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

proprietary post-tensioning anchors and thus is of limited applicability in the case ofpretensioned beams (in which the prestressing force is developed by bond, not anchors).Similar to their previous study, Zieliński and Rowe observed cracking to occur athigher levels of prestress than those predicted considering splitting tensile strength data.In this case, however, the difference was much smaller—beams cracked at strains30 to 50% higher than expected. The researchers offered the following explanation:In end-blocks, by virtue of the complex stress system that exists, the stress-strainrelation of the concrete in tension is modified, the strain capacity before crackingbeing greater than in normal bending tension (p. 39).In a later study, also of eccentrically post-tensioned I-beams, Breen et al. (1994)offered verification of the above hypothesis. Comparing their experimental results toresults from FEA, the researchers found that a concrete tensile-strength modelincorporating the effects of “the complex stress system” could predict observed crackingloads with greater accuracy, though less conservatism, than a model assuming a nominaltensile strength.2.4.1.3 Marshall & Mattock, 1962Though their major contribution to the area of bursting and spalling behavior layin a study of end-region reinforcement strains (discussed in Section 2.4.2.1), Marshalland Mattock were involved in a preliminary study in which concrete surface strains weremeasured for 10 pretensioned I-beams of 23 in. depth. Foil strain gages were applied tothe beam end over the depth of the web, and along the beam length at the height of thesection centroid. Typically, the maximum transverse tensile strain was found at the endface near the centroid (Figure 2.20), and the tensile strains decayed to negligible levelsapproximately h/3 from the member end (Figure 2.21).32