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Incremental Dynamic Analyses on Bridges on various Shallow Foundations. Lijun Deng PI’s: Bruce Kutter , Sashi Kunnath University of California, Davis. NEES & PEER annual meeting San Francisco October 9, 2010. Outline. Introduction and centrifuge model tests

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incremental dynamic analyses on bridges on various shallow foundations

Incremental Dynamic Analyses on Bridges on various Shallow Foundations

Lijun Deng

PI’s: Bruce Kutter, SashiKunnath

University of California, Davis

NEES & PEER annual meeting

San Francisco

October 9, 2010

outline
Outline
  • Introduction and centrifuge model tests
  • Incremental Dynamic Analysis (IDA) model
  • Preliminary results of IDA
    • Maximum drift
    • Instability limits of rocking and hinging systems
    • Residual drift
  • Conclusions
rocking foundation centrifuge tests
Rocking Foundation Centrifuge Tests

Gazli earthquake, pga= 0.88 g

hinging column centrifuge test
Hinging Column Centrifuge Test

Gazli earthquake, pga= 0.88 g

collapse of hinging column
Collapse of hinging column
  • SDOF bridges on rocking foundation survived after 20 scaled GM’s, but the one on fixed foundation and hinging column collapsed
opensees model for ida and parametric study
OpenSeesmodel for IDA and parametric study

Mass = m

Column hinge spring

Column: Stiff elasticBeamColumn

Hc

Foundation: zerolength elements

Moment

Fixed ground center

Rotation

Footing center

Footing mass = m*rm

ki

xi

Lf

validate model through centrifuge data
Validate model through centrifuge data

Centrifuge model

(Cy/Cr=5, T_sys=1 s, FSv=11.0)

input parameters in ida model
Input parameters in IDA model
  • Cy, Cr: base shear coefficients for column or rocking footing
  • Two yielding mechanisms:
    • Cr > Cy Hinging column system;
    • Cy > Cr  Rocking foundation system

(Column hinge strength)

(Foundation element stiffness)

Equally spaced foundation elements

(Column hinge stiffness)

Ac/A=0.2, rm=0.2

(Footing length)

(Foundation element strength)

input parameters in ida model1
Input parameters in IDA model
  • Input ground motions from PEER database

Forty pulse-like ground motions at soil sites(Baker et al. 2010)

ida results sa t t sys vs max drift
IDA results: Sa(T=T_sys) vs. max drift

Rocking Footing (Cy=0.5, Cr=0.2, T_sys=0.85 s)

Hinging column (Cy=0.2, Cr=0.5, T_sys=0.85 s)

0.2 g

Failure zone

0.2 g

Failure zone

Nonlinear zone

Nonlinear zone

Elastic zone

Elastic zone

Instability limit

~=2 m

Instability limit

~=2.2 m

collapse mechanisms
Collapse mechanisms
  • A hinge is a hinge
  • Hinges can be engineered at either position
    • A hinge forms at the edge when rocking occurs
  • P-delta is in your favor for rocking – recentering
  • Instability limits are related to Cy and Cr values

P

P

D

D

selected animations
Selected animations
  • Cy=0.2, Cr=0.5, T=0.85 s (Hinging column)
  • Cy=0.5, Cr=0.2, T=0.85 s (Rocking foundation)

Collapse case

On-verge-of-collapse case

On-verge-of-collapse case

Collapse case

ida results sa t t sys vs max drift1
IDA results: Sa(T=T_sys) vs. max drift
  • 50% median of Sa vs. max drift and +/-σ

50% Median

compare medians of sa vs max drift for various t sys
Compare medians of Sa vs. max drift for various T_sys
  • Longer periods lead to higher drift
  • The max drift is not sensitive to Cy/Cr ratio
  • The max might rely on min{Cy, Cr}, to be confirmed with further study
ida results sa t sys vs residual rotation1
IDA results: Sa (T_sys) vs. Residual rotation
  • Bridge with rocking foundation have smaller rotation than hinging column  re-confirm the recentering benefits
conclusions
Conclusions
  • Rocking foundations provide recentering effect that limits the accumulation of P-D demand (i.e., much smaller residual rotation)
  • Experiments and IDA simulations show column with rocking footing is more stable than hinging column (i.e., fewer collapse cases)
  • ESA approach is not conservative for highly nonlinear cases
  • Analysis is ongoing, and fragility functions are being developed from the results. We are also evaluating the adequacy of Sa(T_sys) as an Intensity Measure of ground motions
collaborators
Collaborators

CONSTRUCTION

DESIGN

THEORY

Khojasteh

Shantz

GEOTECHNICAL

Kutter

Martin

Mejia

Desalvatore

Stewart

Jeremic

CODE DEVELOPERS

Hutchinson

Panagiotou

Browning

Kunnath

Comartin

Mahin

Moore

McBride

Ashheim

Mar

Mahan

BRIDGES

STRUCTURAL

BOTH

BUILDINGS

acknowledgments
Acknowledgments
  • Current financial support of California Department of Transportation (Caltrans).
  • Network for Earthquake Engineering Simulation (NEES) for using the Centrifuge of UC Davis.
  • Other student assistants: T. Algie (Auckland Univ., NZ), E. Erduran (USU), J. Allmond (UCD), M. Hakhamaneshi (UCD).
ida results sa t t sys vs max drift2
IDA results: Sa(T=T_sys) vs. max drift

Rocking Footing (Cy=0.5, Cr=0.2, T_sys=0.85 s)

Hinging column (Cy=0.2, Cr=0.5, T_sys=0.85 s)

  • Equivalent Static Analysis (ESA) commonly used in codes may underestimate the displacement.