Aspects of Earthquake Engineering of Concrete Bridges

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Aspects of Earthquake Engineering of Concrete
Bridges

• M. Saiid Saiidi, Professor
• University of Nevada, Reno
• Illini MS-77, PhD-79

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Topics

• Simple Direct
• Beyond what the Eyes See
• How to Retrofit A Column in A Closet
• Plucking Brides Eyebrows

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Two Sub-Topics

• Confinement Steel Design in Spiral Columns
  (FHWA/NDOT)
• Constitutive Modeling of CFRP-Confined Concrete
  (NSF/Caltrans)

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Basic Equation for Confinement in Spiral Columns

• ?s 0.45 ( Ag / Ac ) 1 (fc / fy )

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Factors Not Included in the Basic Equation
1- Axial load effect 2- Vertical steel ratio 3-
Dominance of flexure
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Fix 1 Add axial load

• ?s 0.45 ( Ag / Ac ) 1 (fc / fy )

?s 0.45 ( Ag / Ac ) 1 (fc / fy )
0.5 1.25 P / (f fc Ag )
No longitudinal steel effects!
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Fix 2 Add long. Steel ratio
?s 0.45 ( Ag / Ac ) 1 (fc / fy )
?s 0.16 (fc / fy ) 0.5 1.25 Pe / (f fc Ag
) 0.13 ( ?l 0.01 )
Not Simple Not Direct! Not useful for PBD
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Curvature Ductility
Previous work
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Objective

• Develop a new method for confinement spiral steel
  design to account for
• Target curvature ductility
• Axial load
• Longitudinal steel ratio
• Geometric and material properties

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Advantages of the New Method

• Uses curvature ductility
• It is a performance-based design method
• Can be used for areas of moderate as well as high
  seismicity

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PBD Method

• Five Simple Steps
• Perform moment curvature analysis
• Choose a target curvature ductility
• Calculate ultimate curvature
• Determine ultimate concrete strain
• Calculate spiral steel ratio

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PBD Method
Step 5

• Avoids iterations
• Accounts for increase in strength due confinement

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PBD Design
Target Curvature ductility 12
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Test Specimens
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Axial load 0.23 fc Ag
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Caltrans Column
2.25 x ATC-32-D
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PBD Column
2.25 x ATC-32-D
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Moment Curvature Comparison
Curvature (rad/mm)
0.00000
0.00008
0.00016
0.00024
0.00031
0.00039
0.00047
0.00055
1600
180766.72
1400
158170.88
1200
135575.04
1000
112979.2
Moment (kip-in)
Moment (kN-mm)
800
90383.36
Curvature Ductility SC-CAL 9.1 SC-PBD 11.7
600
67787.52
400
45191.68
SC-CAL
200
22595.84
SC-PBD
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0
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
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Curvature (rad/in)
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Conclusions
Proposed PBD method for spiral steel is a
simple, direct, and rational method that can
tailor the response to a target curvature
ductility level. Columns designed based on
the proposed method overcome problems with low
ductility under high axial loads.
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Stress-Strain Relationship for Concrete Confined
by CFRP
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Features of Existing Models
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Confinement Models
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Proposed Simple Model

• Eliminate the transition zone and use a
• bilinear relationship.
• Simplify the equations with conservative
• assumptions and coefficient down to 1 or 2
• significant figures

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Evaluation Determine M-? curves for typical
columns
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Sample Result 4-diam. Col.
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Beyond what the Eyes See
Retrofitting Two-Column Bridge Piers 1-
Steel or FRP jackets on the columns
with enlargement and/or prestressing of the beam
2- RC in-fill walls
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Constructibility Problems (California

contractors) gtgt Upward drilling into the beam
for
vertical dowels gtgt Placement of concrete
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What the eyes see
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As-Built Model Performance
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Loading Direction
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How to Retrofit A Column in A Closet or A Fish
Tank
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Message

• Non-technical factors can control the final
  decision on seismic retrofit.
• Two Examples
• Airport Viaduct
• Nugget Viaduct

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The Airport Viaduct
- 16-span freeway bridge

• Columns deficient
• in shear

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• Retrofit Choice 1 Steel Jacket
• Preferred by NDOT because
• of past experience
• Constraints
• gtgt Tight working space
• gtgt Minimize interruption to the
• operation of RTC
• gtgt Avoid foundation work
• Other Choices
• GFRP and CFRP jackets

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Four 0.3-scale models As-built, Steel, GFRP,
and CFRP jackets
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Load vs. Displacement
Steel
GFRP
CFRP
As-Built
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Columns in Fish Tank
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Column Features

• Pinned at the base
• Short anchorage length for column bars at the
  top.
• Low confinement and shear capacity in plastic
  hinge (top)

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Constraints

• Avoid or minimize interruption to the hotel
  operation or the freeway.
• Avoid foundation work.
• Tight working space in the upper parts of the
  columns

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Solution Shift plastic hinge to
increase longitudinal bar anchorage. Use GFRP
jacket to enhance shear strength and confinement

gtgt No interruptions gtgt No excavations
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Plucking Brides Eyebrows

• Persian saying
• In the process of beautifying brides eyebrows,
  they made her blind.
• Retrofit should not take away good features of
  the existing structure.

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Failure of Mission-Gothic Bridgeduring
Northridge 94 Earthquake
Good News No damage to the top connection or
the superstructure. Shifted Plastic Hinge
Bad News Shear failure due to
flexural overstrength at the flares
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Retrofit Objective

• Fix the eyebrows but do not take the eyesight
  away!
• In earthquake engineering terms
• Increase the shear capacity, but maintain
• the location of the plastic hinge.

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