Probabilistic Evaluation of 2015 NEHRP Soil-Structure Interaction Provisions

Abstract

This paper reveals the consequences of practicing the 2015 National Earthquake Hazards Reduction Program (NEHRP) soil-structure interaction (SSI) provisions, which form the basis of the 2016 edition of the seismic design standard provided by the ASCE. For this purpose, the probability that the practice of SSI provisions, in lieu of fixed-base provisions, increases the ductility demand of the structure computed. It is subsequently investigated whether the NEHRP provisions are indeed an improvement upon the SSI provisions of the current ASCE seismic design standard. To this end, 720 soil-structure systems with different numbers of stories, structural systems, aspect ratios, and foundation embedment ratios on various site classes are studied. A probabilistic framework is employed to account for prevailing uncertainties in ground motion and in the properties of the soil-structure system. It is concluded that both NEHRP and the current provisions result in unsafe designs for structures with surface foundation on moderately soft soils, but NEHRP slightly improves upon the current provisions for squat structures. For structures on very soft soils, both provisions yield conservative designs where NEHRP is even more conservative. Finally, both provisions yield near-optimal designs for other systems.