50th indian geotechnical conference thigs/ldh/files/igc 2015 pune/theme 14... · on isolated...

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INDIAN GEOTECHNICAL CONFERENCE

17th

– 19th

DECEMBER 2015, Pune, Maharashtra, India

Venue: College of Engineering (Estd. 1854), Pune, India

Soil Structure Interaction for RCC Framed Structure - A Case Study

Dr. G Ravi

1, Dr. H S Prasanna

2, Vinay M L Gowda

3

ABSTRACT

This paper describes some aspects and applications of Soil-Structure Interaction (SSI) approach in

geotechnical engineering. Soil-structure interaction is an interdisciplinary field which involves structural

and geotechnical engineering. The focus of this study is on potential effects of SSI on framed structure

with shallow foundation resting on clayey soils. The main advantage of SSI approach is to combine the

principles of soil mechanics and structural analysis to arrive at acceptable and viable solution.

In defining characteristics parameters which control SSI, the effect of gravity loads action on structure is

often neglected. In fact SSI is more important in tall but relatively rigid structures founded on soft soils,

where as gravity effect becomes more pronounced in flexible structures. The seismic effects are signified

by huge amount of energy release which affects the structure. Hence the behavior of the structure is

surely affected by seismic activity. Two aspects of foundation response are considered. Firstly the effect

of SSI on shallow foundation is investigated for gravity loads only and then the effect of SSI considering

seismic effect on the same soil profile is investigated and compared with non SSI models. Hence, an

effort is made to evaluate and compare SSI effect in both cases.

For this purpose an RCC framed structure is considered for further study. The frame is modeled and

analysed, employing Finite Element Method using ETABS software under two different boundary

conditions: (i) considering fixed base (rigid condition) and (ii) considering SSI (flexible condition). The

analysis is carried out changing the ratio of area of footing size. Foundation soil behavior is assumed to be

nonlinear while structures are assumed to behave in elastic range.

The study shows the effect of SSI with regard to structural behavior for gravity loads as well as seismic

loads. The inclusion of soil in the analysis provides results in the form of stresses and displacement

values, deformations, story drift which are realistic values than those provided by analysis of a fixed-base

structure. Therefore, considering SSI effects in seismic design of tall concrete building frames resting on

soft soil deposit is vital to have realistic analysis.

1Soil Structure Interaction for RCC Framed Structure – A Case Study_Dr. G Ravi, Professor, Civil Engineering Department,

NIE, Mysore, India, [email protected] 2Soil Structure Interaction for RCC Framed Structure – A Case Study_Dr. H S Prasanna, Professor, Civil Engineering

Department, NIE, Mysore, India, [email protected] 3 Soil Structure Interaction for RCC Framed Structure – A Case Study_,Vinay M L Gowda, PG Student, Civil Engineering

Department, NIE, Mysore, India, [email protected]

`

mailto:[email protected]



Dr. G Ravi, Dr. H S Prasanna & Vinay M L Gowda

Keywords: clayey soils, framed structure, finite element method, flexible, shallow foundation, Soil-

Structure Interaction

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Soil Structure Interaction for RCC framed structure - A case Study

1.Dr. G. Ravi, Professor, Department of Civil Engineering, The National Institute of Engineering, Mysore-

570008, Karnataka, India. E-mail: [email protected] 2.Dr. H.S.Prasanna, Professor, Department of Civil Engineering, The National Institute of Engineering,

Mysore-570008, Karnataka, India. E-mail: [email protected]

3.Vinay M L Gowda, PG Student, Department of Civil Engineering, The National Institute of Engineering,

Mysore-570008, Karnataka, India. E-mail: [email protected]

ABSTRACT: The effect of Soil Structure Interaction (SSI) may significantly affect the response of structure and

neglecting SSI in analysis may also lead to conservative design. In the conventional analysis the stress-strain

response of soil medium is not considered. Though the equilibrium equations are satisfied the compatibility is not

considered. In reality the soil, the foundation and the frame together act as a single unit. The effect of flexibility in

soil and non-linear response of soil is accounted through Winkler’s spring approach. The non-linear response of

soil is included in the analysis through multi linear isotropic (MISO) model. The effects of SSI are analysed for 10

storey RCC frame building when column resting on isolated foundation. Two methods of analysis are used to

evaluate foundation response, one by gravity loads and other by response spectra method. The inclusion of soil in

structural analysis provides values of stresses, displacements, base shear, story drift, natural period & frequency,

which are closer to the actual behaviour of the structure than those provided by the analysis of a fixed base

structure.

INTRODUCTION

Interest in the behavior of engineering systems

having several alternative scenarios indicates

importance of the use of a model as a common

tool in structural engineering. However, the

problem that usually arises is how to select the best

possible model from the pool of those available in

order to correctly estimate the design force

quantities.

In the conventional analysis of any civil

engineering structure the super structure is usually

analyzed by treating it as independent from

foundation and soil medium on an assumption that

no interaction takes place. This usually means that

by providing fixity at the support, Structural

analyst simplifies soil behaviour, while

Geotechnical Engineer neglects the structural

behavior by considering only the foundation while

designing [1].

When a structure is built on soil some of the

elements of the structure are in direct contact with

the soil. When the loads are applied on the

structure, internal forces are developed in both the

structure and as well as in soil. This results in

deformations of both the components (structure

and soil) which need to be compatible at the

interface as they cannot be independent of each

other [2]. Because of this mutual dependence,

which is termed as interaction, the stress resultants

in structure and, stresses and strains in soil are

significantly altered during the course of loading.

Therefore it becomes imperative to consider the

structure-foundation and soil as components of a

single system for analysis and design of the

structure and its foundation [3].

The analysis that treats structure-foundation-soil as

a single system is called as Soil Structure

Interaction (SSI) analysis.

The effect of soil flexibility is accounted through

consideration of springs of specified stiffness to

represent soil. The present study aims to

understand the effect of soil flexibility on various

parameters of idealized building frame.

METHODOLOGY

Symmetric RCC frame buildings of 2x6 bay with

10 storeys, with isolated footings resting on

different types of soil in layered soil stratum are

considered in the study. The frames are considered

with fixed base and with support on flexible base





represented by layered soil models. The frames are

analysed for both gravity loads and seismic loads

for static and dynamic analysis. The effect in SSI is

analysed by altering the ratio in area of footing.

The design response spectrum suggested by Bureau

of Indian Standards in IS 1893 (Part I): 2002 [4] is

used for dynamic analysis. The building is assumed

to be situated in Zone IV. Medium, Hard and

Sandy are three types of clay soil upon which

structural frames are considered to be resting. The

soil flexibility is incorporated in the analysis using

Winkler approach (spring model). Direct method

of analysis is considered and carried out. Different

combinations of dead load, imposed load and

seismic load as per IS1893 (Part I):2002 [4] are

considered and the critical among them is reported.

Properties of RCC frame

An idealised 2 bay X 6 bay building with 10 storey

on isolated footing building with brick masonry

wall has been considered. The height of each

storey is taken as 3.5m and the longitudinal and

transverse dimension of building is taken 9m &

3.5m respectively. The beams and columns are

modelled as 3D frame element. The element is

defined by two nodes with the input of the cross-

sectional area, and material properties [5]. The plan

of the structure is shown in Figure 1. The

geometric properties of frame and material

properties adopted in the analysis is presented in

Table 1.

Figure 1: Plan of RCC frame structure considered

Table 1 Geometry & Material properties of frame

sections

COMPONENT DESCRIPTION DATA

Frames No. of storeys 10

No. of bays in

X direction

6

No. of bays in

Y direction

2

Story height 3.5m

Bay width in

X direction

3.5m

Bay width in

Y direction

9m

Wall thickness 230mm

Size of beam 0.23m x

0.45m

Size of column 0.23m x

0.50m

Thickness of slab 125mm

Concrete M30

Grade

Weight per unit

volume 25 kN/𝑚3

Poisson’s ratio 0.2

Masonry Weight per unit

volume 20 kN/𝑚3

Poisson’s ratio 0.25

Soil & Foundation characteristics

The interaction between foundation and soil

depends on the elastic properties of soil and

foundation dimensions. The structure is analysed

having isolated footing resting on soil. The

flexibility of soil is usually modelled by inserting

springs between foundation member and soil

medium. The soil layer is assumed to have a

layered structure resting on rigid bed rock. In this

study foundations are considered to be resting on

three types of clayey soil namely Medium (M),

Hard (H) and Sandy (S). The properties of these

soils are shown in Table 2. A total 6 different soil

models are formulated as layered soil with a sub-

grade at bottom i.e., HHH, MHH, MSS, HSS,

MHS & HHS respectively. The allowable safe

bearing capacity of the clay soil is considered as

440 kN/m2. The values of SBC is considered

according to National Building Code of India [6].

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The foundation flexibility in the analysis is

considered by means of replacing the foundation

by statically equivalent springs. The effect of SSI

in the current study is considered using Winkler

spring. Winkler’s idealization represent soil

medium as system of identical but mutually

independent, closely spaced, discrete, elastic

springs. Since, soil can take only compression

during loading this is modelled as linear

compression support spring only. The winkler’s

springs are considered to be discrete, independent

and linearly elastic springs. The spring stiffness

constants are considered, according to equations

given by John Wolf [3,8].

The soil medium below the isolated footing is

idealized by 8 noded brick elements [5]. In order to

find the width of soil region to be used in study,

many trial analysis are carried out. The width of

soil below each isolated footing is calculated by

considering stress distribution transferred into soil

from footing. The depth of soil below footing is

assumed to be 1.5 to 2 times the width of footing.

Due to overlapping of soil stress distribution from

adjacent footing a single large uniform soil bed is

modelled below frame by extending its length on

either sides of frame plan by 2.5m. Soil size of

26m x 23m with two layers each of 2m & 3m

respectively is considered in analysis. The building

frame is placed centrally with a projection of 5m

on either side in plan. The soil elastic constants are

taken from literature and given in Table 2 [9].

Table 2: Properties of clay soil considered

Soil Type E (N/𝒎𝒎𝟐)

G (N/𝒎𝒎𝟐)

Poisson’s

Ratio

µ

Medium

Clay

50 18.8 0.33

Hard Clay 100 35.71 0.4

Sandy

Clay

250 86.21 0.45

The unit weight per volume of soil is assumed as

18.83 kN/𝑚3. Different types of soil profile models

are defined and considered for the SSI analysis. A

total of 6 different soil models are considered in

analysis by altering layers in soil profile by

Medium, Hard & Sandy of clay soil classification.

Soil models are defined as MHS, HHS, MSS, HSS,

MHH & HHH respectively.

The numerical values of spring constants for

different type of foundation soil for isolated

footing are summarized as in Table 3.

Table 3: Spring stiffness values of the clay soil

Type of

Soil

Kx

(kN/m)

Ky

(kN/m)

Kz

(kN/m)

Medium 52380 52380 63770

Hard 101796 101796 134315

Sandy 274666 274666 353737

The footings are defined at a depth of 1 m below

ground level. The dimensions of isolated footings

are grouped and summarized in Table 4.

Table 4: Dimensions of isolated footing

Footing group Area of footing (m2)

F1 2.8 x2.8 = 7.84

F2 3.1 x 3.1 = 9.61

F3 3.5 x 3.5 = 12.25

F4 3.8 x 3.8 = 14.44

The footing dimensions are defined from design

values calculated from reactions obtained at base

of structure. Groups F1 defines isolated footing at

nodes 1, 7, 15 & 21, the group F2 defines isolated

footing at nodes 2, 3, 4, 5, 6, 16, 17, 18, 19 & 20,

the group F3 defines isolated footing at nodes 8 &

14 and the group F4 defines isolated footing at

nodes 9, 10, 11, 12 & 13. The plan representing

node junctions for frame supporting isolated

footings are shown in Figure 2.


Figure 2: Plan showing node junctions for frame

supporting isolated footings

COMPUTATIONAL MODEL

The numerical models are generated using

computer program ETABS.

Figure 3: SSI model generated for gravity load and

seismic load analysis.

Type-I Model is generated to simulate soil-

structure interaction and includes the structure,

foundation and subsurface conditions. The

structure and foundations are modelled with finite

elements, while subsoil conditions are modelled

with springs. The idealised Type-I model is shown

in Figure 3.

Figure 4: Non-SSI model generated for gravity

load and seismic load analysis

Type-II Model is generated for comparison

purpose and considers a fixed base condition. The

models are also referred to as Non-SSI models.

The idealised Type II model is shown in Figure 4. RESULTS & DISCUSSION

Three primary factors control soil-structure

interactions: (i) soil geometry (geologic profile

geometry), (ii) soil material property, and (iii)

ground motion. Generally, the influence of these

three factors is complex. It is often difficult to

evaluate the effects of one factor on the response of

a given structure independent of the others. Hence,

analysis is done considering all the three factors.

Gravity load analysis and Seismic load analysis

using response spectrum method have been

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conducted on building frame with isolated footing

resting on two layered soil medium with subgrade.

The results of gravity load analysis for both SSI

and Non-SSI conditions are generated considering

loading combinations of 1.5 (DL+LL). The normal

stresses and vertical displacements at the

interaction layer between soil and structure are

tabulated and compared are presented in Table 5 &

Table 6.

In the seismic analysis, soil structure interaction

effect during earthquake shows significant changes

in the result on the design parameters of the

structure. The average maximum lateral deflections

(storey drift), natural period and frequencies for

SSI and Non-SSI conditions of structure are the

most important parameters considered and

determined.

Normal Stresses

Variation of normal stresses in Non-SSI models are

compared with SSI models. Stresses of SSI models

chosen and stresses of Non-SSI model are

tabulated in Table 5.

Table 5: Stress values from gravity loads at the

interface of soil and structure

Join

t

Non-SSI

(MPa)

MHH

(MPa)

HHH

(MPa)

HSS

(MPa)

1 0.446 0.88 0.9 0.83

2 0.434 0.34 0.34 0.39

3 0.437 0.86 0.85 0.82

4 0.439 0.39 0.4 0.41

5 0.437 0.86 0.85 0.82

6 0.434 0.34 0.31 0.39

7 0.446 0.88 0.9 0.83

8 0.435 0.33 0.35 0.33

9 0.438 0.16 0.16 0.17

10 0.444 0.31 0.33 0.32

11 0.445 0.37 0.37 0.39

12 0.444 0.31 0.33 0.32

13 0.438 0.16 0.16 0.17

14 0.435 0.33 0.35 0.33

15 0.446 0.88 0.9 0.83

16 0.434 0.34 0.34 0.39

17 0.437 0.86 0.85 0.82

18 0.437 0.39 0.4 0.41

19 0.437 0.86 0.85 0.82

20 0.434 0.34 0.34 0.39

21 0.446 0.88 0.9 0.83

It is seen that the values of stresses at the interface

of footing and soil medium is generally seen to be

lesser in SSI model when compared to Non-SSI

model. Further it is also seen that the values of

stresses are not affected by type of soil medium. At

the exterior alternate nodal interface of soil and

footing with node no.’s 1, 3, 5, 7, 15, 17, 19 & 21

the stresses in SSI model is observed to be greater

than Non-SSI model.

Vertical Displacements

Vertical displacements are observed at the

interaction layer between soil and structure.

Table 6: Vertical deflection values from gravity

loads at the interface of soil and footing

Joint MHH

(mm)

MHS

(mm)

HHS

(mm)

HSS

(mm)

1 -153.2 -144 -94.8 -72.5

2 -153.7 -144.5 -95.3 -73

3 -153.3 -144 -94.9 -72.6

4 -153.5 -144.2 -95.1 -72.8

5 -153.3 -144 -94.9 -72.6

6 -153.7 -144.5 -95.3 -73

7 -153.2 -144 -94.8 -72.5

8 -153.4 -144.1 -95 -72.7

9 -153.5 -144.3 -95.1 -72.8

10 -153.5 -144.2 -95.1 -72.8

11 -152.4 -143.1 -94.2 -71.7

12 -153.5 -144.2 -95.1 -72.8

13 -153.5 -144.3 -95.1 -72.8

14 -153.4 -144.1 -95 -72.7

15 -153.2 -144 -94.8 -72.5

16 -153.7 -144.5 -95.3 -73

17 -153.3 -144 -94.9 -72.6

18 -153.5 -144.2 -95.1 -72.8

19 -153.3 -144 -94.9 -72.6

20 -153.7 -144.5 -95.3 -73

21 -153.2 -144 -94.8 -72.5


The variation in displacements are compared in

SSI models. Displacements of SSI model with least

and maximum values and displacements of Non-

SSI model are tabulated in Table 6. The vertical

displacements in Non-SSI is completely zero due

to consideration of fixity. It is seen that the values

of deflections at the interface of soil and footing

are influenced by the type of soil medium. The

values of deflections are in the range of 72.5 mm to

153.7 mm. Further it is also noted that the

minimum deflection is observed in HSS model.

The deflections can be optimised to minimum

values by increasing the spring stiffness value,

which can be attained in practice by ground

improvement techniques.

Natural Time Period

The variation in Natural Time Period of structure

of fixed base and flexible base models are

presented in the analysis. Time period of SSI

chosen models with least and Non-SSI model are


Table 7: Natural Time Periods values from seismic

loads for Non-SSI model

Mode Non-SSI (seconds)

MHH (seconds)

HHS (seconds)

HSS (seconds)

1 0.016 1.362 1.024 0.988

2 0.015 1.35 1.022 0.988

3 0.014 0.511 0.414 0.38

4 0.01 0.363 0.307 0.257

5 0.01 0.351 0.298 0.243

6 0.008 0.233 0.206 0.16

7 0.007 0.215 0.186 0.152

8 0.006 0.197 0.172 0.137

9 0.006 0.196 0.17 0.135

10 0.006 0.196 0.169 0.135

11 0.006 0.187 0.163 0.132

12 0.006 0.18 0.155 0.126

It is seen that fundamental time period for MHH

model with 1.362 seconds (Mode 1) from Table 7

is the highest. Since time period and frequency are

inter related, least value of frequency is obtained in

MHH model.

Storey Drift

The variation in Storey Drift of structure of fixed

base and flexible base models are presented in the

analysis. Storey drift ratio is the maximum relative

displacement of each floor divided by height of

same floor.

Drift = (𝑑𝑖+1- 𝑑𝑖)/h

where 𝑑𝑖+1 is deflection at i+1 level, 𝑑𝑖 is

deflection at I level, h is the storey height.

The storey drift increases with soil flexibility. The

spring models reflect the flexibility with high

precision due to the idealization of six DOF. The

storey drift increases with higher rate with increase

in softness of soil.

Non-SSI models

The storey drift is calculated for load combination

of 1.5(DL+EQy) which is defined for maximum

values. The storey drift values of Non-SSI model is


Table 8: Storey drift values from seismic loads for Non-SSI model

Storey Elevation X-Dir Y-Dir

M

Storey10 35 9.517E-08 4.151E-07

Storey9 31.5 4.003E-08 4.231E-07

Storey8 28 3.972E-08 4.59E-07

Storey7 24.5 4.069E-08 4.841E-07

Storey6 21 4.366E-08 4.945E-07

Storey5 17.5 4.572E-08 4.874E-07

Storey4 14 4.198E-08 4.599E-07

Storey3 10.5 2.511E-08 4.116E-07

Storey2 7 6.31E-08 4.131E-07

Storey1 3.5 2.513E-07 0.000001

Base 0 0 0

The graphical representation of storey drift for

Non-SSI model is presented in Figure 4.

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The drift is maximum at storey 1 with ratio of

2.513E-07 along X-direction and 1.0E-05 along Y-

direction. The maximum drift varies from base to

storey 2. The drift values from storey 2 to storey 10

along X & Y direction is almost same and

continues to be linear almost as a straight line.

SSI Model

The storey drift is calculated for load combination

of 1.5(DL+EQx) which is defined for maximum

of 1.5(DL+EQx) which is defined for maximum

values. The storey drift in SSI models observed to

critical in MHH model. The storey drift values are

tabulated in Table 9. The drift is maximum at

storey 3 with ratio of 0.00481 along X- direction

and 0.000619 along Y-direction. The drift values

from storey 4 to storey 13 along X & Y direction is

almost same and continues to be linear almost as a

straight line.

Storey10

Storey9

Storey8

Storey7

Storey6

Storey5

Storey4

Storey3

Storey2

Storey1

Base

Y-dir 4.15E 4.23E 4.59E 4.84E 4.95E 4.87E 4.60E 4.12E 4.13E 1E-06 0

X-dir 9.52E 4.00E 3.97E 4.07E 4.37E 4.57E 4.20E 2.51E 6.31E 2.51E 0

0.00E+002.00E-074.00E-076.00E-078.00E-071.00E-061.20E-061.40E-06

Dri

ft V

alu

es

Maximum Storey Drift

Storey 13

Storey 12

Storey 11

Storey 10

Storey 9

Storey 8

Storey 7

Storey 6

Storey 5

Storey 4

Storey3

Storey 2

Storey 1

Base

Y-dir 1.00E 7.53E 7.65E 7.58E 7.25E 6.40E 4.94E 7.83E 1.90E 1E-06 0.000 0.002 0.000 0

X-dir 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.004 0.002 0.001 0

0

0.001

0.002

0.003

0.004

0.005

0.006

Dri

ft V

alu

es

Maximum Storey Drift

Figure 4: Graphical representation of storey drift in Non-SSI models.

Figure 5: Graphical representation of storey drift in SSI (MHH) models.


Table 9: Storey drift values from seismic loads

for MHH model in SSI

Storey Elevation X-Dir Y-Dir

M

Storey13 41 0.002101 9.997E-08

Storey12 37.5 0.002101 7.531E-08

Storey11 34 0.002101 7.648E-08

Storey10 30.5 0.002101 7.575E-08

Storey9 27 0.002101 7.245E-08

Storey8 23.5 0.002101 6.396E-08

Storey7 20 0.002101 4.94E-08

Storey6 16.5 0.002101 7.827E-08

Storey5 13 0.002101 1.895E-07

Storey4 9.5 0.002101 0.000001

Storey3 6 0.00481 0.000619

Storey2 5 0.002366 0.00248

Storey1 3 0.001008 0.000928

Base 0 0 0

Base Shear

The base shear of structure of fixed base and

flexible base models are presented in the analysis.

The variation in base shear is due to the effect of

varying stiffness of soil. It is seen from the table

that value of base shear in SSI models decreases

marginally when compared to Non-SSI models.

The values of base shear for Non-SSI models and

SSI models are tabulated in Table 10.

Table 10: Effective values of base Shear in SSI

and Non-SSI models

Model Base Shear

along

X-direction

Vx (kN)

Base Shear

along

Y-direction

Vy (kN)

Non-SSI 3174.7132 3174.7132

HHH 3094.5708 3094.5708

MHH 3094.5904 3094.5904

MHS 3094.5687 3094.5687

MSS 3094.5658 3094.5658

HHS 3094.5586 3094.5586

HSS 3094.5512 3094.5512

Ratio of Area of Footing

The effect of change in area of footing is an

important parameter considered in the present

study. The effect of SSI by altering the area of

footing in normal stresses, vertical displacements,

natural period, storey drift and base shear is also

determined. The ratio of change in area of footing

is considered from 1.1 to 1.9. The change in

dimension of footing is found to be negligible on

behaviour of SSI. The normal stresses values for

ratio 1.1, 1.3, 1.5, 1.7 & 1.9 for HHH model are

tabulated in Table11.

Table 11: Normal stress values for HHH model

considering the change in ratio of footing. Joint Ratio

1.1

(MPa)

Ratio

1.3

(MPa)

Ratio

1.5

(MPa)

Ratio

1.7

(MPa)

Ratio

1.9

(MPa)

1 0.9 0.9 0.9 0.9 0.9

2 0.34 0.34 0.34 0.26 0.34

3 0.85 0.85 0.85 0.85 0.85

4 0.4 0.4 0.4 0.4 0.4

5 0.85 0.85 0.85 0.85 0.85

6 0.31 0.34 0.34 0.28 0.34

7 0.9 0.9 0.9 0.7 0.9

8 0.35 0.35 0.35 0.28 0.35

9 0.16 0.16 0.16 0.16 0.16

10 0.33 0.33 0.33 0.33 0.33

11 0.37 0.37 0.37 0.37 0.37

12 0.33 0.33 0.33 0.33 0.33

13 0.16 0.16 0.16 0.16 0.16

14 0.35 0.35 0.35 0.35 0.31

15 0.9 0.9 0.9 0.9 0.9

16 0.34 0.34 0.34 0.34 0.34

17 0.85 0.85 0.85 0.85 0.85

18 0.4 0.4 0.4 0.4 0.4

19 0.85 0.85 0.85 0.85 0.85

20 0.34 0.34 0.34 0.34 0.34

21 0.9 0.9 0.9 0.9 0.9

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CONCLUSIONS

The study on gravity and seismic response of RCC

frames with isolated footing on shallow stratum of

layered soil has been carried out, to determine the

variation in normal stresses, vertical displacements,

natural period, storey drift and base shear due to

the effect of varying stiffness in soil model profile

considered. Results indicate that,

The response of the structure changes

significantly in the soil-structure-interaction

analysis when compared to the non-

interactive analysis.

The Normal stresses at the interaction

surface between soil and structure

decreases in SSI analysis, when considered

for gravity loads only. This is due to

coupling of horizontal displacements

between the footing and soil.

The decrease in stress value range from

10.68% to 24.29%. The decrease in stresses

is observed to be least in HSS model and

maximum in MHH model.

The stresses at the exterior footing and soil

interaction junctions with node no.’s 1, 3, 5,

7, 15, 17, 19 & 21 increases in SSI analysis

by 60% to 67% when in comparison to

Non-SSI analysis. This might be due to

effect of separation of soil and structure at

these nodes.

It can be also seen that change in clay soil

profile type has negligible or marginal

effect on stress values.

The effect of vertical displacement at

interaction layer of soil and structure is

observed under effect of gravity loads only.

In a Non-SSI model structure is presumed

to be fixed.

In SSI model the vertical displacement

varies from 72.5mm to 153.2mm. The

displacement is observed to be least in HSS

model and maximum in MHH model.

Natural period of the system in SSI model

increases by 1.9 times. The time period

increases when the soil becomes softer.

The story drift values in SSI analysis

increases by 2 times along X-direction and

by 1.2 along Y-direction when compared to

Non-SSI effect. The story drift can be

reduced and optimized by increasing

stiffness in soil i.e., by ground

improvement techniques.

Story drift and Natural time period values

increases when the type of soil changes in

layer from Sandy to Hard and from Hard to

Medium.

Variations in base shear in models with and

without SSI effect is also considered. Base

shear decreases marginally by 2.55% when

SSI effect is considered.

The SSI analysis considering the effect of

normal stresses and vertical displacements

by altering the ratio in area of footing from

1.1 to 1.9 is done. It is seen that change in

dimension of footing is found to be

negligible on behaviour of SSI.

REFERENCES

1. Edward Tsudik. (2013), Analysis of Structures

on Elastic Foundation, J Ross Publishing.

2. Selva Durai, A.P.S. (1979), Elastic analysis of

Soil Foundation Interaction, Elsevier Scientific

Publishing Company.

3. Wolf, J.P. (1985), Dynamic Soil Structure

Interaction, Prentice Hall, Englewood Cliffs,

N.J.

4. IS: 1893 (Part 1): 2002, Criteria of Earthquake

Resistant Design of Structures – General

Provisions and Buildings, Fifth Revision, BIS

New Delhi.


5. Halkude, S.A., Kalyanshetti, M.G. and

Barelikar, S.M. (2014), Seismic Response of

R.C. Frames with Raft Footing Considering

Soil Structure Interaction, International Journal

of Current Engineering and technology.

6. National Building Code of India (1983).

7. Subramanian, N. (2008), Design of Steel

Structures, Appendix C, 1396-1400.

8. Pandey, A.D., Prabhat Kumar and Sharad

Sharma. (2011), Seismic Soil-Structure

Interaction of Buildings on Hill Slopes,

International Journal of Civil and Structural

Engineering, Vol. 2, 544-555.

9. IS 456:2000. Plain and Reinforced Concrete-

Code of Practice, BIS New Delhi.

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