Overview of the “Recommended LRFD Seismic Design Specifications for Highway Bridges” Ian M....
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Transcript of Overview of the “Recommended LRFD Seismic Design Specifications for Highway Bridges” Ian M....
Overview of the Overview of the “Recommended LRFD Seismic “Recommended LRFD Seismic
Design Specifications for Design Specifications for Highway Bridges”Highway Bridges”
Ian M. Friedland, P.E.Ian M. Friedland, P.E.Bridge Technology EngineerBridge Technology Engineer
Federal Highway AdministrationFederal Highway Administration
Background
Project requested by AASHTO Bridge Committee in 1997 to update existing seismic design specifications
Initiated in August ’98, and conducted under TRB’s National Cooperative Highway Research Program (NCHRP Project 12-49 by ATC/ MCEER Joint Venture)
NCHRP 12-49 completed in 2001; developed LRFD specification “cut and paste” provisions
Background, continued
AASHTO subsequently requested standalone “guide spec” version of the “cut and paste” provisions, similar to Division I-A
MCEER/FHWA funded rescoping effort to prepared stand-alone “Recommended Guidelines”
Background, continued
MCEER/FHWA initiated trial design project in December ’01 to test and validate the stand-alone Guidelines
13 states and FHWA Federal Lands Highway Division conducted trial designs
Specification Philosophy
Minimize loss of life/injury from unacceptable bridge performance
Allow bridge damage (possibly require replacement) but limit potential for collapse
Critical (lifeline) bridges should remain functional after a major earthquake
Philosophy, continued
Upper level ground motions with low probability of exceedance during 75-year bridge design life
Provisions applicable to all regions of U.S.
Designer encouraged to consider and employ new concepts, design approaches, and structural details
Deficiencies in Current Provisions
Based on ATC-6 seismic design guidelines developed in the late 1970’s
Seismic hazard based on 1988 national seismic hazard maps which are no longer considered adequate or correct
Soil site factors which have been demonstrated in many recent earthquakes as being incorrect and inadequate
Deficiencies, continued
Response spectra curve construction that decreases as 1/T 2/3 rather than 1/T in long-period part of the curve
Effectively address only concrete design – no provisions specific to steel or wood super- or substructures
Period (seconds)
Sei
smic
Co
effi
cien
t
1/T 2/3
New Concepts and Major Additions
1996 USGS maps
Performance objectives and design earthquakes
Design incentives and revised R-Factors
Improved/validated soil site factors
Improved spectral shape
Earthquake resisting systems and elements
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
New Concepts andMajor Additions, continued
Explicit steel design provisions
Enhanced concrete design provisions
Superstructure design provisions
Bearing design and testing requirements
Seismic isolation provisions
Liquefaction case studies
Features of the New Provisions
Based on best scientific and engineering approaches and technologies currently used worldwide for building and bridge construction
Reviewed by broad cross-section of State bridge engineers and consultants, earthquake engineers, experts from various industries and technologies
Comprehensive parameter study and trial design program produced bridge designs that are in keeping with existing AASHTO specifications, while providing significantly higher levels of performance
Features, Continued
Include a “no seismic analysis” design approach based primarily on good detailing practice, and which should be applicable to large regions of the United States
Provide substantially more guidance on soil liquefaction and lateral spread
Specific provisions for the design of steel super- and substructures
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 ft
Lengths – 133 ft to 1320 ft
Trial Design Locations – Lower 48
Trial Design Locations - Alaska
Summary of Design Impacts
Format – similar to Division I-A Hazard – location and soil based No-Analysis – provides simplifications
for some regular bridges Capacity Spectrum – regular bridges Displacement Verification – codified Two-Level Design – frequent and rare
earthquakes Geotechnical – more guidance provided
Design Impacts, continued
Load Combinations – kept simple R-Factors – retained, but revised Breadth 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
Status
Reviewed by AASHTO Bridge Committee in May 2002; to be considered for adoption as a Guide Specification in 2003
Will need to develop and make available formal training courses (e.g., via FHWA/NHI)
Develop and publish design aids and design examples