Seismic Design of Bridges

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Seismic Designof Bridges

• Lucero E. Mesa, P.E.

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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
CarolinaMagnitude7.3M, 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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SC-38 over I-95

• 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

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Maybank Highway Bridgeover the Stono River
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Maybank Highway over Stono RiverDescription of
project

• 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

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Maybank Highway over Stono River

• 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.

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Maybank Highway over Stono River

• Original Seismic Design
• Soil Classification Type II

• Trial Design Example
• Stiff Marl classified as Site Class D

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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 T0.2 sec
• Sa 0.65 at T1 sec
• Liquefaction

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Cooper River Bridge 2500 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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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

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SCDOT Seismic Design Specifications October 2001

• Large Earthquakes
• Life Safety
• No Collapse
• Serviceability
• Detectable and Accessible Damage
• Safety Evaluation Earthquake
• (SEE) or 2 in 50 yr. event

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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

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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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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 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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Seismic Designof Bridges
Thanks

• Lucero E. Mesa, P.E.