Overview of the “Recommended LRFD Seismic Design Specifications for Highway Bridges” Ian M. Friedland, P.E. Bridge Technology Engineer Federal 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 by ATC/ MCEER Joint Venture)   NCHRP 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

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 and Major 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 and Major 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