Confined concrete behavior in hollow columns

Abstract

<p>With the development of new design and construction methods, bridge design and analysis have been continuing to evolve, in an effort to facilitate fast, efficient, and cost-effective bridge construction. Two such issues addressed in this dissertation are use of hollow concrete bridge columns, and accelerated bridge construction (i.e., ABC) methods in seismic regions. Current seismic design specifications (e.g., Caltrans Seismic Design Criteria and AASHTO LRFD Bridge Design Specifications) require pushover analyses to be performed in order to accurately assess the displacement capacities of bridges. In pushover analyses, accurate models to capture the nonlinear behavior of critical members in a bridge system is the most important and challenging part. The overall goal of the current research is to advance the seismic analysis capability in order to enhance the design and construction practices of bridges in seismic zones. In consideration of hollow bridge columns design and accelerate bridge construction method, the focus of the current study is to understand the confinement effect in hollow bridge columns and the load transfer characteristics of unstressed strands used as connection reinforcement for precast concrete members.</p> <p>The confinement effect in both circular and square hollow bridge columns was investigated through detailed analytical studies by employing finite element analysis (FEA) method. The main variables were confinement configuration, wall thickness and confinement reinforcement amount. Understanding of confinement effect in both solid and hollow concrete columns was improved using fundamental key variables such as concrete dilation and confining pressure. It was shown that the effects of confinement in solid and hollow concrete columns are very different. The difference in behavior was due to variations in concrete dilation and the distribution of confining pressure across the wall thickness. Furthermore, it is shown that the effectiveness of confinement in hollow concrete columns could be quantified using confinement configuration, wall thickness, and confinement reinforcement ratio as the main variables.</p> <p>After realizing that the confinement effect in hollow concrete columns with a single layer of transverse reinforcement is not as good as that established for solid concrete columns, a modeling method was proposed to account for this reduction of confinement effectiveness. Suitable modifications were introduced to an existing, widely-used confined concrete model (i.e., Mander’s model) based on a detailed FEA. The force-displacement responses predicted using the modified confined concrete model, which is applicable for hollow columns with a single layer of reinforcement, were compared to both previous research and the experimental responses in the current study. Satisfactory agreement was found.</p> <p>Finally, the load transfer characteristic of unstressed strands was examined based on pullout tests. With the development of ABC, strands provide an economical and practical solution as connection reinforcement of precast concrete members and could be used to connect a hollow column to cap beam or foundation. The unstressed strands were embedded in large concrete blocks with different anchorage details and various embedment lengths. The relationship between strand stress and loaded-end slip was developed and several design recommendations have been made regarding the use of unstressed strands for connection reinforcement of precast concrete members.</p>