Strength and Ductility of a Three-Span Externally Post-Tensioned Segmental Box Girder Bridge Model

R.J.G. MacGregor, M.E. Kreger and J.E. Breen


This report is the third and final report in a series outlining a major study of the behavior of post-tensioned concrete box girder bridges with post-tensioning tendons external to the concrete section. It summarizes thd design, construction, testing and interpretation of a very comprehensive three-span externally-post-tensioned box girder bridge model. The model was constructed from precast segments using the span-by-span construction procedure. Careful measurements were made during construction to document the actual stresses and prestress losses occurring. One span of the model had dry joints while the other two spans had epoxy joints.

Loading was applied at design service load levels, design factored load levels, and ultimate load cycles for both maximum flexure and maximum shear loading configurations. Careful observations were made of deformations, tendon stress changes, joint openings and reaction changes. Companion analysis was performed to assist in development of the model and in the interpretation of the test data.

The model bridge was very stiff at service load conditions and exhibited linear behavior to loads higher than the factored design load. The cracking load for epoxy-jointed spans was approximately twice the load required to decompress the flexural tension fiber and begin to open a previously cracked joint. This suggests that epoxied joints can provide a reasonable factor of safety against joint opening and that this same factor of safety can be provided in dry-jointed spans only by applying additional prestress force. Both the dry and epoxy-jointed spans displayed considerable ductility during flexural strength tests.

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