Modified endurance time method for seismic performance assessment of base-isolated structures

Endurance time (ET) method is a novel Time History Analysis (THA) approach in which structures are subjected to a set of intensifying excitations called endurance time excitation functions (ETEF). Although various studies show the efficiency of the ET method for the seismic evaluation of fixed base structures, this method has been less employed in isolated structures. The lack of coverage of high-intensity measures by existing ETEFs is the major obstacle to adopting the ET method in high-period structures such as isolated structures. It must be noted that high-intensity measures that are associated with large earthquakes are of paramount importance in performance assessment. This paper introduced a novel method called the modified endurance time (MET) method based on scaling existing ETEFs to alleviate this problem. By averaging multiple scaled ETEFs, the accuracy of the ET method is improved. Three steel structures with a different number of stories (6, 9, and 12), including Intermediate Moment Frames (IMFs), mounted on Triple Friction Pendulum Isolators (TFPIs) are considered to demonstrate the efficiency of the proposed method. These structures are analyzed under ETEFs with intensity multipliers 1 to 4 for three seismic hazard levels with return periods of 72, 475, and 2475 years. The effect of changing parameters such as isolator damping and period is also investigated on the results. The seismic responses obtained from the Endurance Time Analysis (ETA) are compared with the responses obtained from the nonlinear THA and pushover analysis. Results show that increasing the intensity multiplier of ETEFs can greatly improve the accuracy of results. However, it should be careful in the procedure of selecting the appropriate intensity multiplier. Because of the unbalanced changes in some ETEFs parameters such as PGV/PGA, the accuracy of results may also reduce by increasing the intensity multiplier. Therefore, the use of different combinations of results, obtained from several intensity multipliers, is investigated and proposed to reduce the error in results.