New Site Classification Procedures for Seismic Design for US Building Codes with Applicability to NZ

Abstract: The procedures used to classify sites were revised in the 2020 NEHRP Provisions and the 2022 ASCE 7/22 documents, which form the basis for US building codes. This presentation reviews the historical procedures used for site classification, problems with those procedures, and revised site classes intended to address those problems. A key feature of the revised procedures is their requirement that a site shear wave velocity profile be developed. Various ways of developing shear wave velocity profiles, either through measurement or estimation from other parameters, are described. A seemingly benign detail of the site classification is that it is based on a time-averaged velocity over a depth interval of 0 to 30 m. The NEHRP and ASCE documents have added a specific definition of that depth interval due to practices for some recent projects where engineers have redefined the parameter to apply over a deeper depth interval for applications to embedded structures. The speaker will explain why this practice is unconservative, which motivated the specific depth range definition in the code documents. Alternative procedures to derive ground motions for the foundation levels of structures that are allowable in the code are described.
Bio: Jonathan P. Stewart is a Professor in the Samueli School of Engineering at UCLA, and at the time of filming was on sabbatical leave at the University of Canterbury (Feb - May 2024). The work of his research group focuses on geotechnical earthquake engineering and engineering seismology, with emphases on seismic soil-structure interaction, earthquake ground-motion characterization, site response, seismic ground failure, and the seismic performance of earth structures. Findings from his research have been widely utilized in engineering practice, including through the National Seismic Hazard Model, produced by the US Geological Survey, and various engineering guidelines documents. He maintains an active consulting practice related to seismic hazard analysis, site response, seismic performance assessment, and geotechnical engineering. His work has been recognized with best-paper awards, honorary lectures, the UCLA Distinguished Teaching Award, and election to the National Academy of Engineering.