CCEER-14-02: Performance-Based Probabilistic Damage Control Approach for Seismic Design of Bridge Columns

Abstract

Probabilistic seismic design is a relatively new concept for seismic design of bridges subjected to earthquakes. Through this method, uncertainties in seismic response and seismic demand are taken into account. The concept can be extended to control damage by designing for certain target apparent damage with an associated probability of exceedance. Through a study funded by the California Department of Transportation (Caltrans), a performance-based probabilistic damage control approach (PB-PDCA) was developed by incorporating the extent of lateral displacement nonlinearity defined by "Damage Index" (DI). The performance objective was defined based on predefined apparent damage states and the damage states were correlated to damage indices based on a previous study at the University of Nevada, Reno. The correlation between DI and DS was determined from a statistical analysis (resistance model) of over 140 response data measured from testing of 22 bridge column models subjected to seismic loads. Extensive analytical modeling of seismic response of single column and multicolumn bents was conducted. A wide range of variables was included in the study to address the effect of aspect ratio, longitudinal steel ratio, site class, distance to active faults, earthquake return period, and number of columns per bent. Each column was analyzed under 25 near-field and far-field ground motions. A statistical analysis of the demand damage index (DIL) was performed to develop fragility curves (load model) and to determine the reliability index for each DS. The results of reliability analysis were analyzed and a direct method was developed to calibrate design DI to obtain a desired reliability index against failure. The calculated reliability indices and fragility curves showed that the proposed method could be effectively used in seismic design of new bridges as well as seismic assessment of existing bridges. Included in the current project was an exploratory study to extend the PB-PDCA to earthquake-damaged columns that have been repaired. Like conventional original (not repaired) columns, different damage states (DSs) were defined for repaired columns associated with varying degree of damage. The PB-PDCA that was developed and investigated provides design tools enabling designers and researchers to detail bridge columns for a target expected damage with an associated probability of occurrence and a reliability index.