04-06-2014
Indo-Norwegian Training Programme on Seismic Design of Multi-storey
Buildings: IS 1893 vs. Eurocode 8
May 26-28, 2014 at New Delhi
OUTLINE
• Rigid Base Model
• Flexible Base Model
• Modeling of Shallow foundation
• Modeling of Raft foundation
• Modeling of Pile foundation
MODELLING OF SOIL-FOUNDATIONSTRUCTURE SYSTEM
by
SHRABONY ADHIKARY
Research Scholar
(Department of Earthquake Engineering, IIT Roorkee)
FOUNDATION MODELING
ASSUMPTION(FEMA-440)
Flexible Base Model
Geotechnical
components of
foundation
Rigid Base Model
Structural
components of
foundation
Infinitely rigid foundation and soil
Ug=free field motion (FFM)
with conventional damping
Ug=free field motion (FFM)
with conventional damping
• Inappropriate for Structural systems that incorporate stiff vertical system
for lateral resistance(e.q, shear walls,braced frames)
• Allowable for moment frames
•
•
•
•
Base rotations and translations
Predicted period of the structure lengthens
Distribution of forces among various element changes
Realistic evaluation of the probable structural behavior
Soil Flexibility -ASCE 41
METHOD 1
For Rigid shallow foundation and flexible soil system
SOIL MODELLING FOR
SHALLOW FOUNDATION
P
ksr
M
ksh
H
ksv
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Stiffness of foundation at surface
Correction Factor for Embedment
ASCE 41
ASCE 41
Effective shear modulus
Flexible Base properties and Soil
Properties
Foundation dimensions
• Length, L
• Width, B
• Thickness, d
• Depth, D
METHOD 2
For Rigid shallow foundation and flexible soil system
Winkler Model
•
Coefficient or
Modulus of Subgrade Reaction
k end =
6 .83 G
(1 − ν )
Stiffness per unit length
ASCE 41
k mid =
0 .73 G
(1 − ν )
Component Stiffness
Modulus of Subgrade Reaction
IS 9214 (1979): Method of determination of modulus of subgrade reaction (ksvalue) of soils in field
Pressure, σ
Initial Tangent
or
initial secant line
Δσ
ks =Δσ/Δδ
Empirical Relationships
(Terzaghi Equation)
(Vesic Equation,1961)
For Footing on clay
ks = k1 (B1/B)
ks=1.33* kP * (Bp/B) 0.73 Moayad and Janbaz(2008)
ks = Es/B(1-µ2)
Δδ
For Footing on sand
Deformation, δ
ks = k1 (B+B1/2B)2
ks value is taken as the slope of the line passing through the origin and the point on
the curve corresponding to 1.25 mm settlement
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METHOD 3
For flexible shallow foundation and soil system
unit subgrade spring coefficient, ksv
k sv =
1 . 3G
B (1 − ν )
Poisson’s ratio ν
v = 0.50 for saturated clay soils
ν = 0.25 for all other soils
SOIL MODELLING FOR RAFT
FOUNDATION
Rigid Foundation (Conventional Method)
a) The foundation is rigid relative to the supporting soil and the compressible soil
layer is relatively shallow.
b) The contact pressure variation is assumed as planar, such that the centroid of
the contact pressure coincides with the line of action of the resultant force of all
loads acting on the foundation.
Flexible Foundation (Simplified Method)
It is assumed that subgrade consists of an infinite array of individual elastic springs
each of which is not affected by others. The spring constant is equal to the
modulus of subgrade reaction ( k ). The contact pressure at any point under the
raft is, therefore, linearly proportional to the settlement at the point.
Calculation of node springs
Column
IS : 2950 (Part I) – 1981, Code of Practice for Design and
Construction of Raft Foundations, Part 1 Design
Modeling of Raft Foundation
Decoupled Winkler spring model similar to Method 2
Estimate of Modulus of subgrade
reaction ks for Sandy soil
Loose Sand
Medium Dense Sand
Dense Sand
Clayey Medium Sand
Ki(kN/m) =ks (kN/m3)×Area(m2)
Silty Medium Dense Sand
4800-16000 kN/m3
9600-80000 kN/m3
64000-128000 kN/m3
32000-80000 kN/m3
24000-48000 kN/m3
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Estimate of Modulus of subgrade
reaction ks for Clayey soil
Clay, qa < 200kPa
Clay, 200kPa < qa <800kPa
Clay, qa > 800kPa
12000-24000 kN/m3
24000-48000
kN/m3
>48000 kN/m3
Determination of Depth of Fixity
IS : 2911 (Part 1/Sec 4) - 1984
Winkler Model
SOIL MODELLING FOR PILE
FOUNDATION
Determination of Lateral deflection and
Maximum moment in the Pile
IS : 2911 (Part 1/Sec 4) - 1984
Calculation of spring coefficients
Silva 2008
Silva 2008
Horizontal soil spring stiffness at any depth was obtained according to the relation
Vertical end bearing soil spring stiffness at bottom was obtained according to the
relation
Vertical skin friction spring stiffness along length of pile
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