Ijciet 08 02_034

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International Journal of Civil Engineering and Technology (IJCIET)
Volume 8, Issue 2, February 2017, pp. 316–327 Article ID: IJCIET_08_02_034
Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=2
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication Scopus Indexed
SEISMIC ISOLATION OF RC FRAMED STRUCTURE
WITH AND WITHOUT INFILLS
Santhosh H.P
Research Scholar, Department of Civil Engineering,
Sahyadri College of Engineering and Management, Mangaluru, India.
Harsha M.S
Student, Department of Civil Engineering,
Manohar K
Assistant Professor, Department of Civil Engineering,
Pradeepa B.B
Student, Department of Civil Engineering,
ABSTRACT
Earthquakes are the nature’s greatest hazard which cannot be exactly predicted by human
beings. Earthquake is a sudden movement of earth’s surface with the release of massive energy
in the form of seismic waves. There are so many methods to control the vibration of the
buildings like passive, active and hybrid control systems.[1] Base isolation is a passive control
system. In the present work, a (G+3) structure considering with and without infill’s subjected
to seismic force along with static gravity loads are analyzed using Response Spectrum Method
in ETABS-2015 package. From the study, the response of the structure obtained is time period,
base shear, story displacement and inter-story drifts and also the comparison is drawn for the
same parameters considering structure as bare and infill frames. The variation in percentage
of steel is also presented. It is found that the time period and story displacement increases
while base shear and story drifts gets reduces with the provision of effective base isolators.
Key words: Base isolation, Base shear, Story- drift, Acceleration, Infill.
Cite this Article: Santhosh H.P, Harsha M.S, Manohar K and Pradeepa B.B, Seismic Isolation
of RC Framed Structure with and without Infills. International Journal of Civil Engineering
and Technology, 8(2), 2017, pp. 316–327.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=2

Santhosh H.P, Harsha M.S, Manohar K and Pradeepa B.B
1. INTRODUCTION
Earthquake is a natural calamity that has taken the lives of millions of people through the ages in the
recorded and unrecorded human history. A disturbance that causes shaking on the surface of the earth
due to underground movement along the fault plane or through volcanic activity is called an
earthquake. In the seismic design of low to medium rise buildings, the fundamental frequency of
vibration is much lower than the earthquake force that means building acts as an amplifier and the
acceleration experienced at each floor level increases to the top. Because of this, inter-story drift and
stresses in the member’s increases and damage to the column occurs between the floors. These
accelerations can cause severe damage to the occupants and the contents of the floors without causing
much damage to the structure. Only way to achieve flexibility in low to medium rise buildings is by
the use of base-isolation method. In recent years, this new technology has emerged as an alternative
solution to the conventional seismic strengthening. This technology has attained professional and
academic interest throughout the world and many hundreds of buildings were built in U.S.A, New
Zealand, China and Japan using seismic isolation technology. In India, base-isolation technique is
rarely used in some public and residential buildings like Bhuj hospital building and an experimental
building at IIT, Guwahati.[2]
1.1. Mechanism of Base-Isolation
During the strong ground vibrations a base isolated structure undergoes decoupling i.e, the
superstructure is decoupled from the energy absorbing substructure. Period shift is the main
mechanism in the base isolation system. A building is subjected to many frequencies during the
seismic activity. The lowest first natural frequency of the structure is the most predominant one and it
causes severe damage to the structure during vibration. The frequency of the isolated structure is less
than the fixed base frequency of the structure and hence one can observe the reduced response of the
superstructure. So, the fundamental frequency of the structure is reduced, i.e. time period of the
structure is increased. [1]
Figure 1 Response of the structure with fixed base and base isolation.
1.2. Structural Action of Infill Frames
Under very low lateral loads masonry infill’s remains in contact with the RC frame and hence one can
accept there is a composite action between RC frame and infill’s. But when the lateral load increases
at the interface between RC frame and masonry infill, masonry infill starts to crack. Further, when the
lateral load increases there is a separation between masonry infill and RC frame. Masonry is very
weak in tension because bond between them is weak and hence when subjected to lateral loading it
does not behave elastically even for small deformations. Therefore, masonry always resists

Seismic Isolation of RC Framed Structure with and without Infills
compressive forces. However, by providing reinforcement between the two courses, tensile stresses
can be eliminated.[3]
Figure a. Predominant frame action Figure b. Predominant truss action
Figure 2 Lateral load transfer mechanism of frame system having with and without infill.
1.3. Objectives of the Present Study
The main analysis of the study is to compare the results for the parameters Base shear, Time period,
Story displacement, Inter – Story drifts and variation of percentage of steel for ,
 Fixed base and isolated base structures having bare frame.
 Fixed base and isolated base structures having infill frame.
2. DESCRIPTION OF THE BUILDING
The description of the building used in the analysis is as shown in the fig 3.3. A (G+3) storey building
located in Zone V on medium soil is taken and analysis of all the structural members including seismic
evaluation is done as per IS 456: 2000 and IS 1893 (part 1): 2002, using ETABS 2015 software. The
typical floor plan of the building is as shown. All dimensions are in m.
Figure 3 Plan of the building.

Table 1 Details of the RC frame structure.
2.1. Design of Lead Rubber Bearing
For designing of isolators the loads at the base of the column obtained from the ETABS is considered.
These loads are grouped into three categories for the purpose of isolator design.
Isolator 1 loads considered 1233.47 kN, 1319.29 kN and 1400.14 kN.
Isolator 2 loads considered 1566.54 kN, 1608.89 kN, 1641.49 kN and 1635.66 kN.
Isolator 3 loads considered 1719.09 kN and 1783.5 kN.
Three isolators were designed and maximum load from each group is considered. In the present
work the design procedure for isolator 3 for a load of 1783.5 kN is discussed. The following data’s
were assumed for the design of LRB.[2]
1) The design target period, TD of the isolated structure should be greater than 3times the fixed
base period.
For fixed base structure, for mode 1 time period is 0.826 sec.
Therefore, for isolated structure, TD = 0.826 X 3 = 2.478 sec = 2.5 sec.
Type of frame Ordinary moment resisting frame.
No of bays along X direction 5
No of bays along Y direction 5
No of storey’s G + 3
Storey height 3.5 m
Seismic Zone V
Zone factor 0.36
Soil type Type ɪɪ (Medium)
Importance factor 1.0
Response reduction factor 3.0
Damping of the structure 5%
Cross- sectional properties
Flexural member
Compression member
Depth of slab
0.2 X 0.6 m
0.3X 0.75 m
0.15 m.
Concrete
Density of concrete ( self- weight)
Modulus of Elasticity of concrete.
Poisson’s ratio of concrete
25 kN/m3
(As per IS:875(Part-ɪ) – 1987)
=27386.128N/mm2
.
0.2
Masonry
Masonry (Infill) thickness
Density of Masonry (self-weight)
Compressive strength of Masonry
Elastic modulus of infill
Poisson’s ration of masonry
0.2 m.
20 kN/m3
(As per IS:875(Part-ɪ) – 1987)
10 N/mm2
(taken).
=5500 N/mm2
.
0.16
Unit Weight of rebar 78.5 kN/m3
Imposed load 2 kN/m2
(As per IS:875(Part-ɪɪ) –1987).
Floor finish 1.25kN/m2
(As perIS:875(Part-ɪ)– 1987)
Earthquake analysis IS: 1893 (Part-ɪ) – 2002.
Design Philosophy Limit State Method. (IS: 456 – 2000).

2) Max. Shear strain, γmax = 50 %.
3) Effective damping ratio, ζeff= 10 %.
4) Damping ratio, BD = 1.2 (Table 1623.2.2.1 of IBC 2000).
5) Seismic coefficient, SD = 0.4(Table 1615.1.2.2 of IBC 2000).
2.1.1. Analysis
1) Effective horizontal stiffness of the isolator,
Keff= (W/g) (2π/ TD) 2
= (1783/9.81) (2 π/ 2.5) 2
= 1148 kN/m = 1.148 kN/m.
2) Design displacement,
DD= (g/4π2
) (SDx TD/BD) (Equation 16.79 of IBC 2000).
= (9.81/4 π2)
x (0.4 x 2.5) /1.2
= 0.207 m ˂ 0.3 m.
3) Short term yield force,
QD = WD/4DD
= π/2 x Keffxζeffx DD
= (π/2 x 1148 x 10 % x 0.207)
= 37.33 kN.
4) Stiffness of the lead core,
= QD / DD
= 37.33 / 0.207 = 180.34 kN/m.
5) Post yield horizontal stiffness ,
Kd= Keff- QD / DD
= (1148 – 180.34)
Kd = 967.66 kN/m.
6) Pre yield stiffness ,
Ku = 10 Kd
= (10 X 967.66)
Ku= 9676 kN/m.
2.1.2. Design
1) Design of lead core.
Assuming the yield strength of the lead core to be fpy = 8.82 x 103
kN/m2.
The required lead area,
Ap = QD / fpy
Ap = (37.33 / 8.82 x 103
) = 4.232 x10 -3
m2
.
Therefore, use diameter dP = 7 cm.
2) Design the area and dimensions of rubber layer.
a) Total height of rubber layer ,
tγ= DD / γmax
= (0.207 / 50 %)
tγ = 0.414 m.

b) Select the rubber properties , assuming the hardness of the rubber to be
IRHD – 60.
Hardness = IRHD – 60.
Elongation at break, €b = 500 %.
Young’s modulus, E = 4.45 x 103
kN/m2
.
Shear modulus, G = 1.06 x 103
kN/m2
.
Modified factor, K = 0.57.
c) Shape factor , S
E (1+ 2KS2
) / G ≥ 400
= (445 (1+ 2 x 0.57 x S2
) / 106 ≥ 400)
S = 9.09 Use S = 10.
Ec = E (1+ 2KS2
)
= (4.45 x 103
(1+ 2 x 0.57 x 102
))
Ec = 511750 kN/m2.
d) The effective area Ao of the bearing based on the allowable normal stress Ϭcfor the
vertical load case,
Ϭc= P / Ao≤ 7.84 x 103
kN/m2
= 1783/ Ao= 7.84 x 103
Ao = 0.227 m2
.
e) The effective area, At of the bearing for the shear strain condition for the vertical load
case,
6S (P/ Ecx At) ≤ €b / 3
= (6 x 10 x (1783 / 511750 x At) ≤ 500% /3)
At = 0.12 m2
.
f) Elastic stiffness Kr of the bearing,
Kd= Kr (1+ 12(Ap / Ao ))
967.66 = Kv(1+ 12(4.232 x10 -3
/ 0.227))
Kr = 793 kN/m.
g) Based on the shear failure condition , the effective area A of the individual rubber layers,
G = Kr x tγ/ Aeff
(1.06 x 103
) = 793 x 0.5 / Aeff
Aeff= 0.374 m2
.
For a circular bearing, the diameter corresponding to the area 0.374 m2
is d = 0.69m.
Effective area can be calculated as A2, d2
/4 (β – sinβ)
Where, β = 2 cos-1
( DD/d)
= 2 cos-1
(0.207/0.69)
β = 2.532.
A2 = (0.692
/4 (2.532 – sin 2.532))
A2 = 0.296 m2
.
A = max. (Ao, At, A2) = (0.227, 0.12, and 0.296) = 0.296 m2
.
h) The size and dimensions of rubber layer for a circular bearing ,
Are = d2
/4 (β – sinβ)

Where, β = 2 cos-1
( DD/d)
= 2 cos-1
(0.207/0.7)
β = 2.54.
Are = (0.72
/4 (2.54 – sin 2.54))
Are = 0.24 m2
.
i) For a circular bearing, single layer thickness t, and no. of layers N.
S = d / 4t
10 = 0.7 / 4t
t = 0.0175m = 2 cm = 20 mm
No. rubber layers, N = tγ/ t
= 0.414 / 0.02
N = 21 nos.
j) Steel plate thickness , ts
ts≥ 2 (ti + ti+1) P / Area x Fs
Fs= 0.6 Fy = 0.6 x 274.4 = 164.64 MN/m2
ts = 2( 0.02 + 0.02) x 1.783 / (0.224 x 164.6)ts= 2mm.
K) Total height of the bearing,
Assuming the thickness of the top and bottom cover plates to be 2.5 cm.
Total height is, h = tγ+ 21 tsx + 2 x 0.025
h = 0.592 m = 0.6 m = 600 mm = 60 cm.
L) Shear strain and stability condition.
a) Verticals load,
γsc = 6S x P / (Ec x A )
= (6 x 10 x 1783) / (511750 x 0.384)
= 0.544 ≤ €b/3
γsc = 0.544 ≤ 1.67. Hence ok.
a) Stability check,
Ϭc = P/ A
= 1783/ 0.384 = 4643 kN/m2
≤ G x S x L / (2.5 tγ)
≤ (1.06 x103
x 10 x 0.7 (2.5 x 0.414))
4643 ≤ 7169 kN/m2
.
b) Vertical stiffness , Kv = ( 6 x G x s2
x A) / tγ
= (6 x 1.06 x103
x 102
x 0.296)/ 0.414
Kv= 454724 kN/m.
And the Width of diagonal strut is calculated as per the Main- stone’s equation suggested in
various literatures. The values of equivalent strut width for different bay sizes were shown in the table
2 below.
Table 2 Values of equivalent strut width.
Bay size
(m)
Dimension of the infill
Width of infill(m) Depth of infill(m)
3.5 x 3 0.2 0.545
3.5 x 4 0.2 0.628
3.5 x 5 0.2 0.724

2.2. Modeling of LRB in ETABS 2015
The values of the rubber isolator are designed as a two joint link property and assigned at the base of
each column to act as a base isolator. The calculated stiffness values of LRB are entered in ETABS
2015 as shown below.[2]
For Isolator 1,
 U1 Linear effective stiffness = 484760 kN/m.
 U2 & U3 Linear effective stiffness = 901.44 kN/m.
 U2 & U3 Non Linear effective stiffness = 7598.4 kN/m.
 U2 & U3 yield strength = 29.31 kN.
 U2 & U3 post yield stiffness ratio = 0.1
 Effective damping = 0.05
2.3. ETABS Models
Figure 4 3-D View of Bare frame (Fixed base). Fig.3.5 3-D View of infill frame (Fixed base).
Figure 5 3-D View of bare frame model (Isolated base).

3. RESULTS
Table 3 Comparative study of Time period for fixed and isolated base for Mode 1.
Framing Type
Time Period (Sec.)
Percentage
Difference
Fixed base Base isolated
Bare Frame 0.826 2.083 60.35 %
Infill Frame 0.386 1.982 80.52 %
Graph 1 Comparison of Time Period of fixed and base isolated models for Mode 1.
Table 4 Comparative study of Base shear for fixed base and base isolated structures having bare and infill
frames.
Graph 2 Comparative study of Base shear (kN) for fixed base and isolated base.
Bare Frame 0.539 1.809
Infill Frame 0.237 1.733
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
TimePeriod(Sec.)
Bare Frame
Bare Frame 3925 1155
With Infill 4236 1260
0
500
1000
1500
2000
2500
3000
3500
4000
4500
BaseShear(kN)
Bare…
Framing Type
Base Shear (kN)
Percentage
Difference
Bare Frame 3925 1155 70.57 %
Infill Frame 4236 1260 70.25 %

Graph 3 Variation of Story displacements for bare frame in X- direction.
Graph 4 Variation of Story displacements for infill frames in X- direction.
Graph 5 Variation of Inter – Story drifts for bare frame in X- direction.
0
1
2
3
4
5
0 10 20 30 40 50
No.ofstoreys
Displacement (mm)
Fixed Base
0
1
2
3
4
5
0 10 20 30 40 50
No.ofstoreys
Displacement (mm)
Fixed Base
0
1
2
3
4
5
0 2 4 6 8 10
No.ofstoreys
Storey Drift (mm)
Fixed Base
Base isolation

Graph 6 Variation of Inter – Story drifts for infill frames in X- direction.
3.1. Percentage of Steel Variation in Columns
The percentage of steel in case of isolated building is 0.8 % in all the floors except in GF. And also the
percentage of steel in the models like fixed with infill and isolation with infill is 0.8%. Therefore only
the percentage of steel in GF in both fixed base and base isolated building with bare frame is shown
below.
Table 5 Percentage of Steel variation.
Column no. Percentage of Steel
Fixed Base Base Isolation
C1, C31 ,C11, C36 4.33 1.28
C4, C25, C12, C30 4.30 0.90
C13, C19, C18, C24 3.99 0.86
C2, C32, C7, C35 4.21 1.32
C3, C26, C10, C29 4.19 0.94
C14, C20, C17, C23 3.91 0.86
C5, C33, C6, C34 4.16 1.25
C8, C27, C9, C28 4.18 1.07
C15, C21, C16, C22 3.97 0.88
4. CONCLUSIONS
 From the results it is clear that with base isolation the natural period is shifted towards the target time
period of T = 2.5 sec, distancing from the predominant period of earthquake. This increases the
response of the structure by preventing the structure resonating with frequencies of earthquake. From
the results it is clear that the time period increases by 60.35% for the isolated structure.
 Isolators induce a large flexibility to the structure at the isolator level, thus it is evident that a large
reduction in base shear in case of bare and infill frame structures as compared with fixed base structure.
The reduction in base shear is 70.57% in case of isolated structures.
 The increase in the flexibility of the bare and infill frame structures with isolation has increased the
total displacement of the structure. However this displacement is concentrated only at the isolator level
and hence the displacement between the base and top story of the structure is well within the limits as
per IS 1893 (Part -ɪ): 2002, clause 7.11.1. The increase in the displacement for the isolated structure for
the bare frame is 53.57%.
0
1
2
3
4
5
0 2 4 6 8 10
No.ofstoreys
Storey Drift (mm)
Fixed Base
Base isolation

 With the base isolation system, inter story drifts are reduced or almost negligible. This reduced story
drifts enables both bare and infill frame structures to behave ideally stiff resulting in less damage to the
structural and non structural components. The story drifts follows a non-liner pattern which can be
observed in the graphs, however this non-linearity decreases in case of structures with infill compared
to structures with bare frame. This is because infill’s increases the rigidity of the structure and hence
one can observe the comparative reduction in the drift. The story-drifts obtained for various models are
well within the limit as per IS 1893 (Part -ɪ): 2002, clause 7.11.1. The reduction in story-drift for the
isolated structure is 65.47%.
 Introduction of isolation system resulted in the reduction of base shear as a result one can observe a
large reduction in the percentage of steel in case of isolated structures compared with fixed base
structures. The reduction in percentage of steel for the isolated structure is 74.88%.
4.1. Scope for Further Study
 The same structure may be analyzed by considering the shear wall to know the cost differences and the
behavior for isolated structure and structure with shear wall.
 By considering infill’s with different percentage of openings the structure may be analyzed.
 The structure may be analyzed by considering the soil- structure interaction system.
REFERENCES
[1] Tanmay Ramani (2015), “Smart Base Isolators for Seismic Control of Structures”, Vellore Institute
of Technology, Chennai.
[2] Ganga Warrier A and Dr. Sathish Kumar K.(2015), “Response Control of Structures Using Base
Isolation”.
[3] Mr. V. P. Jamnekar1 and Dr. P. V. Durge (2013), “Seismic Evaluation of Brick
[4] Masonry Infill”, (IJETET) Vol. 02, No. 01, 2013 ISSN No. 2248-9592.
[5] IS 456:2000, “Plain and Reinforced Concrete – Code of Practice”, BIS, New Delhi, India.
[6] IS: 1893 (part 1): 2002, “Criteria for Earthquake Resistant Design of Structures”, BIS, New Delhi,
India.
[7] UBC-97, “Uniform Building Code”, Dynamic methods of design complex.
[8] ASCE 7: “Minimum Design Loads for Buildings and other structures”.
[9] Prerna Nautiyal, Saurabh Singh and Geeta Batham, A Comparative Study of the Effect of Infill
Walls on Seismic Performance of Reinforced Concrete Buildings. International Journal of Civil
Engineering and Technology, 4(4), 2013, pp.208–218.
[10] Dr. D. V. Prasada Rao and G. Sulochana, Modelling of an Infill Wall for the Analysis of a Building
Frame Subjected to Lateral Force, International Journal of Civil Engineering and Technology, 7
(1), 2016, pp. 180-187.

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