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Seismic Design of a Light Rail Transit Bridge with Fault
Rupture Crossing
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Presentation Outline
1. Project Overview2. Site-Wide Fault Mapping3. Field Exploration at Three Bridge Sites4. Design Fault Rupture Displacements5. Faulting Through Foundations 6. Bridge Design for Fault Rupture
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Project Overview
11 miles of new light rail in S. California9 stations7 bridges>4 miles of elevated viaductSeveral miles of retaining walls$2.1B total cost4 kilometers of the alignment affected by surface fault rupture hazard
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Regional Faulting
Project Alignment
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Desk Top Study – Vintage Stereoscopic Aerial Photo Interpretation
Interpretations by Scott Rugg and Tom Rockwell (Kleinfelder 2013)
Station
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Detailed Field Exploration Programs at Three Bridge Sites
Bridge Site
Bridge Site
Bridge Site
LEGEND
Project Alignment
Interpreted Fault Locations:Kleinfelder (2013)Alquist-Priolo (CDMG 1991)City of San Diego (2008)
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Field Exploration and Fault Mapping at LRT Overhead Bridge Site – Plan View
Proposed LRT Overhead Bridge
Primary Active Fault
Secondary Active Faults0 200 400
SCALE (FEET)
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Geologic Mapping of Cut Surface
Secondary Active Faults
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Design Fault Displacements –Deterministic and Probabilistic Analyses
4 ft = 1.2 m selected for design
Logic tree used in PFDHA
Hazard curve with deterministic values overlain
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Fault Rupture Design Scenario
Proposed LRT Overhead BridgeProposed Rail Bridge
4 ft
1.2 ft1.2 ft
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Fault Rupture Design Scenario
Ben
t 2
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Foundation Design StrategyUnusual situation of faulting through foundations Avoid primary fault where possibleLarge Diameter CIDH Piles or mat-footingsModeling to evaluate foundation behavior and displacements
Desirable foundation behavior Undesirable behavior
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Modeling of Soil-Fault Foundation System
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Modeling of Soil-Fault Foundation System
Pile Model
Concrete Cracking
Rebar Stresses
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Soil Model Calibration and Validation
Centrifuge test data from Loli et al. (2009)
Model simulation results
Constitutive model approach of Anastasopoulos et al. (2007)
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Modeling of Soil-Fault Foundation System
6 543210
Horz. Disp. (feet)
Shorter more rigid pile moves same as ground
Longer more slender pile moves more than ground
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Abutment Modeling Results
6 543210
Horz. Disp. (feet)
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Pile Performance
-5,000 0 5,000
Shear (kips)
-40,000 0 40,000
Moment (kip-ft)
0
10
20
30
40
50
60
70
80
0 0.5 1
Dep
th B
GS
(ft)
Pile Deflection (ft)
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Fault Rupture Design Displacements
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Bridge DesignPerformance Objectives:1. Performance Level (No collapse)
Higher Level Project-Specified Ground Motion (Caltrans Design Spectrum)
2. Service Level (Minimally serviceable to unserviceable after event) Lower Level Project-Specified Ground Motion
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Bridge DesignBridge Demand:
𝑢𝑢𝑜𝑜𝑜𝑜 = 𝑢𝑢𝑜𝑜𝑜𝑜 + 𝑢𝑢𝑜𝑜𝑜𝑜Peak Seismic Response of the Bridge
Peak Quasi-Static Demand
Peak Dynamic Demand
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Bridge DesignBridge Alternatives
Deformed Shape of a Continuous Bridge Due to Surface Fault Rupture (Integral Bent Cap)
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Bridge DesignBridge Alternatives
Deformed Shape of a Continuous Bridge Due to Surface Fault Rupture (Dropped Bent Cap)
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Bridge DesignBridge Alternatives
Deformed Shape of a Simply Supported Bridge Due to Surface Fault Rupture
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Bridge Design
Simple spans with pre-cast girdersWidened seatsArticulationCompression: gap at Abut + Pin Fuse at B2
Gap allows compression
Shear Pin Fuse
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Abutment DesignShear Key
5 ft compression vault
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Details at Bent 2
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ConclusionsSurface fault rupture hazard assessment and mitigation requires multi-discipline approachTranslation of hazard into design scenarios requires engineering insight and judgmentFoundations intersected by faults can be designed for ductile behavior and to perform satisfactorily despite severe fault load demandsBridge can be designed for no-collapse using articulation and ductility, but severe damage should be expected.
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THANK YOU