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Recommendations for Improved AASHTO Bridge Design Specifications

by Christopher Rojahn, Ronald L. Mayes, and Richard V. Nutt
The Applied Technology Council recently completed a project to review currently available seismic design criteria and specifications for highway structures worldwide and to provide recommendations for improved national seismic design specifications for highway bridges (ATC-18). These recommendations were made by a Project Engineering Panel, whose members were Ronald Mayes (Chairman and Project Director), Donald Anderson, John Clark, Ian Buckle, John Hom, Richard Nutt, Michael O'Rourke and Charles Thornton. The recommendations embody significant changes to current specifications. For further information, contact Chris Rojahn at ATC, phone: (415) 595-1542 or via email at crojahn@atcouncil.org.

Background

While the overall scope of work on Project ATC-18 pertained to all types of highway structures, particular attention was given to bridge structures and foundations. The ATC-18 project team initially focused on a review of current design practice and criteria for new bridge design, as well as the philosophies on which they are based. This involved a review of existing U.S. standards along with the latest codes of Japan, New Zealand, and Europe. Guidelines developed for the Transportation Corridor Agencies (TCA) for Orange County, California, the new AASHTO Load and Resistance Factor Design (LRFD) Bridge Specifications, and information emanating from the ATC-32 project were also reviewed. The final phase of the ATC-18 project focused on the development of recommendations for the future direction of seismic code requirements for bridge structures in the United States. An important aspect of the recommendations is a two-level design approach. The following paragraphs are specific recommendations for future bridge seismic design code development.

Performance Criteria

ATC-18 recommends that the performance criteria to be included in any future code should be approved by a group that includes legislative policy makers. In order for this review to be effective, a significant amount of cost and technical data will need to be developed before the performance criteria can be intelligently discussed by a non-technical panel. The other key requirement for the recommended performance criteria is a specific and unambiguous definition of Important and Ordinary bridges. Future codes may have significantly different design requirements for these two bridge categories.  
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    Table 1: Recommended Seismic Performance Criteria (ATC 1996a)
    Ground Motion at Site 
    Ordinary Bridges 
    Important Bridges 
    Functional-Evaluation 
    Ground Motion 
    Service Level-Immediate 
    Damage Level- 
    Repairable Damage 
    Service Level-Immediate 
    Damage Level- 
    Minimal Damage 
    Safety-Evaluation 
    Ground Motion 
    Service Level-Limited 
    Damage Level- 
    Significant Damage 
    Service Level-Immediate 
    Damage Level- 
    Repairable Damage 
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Design Approach

It is recommended that the current AASHTO Seismic Performance Category approach be continued in future codes, since it is a good method of varying design requirements in different seismic zones. It is also recommended that a two-level design approach be adopted at least for Important bridges in higher seismic zones. The lower-level design requirements should be based on elastic design principles to ensure that there is no damage. The upper-level analysis should be deformation-based using nonlinear static (pushover) analysis procedures with strength and stiffness requirements being derived from appropriate nonlinear response spectra.

If a single-level design procedure is adopted for Ordinary bridges, it is recommended that the design approach include a nonlinear static analysis as part of the design procedure.

References

AASHTO, (1995), Standard Specifications for Highway Bridges, 16th Edition, American Association of State Highway and Transportation Officials.

ATC, (1981), Seismic Design Guidelines for Highway Bridges, Report No. ATC-6, Applied Technology Council, Redwood City. Also published by Federal Highway Administration as FHWA/RD-81/081, Washington, DC

ATC, (1996a), Improved Seismic Design Criteria for California Bridges: Provisional Recommendations, Report No. ATC-32, Applied Technology Council, Redwood City.

ATC, (1996b), Seismic Design Criteria for Highway Structures: Current and Future, Report No. ATC-18, Applied Technology Council, Redwood City.

BSSC, (1994), NEHRP Recommended Provisions for the Development of Seismic Regulations for New Buildings, Building Seismic  Safety Council, Washington, DC

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