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Overview of the
Recommended LRFD SeismicDesign Specifications for
Highway Bridges
Ian M. Friedland, P.E.Bridge Technology Engineer
Federal Highway Administration
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Background
Project requested by AASHTO BridgeCommittee in 1997 to update
existing seismicdesign specifications
Initiated in August 98, and conducted underTRBs National
Cooperative Highway ResearchProgram (NCHRP Project 12-49 by
ATC/
MCEER Joint Venture)NCHRP 12-49 completed in 2001; developedLRFD
specification cut and paste provisions
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Background, continued
AASHTO subsequentlyrequested standalone
guide spec version ofthe cut and pasteprovisions, similar
toDivision I-A
MCEER/FHWA fundedrescoping effort toprepared stand-alone
RecommendedGuidelines
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Background, continued
MCEER/FHWA initiated trial design project inDecember 01 to test
and validate the stand -alone Guidelines13 states and FHWA Federal
Lands HighwayDivision conducted trial designs
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Specification Philosophy
Minimize loss of life/injury from unacceptablebridge
performance
Allow bridge damage (possibly requirereplacement) but limit
potential for collapse
Critical (lifeline) bridges should remainfunctional after a
major earthquake
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Philosophy, continued
Upper level ground motions with low probabilityof exceedance
during 75-year bridge design life
Provisions applicable to all regions of U.S.Designer encouraged
to consider and employnew concepts, design approaches,
andstructural details
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Deficiencies in Current Provisions
Based on ATC-6 seismic design guidelinesdeveloped in the late
1970s
Seismic hazard based on 1988 national seismichazard maps which
are no longer consideredadequate or correct
Soil site factors which have been demonstratedin many recent
earthquakes as being incorrectand inadequate
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Deficiencies, continued
Response spectra curve construction that decreasesas 1/T 2/3
rather than 1/T in long-period part of thecurve
Effectively address only concrete design noprovisions specific
to steel or wood super- or
substructures
Period (seconds)
S e
i s m
i c C o e f f
i c i e n
t
1/T 2/3
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New Concepts andMajor Additions, continued
No analysis design concept
Capacity spectrum design procedure
Displacement capacity verification analysis pushover
analysis
Improved foundation design provisions
Improved abutment design provisions
Formal liquefaction assessment and mitigation
design procedures
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New Concepts andMajor Additions, continued
Explicit steel design provisions
Enhanced concrete design provisions
Superstructure design provisions
Bearing design and testing requirements
Seismic isolation provisionsLiquefaction case studies
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Features of the New Provisions
Based on best scientific and engineeringapproaches and
technologies currently usedworldwide for building and bridge
construction
Reviewed by broad cross-section of State bridgeengineers and
consultants, earthquake engineers,experts from various industries
and technologies
Comprehensive parameter study and trial designprogram produced
bridge designs that are inkeeping with existing AASHTO
specifications,while providing significantly higher levels
ofperformance
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Features, Continued
Include a no seismic analysis design approachbased primarily on
good detailing practice, and
which should be applicable to large regions ofthe United
States
Provide substantially more guidance on soilliquefaction and
lateral spread
Specific provisions for the design of steel super-and
substructures
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Trial Design Program Overview
13 States + FHWA FLHD participated
19 trial designs produced
Nationwide effort
Broad range of seismic hazard
Spans 46 ft to 216 ftLengths 133 ft to 1320 ft
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Trial Design Locations Lower 48
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Trial Design Locations - Alaska
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Summary of Design Impacts
Format similar to Division I-AHazard location and soil
basedNo-Analysis provides simplifications forsome regular
bridgesCapacity Spectrum regular bridgesDisplacement Verification
codifiedTwo-Level Design frequent and rareearthquakesGeotechnical
more guidance provided
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Design Impacts, continued
Load Combinations kept simpleR-Factors retained, but
revisedBreadth and Depth more guidance more design approach
flexibility more concept flexibility
Summary some learning curve, but provides logical
transition from Division I-A to more
advanced methods
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Status
Reviewed by AASHTO Bridge Committee inMay 2002; to be considered
for adoption as aGuide Specification in 2003
Will need to develop and make availableformal training courses
(e.g., via FHWA/NHI)
Develop and publish design aids and designexamples
