Performance of Reinforced Concrete Bridge Columns with Various Reinforcement Details Subject to Long-Duration Earthquakes

Abstract

Devastating long-duration earthquakes such as 2011 Tohoku Earthquake in Japan, and 2010 Chile Earthquake have proved the importance of considering ground motion duration for seismic demand assessment. Thus, it is important to understand the design implications of long duration earthquakes. This research focuses on using both analytical and experimental methods to study the effect of different design details (i.e. confinement spacing ratio and longitudinal bar debonding) and different reinforcement (i.e. 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 a shake table at the University of Nevada, Reno.The first phase included three specimens designed using conventional Grade 60 ASTM 706 reinforcing bars and tested under a sequence of long duration earthquakes (Tohoku, Japan 2011 EQ). All three columns had same longitudinal reinforcement ratio. However, the second column had different confinement spacing ratio compared to that of the first column. The third column on the other hand, considered debonding of longitudinal reinforcement at the column-footing interface. The other three specimens in the second phase were reinforced longitudinally with high strength grade 100 ASTM A1035 MMFX steel. The columns were tested under short and long duration motions to leverage research on cyclic deterioration and help to qualify the use of high strength reinforcement in seismic design of bridge columns. The pre-test analyses, design and construction of the specimens as well as the results of the shake table tests and a comparison of the global and local seismic response of the six columns are presented in this document. The global responses include the force and displacement capacities and mode of failure. On the other hand, the presented local responses include the strain in both transverse and longitudinal rebars and the columns’ curvature within the plastic hinge zone. The experimental results showed that higher concrete confinement (i.e. smaller tie spacing) and longitudinal bars debonding both are effective to improve the column performance under long-duration earthquakes. However, the effect of confinement is more significant. The pre-test analysis was conducted using a computational model which was initially calibrated against the 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 the modeling recommendations. In addition, to refine the high strength reinforcement material, a set of material tests testing was conducted to investigate the effect of high strain rate on these reinforcing bars. The results revealed significant increase in the yield stress and reduction in fracture strain due to high strain rate effect. Furthermore, an analytical study on two-span two-column bent archetype bridges was conducted to make recommendations for the current seismic provisions to consider the duration effect in design of the bridges at the sites with potential occurrence of long duration earthquakes. New site-specific design criteria were developed for multi-column bent bridges considering the sites in the U.S. Pacific region and Alaska to mitigate the duration and spectral shape effects. 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.