Bursting and Spalling in Pretensioned U-Beams - Ferguson ...

tested at transfer and under shear loads in the near future under Research Project5831, with these test results determining the adequacy of such details.5. Measured stresses in bottom-flange lateral bars never exceeded the AASHTO limit(20 ksi), despite the fact that the reinforcement provided in this zone was 30% of thatrequired by AASHTO LRFD. As the recent code change (2008) requiring “splitting”reinforcement in this zone seems to have been based on Washington State DOTstandard practice rather than the results of a specific research program, the coderequirement may be overly conservative. Similar results from other beam types (e.g.box beams, as planned in later portions of Research Project 5831) would be necessaryto substantiate these findings, however.6. No trend in the measured bursting and spalling stresses could be linked to the endblockdesign alternative (large triangular or small skewed end block) chosen for theskewed end; neither end block can be recommended as providing superiorperformance at prestress transfer. Should the finding of structural equivalence bereplicated in shear testing and further bursting/spalling testing, the small skewed endblock may be recommended for temperature-related reasons.7. Bursting reinforcement should be provided in all locations at which strands first bondto concrete, be they near the beam end as in the usual case, or further from the beamend in the case of debonded strands or dapped ends. With limited experimental workthus far conducted examining the effects of different levels of bursting reinforcement,it is recommended that bursting reinforcement should be designed similarly to the“splitting” reinforcement in AASHTO LRFD, to resist 4% of the applied prestressingforce (or increment of applied prestressing force newly bonded at that section) withan allowable design stress of 20 ksi for the transverse reinforcement.190