Seismic Vulnerability Study of the Alaskan Way Viaduct: Typical Three-Span Units

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Seismic Vulnerability Study of the Alaskan Way
Viaduct Typical Three-Span Units

• Marc Eberhard
• (J. De la Colina, S. Ryter, P. Knaebel)
• Lacey, Washington

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Outline

• Scope of Study
• Description of Typical WSDOT-Designed Unit
• Analyses of WSDOT Unit
• 3D Response Spectrum
• 2D Nonlinear Static
• Vulnerability Assessments of WSDOT Unit
• Flexure
• Shear
• Anchorage
• Splices
• Joints
• Footings
• Comparison of SED and WSDOT Typical Units

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Scope of Structural Evaluation

• WSDOT and SED Typical Three-Span Sections
• Ground Motions
• ATC-6 spectrum, Type III soil with Ag 0.25g
• Site-specific spectra for 20-ft fill/20-ft tidal
  deposit
• Analyses
• 3D Response Spectrum Analysis
• Nonlinear 2D Static Analysis

• ATC-6-2 and Priestley Procedures for
• Flexure
• Shear
• Anchorage
• Splices
• Joints
• Pile-Supported Footings
• As-Designed Unit (no deterioration or
  construction errors)
• Retrofit Priorities

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Outside Scope

• Failure Modes
• Torsion
• Effect of anchorage deficiencies on beam/slab
  shear capacity
• Effect of splice failure on shear resistance
• Piles
• Fracture at welded lap splices
• Behavior of square bars
• Procedures Developed Since 1994
• Specific Retrofit Recommendations

• Atypical Sections
• North-end and South-end single-deck structures
  (Bents 1-53)
• Double-deck to single-deck transition structures
  (e.g., outrigger bents)
• Ramps
• Curve
• Interactions among three-span frames
• Variations in Properties
• Material properties
• Reinforcement

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WSDOT Unit Longitudinal Elevation
Interior Frame
Interior Frame
Exterior Frame
Exterior Frame
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WSDOT Interior Bent Transverse Elevation
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WSDOT Interior Bent Column Cross-Section
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Analyses

• Assumed Material Properties
• Description of Typical WSDOT-Designed Unit
• Analyses of WSDOT Unit
• 3D Response Spectrum
• 2D Nonlinear Static
• Vulnerability Assessments of WSDOT Unit
• Flexure
• Shear
• Anchorage
• Splices
• Joints
• Footings
• Comparison of SED and WSDOT Typical Units

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Model for Dynamic Analysis
Slab (Shell Elements)
Transverse Beams
Exterior Girders
Interior Stringers
Columns (Frame Elements)
WSDOT UNIT
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Modal Periods
20' Fill, 20' Tidal
30' Fill, 30' Tidal
40' Fill, 40' Tidal
38' Fill, 26' Tidal
ATC-6
WSDOT-UNIT
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Summary of Minimum Flexural C/D Ratios
Response Spectra Direction Rec, First Story Rec, Second Story Rec, Beams, Girders
ATC-6 Transverse 0.36 1.17 0.43
Longitudinal 0.31 0.92 0.75
Site-Specific Transverse 0.22 0.80 -
Longitudinal 0.22 0.59 -
Depending on site and direction, expect
first-story column displacement ductility demands
in the range of 2-4. Location of max. demands
depends on column base-fixity. Low ductility
demands in 2nd story, with exception of M in
some beams.
WSDOT UNIT
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Two-Dimensional, Nonlinear Analysis
WSDOT-INT FRAME
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Force-Deflection Relationship
Total
Wunit 4800 kips Vtrans/Wunit 0.25
One Interior Frame
One Exterior Frame
WSDOT-INT FRAME
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Column Moments
WSDOT-INT FRAME
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Beam Bending Moment (first level)
WSDOT-INT FRAME
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Curvature Ductility Demands at Vu (?_at_Vmax/?y,
Fixed Base)
0.7
1.0
0.5
0.8
0.7
0.2
1.0
10.0
0.7
1.0
3.9
5.6
Interior Frame
Unclear if first-story or beam mechanism
controls. First-story controls for pinned base
and for longitudinal frames.
WSDOT Unit
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Flexural Failure C/D Ratios
Response Spectra Direction C/D (Fixed) C/D (Pinned)
ATC-6 Transverse 1.07 0.75
Longitudinal 1.43 0.97
Site-Specific Transverse 0.64 0.53
Longitudinal 0.89 0.70
?cu0.005 corresponds to low level of damage.
Depending on site and base fixity, expect low to
moderate level of flexural damage. Unlikely to
lead to catastrophic collapse of unit.
WSDOT UNIT
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Shear Failure
2
4
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Shear Failure (Min. C/D Ratios)
Story Direction C/D (?2) C/D (?4)
First Transverse 2.28 1.23
Longitudinal 2.64 1.34
Second Transverse 0.95 0.51
Longitudinal 1.39 0.70
First-story columns probably OK, but margin of
safety is small at large ductility demands.
Second story columns OK as long first story is
not strengthened. Could check with updated
shear-strength procedures
WSDOT UNIT
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Anchorage

• Assume hooked bars are OK
• Follow Priestley (1992) recommendations (No
  method available to account for square bars in
  SED Unit).
• Bar development OK if splitting failure
  suppressed
• If not

ls
ls
ls
ls
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Ts/Tu
ls
ls
0.8-1.3
0.9-1.3
ls
ls
0.8-0.9
1.4
Bottom reinforcing bars of first-level beams have
the most critical anchorage conditions.
Consequences on vulnerability (e.g., shear
resistance) unclear. Also need to look at
development of top-story column bars through
joint.
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Lap Splices

• Short (20-22 Db)
• Poorly confined
• Located at base of first- and second-story
  columns

ATC-6-2 (rcs) Priestley (Mb/My) Priestley (Mb/Mu)
First Story 0.02-0.04 1.2 1.0
Second Story 0.05-0.15 1.2 1.0
Bottom splice critical, because even if Priestley
procedure is correct, deformation demands will be
large. Top splice probably OK. (Interaction
with joint?)
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Joints

• Priestley (1992) suggests limit on maximum
  tensile stress of 3.5(fc).5 to prevent joint
  cracking. Others limit shear stress.
• Did not consider results of recent tests by
  Stanton and Lehman (2000)
• Calculate
• with and without slab reinforcement
• at ultimate conditions and using forces from
  nonlinear analysis
• Neglect interaction with splice
• Assume only part of joint is effective

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Joint Effective Areas
Transverse
Longitudinal
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Normalized Joint Stresses (Transverse)

2.7
6.5
8.3
2.7
Normalized vult
Similar for interior and exterior WSDOT frames
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Normalized Joint Stresses (Longitudinal)

5.0
4.6
4.6
5.0
5.0
8.7
5.2
8.7
Normalized vult
Joint shear cracking is expected in transverse
direction (1st and 2nd level) and in longitudinal
direction (1st level only). Cracking does not
necessarily lead to collapse.
WSDOT Unit, including slab reinforcement
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Footings

• Flexure
• Shear (ACI-ASCE 426)
• Anchorage
• Joints (3.5(fc).5)

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Footings C/D Ratios
C/D min Transverse C/D min Longitudinal
Flexure 0.9 on My 1.1 on Mu 1.3 on My 1.6 on Mu
Shear 0.31 0.53
Anchorage of Starter Bars 0.99 0.91
Joints 0.60 0.70
Using conservative criteria, it appears that the
WSDOT footings are vulnerable to shear failure.
Detailed analyses and testing might change that
diagnosis. Few footings have failed during
earthquakes.
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Priorities for Retrofitting
Location Failure Mode Likelihood Consequence Cost
1st-Story Splices Flexure Shear 1 ? 2 1 1 1
Joints Diag. Tens. 2 1 3
Columns Flexure Shear 2 2-3 4 1 1-2 2
Footings Shear 2 2 3
2nd-Story Splices Flexure 4 2 2
Modified