Abstract:
Deep prestressed concrete bridge girders are becoming increasingly popular due to their ability to span longer distances and reduce the total cost of bridge projects. However, these girders have frequently been subject to end zone cracking during the transfer of prestress forces despite being designed to current AASHTO specifications. Previously, the Alabama Department of Transportation (ALDOT) has designed deep prestressed girders which can span up to 165 ft. During the fabrication of these girders, crack formations in the end zone were typically noticed. To address this concern, longitudinal reinforcement was added to the end zones. This solution controlled cracking to some extent but could not completely eliminate cracking. An experimental study was conducted to find a practical engineering solution to the problem of end zone cracking, as well as to develop a 78 in. deep prestressed bulb-tee girder design to reach a span length of 180 ft. 3D finite element modeling was used to find three practical alternative end zone modifications to the standard design. The modified designs included a lowered draping angle, partial debonding of the strands, and a combination of the two. Four 54 ft. long specimens, including three with end zone modifications, were fabricated at Hanson Pipe & Precast in Pelham, Alabama, and monitored during the detensioning process. The end zones were instrumented with steel and concrete strain gauges to better understand the complex behavior of girder end zones. External DEMEC instrumentation was also included at the girder ends to measure the transfer length of the strands in each specimen. The specimens were then load tested at the UA Large Scale Structures Laboratory (LSSL) to determine the effects of the modified end zone details on the girder capacity. Based on the study, modified girder end zone details are recommended to ALDOT for implementation.
