Function and design of confinement reinforcement in pretensioned concrete I-girders.

摘要

Confinement reinforcement is placed near the end of pretensioned concrete I-girders to enclose prestressing strands in the bottom flange. Experimental and analytical test programs were conducted to investigate the function of confinement reinforcement, and to provide the basis for a confinement reinforcement design model. Five 54-in. deep Florida I-Beam (FIB-54) girders were fabricated and load tested in the experimental program. Each end of each girder had a different combination of variables, which resulted in ten unique test specimens. Variables included: presence or absence of embedded steel bearing plates, quantity and configuration of confinement reinforcement, strand bond pattern, strand quantity, and quantity of horizontal and vertical end region reinforcement. Data were collected during and after prestress transfer to evaluate the effects of test variables on bottom flange cracking. Load tests were then conducted on each specimen (end) to determine the effects of test variables on girder behavior and capacity. Specimens were loaded in three-point bending at a shear span-to-depth ratio of 2.0. Failure modes in the test program included web-shear, bond-shear, and lateral-splitting. Data from fabrication and load testing were used to validate linear-elastic finite element models. Validated models were then used to investigate additional variables such as: Bearing pad geometry and stiffness, cross-section geometry, prestress release sequence, and strand transfer length. Building on the experimental and analytical results, serviceability and ultimate strength design models were created for the bottom flange of pretensioned I-girders. The serviceability model is specific to FIB cross-sections and can be used to evaluate transverse splitting stresses which cause bottom flange cracking. The ultimate strength model is general for any pretensioned I-girder cross-section and can be used to design bottom flange confinement reinforcement to mitigate lateral-splitting failure. Both the serviceability and ultimate strength models were found to be in good agreement with experimental data. Other primary outcomes of the research include an improved understanding of the function of confinement reinforcement during prestress transfer and at ultimate load, and an improved understanding the interaction between confinement reinforcement and the other test variables.