Shear behavior of prestressed concrete U-beams

An experimental study was conducted at the Ferguson Structural Engineering Laboratory in order to investigate the shear behavior of 54-inch deep prestressed concrete U-beams. The primary goal of this research was to improve the design and detailing of the skewed end-blocks commonly used in these beams. As U-beams had been in service for several decades without incident, it was anticipated that there would be little need for change in the design, and the findings of the research would involve a slight tweaking to improve the overall performance. Unfortunately, during the first phase of shear testing (testing of the current design standard) it was found that the U-beam was not reaching the code calculated shear capacity. During this phase of testing the premature failure mechanism was isolated as the breakdown of the web-to-flange interface in the end region of the girder. Therefore, the second phase of testing sought to prevent the breakdown of this boundary by three options: (1) increasing the web width while maintaining current levels of mild reinforcement, (2) increasing the web width while also increasing the amount of reinforcement crossing the web-to-flange boundary, or (3) by increasing the amount of reinforcement at the boundary while maintaining the current web width. Two acceptable solutions to the premature failure method were developed and tested during this phase both of which included an increase in the amount of mild reinforcement crossing the web-to-flange interface (with and without an increase in web width). The research into refining of these new details is ongoing as part of the Texas Department of Transportation’s Research Project number 0-5831.