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10 CHAPTER 2 Data Collection and Review The goal of Task 1 of the NCHRP 12-70 Project was to col- tions will vary depending on the function of the retaining wall, lect, review, and interpret relevant practice, performance data, slope and embankment, or buried structure. research findings, and other information needed to establish With the exception of California, the standard approach a starting point for subsequent phases of the Project. The work within AASHTO at the time of the NCHRP 12-70 Project performed within this task included review of the current sta- involved use of a 500-year design earthquake (that is, approx- tus the NCHRP 20-07 Project; literature searches; and con- imately 10 percent chance of exceedance in a 50-year period). tacts with individuals involved in the seismic design of retain- Individual states could adopt more stringent requirements ing walls, slopes and embankments, and buried structures. for critical bridges. For example, the design basis used by the Realizing that the final product for the Project needed to be a Washington Department of Transportation (WSDOT) for set of specifications that can be implemented by practicing the new Tacoma Narrows Bridge was 2,500 years (that is, engineers, the focus of this task was on the identification of approximately 2 percent probability of exceedance in 50 years), approaches or ideas that could be implemented on a day-to- as this bridge was considered a critical structure. Under the day basis by practicing engineers, rather than highly rigor- standard design approach, the structure (normally a bridge ous or numerically intensive methods that would be more and its related abutment and wing walls) was designed to suited for special studies. The results of this data collection withstand the forces from the design earthquake without and review task are summarized in four sections consisting collapse, albeit damage could require demolition following of discussion of the earthquake design basis, key observa- the design event. tions from the literature review, results of contacts with var- The NCHRP 12-49 Project (NCHRP Report 472, 2003) ious individuals engaged in design, and a summary of con- attempted to increase the minimum design basis within clusions reached from this phase of the Project. Although AASHTO LRFD Bridge Design Specifications to a 2,500-year this task was largely complete early in the Project, limited return period for the collapse-level event. The 2,500-year data collection and review continued throughout the dura- return period event has approximately a 2 percent probabil- tion of the Project. ity of exceedance in 50 years. However, the recommended increase was not adopted for several reasons, including the potential cost of designing for the longer return period and a 2.1 Earthquake Design Basis concern about the complexity of the recommended design One of the key requirements for this Project was the deter- process. A follow-up effort was undertaken by Dr. Roy mination of an earthquake design basis. The earthquake design Imbsen of Imbsen & Associates to modify the previous NCHRP basis was important because it defined the level of ground 12-49 work, referred to as the NCHRP 20-07 Project (Imbsen, motion that will occur at a site. The level of ground motion 2006). As part of this effort, the design return period was creates the "demand" side of the basic LRFD equation. As reconsidered. A consensus was reached by Dr. Imbsen and the earthquake design basis increases, the demand (or load) the AASHTO Highway Subcommittee on Bridges and Struc- increases; and the capacity of the foundation needs to be pro- tures on the earthquake design basis for both new and retro- portionately larger to limit displacements and forces to accept- fitted structures. This consensus involved a single level design able levels. The earthquake design basis also established the with a return period of 1,000 years. performance expectations--for example, the amount of dis- The decision on the design return period established a basis placement that was acceptable. These performance expecta- for determining the approach to seismic design for the NCHRP