Seismic Design and Detailing of Bridge Columns to Account for Ground-Motion Duration
Authors: Amiri S. Mojtaba , Moustafa Mohamed , Saunders David 
Published: March 2022

Devastating, long-duration earthquakes such as 2011 Tohoku, Japan, earthquake, and 2010 Maule, Chile, earthquake have proved the importance of considering the duration of ground motion in conducting a seismic demand assessment. This research focuses on using both analytical and experimental methods to study the effect of different design details—confinement spacing ratio and longitudinal bar debonding—and different reinforcement strategies—conventional and high-strength reinforcement—on the seismic response of reinforced concrete (RC) bridge columns under long-duration ground motions. In this study, six large-scale RC bridge column specimens were designed, constructed, and tested in two phases on the shake table at the University of Nevada, Reno. The first phase included three specimens designed using conventional Grade 60 ASTM 706 reinforcing bars tested under a sequence of long-duration earthquakes (2011 Tohoku earthquake). All three columns had the same longitudinal reinforcement ratio. Column #2 had a different confinement spacing ratio compared to Column #1. In contrast, Column #3 considered debonding of longitudinal reinforcement at the footing interface. Columns #4, 5, and 6 tested in the second phase were reinforced longitudinally with high-strength grade 100 ASTM A1035 MMFX steel. These columns were tested under short- and long-duration motions to study the cyclic deterioration of high-strength reinforcement and quantify the response of bridge columns under seismic events. Presented herein are the pre-test analyses, design, and construction of the specimens, the results of the shake table tests, and a comparison of the global and local seismic response of the six columns tested. The global responses include the force and displacement capacities and mode of failure. Local responses include the strain in both transverse and longitudinal rebars and the curvature of the columns within the plastic hinge zone. The experimental results demonstrate that although both the higher concrete confinement (i.e., smaller tie spacings) and longitudinal bars debonding are effective in improving the performance of columns subjected to long-duration earthquakes, the smaller tie spacings is more effective. The pre-test analysis was conducted using a computational model that was initially calibrated against a previous experimental study. The model was then assessed using the shake table test data, refinements were conducted, and new modeling values/parameters/equations were obtained and proposed as modifications to the model. In addition, a set of material tests was conducted to refine the high-strength reinforcement material to investigate the effect of high strain rates on these reinforcing bars. The results revealed a significant increase in the yield stress and reduction in fracture strain due to high-strain-rate effect. Finally, an analytical study on two-span, two-column bent archetype bridges was conducted with recommendations to amend the current seismic provisions in the design of the bridges at those sites with potential for long-duration earthquakes. To mitigate the damage from long-duration seismic events and spectral shape effects, new site-specific design criteria were developed for multi-column bent bridges located at sites in the U.S. Pacific Northwest and Alaska. The results of the experimental and analytical studies can help assess the effectiveness of the varied design details and provide a foundation for future design guidelines to account for longer duration earthquakes.