Tensile Stresses in the End Regions of Pretensioned I-Beams at Release

Tensile Stresses in the End Regions of Pretensioned I-Beams at Release

An experimental research study was conducted at the Phil M. Ferguson Structural Engineering Laboratory at the University of Texas at Austin to investigate the cracking in end regions of pretensioned I-beams at prestress transfer. This horizontal cracking near the end face of the girder is caused by the tensile forces that develop perpendicular to the line of action of the prestressing force after it has been applied. After seeing this cracking occur in multiple girders throughout the state of Texas and in the process of developing design standards for a new family of prestressed concrete I-beams (Tx family of beams), the Texas Department of Transportation sponsored this research to investigate this end region cracking problem. The goals of this research were to:

i. Ensure that the cracks forming in the end regions of new I-Beams are comparable to or less severe than those in conventional AASHTO girders or TxDOT beams.

ii. Quantify the amount of end region reinforcement that should be used in the new I-Beams to ensure that the quality of the new I-Beams, that have larger bottom flanges and greater prestressing force, is comparable to or better than the conventional AASHTO girders or TxDOT beams.

To achieve these research objectives, four full scale beams from the new Tx family of girders were fabricated. More specifically, two 28”, one 46”, and one 70” deep girders were tested to examine transverse stresses in the end regions of pretensioned beams. The testing of four 30-ft-long full-scale beams resulted in 8 test regions. The test regions at the two ends of each girder had different transverse reinforcement details. Each end region was comprehensively instrumented by strain gauges to capture the most critical strains. The strain gauge data collected at the release of the prestressing trands was used to prepare new reinforcement details to control the widths of the cracks that form in the end regions of pretensioned I-girders at release.