Seismic Analysis of Multistoreyed RC Building Due to Mass Irregularity by Time History Analysis

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 04 Issue: 08 | Aug -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 621
SEISMIC ANALYSIS OF MULTISTOREYED RC BUILDING DUE TO MASS
IRREGULARITY BY TIME HISTORY ANALYSIS
Sampath Nagod1 , Prof. A.J.Zende2
¹Student of Master of Technology in Structural Engineering
BLDEA’s V.P. Dr. P.G Halakatti College of Engineering and Technology, Vijayapur - Karnataka, India.
²Professor at Civil Engineering Department
BLDEA’s V.P. Dr. P.G Halakatti College of Engineering and Technology, Vijayapur - Karnataka, India.
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Abstract - From earthquakes history, it can be observed
that if the structures are not appropriately analyzed and
constructed with necessary quality, then it may lead great
damage and loss to human lives. We can see many of
structures are fully or partially damaged due to earthquake.
So, there is need to establish seismic responses of buildings.
Seismic analysis of the structure is done to determine seismic
responses by time history analysis which is one of the
important techniques for structuralseismicanalysisespecially
when the evaluated structural response is non- linear in
nature. To perform such an analysis, a representative
earthquake time history is required for the structure being
evaluated. In this present work non-linear dynamicanalysis of
12 storeyed RC building having mass irregularity considering
Bhuj earthquake time history is carried out using ETABS
software. Then, Lead Rubber Bearing is manually designed
and isolator properties are assigned to the building. Various
parameters like Base shear, Time period and Storey
displacement are determined for regular and mass irregular
buildings with fixed base and base isolated condition and
compared with each other.
Key Words: Base isolation, Mass irregularity, Time
history analysis, Bhuj earthquake.
1. INTRODUCTION
1.1 General
All over the world, there is much need of construction of
high-rise multi-storey buildings due to the urbanizationand
increase in population. These multi-storey structures are
unsafe when they are subjected to the earthquakes. While
earthquake engineering is noticeably progressed,structures
failures are found wherever strong disturbances caused due
to the motion of the earth. Earthquake forces are
unpredictable. Environmental loads like an earthquake are
harmful and last only for the small durationbutcausesevere
damage to the structures and harm lives of people. Yearly
near 1.5 crore, people lose their lives due to the earthquake
that strikes the earth. The multi-storey structure generally
subject to failure due to seismic forces at the location where
there is a weakness. The weakness of structuresisduetothe
presence of irregularities in stiffness, strength and mass.
Earthquake code IS 1893 is used to design multi-storey
buildings. But Ahmedabad during Bhuj earthquake, because
of mass irregularity the buildings got severely damaged.
Excess mass leads in reduction of ductility of vertical load
resisting elements and increase inertia forces and thus
increase the tendency towards collapse. Excess mass on
upper floors has a more unfavorable effect than those at
lower floors. Thus there is the necessity of designing these
structures for earthquake loading so that they sustain
moderatetostrongearthquakeforces.Multi-storeybuildings
and their structural members can be protected against
earthquake forces by installing structural isolation devices.
The technique used to avoid earthquake damage by
separating the structure from the ground known as base
isolation.
1.2 Base Isolation
Base isolation is a technique in which during an earthquake,
separating the superstructure or reducing the lateral
movements of building superstructure from the movement
of ground or foundation is done. The bearings of base
isolation are designed in such a way that they are stiff
vertically and flexible horizontally to allowforthedifference
in lateral movement while still supporting the
superstructure. The structures with base isolation are
different than that of fixed base structure, in which the
connection between the superstructure and the foundation
are rigid and the superstructure translationinall directionis
constrained. Behavior of the building with base isolators is
shown in below fig
Figure 1.1: Behaviour of the buildingwith BaseIsolators

The main aspire of base isolation is to decrease the
earthquake force produced on building’s superstructure. To
some amount by reducing the superstructure’s spectral
acceleration, the lessening in seismic forceatsuperstructure
is achieved. By mounting the base isolated structure
fundamental period and through damping caused by
dissipation energy within bearing the accelerations are
reduced. The effect of base isolated and fixed base building
on spectral acceleration is shown in fig below.
Figure1.2: Effect of Seismic Isolation on Spectral
Acceleration
Due to the reduced lateral stiffnessofisolatedstructures, the
effective period of the superstructure is increased. The stiff
structures with the small period the base isolation are most
effective. Below equation shows the relation between
effective period and the stiffness.
T = 2π√ (M/K)
As, ↓K => ↑ T
Where, T = Period (sec.)
K = Stiffness (kN/mm)
M = Mass (N –Sec.2/mm)
1.3 Objectives of the Study
In the present study, the work includes the analysis of a 12
storey reinforced concrete mass irregular building in
accordance with IS1893-2002 provisions; one with fixed
base and other with base isolated.
The objectives of the study are as follows;
1. To carry out modeling and analysis of multi-
storeyed RC building with and without mass
irregularity by using ETABS software and study the
effect of seismic forces on different models.
2. To design and study the effectivenessofleadrubber
bearing used as a base isolation system
3. To carry out comparison between fixed base and
base isolated building by Time history analysis
considering Bhuj earthquake record and obtaining
different parameters such as Base shear, Time
period and Storey displacement.
2. METHODLOGY
In this study, 12 storied reinforced concrete building with
and without base isolation are considered. In addition to
that, mass irregularity is also taken into consideration by
providing heavy mass at 11th floor. The RC frame with infill
panels is modeled for Indian seismic zone V IS: 1893-2002
having medium stiff soil. Time history analysis is performed
on buildings using ETABS software.
2.1 Models considered for analysis
 Model 1: Building with fixed base
 Model 2: Building with base isolation
 Model 3: Mass Irregular Building with fixed base
 Model 4: Mass IrregularBuildingwithbaseisolation
2.3 Structural specifications
2.3.1Member properties
Table 2.1: Member properties
No of storey 12
Size of Building 20 x 12m
Spacing of frames in X direction 5m
Spacing of frames in Y direction 4m
Size of Beam 300 x 450mm
Size of Column 300 x 500mm
Storey height 3m
Thickness of slab 150mm
Thickness of wall 230mm
Height of parapet wall 1m
2.3.2Load intensities
Table 2.2: Load intensities
Live load on Roof 1.5 kN/m²
Live load on Floor 3.0 kN/m²
Floor finish 1.0 kN/m²
Swimming pool 18 kN/m²
2.3.3Material properties
Table 2.3: Material properties
Grade of Concrete M25
Grade of Steel Fe415
Figure 2.1: Plan of Building

Figure 2.2: 3D view of Building
Lead Rubber Bearing is used as a baseisolator.Afteranalysis
of fixed base RC building with and without mass irregularity
using E-TABS software, the maximum vertical reaction is
obtained for respective models. By using this vertical
reaction lead rubber bearing is designed manually and the
properties obtained by design are used as base isolator
properties for model 2 and model 4.
2.4 Lead Rubber Bearing Properties
Table 2.4: Summary of Lead Rubber Bearing Properties
Properties
Building without
mass irregularity
Building with
mass irregularity
Axial load on
column
4981kN 5330kN
Required
stiffness (keff)
5011.26 kN/m 5362.38 kN/m
Bearing
horizontal
stiffness (Kb)
1005.93 kN/m 1078.93 kN/m
Vertical stiffness
(Kv)
889963.50 kN/m 992048.12kN/m
Yield force (F) 75.13 kN 80.40 kN
Stiffness ratio 0.1 0.1
Damping 0.05 0.05
3. RESULTS AND DISCUSSION
3.1 Base Shear
From the obtained results, it is observed that for a 12 storey
RC Base Isolated buildingwithoutmassirregularityanalyzed
by Time history analysis (Bhuj earthquake), Base Shear is
decreased by 38.68% in the X direction and by 49.47% in Y
direction compared to a fixed base building without mass
irregularity. Similarly, Fixed base building with mass
irregularity analyzed by Time history analysis (Bhuj
earthquake), Base Shear is increased by 31.08% in X
direction whereas decreased by 14% in Y direction
compared to a fixed base building without mass irregularity.
Figure 3.1: Base Shear Comparison
3.2 Time Period
by Time history analysis (Bhuj earthquake), Time Period is
increased by 26.77% in the X direction and by 28.46% in Y
irregularity. Similarly, Fixed base building with mass
irregularity analyzed by Time history analysis (Bhuj
earthquake), Time Period is increased by 8% in X and Y
irregularity.
Figure 3.2: Time Period Comparison

3.3 Storey Displacement
by Time history analysis (Bhuj earthquake), storey
displacement is increased by 25.147% in the X direction
whereas decreased by 31.964% in Y directioncomparedtoa
fixed base building without mass irregularity. Similarly,
Fixed base building with mass irregularity analyzedbyTime
history analysis (Bhuj earthquake), storey displacement is
decreased by 7.199% in X direction and by 4.472% in Y
irregularity
Table 3.1: Storey Displacement along X direction
Storey Model 1 Model 2 Model 3 Model 4
12 101.4 126.9 93.8 106.6
11 98 123.6 91.9 103.7
10 93.2 118.2 88.4 98
9 87.5 110.7 83.5 90.7
8 79.6 101.9 78 82.1
7 69.4 92.1 73.8 72.9
6 58.5 81.6 69.2 66.8
5 49 71 62.4 61.5
4 38.7 61.6 52.8 56.2
3 28.4 52.7 40.3 50.7
2 18.3 43.7 25.4 43.6
1 7.3 33.2 10 33.8
Base 0 13.7 0 15
Figure 3.3: Storey Displacement in X direction
Table 3.2: Storey Displacement along Y direction
Storey Model 1 Model 2 Model 3 Model 4
12 136.4 92.8 130.3 98.9
11 131.2 91.1 125.1 96.9
10 121.9 88.5 113.8 93.2
9 109.1 84.9 99.5 88.6
8 94.3 80.4 84 83.6
7 80.6 75.1 69.1 78.2
6 71.9 69.2 57.5 72.4
5 65.6 62.8 50.3 66.1
4 56.3 56.1 42 59.6
3 43.8 49.9 32.2 52.7
2 28.9 43.3 21.5 45
1 12.6 34.4 9.6 35.4
Base 0 10 0 10.5
Figure 3.4: Storey Displacement in Y direction
4. CONCLUSION
From the analysis and results, it is observed that the base
isolator and mass irregularity in a building influence on the
seismic response of a building. Bycomparingtheresponseof
the structure, following conclusions are made.
1. The Base shear is decreased by about 40% in the
base isolated building when compared to the fixed
base building. It is also observed thattheBaseshear
is increased by about 30% in mass irregular
building compared to the regular building.
2. As compared to all four models studied, Base shear
is found to be minimum in a regular building with
base isolators.

3. Time period of a building increased by about 27%
by the use of base isolators which leads to less
transfer of lateral forces at the time of an
earthquake. It is also observed that mass irregular
building have about 8% increased time period
compared to the regular building.
4. At the base, fixed base buildings have zero
displacementswhereasbase isolatedbuildingshave
a considerable amount of bottom displacement.
5. Storey displacement is increased by 25% in the
buildings with base isolators compared to the
buildings with a fixed base.Displacementisreduced
by 7% in mass irregular buildings compared to
regular buildings.
Finally, it can be recommended that Time History analysis
shall be carried out for the multi-storeyed buildings before
their construction for earthquake function with a high
magnitude such as Bhuj earthquake time history as
considered in present study along with consideration of
mass irregularity in buildings.
5. REFERENCES
1. IITK-BMTPC EQTips – National Information Centre
of Earthquake Engineering (www.nicee.org) EQ
Tip6
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supplemental viscous dampers on seismic isolated
buildings under near-fault excitations” Engineering
Structures ELSEVIER 2007
3. Naveen K, Dr. H.R Prabhakara and Dr. H Eramma
“Base Isolation of Mass Irregular RC Multi-Storey
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Pantawane “Seismic AnalysisofFixedBaseandBase
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5. Mr. Pathan Irfan Khan, Dr. Mrs. N.R.Dhamge
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