Many retrofit measures have been proposed and then implemented into existing highway bridges. The goal of seismic strengthening is not intended to retrofit a bridge to be "earthquake-proof", but to minimize the likelihood of structural collapse. An amount of acceptable damage may occur during a design level earthquake. The combination of retrofit measures and the acceptable damage greatly complicates the structural properties for a bridge. It is necessary to use analytical and experimental means to verify the effectiveness of various retrofit combinations.
The objectives of this research are: (1) to investigate analytically the feasibility and advantages of applying the retrofit measures developed for single-column bent bridges to multi-column bent bridges; (2) to evaluate analytically the effects and benefits of current column retrofit strategies for multi-column bridges and propose the most effective measures for strengthening bridges; (3) to evaluate the performance of earthquake restrainers and find the change of seismic loads and displacements caused by their installation.
To achieve the objectives, an existing nonlinear dynamic bridge analysis program with elastic-perfectly plastic column behavior and a conventional hysteresis model was modified in order to include softening behavior and a more realistic hysteresis rule for cyclic loading.
Both two- and three-dimensional structural models for two actual bridges from Washington were analyzed by inputting a typical seismic record. The two-dimensional models were used to evaluate column retrofitting measures, and the three-dimensional model was used to evaluate the performance of longitudinal earthquake restrainers. Both partial and full column retrofit strategies were shown to result in decreased maximum earthquake response and decreased plastic deformation of columns for the bridge bent compared to the case without retrofitting. Therefore, it was concluded that the partial column retrofit strategies were feasible after a ductility capacity of the bridge is exactly defined. The opening displacements at expansion joint hinges were decreased due to the installation of longitudinal restrainers. Additionally, the redistribution of earthquake forces caused by their installation was not significant.