Optimal Design of Negative Stiffness Devices for Highway Bridges Using Performance-Based Genetic Algorithm

Parameter optimization analysis on the negative stiffness device (NSD) installed in the benchmark highway bridge is carried out in this study. Key parameters and constrain conditions are determined in accordance with the characteristics of NSD, and an objective function is designed with safety and comfort being considered. Individual fitness value–related cross and mutation operators are designed to protect excellent chromosome and improve the efficiency and convergence of computation. The genetic algorithm is used to realize parameter optimization of the NSD used in a benchmark highway bridge. Dynamic responses of structure without NSD, with random NSD, and with optimal designed NSD are compared. By analyzing the time history of displacement and acceleration, it can be concluded that dynamic responses of the structure decrease obviously when the NSD is added, and a better seismic reduction effect can be reached when the NSD is designed optimally in accordance with the optimization method and different earthquake excitations have slight influence on the optimization results.

Multi-Objective Optimization of Three Different SMA-LRBs for Seismic Protection of a Benchmark Highway Bridge against Real and Synthetic Ground Motions

Many researchers have taken advantage of adding shape memory alloy (SMA) wires to base isolators to control displacements and residual deformations. In the literature, different arrangements of SMA wires wrapped around the rubber bearings can be found, as examples, straight, cross and double-cross arrangements. SMA wires with various configurations and radii lead to the different characteristics of the isolator system and thus various shear hysteresis. Therefore, the aim of this study is to evaluate the performance of these three SMA wire’s configurations in the seismic retrofitting of a benchmark highway bridge by implementing them in the bridge’s existing lead rubber bearings (LRB). This system is referred to as SMA-LRB isolator. Firstly, because of the crucial influence of the wire’s radius, this parameter is determined using a multi-objective optimization algorithm (non-dominated sorting genetic algorithm (NSGA)-II). This algorithm simultaneously minimizes the deck acceleration and mid-span displacement. Secondly, the optimized SMA-LRBs are implemented in the highway bridge and nonlinear dynamic analysis is conducted. For the nonlinear response history analysis, two strong ground motion records are selected from the PEER database, by studying the site’s conditions. In addition, ten synthetic ground acceleration time histories are generated. The result illustrates that the double-cross SMA-LRB reduces the maximum and residual displacements more than two other devices; however, it causes the largest base shear force and deck acceleration. Besides, the cross-configuration results in the least displacement reduction and has the least shear force and acceleration. To find SMA-LRB with the best overall performance, a multi-objective decision-making method is utilized and the straight SMA-LRB is recognized as the most effective isolator.

Seismic Assessment of a Benchmark Highway Bridge Equipped with Optimized Shape Memory Alloy Wire-Based Isolators

In this paper, an evolutionary multi-objective optimization algorithm named NSGA-II was used to determine the optimum radius for shape memory alloy (SMA) wires employed in conjunction with the lead rubber bearing (LRB), referred to as an SMA-LRB isolator. This algorithm simultaneously minimizes the mid-span displacement and the base shear force. Then, the optimized SMA-LRBs were implemented in a benchmark bridge to reduce excessive displacements. The results obtained from the nonlinear dynamic analysis show that the implemented approach could effectively optimize the SMA-LRBs. These improved smart isolators can noticeably reduce the maximum displacements and residual deformations of the structure; meanwhile, the base shear and deck acceleration remain less than those of the non-isolated benchmark bridge. This isolator can reduce the maximum mid-span displacement of the bridge by up to 61%, and the mid-span residual deformations by up to 100%, compared to an uncontrolled isolated bridge under different ground motions. This optimized passive system was compared with nonlinear dampers, passive SMA dampers, and a negative stiffness device. The results indicate that the optimized SMA-LRB isolators are generally more successful in reducing and recovering displacements than the other controllers.