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
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