Iowa Department of Transportation
Iowa Highway Research Board
Behrouz Shafeishafei@iastate.edu email >
Structural Engineer, BEC
About the research
Shrinkage and temperature forces are known to have short- and long-term effects on both the superstructures and substructures of bridges. In the substructure, such effects are more pronounced if frame piers are used, given their volumetric change is often restrained.
The main objective of this research was to investigate the forces developed in frame piers and their supporting foundations due to volumetric changes caused by thermal and shrinkage effects. For this purpose, a set of finite element (FE) models capable of simulating shrinkage strain, creep strain, thermal strain, strength development of concrete, and nonlinear behavior of concrete were developed and calibrated using experimental test results. Field data were then collected from bridges instrumented with vibrating wire strain gauges embedded in the frame piers at the time of construction. Further to obtaining firsthand information from the field, the FE models were validated using collected field data. Various frame pier geometries were then analyzed using the validated model to identify the most susceptible geometries.
The results of the study indicated that frame piers cast in Iowa on warm summer days, particularly in June and July, experience the most demand from temperature and shrinkage effects compared to frame piers cast at other times of the year. The most critical factors affecting frame pier susceptibility were found to be column stiffness, length of the cap beam, and flexural stiffness of the cap beam. Column stiffness was observed to be the most impactful factor 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, were found to be not accurate enough to predict susceptibility, as accurate susceptibility metrics must account for column stiffness and column restraint factors. These results led to the development of two-dimensional linear elastic models that simplified the assessment process without losing accuracy. Overall, the requirements set by the Iowa DOT’s Bridge Design Manual were found to be adequate in capturing the performance of frame piers subjected to temperature and shrinkage forces.