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Seismic Design of Bridges PowerPoint PPT Presentation


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Lucero E. Mesa, P.E. Seismic Design of Bridges. SCDOT Seismic Design Of Bridges Overview. AASHTO - Division IA Draft Specifications, 1996 SCDOT 2001 Seismic Design Specifications Comparison Between LRFD & SCDOT Specs. SCDOT Seismic Hazard Maps Training and Implementation Conclusions.

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Seismic Design of Bridges

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Lucero E. Mesa, P.E.

Seismic Designof Bridges


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SCDOT Seismic Design Of Bridges Overview

  • AASHTO - Division IA

  • Draft Specifications, 1996

  • SCDOT 2001 Seismic Design Specifications

  • Comparison Between LRFD & SCDOT Specs.

  • SCDOT Seismic Hazard Maps

  • Training and Implementation

  • Conclusions


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AASHTO Div IA

  • USGS 1988 Seismic Hazard Maps

  • Force based design

  • Soil Classification I-IV

  • No explicit Performance Criteria

  • Classification based only on acceleration coefficient


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CHARLESTON, SOUTH CAROLINAAugust 31, 1886 (Intensity IX-X)


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Earthquake of August 31, 1886 Charleston, South CarolinaMagnitude=7.3M, Intensity = X


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Draft Specifications

  • 1996 USGS Seismic Hazard Maps

  • Difference in spectral acceleration between South Carolina and California

  • Normal Bridges : 2/3 of the 2% in 50 yr. Event

  • Essential Bridges: Two-Level Analysis


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Draft Specifications

  • Force based specifications

  • N (seat width)

  • Soil classification: I – IV

  • Draft Specifications Version of 1999


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Site Specific Studies

  • Maybank Bridge over the Stono River

  • Carolina Bays Parkway

  • Broad and Chechessee River Bridges

  • New Cooper River Bridge

  • Bobby Jones Expressway


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SEISMIC DESIGN TRIAL EXAMPLES

  • SC-38 over I-95 - Dillon County

  • Maybank Highway Bridge over the Stono River - Charleston County


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SC-38 over I-95

Description of Project

  • Conventional bridge structure

  • Two 106.5 ft. spans with a composite reinforced concrete deck, supported by 13 steel plate girders and integral abutments

  • The abutments and the interior bents rest on deep foundations


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Original Seismic Design

SCDOT version of Div-IA AASHTO (Draft)

2/3 of 2% in 50 yr

1996 USGS maps used

PGA of 0.15g, low potential for liquefaction

Response Spectrum Analysis

Trial Design Example

Proposed LRFD Seismic Guidelines

MCE –3% PE in 75 yr.

Expected Earthquake – 50% PE in 75 yr.

2000 USGS maps

PGA of 0.33g, at MCE, further evaluation for liquefaction is needed.

Response Spectrum Analysis

SC-38 over I-95


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Maybank Highway Bridgeover the Stono River


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118 spans

1-62 flat slab deck supported by PCP

63-104 /33 -meter girder spans and 2 columns per bent supported by shafts.

The main span over the river channel consists of a 3 span steel girder frame w/ 70 meter center span.

105-118 flat slab deck supported by PCP

Maybank Highway over Stono RiverDescription of project


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Original Seismic Design

SCDOT version of AASHTO Div. I-A (Draft)

Site Specific Seismic Hazard

Bridge classified as essential

Project specific seismic performance criteria

Two level Analysis:

FEE – 10% in 50 yr. event

SEE - 2% in 50 yr. event

Trial Design Example

Proposed LRFD Guidelines -2002

Two Level Analysis:

Expected Earthquake - 50% in 75 yr.

MCE – 3% in 75 yr.

Maybank Highway over Stono River


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Original Seismic Design

Soil Classification: Type II

Trial Design Example

Stiff Marl classified as Site Class D

Maybank Highway over Stono River


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  • The SCDOT 's new specifications adopted the NCHRP soil site classification and the Design Spectra described on LRFD 3.4.1

  • If this structure were designed using the new SCDOT Seismic Design Specifications, October 2001, the demand forces would be closer if not the same to those found using the Proposed LRFD Guideline -2002 .


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Cooper River BridgeCharleston Co.

  • Seismic Design Criteria- Seismic Panel

  • Synthetic TH

  • PGA - 0.65g

  • Sa 1.85 at T=0.2 sec

  • Sa 0.65 at T=1 sec

  • Liquefaction


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Cooper River Bridge2500 Yr - SEE for Main Piers


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Need for:

  • New Specifications

  • South Carolina Seismic Hazard Maps


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SCDOT Seismic Design Specifications

October 2001

  • The new SCDOT specifications establish design and construction provisions for bridges in South Carolina to minimize their susceptibility to damage from large earthquakes.


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PURPOSE & PHILOSOPHY (1.1)

  • SCDOT Seismic Design Specifications replace AASHTO Division I-A SCDOT Draft

  • Principles used for the development

    • Small to moderate earthquakes, FEE, resisted within the essentially elastic range.

    • State-of-Practice ground motion intensities are used.

    • Large earthquakes, SEE, should not cause collapse.

  • Four Seismic Performance Categories (SPC) are defined to cover the variation in seismic hazard of very small to high within the State of South Carolina.


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New Concepts and Enhancements

  • New Design Level Earthquakes

  • New Performance Objectives

  • New Soil Factors

  • Displacement Based Design

  • Expanded Design Criteria for Bridges


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New USGS Probabilistic Seismic Hazard Maps

New Design Level Earthquakes

New Performance Objectives

A706 Reinf. Steel

New Soil Factors

Displacement Based Design

Caltrans (SDC) new provisions included

SCDOT Seismic Design Specifications

Background (1.2)


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Upgraded Seismic Design Requirement

(1.3)

  • New Provisions meet current code objectives for large earthquakes.

    • Life Safety

    • Serviceability

  • Design Levels

    • Single Level – 2% / 50 years

      • Normal Bridges

      • Essential Bridges

    • Two Level : 2% / 50 years and 10% / 50 years

      • Critical Bridges


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SCDOT Seismic Design Specifications

Seismic Performance Criteria

III

II

I


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SCDOT Seismic Design Specifications

October 2001


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Site Class

Design Spectral Acceleration at Short Periods

SS 0.25

SS=0.50

SS=0.75

SS=1.00

SS1.25

A

0.8

0.8

0.8

0.8

0.8

B

1.0

1.0

1.0

1.0

1.0

C

1.2

1.2

1.1

1.0

1.0

D

1.6

1.4

1.2

1.1

1.0

E

2.5

1.7

1.2

0.9

a

F

a

a

a

a

a

VALUES OF Fa AS A FUNCTION OF SITE CLASS AND MAPPED SHORT-PERIOD SPECTRAL RESPONSE ACCELERATION SS (TABLE 3.3.3A)


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SCDOT Seismic Design Specifications

October 2001


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DESIGN SPECTRA FOR SITE CLASS A, B, C, D AND E, 5% DAMPING (3.4.5E)

SDI-SEE


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APPLICABILITY (3.1)

  • New Bridges

  • Bridge Types

    • Slab

    • Beam Girder

    • Box Girder

  • Spans less than 500 feet

  • Minimum Requirements

  • Additional Provisions are needed to achieve higher performance for essential or critical bridges


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DESIGN PHILOSOPHY AND STRATEGIES

  • Specifications can be used in conjunction with rehabilitation, widening, or retrofit

  • SPC B demands are compared implicitly against capacities

  • Criteria is focused on member/component deformability as well as global ductility

  • Inherent member capacities are used to resist higher earthquake intensities

  • Using this approach required performance levels can be achieved in the Eastern US


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Design Approach

Ductility Demand

Protection Systems

Reparability

Minimal Plastic Action

Limited

May be Used

Not required to Maintain

Moderate Plastic Action

Limited

May be Used

May require closure of limited usage

Significant Plastic Action

May be higher

Not warranted

May require closure or removal

Design Approaches (4.7.1)


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Other New Concepts and Improvements

  • Plastic Hinge Region Lpr (4.7.7)

  • Plastic Hinge Length (4.7.7)

  • Seat Width SPC A and B, C, D(4.8.2)

  • Detailing Restrainers (4.9.3)

  • Butt Welded Hoops

  • Superstructrure Shear Keys(4.10)


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Lucero E. Mesa, P.E.

Seismic Designof Bridges

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