Seismic Design of a Light Rail Transit Bridge with Fault

Rupture Crossing

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

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

Regional Faulting

Project Alignment

Desk Top Study – Vintage Stereoscopic Aerial Photo Interpretation

Interpretations by Scott Rugg and Tom Rockwell (Kleinfelder 2013)

Station

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)

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)

Geologic Mapping of Cut Surface

Secondary Active Faults

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

Fault Rupture Design Scenario

Proposed LRT Overhead BridgeProposed Rail Bridge

4 ft

1.2 ft1.2 ft

Fault Rupture Design Scenario

Ben

t 2

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

Modeling of Soil-Fault Foundation System

Modeling of Soil-Fault Foundation System

Pile Model

Concrete Cracking

Rebar Stresses

Soil Model Calibration and Validation

Centrifuge test data from Loli et al. (2009)

Model simulation results

Constitutive model approach of Anastasopoulos et al. (2007)

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

Abutment Modeling Results

6 543210

Horz. Disp. (feet)

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)

Fault Rupture Design Displacements

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

Bridge DesignBridge Demand:

𝑢𝑢𝑜𝑜𝑜𝑜 = 𝑢𝑢𝑜𝑜𝑜𝑜 + 𝑢𝑢𝑜𝑜𝑜𝑜Peak Seismic Response of the Bridge

Peak Quasi-Static Demand

Peak Dynamic Demand

Bridge DesignBridge Alternatives

Deformed Shape of a Continuous Bridge Due to Surface Fault Rupture (Integral Bent Cap)

Bridge DesignBridge Alternatives

Deformed Shape of a Continuous Bridge Due to Surface Fault Rupture (Dropped Bent Cap)

Bridge DesignBridge Alternatives

Deformed Shape of a Simply Supported Bridge Due to Surface Fault Rupture

Bridge Design

Simple spans with pre-cast girdersWidened seatsArticulationCompression: gap at Abut + Pin Fuse at B2

Gap allows compression

Shear Pin Fuse

Abutment DesignShear Key

5 ft compression vault

Details at Bent 2

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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