Nonlinear analyses of soils, structures, and soil-structure systems offer the potential for more accurate characterization of geotechnical and structural response under strong earthquake shaking. The increasing use of advanced performance-based design and evaluation procedures will require consideration of long-return-period motions for all structures, especially in western Washington where high seismicity is a concern and long-return-period motions are likely to be strong enough to induce nonlinear, inelastic response in soil deposits and structures.
Nonlinear analyses require the specification of acceleration time histories as input; this requires the analyst to identify input motions that are consistent with the ground motion hazards at the site of interest. A considerable level of research effort has been directed toward the development of procedures for selection and scaling of earthquake ground motions for the purpose of using them in nonlinear structural analysis. This research has shown that structural response of buildings can be quite sensitive to the selection and scaling of ground motions used in nonlinear analyses. While the sensitivity of bridge structures to input motion characteristics has not been studied as explicitly as that of building structures, the response of bridges is also expected to be significantly influenced by input motion characteristics.
As a result, engineers have identified the need for software tools that will automate, to at least a large degree, the process of identifying suites of ground motions that are most appropriate for use in nonlinear response analyses. Along with this report, a piece of software, SigmaSpectraW, was created for WSDOT to do just that.