Catherine W. French, Principal Investigator
Carol Shield, Co-Principal Investigator

Pre-Release Cracks in Pre-Stressed Girders; Finite-Element Analysis of Integral Abutment Bridges

Effect of pre-release crack closure on girder stresses.

The objective of this project was to investigate the effect of vertical prerelease cracks on the behavior of prestressed bridge girders. As high-strength concrete becomes more popular, heavily reinforced sections with longer span lengths are being used in pre-stressed concrete bridge girders. During the production of these girders, it has been observed that vertical cracks near the mid-span of the girders may develop during the curing process if the girders are left on the casting bed for a long time without detensioning the pre-stressing strands. Cracks have been observed to begin at the top flange and propagate downward in the depth of the section. In some cases they have been observed to extend through the entire depth of the section. The cracking is attributed to restrained shrinkage and thermal effects during the curing period of the girders prior to release of the pre-stressing strands. Following the release of pre-stressing strands, the cracks may close completely due to the effects of pre-stressing force and girder self weight. The cracks were suspected to cause a reduction in the loads required to initiate the flexural cracking in the girder. A compatibility theory was developed to explain this behavior.

The team has made significant progress towards discovering the effect of pre-release cracks on pre-stressed concrete bridge girders. Analyses done using ABAQUS showed the effects that pre-release cracks have on local girder stresses, flexural cracking load and girder camber.

The current phase of the study includes the experimental validation of the previous analytical study results. ABAQUS was used to optimize the design and instrumentation of test specimens. Models replicating two actual girders have been prepared by abaqus to compare the results of the analytical study to available experimental data. The ABAQUS results for camber and stress were found to compare well with the test data. The results of the present analytical and experimental studies will be used to develop recommendations for determining pre-release crack width, depth, and spacing that may present a potential concern for strand fatigue.

This research team is also investigating the behavior of integral abutment bridges using the finite-element program ansys. For three years, the team has collected field data from an integral abutment bridge to investigate the effects of environment and vehicle loading on the bridge response. The next step is to conduct an analytical study to develop appropriate models to characterize the bridge’s behavior. A key component of this study is the soil-structure interaction caused by seasonal temperature changes and the integral effects of the whole bridge. The results of the analytical study will be compared with the experimental study.

The team has made progress in developing a Finite Element Model (FEM) of the whole bridge using Subgrade Reaction Method to model the soil-structure interaction. The FEM uses shell and beam elements to simulation the superstructure of the bridge, and uses nonlinear springs to model the structure-soil interaction. Current work focuses on comparing FEM results with live load test results. Parametric analyses will also be conducted.

The future research plan continues with both field measurements and numerical studies.

Research Group

Eray Baran, Graduate Student Researcher
Jimin Huang, Graduate Student Researcher
Tina Wyffels, Graduate Student Researcher