SEISMIC PERFORMANCE OF SETBACK RC BUILDINGS WITH SOIL STRUCTURE INTERACTION

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 09 Issue: 03 | Mar 2022 www.irjet.net p-ISSN: 2395-0072
© 2022, IRJET | Impact Factor value: 7.529 | ISO 9001:2008 Certified Journal | Page 1633
SEISMIC PERFORMANCE OF SETBACK RC BUILDINGS WITH SOIL
STRUCTURE INTERACTION
RASHMI J1, Prof SHIVASHANKAR K M2
1PG Student, Dept. of Civil Engineering, ACS College of Engineering, Bangalore
2Professor, Dept. of Civil Engineering, ACS College of Engineering, Bangalore
----------------------------------------------------------------------***---------------------------------------------------------------------
Abstract - In high rise framed structures, harm from
seismic activity ground motion generally initiates at
locationsofstructuralweaknessespresentinthelateralload
resisting frames .The performance of high rise framed
structures during strong seismic activity depends on the
factors including distribution of mass, stiffness, and
strength among the horizontal and vertical planes of
structures. In certain cases, these weaknesses may arise
due to discontinuity in stiffness, mass among adjacent
storeys. Such discontinuity among stories are frequently
associated with abrupt variations in the frame geometry
alongside the height of structure. A common variety of
verticalgeometricalirregularityinbuildingstructuresarises
is the presence of setbacks, i.e. the presence of abrupt
decrease of the lateral measurement of the structure at
specific levels of the elevation. This structure group is
recognised as ‘setback building’. This structure form is
attractive and more popular in current high rise structure
building mostly since of its functional and artistic
architecture. Stepped structure is the one with vertical
geometric irregularity, where the horizontal dimension of
the lateral force resisting system in any storey is extra than
150% of that in adjacent storey. These structural
irregularity isn’t acceptable from stability point of view, as
seismic activity has proven to affect the structure in case of
seismic activity. All the buildings during seismic activity is
proven to be susceptible but the structures with softstorey
configuration are being found to be mostly susceptible
during seismic activity. Shortage of plain ground in
mountainous regions and urge of mining, lead us to build
asymmetrical constructions in the hills. Thus, the danger
factoroftheseasymmetrical buildingsrisessharplyaseven
the base of the buildings becomes inclined at slope. This
deadly combination of geometric irregularity, mass
irregularity, stiffness irregularity makes the buildings too
much weak to last during seismic activity. Hence, it is
important to study the responses of such buildingstomake
suchbuildingsseismic-resistantandavoidtheirdownfallto
savethe damage of life and property.
Key Words: Stiffness,lateral,setbacks,irregularity etc...
1.INTRODUCTION
A building having difference among the center of mass
and center of resistance is named as irregular building.
Irregularity among the buildings are classified as
1. Stiffness irregularity
2. Mass irregularity
3. Vertical geometric irregularity
4. In plane discontinuity in vertical elements
resisting lateral force
5. Discontinuity in capacity
Stiffness Irregularity- It is one in which the lateral
stiffness is less than 70 percent of that in the storey above.
Mass Irregularity- It is considered when the seismic
weight of the story is extra than 200 percent of that of its
contiguous storeys.
Vertical Irregularity- A structure is said to be vertically
irregular when the horizontal dimension of lateral force
resisting system in the story is other than 150 percent of
that in its adjacent storey.
In plane discontinuity- The in plane discontinuity is
considered to exist in the primary element of lateral force
resisting system whenever a lateral force resistance
element is existing in one story but does not continue
Discontinuity in capacity- Discontinuity in capacity is
consideredwhenthestoryshearstrength is fewer than the
story above.
1.1 Literature review
Chandler and Mendis (2000), studied the force based
seismic design system and also the displacement based
seismic assessment methodology. They also presented a
case study forreinforcedconcretemomentresistingframes
according to European and Australian code provisions
having low, medium and high ductility capacity. They used
Elcentro NS earthquake ground motion as the seismic
participation to get the performance features.
Roy and Chandrasekaran (2006): The authors
analysed 10storeyR C framed structure from dynamic and
static analysis. They have reported that base shear, storey
shear and storey drifts was increased as the height of
structure increased. They have reported that predominant
of hinge formation in pushover analysis.

Poursharifi and Yasrebinia (2012): The authors
analysed unsymmetrical concrete structures using
pushover analysis and time history analysis for dissimilar
stories (4, 6 and 8) with fixedbase. They havereportedthat
both pushover analysis and time history analysis had
extreme lateral storey displacement and maximum base
shear and pushover analysis was very sensitive for the
given loading. The researchersalsostatedthatthedirection
of loading plays an significant role for determining the
critical condition of the buildings.
Literature outcome
From the above literature review, we can see many
researches have analysed for different configurations such
as set-back buildings, from the study done it is observed
that setback building performs better under seismic loads.
Also it is observed that pushover results were accurate
enough for design applications. And the outcome of soil
conditions also affects the structure majorly.
1.2 Objectives of the study
In the present study,Pushoveranalysishasbeencarried
out on the 23 models consistingofsetbackbuildingwiththe
different foundation properties.
1. To analyze the nonlinear static behavior of the
vertical irregular structures.
2. To study the seismic performance of set-back
building on varying soil conditions by linear static
method.
3. To learn the outcome of number of bays and height
of the building which is resting on different soil
conditions.
2. Modelling and Analysis
In the present thesis, 92 models of setback and step back
were modelled and analysed for the different foundation
conditions using ETABS
Table 3.1: The SET BACK models considered for the
analysis
Sl No Model Foundation Properties Description of the
Model
1 R-6 Fixed Foundation 6 storey building –
no bays were
removed in any
floor
Hard soil
Medium soil
Soft soil
2 R-8 Fixed Foundation 8 storey building –
no bays were
removed in any
floor
Hard soil
Medium soil
Soft soil
3 R-10 Fixed Foundation 10 storey building
– no bays were
removed in any
floor
Hard soil
Medium soil
Soft soil
– no bays were
removed in any
floor
Hard soil
Medium soil
Soft soil
– no bays were
removed in any
floor
Hard soil
Medium soil
Soft soil
– no bays were
removed in any
floor
Hard soil
Medium soil
Soft soil
7 S1-6 Fixed Foundation 6 storey building –
one bay on 4th, two
bays on 5th floor
and three bays on
the 6th floor has
been removed
Hard soil
Medium soil
Soft soil

Fig 3.1: Model of R-6
8 S1-8 Fixed
Foundation
8 storey building – one bay
on 6th, two bays on 7th floor
and three bays on the 8th
floor has been removed
Hard soil
Medium soil
Soft soil
9 S1-10 Fixed
Foundation
10 storey building – one
bay on 8th, two bays on 9th
floor and threebays on the
10th floor has been
removed
Hard soil
Medium soil
Soft soil
10 S1-12 Fixed
Foundation
bay on 10th, two bays on
11th floor and threebayson
the 12th floor has been
removed
Hard soil
Medium soil
Soft soil
11 S1-15 Fixed
Foundation
removed
Hard soil
Medium soil
Soft soil
12 S1-18 Fixed
Foundation
removed
Hard soil
Medium soil
Soft soil
13 S2-6 Fixed
Foundation
on 1st and 2nd, two bays on
3rd and 4th floor and three
bays on the 5th and 6th floor
has been removed
Hard soil
Medium soil
Soft soil
14 S2-8 Fixed
Foundation
on 3rd and 4th, two bays on
5th and 6th floor and three
bays on the 7th and 8th floor
has been removed
Hard soil
Medium soil
Soft soil
15 S2-10 Fixed
Foundation
bay on 5th and6th,twobays
on 7th and 8th floor and
three bays on the 9th and
10th floor has been
removed
Hard soil
Medium soil
Soft soil
16 S2-
12
Fixed
Foundatio
n
bay on 7th and 8th, two
bays on 9th and 10th floor
and three bays on the
11th and 12th floor has
been removed
Hard soil
Medium
soil
Soft soil
17 S2-
15
Fixed
Foundatio
n
bay on 10rh and 11th, two
bays on 12th and 13th
floor and three bays on
the 14th and 15th floor
has been removed
Hard soil
Medium
soil
Soft soil
18 S2-
18
Fixed
Foundatio
n
bay on 13th and 14th, two
bays on 15th and 16th
floor and three bays on
the 17th and 18th floor
has been removed
Hard soil
Medium
soil
Soft soil
19 S3-
8
Fixed
Foundatio
n
bay on 1st and 2nd, two
bays on 3rd and 4th and
5th floor and three bays
on the 6th and 7th and 8th
floor has been
removed
Hard soil
Medium
soil
Soft soil
20 S3-
10
Fixed
Foundatio
n
bay on 2nd and 3rd and 4th
, two bays on 5th and 6th
and 7th floor and three
bays on the 8th and 9th
and 10th floorhas
been removed
Hard soil
Medium
soil
Soft soil
21 S3-
12
Fixed
Foundatio
n
bay on 4th and 5th and 6th
, two bays on 7th and 8th
and 12th floor has
been removed
Hard soil
Medium
soil
Soft soil
22 S3-
15
Fixed
Foundatio
n
bay on 7th and8thand9th ,
two bays on 10thand 11th
and 15th floor has been
removed
Hard soil
Medium
soil
Soft soil

Fig 2.3: Model of S
Fig 2.4: Model of S2-8

Fig 3.23: Model of S3-18
Modelling and Analysis of the Structure
The stages involved in the geometric modelling and
analysing of the structure
1. Developing a Geometrical model
2. Define and Assigning the Material property and
Sectional property
3. Define and Assigning the Foundation properties
4. Define Response properties
5. Define the Load patterns
6. Define Load cases
7. Run the Analysis
Creating a geometrical model
A building of 6, 8,10,12,15 and 8 stories have been
modelled with the plan dimension of 24mx24m with a bay
width of 6m on either side of the model. The height of each
floor is 3m.
Defining and Assigning the material property and
sectional property
Table 3.2: Materials properties and dimension
considered for modellingDefine the Load patterns
Define the Load patterns
The dead load, live load, wall load and
earthquake loads are defined. The
earthquake loads are defined by the code
provision of IS 1893. The lateral load is auto
generated in the software itself.
Live loads:
Imposed loads are those loads whose
position can be change from one position to
another position. Imposed loads can
generally be taken from the codeIS875(part
2).
Table 3.5 Live loads on the structure
Grade of concrete M
2
5
Grade of steel for main
reinforcement
Fe 415
Grade of steel for transverse
reinforcement
Fe 250
Column dimension 450mmx50
0mm
Beam dimension 450mmx50
0mm
Slab thickness 125mm
Covers – Beam
– Column
25mm
40mm
Level Live
loads
(KN/m2)
Roof
level
1.5
Floor
level
3

ead loads:
Deadloads are thestationaryor permanentloadswhich
are due to its own weight of members, which are going to
stay throughout the lifespan of the structure. This load
intensity depends on the type of the material, as earlier
mentioned it varies based on its density. As the density
increases, its self-weight also increases. Dead loads
generally be determined according to IS 875 (Part-1)
Wall Load
Wall load on the floors = 19.2 * (3 – 0.45) * 0.23
= (density) * (height) * (wall thickness)
= 11.26 kN-m
Parapet wall load on the roof = 19.2 * 1.5 * 0.23
= (density) * (height) * (wall thickness)
= 6.624 kN-m
Consider 25% of the live load and 100% contribution
for the other loads.
Analysis
The model has been analysedfor the given combination
and the results are obtained forstoreydisplacement,storey
drift, storey shear, overturning moments. The graph has
been drawnwith respect to available results.
3.Results and discussions
In the current thesis work, the outcomes were
evaluated for different models and the Displacement,
Drift, Shear, overturning moments are computed for the
setback buildings. These models were analysed for
differentflexiblefoundationconditionofzoneⅣasperI
S
1839- 2016 (part-Ⅰ).
Comparison of storey displacement for all the setback
models along X direction
Fig 3.1: Storey displacementof R-6 along X direction
Fig 3.2: Storey displacement of R-8 along X direction
Fig 3.3 story displacement of R-10 along x-direction

Comparison of storey displacement for all the
setback models along Y direction
Fig 3.4: Storey displacement of R-6 along Y direction
Comparison of storey drift for all the setback models
along X direction
Fig 3.7: Story Drift of R-6 along X direction

4. CONCLUSIONS
In the present analysis of setback buildings (92) models
were analysed and has beendiscussed in chapter3.Based on
the outcomes discussed from chapter 4, the subsequent
inferences are drawn.
1. The displacement is maximum when the soil is
soft (i.e. Soft soil). The displacement is maximum in
the top storey of the structure. The variation of
displacement on both the directionsofthestructure
is too small (3% to 4%) for all types of foundation,
this is due to the effect of variation of mass and
stiffness.
2. The storey drift is maximum when the soil is soft
(i.e. Soft soil). The storey drift is maximum at the 1st
storey of all the setback models except for the fixed
foundation condition.Thevariationofdisplacement
on both the directions of the structure is too small
(3% to 4%) for all types of foundation, this is due to
the effect of flexibility of soil.
3. The storey shear is maximum for the fixed
foundation condition. The story shear is maximum
at 1st story for all conditions. Variation of
displacementonboththedirections of the structure
is too small (3% to 4%) for all types of foundation,
this is due to the effect of excessive force generated
in some of the members.
4. The overturning momentsis maximumforall the
fixed foundation condition. These moments are
maximum at the base of the structure for all the
foundation conditions. variationofdisplacementon
both the directions of the structure is too small (3%
to 4%) for all types of foundation, this is due to the
effect of irregularity.
5. The Pushover Curve is maximum in Base Shear
for all the fixed foundation condition. Variation of
Base Shear on both the directions of the structureis
too small (3% to 4%) for all types of foundation,
this is due to the effect of irregularity.
6. The time period is maximum when there is soft
soil compared to fixed support this is becauseofthe
property of soil which changes resulting in
maximum time period.
7. Themass is maximum for regular building this is
because the mass is distributed along the members
of frame.
Scope of future work
1. The study may be continued for time history
analysis.
2. Thestudymaybecarriedoutfordifferentzones
of earthquake and different soil conditions.
3. Pushover analysis on retrofitted structures for
different irregularities and soil conditions can
be studied.
References
1. Bahram M. Shahrooz and Jack P. Moehle (1990),
“Seismic Response and Design of Setback Buildings”,
Journal of Structural Engineering, ASCE, Vol-116, pp
1423-1439.
2. Chandler, A.M. andMendis,P.A.(2000).“Performance
of Reinforced Concrete Frames Using Force and
displacement Based Seismic Assessment Methods”.
Engineering Structures. Elsevier 22, 352-363.176
3. Martino.R, Spacone.E, Kingsley.G(2000) “Nonlinear
Pushover Analysis of RC Structures” Earthquake
Engineering 34, DOI 10.1007/978-3-319-07118
4. Kraavasilis T. L, Bazeoes N and Beskos D. E (2007),
“Seismic Response of Plane Steel MRF WithSetbacks:
Estimation of Inelastic Deformation Demands”,
Journal of constructional steel research,
ScienceDirect, pp 644-654.
5. Athanassiadou, C.J. (2008) Seismic performance of
R/C plane frames irregular in elevation. Engineering
structures. Elsevier 30, 1250-1261.
6. Pradip Sarkar, A. Meher Prasad and Devdas Menon
(2010). “Vertical Geometric Irregularity in Stepped
Building Frames”, Engineering structures, Elsevier,
pp2175-2182.
7. Pandey A.D, Prabhat Kumar and Sharad Kumar
(2011), “Seismic Soil Structure Interaction of
Buildings onHill Slopes”, Internationaljournalofcivil
and structural engineering, Vol- 2, pp 544-555.
8. Pradip Sarkar, Devdas Menon, and A. Meher Prasad
(2012), “Seismic Evaluation of RC Stepped Building
Frames”, DOI 10.1007/978-81-322-0757-3_82, #
Springer India 2013
9. Sehgal.V.K and Ankush Mehta (2014) “Pushover
Analysis of Symmetric and Asymmetric Reinforced
Concrete Buildings” Advances in Structural
Engineering, DOI 10.1007/978- 81-322-2187-6167
10. Yang Liu and Kuang.J.S (2017) “Spectrum-based
pushover analysis for estimating seismic demand of
tall buildings” Bull Earthquake Eng DOI
10.1007/s10518-017-0132-8

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SEISMIC PERFORMANCE OF SETBACK RC BUILDINGS WITH SOIL STRUCTURE INTERACTION