Centrifuge Models Design and Execution: Some Practical Considerations

Student Researchers: Gabriel Candia & Roozbeh Geraili

Advisor: Nicholas Sitar

University of California, Berkeley

Department of Civil and Environmental Engineering

WHY CENTRIFUGE MODELING?

2

Advantages

•Correct scaling of stresses

•Repeatable and cost effective

•Ability to model large

structures with reasonable

boundary conditions

Limitations and challenges

•Non uniform gravity field•Need for high speed

instruments•Ability to reproduce recorded

ground motions•Boundary conditions

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

3

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

4

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

1.- INPUT GROUND MOTION

DESIGN AND EXECUTION ISSUES

5

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

10-1

100

0

0.4

0.8

1.2

1.6

2

Period (s)

Sa

(g

)

Original RecordLomaPrietaSC - 1

Loma Prieta SC - 5% Damping Response Spectrum

DESIGN AND EXECUTION ISSUES

2.- INSTRUMENTS LIMITATIONS

6

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

AccelerometersStrain Gages

Disp. Transducers Pressure sensors

0 100 200 300 400 500 600 700 800 900 1000

0 100 200 300 400 500 600 700 800 900 1000

Acceleration

Load Cells

7

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

PCB Piezotronics0.3-12000Hz

Interface Load Cell0-2000Hz

Frequency (Hz)

2.- INSTRUMENTS LIMITATIONS

0 100 200 300 400 500 600 700 800 900 1000

0 100 200 300 400 500 600 700 800 900 1000

Earth Pressure

Acceleration

8

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

Tactilus Free Form0-100Hz

Frequency (Hz)

PCB Piezotronics0.3-12000Hz

2.- INSTRUMENTS LIMITATIONS

9

Direct measurement Pressure Cells

max? max?

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

3. OUR CHALLENGE - EARTH PRESSURE IN RIGID BASEMENT

How to measure seismic earth pressure on rigid buried structures?

Indirect measurement Strain Gages

10

M+

M-M+

M-

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

3. OUR CHALLENGE - EARTH PRESSURE IN RIGID BASEMENT

11

Trend from Pressure Cells

Magnitude from Load Cells

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

Our approach Replace roof and floor with Load Cells

3. EARTH PRESSURE IN RIGID BASEMENT

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

12

0 5 10 15 20 25 30400

600

800

1000

1200

1400

Time (s)

Lo

ad

(lb

f/in

)

0 5 10 15 20 25 300

2

4

6

8

10

Time (s)

Pre

ssur

e (

psi

)

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

4.- BOUNDARY CONDITIONS – INFLUENCE OF MODEL GEOMETRY

13

Input acceleration

Flexible Shear Beam Container

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION

14

0 0.5 1 1.5 20

1

2

3

4

Period

Sa(

g)

Soil

Ring

0 0.5 1 1.5 20

1

2

3

4

Period

Sa(

g)

Soil

Ring

2nd ring

5th ring

8 10 12 14 16 18-1

-0.5

0

0.5

1

Time (s)

Acc

(g)

Vertical Acc

Horizontal Acc

Vertical Input

EFFECTS OF MASS ASYMMETRY

LESSONS LEARNED

Equipment and Instrument Limitations•Instrument frequency response is an important consideration.

Earth Pressure in Rigid Structures•Currently available pressure sensors do give direct measurement, but are inconsistent•Use of redundant instrumentation is essential.

Boundary Conditions•Interaction with container can be an issue for large models•Check plain-strain assumptions

OVERVIEW EXPERIMENTS PERFORMED DESIGN AND EXECUTION