Effect of foundation flexibility on dynamic behaviour of asymmetric building frame with isolated footing

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 04 Issue: 06 | June-2015, Available @ http://www.ijret.org 41
EFFECT OF FOUNDATION FLEXIBILITY ON DYNAMIC
BEHAVIOUR OF ASYMMETRIC BUILDING FRAME WITH
ISOLATED FOOTING
Praveena E1
, G Narayana2
1
Post graduate student, Dept. of civil Engineering, SJCIT, Chickaballapur, Karnataka, India
2
Professor& Head of the department, Dept. of civil engineering, SJCIT, Chickaballapur, Karnataka, India
Abstract
In general the seismic design of building frame structures the designers will consider only the results of fixed base condition the
effect of flexibility is ignored. In post-earthquake study the framed structure reveals that the interaction of soil and foundation
plays an important role in damage of the building frame structures. In this regard a literature survey has been done on frame
structures supported on various foundations such as isolated, combined, raft & pile foundations. To examine the literature revels
the few investigations were done on asymmetric building frame structure is supported on isolated footing. So in this paper is an
attempt to the study of dynamic behavior of asymmetric building frame structure is supported on isolated footings. The modeling
and analysis is done by using “finite element method software” SAP2000 VERSION 14, by considering the different soil
conditions, (soft, medium, hard) different soil parameters (passion’s ratio, young’s modulus, dynamic shear modulus) different
height ratio’s, different span ratio’s & fixed base conditions. The response of the building frame structure is obtained in terms of
fundamental natural period, lateral displacement and seismic base shear.
Keywords: Soil structure interaction, Fundamental natural period, Base shear, Lateral displacement….
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1. INTRODUCTION
Soil structure interaction is a most important criterion for
design of building frame structure. Most of the civil
engineering structure involves some type of structural
elements are directly contact with ground, when the external
forces, such as earthquake, vibrations, wind, acts on this
system neither the ground displacement or nor the structural
displacement are independent of each other. The process in
which the response of the soil influences the motion of the
structure and the motion of the structure will influences the
response of soil is known as “soil structure interaction”. In
common design practice for dynamic loading assumes the
asymmetric building frame structure is to be fixed at their
bases. When the load coming from the structure is transfer to
the footings then transfer to the hard soil strata, this
assumption is applicable only when super structure is more
flexible then the hard soil strata, then the reversal is true,
when the response of the super structure will also influenced
moment in soil to some extent.
2. MODELING AND ANALYSIS METHODS
The modeling is done by using FEM software SAP2000
version14, in interaction analysis super structure is
considered as 3D space frame including columns and beams
in each storey slab at each floor levels. Isolated footing and
soil is modeled as a single unit, the soil is considered as a
homogeneous, isotropic and elastic half space medium with
different input data, such as, dynamic shear modulus and
Poisson’s ratio. The soil medium beneath the isolated footing
as modeled as an eight nodded solid element having a three
degree of freedom of translation each in the respective co-
ordinates at each node, the finite element mesh is generated
at the aspect ratio is in between 1.25 to 1.5.when the plan and
depth of the soil is considered as per BOUSSINESQ’S
equation, the plan of the soil is taken as a 1.5 to 2.0times of
the width of the footing and depth of the soil is taken as
2.5times of the width of the footing. The flexible floor
diaphragm for each slab is modeled as thin shell with four
noded plate element with four degree of freedom system at
each node such as three translational and three rotations in
their respective co-ordinates. The dynamic response spectrum
analysis is done as per IS1893:2000 when the structure is
modeled as fixed and flexible base conditions using SAP V14
software package.
2.1 Soil Parameter and Building Design Input Data
for the Present Study with Isolated Footing
According to Bowles J E and Based on dynamic shear
modulus soil is classified as hard, medium, soft, by using
various parameters as shown below.
Table-1: Soil parameters
Sl
no
Type of soil Shear
modulus
(G) KN/m2
Elastic
modulus
(E)KN/m2
Poisson
’s ratio
(µ)
1 Hard(Type-1) 30,000 72,000 0.2
2 Medium(Typ
e-2)
20,000 50,000 0.25
3 Soft(Type-3) 10,000 26,000 0.3

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The members properties are Material properties are
Column size: 350mmX500mm Grade of concrete: M25
Beam size: 250mmX600mm, Young’s modulus: 25X106
KN/m2
Thickens of slab: 150mm, Density of concrete: 25 KN/m3
Earthquake live load on slab, Density of brick masonry: 20
KN/m3
Roof: 0.25X1.5=0.375 KN/m2
Load intensities are
Floor: 0.25X3.0=0.75 KN/m2
Floor dead: 1.0 KN/m2
Seismic data Roof dead: 2.0 KN/m2
Seismic Zone: V Floor live: 3.0 KN/m2
Zone factor: 1 Roof live: 1.5 KN/m2
Response Reduction factor: 5 Critical damping: 0.05
Earthquake loads are consider as per IS 1893:2000(part 1)
clause7.3.1, clause7.3.2 and response spectrum data are
consider in IS 1893:2000(part 1). For the present study Two
bay Two bay six, eight, ten storied moment resting frame
with uniform span ratio and height ratio is 1.0 (Height ratio is
defined as the ratio of base storey height to upper storey
height), and then considered with and without soil flexibility
and also analysis is done by considering fixed base and
flexible (soil continuum) base conditions.
Fig-1: 2X2 10 Storey with Fixed Base Condition
Fig-2: 2x2 10 Storey with Considering Soil Continuum
3. RESULTS AND DISCUSSION
The obtained results are Fundamental natural period, Base
shear and maximum lateral displacements are comparing
Interaction Analysis (IA) and Non-Interaction Analysis
(NIA)
3.1 Fundamental Natural Period (Sec):
Fundamental natural period variation are considered with
shear modulus of three type of soil for zone V and by
considering Sr=1.0 and Hr =1.0 by increasing the number of
stories. From table 2: The compression is made by Non-
interaction and interaction values of the fundamental natural
period of interaction values are less than the Non-interaction
values. From the fig 4: Fundamental natural period is
increases by increasing the number of stories.
Table-2: Fundamental natural period (sec)
Description Base Shear Non –
Intera
ction
S
l
Frame
Type
IS1893
:2002
Soft Medi
um
hard
1 2X2 6
STOREY
Zone V 0.018 0.014 0.010 0.020
2 2X2 8
STOREY
Zone V 0.020 0.016 0.011 0.027
3 2X2 10
STOREY
Zone V 0.023 0.018 0.013 0.036

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Fig-3: Variation of Fundamental natural period with Shear
Modulus
Fig-4: Variation of Fundamental natural period with storey
variation for different type of soil and fixed base condition
3.2 Base Shear
Base shear variation are considered with shear modulus of
three type of soil for zone V and by considering Sr=1.0 and
Hr =1.0 by increasing the number of stories from table 3:
base shear is decreases by increasing the stiffness of the soil
and storey height increases by decreasing the base shear.
From Fig 6: Base shear is increases by increasing the number
of stories.
Table-3: Base shear (KN)
Description Base Shear Non -
Interac
tion
Sl Frame
Type
IS189
3:200
2
Soft Mediu
m
hard
1 2X2 6
STOREY
Zone
V
1084.4 906.3 674.
7
674.5
2 2X2 8
STOREY
Zone
V
1271.9 1060.
3
790.
6
668.9
3 2X2 10
STOREY
Zone
V
1407.7 1175.
8
860.
5
659.1
Fig-5: Variation of Base shear with Shear Modulus
Fig-6: Variation of Base shear with storey variation for
different type of soil and fixed base condition
3.3 Maximum Lateral Displacement
Maximum lateral displacement variations are considered with
shear modulus of three type of soil for zone V and by
considering Sr=1.0 and Hr =1.0 by increasing the number of
stories from table 4: Base shear is decreases by increasing the
stiffness of the soil, the maximum value will show in fixed
base condition. From fig 8: Maximum lateral displacement is
increases by increasing the number of stories.
Table-4: Maximum lateral displacement (mm)
Description Base Shear Non -
Inter
actio
n
S
l
Frame
Type
IS189
3:200
2
Soft Medi
um
Hard
1 2X2 6
STOREY
Zone
V
0.018 0.014 0.010 0.020
2 2X2 8
STOREY
Zone
V
0.020 0.016 0.011 0.027
3 2X2 10
STOREY
Zone
V
0.023 0.018 0.013 0.036

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Fig-7: Variation of Maximum lateral displacement with
Shear Modulus
Fig-8: Variation of Maximum lateral displacement with
storey variation for different type of soil and fixed base
condition
4. CONCLUSION
In present the effect of soil flexibility on dynamic behavior of
asymmetric building frame resting on isolated footing, such
as fundamental natural period, base shear and maximum
lateral displacement. First number of stories increases by
increasing the fundamental natural period and the non-
interaction analysis values are slightly lesser then its
interaction analysis values. Second number of stories
increases by increasing the base shear values and the non-
interaction analysis values less as compared to interaction
analysis values. Third also number of stories increases by
increasing the displacement values and the maximum lateral
displacement is obtained in non-interaction analysis.
ACKNOWLEDGEMENTS
My humble and heart-felt thanks to my guide Dr. G Narayana
professor and head of the department. Department of civil
engineering, S J C Institute of Technology. Chickaballapur.
Thanks to my dear friends who have supported me to
complete this work.
REFERNCES
[1]. Dr. G Narayana, Dr. H Sharada Bai, & Mrs. M P Jyothi
“Dynamic soil structure interaction behavior of building
frame with raft foundation”
[2]. Vivek Garg & M S Hora “Interaction effect of space
frame-strap footing-soil system on forces in super structure”
vol.7, No. 11, November 2012.
[3]. Pandey A.D, Prabhat Kumar & Sharad Sharma “seismic
soil- structure interaction of building on hill slope” Volume
2, No 2, 2011.
[4]. Vivek Garg & M S Hora “A review on interaction
behavior of structure-foundation-soil system” Vol.2, Issue 6,
November-December 2012.
[5]. IS1893-2002(Part-1), Criteria for Earthquake Resistance
Design of structures, Bureau of Indian Standard, New Delhi.
[6]. IS:456:2000, “Code of Practice for Plain & Reinforced
Concrete” Bureau of Indian Standard, New Delhi, India
[7]. Bowles J E. “Foundation Analysis and Design” 5th
Edition, McGraw-Hill International Editions, Civil
Engineering Series, New York, 1996
BIOGRAPHIES
I am Praveena E in pushing my master of
technology in structural engineering from
S J C Institute of technology,
Chickaballapur. I have one year
experience as a site engineer. I am doing
my dissertation work under the guidance
of Dr. G Narayana. I have pushed my B E, civil engineering
from S L N college of engineering, Raichur.
He Mr. Dr. G Narayana, Professor and
Head of Department, Department of civil
Engineering. S J C Institute of
Technology, Chickaballapur. He has wide
experience in teaching & research in the
field of structural engineering, He also a
structural designer consultant for many project.

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