Recent Advances in Non-linear Soil Structure Interaction Analysis using LS-DYNA

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1 Recent Advances in Non-linear Soil Structure Interaction Analysis using LS-DYNA Michael Willford Richard Sturt Yuli Huang Ibrahim Almufti Xiaonian Duan

2 Arup Global Firm of Design, Planning and Management Consultants 10,000 staff worldwide

3 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

4 Impact Nuclear Transport Containers Automotive Crashworthiness Impact and Penetration

5 Seismic performance of structures

6 Incorporating Non-linear SSI Direct Method Distant Soil Domain Edge Moving as Free-Field Nonlinear Structure Basement and Piles Soil is non-linear, can be layered and site specific Mesh density designed to transmit frequencies desired Motion input via Lysmer dampers at bedrock Soil FE Mesh Horizontal Input Force Time History C h 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

7 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

8 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

9 Porewater pressure generation - validation Example - Dobry et al (1995) centrifuge test on sloping site Excellent simulation of generation and dissipation of pore pressures

10 Validation: Reinforced Concrete Simulation UCSD full scale 7 story rc shear wall shake table test

11 Comparison of test and simulation Progressive stiffness and strength degradation under successive cycles Crack intensity also well predicted

12 Validation: Squat Shear Wall NUPEC shake table test (c.1994) Cyclic degradation Shear failure

13 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

14 Point Fortin LNG Tanks, Trinidad (1995) Two 72m dia. Tanks Hazardous product Total mass 100,000t each Shallow soft-soil layer

15 Point Fortin LNG Tanks, Trinidad 1-D soil column site response analysis

16 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

17 Point Fortin LNG Tanks, Trinidad Effect of pile group on site response 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

18 Point Fortin LNG Tanks, Trinidad Effect of pile group on site response

19 Point Fortin LNG Tanks, Trinidad Effect of tank and contents Soil is highly non-linear and inertia forces are very high Add Housner mass-spring analogy for tank slosh and impulsive Assume rigid basemat

20 Point Fortin LNG Tanks, Trinidad Include tank inertia forces with complete SSI Model

21 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

22 LNG Tank: 3D SSI simulation with explicit fluid and tank wall uplift (2004) Outer tank wall Soil mesh Symmetry plane Distant side boundaries Piles and nonlinear pilesoil interaction springs Bedrock level transmitting boundary earthquake motion input system

23 Analysis now performed in 3D with explicit modeling of tank wall, base and LNG

24 Cross-section overview of response Sensitivity to edge boundary distance checked Cf. Wolf s cone analogy for practical purposes

25 Detail of uplift of flexible base plate

26 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

27 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

28 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

29 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

30 Visualization of Simulation Vertical deflection contoured Green/Blue =settlement Orange/Red/Purple =heave

31 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

32 Strut Force, kips/strut Use in design Permanent increase in prop forces due to earthquake Legend Strut 1 Strut 2 Strut 3 Strut 4 Buttress design is Time, s optimized to control movements Time, s Effect of buttress and time varying soil properties are incorporated in seismic response

33 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

34 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

35 Malampaya CGS, Philippines FEED Study - 3D Model

36 Malampaya CGS, Philippines without seismic isolators with seismic isolators

37 Malampaya CGS, Philippines SSE response analysis Seismic isolator hysteresis sliding soil layer hysteresis Similar beneficial effect on topsides acceleration

38 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

39 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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42 JFK Airport - Bridge Structures LRS Column (Modelled using Seismic Beam Elements) Pile Cap LRS Mass Bridge Bearing Description of SSI Model Soil Layers (Hysteretic Soil Model) Piles Embedded in Soil (Modelled using Seismic Beam Elements) 29.5m Bedrock Motion Applied at Base 27.5m 10.0m Free Field Motion Applied to End Boundaries

43 LS-DYNA - Validation of Soil Model Pile - soil interaction Pile group lateral load test simulation at JFK Airport Quadrant 4 Required by client to demonstrate capacity of pile group. Test examining static push-over condition. Loading Direction We used same DYNA model as for dynamic study to examine pile group test.

44 LS-DYNA - Validation of Soil Model Pile group test simulation -comparison of Results Leading Piles Push-over Analyses Pile Test Trailing Piles