Soil Structure Interaction Method Comparison -...

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Soil Structure Interaction Method

Comparison

Angel Francisco MartinezApplication EngineerMIDAS ITEstados Unidos



Contents

1. Introduction

2. Soil Structure Interaction Substructure Method

3. Ground Response Analysis for Calibration

4. Soil Structure Interaction Direct Method

5. Results Comparison Between SSI Methods

GTS NX

3

Soil Structure Interaction

SSI: The phenomenon in which the response of soil and movement of the structure influence each

other. When external forces, such as earthquakes, act on these systems, neither structural

displacements nor ground displacements are independent of each other. Unlike an above-ground

structure, the response of an underground structure subjected to an earthquake is mainly controlled

by the ground displacement. The ground displacement in turn is largely influenced by the dynamic

properties of the ground and its modeling method. The ground exhibits a complete nonlinear

behavior depending on the strains developed in the ground due to the dynamic load. It is therefore

important to closely examine the nonlinearity of the ground relative to strains.

GTS NX

4

Substructure Method Direct Method

Dynamic SSI




by Substructure Method

GTS NX

6

Initial Gen model dimensions

Initial pile size is large to show optimal design difference

1. Open Strat file Gen model:• 13 stories: 48 m tall building• 9 piles: -10 m deep, 0.5m diameter• 9m X 9mX 0.5m thick slab

Start FILE midas Gen

1

GTS NX

7

Kh = 0.56 (500,000) / 0.5 = 560,000 Kn/m^3

Add Soil Springs Supports in Gen

1. Right click fixed Boundaries in Tree Menu and Delete

2. Add PY soil springs supports to Gen piles and raft.

Apply Soil Springs in Gen

2

1

GTS NX

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Time History Analysis Case

3

1

2

GTS NX

9

Results for TH in Gen using PY springs

• Check TH Max case for moments YMax : 85 kN/m

• Check TH Max case for axial forcesMax: 1269 kN

• Check TH Max load case for displacementsMax 1.12 mm




by Direct Method

GTS NX

11

GTS NX

Linear Equivalent

Time History

Response Spectrum

Eigen Value

Dynamic Analysis

• Free Field

• Linear (Direct)

• Linear (Modal)

• Non linear (Direct)

• Non linear + SRM

• Liquefaction

• Moving Load

• 2D Linear Equivalent



1D Free Field Analysis

GTS NX

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Define 1D FFA Soil Layers

GTS NX

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In order to compare results of 1D and 2D equivalent linear analysis, check the maximum acceleration value. 1. Check the Maximum Accel. of

Layer-1 by clicking Main Menu > Result > Result Table > Motion Result Table. Max = 14.99m/sec^2

2. Bed rock acceleration = 7.525m/sec^2

3. Divide by gravity to get value in g = 0.767

Procedure

The absolute Maximum Acceleration value

converged in the bedrock will be used in

order to correct the maximum ground

acceleration when defining the ground

acceleration for the 2D equivalent linear

analysis. • The procedure is for correcting

the seismic wave magnitude.

1D FFA Results

1

2

GTS NX

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1. Result > Depth Result2. Acceleration results

Procedure1

2

1D ground response analysis cannot readily check a structure, which exists in ground. In such a case, 2D equivalent

linear analysis can be used to check ground – structure interaction.

1D FFA Results



2D Linear Equivalent Analysis

GTS NX

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The use of quadrilateral elements or map-mesh is recommended

since shear forces will occur between elements while seismic wave

will be applied at ground

Mesh 2D Linear EQ Model (Ground Only)

GTS NX

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Define 2D Linear EQ Boundary (Ground Only)

GTS NX

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1. Compare with results related ABS for 2D linear equivalent analysis since results of 1D linear equivalent analysis are absolute.

2. Post Works Tree > EQ. LINEAR - ABS MAX > relative acceleration > Total rel.Max = 14.89m/sec^2

Procedure

2

2D Linear EQ Results (Ground Only)

GTS NX

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Compare results between 1D and 2D (Ground Only)

Results match between 1D FFA and 2D

Linear EQ. models within 5% at

14.9m/sec^2 acceleration at surface.

GTS NX

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Find the Max Shear Strain value and at the surface.

Max value = 0.0124

2D Linear EQ Results (Ground Only)

GTS NX

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Shear Strain G/G0 New G D

0.01 0.3257 34,459 0.1012

0.0124 0.31025 32,824 0.1035

0.02 0.2639 27920 0.1104

Shear Strains from Dynamic curve (Ground Only)

Make table of Max shear strains bands and used the dynamic curve to get new values for

G and D

Initial Shear Modulus (G0) = 105800

Shear Modulus Reduction Factor (G/G0) = 0.31025

New Shear Modulus (G) is = 105800 * 0.3257 = 32,824



3D Time History Analysis

GTS NX

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Use same dynamic curve and time

history function

Dynamic SSI

Free Field 1D

Linear Equivalent 2D

Time History 3D

GTS NX

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Mesh 3D Geometry

GTS NX

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2

3D LTH Loads and Boundaries (ground only)

1 45

6

3

GTS NX

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3D Nonlinear Time History Results (ground only)

Results for Nonlinear Time History is within 5%. Max of 14.53 m/sec^2

1. Main Menu> Analysis Case > Perform…

2. Select 3D Nonlinear Time History ground only>Ok

3. Results > Abs Max > Relative Acceleration Total

Procedure

3

1

GTS NX

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Free Field 1D vs 2D Linear Equivalent

3D SSI with Tunnel

2D Linear Equivalent with tunnel

2D Time History with Tunnel 3D Time History With Tunnel

Ground accelerations match between the 4 cases

GTS NX

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Axial Forces match for 3

cases

3D SSI with Tunnel

2D Linear Equivalent with tunnel

2D Time History with Tunnel 3D Time History With Tunnel

GTS NX

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• Análisis Pushover de un marco tridimensional usado para diseño. Puede ser utilizado para concreto reforzado, acero

estructural y secciones compuestas de concreto-acero.

• Diseño basado en desempeño por FEMA, Eurocódigo 8 y Mampostería

Análisis Pushover para Mampostería

Definir materiales no lineales de rotulasEstado de Rótulas y Curva de Capacidad

Interaction with other software

GTS NX

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Import model to GTS NX

1. Export file from midas GEN in MXT format

2. Open and Import structure3. Open CAD file into midas GTS

NX

Create GTS NX file with building

1

3

2

2

3

GTS NX

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Apply Boundary Conditions

1. Boundary > Constraint > Advanced: Assign Z rotation restriction to piles for stability

Boundary Conditions

2

1

GTS NX

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SSI Results

Results for Nonlinear Time History is within 5%. Max of 15.5 m/sec^2

1. Main Menu> Analysis Case > Perform…

2. Select Nonlinear Time History>Ok3. Results > Abs Max > Relative

Acceleration Total

Procedure

3

1

GTS NX

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TH Results for GTS NX

• Check Max case for moments YMax 156 kN/m

• Check Max case for axial forces Max: 1702 kN

• Check Max case for displacementsMax: 12.49 cm



Results comparison between SSI Methods

GTS NX

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Compare results on foundation

Results comparison between methodsMax GEN GTS NX % Diff

Axial Force (kN) 1269 1702 34%

Moments Y (kN/m) 1109 1487 34%

GTS NX

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Compare results on full structure

Results comparison between methods

Max GEN GTS NX % Diff

Displacements (mm) 425 418 1%

Axial Force (kN) 3915 3962 1%

Moments Y (kN/m) 1109 1487 34%

Structure Period (s) 1.501 1.625 8%

GTS NX

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3D SSI

GTS NX

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Initial Civil model dimensions

GTS NX

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Add Soil Springs Supports

GTS NX

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Time History Analysis Case

3

1

2

GTS NX

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Results for TH in Civil using PY springs

Max Axial forces on structure (kN)

Max moments on structure (kN/m)

GTS NX

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Results for TH in Civil using PY springs

Max displacements (cm)

Max Shear Forces (Kn)

GTS NXSubstructure/Indirect Method

The substructure-only approach:http://Northamerica.midasuser.com > Training > Review Courses

http://Latinamerica.midasuser.com > SesionesWeb> Anteriores

http://northamerica.midasuser.com/

http://latinamerica.midasuser.com/

GTS NX

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Import model to GTS NX

1

3

2

GTS NX

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Mesh CAD in 3D elements

GTS NX

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Dynamic results for imported bridge

Max Axial forces on structure

Max moments on structure

GTS NX

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Results Comparison

GTS NX

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Conclusion

• Midas Gen has internal PY curves in order to easily apply soil springs and run soil structure interaction through substructure method. For Static Load Cases, this is sufficient.

• Substructure Method cannot accurately capture the non linear behavior experienced by the foundation and surrounding ground during an earthquake. Results are underestimated by 1/3.

• Eigenvalue period of main vibration mode shows ~10% difference.

• Midas GTS NX can import full structure from midas Gen to run SSI through Direct Method. However, since the ground is modeled in 3D, it must first be calibrated through comparison of ground response analysis like Free Field and Linear Equivalent.

GTS NX

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END

Soil Structure Interaction Method Comparison -...

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