Effect of Infill and Mass Irregularity on RC Building under Seismic Loading

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 02 | Feb -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 176
Effect of Infill and Mass Irregularity on RC Building under Seismic
Loading
Oman Sayyed1, Suresh Singh Kushwah2, Aruna Rawat3
1Post Graduate Student, Department of Civil Engineering, University Institute of Technology (RGPV), Bhopal, India
2Professor & HOD, Department of Civil Engineering, University Institute of Technology (RGPV), Bhopal, India
3 Asst. Prof. Department of Civil Engineering, University Institute of Technology (RGPV), Bhopal, India
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Abstract – If there is a sudden change in mass, stiffness
and strength along the vertical or horizontal plane of a
building then it may suffer serious damages due to seismic
loading. Such a building which has an irregular distribution
of mass, strength and stiffness along the building height can
be termed as vertically irregular building. These
irregularities cause deterioration of the building which
leads to collapse. In this paper, the focus is made on the
performance & behavior of regular & vertical irregular
G+10 reinforced concrete (RC) buildings under seismic
loading. Total nine building models having irregularity due
to partial infill and mass irregularity are modeled &
analyzed. Response spectrum analysis (RSA) is carried out
for these building models for seismic zone V and medium soil
strata as per IS 1893:2002 (part I). Seismic responses like
storey displacement, storey drift, overturning moment,
storey shear force, storey stiffness are obtained. By using
these responses comparison is made between the regular
and irregular building models. This study focuses on the
effect of infill and mass irregularity on different floor in RC
buildings. The results conclude that the brick infill enhances
the seismic performance of the RC buildings and poor
seismic responses are shown by the mass irregular building,
therefore it should be avoided in the seismic vulnerable
regions.
Key words: Equivalent Diagonal Strut, Infill
irregularity, Masonry infill, Mass irregularity, Response
spectrum analysis, Vertical Irregular.
1. INTRODUCTION
In the past, a number of major earthquakes have
uncovered the deficiency in buildings. This weakness
causes deterioration of the building which leads to the
collapse. This weakness mostly occurs due to the presence
of irregularities in a building system. It has been observed
that regular buildings perform better than irregular
buildings under seismic loading. The irregularities in the
buildings are present due to irregular distribution of mass,
strength and stiffness along the height and plan of
building. Mainly these irregularities are classified into two
types as shown in Fig. 1 and the irregularity limits
prescribed by IS 1893:2002 (part I) are given in Table 1.
Fig.1: Detail of irregularities in RC buildings.
Table 1: Vertical irregularity limits prescribed by IS 1893:2002
Type of Vertical
Irregularity
Prescribed Limits
Stiffness Irregularity -
Soft Storey
A soft storey is one in which the lateral
stiffness is less than 70 percent of that in
the storey above or less than 80 percent of
the average lateral stiffness of the three
storeys above.
Stiffness Irregularity -
Extreme Soft Storey
A extreme soft storey is one in which the
lateral stiffness is less than 60 percent of
that in the storey above or less than 70
percent of the average stiffness of the three
storeys above.
Mass Irregularity
Mass irregularity shall be considered to
exist where the seismic weight of any
storey is more than 200 percent of that of
its adjacent storeys.
Vertical Geometric
Irregularity
Vertical geometric irregularity shall be
considered to exist where the horizontal
dimension of the lateral force resisting
system in any storey is more than 150
percent of that in its adjacent storey.
In-Plane Discontinuity in
Vertical Elements
Resisting Lateral Force
An in-plane offset of the lateral force
resisting elements greater than the length
of those elements.
Discontinuity in Capacity-
Weak Storey
A weak storey is one in which the storey
lateral strength is less than 80 percent of
that in the storey above.
Irregularities
Vertical irregularity
•Irregulardistribution of stiffness, mass and
strength along the height.
•Setback
Horizontal irregularity
•Irregulardistribution of stiffness, mass and
strength along the plan.
•Diaphragm discontinuity
•Re entrained corners
•Asymmetrical plan shapes

2. LITERATURE REVIEW
The various research works which have been carried out
on performance of infill and mass irregular buildings are
as follows:
Tamboli and Karadi (2012) performed a seismic analysis
using equivalent lateral force method for different RC
frame building models that include bare frame, infilled
frame and open first storey frame. They concluded that
infilled frames should be preferred in seismic regions than
the open first storey frame.
Shaikh and Deshmukh (2013) performed linear static &
dynamic analysis on a G+10 vertically irregular building,
as per IS 1893:2002 (part I) provisions. The building was
modeled as a simplified lump mass model having stiffness
irregularity at fourth floor. The results show that, a
building structure with stiffness irregularity provides
instability and attracts huge storey shear.
Hawaldar and Kulkarni (2015) considered G+12 storey
building models with and without infill and carried out
time history analysis for Bhuj and Koyna earthquake
functions using ETABS 2013 software. They concluded
that the displacement values for Bhuj function are higher
than the displacement values for Koyna function and those
for infilled buildings are less than without infilled
buildings which suggest that as the infill stiffness
increases the top storey displacement reduces.
Vijayan and Prakash (2016) analyzed a multi storied RC
building of earthquake intensity III by time history
analysis (THA) and the effects of seismic behavior on the
building in terms of seismic responses such as storey
displacement, storey drift and base shear of the structure
was calculated. They concluded that building without any
mass irregularities are the better structures for resisting
seismic loads. If any mass irregularities exists that must be
concentrated on bottom, to or top or any central areas of
building.
Objectives of the present study are (i) to carryout response
spectrum analysis (RSA) of various regular and irregular
G+10 RC buildings as per IS 1893:2002 (part I) criteria
using CSI ETABS 2015 software considering seismic zone V
and medium soil strata for all the cases, (ii) to evaluate
various seismic responses like storey displacement, storey
drift, overturning moment, storey shear force, and storey
stiffness of the regular and vertical irregular buildings and
(iii) to make the comparison between the regular and
irregular buildings on the basis of these responses.
3. METHODOLOGY
The study involves seismic analysis of various nine regular
and irregular G+10 storey RC buildings. Two types of
vertical irregularities are considered namely irregularity
due to partial infill and mass irregularity. The regular
building (bare frame model) in both the cases are same.
Changes in seismic responses due to the variation of
irregularities along the height of the building have been
studied. The plan and elevation of bare frame (model B) is
given in Fig. 2 and Table 2 shows its structural details.
TABLE 2:Structural details of bare frame (model B)
Specification Structural Detail
No. of storeys G+10
Storey height 3 m
No. of bays in X and Y direction 3
Spacing of frame in X and Y direction 4 m
Grade of concrete M 25
Thickness of slab 0.125 m
Beam size 0.45 m × 0.30 m
Column size 0.45 m × 0.45 m
Thickness of Outer wall 0.23 m
Thickness of Inner wall 0.115 m
Poisson ratio of brick masonry 0.198
Modulus of elasticity of concrete 25×103 MPa
Modulus of elasticity of brick
masonry
4.125×103 MPa
No. of mode used 30
Damping ratio 5%
Seismic zone V
Response reduction factor (R) 5
Soil type Medium
Zone factor (Z) 0.36
Importance factor ( I ) 1
Fig.2: Plan and elevation of bare frame (model B)

3.1 Infill irregular buildings (partial infill)
In this study eight RC G+10 storey buildings are
considered as shown in Fig.3. Seismic responses of the
buildings due to variation of infill along the height are
calculated. The infill wall is modeled as a single equivalent
diagonal strut pinned at both ends.
Model B: Bare frame
Model R: Infilled building
Model I1: Infill upto 8th storey
Model I4: Infill upto 3rd storey
Model I5: Open ground storey building
Model I6: Open 5th storey building
Diagonal strut
Model R Model I1
Model I2 Model I3
Model I4 Model I5
Model I6
Fig.3: Elevation of irregular building models
3.1.1 Equivalent diagonal strut
The equivalent diagonal strut method is used for modeling
the brick infill wall according to FEMA273. In this method
the infill is replaced by an equivalent diagonal strut which
is pin jointed at the corners of the RC frame and can only
resist compressive force and all the moments and shear
forces are released from it. The detail of strut is given in
Table 3.
The present study uses the empirical equation as given by
Mainstone and Weeks (1970) and Mainstone (1971) for
calculating the width of the equivalent strut. The width of
strut ‘w’ is given by,
(1)
where,
λh = coefficient used to determine equivalent width of
infill strut, which is given by
(2)
hcol = Column height between center lines of beams, mm.
h = Height of infill panel, mm.
Ec = Expected modulus of elasticity of column, MPa.
Em = Expected modulus of elasticity of infill, MPa
Ic = Moment of inertia of column, mm4.
d = Diagonal length of infill panel, mm.
t = Thickness of infill panel and equivalent strut, mm.
θ = Angle between diagonal of infill wall and the
horizontal, radian.

Table -3: Detail of diagonal strut
3.2. Mass irregular building
In present case the irregular model is same as regular
model (bare frame model B) but refuge area is provided at
fourth storey and eight storey of the building and other
geometry remains the same as shown in Fig. 2 and Table 2.
Loading due to refuge area is 15 kN/m2.
Model B: Regular bare frame
Model M: Mass irregularity at 4th and 8th floor
3. RESULTS AND DESCUSSIONS
3.1 Infill irregular building (partial infill)
After performing response spectrum analysis, responses
at each storey are shown in Figs. 4 to 8 along the storey
height.
Fig.4: Comparison of storey displacement.
Fig.5: Comparison of storey drift
.
Fig.6: Comparison of overturning moment.
Fig.7: Comparison of storey shear force.
Fig.8: Comparison of storey stiffness.
Storey displacement is maximum in case of bare frame (B)
and minimum in case of infilled frame (R). In the storeys
which are not infilled, a sudden extreme change in the
slope of the displacement curve has been observed as
shown in Fig.4. Storey drift is maximum in case of bare
frame (B). A sudden extreme change in the drift is
observed due to absence of infill as shown in Fig. 5. The
overturning moment and storey shear is maximum in case
of infilled frame (R) and it decreases as the percentage of
infill in the building decreases as shown in Figs. 6 and 7.
The stiffness is maximum in case of infilled frame (R) and
it decreases due to absence of infill as shown in Fig. 8.
Wall h
(mm)
d
(mm)
Θ
(degrees)
λh w
(mm)
Size of
diagonal
strut in
mm
Outer
wall
3000 5000 36.86 0.97 570
570mm
×230mm
Inner
wall
3000 5000 36.86 0.97 570
570mm
×115mm
Storeyheight(m)
Dispalcement (mm)
Storey Displacement
B
R
I1
I2
I3
I4
I5
I6
Storeyheight(m)
Shear force (KN)
Storey Shear Force
B
R
I1
I2
I3
I4
I5
I6
Storeyheight(m)
Moment (kN-m)
Overturning Moment
B
R
I1
I2
I3
I4
I5
StoreyHeight(m)
Drift
Storey Drift
B
R
I1
I2
I3
I4
I5
I6
Storeyheight(m)
Stiffness (kN/m)
Storey Stiffness
B
R
I1
I2
I3
I4
I5
I6

3.2 Mass irregular building
The analyses of models for mass irregular buildings are
evaluated and the responses are shown in Figs. 9 to 13.
The top node displacement in case of mass irregular
building is greater than that of the regular building. But in
lower storeys it is approximately same as that of regular
building as shown in Fig. 9.
Fig.9: Comparison of storey displacement.
The storey drift is greater in case of mass irregular
building in the intermediate storeys, but in top and bottom
storeys it is same as that of regular building as shown in
Fig.10.
Fig.10: Comparison of storey drift.
Fig.11: Comparison of overturning moment.
Fig.12: Comparison of storey shear force.
The overturning moment and storey shear force has been
found maximum in ground storey and it decreases
towards the top of the building. The mass irregular
building has more overturning moment and storey shear
force in lower storeys as compare to regular building, but
in top storeys it is approximately same as that of regular
building as shown in Figs. 11 and 12, respectively.
Fig.13: Comparison of storey stiffness.
Fig.13 shows that due to mass irregularity the stiffness of
the irregular building gets marginally affected in the top
storeys. But in the bottom storeys, stiffness of mass
irregular building is exactly same as that of regular
building.
Storeyheight(m)
Dispalcement (mm)
Storey Displacement
B
M
Storeyheight(m)
Drift
Storey Drift
B
Storeyheight(m)
Shear force (kN)
Storey Shear Force
B
M
Storeyheight(m)
Stiffness (kN/m)
Storey Stiffness
B
M
Storeyheight(m)
Moment kN-m
Overturning Moment
B
M

4. CONCLUSIONS
The performance and behavior of regular and vertical
irregular for infill and mass irregularities G+10 RC
buildings are studied under seismic loading and following
conclusions are made:
1. The displacement results show that the storey
displacement is maximum in case of bare frame (B)
and minimum in case of infilled frame (R). The
storey drift is maximum in case of bare frame (B)
and it suddenly increases in the storeys having no
infill. Therefore, infilled building frame should be
preferred in the seismic prone areas as compared
to the bare building frame.
2. Infilled frame (R) has maximum overturning
moment and storey shear force in the lower
storeys. The overturning moment and storey shear
is maximum in case of infilled frame (R) and it
decreases as the percentage of infill in the building
decreases. The result shows that the presence of
brick infill increases the stiffness and strength of
the RC buildings.
3. The mass irregular building undergoes more
displacement as compare to regular building in the
top storeys. But in lower storeys the displacement
in mass irregular building is approximately same as
that of regular building. The storey drift is more in
case of mass irregular building in the intermediate
storeys, but in top and bottom storeys it is same as
that of regular building.
4. The overturning moment and storey shear force is
maximum in ground storey in case of mass
irregular building and it decreases towards the top
of the building. A marginal effect on stiffness in the
top storeys of the building due to mass irregularity
has been observed. It can be concluded that in
seismic regions poor performance is observed in
case of mass irregular buildings. Therefore, for
resisting seismic loads buildings without any mass
irregularities are the better buildings.
REFERENCES
[1] R.J Mainstone and G.A.Weeks (1970). “The Influence of
Bounding Frame on the Racking Stiffness and Strength
of Brick Walls”, 2nd International Brick Masonry
Conference, Stoke-on-Trent, UK.
[2] R.J. Mainstone (1971). “On the Stiffness and Strength
of Infill frames”, Institution of Civil Engineers,
Supplement IV, pp. 57-90.
[3] FEMA-273 (1997). “NEHRP Guidelines for Seismic
Rehabilition of Buildings”, Building Seismic Safety
Council, Federal Emergency Management Agency,
Washington, D.C, USA.
[4] H.R.Tamboli and U.N. Karadi (2012.), “Seismic
Analysis of RC Frame Structure with and without
Masonry Infill Walls”, Indian Journal of Natural
Sciences, Vol 3, Issue 14, pp. 1137-1148.
[5] A.R. Shaikh and G.Deshmukh (2013) “Seismic
Response of Vertically Irregular RC Frame with
Stiffness Irregularity at Fourth Floor”, International
Journal of Emerging Technology and Advanced
Engineering, Vol 3, Issue 8, pp. 377-385
[6] J.C.Hawaldar and D. K. Kulkarni (2015), “Earthquake
Analysis of a G+12 Storey Building with and without
Infill for Bhuj and Koyna Earthquake Functions”,
International Research Journal of Engineering and
Technology, Vol 02, Issue 05,pp. 525-531
[7] A.Vijayan and A. Prakash (2016), “Study on Seismic
Analysis of Multi Storied Reinforced Concrete Building
with Mass Irregularities,” International Journal of
Science and Research, Vol 5, Issue 7,pp.1196-1198
[8] IS 1893:2002 (Part 1). “Indian Standard Criteria for
Earthquake Resistant Design of Structures”, Bureau of
Indian Standards, New Delhi.

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Effect of Infill and Mass Irregularity on RC Building under Seismic Loading