Seismic Designof 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 http://www.tekniksipil.org/civil-
engineering
AASHTO Div IA
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CHARLESTON, SOUTH CAROLINAAugust 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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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-95Description 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 Bridgeover the Stono River
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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 RiverDescription 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)
Spec
tral
Acc
eler
atio
n, S
a (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 BridgeCharleston 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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0
0.5
1
1.5
2
2.5
0 1 2 3 4 5
Period, sec
Sp
ect
ral
Acc
ele
rati
on
, g
Cooper River Bridge2500 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 developmentSmall 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. 26
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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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)
SiteClass
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
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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_4ASD_4BSD_4CSD_4DSD_4E
Periods T (sec)
Site Class A B C D E
SDI-SEE
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APPLICABILITY (3.1)
New Bridges Bridge TypesSlabBeam GirderBox 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 warrantedMay be higher
Significant Plastic Action
May require closure of
limited usage
May be UsedLimitedModerate Plastic Action
Not required to Maintain
May be UsedLimitedMinimal Plastic Action
ReparabilityProtection Systems
Ductility Demand
Design Approach
2D