AASHTO Guide Specifications for LRFD Seismic Bridge Design 2011 2nd Ed

Abstract

The state of practice of the seismic design of bridges is continually evolving, and the AASHTO Guide Specifications for LRFD Seismic Bridge Design was developed to incorporate improvements in the practice that have emerged since publication of ATC 6, Seismic Design Guidelines for Highway Bridges, the basis of the current AASHTO seismic design provisions. While small improvements have been incorporated into the AASHTO seismic design procedures in the intervening years since ATC 6 was published in 1981, these Guide Specifications and related changes to the current AASHTO LRFD Bridge Design Specifications represent the first major overhaul of the AASHTO procedures. The development of these Guide Specifications was performed in accordance with the recommendations of the NCHRP 20-07/Task 193 Task 6 Report. The Task 6 effort combined and supplemented existing completed efforts (i.e., AASHTO Standard Specifications Division I-A, NCHRP 12-49 guidelines, SCDOT specifications, Caltrans Seismic Design Criteria, NYCDOT Seismic Intensity Maps (1998), and ATC-32) into a single document that could be used at a national level to design bridges for seismic effects. Based on the Task 6 effort and that of a number of reviewers, including representatives from State Departments of Transportation, the Federal Highway Administration, consulting engineers, and academic researchers, these Guide Specifications were developed. Key features of these Guide Specifications follow: This commentary is included to provide additional information to clarify and explain the technical basis for the specifications provided in the Guide Specifications for LRFD Seismic Bridge Design. These specifications are for the design of new bridges. The term “shall” denotes a requirement for compliance with these Specifications. The term “should” indicates a strong preference for a given criterion. The term “may” indicates a criterion that is usable, but other local and suitably documented, verified, and approved criterion may also be used in a manner consistent with the LRFD approach to bridge design. The term “recommended” is used to give guidance based on past experiences. Seismic design is a developing field of engineering that has not been uniformly applied to all bridge types; thus, the experiences gained to date on only a particular type are included as recommendations. Adopt the seven percent in 75 yr design event for development of a design spectrum. Adopt the NEHRP Site Classification system and include site factors in determining response spectrum ordinates. Ensure sufficient conservatism (1.5 safety factor) for minimum support length requirement. This conservatism is needed to accommodate the full capacity of the plastic hinging mechanism of the bridge system. Establish four Seismic Design Categories (SDCs) wit