UCLA

In order to improve the seismic performance of structures and to reduce the total cost (both direct and indirect) due to earthquake damages, structural control through seismic protective devices in either passive or semi-active forms is essential to achieve the desired performance goals. This research intends to develop optimal design and placement of seismic protective devices for improving structural performance of buildings and bridges. This is accomplished by deriving (a) optimal nonlinear damping for inelastic structures, (b) hybrid numerical simulation framework to facilitate nonlinear structural control analysis and (c) efficient seismic protective scheme for bridges using base isolation, nonlinear supplemental damping and semi-active MR dampers.

Supplemental energy dissipation in the form of nonlinear viscous dampers is often used to improve the performance of structures. The effect of nonlinear damping is a function of structural properties, ground motion characteristics and performance objectives. In order to quantify the optimal amount of nonlinear damping needed for inelastic structures, a novel dimensionless nonlinear damping ratio is first proposed through dimensional analysis of inelastic SDOF structures. Subsequently, an equivalent SDOF inelastic system is derived to represent the general MDOF inelastic structures. Based on this equivalency and the help of the nonlinear damping ratio definition, the optimal damping and damper placement for MDOF inelastic structures are developed using genetic algorithms. It's demonstrated that the added nonlinear damping is not always beneficial for inelastic structures, i.e. resulting in the increase of total acceleration response under certain ground motions. A critical structure-to-input frequency ratio exists, upon which an optimal nonlinear damping is needed to balance between the increase of total acceleration and the reduction of structural drift.

Secondly, to facilitate the nonlinear control simulation of complex structures, an existing hybrid testing framework (UI_SimCor) is adopted and modified to enable the dynamic analysis of nonlinear structures equipped with seismic protective devices, including nonlinear viscous dampers, base isolators and MR dampers. Under this framework, inelastic structures can be modeled realistically in general FEM platform (e.g. OpenSees) while the seismic protective devices can be modeled numerically in a different software (e.g. Matlab). Furthermore, control algorithms can also be implemented easily under this hybrid simulation scheme. To validate the hybrid simulation approach, an experimental program is implemented on a scaled 3-story steel frame structure controlled by a semi-active MR damper. Both real-time hybrid simulation and shake table tests were performed and compared. The good agreement between them verifies the accuracy and efficiency of the hybrid simulation scheme. In addition, for application to bridges, special scheme to incorporate multi-support input earthquake motions is also developed so that the significant soil-structure interaction effects on bridges can be simulated.

Finally, the efficient seismic protective scheme for bridges is explored using the hybrid simulation scheme developed. A real highway bridge, the Painter Street Bridge (PSB) is modeled realistically in OpenSees including soil-structure interaction effects while the seismic protective devices and control algorithm are implemented separately in Matlab. Clipped-optimal control algorithm based on LQG regulator and Kalman filter is adopted to derive the optimal structural response of PSB with base isolation and semi-active controlled MR dampers. Eventually, an equivalent passive form of MR dampers is developed, which can mimic the effects of semi-active control to achieve the optimal design of seismic protective devices for highway bridge applications.