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

or #2 deformed bar. Due to the detailing of this reinforcement, in addition to controllinghorizontal cracking, the stirrups served to confine the prestressing force (Figure 2.32).Figure 2.32 Stirrups serving as both transverse & confining reinforcement(Gergely, Sozen & Siess, 1963)Stirrups were instrumented along one line, which represented the height of thecrack predicted by Gergely-Sozen analysis (previously described in Section 2.3.1). Asthis height was just below the centroid, spalling behavior was measured.Strains in transverse reinforcement increased with applied prestress. Interestingly,Gergely, Sozen and Siess reported that the opening of cracks had “no noticeable effect”on the forces carried by the transverse reinforcement (p. 29). This finding either indicatesthat reinforcement is much more effective at preventing cracking than previously (orsince) supposed, or it confirms the finding from the surface strain section that fine cracks(or microcracks) are very difficult to observe in the laboratory. The widest cracksobserved in the absence of stirrup yielding were on the order of 0.003 in.Beams reinforced with two stirrups (rather than one) saw the total spalling forcesplit relatively evenly over the stirrups, with each bar taking approximately 75% of thatseen in single-stirrup beams. This uniform contribution from each transverse bar countersMarshall and Mattock’s observation (Section 2.4.2.1) that spalling stresses decreasedlinearly with distance from beam end face; however, it should be noted that all of the49