Seismic Virtual Team

Overview of the "Recommended LRFD Seismic Design Specifications for Highway Bridges" (slide 1)

Ian M. Friedland
Bridge Technology Engineer
Federal Highway Administration

PowerPoint version (5 mb)

Background (slides 2-4)

• 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
• AASHTO subsequently requested stand-alone "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"
• 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 (slides 5-6)

• 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
• 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 (7-8)

• 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
• 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

New Concepts and Major Additions (slides 9-11)

• 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
• "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
• 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 (slides 12-13)

• 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
• 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 (slide 14)

• 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 (slide 15)

Trial Design Locations - Alaska (slide 16)

Summary of Design Impacts (slide 17-18)

• 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
• 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 (slide 19)

• 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

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