This paper investigates the efficiency of the proposed equations in the last revision of the "Iranian code of practice for seismic resistant design of buildings" (also known as "Standard 2800") and the Iranian "Instruction for Seismic Rehabilitation of Existing Buildings" (known as Code 360), by concentrating on the maximum displacement demands for Single-Degree-Of-Freedom (SDOF) systems. In this regard, a group of SDOF systems with an Elastic-Perfectly-Plastic (EPP) behavior and the fundamental vibration periods of 0.1-2.0 s was considered with the lateral strength ratio of 2-8. SDOF systems were analyzed under the action of natural ground motions that are compatible with the elastic design spectrum of local soil types. In order to investigate the applicability of synthetic earthquakes in demand estimation, the spectrum of these records was matched artificially to those in the code and similar analyzes were accomplished. Statistical analyses were conducted to minimize the variability in results for each group of selected records. Moreover, displacement demands with a 95% confidence interval were obtained from the nonlinear time-history analysis, and the results were compared to those from estimating equations in the prementioned codes. Based on the obtained results, it was observed that despite the compatibility between the design spectrum and records spectrum, the variation in the displacement demands from different record sets is high. However, smaller dispersion is observed in displacement demands from artificially matched-spectrum ground motions in comparison with natural ones. Also, artificially matched-spectrum records lead to underestimated demands in SDOF systems for soil site class I, but higher demands are obtained for softer soil types. A comparison between demands from the estimation equation in the nonlinear static procedure and those from the nonlinear time-history analysis shows the inefficiency of the predicting equation. Hence, a procedure was introduced for modifying equations in order to get more accurate results.
