APPLICATION OF DIGITAL CELLULAR RADIO FOR MOBILE LOCATION ESTIMATION

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

DIMINISHING SEISMIC EFFECT ON BUILDINGS

USING BEARING ISOLATION

A. B. M. SAIFUL ISLAM

Department of Civil & Construction Engineering,

College of Engineering, Imam Abdulrahman Bin Faisal University,

Dammam 31451, Kingdom of Saudi Arabia.

*
Corresponding author: asislam@iau.edu.sa

(Received: 17th May 2017; Accepted: 28th Feb 2018; Published on-line: 1st June 2018)

https://doi.org/10.31436/iiumej.v19i1.844

ABSTRACT: Adopting suitable seismic protection techniques is presently a foremost

concern worldwide and has become a governing principle in the growing construction

industry globally. Thus, a rapid upsurge in infrastructure development in seismic-prone

areas requires proper treatment for building structures. Therefore, the aim of the study is

to incorporate a rubber bearing isolation device in a building base in order to diminish

the seismic effect on the superstructure. The changes of structural parameters and

responses of fixed-based buildings for applying High Damping Rubber Bearing (HDRB)

are investigated under site-specific ground excitation. Twenty representative buildings

have been used to examine the responses employing four types of HDRB systems. The

study reveals that the HDRB makes the structure more flexible, offering reduced

structural responses. The introduction of HDRB may help to decrease floor moment by

31~55%, which can allow the structures to withstand comparatively high seismic tremors

safely and efficiently. The base isolated structures experience significant lateral shift

between 87.15 and 130.15 mm, and relative floor displacement is below 3% because of

additional flexibility. The effective inertia height for the BI buildings remains under two-

thirds of building elevation showing triangular distribution concentrating to the top story

level. The reduction in forces, moments, and relative displacements of the structural

members by applying HDRB can ensure economic design and higher structural safety

against seismic excitation.

ABSTRAK: Penerapan teknik yang sesuai bagi pelindungan gelombang seismos menjadi

penekanan semasa di seluruh dunia dan telah menjadi prinsip penentu dalam industri

pembinaan secara global. Oleh itu, kenaikan yang cepat dalam pembangunan infrastuktur

dalam kawasan terdedah kepada gelombang seismos memerlukan penyelenggaraan yang

betul bagi struktur bangunan. Jadi, matlamat kajian ini adalah bagi menggabungkan

getah alat pengandar asing dalam teras bangunan bagi mengurangkan kesan seismos

pada struktur asas. Perubahan pada parameter struktur dan tindak balas tapak-tetap

bangunan bagi penggunaan teknik High Damping Rubber Bearing (HDRB) dikaji di

bawah setiap kenaikan getaran gelombang sebenar bawah tanah. Dua puluh bangunan

telah digunakan bagi menilai tindak balas terhadap penggunaan empat jenis sistem

HDRB. Kajian menunjukkan HDRB membuatkan struktur lebih fleksibel, dan

mengurangkan tindak balas terhadap struktur. Penggunaan HDRB dalam pembangunan

membantu mengurangkan momen lantai sebanyak 31~55%, di mana struktur bangunan

lebih tahan terhadap gegaran tinggi secara selamat dan berkesan. Struktur asas yang

terasing mengalami peralihan sisi yang ketara antara 87.15 dan 130.15 mm, dan anjakan

lantai relatif di bawah 3% dengan penambahan sistem fleksibel ini. Ketinggian inertia

berkesan bagi bangunan BI kekal di antara dua pertiga ketinggian bangunan. Ini

menunjukkan pengagihan segitiga tertumpu pada aras atas bangunan. Pengurangan pada

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

daya, momen dan perubahan relatif struktur menggunakan HDRB dapat memastikan

reka bentuk lebih ekonomi dan selamat pada bangunan tinggi daripada sebarang

gelombang seismos.

KEYWORDS: bearing isolation; high damping rubber bearing; seismic prone building;

structural flexibility

1. INTRODUCTION

Earthquakes are sudden and overwhelming natural calamities. During earthquakes,

extreme ground motion may cause severe damage to structures. The vulnerability of

structures to seismic damage has been highlighted by numerous recent earthquakes

worldwide. They cause inertia forces on the building structures that are a function of

ground accelerations induced by the earthquake and the building mass. When the ground

movement accelerates, the strength of the structure must be augmented to prevent the

structural damage. However, continuous increase of building strength is not a practical

way to solve the issue. It is neither easy nor economic to design seismic-prone buildings

for such strength levels. Conversely, instead of increasing the strength capacity, seismic

base isolation can give a potential solution by reducing the structural responses. Though

the occurrence of a natural earthquake cannot be controlled, its influence on structures can

be diminished by separating superstructures from their foundations. The isolation system

provides additional flexibility as well as energy dissipation capability by offering such

separation [1]. A new generation device, the high damping rubber bearing (HDRB), has

brought an innovative aspect for analyzing and designing base isolated (BI) structures [2-

6]. The HDRB rheology model [7], mechanical models [8] and the stiffening importance

of HDRBs [9] are some of the investigations done to predict structural responses of BI

buildings.

Even though the use of isolators is well known, there is a lack of appropriate research

on the efficient practical implementation of high damping rubber bearing devices. The

actual in-situ earthquake simulations have rarely been done because of lengthy analysis for

dynamic response and expensive computation process. Due to advancements in computer

software and hardware, it has become relatively easier to conduct the numerical

investigation [10] precisely.

Therefore, the present study evaluates the viability of using rubber bearing isolation

devices, HDRB, at the base of the buildings and evaluates its effect on superstructure.

Static and dynamic analyses have been carried out inserting isolators for different

configurations of the structures. Design parameters of the isolators required for the

selected buildings are assessed. Moment and displacement behavior of fixed base (FB) and

BI buildings have been evaluated. Furthermore, the effective height of inertia forces has

been appraised.

2. MATERIALS AND METHODS

2.1 Idealization of Building Frames

Reinforced concrete (RC) multi-storied building frames with plan areas and

elevations, as shown in Fig. 1, are modelled. Four moment resisting frame (MRF)

buildings of 4 bays @ 7.62m at x and y directions comprising story height @ 3.05m with

gradual increase of numbers of stories to 4 (four), 6 (six), 8 (eight) and 10 (ten) have been

chosen. For each building, the total seismic weight is disseminated in equal manner over

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

all the floors. Static analyses are carried out initially to get the required loading for the

isolation design. For the conventional state, the building natural time periods are 0.50 (4

Story), 0.75 (6 Story), 0.80 (8 Story) and 1.00 (10 Story). Four types of HDRB system

have been used for all the buildings leading to a total number of 16 scenarios. Base

isolators are designed and evaluated for all the four variations of the studied building. The

isolators are attached at the base level confirming all the properties. The building

structures without and with HDRB are also analyzed for dynamic responses by using

SAP2000 [11].

(4@7.62 m bay both ways)

Fig. 1: Plan of the selected multi-storied buildings.

2.2 Design of HDRB

In this study, the HDRB isolators have been designed according to the procedure

mentioned by Kelly et al. [12]. A computer code, HDRB-NONLIN, has been produced to

design HDRB iteratively. The code requires initial input of total seismic weight,

dimension of bearing, and number and thickness of bearing layers. The high initial

stiffness, post elastic stiffness, effective damping, yield strength, as well as post yield

stiffness ratio of the isolation device are computed using this code. The bearings are

attached at the bottom of every column defining these parameters in the analysis package.

The flow chart in Fig. 2 shows the sequential process of designing HDRB. For smaller

dimensions, HDRB offers higher shear strain. Because of large vertical stiffness, HDRB

can withstand substantial structural loads [13]. The characteristics of materials and

parameters for designing the HDRB isolators are given in Table 1. The HDRBs are

demarcated by the circular shape, size of its plan, configurations of rubber layers, as well

as steel plates.

Table 1: Salient features of HDRB.

Elastomer Properties Unit Value

Modulus of elasticity KPa 1350

Shear Modulus KPa 400

Material Constant, k --- 0.87

Ultimate Elongation % 650

Designed parameter

Shape --- Circular

No. of bearings --- 25

Diameter of bearing mm 950

Thickness of rubber layer mm 10

Height of cover plate mm 40

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

Fig. 2: Flow chart of HDRB design.

Damping has been varied for different bearings. The characteristics of the bearings

are given in Table 2. The HDRB is assigned to the building columns’ base. The bearings

contain alternating layers of high damping rubber in thin layers and steel plates. The low

shear modulus of the elastomers controls the bearing’s horizontal stiffness. Furthermore,

high vertical stiffness is the contribution of steel plates which obviously preclude rubber

bulging. Lower horizontal stiffness of HDRB devices ensures the natural periods to be

higher. High non-linearity of stiffness and energy dissipation is the salient behavior of

HDRB, which is shear strain dependent.

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

Table 2: Bearing characteristics

Bearing

Device

Period of

Isolator (sec)

Damping

(%)

Initial lateral

Stiffness (KN/mm)

Post-elastic

stiffness (KN/mm)

HDRB1 1.5 15 11.085 3.034

HDRB2 2.0 16 10.977 2.452

HDRB3 2.5 17 10.932 2.001

HDRB4 3.0 19 10.921 1.557

2.3 Model of HDRB

The effective horizontal stiffness of HDRB is contemplated in terms of Kr. Equivalent

viscous damping is employed. The bearing’s force-deformation has been considered in the

modelling as equivalent to linear. The shape of force-deformation of HDRB under loading

is shown in Fig. 3. The post elastic stiffness has been derived as Eqn. (1).

r
T

AG
K


 (1)

The hysteresis loop area obtained for the respective shear modulus and equivalent

viscous damping provides the hysteresis loop area. The parameter shear modulus, Gγ is a

function of shear strain. The unloading stiffness is named elastic stiffness, Ku and is

defined by Eqn. (2).

ru
KK  (2)

Fig. 3: Deformation pattern of HDRB (not to scale).

The force intercept, Q, follows the Eqn. (3), which is estimated from effective

stiffness, damping
eff

 , maximum displacement  and yield displacement
y

 [14].

  
yeffreff

KQ  *2/
2

 (3)

2.4 Structural Analysis

Simple linear static analyses have been performed with minimum complexity. The

lateral loads for ground excitation are obtained using structural parameters such as

modification factor response, seismic zone factor, soil profile, etc. In addition to this, the

lateral shear force imposed by wind loading has been determined from the related

coefficients and code requirements for earthquake and wind analysis that follow the code

UBC 1997 [15]. The prototype buildings have been simulated and loaded in the finite

element package. The equivalent static analysis considers RI = 8.0 for the traditional FB

buildings. On the other hand, a reduced response modification factor of RI = 2.0 has been

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

considered for the BI buildings with the structural coefficient of 1.0 [15] as per the

occupancy.

Dynamic analysis has been carried out using the response spectrum analysis (RSA).

The FB structures are investigated by RSA along with the equivalent static analysis. The

structures are then linked to the isolator system and such BI structures are analyzed by

RSA to predict the dynamic behavior. The site-specific Dhaka earthquake seismic record,

generated from the recent nearby earthquake occurrence of 2009, has been chosen for

seismic analysis to assess the isolation effect especially at the site condition. The design

response spectra [16] of this selected earthquake for fixed and base isolated conditions are

shown in Fig. 4.

Fig. 4: Response spectra of the selected earthquake.

The dynamic equation for the building super structure after the bearing insertion can

be designated [2, 10] by means of in Eqn. (4).

[𝑀]{�̈� + �̈�𝑏} + [𝐶]{�̇�} + [𝐾]{𝑦} = −[𝑀][𝑇𝑔]{�̈�𝑔} (4)

where, [C] is damping matrix; [K] is stiffness matrix and [M] is mass matrix of the

building superstructure for respective DOF at the slabs. [Tg] is the coefficient matrix of

earthquake effect. In addition, {y} = [yx , yy , yz]
T is the displacement vector at the

structure’s floor levels associated with the base mass; {yb} = [ybx , yby , ybz]
T is the base

displacement vector relative to the ground and }{
g

y is the ground acceleration vector.

This analysis technique uses the design response spectra engendered from seismic

excitation of FB buildings. However, for BI buildings, the design response spectrum has

been adapted to cope with the damping offered by bearing devices by using a composite

spectrum. The B factor has been used to reduce the 5% damped composite spectrum in the

isolated modes (Fig. 4).

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.5 1 1.5 2 2.5 3 3.5 4

S
p

e
c
tr

a
l

A
c
c
e
le

ra
ti

o
n
(

g
)

Time Period (sec)

Response Spectrum for FB Building

Response Spectrum for BI Building

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

The motion equations have been transferred for the dynamic analysis into a normal

coordinate system. Modal superposition technique is employed to perform the RSA. The

values of modal superposition have been combined according to the complete quadratic

combination (CQC) approach. The square root sum of the squares (SRSS) progression

provides the directional combination of the modal values.

2.5 Numerical Study

All sixteen (16) base isolated buildings of varying height comprising the four

variations of HDRB isolation systems as well as four fixed based buildings of different

elevations have been investigated. Each isolation system is incorporated to satisfy the

stiffness and strength properties essential for the assessment. The HDRB systems have

been designed for 1.5 s, 2.0 s, 2.5 s and 3.0 s effective isolation periods.

The properties of HDRB are functions of shear strain applied with around 3 MPa shear

modulus for very little strains dropping to 0.75 MPa at 250% strain. Yield displacements,

Δy, is chosen as 0.05~0.10 times the rubber thickness. Relying on the individual isolation

periods, the damping percentage ranges in between 15%~19%.

Dynamic analyses of the three-dimensional (3D) buildings are carried out with

consideration for the associated nonlinearities. After assigning stiffness values to the

isolators, all the isolated structures have been analyzed in SAP2000 [11] and the results

are critically evaluated.

3. RESULTS AND DISCUSSION

Due to the additional flexibility of the superstructure, the structural behavior of the

base-isolated structure changes. The following sections describe the viability of bearing

incorporation as well as improvement of structural responses by applying various HDRB

systems.

3.1 Viability of HDRB Incorporation

The fundamental time periods of the selected FB buildings for isolation are ≤1.0

second, which can be considered suitable and within reasonable limits [12]. The horizontal

base displacements of the buildings, as per site requirements, are within the allowable

limit of 200 mm. Furthermore, the lateral shear force imposed by wind is clearly less than

10% of building weight [17], as required. From the static analysis of the fixed base

building, it is observed that the maximum wind-induced shear experienced by the 10-story

building is around 2.11 % of the building weight. This tendency is closely followed by all

other scenarios analyzed. Thus, isolators could be inserted at the structural base as an

alternative to the conventional fixed base strategy.

The isolators’ designed parameters need to meet two conditions: the isolation bearings

are capable of securely withstanding the imposed loads and overall performance of the

bearings has to be satisfactory. The ability of isolation bearings to carry the loads has been

checked using factors of safety (FS). As FS is more than 1.0, the ability of the HDRBs for

carrying loads safely can be considered satisfactory. The performance of BI buildings is

assessed for both the design basis earthquake (DBE) and maximum credible earthquake

(MCE) choosing soil profile S3 with the zone factor Z = 0.15. The assessments for

earthquake levels, DBE and MCE have been found to be satisfactory. Each and every

value of maximum displacements remains within the allowable isolator design static

displacement (292.61 mm) under the MCE. Therefore, the properties of the HDRB devices

maintain good agreement and can be reasonably employed.

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

3.2 Moment Behavior

Reduction of seismic damage by inserting a bearing device includes both the

structural along with the non-structural systems of buildings. Decreasing a building’s non-

structural damage requires reducing the floor moment. As a lateral force, an important

response indicator of FB and BI buildings is the moment in superstructure. The

distribution of moment force along the height of the superstructure in base-isolated

buildings indicates the enhancement of the behavior of the structure. Figure 5 shows the

improvement of moments for different buildings. It has been seen that moment might be

overestimated if HDRB is not incorporated at the structural base.

(a) 4 story building

(b) 6 story building

(d) 10 story building

Fig. 5: Moment distribution of FB and BI buildings.

Crucial reduction of moment of mostly 31~55% at the upper stories (Fig. 6) indicates

the improvement of structural responses leading to economic design of structure.

Structural safety against seismic excitation can also be obtained following such force

reduction of structural members. Especially for the bottom stories, the decrement is very

momentous, as clearly shown in Table 3. Nevertheless, for high-rise buildings, especially

for a 10 story case, greater moment values are seen for a few floors and less reduction for

some other floors due to the variation in mass participation and nonlinear dynamics of the

structural system. This is the gorgeousness of HDRB which attracts the construction

industry to keep up with this innovative technique for building construction.

0 50 100 150

Moment (KN-m)

HDRB1

No Bearing

0 50 100

Moment (KN-m)

HDRB2

No Bearing

0 50 100 150 200

Moment (KN-m)

HDRB3

No Bearing

0 100 200 300

F10

Moment (KN-m)

HDRB4

No Bearing

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

A high damping bearing device can be chosen to reduce the forces, moments, and

relative displacement of structural elements that can lead to economic design.

(a) 4 story building

(b) 6 story building

(d) 10 story building

Fig. 6: Change of moment after HDRB isolation.

Table 3: Percentage reduction of moment by using HDRB.

Floor

level

Reduction of Moment (%)

4-story 6-story 8-story 10-story

F0 79.40 82.64 87.59 92.43

F1 53.29 54.98 9.87 -6.01

F2 36.98 53.06 18.13 15.12

F3 37.64 25.82 99.10 -17.87

F4 31.05 41.94 28.51 22.21

F5 --- 31.57 3.97 -16.67

F6 --- 11.60 46.13 11.32

F7 --- --- 33.77 4.23

F8 --- --- -7.60 8.20

F9 --- --- --- 14.76

F10 --- --- --- -6.01

3.3 Displacement Behavior

For the fixed base structures, the tendency of inertia is to keep structures in place at

the time of ground excitation resulting in large displacements at different stories in

0 50 100 150

Change of Moment (KN-m)

Moment reduction(HDRB) No Bearing

0 50 100

Change of Moment (KN-m)

Moment reduction(HDRB) No Bearing

-50 0 50 100 150 200

Change of Moment (KN-m)

Moment reduction(HDRB) No Bearing

-50 0 50 100 150 200

F10

Change of Moment (KN-m)

Moment reduction(HDRB) No Bearing

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

structures. Nonetheless, for base isolated structures, displacement occurs almost uniformly

in the whole upper structure and the displacement at the base remains within acceptable

limits. As the relative displacement of adjacent stories is minimal, such structures can

resist high seismic tremors in a safe and efficient manner against seismic ground

excitation.

The key points in investigating the dynamic responses of base-isolated buildings are

the base displacements and story forces. Here, base displacement indicates the

superstructure translation in isolation interface. The analyses reveal that the structure is

shifted by 87.15 mm ~ 130.15 mm while the bearing is inserted. For BI buildings, the

superstructure gets such significant displacement even at base namely, at isolation

interface. But in case of FB structures, base displacement is zero and the floor

displacements vary nonlinearly commencing from zero to maximum at top story.

Conversely, the variation of horizontal translation in between floor to floor for BI

buildings are not substantial (below 3%), ensuring almost uniform shape of distribution.

Figure 7 plots the variation of horizontal translation of different number of stories

using a unique type of high damping rubber bearing HDRB1. It is logical that with

increase of building elevation, the displacements at bearing interface are gradually

increased. The 4-story building shows around 33.04% less shift than that of the 10-story

building. However, the rate of increment from 8- to 10-story structure is 10%, which is

less than the rate of increment for lower story buildings of 16~17%. The observation

suggests that the pattern of results is comparatively oblivious to the structural period and

so representative HDRB1 induced responses are described. The higher the isolator period,

the larger the displacement and lesser the equivalent viscous damping. Again, to compare

the structural response for different bearings, the scaling of the selected ground motions

and the choice of the damping reduction factor are of great importance. If both are

addressed well, the model developed could be a very useful tool for base isolated

buildings, especially in the preliminary stage of structural design.

Fig. 7: Superstructure Displacement of BI buildings at bearing interface.

3.4 Inertia Behavior

The shear force of buildings is maximum at base level, which is computed from the

concurrent summation of maximum inertia forces at each floor above bearing level. The

modal inertia force is a function of structural mass, spectral acceleration, and the

participation factor at the respective level. The inertia forces at the respective floor denote

the design shears at each level, leading to computation of base shear as well as overturning

moment of the building.

100

120

140

4 Story 6 Story 8 Story 10 Story

D
is

p
la

c
e
m

e
n
t

(m
m

)

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

The distribution of inertia has been computed as the effective inertia height, HC which

is the ratio of highest overturning moment by the product of design base shear and

building height. This effective height indicated linear inertia distribution when the value is

0.5 and triangular for 0.67 magnitude. The centroids of inertia loads are at half and two-

third of the buildings’ height respectively. For HC equaling to 1.0, distribution of inertia

forces concentrates at the top story. Figure 8 plots these distributions for four building

configurations comprising four types of HDRB as well as fixed based buildings in terms

of normalized values. It is observed that without bearing the effective height of building

inertia is around 0.5 tending to be triangularly distributed. Accordingly, it generates a

conformist moment for most of the buildings. For the BI buildings, the effective inertia

height HC ranges between the two-thirds of building elevation (Fig. 8) showing triangular

distribution concentrating at top story level except the low-rise building. This trend is

followed by all types of HDRB. It is also revealed that in general, the lengthier the

isolation period the more flexible the structures are.

Fig. 8: Normalized height of inertia force for different buildings.

This behavior is reasonable because of the fundamental mode shape produced by the

bearing devices. Unfortunately for the 4-story building, the HDRBs provide effective

inertia height exceeding building height by a large extent as 1.63~2.67 times. The

tendency of effective inertia height exceedance increases with the increase of the isolator

period. It indicates the moment distribution has very high shear at the top story. The

phenomena of increasing top story shear and moment is also supported by the effective

inertial height where the distribution offers a high moment corresponding to the shear.

Obviously, it confirms the kicking back of building at the base.

The consequences of the analyses disclose that the high damping bearing device is

expected to diminish the corresponding moments at different floor levels, the

superstructure’s relative displacement as well as upsurge to the effective inertia height.

The superstructure-foundation contact part acts as a passive control system, through the

HDRB device, and can be the main reason behind the stability of the building structures in

seismic-prone areas.

0.5

1.5

2.5

4 Story 6 Story 8 Story 10 Story

H
e
ig

h
t

o
f

In
e
rt

ia
L

o
a
d

No Bearing HDRB1 HDRB2 HDRB3 HDRB4

IIUM Engineering Journal, Vol. 19, No. 1, 2018 Islam

4. CONCLUDING REMARKS

Subsequent findings have been outlined from the analyses conducted in the present

study.

 Anticipated base isolation technique is intended to draw attraction of using the
high damping rubber bearing at structural base in seismic-prone areas as a suitable

alternative to structural retrofitting.

 The moment will be highly overestimated by 31~55% if the bearing is not
incorporated in building base. Reduction of moments allow the structures to

withstand comparatively high seismic tremors in a safe and efficient manner.

 For HDRB isolated structures, the superstructures experience significant lateral
shift as 87.15 mm ~ 130.15 mm due to structural flexibility. The increase of

superstructure shift is 16~17% for low-rise but around 10% between high-rise

buildings. The relative floor displacement remains below 3% of adjacent floor.

 The lengthier the isolation period, the more flexible the structures are.

 The effective inertia height for the BI buildings remains between two-thirds of
building elevation showing triangular distribution concentrating to the top story.

 The distribution of effective inertia offers high moment corresponding to the shear
at top story. Obviously, it confirms the kicking back of the building at the base.

 The superstructure-substructure contact part acts as a passive control system,
through the HDRB device, and might be the main reason behind the stability of

these structures in active seismic zones.

 High damping bearing device can reduce the forces, moments and relative
displacement of structural members which can lead to economic design and

structural safety against seismic excitation.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the Project 2017-212-Eng, Deanship of Scientific

Research (DSR), Imam Abdulrahman Bin Faisal University (IAU) for successful

completion of the study.

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