthe same trans lati ons as the centr e ofgravi ty. Thi s may be achiev ed by a supportwhich give s an effecti ve period of

T e = 0.85 T,where T is the fundamental period of thebuil ding . (The relat ionsh ip may be deriv edfrom Raleig h's period formula when suitableapproximations are m a d e ) . The accu racy ofthe sing le-m ass model is increase d by largeinelastic deformations of the isolator, andthe mod el is not invalida ted by mod erat einelastic deformations of the building.When the max imu m base shear has bee nobtained by dynamic analy sis then themaximum mem ber loads and the maximumdeformatio ns may be determined accuratelyby static calculation, with the base she ar-force distribu ted uniformly ov er thebuilding.

The choice of the flexibility of thebase mou nts and of the effec tive f orce ofthe hyst eret ic damper s depe nds on thesizes of the desi gn eart hqu ake s, and on thechar act eris tic s and installed costs of thebase-isolat or component s. A suitablecompromise between building protection andisolator cos ts may be achie ved with flexi blemoun ts of laminate d rubber having aneffe ctive ru bber height of 6 inc hes , andwith steel-bar hysteretic dampers whichprovide an effective damper force of 5% ofthe building weigh t. Such laminatedrubber moun ts can be selected to give to arigid build ing a period of 2.0 s eco nds , inthe absence of the hysteretic damper s. Thenfrom the peri od formula for a single -massreso nato r it is found that the mou nt st iffness is0.0255 W in whe re W is the wei ghtof the build ing. The two stiffness valuesfor the biline ar loop, which approxim atesthe load-deflection cur ves of typicalsteel-beam dam pers , designed for maximumdeflec tions of 8 inches , are 2.94Q in ~1and 0.18Q in _ 1 , wh ere Q is the ef fect ivedamper forc e. The results presented inthis paper are based on isola tors havin gthe above fe atures.4. RESPONSE OF BASE-ISOLATE D STRUCTURES

Fig .l pres ents the base shears compu tedfor a single-mass model of a linear elasticbuil ding of period T, moun ted on theisolat or descri bed in the last sectio n andthen subject ed to P a times the acceleratio nsrecorded at El Centr o, May 1 940 , N Scompon ent; a typical type 2 earthq uake.In the follo wing t his record will bereferr ed to as the El Centr o eart hqu ake .An overa ll visco us damping of 0.03 ofcrit ical was assumed for the build ing andthe mo un ts . It is seen that for P a = 1.0,1.5 and 2. 0, the maxi mum base share s area p p r o x i m a t e l y 0.15W 0.20W and 0.29Wrespec tively . The correspon ding basetran slat ions , which may be derived fromthe loads require d to defor m the iso lato r,

are 2.9 inch es, 4.4 inc he s, and 7.0 inc hes ,respectively. For comparison Fig.l alsogives the corre spon ding bas e shares fornon-isolated sin gle-mass resonator s with aviscous dampin g of 0.05. As discu ssed belo wthese curves do not adequate ly repr esent t heseverity of the attack on short-p eriodbui ldi ngs (less than 0.5 seconds ).It is evid ent from Fig.l tha t prac tica ldesigns for buil ding s having an elasticresponse only wil l normally be confined tothose with base is olation, when hig h-strength low-d uctil ity types of struct uremay be employe d such as exte rior framesof deep beams and wide col umn s, shear wal ls,or frames with diagonal brace s. Pre-stressed membe rs, with their high strengthand low damp ing, are appro pria te for usein a base-isolated structure.An attracti ve solut ion for a framebuilding which contains a few shear walls

is the provisio n of suppor t for the shearwalls by vertical solid-steel b a r s , 3 to4 feet in leng th, with the uppe r andlower ends of the bars rig idly a nchored toa shear wall and to the fo undat ionsrespe ctivel y. The colu mns of the framesare supported on laminated rubb er mou nts .For horiz ontal tra nsl atio n of the buildingthe solid steel bars act as vertical-cantil ever dam pe rs, and they also act asties to preve nt rock ing of the walls dueto building overturn ing mome nts. Thetransve rse stiff ness of the rubbe r mount sprovides adequate resistance against theP - A forces aris ing from transl atio n ofthe short steel bars.The ductility dem ands which arise whe na yield ing build ing of 0.35 seconds p eriodis mount ed on the base isola tor are g ivenin Fig.2. The load-def ormation charac teris tics of the buil ding wer e represen ted bybilinear hysteresis loops with slope ratio s,R, whi ch is the ratio of slope in theplastic rang e to slope in the elastic ra nge ,of 0.1, 0.15 and 0.2. The ducti lity d emand swere computed for accel erat ion s of 1.5 and2.0 times those of the El Cent ro ea rthq uake .It is seen that, for a buildin g with abiline ar slope ratio of 0.15, yield forcelevel s of 0.13W and 0.17W re str ict th eductility demand to 4.0 for earthquakeamplit ude mul tip lier s of1.5 and 2.0

respe ctivel y. The curv es of Fig. 2 havebeen calcul ated spe cific ally for a buil dingelastic period of 0.35 se con ds; howeverthey should apply approximately to allshort-period buildings.It may be s hown t hat the att ack of atype 1, impuls ive, earthquake on a base-isolated buildi ng is a little le ss severethan the attack of a type 2 eart hqua ke ofthe same maxi mum groun d veloc ity andacceleration.It is evid ent fro m Fig.2 that thebuildi ng bilin ear slope ratio has an impo rtantinfluence on the duct ilit y deman ds on a ba se-isolated building . While tests on reinforced

concrete beam-colu mn connect ions suggest aslope ratio of 0.1 or less for a rein for cedconcrete frame, tests on complete reinforcedconcrete buildings give much higher va lueswhen the ductility dem ands are moderat e.This high slope ratio is pres umab ly causedpartly by progressive formatio n of member


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9 7triangular load distributi on, then theelastic r eserv e may be taken as 50% and theeffec tive base yield level as 0.18W. FromFigs. 1 and 2 it is found th at the bui ldi ngremains elastic until the ground accelerations reach 1,2 tim es those of the El Centroearthquake. For 1.5 and 2.0 times the ElCentro earthquake the ductility de mands are1.5 and 3.7 respectively, assuming a bilinearstiff ness ratio of 0.15.

For comparison with the base-iso latedbuilding, the ductility demand s are givenfor the building with out base isolation,with a des ign base share of 0.16W and witha visc ous damping of 0.05. For an ove rcap aci ty fa ctor of 1.25 the yie ld load is0.2W. The equi valen t wei ght W p of a sing le-mass system may be taken as 90 of thebuild ing wei ght , so that the yield load is0.22 W e . From Figs 1 and 4 it is foundthat the building re ache s its yield levelfor acce lera tion s of 0.25 times the ElCentr o eart hqua ke and that the ducti litydemands for 1.0, 1.5 and 2.0 times the ElCentro acce lera tion s are 6.2, 11.5 and 18.5respective ly. The ratio of maximum memberductility to the above overall ductilitieswill be muc h higher and mor e vari able fora range of earthqu akes than the corres pondi ngratio for base-isolated bu ildin gs, for thereasons enumerated ear lier.

The high ductili ty deman ds on the nonisolated building, when under severeeart hqua ke attack, woul d lead rapidly tolower yiel d levels and to nega tive bilin earslope rati os whi ch woul d furthe r incr easeduct ilit y demand s and lead to rapid fai lure .The ductilit y deman ds for the isolated andthe non-i sola ted bui ldin gs are given in Fig.5. Since the domi nant peri ods of earth quakemotions tend to increase with earthquakemag nit ude the result s of perio d increas esof 1.25 and 1.5, for ear thqu akes withamplitudes of 1.5 and 2.0 times theaccele rations of the El Centro earthqu ake,are also included on Fig 5. Whi le thelargest earthquakes considered will occurvery infrequently it is desirab le thatbui ldi ngs should have a good prob abili ty ofsurviving them witho ut collaps e. If thebase yield level s adopted abov e are usedwit h bui ldi ngs of peri ods less than 0.25secon ds, then with base isolatio n thereare no incr ease s in duct ili ty deman d, butwithou t isolati on there may be largeincreases.

If the two bui ldi ngs of Fig. 5 have thesame desi gn load deformat ions then, for anygiven eart hqua ke, the non-isolat ed bui ldingsuffers more than 5 times the deformati onsof the isola ted bui ldi ng. If the stiffe rhigh-strength low-ductility forms appropriateto base isol ati on are adop ted then ev ensmaller build ing deformati ons will result.These small deforma tions should greatlyreduce non-structural damage and the costof provid ing for buildi ng defor matio ns.C O N C L U S I O N

compo nents are a standard rang e of devi ceswith reliable performance which can bethorou ghly checked in the labora tory.The build ing loads are appr oxima tely staticin thei r effects and their distri but ion isaccurately defined. Hence the demands onthe buil ding comp onent s can be comp utedby straight-forward static techniques.Base isolation suppresses severalfactors which act as severe c onstraint son the architectural design of a nonisolated building. These factors includethe prov ision of a high overall duct ilit yfactor, the dynamic effects of irreg ularities and appen dages , and provision forsubstantial building deformat ions.Cert ain type 3 eart hqua kes can beexpec ted to exten d the very severe duct ilit ydemand s, encountered in the analysi s ofshort-period non-isolated buil ding s, tobuildin gs of longer period. Base isolati on

should prove particularly effective inproviding earthquake resistance for longer-period buildings in microz ones which givesuch type 3 earthquakes.REFERENCES1. New mar k, N.M. and Ros enb lue th, E. :Fundamen tals of Earthquak e Engin eerin g,Prentice-Hal l 1971, pp. 225-2 28, and343-345.2. Skinner , R.I ., Kelly, J.M., and Hei ne,A.J., Hysteretic Dampers for Earthquake-Resistant Struc tures , Int. J. Earthq.Engng St ruct. Dyn., Vol . 3, No .3 ,

1975.3. Skinner, R. I. , Kelly, J.M., and Hein e,A.J. , Energy Absorption Devices forEarthquake Resistant Structu res, Proc .5th Wld. Conf. Earthq. Engng, Session8C, Rome, Italy (1973).4. Robin son W.H . and Gree nban k L.R., AnExtrusion Energy Absorbe r Suitablefor the prot ecti on of Str uctu resDurin g an Ear thqu ake . In pre ss - Int.J. Eart hq. Engng Struc t. Dyn . 197 5.5. Robin son W. H., Greenba nk L.R.,Properties of an Extrusion EnergyAbsorb er. This Confe rence , May 1975.6. Lindley, P.B., Engineering Design withNatural Rub ber, N.R. Technical Bull.,3rd edn, Natural Rubber P roducer sResearch Association, London, 1970.7. Skinne r, R.I ., Beck, J.L. and Byc rof t,G.N., A Prac tica l System for Iso lati ngStructures from Earthquake Attac k, Int.J. Eart hq. Engng Stru ct. Dyn. Vol 3,No . 3, 197 5.

The design of short-period buildingsis much mor e accurate and controlled withbase isolati on than with out isola tion.The long effective perio ds and high dampingsstandar dize the earthq uake attacks whilebase- isola tion simplif ies and standardi zesthe buildi ng respo nse. The main inelastic


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F I G U R E 2 : S I N G L E - M A S S S Y S T EM W I T H B I L I N E A R H Y S T E R E T I C S U P P O R T S Y I E L D F O R C E F S T I F F N E S S R A T I O RE L A S T I C P E R I O D 0 .3 5 S E C D A M P I N G 0 .0 5 . B A S E I S O L A T E D A N D S U B J E C T E D ~t P T IMES THEA C C E L E R A T I O N S R E C O R D E D A T E L C E N T R O 1 9 4 0 N S .


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FIGURE 4 : R ATIO OF DUC TIL ITY TO LOAD RATIO FOR THE NON-ISOLATEDBUILDINGS OF FIGS. 1 AND 2 . LOAD RATIO = ELAST IC BASES HE A R/ Y I E L D L O A D. COCD


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J L0 0 - 5 1 0 1 -5 2 0

E L C E N T R O M U L T I P L I E R , PQF I G U R E 5 : D U C T I L I T Y D E M A N D ON A B I L I N E A R H Y S T E R E T I C B U I L D I N G ; P E R I O D 0 .2 5 S E C

V I S C O U S D A M P I N G 0 . 0 5 . I S O L A T E D ; S T I F F N E S S R A T I O 0 . 1 5 F =. 0 . 1 8 W .N O N - I S O L A T E D ; S T I F F N E S S R A T I O 0 . 1 F = 0 .2 W . E A R T H Q U A K E ; E L C E N T R O1 9 40 N S A C C E L E R A T I O N S M U L T I P L I E R ^ P E R IO D S M U L T I P L I E R P .

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F I G U R E 6 : T O R S I O N A L - B A R H Y S T E R E T I C D A M P E R .


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B A S E

F I G U R E 7 : F L E X U R A L - B A R H Y S T E R E T I C D A M P E R .

F I G U R E 8: C O M P O N E N T S O F B A S E I S O L A T O R U S I N G F L E X U R A L - B A R D A M P E R SA N D L A M I N A T E D - R U B B E R M O U N T S .

base isolation for increased earthquake resistance of buildings

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