EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, VOL. 19,377 388 (1990)

FLOOR RESPONSE SPECTRA FOR BASE-ISOLATED MULTI-STOREY STRUCTURES

FA-GUNG FAN AND GOODARZ AHMADI Department of Mechunicai and Industrial Engineering. Clurkson University. Potsdam, N Y 13676. U.S .A

SUMMARY A study of floor response spectra for a base-isolated multi-storey structure under sinusoidal and seismic ground excitations is carried put. Several base isolation systems including the laminated rubber bearing, the pure-friction, the resilient-friction, the Electricite de France and the sliding resilient-friction systems are considered. A sinusoidal ground acceleration and several earthquake accelerograms (including those of El Centro 1940, Pacoima Dam 1971 and Mexico City 1985) are used to evaluate the floor response spectra. The characteristics of the spectra generated by different base isolation systems are studied, and the results are compared with those for the fixed-base structure. It is shown that the structural contents can be protected against earthquakes by the use of properly designed base isolation systems. In particular, the laminated rubber bearing system appears to be remarkably effective in protecting the secondary systems under a variety of conditions.

INTRODUCTION

The goal of aseismic design is to eliminate or minimize damage to the structures and their contents during earthquakes. Aseismic base isolation of structures has attracted considerable attention in recent years owing to its significant capability in protecting buildings from earthquake attack. One main concept in base isolation is to reduce the fundamental frequency of structural vibration to a value lower than the energy containing frequencies of earthquake ground motions. The other purpose of an isolation system is to provide means of energy dissipation and, thereby, reducing the transmitted accelerations to the superstructure. Accordingly, by using base isolation devices in the foundation, the structure is essentially decoupled from ground motion during earthquakes.

A number of base isolation systems for low to medium rise structures have been proposed, and numerous studies have been carried out in order to understand the performances of these different devices under various conditions. Extensive reviews on the earlier works and recent developments on the subject were provided by Kelly.' Numerical simulations and shaking table experiments show that the peak transmitted accelerations and the deflections generated in the structure are dramatically reduced by using properly designed base isolation systems. (See for example References 2-7.) However, the consequences of using various base isolation devices on responses of secondary systems are not well understood. Recent work on responses of secondary systems for fixed-base structures was reported in References 8-10, and an extensive state-of-the-art review on the subject was provided by Chen and Soong." Studies on the responses of secondary systems for base-isolated structures are rather scarce. Recently, a series of shaking table studies on the influence of base isolation on light secondary equipment was reported in Reference 12. Buckle et ~ 1 . ' ~ studied floor response spectra for a fast breeder reactor. Kelly and Tsai14 described the responses of light secondary systems in a five-storey building with a laminated rubber bearing base isolation system. Studies of Fan et aL6S7 showed that the acceleration Fourier spectra at various floors of a structure with different base isolation systems vary significantly. In particular, frictional base isolation devices could generate high frequency components in the acceleration transmitted to the superstructure which could be quite damaging '0 certain secondary systems.

To understand the performance of various base isolation systems in protecting the structural contents during an earthquake, extensive studies are needed. A step in this direction concerning the evaluation of floor

0098 -8 847/90/0303 77- 1 2$06.00 0 1990 by John Wiley & Sons, Ltd.

Received 17 March 1989 Revised 10 October 1989

378 F.-G. FAN AND G. AHMADI

response spectra is undertaken in this work. A three-storey building is used as the structural model. Several base isolation systems including the laminated rubber bearing, the pure-friction, the resilient-friction with or without upper sliding plate and the Electricite de France systems are considered. A sinusoidal ground acceleration and several seismic accelerograms including the NOOW component of the El Centro 1940 earthquake, the S16E component of the Pacoima Dam 1971 earthquake, and the N90W component of the Mexico City 1985 earthquake are used in this study. A light secondary system, modelled by a single-degree- of-freedom (SDOF) mass-spring, is assumed to be attached to various floors of the structure. The interaction between the secondary system and the primary structure is neglected in this study. Peak acceleration responses of the secondary system for a range of frequencies under different conditions are evaluated and the results are presented in the form of acceleration floor spectra.

BASE ISOLATION SYSTEMS AND THEIR GOVERNING EQUATIONS

This section provides brief descriptions of the base isolation systems considered in this study. The equations governing the motion of various base isolation systems and the primary structure are also presented and discussed.

The laminated rubber bearing (LRB) is the most common base isolation system which has been studied extensively by Kelly and co-workers.'*' This system has been used in a number of structures all around the world including the Foothill Communities Law and Justice Building in San Bernardino County, California. A laminated rubber bearing consists of alternating layers of rubber and steel with the rubber being vulcanized to the steel plates. The bearing is rather flexible in the horizontal direction but quite stiff in the vertical direction. With its horizontal flexibility, the LRB provides protection against earthquakes by shifting the fundamental frequency of vibration to a much lower value and away from the energy containing range of the earthquake ground motion. Most LRB systems are designed to have a natural period in the range 1.5 to 2.5 sec. In this study, a natural period of 2 sec is selected. The damping coefficient of rubber varies significantly with the type of rubber and the strain level. Here a representative value of 0.08 is used. Figure l(a) shows a schematic diagram for a laminated rubber bearing system.

Base isolators in which the only isolation mechanism is sliding friction are classified as pure-friction (P-F) base isolation systems. In this class of idealized base isolators, the horizontal friction force offers resistance to motion and dissipates energy.15 A schematic diagram for the behaviour of the P-F base isolator is shown in Figure l(b). The use of a layer of sand in the foundation of a building as a pure-friction base isolation system was described in Reference 16. It is well known that the friction coefficient not only varies significantly with the type of friction surface, but is also velocity dependent and could be affected by ageing. For the P-F base isolator, a coefficient of friction less or equal to 0.2 is usually recommended. Here a typical value of 0 1 is selected.

A type of base isolation device which is referred to as the resilient-friction base isolation system (R-FBI) was developed in References 17 and 18. It is composed of several layers of Teflon coated friction plates with a central core of rubber or steel-reinforced laminated rubber. The rubber core provides the resilient force for the system while energy is dissipated by the friction forces. The mechanical behaviour of the R-FBI system is illustrated in the schematic diagram shown in Figure l(c). Values of natural period around 4 sec and friction coefficient about 0.03 to 0.05 for the R-FBI system were recommended in References 17 and 18.

A system which was designed mainly for base isolation of nuclear power plants in regions of high seismicity was developed under the auspices of Electricitk de France (EDF).3 An EDF base isolator unit consists of a laminated (steel-reinforced) Neoprene pad topped by a lead-bronze plate which is in frictional contact with a steel plate anchored to the base raft of the structure. The behaviour of the EDF base isolator is shown schematically in Figure l(d). For the systems constructed so far, a natural period of about 1 sec and a friction coefficient of about 0.2 were used.

By adding frictional elements to the Neoprene pad of the EDF base isolator or a sliding plate to the top of the R-FBI system, a new system with double sliding elements was proposed in Reference 19 which is referred to as the sliding resilient-friction (SR-F) base isolator [Figure l(e)]. Whenever there is no sliding in the upper friction plate, the SR-F base isolator behaves as a R-FBI unit. For high intensity earthquake ground

FLOOR RESPONSE SPECTRA 379

- XY - 5 9

(a) LRB System (b) P-F System

w m j<y--D OLE C,

@ZL- I [ 1

I’ I - =9 - I Y

( c ) R-FBI System ( d ) EDF System

GI

- l 8 ( e ) S R ~ F Syst rm

Figure 1. Schematic diagrams of isolation systems

accelerations, sliding in the upper friction plate occurs which provides an additional mechanism for energy dissipation and increases the effectiveness of the base isolation system. In this study, natural periods of 2 sec for the SR-F system are considered.

The equations governing the motion of a multi-storey structure with the LRB, the P-F or the R-FBI base isolation systems are given as

[m] {.x} + [c] { a } + [ k ] {x} = - ( i+Xg)[ rn] { l }

s + h(s, S ) + 1 E l x i = - Xg

( 1 )

( 2 ) n

i = 1

with the function h(s , S) being defined as

for the LRB system for the P-F system

2c00nS +pg 3 (S) + w ; s for the R-FBI system ( 3 )

In these equations, s is the relative displacement between the base of structure and the ground, x i is the displacement of the ith floor relative to the base as shown in Figure 2, x, is the horizontal ground acceleration, p is the friction coefficient, g is the acceleration of gravity, [m], [c], [ k ] represent mass, damping and stiffness matrices of the structure, respectively, and the column vector { l ) is a vector all of whose elements are unity. The natural circular frequency of the aseismic bearing wo, and its effective damping ratio i0 are defined as

380 F . G . FAN AND G. AHMADI

3rd FLOOR m3

1st FLOOR

BASE RAFT

ISOLATION SYSTEM

/,’/// /

’?///////////////3< FOUNDATION

Figure 2. Schematic diagrams of primary and secondary systems

where c, and k , are the damping and the horizontal stiffness of the bearing, respectively. The mass-ratio parameter ai is defined as

mb mi n a,=-

I M ’ %==, M = m b + C mi i = 1

where mi is the mass of the ith floor, mb is the mass of the base and M is the total mass of the structure. In equation (3), @i (S) is a function which is equal to + 1 when S is positive, and - 1 when S is negative. The value of sB(0) is undefined but is bounded between - 1 and + 1. For friction-type base isolators, the base of the structure may stick to the foundation for certain time durations. During the stick conditions, 5 = S = 0 and the motion of the structure is given by equation (1).

The equations of motion of the SR-F base isolator during the sliding condition are given by

2i0003 + 4 Y + Plg 6@ (3) = Pg a (S - 3) (6)

where y is the deflection of the rubber, s is the total relative displacement between the base raft of the structure and the ground. The deflection of the structure for both sliding and non-sliding conditions is governed by equation (1). For pl =0, equations (6) and (7) reduce to the equations governing the motion of the EDF system. Further details of the equations of motion for various base isolation systems and the criteria for motion transitions of friction-type systems were discussed at length in Reference 6.

FLOOR RESPONSE SPECTRA 38 1

STRUCTURAL MODEL

In this study, a three-storey building as shown in Figure 2 is used as the structural model. It is assumed that the masses of different floors and the base raft are identical and the stiffness and damping of columns of the various storeys are also the same. The explicit expressions for mass, damping and stiffness matrices and the corresponding modal equations, the modal matrix and the relations between natural frequencies of the structure were described in References 6 and 7. Accordingly,

w2 = 2.802~, , w3 = 40490, (8)

t2 = 2.8021,, c3 = 4.0495, (9)

and

in which o is the natural circular frequency and 5 is the modal damping ratio and subscripts 1, 2, and 3 identify the mode of vibration. In the subsequent analyses, a fixed fundamental natural frequency of 3.33 Hz and a damping ratio of 0.02 for the fundamental mode are used to represent a typical three-storey shear frame structure.

Typical values of natural periods To, damping ratios C0, and friction coefficients p for various base isolators as listed in Table I are used in this work. Except for the SR-F system, the same values of parameters for various base isolation systems were used in the earlier studies of References 6 and 7, where the acceleration Fourier spectra for different floors were evaluated; therefore, a direct comparison is possible.

FLOOR RESPONSE SPECTRA

For different ground motions, the absolute floor accelerations at different levels of the structure, x, + S +xi, for various base isolation systems are first evaluated. These floor accelerations are then used as the base excitations for a light single-degree-of-freedom (SDOF) secondary system as shown schematically in Figure 2. When the secondary system is located on the ith floor of the building, its equation of motion is given as

z + 2l&w,i f w,2z = -(Xg + S +Xi) (10) where z , 1, and o, are the deflection, the damping ratio and the natural frequency of the secondary system, respectively. A fourth-order Runge-Kutta scheme is used for integrating the governing equations of motion of the structure, the base isolation systems and the secondary system. A time step of 0.00025 sec is used for evaluating the floor accelerations of the structure and the responses of the secondary system.

For a light SDOF secondary system with 5,=0.02, the peak absolute acceleration, Jj;g+S+ii+ZJ,,,,

responses for a wide range of frequency,f, = w,/2n are evaluated. The resulting acceleration floor response spectra are presented and discussed in this section. The deflection and velocity floor spectra are omitted from the presentation owing to space limitation. Furthermore, since the secondary system considered is linear, the pseudo-velocity and pseudo-deflection floor spectra may be evaluated easily from the corresponding acceleration floor spectra.

Table I. Values of parameters used for various base isolators

Base isolation system

Damping ratio

Friction coefficient

P ( P J P )

Laminated rubber bearing (LRB) 2 Pure-friction (P-F) - Resilient-friction (R-FBI) 4 Electricite de France (EDF) 1 Sliding resilient-friction (SR-F) 2

0.08

0.08 008 0.08

- 0.1 005 0.2

0.04/0.1

382 F.-G. FAN AND G . AHMADI

10 -‘

Sinusoidal ground excitation Under a sinusoidal ground acceleration with an amplitude a=0.5g and a frequency off,= 1.25 Hz, i.e.

xp = a sin (27c.t) (1 1)

the acceleration floor response spectra are evaluated. The first 10 sec segment of response time histories are used for obtaining peak responses of the secondary system for a wide range of natural frequencies.

Figure 3 shows the acceleration floor spectra for a secondary system which is attached to the top floor of the structure for several base isolation systems. I t is observed that the floor spectrum for the fixed-base (F-B) structure has a very high peak at the frequency of excitation (1.25 Hz). The response spectra for various base isolation systems also have sharp peaks at this frequency; however, they have much lower amplitudes. These peaks are due to the reasonance between the secondary system and the floor acceleration. It is also observed that the spectrum for the fixed-base structure has a secondary sharp peak at the natural frequency of the structure ( x e 3 . 3 3 Hz), and remains a constant of about 0.99 forfi>8 Hz. The floor spectrum for the LRB system shows a peak atf,’- 1.25 Hz with an amplitude of about 29 and remains a constant of about 0.39 for 1, > 3 Hz. Furthermore, for 4 <L < 13 Hz, the acceleration floor spectra for the frictional base isolation systems contain a number of peaks. For a certain frequency range, the amplitudes of the spectra for these frictional systems are higher than that for the fixed-base structure. This is due to the high frequency contents in the acceleration responses generated by the frictional stick -slip transitions of these isolators.

The acceleration floor spectra for various floors for a structure with different base isolation systems are shown in Figure 4. It is observed that, for the L R B system, the shapes of the spectra curves for various floors are almost identical. This latter observation further verifies that the base isolated structure essentially vibrates in its rigid body mode. The floor response spectra for the P-F and the R-FBI systems, however, contain a number of peaks in the frequency range 4 to I3 Hz with the highest one occurring at a frequency of about 6.25 Hz. For the EDF system, it is observed that several peaks occurs for frequencies less than 10 Hz, with a dominant one atA= 1.25 Hz.

I

i ~ l l ~ i l l l ] l i l i ~ l l i i

0 5 10 15 2 3

FLOOR RESPONSE SPECTRA

- 4 - 0.

v

5 3 -

383

... 5 ( a ) LRR .. __ 3rd FL.

2nd FL - 4 m 2nd FL. 3rd FL

1st FL. Ears FL

l S l FL Bare FL

. . . . _.

..... __._. ........... v

.- 6 3 I e 8 2

8 1 a

- Y Y

8 1 1

0 0 5 10 15 20 25 0 5 10 15 20

Frequency, 1. ( H r ) Frequency.!. ( Hz )

3rd FL 2nd FL. 1st FL

. . . . Base F l

___. .

. . . . .

-.. , , , , 1 , I , - - T I , 1 I I < I 1 I I I

5 10 15 20 Frequency, 1. ( Hz )

___-- (d) EDF

~ 3rd FL 2nd FL. 1st FL

..... Bop&. FL ~ ~ ~ . .

5

3

Figure 4. Acceleration floor spectra at various floors for sinusoidal ground excitation

Figures 3 and 4 show that base isolation systems effectively reduce the amplitude of the resonance peak 01 the floor spectra. Furthermore, the structure with the LRB base isolation system, generally, leads to the lowest peak acceleration responses for the secondary systems.

The floor spectra presented here are fully consistent in their features with the corresponding Fourier spectra described in Reference 6 . In particular, similar high frequency energy contents in the accelerations transmitted by the frictional base isolators were noticed in the corresponding Fourier spectra.

El Centro eurthquuke Using the NOOW component of the El Centro 1940 earthquake record, the acceleration and deflection

floor spectra for the base-isolated structure and the fixed-base one are evaluated. The first 20 sec of different floor accelerations are used for response analyses of secondary systems.

Figure 5 shows the acceleration floor spectra for the top floor of the structure with different base isolation systems and the fixed-base one. I t is observed that the floor spectra of the fixed-base structure has a sharp peak with an amplitude more than 1Oy at a frequency of about 3.33 Hz. This is because considerable energy is channelled into the fundamental natural frequency of the structure under this broad-band earthquake ground excitation. For frequencies more than 5 Hz, the amplitudes of a spectrum vary between about 1 and 2g. I t is also noticed that the use of base isolation systems eliminates the resonance peak observed for the fixed-base structure, and reduces the peak spectral amplitude by a factor of more than ten. The exception is the P-F system, which leads to peak acceleration responses which are comparable to that for the fixed-base structure.

Figure 5 also shows that the acceleration floor spectrum for the R-FBI system has a trend similar to that of the P-F system; however, it has a much lower amplitude. The acceleration spectrum for the SR-F system is generally lower than that for the R-FBI system for the entire range o f 1 considered. Except for a sharp peak

384 F.-G. FAN AND G. AHMADI

0 5 10 15

at N 1 Hz, which corresponds to the natural frequency of the isolator, the spectral amplitude for the EDF system varies between 0.3 and lg. It is also noticed that the acceleration floor spectrum for the LRB system has a peak at J-0.5 Hz, corresponding to the natural frequency of the bearing used. Away from this peak, the spectral amplitude is about 0-1 5 to 0 . 3 ~ which is the lowest among the isolation systems considered.

The acceleration spectra for different floors and for various base-isolated structures and the fixed-base one are shown in Figure 6. For the fixed-base structure, it is observed that the base floor has a low amplitude broad-band spectrum corresponding to the acceleration response spectra of the El Centro 1940 earthquake. The spectra for the higher floors, however, contain sharp peaks at a frequency of 3.33 Hz whose amplitudes

3

3rd FL - 2nd FL 1

0 5 10 15 20 Frequency.!. ( Hz )

__ ~~ .~

- 3rd FL

1rt FL. ~~

-_.. m

. . . . . . . . . . . v 2.0

5 10 15 20 Frequency. 1. ( HZ )

3 0 (b) LRB

v 25 2.0 1 __ 3rd FL 2nd FL 1st FL Base FI

_ _ _ _ . . . . . . . . .

i c 0 ._ 1.5

U -

a" 0.5 l.O I L 0.0 I

0 5 10 15 I , , I , ' , I - 7 l r -

Frequency, 1. ( Hz )

__ 3rd FL 2nd FL 1st FL Bore FL

_ _ _ ~ . . . . . . .. . .

0 5 10 15 Frequency./. ( Hz )

3

0

Figure 6. Acceleration floor spectra at various floors for El Centro 1940 earthquake

FLOOR RESPONSE SPECTRA 385

increase with elevation. That is, the structure transforms the broad-band ground excitation into a high amplitude narrow-band floor response as the acceleration propagates to the upper floors.

Figure 6 also shows that the amplitudes of the acceleration floor spectra for the structure with a LRB system are generally very low and do not vary with elevation. The floor spectra of the various floors for the structure with the R-FBI system are broad-band and their shapes vary for different floors. Although some amplifications are observed, unlike those for the fixed-base structure, the spectra do not become narrow- band at higher floors. The floor spectra for the structure with the EDF base isolation system are sharply peaked at a frequency of 1 Hz, which corresponds to the natural frequency of the isolator. There are also secondary peaks at L-6 Hz. Away from these two peaks, the spectral amplitudes for the EDF system are rather low and remain essentially constant.

Based on the results presented in this section, it may be concluded that, for the El Centro earthquake ground excitation, the use of base isolation systems provides considerable protection for the secondary systems and the structural contents. Among the isolators considered, the LRB systems leads to the lowest peak acceleration responses in the secondary systems. Comparisons of the floor spectra of this section with those of the previous section show that the base isolation systems are far more effective in reducing the peak responses of secondary systems for a broad-band ground excitation (such as the El Centro earthquake) when compared with the sinusoidal ground acceleration.

Pacoima Dam and Mexico City Earthquakes In this section, the floor response spectra for the top floor of the structure for two extreme earthquake

records are analysed. The S16E component of the Pacoima Dam 1971 earthquake accelerogram with a peak ground acceleration 1.179 and the N90W component of the Mexico City earthquake, with a peak ground acceleration of about 0.1 79 (and considerable energy at a frequency of about 0.5 Hz) are used as the ground excitations.

Figure 7(a) shows the acceleration floor spectra for the structure with various base isolation systems and the fixed-base one for the Pacoima Dam earthquake. The first 20 sec of the response time histories of light secondary systems are used for evaluating the peak responses. It is observed that the spectrum for the fixed- base structure contains an extremely high resonance peak (about 509) atX N 3.33 Hz. Beyond this frequency, the amplitude of the spectrum varies between 4 and 79. It is also observed that the resonance peak is entirely eliminated by the use of a base isolation system. Maximum amplitudes of the floor response spectra for various base isolation systems are 10 to 25 times lower than that for the fixed-base structure. Figure 7(a) also shows that the base isolation systems are highly effective in protecting the structural contents under high intensity ground excitation such as that due to the Pacoima Dam 1971 earthquake. In this case, all base

1

10 -’ &,--,-I 10 -*I 0 5 10 15 20 0 5 10 15

Frequency, 1. ( Hz ) Frequency. 1. ( Hz )

Figure 7. Acceleration floor spectra at the top of structure for Pacoima Dam 1971 and Mexico City 1985 earthquakes

386 F.-G. FAN AND G. AHMADI

l o ~

- 0,

v

C 0 ._ c

0 u

F 1 : - a 0 x

a

l o - '

isolation systems reduce the peak responses of secondary systems over the entire range of frequencies greater than 2 Hz.

Figure 7(b) shows the acceleration floor spectra for the structure with various base isolation systems and for the fixed-base one for the Mexico City earthquake. The first 80 sec of responses are used for evaluating the acceleration floor spectra. It is observed that the spectrum of the fixed-base structure contains two sharp peaks; one is located at A-0.5 Hz with an amplitude of about 1.79, and the other is located at L2.3.3 Hz with an amplitude of about 1 . 5 ~ . Away from these two peaks, the spectral amplitudes are about 0.3g. Under this earthquake ground motion, the LRB system shows a severe resonance peak at L 2: 0.5 Hz with an amplitude of about log. Beyond this frequency, the peak acceleration remains a constant of about 0.89. The severe resonance peak observed for the LRB system is due to the peculiar frequency content of the Mexico City 1985 earthquake which coincides with the natural frequency of the LRB base isolator used. Figure 7(b) also shows that the acceleration floor spectra for a structure with the P-F, the R-FBI and the SR-F base isolation systems are sharply peaked at a frequency of about 6 Hz. The EDF system seems to perform reasonably well when compared with other base isolation systems. While it eliminates the peak at the natural frequency of the structure,f; 2: 3.33 Hz, it does not lead to amplitudes higher than those of the fixed-base ones for > 2 Hz.

Figure 7(b) shows that, for ground excitation with considerable energy at low frequencies, the base isolation systems generally do not perform very well and can not protect the secondary systems. This result further emphasizes the known fact that a base isolation system should not be used in regions which have the potential of experiencing earthquakes with low frequency energy contents.

__ ___----- .~ 10 .

(0) (b) ,-. m v

(, = 0 02

u V a Y 0

a 7,:

r 1 ! 1 l I I I I 1 t I I , I f > 1 1 l o - ' I 1 1 I I I 1 I 1 I , I I I I , , ~

Eflects of damping The results presented in the earlier sections showed that the frictional base isolation systems generate high

frequency components in the floor accelerations of structures. Thus, while these devices could provide considerable protection for the primary structure against earthquakes, there could be intolerable damage to secondary systems and non-structural components. To reduce the detrimental effects of high frequency content, the use of additional damping devices may be considered. In this section, the effects of structural damping and the damping of the secondary system on the acceleration floor spectra are examined. A structure with a R-FBI base isolation system is considered and the NOOW component of the El Centro earthquake is used as the ground excitation. For several values of damping coefficients, the floor spectra are evaluated and the results are discussed in this section.

Figure 8(a) shows the acceleration floor response spectra for the top floor for various structural dampings. It is observed that the spectral amplitudes decrease as the structural damping cl increases. The structural damping has no effect on the floor spectrum for frequencies lower than 4 Hz. This figure shows that the damping of the structure does not affect the floor spectra significantly at low frequencies.

I

FLOOR RESPONSE SPECTRA 387

Figure 8(b) shows the acceleration floor response spectra for a light secondary system attached to the third floor of the structure for several values of damping of the secondary system. Here the structural damping is fixed at =0.02. It is observed that the peak acceleration response of the secondary system is significantly reduced by increasing is forf, < 15 Hz. As [, increases from 0.02 to 02, the maximum value of the spectrum is reduced by a factor of more than three. ForA> 15 Hz, the damping of the secondary system has no significant effect on the acceleration floor spectrum.

Based on the results of this section, it may be concluded that increasing the damping coefficient of the secondary system lead to significant reduction of amplitudes of the acceleration floor spectrum for the frictional base isolation systems. Increasing the structural damping is effective in reducing the peak responses of the secondary system forA>5 Hz. The effect of the damping of the isolator is also studied; however, no significant effect on the floor response spectra is obtained. This latter observation is consistent with the results of Reference 7, where it was shown that the damping of the isolator has negligible effects on the peak floor accelerations of structures with a R-FBI system.

CONCLUSION

A study of the floor response spectra for a multi-storey structure with various base isolation systems under different ground excitations is carried out. The resulting floor spectra are compared with those for the fixed- base structure under a variety of conditions. The effects of structural and secondary system damping on floor spectra for a frictional base isolation system are also studied. Based on the results presented, the following conclusions may be drawn.

1. The peak responses of secondary systems during earthquake ground excitation are generally reduced by using a base isolation system.

2. Use of base isolation systems eliminates the resonance peaks of the floor spectra which occur at the natural frequency of the fixed-base structure for earthquake ground excitations.

3. Among the base isolation systems considered, the LRB system generally leads to the lowest peak responses for secondary systems.

4. The frictional base isolation systems generate high frequency components in the acceleration responses of the structure, which could be damaging to structural contents.

5. For a structure with a frictional base isolation system, the peak responses of secondary systems can be reduced by increasing the damping of the secondary system and the structure.

6. For regions which have the potential of generating earthquakes with low frequency energy content, the use of base isolation systems is inappropriate.

ACKNOWLEDGEMENT

This work is supported by the National Center for Earthquake Engineering Research, State University of New York at Buffalo under Grant Nos. NCEER 87-2007A and 88-2012A.

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388 F.-G. FAN AND G. AHMADI

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