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Seismic Design
of Bridges
Lucero E. Mesa, P.E.
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• 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
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• 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
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CHARLESTON, SOUTH CAROLINA
August 31, 1886 (Intensity IX-X)
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Earthquake of August 31, 1886 Charleston, South Carolina
Magnitude=7.3M, Intensity = X
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Sandblow in Charleston
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• 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
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• Force based specifications
• N (seat width)
• Soil classification: I – IV
• Draft Specifications Version of
1999
Draft Specifications
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• Maybank Bridge over the Stono River
• Carolina Bays Parkway
• Broad and Chechessee River Bridges
• New Cooper River Bridge
• Bobby Jones Expressway
Site Specific Studies
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• SC-38 over I-95 - Dillon County
• Maybank Highway Bridge over the
Stono River - Charleston County
SEISMIC DESIGN TRIAL
EXAMPLES
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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 Bridge
over the Stono River
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Highest Hazard
Lowest Hazard
Highest Hazard
Lowest Hazard
Seismicity of South Carolina 1977 to 1996
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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 River Description 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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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
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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 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
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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
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• New Specifications
• South Carolina Seismic
Hazard Maps
Need for:
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• 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
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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 Design Level Earthquakes
• New Performance Objectives
• New Soil Factors
• Displacement Based Design
• Expanded Design Criteria for Bridges
New Concepts and
Enhancements
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• 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
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• 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
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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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• 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)
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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)
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
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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
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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)
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
SD_4C
SD_4D
SD_4E
Periods T (sec)
Site Class
A
B
C
D
E
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)
May require
closure or
removal
Not
warranted
May be higher
Significant
Plastic Action
May require
closure of
limited usage
May be Used Limited Moderate
Plastic Action
Not required to
Maintain
May be Used Limited Minimal
Plastic Action
Reparability Protection
Systems
Ductility
Demand
Design
Approach
2D
µ <
4D
µ <
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• 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)
Other New Concepts and
Improvements
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Seismic Design
of Bridges
Lucero E. Mesa, P.E.
Thanks