seismic design of bridges - seismic design of bridges overview . 3 • usgs 1988 seismic hazard maps...

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  • 1

    Seismic Design

    of Bridges

    Lucero E. Mesa, P.E.

  • 2

    AASHTO - Division IA

    Draft Specifications, 1996

    SCDOT 2001 Seismic Design Specifications

    Comparison Between LRFD & SCDOT Specs.

    SCDOT Seismic Hazard Maps

    Training and Implementation

    Conclusions

    SCDOT Seismic Design Of Bridges

    Overview

  • 3

    USGS 1988 Seismic Hazard Maps

    Force based design

    Soil Classification I-IV

    No explicit Performance Criteria

    Classification based only on acceleration

    coefficient

    AASHTO Div IA

  • 4

    CHARLESTON, SOUTH CAROLINA

    August 31, 1886 (Intensity IX-X)

  • 5

    Earthquake of August 31, 1886 Charleston, South Carolina

    Magnitude=7.3M, Intensity = X

  • 6

    Sandblow in Charleston

  • 7

    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

    Draft Specifications

  • 8

    Force based specifications

    N (seat width)

    Soil classification: I IV

    Draft Specifications Version of

    1999

    Draft Specifications

  • 9

    Maybank Bridge over the Stono River

    Carolina Bays Parkway

    Broad and Chechessee River Bridges

    New Cooper River Bridge

    Bobby Jones Expressway

    Site Specific Studies

  • 10

    SC-38 over I-95 - Dillon County

    Maybank Highway Bridge over the

    Stono River - Charleston County

    SEISMIC DESIGN TRIAL

    EXAMPLES

  • 11

    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

  • 12

    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

  • 13

    Maybank Highway Bridge

    over the Stono River

  • 14

    Highest Hazard

    Lowest Hazard

    Highest Hazard

    Lowest Hazard

    Seismicity of South Carolina 1977 to 1996

  • 15

    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 River Description of project

  • 16

    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

  • 17

    Table C-1. LRFD Spectral Accelerations and Site Coefficients

    Earthquake Spectral Accelerations Site Coefficients

    SS S1 SDS SD1 Fa Fv

    Maximum Considered 1.43 0.407 1.43 0.651 1.00 1.60

    Expected 0.0503 0.0104 0.0503 0.0167 1.00 1.60

    SEE - Compare LRFD to Original Design Curve

    0

    0.2

    0.4

    0.6

    0.8

    1

    1.2

    1.4

    1.6

    0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0Period, T (sec)

    Spectral Acceleration, Sa (g)

    LRFD CurveSite Specific Original CurveSCDOT Curve, soil type IISCDOT Curve, soil type III

    * The cumulative mass participation for

    mode shapes at periods indicated and

    higher, is approximately 70%.

    * Transverse

    * Longitudinal

  • 18

    Original Seismic Design

    Soil Classification: Type II

    Trial Design Example

    Stiff Marl classified as Site Class D

    Maybank Highway over Stono River

  • 19

    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 .

  • 20

    Cooper River Bridge

    Charleston 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

  • 21

  • 22

    0

    0.5

    1

    1.5

    2

    2.5

    0 1 2 3 4 5

    Period, sec

    Spectral

    Acceleration, g

    Cooper River Bridge

    2500 Yr - SEE for Main Piers

  • 23

    New Specifications

    South Carolina Seismic

    Hazard Maps

    Need for:

  • 24

  • 25

    The new SCDOT specifications

    establish design and construction

    provisions for bridges in South

    Carolina to minimize their

    susceptibility to damage from large

    earthquakes.

    SCDOT Seismic Design Specifications October 2001

  • 26

    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.

  • 27

    New Design Level Earthquakes

    New Performance Objectives

    New Soil Factors

    Displacement Based Design

    Expanded Design Criteria for Bridges

    New Concepts and

    Enhancements

  • 28

    Small to Moderate Earthquakes

    Essentially Elastic

    No Significant Damage

    Functional Evaluation Earthquake

    (FEE) or 10% in 50 yr. event

    SCDOT Seismic Design Specifications October 2001

  • 29

    Large Earthquakes

    Life Safety

    No Collapse

    Serviceability

    Detectable and Accessible Damage

    Safety Evaluation Earthquake

    (SEE) or 2% in 50 yr. event

    SCDOT Seismic Design Specifications October 2001

  • 30

    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)

  • 31

    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

    Upgraded Seismic Design Requirement (1.3)

  • 32

    SCDOT Seismic Design Specifications Seismic Performance Criteria

    III II I

  • 33

    SCDOT Seismic Design Specifications October 2001

  • 34

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

    Site

    Class

    Design Spectral Acceleration at Short Periods

    SS 0.25

    SS=0.50

    SS=0.75

    SS=1.00

    SS1.25

    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

  • 35

    SCDOT Pilot WorkshopImbsen & Associates, Inc.

    I IA

    1-6

    Increasing

    performanceIncreasing earthquake

    hazard

    Recent

    Technology

    bc

    de

    f

    ih

    gCollapse

    Prevention

    Limited

    Damage

    Essent ially

    Elast ic

    2% in 50 Yrs.

    2/3 (2% in 50 Yrs.)

    10% in 50 Yrs.

    Proposed Design or Retrofit Objective

    a

    f, h, ia, b, c, d,

    e, g

    Secondary

    System

    Primary

    SystemDesign or

    Retrofit

    Objective

  • 36

    SCDOT Seismic Design Specifications October 2001

  • 37

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

    Ss=1.00g, SEE(2%/50years)

    0.0

    0.2

    0.4

    0.6

    0.8

    1.0

    1.2

    0 1 2 3 4

    SD_4A

    SD_4B

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