# Presentation: 393777

## Overview of the “Recommended LRFD Seismic Design Specifications for Highway Bridges”

- <date/time>

**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 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

## 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