Transcript Document 7320808

Seismic Design
of Bridges
Lucero E. Mesa, P.E.
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SCDOT Seismic Design Of Bridges
Overview
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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
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AASHTO Div IA
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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
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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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Draft Specifications
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1996 USGS Seismic Hazard Maps
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Difference in spectral acceleration
between South Carolina and California
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Normal Bridges : 2/3 of the 2% in 50 yr.
Event
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Essential Bridges: Two-Level Analysis
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Draft Specifications
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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
Trial Design Example
• 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
• 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 Bridge
over the Stono River
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Highest Hazard
Lowest Hazard
Seismicity of South
Carolina 1977 to 1996
1977 to 1997
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Maybank Highway over Stono River
Description 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
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Two level Analysis:
Trial Design Example
• Proposed LRFD Guidelines 2002
• Two Level Analysis:
• Expected Earthquake - 50%
in 75 yr.
• MCE – 3% in 75 yr.
 FEE – 10% in 50 yr. event
 SEE - 2% in 50 yr. event
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Table C-1. LRFD Spectral Accelerations and Site Coefficients
Earthquake
Maximum Considered
Expected
Spectral Accelerations
SS
S1
SDS
SD1
1.43
0.407
1.43
0.651
0.0503 0.0104 0.0503 0.0167
Site Coefficients
Fa
Fv
1.00
1.60
1.00
1.60
SEE - Compare LRFD to Original Design Curve
1.6
LRFD Curve
Site Specific Original Curve
SCDOT Curve, soil type II
SCDOT Curve, soil type III
Spectral Acceleration, Sa (g)
1.4
1.2
* The cumulative mass participation for
mode shapes at periods indicated and
higher, is approximately 70%.
* Transverse
1
0.8
0.6
* Longitudinal
0.4
0.2
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Period, T (sec)
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Maybank Highway over Stono River
Original Seismic Design
Trial Design Example
• Soil Classification: Type
II
• 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 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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Cooper
River
Bridge
US17 COOPER RIVER BRIDGES
2500-YR
SEE for
for Main
Main Piers
Piers
2500
Yr - SEE
(Vertical Spectra)
Spectral
Acceleration, g
2.5
2
1.5
1
0.5
0
0
1
2
3
4
5
Period, sec
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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
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of South Carolina.
New Concepts and
Enhancements
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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
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)
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New Provisions meet current code
objectives for large earthquakes.
 Life Safety
 Serviceability
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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 SHORTPERIOD 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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Primary
Design or System
Retrofit
a, b, c, d,
Objective
e, g
Secondary
System
f, h, i
c
b
a
f
e
d
i
Recent
Technology
h
g
Collapse
Prevention
Limited
Damage
2% in 50 Yrs.
Essentially
Elastic
2/3 (2% in 50 Yrs.)
10% in 50 Yrs.
Increasing
performance
Increasing earthquake
hazard
Proposed Design or Retrofit Objective
I A I
SCDOT – Pilot Workshop
Imbsen & Associates, Inc. –
1-6
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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)
1.2
1.0
Site Class
SD_4A
A
SD_4B
B
C
SD_4C
D
SD_4D
E
SD_4E
SDI-SEE
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
Periods T (sec)
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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)
Design
Approach
Ductility
Demand
Minimal
Plastic Action
Limited
May be Used
Not required to
Maintain
Limited
May be Used
May require
closure of
limited usage
May require
closure or
removal
Moderate
Plastic Action
D  2
D  4
Significant
Plastic Action May be higher
Protection Reparability
Systems
Not
warranted
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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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Thanks
Seismic Design
of Bridges
Lucero E. Mesa, P.E.
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