Allowable Design Release Stresses for Pretensioned Concrete Beams

A research program was conducted in Ferguson Structural Engineering Laboratory at The University of Texas at Austin to determine whether elevated concrete stresses at extreme flexural fibers, relative to current allowable stresses, can be applied to pretensioned concrete beams at transfer. A thorough review of documents related to allowable stresses in prestressed concrete was followed by the construction of a prestressing bed capable of accommodating six fifteen-foot-long beam specimens.

Five sets of six pretensioned beams were cast and monitored between June 2002 and May 2003. These sets included specimens that were representative of standard U beams, I girders, and double-tee beams. Instrumentation for these beams consisted of strain gauges on prestressing strands and linear potentiometers supported on steel frames to measure changes in camber. At prestress force transfer, extreme fiber compressive stresses in these specimens ranged from 0.46f’ci to 0.91f’ci. Tensile stresses ranged from 1.5 f 'ci to ci 9.3 f ' .

Based on the five sets of pretensioned beams fabricated and monitored in this study, camber increased with increases in maximum compressive stress at release, expressed as a function of f’ci, regardless of the cross-section geometry and type of concrete used to fabricate the specimens. It was concluded that pretensioned concrete beams can be subjected to elevated compressive stress levels at prestress release as long as long-term camber response is adequately predicted and values are acceptable to the engineer of record.