ACI 341-C Retrofit Presentation - Part 1

ACI Committee 341-CState-of-the-Art Summary

Seismic Evaluation and Retrofit

Techniques for Concrete Bridges

Committee 341-CRetrofit of Concrete

Bridges• Sub-committee members:– Dawn Lehman and Sri Sritharan (co-chairs)– Adolfo Matamoros, Anthony Powers, David

Sander (authors)– Ayman Salama, Raj Valluvan, Eric Williamson

• Additional Contributions:– Photographs: NISEE Image Database – Analysis of SR-99: WashDOT

UW: Blake Inouye, John Stanton, Dawn Lehman

http://nisee.berkeley.edu/eqiis.html

1971 San Fernando1971 San Fernando

Bridge Damage in Previous Earthquakes

1989 Loma Prieta1989 Loma PrietaBridge Damage in Previous

Earthquakes

1994 Northridge1994 NorthridgeBridge Damage in Previous

Earthquakes

1995 Kobe1995 Kobe

Bridge Damage in Previous Earthquakes

Report Objectives

• Describe key aspects of seismic retrofit program– General understanding of each phase– Conceptual design and analysis methods

• Emphasize design for structural stability• Rich resource of appropriate references

Resource EvaluationMulti-Phase Program

IMPLEMENTATION

Member ResponseGlobal Response

SELECTION AND DESIGN OF RETROFIT MEASURES

SEISMIC EVALUATIONOF EXISTING

SYSTEM

System CapacitySeismic DemandDemand/Capacity Ratio

Phases of Retrofit ProgramSEISMIC

VULNERABILITYEVALUATION

Seismic Hazard

Structural Vulnerabilities

Socio-Economic Consequences

Phases of Retrofit Program:Seismic Vulnerability

Evaluation• Local Soil Conditions• Soil Response and Failure

Source

Path

Site

Evaluation of Site-Specific Hazard


Evaluation• Geometry• Date of Design and Construction

Evaluation of Structural Vulnerability


EvaluationEvaluation of Socio-Economic Consequences• Casualties• Lifeline Interruption• Economic Impact

Phases of Retrofit Program:Seismic Demand/Capacity

Evaluation• Determine as-built conditions• Existing material properties• Estimate capacity of components

Evaluation of Seismic Capacity

(Priestley et al., 1994)


Evaluation• Established Analysis Methods

• Linear or Nonlinear• Multi-Spectra or Time-History

Evaluation of Seismic Demand

Period

Acce

lerat

ion

T


Evaluation

Determine Demand/Capacity Ratios• Global Displacement• Local Deformations and Forces

Phases of Retrofit Program:Seismic Retrofit Measures

• Based on Demand/Capacity Evaluation• Select at Member and/or System Level• Address Global Response

Phases of Retrofit Program:Implementation• Multi-Phase Retrofit Programs• Depends on State and DOT

Figure 1.2 Typical Cable Restrainer SystemCourtesy of the University of Washington

Figure 1.2 Typical Cable Restrainer SystemCourtesy of the University of Washington

Initial Retrofit Measures

Cable Restrainer

More Costly Measures:

Beam and Column Retrofit

Sri SritharanTony Powers



SYSTEMAdolfo Matamoros

Presentation of ReportSEISMIC


David Sanders

INTRODUCTIONCONCLUSIONS

EDITINGDawn Lehman

Seismic Vulnerability Evaluation

• Bridge Geometry• Structural Redundancy• Expansion Joints• Age of Design ~ Vulnerable

Elements• Structural Condition• Condition of Supporting Soil


Bridge Geometry• Bent Configurations• Degree of Skew or Curvature• Flared Columns • Short Seat Widths• Multi-Level Systems• Multiple

Superstructure Types


Vulnerable Elements• Columns• Cap Beams• Joints• Foundations• Hinges and Supports• Superstructure• Abutments• Inadequate Confinement

• Inadequate Shear Strength• Location and Strength of Lap Splices


Vulnerable Elements• Columns• Cap Beams• Joints• Foundations• Hinges and Supports• Superstructure• Abutments • Reduced Flexural

Strength(Insufficient Bar Anchorage)

• Inadequate Shear Strength

• Inadequate Strength in Torsion


Vulnerable Elements• Columns• Cap Beams• Joints• Foundations• Hinges and Supports• Superstructure• Abutments • Insufficient Bar

Anchorage• Inadequate Shear

Strength• Inadequate Joint

Steel


Vulnerable Elements• Columns• Cap Beams• Joints• Foundations• Hinges and Supports• Superstructure• Abutments • Insufficient Flexural

Strength• Inadequate Shear

Strength• Inadequate

Anchorage


Vulnerable Elements• Columns• Cap Beams• Joints• Foundations• Hinges and Supports• Superstructure• Abutments • Insufficient Seat

Length• Bearing

Instability


Vulnerable Elements• Columns• Cap Beams• Joints• Foundations• Hinges and Supports• Superstructure• Abutments • Lack of Transverse

Shear Keys• Damage from Skewed

Bridges• Settlement

Seismic Evaluation

• Seismic Demand• Seismic Capacity• Demand/Capacity Ratios

Seismic Evaluation:Seismic Demand

• Determine Appropriate Analysis Method– Linear– Nonlinear

• Develop Model• Evaluate Demands for Design

Earthquakes

Seismic Demand Evaluation:Appropriate Analysis

Method• Linear– Single-Mode Response Spectrum

•“Simple” System•Regular Mass and Stiffness

– Multi-Mode Response Spectra•More Complex System• Irregular Mass, Stiffness Geometry

– Time History•Complex System•Soil Springs/Dampers

Seismic Demand Evaluation:Appropriate Analysis

Method• Nonlinear Analysis Methods

– Limit or Pushover Analysis•Demands on System (Target Displacement)•Paired with a Dynamic Analysis

– Stand Alone Frame Analysis•Provides Information on Nonlinear Behavior•Neglects Frame and Abutment Interaction

– Time History Analysis

Example of:Appropriate Analysis

Method• SR-99 Bridge• Partial Retrofit• Different

SuperstructureSystems

• Retrofit OutriggerJoints and Beams?


Method• Time-History Analysis• Gap Elements• Soil Springs

AbutmentAbutment NorthNorth + + Off-rampOff-ramp SteelSteel SouthSouthNorth North SteelSteel

Off-rampOff-ramp

AbutmentAbutment

Steel & South Steel & South Concrete StructuresConcrete Structures


MethodModeling Issues

– Material Strengths– Effective Stiffness Values– Stiffness of Jacketed Columns– Model of Superstructure– Stiffness of Adjacent Structures– Soil Springs and Dampers

Example of Appropriate Analysis Method:

Model Verification

Red Gaps = Closed

Gap ClosuresGap ClosuresPredicted: 72 yr. EQ Actual: Nisqually EQPredicted: 72 yr. EQ Actual: Nisqually EQ


Method Analysis Results:• Drift Demands in Outrigger Joints• ID Yielding Columns

From Capacity Evaluation:• Joint Shear Stress Demands• Beam Torsion Demands• Beam Shear Demands

Seismic Evaluation:Seismic Capacity

• Determine Expected Material Strengths– Overstrength in Concrete: Aging– Overstrength in Steel: Strain-

Hardening, Material• Calculate Element Capacities

– Calculate Flexural Capacities– Calculate Shear Strength– Calculate Anchorage or Development

Strength

Seismic Capacity/Demand Evaluation

1. Calculate D/C Ratios for All Elements

2. Determine Critical Failure Modes/Elements

3. Determine Appropriate Retrofit Measures

Example of:Demand/Capacity Evaluation

• Critical Elements– Beam in Torsion– Exterior

Anchorage in Joint

• Retrofit Measure– Steel Jacketing

Beams & Joints

Sri SritharanTony Powers



SYSTEMAdolfo Matamoros

Presentation of ReportSEISMIC


David Sanders

ACI Subcommittee 341–C STATE OF THE ART SUMMARY ON SEISMIC RETROFIT TECHNIQUES

FOR CONCRETE BRIDGES

Retrofit design philosophyRetrofit design philosophy

Avoid excessive damage to members and prevent

structural collapse of the bridge

Objective

• Satisfy strength and displacement demands expected under the design-level earthquakes.– Ensure a desirable yield mechanism– Limit inelastic actions to preselected locations– Column ends are typically selected in bridges– Avoid non-ductile response modes (e.g.,

shear and bond failure; inelastic response of non-ductile members)

Procedure

• Provide sufficient ductility capacity to the potential plastic hinge regions in columns

• Strengthen other members using capacity design principles using the column overstrength moments.

• Add new elements• Reduce seismic demands to avoid inelastic

response in capacity-protected members

Procedure (Cont..)

• Complete retrofit design at member level• Analyze the retrofitted structure to ensure

adequate response of the system. • If necessary, redesign retrofit measures or

introduce a new retrofit scheme

• Columns• Cap Beams• Joints• Footings• Hinges and Supports• Superstructure• Abutments

Vulnerable Structural Elements

• Inadequate Confinement• Inadequate Shear Strength• Location and Strength of Lap Splices

• Provide uniform pressure• Steel, concrete and advanced composites• Use wraps or jackets• Required over 1.5 to 2 times the length of

the plastic hinge region• Circular or oval shaped sections• Leave a gap between column and wrap• Fill gap with grout or concrete• Leave a gap between the column and joint

Confinement retrofit

(Courtesy of University of California, San Diego)

Confinement retrofit – Circular column

(Courtesy of Jacobs Civil Inc.)


Rectangular column


US40/I64 Double deck seismic retrofit in St. Louis

Active prestressed wire wraps and welded wire fabric


Prefabricated composite jacketing of column

(Courtesy of University of Southern California)

Improved Confinement Detail

• Section with curvature ductility of 20• 10% – 75% increase in the effective

elastic stiffness• The new column stiffness should be

included in the system level analysis of the retrofitted bridge

Non-Prismatic Columns

(Courtesy of University of Nevada, Reno)

U-shaped GFRP straps

FRP straps

Half shell steel jackets

(Courtesy of University of Nevada, Reno)

Flared Columns Retrofitted with U-shaped GFRP Straps

Construction at US 395/I 80 Interchange, Reno (Courtesy of University of Nevada, Reno)

Retrofitted Bent

Multi-Column Bents – Transverse Direction

Column Lap Splice Retrofit• Control dilatation strains• Provide sufficient confinement• Confinement retrofit required for the

inelastic response may be sufficient• Rectangular sections are not effective

– unless spliced bars are welded for continuity

Column Retrofit to improve shear capacity

• Estimate demands– assume full development of column hinge– Include material over-strength

• Most techniques used for confinement retrofit are appropriate

• Retrofit is typically required along the full column height

(Courtesy of FHWA)

CFRP(Courtesy of University of California, San Diego)

Steel Jacket


• Columns• Cap Beams• Joints• Footings• Hinges and Supports• Superstructure• Abutments • Reduced Flexural

Strength(Insufficient Bar Anchorage)

• Inadequate Shear Strength

• Inadequate Strength in Torsion

• Post-tensioning cap beam is an effective retrofit measure – may require an increase in dimensions– may require addition of end blocks– will improve joint performance– will enhance torsional resistance

• Concrete bolsters and new reinforcement• Steel jacket retrofit• FRP wraps

Cap Beam Retrofit Measures

Cap Beam Retrofit – Prestressing

(Courtesy of Jacobs Civil Inc.)(Courtesy of University of California, San Diego)

Concrete bolster

(Courtesy of University of California, Berkeley)

Adding Concrete Bolster

Reducing Seismic Demand



• Columns• Cap Beams• Joints• Footings• Hinges and Supports• Superstructure• Abutments • Insufficient Bar

Anchorage• Inadequate Shear

Strength• Inadequate Joint

Steel

• External prestressing• Complete replacement of the joint region

– increase in dimensions– Increase in column bar embedment length– new joint shear reinforcement

• Jacketing of the joint using concrete, steel or composite materials

• Reduce demand using a link beam

Joint Retrofit Measures

Joint Retrofit

(Courtesy of University of Utah)


Complete Joint Replacement


Documents

ACI 341-C Retrofit Presentation - Part 1