Assessment of Seismic Retrofit Measures for Bridge Bearings

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Assessment of Seismic Retrofit Measures for
Bridge Bearings

• Monique C. Hite, Assistant ProfessorSiddharth
  Srivastava, Graduate Research Assistant
• Zachry Department of Civil EngineeringTexas AM
  University
• April 25, 2008

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Presentation Overview

• Motivation
• Bridge Selection Modeling
• Nonlinear Time History Analyses of Bearings
• Unretrofitted case
• Retrofitted case with cable restrainers
• Conclusions
• Impact Future Research

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Seismic vulnerability of bearings
1/5 Motivation
Bearings as past unstable supports
Mander et al. (1996)
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Seismic vulnerability of bearings
1/5 Motivation
Steel pedestals may behave like steel bearings
when subjected to seismic loads
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Steel Pedestal Performance
1/5 Motivation
strong-axis
initial tangent stiffness
P1-1
secant stiffness
initial tangent stiffness
secant stiffness
P1-2
weak-axis
Hite et al. (2007, 2008)
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1/5 Motivation
Objectives of Investigation

• To compare the performance of influence of
  various types of bearings on the seismic response
  of a bridge
• To evaluate the effectiveness of using cable
  restrainers as a seismic retrofit measures for
  steel pedestals and fixed/expansion bearings
  compared to FP isolation bearings

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2/5 Bridge Selection Modeling
Analytical Modeling
GOAL to determine displacement demands
Hite et al. (2007, 2008)
Force-displacement hysteretic relationships
steel pedestals
fixed and expansion bearings
P (lb)
Mander et al. (2006)
friction pendulum isolation bearings
D (inches)
Liberty County Bridge, Georgia
Dicleli (2002)
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Bridge Model
2/5 Bridge Selection Modeling

• Overview 3D MDOF model developed in SAP2000 six
  spans, L245m skew18.5
• Deck modeled as equivalent steel section that
  includes a 7" reinforced concrete deck linear
  elastic elements used 2 expansion joints modeled
  as gap elements
• Bearings Hysteretic force-displacement behavior
  used to define effective stiffness
• Columns frame elements based on confined
  concrete model (Mander et al., 1988)
• Mass total mass of superstructure and
  substructure based on mass of members

bearings
friction pendulum isolation bearings
fixed and expansion bearings
steel pedestals
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Bridge Model Case 1
2/5 Bridge Selection Modeling
Unretrofitted bridge model with expansion joints
at Bents 2 and 6
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2/5 Bridge Selection Modeling
Seismic Activity in the United States
The dark areas MMI VIII The light areas MMI
VI-VII
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Time histories used for model
2/5 Bridge Selection Modeling
0.2g
1.3g
0.35g
0.8g
PGA (in/s2)
PGA (in/s2)
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Analysis of Results Case 1
3/5 Nonlinear Time History Analyses
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Analysis of Results Case 1
3/5 Nonlinear Time History Analyses
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3/5 Nonlinear Time History Analyses
Retrofit Measures
GOAL to limit displacement of these bearings
using cable restrainers
fixed and expansion bearings
Mander et al. (2006)
steel pedestals
Hite et al. (2007, 2008)
Retrofit using cable restrainers, Kr 30 kN/mm
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Bridge Model Case 2
3/5 Nonlinear Time History Analyses
Retrofitted bridge model with expansion joints at
Bents 2 and 6 using cable restrainers (Kr 30
kN/mm)
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Analysis of Results Case 2
3/5 Nonlinear Time History Analyses
Retrofit using cable restrainers, Kr 30 kN/mm
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Deck Displacement Time History CHS 2475
3/5 Nonlinear Time History Analyses
Case 1 P1-2 Tn0.73 s Case 2 P1-2 Tn0.66 s
Max. Absolute Displacement Case 1 P1-2, D 0.78
m (31") Case 2 P1-2, D 0.61 m (24")
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Bearing Displacement Time History CHS 2475
3/5 Nonlinear Time History Analyses
Case 1 P1-2 Tn0.73 s Case 2 P1-2 Tn0.66 s
Max. Absolute Displacement Case 1 P1-2, D 0.78
m (31") Case 2 P1-2, D 0.61 m (24")
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Column Displacement Time History CHS 2475
3/5 Nonlinear Time History Analyses
Case 1 P1-2 Tn0.73 s Case 2 P1-2 Tn0.66 s
Max. Absolute Displacement Case 1 P1-2, D 0.54
m (21") Case 2 P1-2, D 0.58 m (22")
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Conclusions
4/5 Conclusions

• Experimental test data are used to analytically
  model the behavior of bridge bearings and assess
  their influence of on the seismic response.
• Cable restrainers can be used to effectively
  limit the displacements of steel pedestals
  subjected to synthetic ground motions of
  Charleston, SC and other recorded traces.
• Depending on the seat width, unseating of these
  bearings to large lateral loads may occur.
• FP isolation bearings are quite effective in
  reducing the displacements demands, thereby
  making them viable retrofit measures.

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Impact Future Research
5/5 Impact Future Research

• Provided results to support the use of steel
  pedestals and cable restrainers to limit
  displacements demands in regions expecting
  moderate-to-high seismicity, thereby showcasing
  the wide appeal for their use and ability to also
  increase the vertical clearance height of bridges
  
• Promise lies in more detailed analyses
• Modeling soil-structure interaction
• Conducting parametric studies to determine most
  critical (sensitive) aspects such as
• Limitations for pedestal height and
  configurations
• Effect of varying column lengths
• Optimal retrofit measures such bumper blocks,
  seat extenders, steel pipe restrainers, etc.

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Questions?

• Thank you for your kind attention!

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Experimental Results of Steel Pedestals
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Experimental Investigation (3/5)
P
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Performance-Based Design Criteria
from OpenSees analysis