Optimization of Multiple Shape Memory Alloy Devices by a Genetic Algorithm for Seismic Response of a Tall Structure

The goal of this study is to show advantageous use of superelastic shape memory alloy (SMA) damping devices for amelioration of earthquake response in a tall steel frame structure. Damping systems discussed in this work are performance-based in that the three-story structure and its SMA bracing system are designed to minimize quantities calculated for response metrics due to a special time history of earthquake excitation. That is, a unique acceleration time-history is developed using RSPMatch2005 which adjusts an actual time-history record to fit a given design response spectrum and level of damping by means of a wavelet-based technique. In order for the frame to remain within allowable bounds of displacement and acceleration for the adjusted earthquake record, a number of SMA elements are optimized with respect to their area and location within the structure. Multiple-objective numerical optimization that simultaneously minimizes both structural displacements and accelerations is carried out using a genetic algorithm that employs NSGAII-CE. After design of an optimal SMA damping system is complete, full-scale experimental shake table tests are conducted on a large-scale steel frame that is braced with SMA elements at the National Center for Research on Earthquake Engineering (NCREE) in Taiwan. Then, a fuzzy inference system is developed to simulate the nonlinear dynamic material response of the SMA wires. Experimental and numerical results are compared in order to verify a subset of the results predicted by extensive numerical simulations. Finally, nonlinear dynamic analysis of a chevron-like braced frame is carried out to compare the performance of SMA and steel braces. It is shown that residual lateral displacement of the columns in a structure can be minimized or eliminated through a judicious selection of location and physical characteristics of the SMA devices.