Optimal design of isolation devices for mid-rise steel moment frames using performance based methodology

This paper develops and applies the performance-based analysis and design methodology to assess the seismic vulnerability of mid-rise steel moment frame structures and to optimally design the isolation devices to reduce the direct losses due to earthquake damages. An isolated steel moment frame, originally tested in the 2011 E-Defense blind prediction contest, is selected and modeled in detail. The numerical model and the predicted seismic responses of the structure are validated against the full-scale shaking table test results. Subsequently, the fragility functions are derived for the structure when subject to near-fault ground motions exhibiting distinctive acceleration or velocity pulses and far-field motions with less impulsive characteristics. To quantify the system level damage states of the building, the concept of total loss ratio (TLR) is applied as the performance index to account for the direct loss due to structural, non-structural and isolation components in relation to the total repair cost of the original structure. The TLR considers the failure probability (as defined by fragility functions), the damage percentage and related cost for each damage state. Finally, among various isolation designs, the optimal configuration is derived for cases with the minimum TLR. It is shown that the optimal design can reduce the TLR up to 90% of that of the un-isolated structure and it also outperforms the adopted design in the test program. The study demonstrates a systematic way of achieving the optimal isolation design with considerations of uncertainties in earthquake inputs and the combined structural and non-structural damages.