Synthesis of Information Related to Highway Problems. Topic 42-03. Practice and Procedures for Site-Specific Evaluation of Earthquake Ground Motions

AASHTO specifications for seismic design, including both the 2009 Interim AASHTO LRFD Bridge Design Specifications and the 2009 Guide Specifications for LRFD Seismic Bridge Design, mandate site-specific evaluation of the earthquake design ground motion (i.e. the acceleration response spectrum) for ground conditions termed Site Class F. In the AASHTO specifications, Site Class F soils are soft clay sites. These AASHTO specifications also allow discretionary site specific analyses for other ground conditions and a reduction in mapped design ground motions by as much as 33% if justified by a site-specific ground motion analysis. Some State DOTs are taking advantage of this site response reduction provision, particularly in cases where there is the potential for pore pressure generation leading to liquefaction. Furthermore, there is growing evidence that the AASHTO site factors, used to adjust mapped values of design ground motions for local ground conditions, are inappropriate for short period structures (To < 0.5 sec) at shallow bedrock sites, i.e., depth to bedrock less than 100 ft, and for long period structures (To > 1.0 sec) at deep soil basin sites, e.g. depth to bedrock greater than 500 ft For years, the primary method used in engineering practice for evaluating the influence of local ground conditions on earthquake ground motions on a site-specific basis has been the equivalent-linear site response computer program SHAKE. However, SHAKE is known to be unreliable at (1) sites with ground motions in excess of 0.3 g due to non-linear site response effects, (2) at sites where there is a potential for liquefaction because it cannot consider the effects of pore pressure generation, and (3) at deep soil basin sites because of deficiencies in damping and modulus reduction curves. A number of truly non-linear site response analysis methods have recently become available and are now being used in practice, including methods that account for deep soil basin effects (e.g. DEEPSOIL) and pore-water pressure generation (e.g. D-MOD2000, OPENSEES, and DESRAMUSC). Significant expertise is required to conduct and interpret the results from these newer methods, often leading to questions regarding the validity of results. For instance, experience with the newer nonlinear analysis methods show that strains (and hence stiffness reduction) tend to become more localized than in an equivalent linear analysis (e.g. a SHAKE analysis). As a result, details of the soil profile, particularly soft layers and impedance contrasts, can have a larger effect on the results of a nonlinear analysis than they do on the results of an equivalent linear analysis. Furthermore, with all available methods (including SHAKE), significant expertise and numerous discretionary decisions are required in order to conduct a site response analysis, including the selection of an appropriate suite of time histories, whether to measure the small strain modulus and other soil properties in the field and/or laboratory or obtain them using correlations, to what extent sensitivity analyses are required, and what modulus reduction and damping curves to use. The extent of the required expertise and discretionary decisions is greater with the newer methods than with SHAKE and is greatest with the analyses that consider pore pressure generation and dissipation. Commentary within the AASHTO specifications warns of potential issues when conducting site-specific ground motion studies, but the commentary provides no guidance on how or when to consider these potential issues. This lack of guidance raises concerns as to whether appropriate estimates of site-specific ground motions are being made for design, potentially resulting in either excessive project construction costs when ground motion response is overestimated or unacceptable risk to the public when ground motion response is underestimated. A synthesis study will be performed of current practice and available methods for site specific analysis of earthquake ground motions supplemented with a summary of experience gained in developing and employing these methods, including challenges in their application and perceived advantages and disadvantages of the different methods. This study will help establish and improve the state of practice, providing a summary of best design practices, as well as identifying research and development needs on this important topic.


  • English


  • Status: Active
  • Contract Numbers:

    Project 20-05, Topic

  • Sponsor Organizations:

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  USA  20590

    American Association of State Highway & Transportation Officials

    444 North Capitol Street, NW, Suite 225
    Washington, DC    20001

    National Cooperative Highway Research Program (NCHRP) Synthesis

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC    20001
  • Project Managers:

    Williams, Jon

  • Performing Organizations:

    Transportation Research Board

    500 Fifth Street, NW
    Washington, DC  USA  20001
  • Start Date: 20100701
  • Source Data: RiP Project 26220

Subject/Index Terms

Filing Info

  • Accession Number: 01463956
  • Record Type: Research project
  • Source Agency: National Cooperative Highway Research Program
  • Contract Numbers: Project 20-05, Topic
  • Files: TRB, RiP, USDOT
  • Created Date: Jan 3 2013 2:33PM