Seismic vulnerability assessment of bridges for retrofitting and new design
Creators: name: Farokh, Pedram
Publisher: name: No Publisher Supplied
Year: 2017

description: Many bridges in North Eastern region of U.S. were designed prior to the adoption of the AASHTO LRFD Guide Specifications for seismic design and may be vulnerable to damage during an earthquake event. This study evaluates the seismic vulnerabilities of those bridges and the structural factors that could affect their performance during a seismic event. The effects of load demands and age deteriorations were also studied. Aging of certain bridge components such as bearings, columns, and bent caps can affect the capacity and demands of these components and accordingly might affect the global behavior and capacity of a bridge during an earthquake event. The concept of fragility curves was studied as a potential tool for evaluating the seismic performance of new bridges, existing bridges and retrofitted bridges for various bridge types subjected to different peak ground acceleration levels. Fragility curves represent the probability of a structure to experience damage levels higher than specific damage state at different peak ground acceleration. Possible retrofit measures for various bridge components were reviewed, and analyzed for their effectiveness. These include superstructure restrainers, stoppers, shear keys, isolation bearings, bent cap strengthening and column jacketing. Existing research shows that the concept of fragility curves can be used to identify bridge vulnerability and level of damage. They can also be used to identify performance and level of damage of various retrofit measures. The effect of aging of certain components such as stiffening and locking of bearings and corrosion of confining steel in columns need to be included when evaluating bridge load demands and capacities.  Different types of concrete bridges (typical in North Eastern United States) were analyzed using elastic response spectrum and nonlinear push-over analysis for low, medium-to-high, and high seismicity levels. The effects of pier configuration, continuity between the superstructure and the substructure, and the number of spans were investigated.   Analysis results showed that in the longitudinal direction, the displacement demand increased for multi-column bents compared to single-column bents. However, the overall D/C ratio dropped in both transverse and longitudinal directions. The results also showed that in the longitudinal direction the benefit of having multi-column bent over single-column bents in integral bridges is dependent on the seismicity levels.  The D/C (demand/capacity) ratio for single column bents in the longitudinal direction was much lower for integral (monolithic) bents compared to non-integral bents. In the transverse directions, the difference in the D/C ratio was not significant. For multi-column bents, the percent change by having integral bents over non-integral bents was dependent on the seismicity levels. For high seismicity zones, the benefits of having Integral bents becomes more significant.   This investigation presents guidance on incorporating the effects of aging and retrofitting in the finite element modeling of bridges subjected to various levels of earthquake ground motions.