Site Specific Seismic Ground Motions Analyses

The Jackson-Wilson bridge over the Snake River is expected to be replaced in the FY 2022. This site has a moderate to high seismic hazard and unknown depth of soil above the bedrock. The operational importance of structures deemed critical or essential may require site specific analysis depending on soil classification and seismic design category. Recently a bridge completed over the Snake River under Project N104083 was considered an essential structure. Currently the Bridge department at the Wyoming Department of Transportation (WYDOT) uses the general procedure in the American Association of State Highway and Transportation Officials (AASHTO) Guide Specification for Load and Resistance Factor Design (LRFD) Seismic Design (2015, 2016) to determine the seismic design forces for the structure. However, the AASHTO (2015, 2016) specifications allow for a more advanced analyses that may result in a better prediction of the seismic forces needed to design the structure. In order for WYDOT bridge engineers to perform a site specific ground motion analyses a proof-of-concept project is proposed. This project will be used to establish the procedures, steps and training required for WYDOT personnel to perform site specific ground motion analyses. This two year project is co-sponsored by the geology and bridge departments at WYDOT and will include 1) measurement of the shear wave velocity for the Jackson-Wilson bridge site 2) a site specific ground motions analyses and, 3) Training of WYDOT Personnel on how to use shear wave velocity profiles to determine soil site classification and perform site specific ground motion analysis.