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Bridges & Structures


Overview of the "Recommended LRFD Seismic Design Specifications for Highway Bridges"

Ian M. Friedland
Bridge Technology Engineer
Federal Highway Administration

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

Cover of ATC/MCEER joint venture report: Recommended LRFD Guidelines for Seismic Design of Highway Bridges, Part I: Specifications

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
    Schematic graph showing variation of design coefficient for peak ground acceleration (A) as a function of natural period. The coefficient varies inversely with the period in seconds raised to the two-thirds power (i.e., X 1 / T2/3 ).
  • 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)

National USGS 1996 - 1.0 sec Spectral Acceleration (%g) with 2% Probability of Exceedance in 50 years, site:NEHRP B-C boundary, map. Site locations of AASHTO T-3 Trial Designs shown.

Trial Design Locations - Alaska (slide 16)

Alaska USGS 1996 - 1.0 sec Spectral Acceleration (%g) with 2% Probability of Exceedance in 50 years, site:NEHRP B-C boundary, map. Site location of AASHTO T-3 Trial Design shown.

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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Updated: 05/26/2015
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