Civil Engineering Association

SEISMIC ANALYSIS OF RETAINING WALLS, BURIED STRUCTURES, EMBANKMENTS, AND INTEGRAL ABUTMENTS

In 1998, the National Cooperative Highway Research Program (NCHRP) initiated a project to develop a new set of seismic design provisions for highway bridges intended to be compatible with the AASHTO LRFD Specifications (1) . This project, designated 12-49, was conducted by a joint venture of the Applied Technology Council (ATC) and the Multidisciplinary Center for Earthquake Engineering Research (MCEER). This research project was needed to reflect the experience gained during recent damaging earthquakes, as well as the results of research work conducted in the United States, Japan, and other countries over the last decade (2) . Recommended LRFD Guidelines for the Seismic Design of Highway Bridges (3) were based on NCHRP Project 12-49. The purpose of the new NCHRP 12-49 provisions is to provide seismic design guidelines and performance objectives for bridges in order to ensure the safety of the public, and to minimize structural and non-structural damage. In recent years, several major bridges have collapsed and others have sustained significant damage during earthquakes (2) . The NCHRP 12-49 guidelines adopted the MCE (maximum considered earthquake or 2 percent PE on 50 years) as an upper level event for collapseprevention and adopted the EXP (expected) earthquake (50 percent in 75 years) as a lower level event for which the structure essentially remains elastic. These changes in the newly recommended guidelines will have a major impact on seismic design of bridges in the Eastern United States. Several states, including New Jersey, are evaluating the impact of these changes on their local, state, and federal bridges. In addition, soil amplification factors Fa and Fv have increased dramatically for soft soils, especially when subject to small ground motions. These factors are not site-specific to the Eastern United States and were based on soils and earthquake records predominantly in the Western United States (See references 3,4,5, and 7) . These factors may vary for different soils, geographic locations, and ground motions. Among the other major changes in the new NCHRP 12-49 seismic design provisions are updated seismic maps, new response modification factors ®, detailed performance and hazard level criteria, and design incentives when performing “pushover” analysis. These provisions are intended to help bridge owners and state officials with current designs and provide designers more flexibility in the analysis and design.