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Bridges & Structures

Seismic Design of Bridges

Lucero E. Mesa, P.E.

PowerPoint Version (10 mb)


Slide 2

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

Slide 3

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

Slide 4

CHARLESTON, SOUTH CAROLINA
August 31, 1886 (Intensity IX-X)

Map of mid and Eastern US showing maximum intensity of ground motion felt during August 31, 1886 Charleston, SC earthquake.


Slide 5

Earthquake of August 31, 1886

Charleston, South Carolina
Magnitude=7.3M, Intensity = X

Building damage from August 31, 1886 Charleston, SC earthquake.

Debris from building collapse during August 31, 1886 Charleston, SC earthquake


Slide 6

Sandblow in Charleston

Photo showing a sandblow caused by liquefaction during August 31, 1886 Charleston, SC earthquake.


Slides 7-8

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
  • Force based specifications
  • N (seat width)
  • Soil classification: I - IV
  • Draft Specifications Version of 1999

Slide 9

Site Specific Studies

  • Maybank Bridge over the Stono River
  • Carolina Bays Parkway
  • Broad and Chechessee River Bridges
  • New Cooper River Bridge
  • Bobby Jones Expressway

Slide 10

SEISMIC DESIGN TRIAL EXAMPLES

  • SC-38 over I-95 - Dillon County
  • Maybank Highway Bridge over the Stono River - Charleston County

Slides 11-12

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
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

Slide 13

Maybank Highway Bridge
over the Stono River


Slide 14

USGS seismic hazard map showing location of South Carolina.

Seismicity of South Carolina from 1971 to 1996


Slides 15-16

Maybank Highway Bridge over the Stono River

  • 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
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.

Slide 17
Table C-1. LRFD Special 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

Graph of response spectra developed from LRFD seismic specification (NCHRP 12-49), site specific study (Maybank Bridge Project), and SCDOT draft specifications, 1996, ground motion maps for safety evaluation earthquake.


Slide 18

Maybank Highway over Stono River


Original Seismic Design
  • Soil Classification: Type II
Trial Design Example
  • Stiff Marl classified as Site Class D

Slides 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.

Slide 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

Rendering of an elevation of the Cooper River cable stayed bridge tower and foundation.


Slide 21

View Photo


Slide 22

Cooper River Bridge


2500 Yr - SEE for Main Piers

View Graph


Slide 23

Need for:

  • New Specifications
  • South Carolina Seismic Hazard Maps

Slide 24

Photo of Cover page of Seismic Design Specifications for Highway Bridges - SCDOT 2001

Photo of Preface to SCDOT Seismic Design Specification for Highway Bridges. October 2001 as Amended by October 2002 Interim Revisions


Slide 25

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.

Slide 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.

Slide 27

New Concepts and Enhancements

  • New Design Level Earthquakes
  • New Performance Objectives
  • New Soil Factors
  • Displacement Based Design
  • Expanded Design Criteria for Bridges

Slides 28-29

SCDOT Seismic Design Specifications October 2001

  • Small to Moderate Earthquakes
    • Essentially Elastic
    • No Significant Damage
    • Functional Evaluation Earthquake (FEE) or 10% in 50 yr. event
  • Large Earthquakes
    • Life Safety
    • No Collapse
    • Serviceability
    • Detectable and Accessible Damage
    • Safety Evaluation Earthquake (SEE) or 2% in 50 yr. event

Slide 30

SCDOT Seismic Design Specifications Background (1.2)

  • 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

Slide 31

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

Slide 32

SCDOT Seismic Design Specifications Seismic
Performance Criteria


Table 3.2.1 Seismic Performance Criteria
Ground Motion Level Performance Level Normal Bridges
III
Essential Bridges
II
Critical Bridges
I
Functional-Evaluation Service NR* NR Immediate
Damage NR NR Minimal
Safety-Evaluation Service Impaired Recoverable Maintained
Damage Significant Repairable Repairable

*Functional Evaluation Not Required


Slide 33

SCDOT Seismic Design Specifications
October 2001


Table 3.6 Seismic Performance Category (SPC)
Value of SD1-SEE Importance Classification (IC)
I II III
SD1-SEE<0.30g B B A
0.3g<SD1-SEE<0.45g C C B
0.45g<SD1-SEE<0.6g D C C
0.6g<SD1-SEE D D C

Slide 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 SS>1.25
A 0.8 0.8 0.8 0.8 0.8
B 1 1 1 1 1
C 1.2 1.2 1.1 1 1
D 1.6 1.4 1.2 1.1 1
E 2.5 1.7 1.2 0.9 a
F a a a a a

Slide 35

View Graph


Slide 36

SCDOT Seismic Design Specifications October 2001

Graph of spectral response acceleration vs period, response spectra shape as defined by NCHRP 12-49, and USGS mapped values of 0.2 sec and 1.0 sec spectral accelerations.


Slide 37

DESIGN SPECTRA FOR SITE CLASS A, B, C, D AND E, 5% DAMPING (3.4.5E)
APPLICABILITY (3.1)

View Graph


Slide 38

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

Slide 39

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

Slide 40

Design Approaches .(4.7.1)


Design Approach Ductility Demand Protection Systems Reparability
Minimal Plastic Action Limited μd<2 May Be Used Not Required To Maintain
Moderate Plastic Action Limited μd<4 May Be Used May Require Closure Of Limited Usage
Significant Plastic Action May Be Higher Not Warranted May Require Closure Or Removal

Slide 41

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)

Slide 42

Seismic Design of Bridges

Lucero E. Mesa, P.E.

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Updated: 06/13/2013
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