Reliability-Based Demand Estimation for SDOF Systems: From Assessment to Implementation in Seismic Codes’ Estimating Equations

Abstract This study evaluates the accuracy of displacement demand-estimating equations in seismic design codes by benchmarking them against results from nonlinear time-history analyses. The investigation focuses on single-degree-of-freedom (SDOF) systems, examining maximum displacement demands and conducting a reliability-based assessment. Natural ground motions, whose spectra align with the elastic design spectrum, and synthetic earthquakes (whose spectra matched the elastic design spectrum) are considered. The analyzed SDOF systems cover fundamental vibration periods between 0.1 and 2.0 s, assuming elastic–perfectly plastic behavior with strength reduction factors ranging from 2 to 8. Introducing a wide range of probabilistic structural parameters into analytical models, the results demonstrate that code-based displacement estimates lead to nonuniform reliability levels, strongly influenced by vibration period, soil type, and strength factor. To overcome this limitation, the study introduces reliability-based displacement demand equations designed to provide uniform reliability across different conditions. The proposed framework is capable of enhancing seismic design by ensuring displacement demands correspond to a consistent confidence level, thereby enabling safer and more damage-tolerant structural performance.