Seismic Analysis of Multistorey RC Building with Mass Irregularity using ETABS

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 04 Issue: 07 | July -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 637
“SEISMIC ANALYSIS OF MULTISTOREY RC BUILDING WITH MASS
IRREGULARITY USING ETABS”
Chethan B N 1, Sanjay S J 2,
1Structural Engineer, Rudraprasad Consultants, Bangalore, Karnataka, India
2Assistant Professor, Department of Civil Engineering, PESITM, Shivamogga, Karnataka, India
---------------------------------------------------------------------***---------------------------------------------------------------------
Abstract - The comparison of the seismic evaluationofRC
buildings with and without mass irregularity.Inthisanalysis
for mass irregular buildings with floor mass is varied by
considering the slab thickness and thickness is varied from
0.125m to 0.25m and analysis is done by using ETAB 2015
version. The analysis has been carried out for various
parameters like storey displacements, storey drift, Storey
shear . The results shows that The displacement is high in
model IV compared to remaining models and is minimum in
model V. The storey drift is high in the model II compared to
remaining models and is minimum in model V. Shear is high
in model IV compared to remaining models and is minimum
in model V.
Key Words: ETABS-2015, StoreyDisplacement,StoreyDrift,
Storey Shear, Seismic Evaluation.
1. INTRODUCTION
Earthquakes are one of the most destructive of natural
hazards. Earthquake occurs due to sudden transient motion
of the ground as a result of release of elastic energy in a
matter of few seconds. The impact of the event is most
traumatic because it affects largearea,occursall ona sudden
and unpredictable. Earthquake not only damage villages,
towns and cities but also leads to economic and social
system of a country. The vibration can affects settlement.
Some of the soil types like, alluvial or sandy, silts get fail
during earthquake when compare to othersoils.Earthquake
can be measured by Magnitude (M) which was obtained by
recording the data of motions on seismograms. But shaking
of the ground surface will have different intensities at
different locations for the same magnitude. This can be
measured by MMI scale.
1.1 FLOOR MASS IRREGULARITY
Floor mass irregularity is the occurrence of
large mass on a floor or when one floor is much
more when compare to other floors, e.g., heavy
structures like machinery or a swimming pool
installed on an intermediate floor of a building. In
case of unavoidable situations or non-compliance
the ratio of mass to stiffness of twoadjacentStorey’s
should be made equal. Mass irregularities cause the
dynamic response of the structure by increasing
ductility demands at a few locations and lead to
unexpected higher mode effects.
Figure 1. Mass Irregularities
1.2 OBJECTIVES OF PRESENT WORK
 To study the seismic performance of building without
mass irregularity.
 To study the seismic performanceofbuildingwithmass
irregularity.
 To compare the behavior of building without mass
irregularity and with mass irregularity.
1.3 SCOPE OF WORK
Considering the observations a project study was
undertaken with a view to determine the extent of possible
changes in the seismic performance of low, medium and high
rise RC framed buildings. For the seismic performance of a
different height RC framed building has been considered with
mass irregularity G+10 building with increase in floor mass.
Regular configurations of such buildings taken for study are
provided. The effect mass irregularity in the buildings is
studied in terms of variations in storey drift, base shears, top
roof displacements and performance point.
2. MATERIALS AND METHODOLOGY
Here the layout of the building is regular; hence the building
has been analyzed by a 3D space frame model. Which
consisting of assemblage of slab, beam, and column
elements. Any tensional effectsareautomaticallyconsidered
in this model. The buildings will be designed for gravity
loads and evaluated for seismic forces.

2.1 MODELLING CONSIDERATION
This is based on the following assumptions:
 The floors are rigid in their planes having 3 DOF‟s, to
horizontal translations and a single rotation about a
vertical axis.
 The mass of building and mass moment of inertia are
lumped at the floor levels at the correspondingdegrees
of freedom.
2.2 DISCRIPTION OF BUILDING MODEL
General details of building
Number of Stories: G+10
Bottom storey height: 3.0 m
Storey height: 3.0 m
Building frame system: Special Moment Resisting Frames
(SMRF) Building use: Commercial
Seismic zone: Zone ІѴ
Soil type: Medium soil
Material Properties
Grade of Concrete for column: M25
Grade of Concrete for beam: M25
Grade of Steel: Fe 500
Density of Concrete: 25 kN/m3
Load Intensities
Floor finish: 1 kN/m2
Live Load at Floor: 3.5 kN/m2
Beam size: 250x400mm
Column size: 350x750
Slab thickness: 125mm (for regular building), 250(for mass
irregular building)
Figure 2. Bottom 5 Storey and even floor mass
irregular
Figure 3. Bottom 5 Storey and even floor mass
In the present study reinforced concrete moment
resisting frame building of G+ 10 storeys is considered. The
considered five models which are having different loading
criteria in which four having mass irregularity criteria and
one having regular building, the plan layout, elevations and
3D as depicted below for buildings with and without floor
mass irregularity are as shown in the below Figures. The
different configurations of buildings are modeled by
considering only by varying the slabthicknessandnonlinear
behavior of seismic demands. The first model comesupwith
G+10 and the difference is that the first 5 storey‟s having
slab thickness 250mm and all remaining storey‟s having
125mm thick slab. Second model comes with top 5floorslab
thickness with 250mm thick and remaining floors having
125mm thick slab. Third model is having slab thickness has
been varied in even floors only means in 2, 4, 6, 8,10th floors
slab having 250mm thick. Fourth model is having slab
thickness has been varied in odd floors that is in 3, 5, 7,
9,11th floors the slab is having 250mm thick. And the last
model that is the regular building with uniform slab
thickness 125mm through. Each storey height has kept to
3m and is same for all kind of building models. The building
is considered to be located in the seismic zone IV and
intended for commercial purpose.
 Model-I –Building with floor mass irregularity i.e.,
increase the slab thickness for first 5 bottom floors in
building.
 Model-II-Building with floor mass irregularity i.e.,
increase the slab thickness for top 5 floors inbuilding.
 Model-III-Building with floor mass irregularity i.e.,
increase of slab thickness in even floors only and in
other floors it will be kept to 125mm thickness.
 Model-IV-Building with floor mass irregularity i.e.,
increase of slab thickness in odd floors and in other
floors it will be 125mm thick.
 Model-V – Building without mass irregularity i.e.,
building assemblage of regular.
METHODOLOGY
This is based on the following assumptions
 The floors are rigid in their planes having 3 DOF‟s,
to horizontal translations and a single rotation
about a vertical axis.
 The mass of building and mass moment of inertia
are lumped at the floor levels at the corresponding
degrees of freedom.
3.0 RESULTS AND DISCUSSION
An effort in made to study the behavior of regular RC
buildings in comparison with RC buildings having mass
irregularity at different floor levels. Here in the present
study, the behavior of each models are captured and the
results are tabulated in the form of Base shear, top

displacements and inter Storey drifts, Storey shear in linear
analysis.
3.1. STOREY DISPLACEMENT
3.1.1 DISPLACEMENT IN X- DIRECTION
Table 1: Storey Displacement in X-Direction
Heig
ht of
the
Buil
ding
(m)
Model I
Displac
ement
(mm)
Model
II
Displac
ement
(mm)
Model
III
Displac
ement
(mm)
Mode
l IV
Displ
acem
ent
(mm)
Mo
del
V
Displace
ment
(mm)
33 106.9 110.9 109.1 114.3 100.8
30 102.6 106.1 104.7 109.4 96.6
27 96.8 99.7 98.7 103 91.1
24 89.5 91.6 91.1 94.8 84
21 80.6 81.7 81.8 85 75.4
18 70.3 70.3 70.9 73.7 65.4
15 58.6 57.7 58.6 60.9 54.1
12 45.6 44.1 45.2 47 41.7
9 31.6 30.1 31.0 32.3 28.6
6 17.6 16.5 17.1 17.8 15.8
3 5.6 5.2 5.4 5.7 5
Figure 4. Displacement in X-Direction
3.1.2 DISPLACEMENT IN Y-DIRECTION
Table 2: Storey Displacement in Y-Direction
Heig
ht
of the
Build
ing
(m)
Model I
Displac
ement
(mm)
Model
II
Displac
ement
(mm)
Model
III
Displac
ement
(mm)
Model
IV
Displac
ement
(mm)
Model
V
Displac
ement
(mm)
33 128.1 133.2 130.3 131 120.5
30 124.6 129.1 126.8 126.9 117.2
27 119 122.7 120.9 120.8 111.7
24 111.3 113.9 112.8 112.3 104.2
21 101.6 102.9 102.6 101.9 94.7
18 90.2 90 90.5 89.8 83.5
15 77.1 75.6 76.7 76.1 70.8
12 62.2 60 61.3 60.8 56.6
9 45.6 43.3 44.5 44.2 41.1
6 27.6 25.9 26.8 26.6 24.7
3 10.1 9.4 9.7 9.7 9
Figure 5. Displacement in Y-Direction
3.2 STOREY DRIFTS
Inter Storey drifts for differentmodelsareobtainedfrom the
analysis are shown in Table below. Inter Storeydriftsprofile
can also be observed in Figure.
According to IS 1893(Part1):2002clause7.11.1Storeydrifts
limitations are explained that the Storey drifts in any storey
due to the minimum specified design lateral force, with
partial load factor of 1.0 shall not exceed 0.004,(0.004h)
times the storey height.

3.2.1 STOREY DRIFT IN X – DIRECTION
Table 3: Storey Drift in X-Direction
Height
of the
Building
(m)
Model
I
Drift
Model
II
Drift
Model
III
Drift
Model
IV
Drift
Model
V
Drift
33 0.0018 0.0020 0.0018 0.0020 0.0017
30 0.0024 0.0026 0.0025 0.0026 0.0023
27 0.0030 0.0032 0.0031 0.0032 0.0028
24 0.0035 0.0037 0.0036 0.0037 0.0033
21 0.0039 0.0041 0.0040 0.0041 0.0037
18 0.0042 0.0044 0.0043 0.0045 0.0040
15 0.0045 0.0046 0.0046 0.0048 0.0042
12 0.0047 0.0047 0.0047 0.0049 0.0044
9 0.0047 0.0045 0.0046 0.0048 0.0042
6 0.0039 0.0037 0.0039 0.00405 0.0035
3 0.0018 0.00175 0.00182 0.0019 0.00168
Figure 6. Storey Drift in X-Direction
3.2.2 STOREY DRIFT IN Y – DIRECTION
Table 4: Storey Drift in Y-Direction
Height
of the
Building
(m)
Model
I
Drift
Model
II
Drift
Model
III
Drift
Model
IV
Drift
Model
V
Drift
33 0.0016 0.0019 0.0016 0.0019 0.0016
30 0.0026 0.00289 0.00271 0.00284 0.00247
27 0.0033 0.00364 0.00346 0.00357 0.00319
24 0.0039 0.00424 0.00408 0.00412 0.00376
21 0.0044 0.00476 0.00458 0.0046 0.00423
18 0.0048 0.00516 0.00506 0.00502 0.00465
15 0.0053 0.00547 0.00544 0.00541 0.00501
12 0.0057 0.00571 0.00578 0.00573 0.00532
9 0.0060 0.00585 0.00598 0.00594 0.0055
6 0.0058 0.0055 0.0057 0.0056 0.0052
3 0.00337 0.00312 0.00325 0.00323 0.00299
Figure 7. Storey Drift in Y-Direction

4.3 STOREY SHEAR
1) STOREY SHEAR IN X – DIRECTION
Table 5: Storey Shear in X-Direction
Height
of the
Buildin
g
(m)
Model
I
Shear
(KN)
Model
II
Shear
(KN)
Model
III
Shear
(KN)
Model
IV
Shear
(KN)
Model
V
Shear
(KN)
33 304.27 371.75 277.01 397.60 292.18
30 608.56 678.71 652.05 641.05 580.16
27 798.17 849.04 812.71 839.80 756.65
24 915.16 958.72 947.27 933.42 865.79
21 1009.7 1069.0 1031.0 1032.0 955.68
18 1094.7 1143.6 1142.5 1108.5 1040.0
15 1199.6 1207.9 1213.8 1205.5 1118.9
12 1315.1 1283.1 1321.7 1284.8 1208.3
9 1458.2 1375.0 1418.9 1417.5 1314.5
6 1586.7 1452.5 1536.0 1501.1 1404.0
3 1640.5 1483.5 1571.5 1547.3 1440.1
Figure 8. Storey Shear in X-Direction
2) STOREY SHEAR IN Y – DIRECTION
Table 6: Storey Shear in Y-Direction
Height
of the
Buildi
ng
(m)
Model
I
Shear
(KN)
Model
II
Shear
(KN)
Model
III
Shear
(KN)
Model
IV
Shear
(KN)
Model
V
Shear
(KN)
33 266.28
1
326.69
7
239.08
9
352.82
3
251.24
9
30 499.54
4
557.03
5
533.12
4
532.41
5
469.55
1
27 634.76
4
690.17
4
648.91
4
681.98
3
601.63
4
24 737.24
1
793.41
9
770.23
9
767.08 701.61
8
21 825.50
8
892.39
8
847.56
4
857.16
7
785.91
9
18 908.77
6
960.67
2
952.23
9
932.00
1
866.81
5
15 1009.8
9
1016.4
2
1016.9
5
1018.9
6
936.74
12 1100.5
4
1071.8 1097.5
6
1077.7
6
1003.7
2
9 1188.4
9
1130.4 1158.2
2
1160.4
4
1072.0
2
6 1287.6
5
1197.2
7
1252.7
2
1230.6
2
1145.1
2
3 1356.9
3
1238.6
1
1298.7 1293.1
7
1190.9
8
Figure 9. Storey Shear in Y-Direction

3. CONCLUSIONS
RCC structures with irregular masses, different
stiffness and irregular vertical geometry are been
studied and analyzed in this project. The analysis for
five different models has been carriedoutandvarious
results are obtained. The different parameters are
studied in detail for each building, such parameters
include displacement, storey drift, Storey shear. After
the study we come up with following conclusions,
 When the models are imposed with loads they
tend to displace. The displacements are different for each
storey‟s and each model. Model II possess more
displacements compare to Model I in both axes. The model
provided with irregular masses for odd floors in Model IV
shows more displacements whereas regular models have
minimum displacements.
Storey drifts vary with the floors irregularlythedriftvalue is
more for the models where thicker slabs are provided for
odd floors and it can be seen that minimum storey drifts
occur in regular building in both axes.
 Shear forces occur more in model I compare to
model II and model IV,irregularmodelshavemoreshear
values compared to regular model and the regular
model have minimum shear value in both the axes.
 Considering all the parameters, regular building
exhibit better performance with lesser failure values
than the mass irregular models.
 Among the mass irregular models the models
provided with thicker slabs at odd floors that is model
IV finds to be more inefficient and the buildings
provided with thicker slabs for top five floors that is
model II scores out as the efficient one among irregular
buildings.
REFERENCES
1. A.D‟Ambrisi,M.DeStefano,S.Viti “Seismic
Performance of irregular 3D RC Frames”The 14th
world conference on Earthquake Engineering
October12-17,2008,Beijing,China.
2. PankajAgrawal and Manish
Shrikhande,Earthquake and vibration effect on
Structure:
Basic element of Earthquake Resistant Design,
„Earthquake resistant design of structures, PHI
Learning private limited, New Delhi
3. Andreas.J.Kappos,GeorgiosPanagopoulos(2004)
“Performance-based seismic design of
3D R/C buildings using static and dynamic analysis
procedures”,ISET journal of Earthquake
technology,paper no.444,vol.41,no.1,pp.141-158.
4. ATC-40(1996)”SeismicEvaluationandRetrofitof
concrete Buildings”,Applied Technology
council,Seismic safety commission, Redwood
city,California,volume 1&2.
5. VinodK.Sadashiva ,Gregory A.MacRae& Bruce
L.Deam.”Determination of structural
Irregularity Limits-Mass irregularity
Example”Bulletin of the New Zealand Society for
Earthquake Engineering,Vol.42 No.4 December
2009.
6. FarzadNaeimPh.D, “Performance Based seismic
Engineering”,The seismic design Handbook,
Research and development Los
Angeles,California,pg.no. 757-792.
7. IS 1983-2002(Part-1),”criteria for Earthquake
resistant design of structures”,General provisions
and buildings,Bureau of Indian Standards,New
Delhi.
8. IS 456:2000,”Plain and Reinforced concrete”-
Code of practice, Bureau of Indian Standards,New
Delhi.
BIOGRAPHIES
1. Chethan B N is presently
working as Structural Engineer,
Rudraprasad Consultants,
Bangalore, Karnataka, He
obtained his M.Tech degree in
Structural Engineering from VTU.
2. Sanjay S J is presently working
as assistant professor,Dept. ofCivil
Engineering at PESITM,
Shivamogga, Karnataka. He
obtained his M.Tech degree in
Computer Aided Design of
Structures from VTU. His areas of
research interest include Fiber
reinforced concrete and Cement
mortar.
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