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Recent Advances in Non-linear Soil
Structure Interaction Analysis
using LS-DYNA
Michael Willford
Richard Sturt
Yuli Huang
Ibrahim Almufti
Xiaonian Duan
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Arup
Global Firm of Design, Planning and Management Consultants
10,000 staff worldwide
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LS-DYNA
• Multi-physics simulation software developed by LSTC
• Typical Design Applications
– Impact
– Blast
– Seismic
– Numerical prototyping
• Arup collaborating with LSTC since 1982
• Arup enhancements made in our development version of the
code, later ported to LSTC commercial version
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Impact
• Nuclear Transport Containers
• Automotive Crashworthiness
• Impact and Penetration
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Seismic performance of structures
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Incorporating Non-linear SSI Direct Method
• Soil is non-linear, can be
layered and site specific
• Mesh density designed to
transmit frequencies desired
• Motion input via Lysmer
dampers at ‘bedrock’
• Vertical cut faces of soil are
distant (requiring large model)
and subjected to free-field site
response motions
• wave passage and incoherency
can be included via spatial
variation of input motions
Horizontal InputForce Time History
Ch
Distant SoilDomain Edge
Moving asFree-Field
Soil FE Mesh
Basementand Piles
Nonlinear Structure
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Numerical Simulation of Traditional Site
Response using LS-DYNA
• Non-linear hysteretic soil model of layered site over bedrock
• 1-D Vertically propagating shear wave
• Transmitting bedrock boundary
• Amplitude dependence of stiffness and material damping simulated
Typical soil hysteresis
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Validation of Site Response Simulation
• Comparison with measurements
in the Chiba borehole array
• Similar results to SHAKE for
moderate levels of ground motion
• Excellent comparison with
DeepSoil for strongly non-linear
response
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Porewater pressure generation - validation
Example - Dobry et al (1995) centrifuge test on sloping site
Excellent simulation of
generation and dissipation
of pore pressures
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Validation: Reinforced Concrete Simulation
• UCSD full scale 7 story rc shear wall shake table test
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Comparison of test and simulation
• Progressive stiffness and strength
degradation under successive cycles
• Crack intensity also well predicted
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Validation: Squat Shear Wall
NUPEC shake table test (c.1994)
• Cyclic degradation
• Shear failure
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Project Applications
• LNG tanks
– Soft soil acting like lateral seismic isolation
– Uplift of flexible foundation
• Heavy building subjected to adjacent deep excavation and earthquake
– Effect of initial stress state in soils
– Strain rate effects
– Interaction of adjacent structures
– Permanent deformation
• Offshore Gravity Petrochemical Platform
– Foundation sliding and seismic isolation
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Point Fortin LNG Tanks, Trinidad (1995)
• Two 72m dia. Tanks
• Hazardous product
• Total mass 100,000t
each
• Shallow soft-soil
layer
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Point Fortin LNG Tanks, Trinidad
1-D soil column site response analysis
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Point Fortin LNG Tanks, Trinidad
Site Response Results
• Natural period of
primary inertial
mode c. 0.4 secs
• Soft site provides
natural isolation –
elastic spectral
demands are halved
• But foundation must
support gravity load
and will stiffen site
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Point Fortin LNG Tanks, Trinidad
• Non-linear modeling of soft soil and driven steel pipe piles
• Non-linear local p-y soil-pile springs
• Upper, lower and best estimate soil properties
• Linear elastic halfspace for class B bedrock
Effect of pile group on site response
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Point Fortin LNG Tanks, Trinidad
Effect of pile group on site response
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Point Fortin LNG Tanks, Trinidad
• Soil is highly non-linear and inertia forces are very high
• Add Housner mass-spring analogy for tank slosh and impulsive
• Assume rigid basemat
Effect of tank and contents
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Point Fortin LNG Tanks, Trinidad
Include tank inertia forces with complete SSI Model
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Conclusions
• Non-linear modeling of soft soil enables benefit to be
taken of natural ‘Isolation’
• Steel pipe piles support gravity and overturning and do
not yield in SSE
• Ground improvement would have stiffened site and
increased demand on tanks
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Soil mesh
Outer tank
wall
Bedrock level transmitting boundary
earthquake motion input system
Distant side boundaries
Symmetry plane
Piles and nonlinear pile-
soil interaction springs
LNG Tank: 3D SSI simulation with explicit
fluid and tank wall uplift (2004)
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Analysis now performed in 3D with explicit
modeling of tank wall, base and LNG
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Cross-section overview of response
• Sensitivity to edge boundary
distance checked
• Cf. Wolf’s cone analogy for
practical purposes
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Detail of uplift of flexible base plate
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Effect of construction of large
excavation adjacent to existing tall
building (2009) • Tall reinforced
concrete building
with basement
• Adjacent
excavation 55’ deep
185’ wide
• Secant pile buttress
to be installed to
bedrock to control
movements due to
construction
• Earthquake to be
considered
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Effects of concern
• Movement of existing building due to excavation
• Forces in props across excavation
• Design of ‘buttress’ to prevent slip-circle failure
• Effect of M7.5 earthquake
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Modeling issues
• 3-D problem with non-horizontal surface (after excavation)
• Previous experience shows non-linear soil behavior essential for
accurate ground movement predictions
• Soil properties vary across site due to different effective stress
states (weight of building, unloading beneath excavation)
• Buttress is segmented concrete secant pile wall – potential sliding
interfaces - and
• Soil properties at slow strain rate (excavation) and dynamic strain
rate (seismic) are different
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Sequence of Simulation
• Initialize free field soil pressures
• Simulate construction of existing building
• Simulate construction of secant buttress and shoring walls
• Simulate excavation and insertion of props
• Apply specified earthquake
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Visualization of Simulation
• Vertical deflection
contoured
• Green/Blue
=settlement
• Orange/Red/Purple
• =heave
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Results
• Predicted settlement profile due to construction of existing
building match ongoing measurements very well
• Additional permanent settlement and rotation are induced by
excavation and by earthquake
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Use in design
• Permanent increase in
prop forces due to
earthquake
• Buttress design is
optimized to control
movements
• Effect of buttress and time
varying soil properties are
incorporated in seismic
response
•
0 5 10 15 20 25 30 35 40
Time, s
-0.2
-0.1
0.0
0.1
0.2
Accele
rati
on
, g
0 10 20 30 40 50 60
Time, s
-8,000
-4,000
0
4,000
8,000
Ba
se S
hear,
kip
s
0 10 20 30 40 50 60
Time, s
-800
-600
-400
-200
0
Str
ut
Fo
rce, kip
s/s
tru
t
LegendStrut 1
Strut 2
Strut 3
Strut 4
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Malampaya Gas Platform – Philippines
(1994)
• Massive reinforced concrete
structure to support 13,000t
topsides
• Sea-towed to offshore site
• Seabed leveled with engineered
gravel fill
• Ballasted to seabed
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Malampaya – Seismic design issues
• Conventional design would place ballast offshore to prevent
sliding in design earthquakes (i.e. fixed base)
• This design would require seismic isolation of deck to reduce
equipment responses
• Alternative is to reduce quantity of ballast and permit limited
sliding in SSE
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Malampaya CGS, Philippines
FEED Study - 3D Model
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Malampaya CGS, Philippines
without seismic isolators with seismic isolators
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Malampaya CGS, Philippines
SSE response analysis
Seismic isolator hysteresis sliding soil layer hysteresis
Similar beneficial effect on topsides acceleration
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Outcome
• Isolation associated with sliding on engineered soil layer is
sufficient to control topsides equipment responses
• Sliding deflections easily accommodated in flexible seabed
pipeline design
• Cost is saved by reduction of requirement for offshore placed
ballast, the cost of seismic isolators and multiple flexible
topside connections
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Summary
• We have conducted extensive development and (importantly)
validation of LS-DYNA to improve the design of major
construction projects
• Non-linear soil structure interaction analysis is feasible, and is
being used in design practice to find realistic and economic
solutions to complex design issues
• Non-linear analysis is the only means of predicting important
effects such as permanent deformation, sliding, uplift etc.
• In some cases very significant performance and/or cost benefits
can be realized by taking account of non-linear effects explicitly
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LRS Mass
Bridge Bearing LRS Column
(Modelled using
Seismic Beam
Elements)
Pile Cap
Soil Layers
(Hysteretic
Soil Model)
Piles Embedded
in Soil (Modelled
using Seismic
Beam Elements)
Bedrock Motion
Applied at Base
Free Field Motion
Applied to End
Boundaries
27.5m 10.0m
29.5m
Description of SSI Model
JFK Airport - Bridge Structures
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LS-DYNA - Validation of Soil Model
Pile - soil interaction
• Required by client to
demonstrate capacity of
pile group.
• Test examining static
push-over condition.
• We used same DYNA
model as for dynamic
study to examine pile
group test.
Loading
Direction
Pile group lateral load test simulation at JFK Airport Quadrant 4
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LS-DYNA - Validation of Soil Model
Push-over Analyses
Pile Test
Leading Piles
Trailing Piles
Pile group test simulation -comparison of Results