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Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification
Lisa Anderson Farhang Ostadan Bechtel National, Inc. USDOE NPH Workshop October 2014
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-2
Objective
• Validate Use of SASSI2010 Substructuring Techniques
• Direct Method • Extended Subtraction Method • Subtraction Method
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-3
Structural Model Types
•Three Model-Type Comparisons • Hybrid Lumped Mass Stick Model with Finite
Element Embedment (DOE NPH 2011) • Coarse 3-D Finite Element Model • Complex 3-D Finite Element Model
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-4
Benchmarks
• Three different benchmarks • SASSI2000 direct method analysis • SAP2000 time history analysis • ANSYS harmonic analysis
• Benchmarks listed in order of accuracy/independence • Unlike SASSI2010 direct method, SAP2000 and
ANSYS solutions solve in one analysis step • Hence, no substructuring is involved.
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-5
Coarse 3-D Model - Methodology
• SASSI2010 Analysis • Structural model is generated using shell elements • Soil model is considered using free-field soil layers • Vertical boundary simulated using rigid halfspace
• SAP2000 Analysis • Identical structural model as SASSI2010 • Soil model generated using solid elements • Vertical boundary fixed; horizontal boundary using
rollers
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-6
Coarse 3-D Model - Methodology
• SASSI2010 Analysis • Subtraction Method (SM) implemented with
interaction nodes only at interface of soil and structure
• Control point at base of halfspace • SAP2000 Analysis
• Time history analysis with input motion applied at base of model
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-7
Coarse 3-D Model - Simplifications
• One uniform soil case is considered • Consistent damping for structure and soil • Cutoff frequency approximately 15 Hz • Only horizontal direction compared
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-8
Coarse 3-D Model – Geometry
600 ft
150 ft
Uniform
Vs = 2,500 fps
VP = 4,677 fps
Damping = 5% (structure and soil)
100 ft
600 ft
EL 0 ft - Finished Grade
EL -140 ft - Foundation
EL +50 ft - Roof
EL -60 ft
EL -240 ft “Bedrock” fixed
X
Y Z
Simplified quarter model with deep embedment and large footprint
Uniform medium soil is used with high frequency input motion
Rollers on side perimeters and fixed on bottom perimeter
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-9
Coarse 3-D Model – Result Comparison Comparison of transfer functions for response in the horizontal direction (X)
Comparison of ARS for response in the horizontal direction (X)
Both comparisons indicate close match between SASSI and SAP results
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-10
Complex 3-D Model - Methodology • ANSYS Full Harmonic Response Method used as a benchmark
• Comparison encompasses true problem size and range of inputs anticipated in a full analysis
• Structural model is capable of representing global and local response characteristics
• Soft Soil, Medium Soil, Stiff Soil, and Hard Rock are considered
• Two motions are applied with characteristic of the Western United States (WUS) and Central Eastern United States (CEUS)
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-11
Complex 3-D Model - Methodology • Two sub-structuring methods are used in SASSI:
• Subtraction Method (SM) – Only the nodes on the perimeter of the foundation are considered interaction nodes
• Extended Subtraction Method (ESM) – The perimeter as well as additional horizontal planes are considered interaction nodes
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-12
Complex 3-D Model - Methodology • ANSYS Full Harmonic Response Method utilized
• Harmonic ground motion accelerations are applied independently at the fixed boundary of the model
• This method is selected for the following advantages:
• An efficient method for calculating transfer functions
• Material damping may be specified as frequency independent for each soil layer
• Full matrices are utilized for the solution, thus no mass matrix approximations are involved
• The equation of motion is directly solved in the frequency domain
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-13
For validation of the SASSI solution method, a deeply embedded finite element model is used One plane of symmetry is considered The finite element model is refined to adequately capture global and local responses
Isometric View of Structural Model with Elevations (El. 122 = Grade)
Total Nodes: 87,325 Total Shell Elements: 27,277 Total Brick Elements: 58,932 Total Beam Elements: 1,498 Total Lumped Masses: 21,044
Complex 3-D Model – Structure
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-14
Three soil profiles and one hard rock profile are considered:
• Soft Soil (C2) - average soil column Vs of 1700 fps (Vp = 5400 fps)
• Medium Soil (C5) - average soil column Vs of 3300 fps (Vp = 8500 fps)
• Stiff Soil (C7) - average soil column Vs of 6400 fps (Vp = 13,000 fps)
• Hard Rock (HR) – uniform soil column Vs of 100,000 fps (Vp = 187,000 fps)
For each soil case, strain-compatible soil profiles are used in the SSI analysis
Shear Wave Velocity Profiles
Complex 3-D Model – Soil
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-15
Two sets of input time histories compatible with the foundation level CEUS and WUS input response spectra are utilized
Input motion is defined at a depth of 250’ below grade
Hence, “within” time histories at a depth of 250’ are calculated and used in the SSI analysis
Horizontal Input Motions – Shown as Outcrop at Foundation Depth
Input Acceleration Response Spectra5% Damping
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.1 1 10 100
Frequency (Hz)
Acc
eler
atio
n (g
)
WUS
CEUS
Complex 3-D Model – Motion
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-16
The following SSI analysis cases are performed:
• SASSI2010 Subtraction Method (SM)
• SASSI2010 Extended Subtraction Method (ESM)
• ANSYS Full Harmonic Analysis (ANSYS)
Complex 3-D Model – Analysis
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-17
Extended Subtraction Method (ESM) and Subtraction Method (SM) Boundary Conditions
Excavated Soil Volume
SM Interaction Nodes (5713)
ESM Interaction Nodes (9477)
Two additional horizontal layers
Complex 3-D Model – SSI Boundary
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-18
• For comparison with SASSI2010 HR results, a fixed boundary analysis is completed using ANSYS (no soil around the structure)
• For comparison with SASSI2010 SSI results, a soil island model is included
The mesh size and extent of the model is determined through iterative parametric studies and comparisons with the free-field response as calculated using SHAKE2000
Average soil element size within 100 ft from structure = 5 ft (10 ft elsewhere)
Complex 3-D Model – ANSYS Model
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-19
Complex 3-D Model – ANSYS Model
Elements forming the ANSYS soil island are assigned identical properties to those specified in the SASSI SITE module
The soil model was extended laterally to minimize boundary effects
Fixed boundaries are applied in the base of the soil model at the depth of 250’ which mimics a rigid halfspace as modeled in SASSI2010
Rollers are provided on the sides of the mesh
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-20
Complex 3-D Model – Responses
• Generation of Transfer Functions and Acceleration Response Spectra (ARS)
• SASSI generated transfer functions and ARS are extracted directly using the MOTION module
• ANSYS absolute accelerations are calculated by adding the ground motion acceleration and the ANSYS relative nodal acceleration calculated during the frequency response analysis
• Using the SASSI MOTION Module, ANSYS transfer functions are interpolated and ANSYS ARS are calculated
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-21
Complex 3-D Model – Responses
Comparison of SHAKE2000 and ANSYS Free-Field Response
Free-Field Nodes Case 2 Comparison
Case 7 Comparison Case 5 Comparison
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-22
Complex 3-D Model – Responses
Nodes for Local Comparisons
(Subset of Results Shown for Clarity)
Nodes for Global Comparisons
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-23
Complex 3-D Model – Responses
Global Transfer Function Comparison – SASSI ESM HR vs. ANSYS Fixed-Boundary
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-24
Complex 3-D Model – Responses
Local Transfer Function Comparison – SASSI ESM HR vs. ANSYS Fixed-Boundary
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-25
Complex 3-D Model – Responses Global Transfer Function Comparison – SASSI SM vs. SASSI ESM vs. ANSYS Soft Soil (C2) – X Direction
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-26
Complex 3-D Model – Responses Global Transfer Function Comparison – SASSI SM vs. SASSI ESM vs. ANSYS Soft Soil (C2) – Y Direction
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-27
Complex 3-D Model – Responses Global Transfer Function Comparison – SASSI SM vs. SASSI ESM vs. ANSYS Soft Soil (C2) – Z Direction
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-28
Complex 3-D Model – Responses
Global Transfer Function Comparison – SASSI SM vs. SASSI ESM vs. ANSYS Medium Soil (C5) – X Direction
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-29
Complex 3-D Model – Responses
Global Transfer Function Comparison – SASSI SM vs. SASSI ESM vs. ANSYS Medium Soil (C5) – Y Direction
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-30
Complex 3-D Model – Responses
Global Transfer Function Comparison – SASSI SM vs. SASSI ESM vs. ANSYS Medium Soil (C5) – Z Direction
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-31
Complex 3-D Model – Responses
Global Transfer Function Comparison – SASSI SM vs. SASSI ESM vs. ANSYS Stiff Soil (C7) – X Direction
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-32
Complex 3-D Model – Responses
Global Transfer Function Comparison – SASSI SM vs. SASSI ESM vs. ANSYS Stiff Soil (C7) – Y Direction
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-33
Complex 3-D Model – Responses
Global Transfer Function Comparison – SASSI SM vs. SASSI ESM vs. ANSYS Stiff Soil (C7) – Z Direction
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-34
Complex 3-D Model – Responses Local Transfer Function Comparison – SASSI ESM vs. ANSYS Soft Soil (C2)
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-35
Complex 3-D Model – Responses Local Transfer Function Comparison – SASSI ESM vs. ANSYS Medium Soil (C5)
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-36
Complex 3-D Model – Responses Local Transfer Function Comparison – SASSI ESM vs. ANSYS Stiff Soil (C7)
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-37
Complex 3-D Model – Responses
Acceleration Response Spectra Comparison
SASSI ESM vs. ANSYS
Soft Soil (C2) CEUS Motion
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-38
Complex 3-D Model – Responses
Acceleration Response Spectra Comparison
SASSI ESM vs. ANSYS
Soft Soil (C2) WUS Motion
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-39
Complex 3-D Model – Responses Acceleration Response Spectra Comparison
SASSI ESM vs. ANSYS
Medium Soil (C5)
CEUS Motion
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-40
Complex 3-D Model – Responses Acceleration Response Spectra Comparison
SASSI ESM vs. ANSYS
Medium Soil (C5)
WUS Motion
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-41
Complex 3-D Model – Responses
Acceleration Response Spectra Comparison
SASSI ESM vs. ANSYS
Stiff Soil (C7) CEUS Motion
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-42
Complex 3-D Model – Responses
Acceleration Response Spectra Comparison
SASSI ESM vs. ANSYS
Stiff Soil (C7) WUS Motion
Validation of the SASSI2010 Subtraction Method Using Full Scale Independent Verification, Page-43
Conclusions
• SSI solution using SASSI2010 can be validated using independent solution methods that do not employ substructuring methods
• Model size and extent is restricted only by hardware capabilities
• Simplified and complex model comparisons indicate a close match between the Subtraction Method, Extended Subtraction Method, and independent solutions (i.e. ANSYS harmonic and SAP time history) for the range of inputs considered