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

The requirement for transverse reinforcement sufficient to resist 4% of the appliedprestressing force ( ) can be considered a simplification of Marshall and Mattock’soriginal equation, with the assumption that the beam height-to-transfer length ratio (/ )be approximately equal to two (Equation 2.4).where: 0.021 0.021 2 0.04 Equation 2.4= required area of transverse reinforcement near the member end= prestress force at transfer= design stress in the transverse reinforcement= depth of memberh = strand transfer lengthFountain (1963) agreed with this characterization of the 4% requirement—ascoming from Marshall and Mattock—citing their research when making hisrecommendation for end-region reinforcement. Fountain’s recommendation (below)reads very similar to the provision of the 1961 AASHO Interim Specifications.Regardless of the prestressing steel pattern or computed stresses, a minimumamount of web steel sufficient to resist a total vertical tensile force equal to4 per cent of the total initial prestress force be placed at each end of thebeam between the end face and a vertical plane located one-fourth the depth ofthe beam from the beam end. This reinforcement should start as close to thebeam end as practicable (p. 13).The next major change to AASHO’s treatment of pretensioned beam end regionscame in 1973, when confining reinforcement became mandatory. The relevant codeprovision reads:For at least the distance d from the end of the beam, nominal reinforcement shallbe placed to enclose the prestressing steel in the bottom flange. For box girders,transverse reinforcement shall be provided and anchored by extending the leginto the web of the girder (Section 1.6.15).It is unclear why, behaviorally, prestressing strand in the end region of a boxgirder (or a U-beam, essentially an untopped box girder) should require less confinement75