Seismic protection of low- to moderate-mass buildings using RNC isolator

STRUCTURAL CONTROL AND HEALTH MONITORING

Seismic protection of low- to moderate-mass buildings usingRNC isolator

Mohammed Ismailz, Jose Rodellar�,y and Faycal Ikhouaney

Department of Applied Mathematics III, Control, Dynamics and Application Laboratory, School of Civil Engineering,Technical University of Catalonia, Barcelona, Spain

SUMMARY

The general objective of base isolation is to increase the natural period of vibration of the structures out ofthe typical earthquake dominant periods through increasing their flexibility. This objective is particularlyimportant for light- to moderate-weight structures as low masses imply lower natural periods. This paperpresents the application of a new roll-n-cage (RNC) base isolator to mitigate the seismic response of thisclass of structures. From a functional point of view, RNC is essentially an isolation bearing, while it offersa significant resistance to wind and minor vibrations. Rolling is the basic mechanical principle adopted tooffer a great range of horizontal flexibility, which is complemented with an energy dissipation mechanism.The device is designed to have several practical controls to ensure buffer, re-centering and no uplift duringmotion. In this paper, the principles of operation and force–displacement relationships of the novel RNCisolator are presented, along with its implementation and performance evaluation considering a wide rangeof structural, isolator and ground motion characteristics. The numerical results show that the proposedbearing is a useful tool for isolating light- to moderate-mass systems, and it can mitigate the seismicresponse under a variety of ground motion excitations while exhibiting the robust performance for a widerange of structures. Copyright r 2010 John Wiley & Sons, Ltd.

Received 23 October 2009; Revised 15 August 2010; Accepted 28 August 2010

KEY WORDS: passive control; seismic isolation; rolling; RNC isolator; hysteresis; Bouc–Wen model; lightstructures

1. INTRODUCTION

Designing earthquake-resistant low- to medium-rise structures is problematic in that theirfundamental frequency of vibration lies in the range of frequencies where earthquake energy isthe strongest. Thus, the building acts as an amplifier of the ground vibrations, and the flooraccelerations increase with the height of the building. Ultimately, the seismic design shouldreduce these accelerations below the level of the ground accelerations. To do this, the buildingmust be flexible. For a low- or medium-rise building, the necessary flexibility can only beachieved by using seismic isolation at the foundation level to shift the structural period awayfrom the period range that contains most of the earthquake energy, while leaving the structureto undergo almost a rigid body motion [1,2].

Seismic isolation concept dates back to the 1800s, but it moved into the mainstream ofstructural engineering since the 1970s [3]. The lead rubber bearing (LRB) was invented in the

*Correspondence to: Jose Rodellar, Department of Applied Mathematics III, School of Civil Engineering, TechnicalUniversity of Catalonia, Campus Nord, Module C-2, 08034-Barcelona, Spain.yE-mail: [email protected]. Candidate.yProfessor.

Copyright r 2010 John Wiley & Sons, Ltd.

Struct. Control Health Monit. 2012; 19:22–42Published online 29 October 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/stc.421

1970s, [4–6] and this allowed flexibility and damping to be included in a single unit. Aboutthe same time, the first applications using rubber bearings for isolation were constructed. In theearly 1980s, developments in rubber technology lead to new rubber compounds, which weretermed as high damping rubber (HDR) [7]. Both LRB and HDR bearings have minimumflexibility limit. When it is exceeded, the bearing stability is drastically affected. Therefore, theydo not suite the isolation of low-mass systems. Flat sliding bearings were not used alone asisolation systems because they do not have a restoring force mechanism nor a buffer. Thefriction pendulum system (FPS) [8] does not have the above restrictions, but its high cost hindersits economic use for light buildings [3]. Further, it causes structural uplift and has fixed vibrationperiod [9].

The concept of rolling is an efficient way to achieve the necessary isolation period ifcompared with the sliding motion mechanism. Therefore, rolling-based isolators could be bettercandidates for light systems isolation. Lin and Hone [10] proposed a system of free circularrolling rods located between the base and the foundation. The most attractive feature of thistype of isolator is their low value of rolling friction coefficient. However, such system lacksre-centering and damping capabilities. To provide it with a re-centering mechanism, Jangid andLondhe [11] proposed that the shape of rolling rods should be elliptical rather then circular, butthe system damping is still absent.

Combining the advantage of free rolling rod isolators and FPS isolators, the base isolation bya ball system with restoring property was proposed by Zhou et al. [12]. The resulting system is aself-recentering ball bearing having a fixed vibration period. A similar approach was presentedby Barghian and Shahabi [13], where a mushroom-shaped (pendulum-like) basements were usedunder structures as a base isolation system. Indeed, the idea of the rolling pendulum system isquite old, as it dates back to the US patent [14]. The so-called ball-n-cone isolation bearing forlight-mass systems was proposed and examined by Kasalanti et al. [15,16] and Kemeny [17].The RoBall isolation system that has no displacement limit has been presented in Reference [18].It consists of two opposite saucer-like rubber containers with flat outer surfaces and curvedinner ones. A multi-step rolling isolation system was presented in Reference [19], wherea mutually eccentric rotators isolation system was proposed by Kawai and Shiki [20].A bi-directional rolling pendulum system was proposed by Kim [21]. Some other rolling-basedisolation systems were proposed by authors of References [22–28].

Although many other systems have been promulgated [13,18,21,29–42], based on rollers,springs, cables etc., the market for seismic isolation now is mainly distributed among variationsof LRBs, HDR bearings, flat sliding bearings and FPS. There have been systems proposed toisolate light buildings. However, the fact remains that there are few instances of successfulisolation of light structures.

In previous works, an innovative new seismic isolator has been proposed referred to as roll-n-cage (RNC) isolator [43]. Under different configurations, the efficiency of this device has beenanalyzed to isolate equipments housed in structures [44,45] and to isolate full heavy-massbuildings [46]. In the work presented in this paper, a version of the RNC isolator is designedspecifically to protect structures of light to medium masses from seismic threat. The mainprinciples of operation and physical characteristics (isolation through rolling, buffer, inherentgravity-based re-centering mechanisms and no uplift) are briefly described. A force–displacementrelationship is obtained to characterize the energy dissipation with an input–output manageablemodel. Extensive numerical studies show that the RNC isolator is a useful tool for isolating light-to moderate-mass systems, as it can mitigate the seismic responses under a variety of conditions.

2. THE RNC ISOLATION SYSTEM

The RNC isolation bearing (see Figure 1) is mainly made up of a stiff rolling body (1), whichrepresents the bearing mechanism, placed between two stiff circular bearing plates (2,3) fixed tothe isolated superstructure and its base, respectively. The contact between these three parts takesplace through less stiff plates (4,5) that are perfectly stuck to the upper and lower bearing plates,


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DOI: 10.1002/stc

SEISMIC PROTECTION OF LOW- TO MODERATE-MASS BUILDINGS

respectively. A strong hyperelastic material such as neoprene is preferred for plates (4,5).The energy dissipation takes place through metallic yield dampers (6). These dampers aredesigned and arranged around the rolling body, as a cage, to provide the desired stiffness anddamping. Stiffeners (7,8) are used to enhance the behavior of the metallic yield dampers.A detailed description of the concept and the operation principles are found in a recent patent [43].

From its name, the RNC isolation bearing adopts the rolling mechanism to cut off the loadpath between the superstructure and the base. Such rolling mechanism offers minimal degree ofsuperstructure-base coupling as it requires lower force to roll if compared with slidingmechanism. As a consequence, rolling approaches the ideal isolation concept, which requiresa total horizontal separation. However, a system with minimum resistance to lateralmotion is susceptible to shaking under minor vibrations and may end up in a differentlocation after an earthquake and continue to dislocate under aftershocks. To avoid this inthe RNC isolator, it is provided with a number of metallic yield dampers (6), as a cage aroundthe stiff rolling body, to provide suitable initial stiffness under minor vibrations and damping tolimit the vibrational displacement amplitude. They are shaped and arranged as shown inFigure 1 to exhibit the same shear strain in any horizontal direction and to allow for enoughextension during rolling.

To prevent the vertical elevation of the RNC-supported structure during rolling, the upperand lower stiff plates (2,3) are provided with inner triple curvatures that are carefully designedto exactly absorb the expected elevation during rolling of the elliptical rolling body (1), over thefull time history of excitation. In this case, the hyperelastic neoprene plates (4,5) help increasingthe sliding friction coefficient with the rolling body (1) to force rolling motion and to avoid anyunwanted slipping. Moreover, such curvatures makes rolling of the elliptical roller exactly

Figure 1. (a) Isometric view; (b) Side view; (c) Plan view; and (d) Incomplete sectional elevation of theproposed RNC isolator.


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M. ISMAIL, J. RODELLAR AND F. IKHOUANE

similar to the rolling of the spherical roller, in the sense that elliptical roller consumes someenergy to elevate the supported structure or object at the end of stroke. This forces the isolatedsystem to vibrate as a pendulum having a fixed vibration period, which represents a severepractical difficulty. Such limitation does not exist in spherical rollers nor in the RNC isolatorFigure 2(a)–(c).

As shown in Figure 1(d), the rolling body (1) is provided with two grooves and the bearingplates (2,3) are provided with vertical edge sidewalls. Both the grooves and the side wallsconstitute a fail safe limiting lateral rolling capability during seismic excitations stronger thanthe design earthquake, as illustrated in Figure 2(a), (c). In this case, the rolling body (1) works asa rigid link in compression against the direction of motion. This rigid link is bounded verticallyby the bearing plates (2,3) and bounded horizontally by the vertical edge sidewalls of theseplates, as demonstrated in Figure 2(g), (i). Three things would alleviate the possible impact: (i)the vertical edge sidewalls are covered with the hyperelastic plates (4,5), (ii) rolling onhyperelastic material reduces rolling velocity, (iii) the gradually developed tensile forces thatstretch the metallic yield dampers as the rolling displacement increases help reducing the rollingvelocity before hitting the vertical edge sidewalls Figure 2(a), (c).

The pointwise contact in the case of roller bearings reduces their ability to support heavierloads without flattening at the contact points. This main drawback has been alleviated in theRNC isolator whose bearing mechanism is improved by the following:(i) large radius ofcurvature of the rolling surfaces; (ii) insertion of deformable hyperelastic neoprene plates (4,5)between the rolling body (1) and the upper and lower stiff bearing plates (2,3), which ensuresgetting contact areas instead of contact points. In addition, the plates (4,5) prevent stiff-to-stiffpointwise contact between the stiff rolling body (1) and the stiff bearing plates (2,3). Certainly,this avoids flattening of the stiff materials at the contact points under long-term vertical loading.

Practically, isolation systems should be designed to provide a restoring force to return theisolated structure safely to its original position after earthquake and to reduce or prevent torsionresponse. The RNC isolator is provided with an efficient gravity-based re-centering mechanism

(a) (b) (c)

(d)

(g)

(e)

(h) (i)

(f)

Figure 2. The main available mechanisms in the RNC isolation bearing.


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through the elliptically shaped rolling body (1) along with the weight of supported structure asshown in Figure 2(d)–(f). Such mechanism works as follows: (i) at neutral position, thestructural weight has the same line of action of its reaction, therefore, the developed restoringmoment Mr is zero, keeping steady situation; (ii) as the relative motion between the isolatedstructure and the ground initiates, an eccentricity or offset develops between the two verticallines of action of the downward weight and its upward reaction. This eccentricity is equal to thedeveloped relative displacement between structure and ground. Therefore, a restoring couple Mr

is generated opposite to the motion-causing couple as illustrated by Figure 2(d)–(f).

3. MECHANICAL CHARACTERISTICS

In this study, the rolling body (1), top stiff plate (2) and lower stiff plate (3) are made of steel.The less stiff plates (4,5) are made of neoprene material. The isolator dimensions are chosen toallow for 53 cm as a maximum rolling displacement, beyond which the designed buffermechanism stops the isolated structure with minimal shock. The metallic yield dampers aremade of mild steel. The horizontal and vertical distances between the furthermost points of thebearing are 2.48 and 1.40m, respectively.

To determine the maximum vertical load capacity of the proposed RNC isolator, a trial anderror procedure has been carried out using ANSYS Multiphysics [47], such that the verticalstrain in the neoprene plates (4,5) does not exceed 30% as recommended by [48]. As a result, themaximum vertical load capacity for the RNC isolator is 600 kN per bearing.

The general-purpose finite element code ANSYS is used to enable computer-aided design andtesting of the RNC isolation system. Through this scheme, real-scale models are designed.Extensive and detailed series of tests are carried out in a machine-like environment, whichaccurately simulates the response of the device subjected to a real testing machine. This allowsfully identification of the RNC mechanical characteristics before its construction.

In machine-like testing, the topmost surface of the modeled RNC unit is allowed to movevertically (without rotation) under the structure own weight but it is always kept fixed inhorizontal direction. The lowermost surface of the RNC unit is kept always fixed in the verticaldirection. In horizontal direction, it is kept fixed only during the vertical application of thestructure own weight. When the ground motion is applied, the lowermost surface is releasedhorizontally without rotation. The less stiff plates (4,5) are completely glued to the innersurfaces of the upper and lower steel bearing plates (2,3), while they are always kept in rollingcontact with the rolling body (1).

The ANSYS calculated force–displacement relationship for the RNC is displayed in Figure 3in dashed line. It is referred to as ‘measured’ as it will be considered as the ‘true’ response forcomparisons with a simplified equivalent bilinear approximation and for the furtheridentification of a hysteretic model. The equivalent bilinear approximation [49,50] is plottedin solid line in Figure 3. The following mechanical characteristics are extracted from this bilinearcycle:

1. Pre-yield stiffness, ke.2. Post-yield stiffness, kb.3. Yield displacement, dy.4. Yield force, fy.5. Characteristic strength, Q.

4. HYSTERETIC MODELING OF ENERGY DISSIPATION MECHANISM

The characterization described in Section 3 has shown that the RNC isolator exhibits ahysteretic behavior, as shown in Figure 3. The objective of this section is to obtain aninput–output mathematical model to describe in a reasonable and manageable form the


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force–displacement relationship exhibited by the RNC isolator. The Bouc–Wen model ofsmooth hysteresis is considered.

4.1. The Bouc–Wen model

The Bouc–Wen hysteretic model [51] has been extensively used to describe nonlinear hystereticbehaviors [52], particularly, in seismic isolation devices. The so-called normalized form of themodel proposed by Ikhouane and Rodellar [53] is utilized in this study to capture twoadvantages: (i) the warranty of a unique input/output behavior for each set of parameters,which is ideal for identification purpose; (ii) elimination of parameter redundancy or over-parametrization. The restoring force Fb offered by metallic yield dampers in the proposedisolation bearing can be expressed using the normalized Bouc–Wen model as

FbðtÞ ¼ kx xðtÞ1kw wðtÞ; ð1Þ

_wðtÞ ¼ rð_x� sj_xjjwjn�1w� ðs� 1Þ_xjwjnÞ; ð2Þ

where kx, kw, r, s and n are the shape controlling parameters of the hysteresis loop; and w(t) isan auxiliary variable, which is not accessible to measurement. Furthermore, and to guaranteeBIBO stability, passivity, and consistency with physical asymptotic motion, the lower value ofthe parameter s is limited to 1

2 [53,54].Note that the Equation (2) can be written in the form

_w ¼ rð1� s sgnð_xÞjwjn�1w� ðs� 1ÞjwjnÞ_x; ð3Þ

in which sgn is the signum function. The last equation leads to

dwdx¼ rð1� s sgnð_xÞjwjn�1w� ðs� 1ÞjwjnÞ: ð4Þ

Equations (3) and (4) highlight that, in the Bouc–Wen model, the rate of force ( _w) isdependent on the rate of displacement, whereas the relationship between the force (w) and thedisplacement depends on the direction of the velocity (sgnð_xÞ) only and not on its magnitude.

(a)

(b)

Figure 3. Measured vs equivalent bilinear approximation responses.


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4.2. Bouc–Wen model parameters estimation

The normalized Bouc–Wen form provides an exact and explicit expression for the hystereticlimit cycle [53]. Therefore, by using a periodic input signal x(t) along with analytical descriptionof the limit cycle, a robust parametric nonlinear, nonrecursive identification method or thenormalized Bouc–Wen model was presented [54,55]. This method provides exact values of themodel parameters in the absence of disturbances, and gives a guaranteed relative error betweenthe estimated parameters and the true ones in the presence of the perturbations. Thisidentification method is used in this paper, leading to the following parameters of thenormalized Bouc–Wen model:

kx ¼ 19:315; kw ¼ 16:227; r ¼ 55:641; s ¼ 1:022; n ¼ 2:162:

An efficiency measure of the identified parameters is carried out through the model validation.In Figure 4, the measured (calculated by ANSYS numerical model) output restoring force of

the isolator is plotted against the calculated one via the identified normalized Bouc–Wen model.The good matching shows that the Bouc–Wen model is able to capture the nonlinear hystereticbehavior of the proposed isolator.

4.3. Bouc–Wen model parameters validation

To further check the validity of the identified parameters, an actual random seismicdisplacement (El-Centro) signal is input to the ANSYS and the Bouc–Wen models. Then, thediscrepancy between the measured and predicted outputs, Fm and Fb, is quantified using the L1

and L1-norms and the corresponding relative errors e:

jjf jj1 ¼Z Te

0

jf ðtÞjdt; jjf jj1 ¼ maxt2½0;Te�

jf ðtÞj; e1;1 ¼jjFm � Fbjj1;1jjFmjj1;1

: ð5Þ

The relative error e1 quantifies the ratio of the bounded area between the output curves to thearea of the measured force along the excitation duration Te, while e1 measures the relativedeviation of the peak force.

Figure 5(a) shows the input displacement of El-Centro earthquake record to the ANSYS andBouc–Wen models. As shown in Figure 5(b), the hysteretic Bouc–Wen model can be seen as avery powerful replacement of the experimental prototype for more case studies using the RNCisolator. This is asserted by the relatively small error percentages (e1 ¼ 5:70% and e1 ¼ 3:15%)and the close match of both measured and predicted output curves observed in Figure 5(b).

Figure 4. Measured vs calculated restoring force for 5, 10 and 20% shear strains.


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The hysteretic model identified in this section will be used in the remaining of the paper torepresent in a cost-effective way the restoring force supplied by the isolator.

5. IMPLEMENTATION IN BUILDINGS

An idealized single-bay 5DOFs (including the suspended base) base-isolated concrete moment-resisting frame is considered in this study as the example structure. It is modeled as a shear-typestructure mounted on the proposed isolation bearing, Figure 6, with one lateral degree-of-freedom (DOF) at each floor. All the vibrational modes are included in the analysis. Thefollowing assumptions are made for the structural system under consideration:

1. The superstructure remains within the elastic limit during the earthquake excitation.2. The floors are assumed rigid in its own plane and the mass is lumped at each floor level.3. The columns are inextensible and weightless providing the lateral stiffness.4. The system is subjected to single horizontal component of the earthquake ground motion.5. The effects of soil–structure interaction are not taken into consideration.

The cross-sectional dimensions of the frame columns and beams are 0.70� 0.25m and0.70� 0.25m, respectively. All the stories are 3.0m height and the frame span is 5.0m. Theframe material is normal-weight concrete with a total material volume per frame of 8.60m3.This concrete material has the following isotropic properties:

� Weight per unit volume5 23563.12N/m3

� Mass per unit volume5 2402.77Kg/m3

� Modulus of elasticity5 2.486E110N/m2

� Poisson’s ratio5 0.20� Shear modulus5 1.036E110N/m2

� Specified concrete compressive strength5 27579032N/m2.

The superstructure is idealized as a linear flexible building. The modal frequencies, periodsand the modal mass participation factor of the designed structure are listed in Table I, where the

(a)

(b)

Figure 5. (a) Input displacement into ANSYS and Bouc–Wen models and (b) ANSYS output vs Bouc–Wenmodel output, relative error e15 5.70% and e1 ¼ 3:15%, respectively. ANSYS output (- - -), Bouc–Wen

model output (—–).


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damping ratio for all modes is kept fixed to 2% of the critical damping. The whole weight of theexample frame structure (including the isolation bearings) is 1180 kN.

In this paper, the maximum rolling displacements of the RNC isolators during earthquakeswere determined without using the built-in buffer (stoppers) mechanism, to check whether theyare affordable or not. Then, the buffer is designed to allow for rolling displacements a little bithigher. During stronger earthquakes, the rolling displacement limits may be exceeded but thebuffer prevents such excessive displacements with minimal shock. This last point has not beenstudied in this paper, because there is no available model, at the moment, for the built-in buffermechanism of the RNC isolator. Such study is postponed to the near future experimental work.

5.1. Equations of motion

The equations of motion of an N-story linear shear-type superstructure subjected to earthquakeexcitation is written in the matrix form

Ms €xs1Cs €xs1Ksxs ¼ �Msf1gð€xb1€xgÞ; ð6Þ

(a) (b)

Figure 6. Case study structure.

Table I. Modal properties of fixed-base and isolated example structures.

Mode number

Isolator 1 2 3 4

Fixed-base structureFrequency (Hz) 4.674 13.335 20.066 24.072Period (sec) 0.214 0.075 0.050 0.042Effective modal mass (%) 0.898 0.081 0.018 0.003Base-isolated structureFrequency (Hz) 0.341 7.815 15.023 20.744 24.217Period (sec) 2.932 0.128 0.067 0.048 0.041Effective modal mass (%) 0.999 0.001 0.000 0.000 0.000


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where Ms, Ks and Cs are the N�N mass, stiffness and damping matrices of the superstructure,respectively; xs ¼ fx1; x2; . . . ; xNg

T is the relative displacement vector of the superstructure; _xs

and €xs are the relative velocity and acceleration vectors, respectively; xjðj ¼ 1; 2; . . . ;N Þ is thelateral displacement of the jth floor relative to the base mass; f1g ¼ f1; 1; 1; . . . ; 1gT isthe influence coefficient vector; €xb is the relative acceleration of the base mass and €xg isthe earthquake ground acceleration.

The governing equation of motion for the base mass is given by

mb €xb 1ZFb � c1 _x1�k1x1 ¼ �mb €xg; ð7Þ

where mb is the mass of the base raft; c1 and k1 are the damping and stiffness of the first story,respectively; Z is the total number of isolators and Fb is the restoring force transmitted to thebase mass by a single RNC isolator. This force is expressed using the normalized Bouc–Wenmodel defined in (1) and (2).

5.2. The RNC isolator design sets

To draw relatively general conclusions about the performance of the RNC isolator under avariety of structural and ground motion properties, three different designs of the RNC isolatorshown in Figure 1, ranging from stiff (sets I) to very flexible (sets III) designs, are evaluated inthis study. Table II gives the basic mechanical characteristics of such designs. Thesecharacteristics have been obtained following the scheme described in Section 3.

5.3. Performance measures

The response quantities of interest are the top floor absolute acceleration, building drift, baseshear and the relative-to-ground base displacement. These response quantities are of importancebecause floor accelerations developed in the superstructure are a measure of human comfort andare the main source of damaging housed sensitive equipments. The building drift is the maincause of structural and nonstructural damage. The base shear and base moment govern thecross-sectional dimensions of the lateral force supporting systems in structures, while thebearing (base) displacements are crucial in the design of isolation systems.

5.4. Simulation tool

All the structural response quantities have been obtained by means of the Structural AnalysisProgram SAP2000 advanced [56]. The dynamic response is calculated using the fast nonlinearanalysis (FNA) [57], which is more suitable for structures having limited number of points ormembers in which nonlinear behavior takes place when subjected to static or dynamic loading.This FNA makes the nonlinear analysis almost as fast as a linear analysis keeping the accuracy ofaccurate direct integration methods. The proposed RNC isolator is modeled as a nonlinearsupport, whose dynamic behavior is governed by the hysteretic Bouc–Wen model [51,53,54,58,59],where the rest of the structure is assumed to behave linearly. The parameters of the Bouc–Wenmodel have been identified, in Section 4.2, and incorporated into the simulation code [56]. Themass of the isolator’s components are included in the nonlinear dynamic analysis.

5.5. Numerical study

In the following numerical study, extensive investigation of the RNC isolator is performed toassess its effectiveness. In Section 6, the behavior of the RNC isolator is investigated under

Table II. Characteristics of different isolator sets used in study, kN-m units.

Isolatorset

Isolator period (Tb)(sec)

Elastic stiffness(ke)

Post-yield stiffness(kb)

Yield force(fy)

Characteristicstrength (Q)

I 2.932 7123.87 270.96 96.48 95.25II 4.416 3561.94 135.50 48.24 47.62III 5.863 1780.97 67.75 24.12 23.81


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strong earthquake ground motion, using time history analysis and considering different RNCcharacteristics. The influence of the superstructural flexibility is studied in Section 7, whereas theinfluence of different earthquake characteristics including long-period earthquakes is examinedin Section 8. Finally, the effect of the building height and mass is studied in Section 9.

6. TIME HISTORY ANALYSIS

This section investigates the dynamic response of the isolated structure over the full time historyof excitation. The strong-motion Kobe earthquake, of peak ground acceleration PGA5 0.68 g,is considered. Figure 7(a)–(c) shows the absolute acceleration time history responses at the topfloor considering the three RNC design sets I, II and III, respectively. From Figure 7, it isobvious that the RNC isolator can significantly reduce the structural absolute accelerationsunder such strong-motion earthquake for all RNC sets along the excitation duration. Theresponse reduction becomes more significant as the isolator flexibility increases (Figure 7(b), (c))as the peak acceleration responses are 0.46, 0.32 and 0.21 g for sets I, II and III, respectively,where it was 2.93 g in the case of fixed-base structure. The recorded associated basedisplacement for all cases is always below 0.38m, which is reasonable and can be accommodatedby the RNC isolator without affecting its stability nor reducing its load carrying capacity.

7. INFLUENCE OF SUPERSTRUCTURE FLEXIBILITY

The flexibility in the base-isolated structure is mainly concentrated at the isolation level. As aresult, the response of the base-isolated structure can be investigated by modeling thesuperstructure as rigid [60–62]. However, it will be interesting to compare the seismic responseof a RNC-base-isolated structure in two cases with the superstructure modeled as rigid and

(a)

(b)

(c)

Top absolute acceleration, RNC isolator set I against fixed-base

Figure 7. Acceleration time histories under Kobe earthquake using different RNC isolator sets: (a) Set I;(b) Set II; and (c) Set III.


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flexible, respectively, to study the influence of the superstructure flexibility. The isolationsystems are robust and practically useful when they are effective for different structures withwide range of properties. For dynamic response, the most important property is thefundamental time period of the structure. For most typical building structures, the fundamentaltime period varies between 0.1 and 0.50 sec. Tall buildings and flexible structures, such asbridges, have periods of up to 2 sec or longer. Base isolator derives its effectiveness by increasingthe time period of the structure. It is therefore essential that the fundamental period of thestructure be shorter than the period of the isolator. However, the most suitable structures forseismic isolation are those with short natural period, less than about 1 sec. This is the case forconcrete buildings less than 10 stories and for flexible types of structures, such as steel momentframes, less than 5 stories. In this investigation, the time period of the RNC isolator, Tb, ischosen as 4.15 sec (set II, as the average design set), and a reasonable wide range of structuraltime period from 0.10 to 1.20 sec has been considered along with a perfectly rigid structure.These choices correspond to the most likely practical values of these parameters. The responsequantities of interest are absolute acceleration of the structure and bearing displacement.

Table III shows the variations of top floor absolute acceleration and bearing displacement ofa 5DOFs RNC-base-isolated structure against the superstructure fundamental time period, Ts,under three different ground acceleration records having low (Kern of PGA5 0.179 g),moderate (El Centro of PGA5 0.348 g) and high intensity (Northridge of PGA5 0.883 g).As expected, the isolation bearing performs well as the structural period is lower than 1 sec forall the three earthquakes. However, under low-intensity Kern earthquake, as structuralflexibility continues to increase, the isolation tends to be a burden. This is not the case undermoderate- (El Centro) and high-intensity (Northridge) earthquakes where the proposed isolatoris still effective up to 1.20-sec period. Practically, if the structure has a long period, then nomuch benefit from isolation is expected, although in some cases, energy dissipation at the basemay help. This is used quite often in bridges with a long period, but not so often for buildings.

Further inspection of Table III shows a substantial increase in the top floor acceleration asthe fundamental time period of the isolated superstructure increases. This implies that thesuperstructure accelerations will be under-estimated if the superstructure flexibility is ignoredand it is modeled as a rigid body. The increase in the superstructure accelerations is found to bemore pronounced for high PGA excitation. On the other hand, the bearing displacement is notmuch influenced with the increase in the superstructure flexibility. Thus, the flexibility ofsuperstructure increases the superstructure acceleration but it does not have significant influenceon the bearing displacements. Moreover, the efficiency of the proposed RNC isolator is almostindependent of the structural time period, especially as the excitation intensity increases.

Table III. Effect of superstructure flexibility, isolation set II.

KERN earthquake El-Centro earthquake NORTHRIDGE earthquake

Isolated structureFixed-base S.



Ts

(sec)€xtop

(m/sec2)xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

0.0 1.05 0.032 1.76 1.13 0.066 3.41 1.22 0.096 8.660.1 1.05 0.032 2.48 1.18 0.066 9.74 1.22 0.096 21.530.2 1.21 0.033 7.60 1.62 0.067 11.23 2.24 0.097 39.790.3 1.68 0.035 6.94 2.03 0.067 10.21 3.61 0.099 21.260.4 1.88 0.032 6.43 3.81 0.074 10.19 5.53 0.106 17.420.5 2.41 0.031 5.06 3.48 0.059 13.28 5.40 0.107 12.560.6 2.79 0.028 5.65 3.69 0.069 10.66 4.54 0.093 13.700.7 2.67 0.022 3.89 3.45 0.066 10.37 5.25 0.097 16.910.8 2.69 0.018 3.88 4.26 0.058 8.46 5.09 0.097 17.340.9 3.10 0.017 4.55 4.09 0.045 8.61 5.73 0.083 13.161.0 3.40 0.021 3.11 4.23 0.052 6.32 6.81 0.084 11.341.1 3.70 0.019 3.00 4.45 0.060 6.05 6.24 0.067 9.991.2 3.59 0.017 3.19 4.06 0.054 5.32 5.83 0.076 7.23


33


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8. INFLUENCE OF ISOLATOR PROPERTIES AND EARTHQUAKECHARACTERISTICS

This section is devoted to investigate the robustness of the RNC isolator under differentearthquake characteristics. For relatively general results, the three design sets as well as a widerange of structural fundamental periods are considered. This investigation is carried outfollowing three steps:

� In the first step, the three design sets of the RNC and a wide range of structuralfundamental periods of 0.00–1.20 sec are considered. Three scaled intensities (PGA5 50,100 and 200%) of El Centro earthquake are used in order to investigate the influence ofearthquake amplitude on the RNC behavior. The chosen performance measures are thetop floor absolute acceleration and base displacement. The results of this step are listed inTables IV–VI.

Table IV. Effects of isolator properties and earthquake intensity, utilizing the isolation set I.

El-Centro (50% PGA) El-Centro (100% PGA) El-Centro (200% PGA)




Ts

(sec)€xtop

(m/sec2)xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

0.00 1.92 0.026 1.71 2.07 0.042 3.41 2.35 0.110 6.820.10 1.97 0.026 4.87 2.23 0.042 9.74 2.77 0.110 19.480.20 2.45 0.023 5.61 2.97 0.044 11.22 3.99 0.107 22.440.30 2.70 0.025 5.11 4.68 0.042 10.22 6.02 0.107 20.440.40 3.83 0.025 5.10 6.62 0.045 10.20 10.04 0.130 20.400.50 4.46 0.021 6.64 7.52 0.053 13.28 9.90 0.093 26.560.60 4.17 0.022 5.33 6.51 0.051 10.66 8.10 0.088 21.320.70 4.22 0.018 5.19 7.00 0.037 10.38 9.24 0.092 20.760.80 4.26 0.016 4.23 6.60 0.044 8.46 9.15 0.090 16.920.90 3.62 0.017 4.31 6.53 0.034 8.62 9.26 0.088 17.241.00 3.58 0.011 3.16 6.21 0.026 6.32 10.10 0.079 12.641.10 3.30 0.010 3.02 5.70 0.025 6.04 9.02 0.090 12.081.20 2.96 0.008 2.66 5.68 0.021 5.32 9.51 0.091 10.64

Table V. Effects of isolator properties and earthquake intensity, utilizing the isolation set II.





Ts

(sec)€xtop

(m/sec2)xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

0.00 0.98 0.035 1.71 1.13 0.066 3.41 1.45 0.141 6.820.10 0.98 0.035 4.87 1.18 0.066 9.74 1.54 0.141 19.480.20 1.22 0.033 5.61 1.62 0.067 11.22 2.08 0.141 22.440.30 1.59 0.032 5.11 2.03 0.067 10.22 3.82 0.142 20.440.40 2.17 0.030 5.10 3.81 0.074 10.20 5.67 0.157 20.400.50 2.29 0.027 6.64 3.47 0.059 13.28 5.00 0.139 26.560.60 2.60 0.029 5.33 3.69 0.069 10.66 4.66 0.140 21.320.70 2.44 0.029 5.19 3.45 0.066 10.38 5.91 0.138 20.760.80 2.42 0.024 4.23 4.26 0.058 8.46 6.53 0.149 16.920.90 2.53 0.019 4.31 4.08 0.045 8.62 5.80 0.149 17.241.00 2.48 0.019 3.16 4.23 0.052 6.32 7.19 0.157 12.641.10 2.44 0.019 3.02 4.45 0.060 6.04 6.89 0.140 12.081.20 2.29 0.014 2.66 4.06 0.054 5.32 6.14 0.117 10.64


34


DOI: 10.1002/stc


� In the second step, the influence of the long-period earthquakes on the RNC-isolatedstructure behavior is investigated. The 1985 long-period Mexico City earthquake is usedalong with the same range fundamental periods range of the first step. Only the RNC setII is considered for a single case study structure having a fundamental period of 0.215 sec.The performance measures are chosen as the base displacement, base shear, top absoluteacceleration and whole building drift, which is expressed as the difference between the topfloor and the base mass displacements. The results are shown in Figure 8.

� In the third step, the robustness of the RNC isolator under a wide range of excitations ischecked. Only the RNC set II is considered for a single case study structure having afundamental period of 0.215 sec. A set of 36 earthquakes is used to excite the structure. Theperformance measures are the same as in the second step. The results are listed in Table VII.

From Tables IV–VI, the following conclusions can be drawn from the first step:

1. The RNC isolator efficiency in reducing structural accelerations is higher under higherintensity excitations, and it is more significant at the time period that corresponds to themaximum fixed-base response.

2. Higher intensity excitation widens the period range on which the RNC isolator canmitigate the structural response efficiently.

3. As the excitation intensity is doubled, the fixed-base response is exactly doubled but thefixed-base response is not.

4. With the decrease in the isolator elastic stiffness, ke (moving from set I to set III), the topfloor absolute acceleration decreases substantially. On the other hand, the bearingdisplacement shows marginal increasing trend with the decrease in the isolator elasticstiffness, but decreases with the increase in structural flexibility under the same excitation.This implies that the elastic stiffness of the isolator has significant effects on the responseof the base-isolated structure.

5. With the increase in the isolator characteristic strength Q (the intersection of hysteresis loopwith vertical y axis), the top floor acceleration increases and the bearing displacementdecreases. This is expected, because for higher isolator characteristic strengths, the isolationsystem remains much longer time in the elastic state, which produces less flexibility in thestructural system and thereby less energy dissipation. As a result, the superstructureacceleration increases and the bearing displacement decreases with the increase of theisolator characteristic strength. Thus, it can be concluded that the response of the base-isolated structure is significantly influenced by the characteristic strength of the isolator.

Table VI. Effects of isolator properties and earthquake intensity, utilizing the isolation set III.





Ts

(sec)€xtop

(m/sec2)xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

0.00 0.55 0.047 1.71 0.71 0.091 3.41 1.04 0.153 6.820.10 0.55 0.047 4.87 0.71 0.091 9.74 1.18 0.153 19.480.20 0.60 0.047 5.61 0.80 0.092 11.22 1.66 0.153 22.440.30 0.65 0.047 5.11 1.03 0.092 10.22 1.79 0.153 20.440.40 1.02 0.046 5.10 2.04 0.093 10.20 3.08 0.153 20.400.50 1.12 0.044 6.64 1.76 0.094 13.28 2.27 0.152 26.560.60 1.23 0.045 5.33 1.19 0.092 10.66 2.82 0.154 21.320.70 1.29 0.045 5.19 2.50 0.089 10.38 3.11 0.153 20.760.80 1.46 0.046 4.23 2.28 0.097 8.46 3.53 0.150 16.920.90 1.67 0.036 4.31 2.42 0.092 8.62 4.00 0.153 17.241.00 1.65 0.028 3.16 2.90 0.087 6.32 4.27 0.151 12.641.10 1.84 0.027 3.02 2.82 0.075 6.04 3.40 0.147 12.081.20 1.80 0.027 2.66 2.50 0.080 5.32 4.00 0.148 10.64


35


DOI: 10.1002/stc


Figure 8 displays the four response quantities of interest under the long-period Mexico Cityearthquake. From this figure, the following observations can be made:

1. The bearing displacements are relatively small and can be simply accommodated by theRNC isolator.

2. The RNC isolator reduces the building drift for all structural periods, specially at longstructural periods.

3. The RNC is reasonably effective in reducing the acceleration response at short periods(up to 0.70 sec).

4. Considering the base shear, the RNC exhibits a relatively robust behavior for all theconsidered structural periods.

Therefore, the RNC isolator is a reasonably useful isolation device for a reasonable range offundamental structural periods under long-period earthquakes.

Table VII presents the results of the extensive study under a wide set of distinct earthquakeground motions (third step) using the RNC isolator set II. The same response quantities of interestused in the second step are used as performance measures. The main observation from this table isthe great ability of the proposed RNC isolator to reduce all the four response quantities underconsideration, regardless of the amplitude and frequency content of the ground motion excitation.

In summary, the analysis performed in this section shows that the RNC isolator may be seenas a robust isolation device under variations in earthquake characteristics including long-period

(a) (b)

(c) (d)

Figure 8. Response spectra of RNC-isolated structure under long-period Mexico City earthquake: (a) Basedisplacement; (b) Building drift; (c) Base shear; and (d) Top floor absolute acceleration.


36


DOI: 10.1002/stc


earthquakes, as its efficiency is almost independent of both the amplitude and frequency contentof excitation thereby preserving the most significant advantage of ideal isolation system.

9. INFLUENCE OF BUILDING HEIGHT AND MASS

In this section, the effect of variable building height and its overall mass is studied. Threereduced-height versions of the building shown in Figure 6 are used. They are of three, two andone stories, as shown in Figure 9. Three RNC isolators are designed to support such low-risebuildings, one RNC isolator for each building shown in Figure 9, and their mechanicalcharacteristics are given in Table VIII. A wide range of fundamental vibration periods Ts is

Table VII. Characteristics of RNC isolators used in Section 9.

Base shear (10 kN)Abs. top

acceleration (g) Building drift (m)

No. EarthquakePGA(g) xb (m) Isolated

Fixed-base % Isolated

Fixed-base % Isolated

Fixed-base %

1 ALTADENA 01 0.45 0.0455 13.88 91.33 85 1.93 13.48 86 0.0018 0.0155 882 ALTADENA 901 0.18 0.0146 8.15 24.66 67 0.79 3.78 79 0.0011 0.0044 753 ARRAY06 01 0.38 0.1712 15.22 81.85 81 1.60 11.67 86 0.0019 0.0146 874 ARRAY06 901 0.44 0.3460 20.19 72.11 72 1.99 10.33 81 0.0025 0.0119 795 CORRALIT 01 0.63 0.1011 19.53 125.98 85 2.51 17.25 85 0.0023 0.0209 896 CORRALIT 901 0.48 0.1108 15.42 96.21 84 2.05 15.17 86 0.0019 0.0179 897 HOLLISTE 01 0.37 0.2099 17.40 59.05 71 2.13 6.60 68 0.0023 0.0085 738 HOLLISTE 901 0.18 0.0570 12.28 32.85 63 1.36 4.43 69 0.0016 0.0053 709 LACC NOR 01 0.22 0.0489 12.68 37.17 66 1.25 5.72 78 0.0015 0.0067 7810 LACC NOR 901 0.26 0.0376 12.26 90.57 86 1.18 14.31 92 0.0014 0.0173 9211 LEXINGT 01 0.44 0.1665 18.69 70.46 73 2.01 9.30 78 0.0019 0.0115 8412 LEXINGT 901 0.41 0.1869 19.70 67.75 71 2.03 9.55 79 0.0021 0.0116 8213 LUCERNE 01 0.68 0.0641 13.46 61.90 78 1.34 20.65 94 0.0016 0.0101 8414 LUCERNE 901 0.70 0.0417 15.65 106.81 85 1.65 18.67 91 0.0017 0.0194 9115 NEW HALL 01 0.59 0.2466 21.76 175.66 88 2.90 25.02 88 0.0025 0.0302 9216 NEW HALL 901 0.58 0.1000 19.30 131.95 85 2.06 20.70 90 0.0020 0.0246 9217 OAK WHAF 01 0.29 0.0944 14.19 49.66 71 1.64 7.06 77 0.0018 0.0085 7918 OAK WHAF 901 0.27 0.1401 15.07 44.28 66 1.80 5.76 69 0.0019 0.0070 7319 PETROLIA 01 0.59 0.1160 17.16 107.12 84 2.65 16.47 84 0.0022 0.0196 8920 PETROLIA 901 0.66 0.3015 23.92 82.71 71 2.95 12.38 76 0.0029 0.0131 7821 POMONA 01 0.19 0.0201 10.32 66.08 84 1.00 9.58 90 0.0013 0.0118 8922 POMONA 901 0.21 0.0182 9.36 30.91 70 1.04 5.69 82 0.0013 0.0062 7923 SANTA

MONICA 010.37 0.0449 12.32 85.63 86 1.65 13.58 88 0.0017 0.0160 89

24 SANTAMONICA 901

0.88 0.0980 14.38 266.58 95 2.16 43.02 95 0.0021 0.0505 96

25 SYLMAR 01 0.84 0.3001 27.00 124.98 78 2.55 15.73 84 0.0024 0.0186 8726 SYLMAR 901 0.60 0.1934 17.96 93.82 81 2.36 10.19 77 0.0023 0.0134 8327 YERMO 01 0.15 0.0499 11.46 45.44 75 1.10 6.48 83 0.0014 0.0078 8228 YERMO 901 0.24 0.1079 13.58 45.82 70 1.43 5.76 75 0.0017 0.0071 7629 EL CENTRO 0.35 0.0678 13.54 61.83 78 1.77 9.85 82 0.0017 0.0114 8530 KERN 0.18 0.0334 11.82 51.57 77 1.23 7.88 84 0.0014 0.0094 8531 KOBE 0.68 0.3810 27.92 188.37 85 3.14 28.73 89 0.0030 0.0337 9132 LOMA PRIETA 0.28 0.0874 14.82 58.75 75 1.83 8.01 77 0.0018 0.0098 8233 MEXICO 0.10 0.0697 10.95 13.98 22 1.05 1.41 26 0.0014 0.0019 2634 NORTHRIDGE 0.88 0.0980 14.38 266.58 95 2.16 43.02 95 0.0021 0.0505 9635 PARKFIELD 0.24 0.0210 9.91 44.15 78 1.00 7.26 86 0.0013 0.0082 8436 SAN FERNANDO 1.17 0.3239 24.44 291.55 92 3.28 47.62 93 0.0030 0.0556 95

Averagereduction (%)

77 Averagereduction (%)

82 Averagereduction (%)

83


37


DOI: 10.1002/stc


considered, ranging from perfectly (Ts 5 0.0 sec) rigid to very flexible case (Ts 5 1.2 sec).The same three seismic excitations used in Section 7 having low (Kern earthquake), moderate(El-Centro earthquake) and high (Northridge earthquake) peak ground accelerations are alsoconsidered in this section. The absolute building accelerations at their topmost floors, in fixed-base and isolated cases, along with the isolated base displacements are recorded in Tables IX–XIfor three-story, two-story and one-story buildings, respectively.

From these tables, the following observations can be deduced:

1. The RNC isolator achieved significant acceleration response reductions with veryaffordable base displacements for almost all the considered cases.

2. The response reductions, using the RNC isolator, are high at short periods andreasonable at longer periods.

3. For the same building, the response reductions are higher under stronger seismic excitations.4. For vibrations periods p0.60 sec (that are the best candidates for seismic isolation), the

RNC isolator proved to be a very useful tool for a seismic design of low-rise buildings.5. No response amplification is recorded below the period of 1.0 sec at any case of study.6. The very few cases at which the RNC isolator is inefficient have periods of X1.0 sec,

which represents a very flexible low-rise building. Such cases are rare in practice, and iffound, seismic isolation concept will not apply to them.

This surely proves the ability of the RNC isolator to mitigate structural responses under apractical wide range of vibration periods, no matter the height or the overall mass of thebuilding nor the excitation amplitude.

Table VIII. Characteristics of RNC isolators used in Section 9.

Building heightElastic stiffness

(ke)Post-yield stiffness

(kb)Yield FORCE

(fy)Characteristic strength

(Q)

Three-story building 267.146 10.163 3.618 3.572Two-story building 178.097 6.775 2.412 2.381One-story building 89.049 3.388 1.206 1.191

(a) (b) (c)

Figure 9. (a) Three-story-4DOFs building; (b) two-story-3DOFs building; and (c) one-story-2DOF building.


38


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10. CONCLUSIONS

An innovative seismic isolation bearing, referred to as RNC isolator, has been designed toprotect light- to moderate-weight structures subjected to seismic excitations. Functionally, it is apurely passive isolation bearing that is based on universally accepted principles in practice, justapplied in an innovative manner in a single unit. Rolling is the basic mechanical principle, whichis adopted to offer a great range of horizontal flexibility. It is complemented with an energydissipation mechanism. The device is designed to have several practical controls to ensurebuffer, re-centering and no uplift during motion. By using a numerical scheme that simulates amachine-like environment, essential physical characteristics of the RNC isolator have beenobtained and a hysteretic force–displacement model has been identified. The seismicperformance has been investigated using a wide range of low- to moderate-mass low-rise

Table IX. Influence of building height and mass, three-story building shown in Figure 9(a).





Ts

(sec)€xtop

(m/sec2)xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

0.0 0.99 0.036 1.76 1.08 0.069 3.41 1.16 0.100 8.660.1 1.00 0.036 3.13 1.16 0.069 8.75 1.17 0.101 20.500.2 1.15 0.037 7.85 1.61 0.070 11.56 1.95 0.102 39.540.3 1.42 0.037 6.52 2.02 0.070 9.93 3.05 0.103 21.910.4 1.79 0.034 5.34 3.63 0.075 8.95 5.10 0.104 17.910.5 1.99 0.031 4.95 3.15 0.066 13.35 4.26 0.102 13.080.6 2.44 0.027 5.33 2.97 0.074 10.98 4.43 0.099 15.230.7 2.49 0.022 3.43 3.54 0.065 10.33 4.48 0.119 12.440.8 2.48 0.020 3.54 3.66 0.059 8.51 3.62 0.090 12.780.9 2.99 0.020 4.21 3.66 0.046 7.60 4.56 0.085 10.881.0 3.31 0.021 2.95 3.83 0.056 6.67 4.35 0.082 8.021.1 2.96 0.018 2.40 3.96 0.055 5.73 4.29 0.069 6.611.2 2.39 0.021 3.32 4.10 0.046 5.20 3.79 0.049 3.97

Avg. 2.11 0.03 4.21 2.95 0.06 8.54 3.56 0.09 14.73

Table X. Influence of building height and mass, two-story building shown in Figure 9(b).





Ts

(sec)€xtop

(m/sec2)xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

0.0 0.89 0.038 1.76 0.99 0.075 3.41 1.06 0.099 8.660.1 0.89 0.038 2.38 1.04 0.075 9.47 1.07 0.099 20.780.2 1.09 0.037 6.57 1.64 0.076 10.92 1.52 0.098 37.510.3 1.42 0.036 5.99 2.09 0.078 9.11 2.57 0.097 19.460.4 1.74 0.034 5.15 2.78 0.075 8.94 4.00 0.095 18.750.5 2.26 0.030 4.57 2.59 0.077 11.84 3.69 0.090 12.010.6 2.06 0.026 3.76 2.74 0.074 10.92 3.16 0.085 9.200.7 2.46 0.024 3.49 3.07 0.068 7.96 3.03 0.087 8.380.8 2.70 0.022 3.62 3.16 0.067 6.95 2.56 0.082 7.790.9 2.14 0.028 2.91 3.45 0.068 6.80 2.91 0.095 6.221.0 2.34 0.021 2.02 3.52 0.063 5.18 2.94 0.071 3.551.1 2.51 0.017 2.31 3.37 0.064 4.68 3.07 0.086 2.801.2 2.24 0.031 1.94 3.28 0.055 3.88 2.92 0.107 2.67

Avg. 1.90 0.03 3.57 2.59 0.07 7.70 2.65 0.09 12.14


39


DOI: 10.1002/stc


buildings under many actual ground motion records. Moreover, different isolator characteristicshave been considered. The numerical investigations have confirmed the effectiveness of thedevice by attaining significant reduction of the building accelerations, drifts and base shears,while keeping reasonable base displacement. Even when flexible structures are isolated and longtime-period excitations are considered, the RNC isolator has shown to be highly effective. Allthe design and evaluation procedures have been performed numerically. Further research isbeing developed to build prototypes and perform experimental validations. Some practicalfeatures, like the buffers that limit maximum isolator displacements and the vertical upliftprevention mechanism, have not been specifically analyzed. Although they are inherentlypresent in the device by construction, it is hard to include them in a cost-effective model andhave been left out of the scope of this study. Experimental results will allow to investigate thoseand other practical issues.

ACKNOWLEDGEMENTS

This work has been funded by a project sponsored by the Generalitat de Catalunya, through the ACC10Program for technological development of scientific research, with reference VALTEC09-2-0022.

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Table XI. Influence of building height and mass, one-story building shown in Figure 9(c).





Ts

(sec)€xtop

(m/sec2)xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

€xtop(m/sec2)

xbase(m)

€xtop(m/sec2)

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Avg. 1.19 0.03 2.74 1.80 0.09 6.06 1.61 0.10 9.25


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ASCE/Structures Congress, Portland, OR, April 1997; 1191–1195.16. Kasalanti A, Sanders D, Reinhorn AM, Constantinou M. Experimental study of ball-in-cone isolation system.

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Seismic protection of low- to moderate-mass buildings using RNC isolator