Shear Behavior Of Prestressed Concrete Beams

According to the investigation of long-span box girder bridges, excessive deflections at midspan and diagonal web cracks are always "complications". Diagonal web crack as concrete shear behavior is one of the hotspots in structural field. However, most experimental researches performed pertaining to the shear behavior have focused on the ultimate shear strength rather than the shear deformation, lacking an advanced and all-sided design theory. This paper carries out research on shear strength of reinforced concrete beams and prestressed concrete beams simultaneously, and emphasizes attention to the effect of shear stiffness on deflections at midspan after diagonally cracking, then quantifies the effect by the finite element analysis. The conclusions are listed as follows.(1) A shear database including 216 RC beams with stirrups is compiled, which is utilized to demonstrate the influences of several substantial factors exerted on shear strength, such as shear span-to-depth ratio, effective depth of section, concrete compression strength, longitudinal reinforcement ratio and stirrup characteristic value. Then several representative design specifications are introduced and an expression based on the truss-arch model with the consideration of compatibility is presented. Finally, the proposed method is compared with the current specifications by the methods of statistic eigenvalue and influencing factors analysis. Results indicate that there is a good correlation between the shear strength obtained by the proposed method and the shear database, and the proposed method can reflect precisely the influence of shear span-to-depth ratio.(2) Based on the aforementioned truss-arch model, an expression for shear strength of PC beams is derived with the consideration of the beneficial effects of prestress and compression flange. Then a shear database including 118 PC beams with stirrups is compiled, which is utilized to demonstrate the influences of several substantial factors exerted on shear strength, such as shear span-to-depth ratio, effective depth of section, concrete compression strength, longitudinal reinforcement ratio, stirrup characteristic value and prestress at section center. Finally, the proposed method is compared with the current specifications by the methods of statistic eigenvalue and influencing factors analysis. Results indicate that there is a good correlation between the shear strength obtained by the proposed method and the shear database.(3) The formula of effective shear stiffness is proposed with the effect of tension stiffening taken into account.6 rectangular PC beams and 4 I-shaped PC beams are compiled to identify the applicability of effective shear stiffness and to compare with the flexural deformation calculated by GB 50010-2010. Then the research concerning deformation ratio of experimental beams shows that the shear contribution to deflection after the formation of diagonal cracks is nonignorable, especially for beams with thin webs. Finally, the influences of several substantial factors exerted on shear deformation after diagonally cracking are analyzed, such as angle of draped tendons, longitudinal reinforcement ratio, shear span-to-depth ratio, span-to-depth ratio, and effective prestress, concrete compression strength and stirrup reinforcement ratio.(4) Taking the continuous rigid frame of the Sutong Bridge as background, the influences of concrete shrinkage and creep, prestress loss and diagonally cracking exerted on the deflection of long-span box girder bridges are analyzed by the finite element MIDAS/Civil. Results indicate that shrinkage and creep model has the most significant impact, prestress loss of cantilever tendons exerts more evident influence among tendons, and the shear contribution to deflection after the formation of diagonal cracks is nonignorable.