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