2020 Issue

28 IBC 2018/ASCE 7-16 Seismic Code Changes Byron Foster O n July 1, 2019, Utah adopted the 2018 IBC (based on ASCE 7-16), which brought with it several very significant changes to local seismic practice. These changes, which are summarized below, are impacting ge- otechnical and structural engineers and their clients. They were discussed in greater detail by several presenters at a continuing education workshop on June 10, 2019, at SLCC. When you become familiar with these changes, you need to make sure that you are looking at the final version of ASCE 7-16 and also the December 2018 supplement. The primary changes include: • A penalty for not having detailed site investigations • Changes to the site factor (Fa and Fv) tables • Near-fault site definitions • A vertical ground motion approach (where site-spe- cific procedures are not used) • Site-specific seismic assessments for Site Class D or E when S 1 > 0.2 and for Site Class E when S S > 1.0 unless exceptions are taken. There is a significant impact for longer period structures. From discussions with local engineers who have started to use the new code, it appears the largest changes may be: • Penalties for assuming a Site Class D, or not having measured site properties to define the Site Class, should drive projects to obtain good measured prop- erties routinely. • From structural engineers who have locally compared ASCE 7-10 and the old seismic maps to ASCE 7-16 and the new seismic maps, the ground motions seemed to have increased significantly. • However, since the intent of the code change was to address the unconservative estimates of ground motions for longer periods (taller buildings), it is not clear yet what the impact of the exceptions will be on building design/cost. • Cost increases from the new ground motions used for design could be as high as about 5% of the building costs. • The most significant change will probably be that for almost all of the populated areas of Utah (which are typically Site Class D or E with S 1 > 0.2g), the owner will have to have either a site-specific seismic assess - ment conducted, or the structural engineer will have to use one of the three “exceptions” (potentially penalizing building costs). There has been some confusion in Utah regarding what constitutes a “site-specific seismic assessment.” For example, the spectral acceleration values obtained from the USGS website do NOT constitute a site-specific assess - ment. Similarly, a Deterministic Seismic Hazard Analysis (DSHA), by itself, is not a site-specific assessment. A DSHA is only a check on the Probabilistic Seismic Hazard Analysis (PSHA). There are two types of site-specific assessments defined under the Code: • A Ground Motion Hazard Analysis, which is composed of both a PSHA and a DSHA for a check, and • A Site Response Analysis (SRA) Site Response Analysis and Ground Motion Hazard Anal- ysis are two very different analyses, and each requires