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

Early studies often examined beams in which up to 50% of the prestressing force wasconcentrated in the upper flange. Spalling may be critical for such beams due to“multiple-anchor spalling stresses” (shown previously in Figure 2.4), and as such, thebeams were primarily instrumented in the spalling zone. Some later studies have copiedthis instrumentation approach uncritically, placing gages only in the spalling zone forbeams in which bursting may be critical (e.g. bulb tees with a large prestressing forceapplied to the bottom flange). As such, the experimental database is more complete withregard to spalling than bursting behavior.2.4.2.1 Marshall & Mattock, 1962The first study of end-region reinforcement strains was completed by Marshalland Mattock at the PCA laboratory in 1962. The primary purpose of the study wasprescriptive rather than descriptive: to develop design criteria for the transversereinforcement in the end regions of pretensioned beams. To broaden the applicability ofthe findings, the 25 I-beams examined for the study (Figure 2.28) had a number ofvariables, including• Size and pattern of the prestressing strand;• Magnitude of prestressing force;• Web thickness; and• Size of transverse reinforcement (#2 or #3 stirrups).Transverse reinforcement was provided within h/3 of the beam end and wasinstrumented along the centroidal axis (just below mid-depth). In the case of beamsreinforced with #3 stirrups, the transverse reinforcement provided in the beam end regionwas approximately double that currently required by the AASHTO LRFD. For beamswith #2 stirrups, the reinforcement was 80 to 120% of that now required by AASHTO.43