Structural behavior of frame piers subject to temperature and shrinkage effects

Abstract

Frame piers are a popular type of bridge pier composed of columns that support a cap beam. Frame piers are often the most economic pier choice when a pier does not need to be placed in water or designed for vehicular collisions. However, certain frame pier geometries, such as long frame piers, are susceptible to forces caused by temperature and shrinkage effects. In design, thermal strain, shrinkage strain, creep strain, and even the nonlinear behavior of concrete may be significantly simplified. This can lead to an overdesigned frame pier with diminished economic benefit and longer construction times. Therefore, it is advantageous to develop a model capable of simulating these effects and use the model to identify the most susceptible frame piers to these effects. Furthermore, various metrics predicting the susceptibility of frame piers to these effects are assessed. A finite element model capable of simulating shrinkage strain, creep strain, thermal strain, strength development of concrete, and the nonlinear behavior of concrete is developed and calibrated with published experimental results. Field data is collected from bridges that are instrumented with vibrating wire strain gauges embedded in the frame piers. The finite element model is validated with collected field data from an instrumented frame pier. Various frame pier geometries are analyzed with the validated model to identify the most susceptible geometries. Frame piers cast on warm summer days, particularly in the months of June and July, experience the most demand from temperature and shrinkage effects compared to other times of the year. Frame piers with nonuniform columns, such as larger exterior columns or varying column height, are found not to be more susceptible to these effects than piers with uniform columns. Bay length in also found to have negligible impact on frame pier susceptibility to these effects. The most critical factors affecting frame pier susceptibility are found to be column stiffness, length of the cap beam, and flexural stiffness of the cap beam. Column stiffness is observed to be the most impactful on the susceptibility of frame piers to these effects. Basic susceptibility metrics, such as the length of the frame and the length to height ratio of the frame, are found to be very inaccurate in predicting susceptibility. Accurate susceptibility metrics must account for column stiffness and column restraint.