Isolat in Chile

Analysis, testing, and implementation of seismic isolationof buildings in Chile

Juan C. De la Llera* **, Carl Lüders , Patricio Leigh and Henry Sady

Department of Structural and Geotechnical Engineering, Pontificia Universidad Católica de Chile,Casilla 306, Correo 22, Santiago, Chile

SUMMARY

This article summarizes the work done by the authors in seismic isolation over the past six years inChile. First, a general evaluation of the optimal values of the yield level of the isolation system isperformed, focusing on the idea of, but not restricted to, the use of lead—rubber bearings. These optimalvalues are obtained for two performance objectives: to minimize the base shear in the superstructureand to control the isolator deformation. They were used in the design and construction of two importantisolated buildings that are described herein; a short description of the more relevant aspects of the designand implementation of the isolation system in these two buildings is also presented. Furthermore, resultsfrom a long testing program conducted on more than 260 full-size elastomeric isolators are summarizedand discussed. It is shown that these experimental results enable the elastomeric compounds to becharacterized quite accurately by testing reduced-scale specimens with elastomer thickness identical tothat used in the full-size isolators. Also, results from isolator constitutive modeling, scragging, and creepin the short term are briefly discussed. Inelastic analyses were performed in the structures in order toevaluate realistic interstory drift and floor acceleration response reduction factors due to the isolationdesign used. It is shown that in spite of the isolation system, minor inelastic excursions of the primarystructure are expected, leading to smaller drift and acceleration reduction factors than those obtainedfrom assuming an elastic response of the superstructure. In any case, seismic isolation is shown to bea very competitive alternative, technically and economically, for building design in Chile. Althoughit may be difficult to extrapolate this experience to other environments, the results presented hereindemonstrate that seismic isolation is a technique that can be effectively used to mitigate seismic hazardsin developing countries. Copyright © 2004 John Wiley & Sons, Ltd.

KEY WORDS: seismic isolation; elastomeric bearings; lead—rubber bearings; optimal yield capacity;Chilean code, isolator testing; building implementation

*Correspondence to: Juan C. De la Llera, Department of Structural and Geotechnical Engineering, PontificiaUniversidad Católica de Chile, Casilla 306, Correo 22, Santiago, Chile.tE-mail: [email protected]

Contract/grant sponsor: Chilean National Fund for Research and Technology, FONDECYT; contract/grant number:1020774Contract/grant sponsor: Fund for Foment and Technology, FONDEF; contract/grant number: D9611008

544 J. C. DE LA LLERA ET AL.

INTRODUCTION

Seismic isolation has become a popular earthquake-resistant design technique for buildings andbridges throughout the world. The technical advantages of this technique have been shownby experimental work [1–3] and also by their successful response during earthquakes [4, 5].In spite of this success, seismic isolation has not been incorporated into practice in someearthquake-prone countries at a rate consistent with these advantages. Some arguments thatjustify this are: (i) the natural conservatism of the profession, architects, structural engineers,and codes, to incorporate new technologies in building and bridge design; (ii) the infrequentoccurrence of seismic events in some countries; (iii) cost-related issues regarding the devicesand their implementation; (iv) additional architectural and structural detailing; and (v) the lackof knowledge of the users relative to the current earthquake design philosophy that admitsnon-structural and structural damage during severe ground motions.

Because it is common to see seismic isolation stigmatized as an expensive technique, theapplication of which should be primarily for critical facilities, one objective of this article is toshow the experience with base isolation in a country like Chile, within a framework of seriousbudget limitations and other technological constraints. Although other applications of seismicisolation are also increasing in this country, only its use in new building construction willbe emphasized herein, for which the implementation process is complex and where isolatedsolutions must compete in direct costs with traditional earthquake-resistant schemes.

The first base-isolated building in Chile was designed and built in 1992 [6] as a joint effortbetween the professors and researchers at the Universidad de Chile, the Ministry of Hous-ing, and Professor James Kelly of the University of California at Berkeley. The structure is afour-story masonry building supported on eight medium damping rubber isolators (MDB). Thenext step was a 3-year project funded by the government aimed at fostering the research andimplementation of seismic isolation and energy dissipation techniques in Chilean construction.This project was developed at the Universidad Católica de Chile and led to a new laboratoryfor dynamic testing and vibration control. Several investigations with rubber and frictional iso-lation [7], and metallic and frictional dampers [8]. have been developed since then. As a resultof this latter project, the first seismic-isolated hospital was designed and constructed in Chilein the year 2000 (H-UC). The 6-story R/C building is about 8000 m-, in plan and the isolationsystem consists of 30 high damping isolators (HD) and 22 lead–rubber bearings (LRB). Re-cently, a new seismically isolated institutional building for the Faculty of Engineering of theUniversidad Católica (EF-UC) has been constructed (2002). The 5-story R/C structure is about6000m 2

and has 25 HD isolators, 17 LRB, and 11 steel–teflon frictional sliders. Most recently,a new seismically isolated Military Hospital (MH) has been designed, with construction start-ing in January 2004. This 5-story R/C building has a total constructed area of about 50000m 2

with 164 isolators, 114 MDB and 50 LRB. Several aspects of the devices, earthquake response,and design of the H-UC and EF-UC buildings are described in detail in this investigation.

During the design and construction processes of these buildings, analysis, design, and exper-imental aspects came up that the authors believe may be useful to the engineering professionand scientific community. Because of space limitations only those aspects considered more rel-evant for structural engineering are reported in this article. Hence, construction aspects are notreported; however, they are essential in a proper implementation of seismic isolation. In this ar-ticle, the elastic and inelastic response of simplified models is first used to estimate the optimalyield capacity of LRB for impulsive-type and subduction-type ground motions. Then, selected

545SEISMIC ISOLATION OF BUILDINGS IN CHILE

experimental results aimed at characterizing the horizontal cyclic behavior of MDB and LRBfrom a sample of about 260 full-size seismic isolators are presented. Next, a more detailed de-scription of the two buildings considered as examples is presented together with their expectedinelastic earthquake response. Finally, some practical observations relative to the analysis, de-sign, and implementation of seismic isolation in these structures are presented; some of theseaspects are considered in the proposed building code for seismic isolation in Chile.

OPTIMAL YIELD CAPACITY

The first aspect analyzed is the Optimal Yield Capacity (OYC) required for the isolationsystem of a structure. In the case of LRB, the OYC is primarily related to the amount oflead used in the design of these bearings; for MDB the OYC is defined by the mechanicalproperties of the elastomer compound used. This yield capacity has an important effect onthe maximum base shear, torque, and acceleration of the superstructure, as well as the defor-mation demand in the isolation system. Along this line, Park et al. [9] looked for the OYCof a bridge model subjected to different intensities of the N—S component of the El Centrorecord (1940). On the other hand, Ramallo et al. [10] have made a similar analysis for build-ing models but without varying the ground motion intensity. This motivated the study of asimplified 5-story building model, used in previous isolation investigations [10], subjected toa suite of Chilean records and other well-known recorded ground motions for a wide rangeof ground motion intensities. The objective of this parametric study was to define the OYCfor the particular design of the H-UC and EF-UC buildings but results are extended to otherstructures and ground motion characteristics.

Let us consider first the earthquake response of the simplified 5-story building presented inFigure 1. The parameters of this building are presented in Table I and were used previouslyby Kelly et al. [II]. The building stiffness kb is defined to generate an objective `secant'design isolation period To = 2.5s. Because the isolation modes dominate the response of globalresponse quantities such as the base shear in the superstructure, a shear building model wasconsidered in the analysis with a single lateral degree-of-freedom at each floor level.

The equations of motion of the structure with an inelastic isolation system may be stated as

(1)

where M, C and K are the mass, damping and stiffness matrices of the superstructure, respec-tively; r is the excitation influence vector; L is the kinematic transformation matrix betweenthe isolator deformation v and the degrees of freedom u of the structure, i.e., v = Lu; and

546 J. C. DE LA LLERA ET AL.

Figure 1. Simplified 5-story model used to compute the optimal yield capacity (OYC):(a) base-isolated; and (b) fixed-base.

is the restoring force vector of the isolators. Thus the termto the equilibrium, stated at the degrees of freedom of the structure, of the isolation restoringforces. Equation (1) is frequently written in state-space format so as to integrate the linearpart of the structural response in exact form and the non-linear part by using, say, a predictor–corrector algorithm, or a Runge–Kutta method [12].

For this simplified parametric study, a simplified bilinear force deformation constitutiverelationship is assumed for the isolators. The influence of two parameters is considered,namely the yield force Qy and the ratio between the post-yield and initial isolator stiff-

Let the optimal performance criterion for the OYC of the isolation system be definedby either the minimum base shear of the superstructure, or by limiting the isolator defor-mations. For a given earthquake record, the range of normalized yield force considered is

, where W is the seismic weight of the structure. Three values of kare considered in the analysis, 6, 10 and 15, which are typical for LRB. The OYC of theisolation system is obtained by minimizing the base shear of the structure as a function offor a range of peak ground acceleration (PGA) values. Because shear forces are of interest,the records are scaled in order to obtain a functional relationship between the OYC and thePGA values. A second criterion used was to define the OYC required to limit the deformationdemand in the isolation system, which may be critical in some cases.

is the contribution


Figure 2. Mean values of normalized base shear and isolation deformation as a function of the normal-of the isolation system: (a) Llo Lleo N10E ; and (b) Sylmar N00E .

Figure 2 shows a typical relationship between the normalized base shearof the isolation system obtained for Sylmar (NI OE component)

and Llo-Lleo (N00E component); the curves presented represent average values for the threenormalized stiffness ratios k = 6, 10 and 15. As expected, the OYC in shear increases as thePGA increases. For both records, such increase is almost linear in the space defined by

[9]. The optimal yield values are, however, very different between the Sylmar andLlo-Lleo records. For instance, OYC values for the Sylmar record vary between 4% and 25%of the weight of the structure; the corresponding range for the Llo-Lleo record is between0.5% and 3.5%, i.e., 7 to 12 times smaller. Besides, an increase of only 2% in the normalized

for the Llo-Lleo record leads to a large increase of 0.1 in the normalized based

548 J. C. DE LA LLERA

however, the same increase in for the Sylmar record leads to an increase of onlyshear

0.04 in This implies, that the base shear in the structure is more sensitive to the yieldforce in the former case. Furthermore, since the curves beyond the point of minimum shear

show smaller slopes than the curves before that point, it is always convenient to select aslightly above the optimal value. Moreover, as should be expected, the isolation

deformations tend to decrease as the yield force increases for a given value of the PGA. Thetrends indicated in Figure 2 by these two records turn out to be general for other records ofthe same families, respectively [13].

Both performance criteria of the isolation system may be combined in a single plot as

shown in Figure 3. This figure presents the relationship between the normalized OYC,and PGA. Superimposed also is the normalized yield force required, as a function of thePGA, to maintain simultaneously the isolator deformation demand at a specified value; theserestrictions are obtained from the points identified by the circular markers in the bottom

plots of Figure 2. Notice again that there is about a factor of 10 between thefor the Sylmar and Llo-Lleo records. For both records, the yield force required to controlthe isolation deformation for values above the OYC increases rapidly with the PGA. Thesedeformation iso-performance curves may be used effectively in design in order to balancebase shear requirements and deformation requirements in the isolated structure. For instance,it is apparent that forcing a small isolator deformation value for the true Sylmar record, sayD = 20 cm, would necessarily imply isolation yield forces larger than 30% of the weight ofthe structure. On the other hand, for the Llo-Lleo record the required isolator yield forces areless than 2% to keep a deformation demand of about 15 cm.

A summary of the normalized OYC values,for the nine different ground motions and three soil conditions considered are presented inFigure 4. It is clear that different ground motions may lead to quite different required optimalvalues and trends are hard to generalize. Results confirm, however, that the OYC in thecase of Chilean records and different soil conditions will rarely exceed 3%, associated withisolation deformations of less than 40 cm. For soil conditions I and III (Table II), the OYCis rather insensitive to variations of the PGA and, hence, the lead core is justified only forcontrolling isolator deformations under serviceability conditions and for an eventual torsionalbalance of the building plan. Instead, for soil type II there is a larger dependency of the OYCwith the PGA values. For instance, a value of 2.5% for the OYC would be adequate to keepthe normalized base shear in the superstructure less than 0.12 and isolator deformations lessthan 20 cm for a rather large PGA of 0.8g.

For the other suite of ground motion records considered, Kobe and Newhall show thesmallest OYC requirements, say less than 10%. At the other end, the SCT, Taiwan, andSylmar ground motion records would require OYC values larger than 15% of the weight

would correspond to a PGA of about 0.25y in

the case of the SCT record; however, the sameof 1.2g for the Sylmar record, i.e., about five times larger. As should be expected, thereis a strong dependency of the required OYC values on the frequency distribution of en-ergy of the ground motion. For all ground motions considered, linearity between the OYCand PGA or isolator deformation is apparent and, hence, two design points would suf-fice to extrapolate or interpolate the behavior of the structure for other values of theseparameters.

value of

values

and corresponding isolation deformations

of the structure. For instance,

value would correspond to a PGA


Figure 3. Optimal yield capacity for two performance criteria: (a) minimum base shear; and(b) isolation deformation limit.

EXPERIMENTAL PROGRAM

This section summarizes the results of dynamic testing of rubber isolators and reduced-scalespecimens conducted between 1997 and 2002 intended to select, design, and validate the cyclic


Figure 4. Optimal yield capacity and isolation deformation corresponding to minimum base shear in theisolation system for different ground motion records.

behavior of the isolation system for the H-UC, EF-UC, and MH buildings. Such a programconsidered the testing of 100 by 100 mm square elastomer specimens up to rupture (over200), quality control testing of more than 260 isolators under design conditions, and over 20prototypes of isolators under maximum capable earthquake conditions, including simultaneousshear/tension and shear/compression.

The set-up used initially for testing couples of isolators is presented in Figure 5(b) and(c) and consists of a dynamic 1000 kN MTS actuator and a vertical loading system to apply5000kN in compression and 400kN in tension. A new testing rig has been recently developedto test individual isolators with a maximum axial load of 8000kN (Figure 5(d)). The stroke of


Table II. Seismic records considered in this investigation.

the actuator is 1000mm and is connected to a 280gal/min pump through a manifold with three251. accumulators. The axial load during the test is maintained constant by the use of a passivepressure control system connected to four axial load actuators of 2500kN each. The two beamsof the test rig (Figure 5(c)) are joined by a hinge (Figure 5(c)) that enables 3D rotation of theactuator (primarily about a vertical axis); the beam on the left of the test rig is anchored to theground and the other is free to pivot laterally on top of a teflon interface. The whole system iscontrolled analogically for simple signals with an MTS 407 controller, or digitally for earth-quake and real-time simulation using the software and hardware dSpace [14]. The test rig forindividual isolators (Figure 5(d)) works on a vertical plane; the isolator is fixed in the upperend to the vertical loading frame and on the other end to a displacement controlled sliding tablemounted on rollers. Thus, the 1000 kN shear may be essentially applied to a single isolator.

ranging from 600 mm to 900 mm were installed in theH-UC, EF-UC, and MH buildings and considered in this research program. All these isolatorswere tested up to a design shear deformation

= 200%, which takes the actuator to its maximum shear capacity. Rubber compoundsfor these three buildings were provided by a local manufacturer and based on an exhaustivedestructive testing of reduced-scale 100 by 100 mm specimens and isolator prototypes. Aswill be shown next, the correlation between the mechanical properties of these reduced-scalespecimens and the full-scale isolator properties enables one to predict the behavior of thefull-scale isolators from small to large shear deformations of the elastomer compound.

Figure 6 shows typical results for the secant shear modulusof three reduced-scale specimens with a medium stiffness compound subjected to

Elastomeric isolators of diameter

= 150%; 20 additional isolators were testedup to

and effective dampingratio


= 600 mm isolator;Figure 5. Experimental set-up for shear-compression—tension testing: (a) typical(b) shear-compression test; (c) test rig for isolator couples; and (d) test rig for individual isolators.

= 0.1 Hz. The specimens have two elastomer layersof the same thickness as that used in the full-scale isolators. The testing sequence included

=0.25, 0.5, 0.75, 1, 1.25, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5and 6; this latter value was attained only in a few specimens. A typical shear failure of theelastomer is shown in Figure 6(b); the propagation of rupture in the compound is caused bythe increase in shear deformation and starts for nominal shear strain y >5, which was theminimum performance established for these projects. It is interesting to mention that thesecompounds all have elongations at rupture

sinusoidal cyclic testing at frequency

shear deformation amplitudes

>6. The smaller value of the shear deformation


Figure 6. Typical response of reduced-scale specimen, with a medium-stiffness rubber com-pound: (a) scaled specimen; (b) rubber compound failure; (c) secant shear, modulus,

and (d) effective damping ratio,

capacity as opposed to is attributed to: (i) the progressive damage that occurs in thespecimen as a result of the seven dynamic cycles applied at each deformation level and (ii)the local shear strain concentrations in the corners of the square specimens. Figure 6(c) and

with shear strain for the medium stiffnesselastomeric compound. It is apparent that stiffening of the specimen begins atthat the effective damping ratio always decreases with increasingcompounds were used in the isolation system of the H-UC and EF-UC buildings with nominal

(low stiffness) andBoth compounds contain, in weight, over 90% natural rubber; damping is obtained by the useof special mineral oils and proportions of reinforcing fillers. Although not included here forbrevity, all experimental results lead to trends similar to those presented in this figure.

Similar testing results are presented for full-scale isolators of the H-UC building inFigure 7. Both low- and medium-stiffness elastomer compounds were used in the building.

(d) present a typical variation of1.5 and

Two different elastomer

secant shear values medium stiffness).


Figure 7. Shear modulustual-size isolators with low- and medium-stiffness rubber compounds: (a) low stiffness

compound; and (b) medium stiffness compound.

and damping ratio obtained from testing of ac-

Because isolators used in the EF-UC building were all manufactured with the same mediumstiffness compound, testing results are omitted herein for brevity. The trends for the secantshear modulusthe reduced-scale specimens. It is apparent that the low-stiffness compound used was targetedfor high damping ratios, while the medium-stiffness compound was for medium damping. Atleast in the authors' testing experience, there is almost inevitably a trade-off between larger

and damping ratio of the isolators follow similar trends as those of

damping values, smaller shear deformation capacity and smaller stiffness of the elastomercompound. Hence, the designer should be aware that large damping is harder to obtain withhigh-stiffness compounds and that damping values larger than those presented in Figure 7 maylead to elastomer compounds with reduced shear deformation capacityof low-stiffness compounds.

In light of the results of Figures 6 and 7, a relationship may be found between the propertiesof reduced-scale and full-scale specimens. Such results are presented in Figure 8 where the

as compared to that

secant shear modulus values and damping ratios are plotted against eachother. The results are interesting and show an almost perfectly linear relationship between thetwo. For instance, the suggested equations for the medium stiffness compound are

(2 )


Figure 8. Relationship between secant shear modulus and effective damping ratio determined from themeasured shear response of full-scale isolators and reduced-scale 100 x 100 mm specimens.

and for the damping ratio

(3)

Although these expressions may change among isolator manufacturers, a relationship of thiskind may still exist. This implies that it is possible to predict the values of the secant shearmodulus and effective damping ratios of the full-scale isolators by means of the 100 by100 mm specimens for the range <1.5, which covers the usual design region. Furthermore,these results may probably be extrapolated at least for as well, and enable us toforesee the performance of the isolator for a maximum capable earthquake condition. This iswhy it seems advisable to request a 100 by 100mm specimen for each manufactured isolator;this would eventually make testing of full-scale isolators less necessary in the future.

The effect of axial loads on the cyclic behavior of = 600 mm LRB isolators is evaluatednext. A lead core of diameter mm was introduced in the central orifice of a coupleof isolators with nominal shear modulus Results of testing these isolators atdifferent axial loads and deformation amplitudes are presented in Figure 9. Figure 9(a) showsthe behavior of the isolators for very small deformations ranging from 0.3 mm to 10 mm,three axial loads = 1000, 2000 and 3500 kN, and three sinusoidal excitation frequencies


Figure 9. Constitutive behavior of a couple of LRB =600mm) for small and large deformations anddifferent axial loads: (a) small deformations; and (b) large deformations.

0.2 and 0.5 Hz. Figure 9(b) shows the cyclic behavior for larger deformationsranging between 20 and 280 mm for the same axial loads, and a single excitation frequency

Properties of these force—deformation cycles are summarized and compared for differentaxial loads and excitation frequencies in Figure 10. By comparing the results of the small andlarge deformation cycles, it is apparent that the value of axial load has an effect mainly on the


small deformation cycles (Figures 9 and 10). In the latter case, an increase in compressionforce leads to better confinement of the lead core, thus increasing the normalized isolator stiff-

and effective damping ratioshould be expected, an increase in the excitation frequency leads to an increase in the isolatorstiffness, but less than 10%; it also leads to a slight decrease in the damping ratio. The sensi-tivity of the response to different axial loads reduces with increasing shear deformation for thelevel of deformations considered (Figure 10(b)). Indeed, fordesign purposes essentially identical. Results may also be presented in terms of the normalized

, and normalized shear force,

, but show identical trends [13]. Although these results may be slightly perturbed bythe scragging recovery present in the isolators, they clearly show that for compression stresses

, the effect of axial loads is negligible as seen from theshear force–deformation cycle of these isolators. In spite of this, axial forces are extraordi-narily important in isolation design and evaluation of the maximum shear deformation of theelastomer, stability properties, and long-term properties of the isolators.

Time variation of isolator properties is a permanent concern of users and designers. Someresults of scragging, creep, and time change of mechanical properties in the short term arepresented in Figure 11. Figure 11(a) shows a sequence of cycles performed to evaluate theconstitutive behavior in shear and scragging of the isolators. The elastomeric isolator se-

was subjected to the following tests: (a) one first cycle at0.75, 1, 1.25 and 1.5; (b) after each cycle in (a), seven cycles were performed dynamically

, the last of these seven cycles is presented by dashed lines in thefigure; (c) repeat (b) at increasing values of shear deformation1.5–only as an example, the maximum force attained in the last of these cycles foris indicated by c in the figure; (d) repeat (c) 15 hours later. Notice that if the isolator defor-mation exceeds for the first time the deformation corresponding to that of a previous cycle,the curves catch the envelope of the unscragged shear–deformation constitutive relationship.This implies that scragging is a deformation-dependent phenomenon and will restart everytime the isolator is taken to a new larger deformation. In our example, point a represents theforce for the unscragged envelope (upper bound) and point b is the maximum force where thecyclic behavior becomes stable for a maximum

, is imposed on the isolator, the cyclic behavior forat point c, which is considerably lower than point b. Moreover, if the test is repeated severalhours later, the cycles become stable at point d, which is slightly below c. Thus, although theisolator may have been scragged to a deformation, say

will decrease the isolator forces developed ateffect should be considered in deciding the objective scragging deformation.

A second effect in design is the time recovery of the unscragged properties of the elastomer.Compared in Figure 11(b) are the values of the secant shear modulus G obtained from severaltests of isolators performed about 96 hours apart;modulus, respectively, for the same isolator under the same testing conditions. The apparenttrend is that for smaller deformations the isolator stiffness increases (recovers) with time.Because all the projects presented in this study are very recent there is no further informationthat enables us to confirm this short-term behavior in the long term. Furthermore, typicalresults from creep tests of isolators are presented in Figure 11(c). For these isolators, the final

0.4, the three curves are for all

in the isolators less than 130 kgf,

1.25. However, if a larger deformation,1.25 now becomes stable

scragging at larger deformationsdespite the previous scragging. This

represent the initial and last shear


Figure 11 . Scragging, recovery and creep in the short term for a typical isolator: (a) typical constitutivebehavior of = 900mm rubber isolator; (b) recovery of second shear modulus; and (c) isolator creep.

deformation set values are achieved in less than 30 hours. A large number of short-term creeptests performed on these isolators demonstrate that for long-term compression stresses less than

, short-term creep deformations will rarely exceed 1 mm for these compounds andisolator design. The fluctuations observed in the final deformation values may be attributedto changes in ambient temperature.

130 kgf/


The force–deformation constitutive relationship of a LRB may be cast in terms of a recentlyproposed model [15]. The model has fewer parameters than the original model proposed byPan and Yang [16] and is capable of representing peculiar phenomena of elastomeric bear-ings such as Mullins and scragging effects. In this model, the force–deformation relationshipproposed is defined by a pseudo-linear relationship [15]:

(4)

where are the deformation and deformation-rate dependent stiff-ness and damping of the isolator, respectively; and are the isolator deformationand deformation velocity. The stillness may be computed through

(5)

and the damping coefficient by

(6)

where are parameters that control the force–deformation loop. For instance, the termtimes the integral of the work done by the isolator is intended to represent the loss of

stillness due to scragging of the isolator; the term is related to the energy lost throughcycling of the isolator. Figure 12(a) shows the results obtained from this model for different

predicted and measured traces of the force–deformation relationship is good, except for shear

shear deformations and a constant compression load N = 3500 kN; the selected parametersare defined in the enclosure in units of ton and cm. The matching observed between the

deformations below = 0.25. For other values of the axial load N in ton, just needs to bemodified by considering the relationship =4.188(6.2+0.003N). Also shown in Figure 12(b)is the identified behavior of the lead core of the isolator, obtained by subtracting the measuredconstitutive behavior of the isolator without the lead core from the total shear-deformationbehavior of the isolator including the lead core. It is observed that the identified yield capacityin shear of the lead is about 10 MPa. which is the usual nominal value considered in design.

DESCRIPTION OF BUILDINGS

The first building considered is a new hospital of the Universidad Católica ( H-UC) located onthe skirts of the Andes mountains in the cast part of the city of Santiago (Figure 13(a)). Thebuilding was designed in 1999 and its one-year construction was finished in early 2001. Thebuilding has a total area of about 8000m' distributed in 6 stories (5 above ground level and 1basement) and weighs about 10341 ton above the isolation level. As indicated in Figure 14(a),the building was built as a single unit with no construction joints due to the use of the isolationsystem; its approximate plan dimensions are 78 m by 31 m. Story heights are equal to 3.4 min the upper 5 stories and 4 m at the basement. The structure is formed by ductile moment-resisting frames with 80 cm deep beams with a typical 12 m span and columns with 75 cmsquare sections in the superstructure and 90 cm at the basement. These latter columns are


Figure 12. Comparison between the measured and predicted cyclic behavior of a LRB and identifiedbehavior of the lead-core: (a) LRB constitutive behavior; and (b) identified lead-core behavior.

laterally braced by ribs coming out of the perimeter retaining wall, and interior columns areconnected below the isolation level by 40 x 40cm axial R/C struts. Concrete with a cylindrical

was used inthe construction of the structure. As shown in Figure 14, seismic isolation is placed at thetop of the basement, thus avoiding the construction of an extra floor slab for the building.

strength and steel with a nominal yield stress


Figure 13. Isolated buildings described in this investigation: (a) Hospital building (H-UC); and(b) Engineering faculty building (EF-UC).


Figure 14. Hospital building considered in this study (H-UC): (a) building plan; and(b) moment resisting plane 17.


= 100 mm lead cores, according to the planwise distribution presentedin Figure 14(a). The two elastomeric compounds presented earlier (Figure 7) were selected

Since all isolators were tested prior to their installation in the building, their final locationwas selected in order to balance the structure in torsion. Indeed, the final estimated nominal

The nominalthe corresponding

for the structure, with design shear modulusisolation periods of the structure areperiods of the fixed-base structure are 0.41 s, 0.38 s and 0.23 s, respectively.

eccentricities of the structure with measured stiffness values arewhich are small for such an irregular structure. In order to increase the lateral-to-torsionalfrequency ratio of the structure the isolators with lead cores were placed closed to the

The isolation system consists of 52 elastomeric isolators of diameters 600, 650 and 700 mm,22 of them with

perimeter of the building (Figure 14(a)); the final for the structure is estimated to beabout 1.2. The spectrum selected for the design of the isolation system is based on themeasured ground motion during the 1985 Chile earthquake, and defines for this structurea maximum displacement for the isolator of 35 cm associated with a 10% probability ofexceedance in 100 years. Furthermore, based on the results presented earlier for the OYC, a

was selected for the design of the structure. Although not presented herefor the sake of brevity, architectonic details of this structure are rather complex and a largenumber of interesting solutions were proposed for hanging a cable-stayed bridge used in themain access of the structure, ramps, hanging staircases, hanging elevator shafts, pool, drains,auditorium, piping, and so forth; the interested reader may check them elsewhere [17].

Figure 13(b) shows the new Engineering Faculty building of the UniversidadCatólica(EF-UC), which is located in the San Joaquin campus of this university in a SW districtof Santiago. The building was designed in the year 2000 and constructed during 2001. It

value

has a total surface area of approximately 6000 distributed in 5 stories (4 above groundlevel and 1 basement) and the superstructure weighs about 7456 tons. It has a rectangularplan with plan dimensions 92 m by 14 m and, as in the previous case, it was built as asingle unit with no construction joints due to the lateral flexibility of the isolation system(Figure 15). The structure of the building is based on a combination of R/C shear walls(e =20cm) and moment-resisting frames with reinforcement ductility details corresponding toa zone of moderate seismicity. Such is the case because the shear walls constitute the lateralload-resisting system and the columns are primarily a vertical load-carrying system. A typicaltwo span (9m and 5m) moment-resisting frame in the transverse direction is shown in Figure15(b). Beams are either 20cm by 70cm, or 35cm by 75cm deep, rectangular exterior columnsare 40cm by 60cm, and interior columns are 45cm square sections. Nominal concrete strengthand steel reinforcement yield stress are the same as for the previous structure.

The structure is isolated at the foundation level with 42 identical elastomeric isolators(Figure 15(a)) of diametersteel—teflon—elastomer sliders. The structure has a retaining wall along its perimeter separatedfrom the main building structure by a 32cm gap, which is the maximum displacement expectedfor an earthquake with a 10% probability of exceedance in 100 years. A single G = 7.5kgelastomer compound was used for all isolators and sliders. The 300 mm slider moves on a

= 1200 mm diameter. The measured frictional coefficientvaries approximately from 0.10 to 0.15 for the range of velocities and maximum axial loadof 68 ton assumed in the design. As in the previous case, the natural eccentricity of thesuperstructure was corrected by the proper placement of the isolators in plan, leading to

= 600 mm, of which 17 have = 100 mm lead cores, and 11 are

stainless steel sliding surface with


Figure 15. Engineering Faculty building considered in this study (EF-UC): (a) building plan (basement);and (b) moment resisting plane 7.

a final design with maximum nominal eccentricities ofnominal isolation periods of the structure are the

and = 0.06. The

corresponding periods of the fixed-base structure are 0.189 s, 0.32 s and 0.29 s, respectively.Again, the isolators with lead cores were placed close to the perimeter of the structure leading

= 2.3% was selected in this case.= 1.08. Furthermore, an OYCto


The final costs of the structure, including all aspects of the isolation system, were US$and US$ 125 for the H-UC and EF-UC buildings, respectively. In Chile these costs115 ,

are similar to those of conventional structures, positioning seismic isolation as a competitivetechnique in the market. This leaving aside powerful arguments such as protection of contentsand earthquake-damage reduction in the isolated structure relative to a conventional structure.Currently, three new modules are being constructed to the south of the EF-UC structure,making this the longest building in the country

ELASTIC AND INELASTIC BUILDING RESPONSE

In this section, the inelastic response of typical resisting planes of the H-UC and EF-UC(Figures 14 and 15) is evaluated. A parametric study is performed for 12 recorded impulsiveand non-impulsive ground motions and variable yield capacityranging between 2.5% and 15% of the weight of the structure. The inelastic analyses are per-formed assuming elasto-plastic 2D hinges at the column and beam ends of the superstructure.Plastic behavior in the hinges is defined by an interaction surface between axial load andbending moment computed from nominal section and reinforcement properties of the struc-tural members. Each isolator is modeled by a bilinear constitutive relationship calibrated withtheir experimentally measured constitutive behavior.

Four analyses are performed with different analytical models of the superstructure andground motion records, i.e.: (i) fixed-base structure with linear behavior (L); (ii) fixed-basestructure with inelastic behavior (NL); (iii) linear structure with inelastic behavior of the iso-lation system (LI); and (iv) inelastic structure with inelastic behavior of the isolation system( NLI). Response reduction factors are defined as the ratio of the response of the fixed-basestructure over the corresponding response of the isolated structure; i.e.,quently, two reduction factors are defined: a linear reduction factorthe responses of the linear fixed-base and isolated structure and the inelastic reduction fac-

relating the responses of the inelastic fixed-base and inelastic isolated

ing for three different yield force values,For the EW stiff soil site ground motion Zapallar (Chile, 1985), the isolated structure has amaximum deformation less than 1.5 cm; notice also that the minimum shear value occurs for

= 15% the isolation system remains elastic. The NS component

without construction joints

of the isolation system

Conse -, relating

torstructure.

Figure 16 shows the force–deformation responses of the isolation system of the H-UC build-= 2.5%, 5% and 15%, and three ground motions.

= 2.5% and that forof the Melipilla record (Chile, 1985) is more demanding and for = 2.5%, which leadsto the smallest base shear, the maximum isolator deformation is about 10 cm. It is apparenthow the shear force in the isolation system increases assmall OYC for the Chilean records. Furthermore, the N00E component of the Sylmar record(Northridge, 1994) is quite demanding for the isolation system of this building, leading to

= 2.5%. However, it is interesting to note thatvalues, the maximum base shear demand in the structure is very similar, while the

maximum isolator deformations decrease significantly for largerthe H-UC building were subjected to the Sylmar record, the OYC should have been of theorder of 15% of the weight of the structure instead of the 2.5% used for its actual design.

increases, justifying the use of a

a deformation demand close to 50 cm forfor all

values. Consequently, if


Figure 16. Typical bilinear force–deformation response of an isolator of the H-UC building with inelasticmodel of the isolation system and superstructure.

A comparison between the elastic and inelastic roof accelerations and 2nd-story drift ofthe H-UC building for Melipilla and Sylmar is presented in Figure 17. The use of seismicisolation in the structure always leads to a significant reduction in interstory drift and accel-eration, i.e.,

other hand,

for the Melipilla record; on the

for the Sylmar record. Therefore,inelastic acceleration reduction factors are, as should be, smaller than the elastic ones. Forthe inelastic case and Sylmar (Figure 17(b)), the 2nd-story drift is reduced from 2.3% toabout 0.6% and the maximum roof acceleration from 1 y to 0.7g. By comparing parts (a) and(b), the traces of interstory drift are strongly affected by the inelastic behavior of the su-perstructure. Indeed, for the Melipilla and Sylmar records the fixed-base structure would endup with permanent drifts of the order of 0.15% and 0.3%, respectively. Notice also that forMelipilla the interstory drift demand for the isolated structure is below 0.2% and, hence, onlyminor damage, if any, could be expected in non-structural components for this frame structuresubjected to this ground motion. These observations carry over to other ground motions andto the EF-UC building as well [13].

Furthermore, Figure 18 shows the heightwise profiles of displacement, drift, and accelerationfor the H-UC building subjected to the Melipilla and Sylmar ground motions. The inelasticdrift demand occurs in the second story and is about 1% and 2.3% for these two records,respectively. Seismic isolation in the structure is able to reduce these drifts to about 0.16%and 0.60% (Figure 17). Analogously, floor accelerations in the superstructure are also reducedby seismic isolation from maximum values of about I y for the inelastic case to 0.2g and 0.7yfor Melipilla and Sylmar, respectively. These accelerations increase with increasing values


Figure 17. Response history results for the fixed-base and isolated H-UC building considering a linearand inelastic model of the superstructure: (a) linear model of the superstructure; and (b) inelastic

model of the superstructure.

; accelerations in the level immediately below the isolation interface also increase asa result of this part of the structure being connected to the ground. This implies that thestory below the isolation level might result in larger member forces if the selection of the

/ W =2.5% and the Melipilla record leadsin the superstructure to peak floor accelerations less than 0.20g, story drifts less than 0.2%,and isolation displacements below 10 cm, this value was selected in the actual design of bothbuildings considered in this investigation. This OYC could also work in principle for thestructure subjected to the Sylmar record; however, the isolation displacements would be over50 cm, leading to a large gap and a costly isolation system. Because of this, a larger value ofshear capacity of the order ofdisplacement demand of the isolation system to values below 30cm, peak floor accelerations inthe superstructure less than 0.7g, and story drifts below 0.6%. Consequently, these results showthe sensitivity of an isolation design to the different characteristics among ground motions.Finally, notice that the earthquake responses estimated from linear and non-linear models of

of

OYC is inadequate. Since the yield force value

/W = 15% would work better in such a case, reducing the


Figure 18. Response of the H-UC building subjected to the Melipilla and Sylmar records for differentyield force and modeling of the superstructure: (a) Melipilla record; and (b) Sylmar record.

the superstructure are quite similar. Hence, these results warrant the use in design of an elasticmodel for the superstructure together with an inelastic model of the isolation system.

In order to generalize these results for other ground motions, Figure 19 shows bands forthe ratio of the maxima of the interstory drift, , and floor acceleration, for the two


Figure 19. Band of reduction factors for acceleration and interstory drift in the superstructure of buildingsH-UC and EF-UC (firm soil): (a) Chilean ground motions; and (b) impulsive ground motions.

(2.5%—15%). The results presented are for the actual firmsoil conditions of the two structures and the reduction bands obtained correspond to elasticand inelastic models of the superstructure; results for other soil conditions lead to similar

buildings as a function of


conclusions and are summarized elsewhere [13]. It is apparent that seismic isolation alwaysleads to considerable reductions in interstory drifts that range between 2 and 7 times. Thesereduction factors tend to decrease with increasing values ofto this parameter for Chilean records because the dissipation capacity of the lead cores is moreeffective, as with most energy dissipators, only if the structure goes through multiple vibrationcycles. Furthermore, reduction factors for floor accelerations vary between 1.5 and 4 for bothfamilies of ground motions and always decrease asOYC value less than 4% seems a good recommendation for Chilean records; however, largervalues will be needed for the design with impulsive ground motions in order to control thedeformation demand of the isolators. Finally, reduction factors based on the actual inelastic

are usually, but not always, smaller than reduction factors based

They are also more sensitive

increases. Based on these results, an

response of the structureon the elastic response of the building This should be expected since, for instance, flooraccelerations for the fixed-base inelastic superstructure are smaller than those of the elastic

decreases for the inelastic case.

IMPLICATIONS FOR DESIGN AND PRACTICE

The results presented above have been used in calibrating a proposal for a new seismic isola-tion building code presented by a group of researchers and practitioners to the Chilean Instituteof Normalization [18]. This proposal is strongly based on the ICBO 2000 [19] and other pre-vious design documents [18]—indeed the initial document is a translation of the isolationprovisions of the UBC 97, but includes interesting modifications (some in the commentary)in aspects related to the local seismicity, design spectra definition, damping reduction factors,isolation modeling, optimal values of shear capacity, experimental isolation results, and otherissues to make the text coherent with the local engineering practice.

An important step was to develop a Newmark–Hall-type design spectra (Figure 20) consis-tent with the response spectra obtained for the 1985 Chile earthquake in all frequency regions.Since the tendency has been to calibrate design spectra based on pseudo-acceleration responsespectra, reported spectral ordinates for long-period structures (T >2 s) are usually inadequatesince the exponents of the decaying branches of the spectrum do not satisfy the physicallyexpected trends. The spectra presented in Figure 20 is based on the work of the committee[18] on the response spectra obtained for the 1985 earthquake [18]. Since for dynamic analysisthe whole spectrum is required, a simplified, say, constant pseudo-velocity spectra may leadusually to an over-estimation of floor accelerations and forces in the superstructure.

Because of the organic nature of rubber and the inherent variability in the manufactur-ing process of isolators, testing some bearings and reduced-scale specimens of the elastomercompound seems strictly necessary. In the authors' opinion, based on testing so far over 400different size and shape building and bridge isolators, quality control testing is at least asimportant as prototype testing since it enables one to identify flaws that are common to thefabrication process, such as differences in the properties of rubber compounds coming fromdifferent batches of rubber, improper alignment of steel shims in plan and height (rarely seenfrom the outside in the undeformed isolator due to the cover), local failures and cracks re-lated to the mould and curing process, and more importantly, benchmarking for monitoringthe evolution of the isolation system in time. Although prototype testing does guarantee thestability of the design based on testing a couple of isolators, it is fair to say that failing

superstructure and, hence, the numerator of


zone 2 and damping ratio

in shear a reasonably fabricated natural rubber isolator under maximum capable earthquakeconditions is difficult, even after a large number of cycles. Therefore, since the results ofreduced-scale 100 by 100 mm specimens have shown an excellent agreement with the truebehavior of full-scale specimens, and they are cheaper, they should be required for describinga compound throughout the whole shear deformation range. In any case, the designer mustbear in mind that variabilities in stiffness and damping ratio withinfor the same isolator manufacturer and design.

An existent or induced crack in the elastomer will usually increase progressively as defor-mation increases. Therefore, the initiation of any crack in the elastomer should be delayed bya good geometric design of the isolator mould, including primarily a well-designed foot andrubber vents. Sharp corners and other singularities should be avoided 'since they generate largelocal shear strains that initiate unbonding or cracking of the rubber laminates. From the testsperformed in scaled specimens, final unbonding failures should be precluded since they tendto occur suddenly as opposed to rupture in the elastomer that leads to a failure that requiresseveral cycles to propagate throughout the laminate (Figure 6(b)).

Specific damping reduction factors for non-impulsive ground motions were derived for thiscode proposal. The reduction factors due to damping B for non-impulsive ground motionsmay be computed for the three different soil conditions by (Table III):

Figure 20. Design spectrum proposed for the Chilean code of isolated structures (seismic= 0.05).

15% should be expected

(7)

where a is a coefficient specified in Table III; and is thedamping ratio. These reduction factors were computed by using ground motions compatiblewith the design spectrum associated with the three different soil conditions. Because of thelarger number of strong motion cycles present in non-impulsive ground motions relative to


impulsive motions, these factors are, as should be, larger than those specified in the ICBO2000 code [19].

Although not described herein for reasons of brevity, architectonic aspects in seismicallyisolated structures are of great importance and should not be overlooked during the structuraldesign phase. It is precisely this point that makes isolation projects special in the sense thatthey require a stronger interaction between the architect and structural engineer; a poor designof these architectonic details may counter balance the merits of seismic isolation in terms ofstructural response. Although every structure has specific architectonic details, there are certaintypologies of solutions that tend to repeat in order to accommodate the lateral displacementbetween the fixed-to-ground structures and components and the superstructure. The interestedreader may find a collection of low-cost joint solutions and details for the surrounding gap,facades, staircases, ceilings, handrails, elevators, piping, and other situations elsewhere [17].

As stated before, the cost of the bare structure including isolation was US$ 115 and US$125 per square meter for the H-UC and EF-UC buildings, respectively. This value is withinthe usual range for conventional structures and thus makes isolated structures very competitive.The reader may wonder about the direct cost of these structures without isolation. Althoughestimates for both buildings were generated in terms of preliminary designs to compare be-tween the isolated and conventional solutions, the numbers are always questionable since afinal project would be required for estimating real construction costs in each case. However,it is the authors' opinion that direct costs between the two solutions ended up being quitesimilar mainly because no extra slab was required in the H-UC building, all shear walls wereeliminated, and the building has no construction joint, which is required for structures ofsuch dimensions in plan. For the EF-UC building, extra costs due to isolation were counter-balanced by: (i) the elimination of the construction joint in the structure (ii) a more economicdesign of the foundation system; and (iii) a lower density of shear walls and reinforcementin columns than for conventional structures of this kind in the country.

CONCLUSIONS

This article summarizes the work done by the authors over the past six years in Chilewith seismically isolated structures. The work has included an extensive testing program,design, and implementation of elastomeric isolation in two reinforced concrete structures ofquite different characteristics in structure and foundation soil. Currently, results of this ap-plied investigation will be used in the new Military Hospital, a structure with 164 elas-tomeric isolators, the largest isolated structure in South America. It has been shown by this


structural implementation that seismic isolation is a competitive earthquake-resistant construc-tion technique in this country and it is expected to expand rapidly in the years to come.Furthermore, a new building code for seismically isolated structures is currently approved andwill probably be available for the profession in November 2003. This technology is beingtransferred to the profession through seismic isolation courses taught in the Civil Engineer-ing curricula of the best universities in the country as well as through the interaction inprojects between the different universities and the engineering profession, such as with theMilitary Hospital. It is expected that the experience developed in this country may be of use toother countries in the region which share similar seismic characteristics and social/economicenvironments.

ACKNOWLEDGEMENTS

This investigation has been supported by the Chilean National Fund for Research and Technology,FONDECYT under Grant # 1020774. Part of the research was also supported by the Fund for Fomentand Technology, FONDEF under Grant #D9611008. The authors are grateful for this support.

REFERENCES

Isolat in Chile

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