Vulnerability assessment of tall isolated steel building under variable earthquake hazard levels using endurance time method

In this research, the superstructure behavior in isolated structures has been studied based on their ductility. For this purpose, three tall structures isolated by triple friction pendulum isolators (TFPI) have been designed. The ductility of superstructures is different due to their different seismic force-resisting systems. The performance of the structures has been evaluated by endurance time (ET) method. This method significantly reduces nonlinear dynamic analysis time by optimizing the number of desired earthquake records to one to three endurance time excitation functions (ETEFs). Additionally, the seismic response of structures under different earthquake hazard levels can be achieved by this method. Results show that the change in ratio of the isolator period to the superstructure period (TI/TS) can affect floor acceleration, TFPI displacement and interstory drift ratio (IDR). However, TFPI displacement and IDR are also sensitive to the type of superstructure system, whether it is a braced frame or a moment frame. Moreover, there is a possibility of entering the collapse phase of structural components in tall isolated structures with braced or moment frame which are designed by linear static and spectral methods, under an earthquake with a return period equal to the earthquake hazard level considered in the design step.