Full-scale experimental and numerical investigation of precast double-T beams with combined straight and harped pretensioned strands

To enhance the shear resistance of pretensioned concrete beams and extend their applicability to medium- and long-span bridges, this study proposed a novel pretensioned precast double-T beam incorporating a combination of straight and harped pretensioned strands. This configuration is designed to facilitate industrialized production. To validate the structural performance of the proposed design, full-scale simply supported specimens were fabricated and subjected to four-point bending tests at mid-span, as well as bending-shear coupled loading tests with a shear span-to-depth ratio of 2.5. For safety considerations, the full-scale tests were not continued to complete failure; therefore, supplementary numerical analyses were conducted. The results of both experimental and numerical investigations indicate that under four-point bending, the beam exhibited progressive stiffness degradation following flexural cracking and the yielding of the tensile prestressing strands and longitudinal reinforcement. Failure occurred due to crushing of the compression-zone concrete, with the specimen demonstrating substantial vertical deformation and ductile failure characteristics. Under bending-shear coupled loading, flexural cracks developed primarily near the loading points, while diagonal cracking in the shear span remained relatively limited due to the confining effect of the harped strands. Upon yielding of both the prestressing strands and shear reinforcement in the shear span, the web concrete experienced diagonal crushing, leading to failure. Comparative analysis revealed that, relative to beams reinforced with combined strands, specimens reinforced solely with straight strands exhibited a 23.1 % reduction in cracking load, an 8.3 % decrease in ultimate load, and a 66.7 % reduction in ultimate vertical deformation. These findings confirm that the proposed double-T beam system demonstrates superior flexural and shear performance. Furthermore, its structural capacities can be evaluated using the design methodologies outlined in both Chinese and AASHTO specifications, provided that the principal compressive stress in the web concrete is adequately controlled to prevent premature shear failure.