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

2.3.1.3 Plastic Analysis of Beam End RegionsThe primary plastic method of analysis used is strut-and-tie modeling (STM). Anadvantage to this method is its versatility: it can be used to provide a rational basis fortransverse reinforcement placed anywhere in a beam end region, not just very close to thebeam end. However, in order for the reinforcement to become effective, cracking mustdevelop. If the transverse reinforcement is placed far from the regions of maximumtensile stress from elastic analysis, cracking may be extensive before the rebar isengaged.As the primary purpose of transverse-tension analyses since Gergely, Sozen andSiess (1963) has been to provide sufficient transverse reinforcement to ensure serviceableend regions, the applicability of a method that assumes the plastic behavior of highlycracked concrete may seem limited. The use of STM for conservative predictions ofultimate capacity in post-tensioned concrete beams (Sanders, 1990) does not immediatelylend itself to pretensioned beam serviceability.That said STM has been applied to pretensioned concrete beams by Castrodaleet al. (2002) and Davis, Buckner and Ozyildirim (2005). As the latter model wasexperimentally investigated by Crispino (2007), it is outlined below.The goal of the STM developed by Davis, Buckner and Ozyildirim is to provide arational basis for dimensioning transverse reinforcement beyond the near-end regionhandled by AASHTO LRFD in hopes of controlling long diagonal spalling cracks oftenseen in Virginia bulb tees. The prestressing force in these beams is typically separatedinto two groups, one applied to the top of the web and the other to the bottom flange.While in an actual pretensioned concrete beam, the prestressing force is applied to thesection gradually over the strand transfer length, the model assumes the prestressing forceto act as concentrated load at the member end (like in a post-tensioned beam). The more19