INFLUENCE OF SOIL-STRUCTURE INTERACTION ON RESPONSE OF A MULTI-STORIED BUILDING AGAINST EARTHQUAKE FORCES

INFLUENCE OF SOIL-STRUCTURE
INTERACTION ON RESPONSE OF A MULTI-
STORIED BUILDING AGAINST EARTHQUAKE
FORCES
Submitted by,
G.Kamala Kumari
(14481D8709)

CONTENT
 Abstract
 Introduction
 Literature review
 Methods for SSI
 SSI in Seismic codes
 Proposed work
 Geotechnical classification on study area
 Response of structures against earthquake forces
 Equivalent lateral force method
 Free vibration analysis
 Results
 Conclusions
 Future scope
 References

ABSTRACT
In the analysis of structures subjected to earthquake forces, it
is usually assumed that the structure is fixed at the base to simplify
the mathematical problem. This assumption leads to gross error in
assessment of overall response under dynamic loads. The interaction
phenomenon is principally affected by the mechanism of energy
exchanged between the soil and the structure during an earthquake.
In the present investigation, a multi-storied building which is
located in Amaravathi is chosen as the study area which consists of
different types of soil / rock profiles at different locations. Many high
rise structures are expected in future in the new city.
Earthquake analysis is carried out when similar structure
rests on different types of soils and the results of fundamental time
periods, base shears and displacements are compared with the results
obtained from fixed base condition.

INTRODUCTION:
 In seismically active regions like India, there is potential
risk for multi- storied RC buildings.
 As per the latest seismic zoning map brought out by the
Bureau of Indian Standards (BIS), over 65% of the
country is prone to moderate to high intensity
earthquakes.
 Most of the mega cities in India are in seismically active
zones and many structures in these cities are designed for
gravity loads only.

SOIL-STRUCTURE INTERACTION(SSI)
 Most of the civil engineering structures involve some type of
structural element with direct contact with ground.
 The process in which the response of the soil influences the
motion of the structure and the motion of the structure
influences the response of the soil is termed as soil-structure
interaction.
 In general conventional structural design methods neglect the
SSI effects.
 The effect of SSI, however, becomes prominent for heavy
structures resting on relatively soft soils.

WHY SSI CONSIDERED
 In capital region of A.P majority of soils are of black
cotton, silty clay and loose soils.
 Many high rise structures are expected in the new city in
future.
 This area falls under seismic zone III.

PARAMETERS FOR SSI AND STRUCTURAL
SSI parameters:
 Local soil condition
 Peak ground acceleration
 Shear wave velocity
 Frequency content of motion
 Proximity to fault

Structural parameter:
 Natural time period
 Base shear
 Roof displacement
 Column moment.
 Beam moment

LITERATURE REVIEW
 Response of structures against earthquake forces
considering the effect of soil-structure interaction was
carried out by several authors earlier.
 S.R.K.Reddy et al, Terrain evaluation and influence of
soil-structure interaction on seismic response of
structures in urban environs” seismic waves travel
through different rock or soil media and reach the
foundation layers causing the structure indeed affect the
response of the structure apart from the influence of other
parameter such as the type of ground excitation,
configuration, ductility and quality of construction. Soil is
represented by introducing two additional springs, one in
horizontal and other in rocking mode.

 Milos Novak “Dynamic stiffness and damping of
piles” An approximate analytical approach based on linear
elasticity is presented, which makes it possible to establish
the dimensionless parameters of the problem and to obtain
closed form formulas for pile stiffness and damping.
 Shehata E.Abdel Raheem, Mohamed M. Ahmed and
Tarek M.A.Alazrak ,“Soil-structure interaction effects
on seismic response of multi-story buildings on raft
foundation” The conventional design procedure usually
involves assumption of fixity at the base of foundation
neglecting the flexibility of the foundation, the
compressibility of soil mass and consequently the effect of
foundation settlement on further redistribution of bending
moment and shear force demands.

 Whiteman.R.V. Richart.F. “design procedure for
dynamically loaded foundations” Assuming the foundation
of the structure with isolated footings, translational and
rocking stiffness formulas are suggested and also a fictitious
mass to be added to the soil-structure model in time domain
to reduce the error caused by frequency dependent nature of
the response.
 V.K.Puri and Shamsher Prakash “Foundations for
Dynamic loads” Design of foundation in earthquake prone
areas needs special considerations sallow foundations many
experiences a reduction in bearing capacity and increase in
settlement and tilt due to seismic loading. The response of a
footing to dynamic load is affected by the nature and
magnitude of dynamic load, number of pulses the strain rate
response of soil.

METHODS FOR SSI
Direct method
 Direct approach is one in
which the soil and structure
are modeled together in
a single step accounting
for both inertial and
kinematic interaction.

SUB STRUCTURE METHOD
 Substructure method is one in which the analysis
broken down into several steps.
 That is the principal of superposition is used to
isolate the two primary causes of soil - structure
interaction.

SSI IN SEISMIC CODES
 The important cases in which SSI has a pronounced effect
need to be considered according to part five of Eurocode
8. These cases are as follows:
 Structure where P-Δ effects play a significant role.
 Structures with massive or deep-seated foundations,
such as bridge piers, offshore caissons, and silos.
 Slender tall structures, such as towers and chimneys.
 Structures supported on very soft soils, with average
shear wave velocity less than 100m/s, such as clayey
soils.

FEMA 450
 It has incorporated a procedure to take into account the
flexibility of foundation and soil to evaluate the equivalent
lateral force.
 By finding an effective period considering the motion of
the first mode of vibration a reduction of base shear is
introduced which results to reduction of lateral forces and
overturning moments.

FEMA 273, 1997
 It is also suggests to consider SSI for near-field and soft
soil sites in which the increase in the fundamental period
due to SSI increases.
 But it is commented that soil-structure interaction cannot
be used to reduce component and element actions by
more than 25%.

PROPOSED WORK
 In the present study, a twelve storied building (multi-
storied building), with lower two stories for parking
(soft stories) and the remaining ten stories for
commercial and residential purpose, resting on five
different types of homogeneous soils, is chosen for the
analysis.
 The dimensions and properties and the structural
element of the building are presented in table. The
building consists of 3 bays in X-direction and 6 bays
in Z-direction, plan and elevation of building is shown
in figure.

INFLUENCE OF SOIL-STRUCTURE INTERACTION ON RESPONSE OF A MULTI-STORIED BUILDING AGAINST EARTHQUAKE FORCES

DIMENSIONS OF STRUCTURAL ELEMENTS
Parameter Dimension/s
Height of the building 38.4m
Height of each storey 3.2m
Number of stories 12(2 soft +10 )
Column size 0.23m x 0.5m
Longitudinal beams 0.23m x 0.35m
Transverse beams 0.23m x 0.5m
Plinth beams 0.23m x o.35m
Slab thickness 0.12m
Exterior wall thickness 0.23 m
Interior wall thickness 0.115m
Parapet wall height 1m

PROPERTIES OF MATERIALS AND DIFFERENT LOADS
Grade of concrete M25
Grade of steel Fe 415
Unit weight of RCC 25 KN/ m3
Unit weight of brick work 19 KN/ m3
Live load (floor) 4.00 KN/ m2
Live load (terrace) 1.50 KN/ m2
floor finish 1.0 KN/ m2
Terrace finish 1.5KN/ m2

CALCULATION OF EQUIVALENT U.D.L ON BEAMS
 Loads onto beam are considered as
 Trapezoidal section of slab / triangular section of slab or
both from one side or two sides.
 From Wall, if any
 Self-weight of rib of Beam

Formulae for equivalent U.D.L on beam
B.M CONSIDERATIONS S.F CONSIDERATIONS
FOR TRIANGLE
FOR TRAPEZOIDAL

GRAVITY LOAD CALCULATION ON COLUMN
 Loads onto the columns are considered to be
 Load from the beams that are connected to the column
 Self-weight of column
 Point loads if any
 Loads on critical column
A1 column load = 1059 KN
A2 column load = 1268 KN
B1 column load = 1535 KN
B2 column load = 1833 KN

GEOTECHNICAL CLASSIFICATION IN STUDYAREA
 The study area is covered by different types of
geomorphic units/ landforms. The seismic response of
similar building behaves differently in different soil units
during an earthquake
 Type S1 – Clay, Silty Clay
 Type S2 – Stiff clay
 Type S3 – Coarse gravel & murrum (soft rock)
 Type S4 – Sand stone or laterite
 Type S5 – Hard rock (granite)

GEO-TECHNICAL PROPERTIES OF VARIOUS GEOMORPHIC UNITS
Property of the material
units Soil / rock type
S1 S2 S3 S4 S5
Shear wave velocity Vs m/s 60 150 400 1250 2700
Mass density (ρ) KN-m2/m4 1.7 1.8 1.9 2.1 2.6
Poisson’s ratio (µ) - 0.4 0.4 0.33 0.30 0.30
Shear modulus (Gs ) =
ρVs
2
KN/m2x 105 0.0612 0.405 3.04 32.812 189.54
Young’s modulus (Es) KN/m2x 105 0.1775 1.134 8.086 85.31 492.804
Bearing capacity (p) KN/m2 60 200 300 400 500

Variation of shear modulus (G) with shear wave velocity (Vs)

 In general, Earthquake motion causes both horizontal
and vertical ground motions.
 Usually vertical ground motion has much smaller
magnitudes than that of horizontal. The vertical ground
motion due to the earthquakes can be resisted by the
factor of safety provided in the design of structures.
 The structures which are designed to carry only the
gravity loads will not be able to resist the horizontal
ground motion.
 The horizontal ground motion causes the most
significant effect on the structure by shaking the
foundation.
RESPONSE OF STRUCTURE AGAINST EARTHQUAKE
FORCES

SEISMIC ANALYSIS OF THE STRUCTURE FOR
FIXED BASE CONDITION
 Seismic Weight:
 The Seismic weight of a floor is its full dead load plus
appropriate amount of imposed load, as specified in
Clause 8.3.1 and 8.3.2 of IS 1893 (Part I) : 2002.
 Following reduced live loads are used for analysis:
 Zero on terrace and
 50% on other floors.

STIFFNESS OF EACH STOREY
The total equivalent stiffness of each storey is taken as,
k = ∑ kc + ∑ kw.
Stiffness of column kc =12EI/l3
Stiffness of wall kw = d
m
l
AE 2
cos

MASS AND STIFFNESS VALUES OF STRUCTURE
Stiffness
(KN/m)x106
k1,2 1.02
k3-12 5.11
Mass (KN-sec2/m)
m1 206.3
m2 276
m3-11 343.3
m12 226

EQUIVALENT LATERAL FORCE METHOD
 It is the simplest method of analysis and requires less computational
effort
 The code recommends the use of the design spectrum method for
determination of lateral forces.
 The following is the step-by-step procedure for analysis of the frame :
Step 1: Calculation of Lumped Masses to Various Floor Level.
Step 2: Determination Of Fundamental Natural Period
Ta = 0.085h 0.75 (Moment resisting steel frame building without brick infill walls)
Ta = 0.085h 0.75 (Moment resisting RC frame building without brick infill walls
Ta = (all other buildings including moment resisting RC frame buildings
with brick infill walls)

Step 3: Determination of Design Base Shear
VB = Ah W (Clause 7.5 of IS 1893:2002)
Ah = x x (Clause 6.4.2 of IS 1893:2002)
Zone factor, Z (Clause 6.4.2 of IS 1893:2002)
Importance factor, I (Clause 6.4.2 of IS 1893:2002)
Seismic zone II III IV V
Seismic Intensity
(Z)
Low
0.10
Moderate
0.16
Severe
0.24
Very severe
0.36
Structure Importance Factor
Important service and community buildings, such as
hospitals; schools; monumental structures; emergency
buildings like telephone exchange, television stations,
radio stations, railway stations, fire station buildings;
large community halls like cinemas, assembly halls
and subway stations, power stations.
1.5
All other buildings 1.0

 Step 4 : Vertical Distribution of Base Shear
Qi = VB .

DISTRIBUTION OF DESIGN FORCE
Storey no Wi hi Wi hi
2 Wi hi
2/ΣWi hi
2 Qi (kN) Vi (kN)
12 2217 38.4 3269099.52 0.1581 272.04 272.40
11 3368 35.2 4173086.72 0.2018 347.7 620.10
10 3368 32.0 3448832 0.1667 287.2 907.30
9 3368 28.8 2793553.92 0.1351 232.6 1140.0
8 3368 25.6 2207252.48 0.1067 183.8 1323.7
7 3368 22.4 1689927.68 0.0817 140.76 1464.4
6 3368 19.2 1241579.52 0.0601 103.5 1568.0
5 3368 16.0 862208 0.0417 72 1640.0
4 3368 12.8 551813.12 0.0267 46 1686.0
3 3368 9.6 310394.88 0.015 26 1712.0
2 2703 6.4 110714.88 0.0054 9.3 1721.3
1 2024 3.2 20725.76 0.0010 1.7 1723.0

TYPE OF FOUNDATION
Type of soil Type of foundation
S1 Pile/Mat/isolated footing
S2
S3 Mat/isolated footing
S4 isolated footings
S5

SOIL MODEL
 The dynamic model of soil requires the representation of
soil mass, soil stiffness and damping factors allowing for
strain dependence and variation of soil properties.
 The structure is assumed to rest on uniform elastic half-
space and soil-spring approach is used to model the soil-
structure interaction.
 Since the structures are usually designed for gravity
loads and plan of the building is symmetrical, only
horizontal and rocking springs are considered .

STIFFNESS OF SOIL (ISOLATED FOOTING)
 Horizontal stiffness, kx (KN/m) = 2(1+ ν) Gβx (BL)½
 Rocking stiffness, kψ (KN-m) =[G/(1- ν)] βψ BL2
Constants for rectangular basis (Whitmen and Richart)

CALCULATION OF STIFFNESS OF PILE
 Calculation of stiffness of pile by using Novak and EI-
Sharnouby Formulae (1983)
 Translational stiffness Kx=[ ] *fx
 Rocking stiffness Kψ =[ ] *fψ1
Where Ep = modulus of elasticity of pile material
Ip= moment of inertia of single pile about X
or Y axis
ro = pile radius
fx1 , fψ1 are Novak’s coefficients

MASS AND STIFFNESS VALUES OF SOIL
parameter Type of
foundation
Mo I kx k φ
Units KN.sec2
/m
KN.m.s
ec2
KN/m*
106
KN.m*
106
Type
of
soil
S1 Pile 972 57242 1368 35890
Mat 1110 58901 0.298 19.04
S2 Pile 863 55931 4694 54000
Footing 670 53616 8.57 30.618
S3 Mat 955 57037 13.47 490.9
Footing 530 51937 46.1 255.3
S4 Isolated
footing
431 50749 407 1717
S5 401 50380 2091 5680

FREE VIBRATION ANALYSIS:
Using the combined mathematical model of both
structure and soil with masses and springs , the
equilibrium equations are formulated and put them in
matrix form
..
[M] x + [k] x = 0
Where [M] – Mass matrix,
[k] – Stiffness matrix,
..
x – Horizontal acceleration,
x – Horizontal displacement

MASS MATRIX
I-mass Moment Of Inertia Of All Storey Masses

RESULTS
Parameter Frequency
(Hz)
Time
period(sec)
Displac
ements
(mm)
Shears
absorb by
soil (kN)
Net base
Shears
(kN)
S1 Pile 1.628 0.614 5.0 224.2 1498.8
Mat 0.436 2.29 63.5 261.2 1461.8
S2 Pile 1.629 0.6138 5.2 214.1 1508.9
Footing 0.558 1.79 37.2 217 1506.0
S3 Mat 1.35 0.7394 6.57 242 1481.0
Footing 1.2 0.8332 7.97 201 1522.0
S4 Footing 1.54 0.6485 5.1 198 1525.0
S5 1.6 0.6235 4.8 194 1529.0
Fixed base 1.63 0.6126 4.5 172 1551.0

Graph 1(a) time period Vs shear wave velocity

Graph 1(b) storey shear Vs shear wave velocity

Graph 1(c) rocking stiffness vs shear wave velocity(pile to footing)

Graph 1(d) rocking stiffness vs shear wave velocity (mat to footing)

Graph 1(e) horizontal stiffness vs shear wave velocity

0
1
2
3
4
5
6
7
8
9
10
D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14
pile (S1)
mat (S1)
Displacements at storey level
displacements(mm)

0
1
2
3
4
5
6
D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14
pile (S2)
isolated footing(S2)
Displacements at storey level
displacements(mm)

0
0.2
0.4
0.6
0.8
1
1.2
D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14
mat (S3)
isolated footing (S3)
Displacements storey level
displacements(mm)

0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14
isolated S4
isolated S5
fixed base
storey level
displacements
(mm)

CONCLUSION
 The shear wave velocity influences significantly in
changing the shear modulus of different soils from static
to dynamic state. It is noticed that dynamic shear modulus
exponentially increases with the increase of shear wave
velocity.
 The horizontal and rocking soil spring constant values
increase when type of soil varies from loose soil to hard
rock. It is also noticed that these values are high in case of
pile foundations when compared to the isolated footing
type of foundation.

 Fundamental time period of the building invariably decreases
with the increase of soil stiffness. In loose soils like silty clay
and silty sand, where normally pile foundations are preferred,
these time period values decrease compared to the values
obtained when isolated footing type and mat foundation is
provided.
 The base shear values obtained are the lateral shears transferred
from soil to the base of the structure due to effect of soil –
structure interaction.
 It is noticed that the shears and displacements are high in case
of loose soils compared to those of very stiff/ hard soils.
 In general, it can be concluded that structure resting on stiff
soils or rock behave well during earthquake than structures
resting on loose soils.

FUTURE SCOPE
 In this experimental study a multi storey building rests on
different soil conditions (hard rock granite, coarse gravel, silty
clay) have been considered, for further experimental
investigations different combinations of soil conditions can be
carried out.
 In this present study the soil in and around the foundation has
been considered as a spring one in horizontal and rocking mode,
for further study spring can modeled including torsion effects
this can modeled as used in different softwares like
ANSYS,SAP200, and ETABS or inelastic continuum method.
 The soil is considered as single media but layered soil types
exist below is not single, so different linear equilibrium effects
can also be considered.

REFERENCES
[1] S.R.K.Reddy, et al, “Terrain evaluation and influence of soil-structure
interaction on seismic response of structures in urban environs” proc of 3rd
International conference (2006)Venice, Italy, pp235-242.
[2] Whiteman. R.V, Richart.F.E, “Design procedure for dynamically loaded
foundations” journal of soil mechanics and foundation engg., Division, ASCE
93, 1967, pp167-191.
[3] Shehata E.Abdel Raheem, Mohamed M. Ahmed and Tarek M.A.Alazrak, “Soil-
structure interaction effects on seismic response of multi-story buildings on raft
foundation”, Journal of Engineering aSciences Assiut University Faculty of
Engineering vol.42, No. 4, pp 905-930, July 2014.
[4] P.A Dode, H.S.Chore and D.K.Agraual, “Space frame – pile foundation –soil
interaction analysis”, Journal of structural engg. Vol.42, No.3, August –
September 2015.pp.246-255.
[5] Vidya.V, B, K, Raghu Prasad and Amarnath.K, “Seismic response of high rise
structure due to interaction between soil and structure”, International Journal of
Research in Engineering and Applied Sciences, Volume 5, Issue 5-May-2015.
[6] Milos Novak “Dynamic stiffness and damping of piles” (1974) Canadian Geo
Technical Engineering, journal, 1974, pp 574-598.
[7] V.K.Puri and Shamsher Prakash “Foundations for Dynamic loads”.

[8]A Criterion For Considering Soil-structure Interaction Effects In
Seismic Design Of Ductile Rc-mrfs According To Iranian Codes
By Massumi 1 And H.R. Tabatabaiefar 2
[9] Bharath Bhushan Prasad, “Fundamentals of Soil Dynamics and
Earthquake Engineering’’, PHI learning private limited,2009.
[10] Joseph E Bowles, “Foundation analysis and design”, The
McGraw-Hill Companies, Inc.,5th Edition, pp. 501-505, 1996.
[11] Ketan Bajaj, JiteshT Chavda, Bhavik M Vyas, “seismic behaviour
of buildings on different types of soil”.
[12] Bureau of Indian Standards IS 1893 (Part I): 2002. Criteria for
earthquake resistant design of structures. Part I General provisions
and Buildings, 2002.
[13] IS 456:2002 “Plain and Reinforced Concrete - Code of Practice”
fourth revised edition.
[14] IS 13920 (1993)”Ductile detailing of reinforces concrete structures
subjected to earthquake forces”
[15] IS 875(PART 1 & 2) : 1987 code of practice for design loads for
building and structures.
[16] Soil mechanics and foundation engineering DR.K.R.Arorass
[16] Earthquake resistant design by Punkaj Agarwal.

INFLUENCE OF SOIL-STRUCTURE INTERACTION ON RESPONSE OF A MULTI-STORIED BUILDING AGAINST EARTHQUAKE FORCES

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INFLUENCE OF SOIL-STRUCTURE INTERACTION ON RESPONSE OF A MULTI-STORIED BUILDING AGAINST EARTHQUAKE FORCES