Seismic behavior of reinforced concrete beam-column joints with unstressed steel strands fully or partially used for beam longitudinal reinforcement

Abstract

Five beam-column joint specimens: three exterior and two interior specimens were subjected to cyclic loading to investigate the effectiveness of using unstressed Grade 1860 MPa steel strands as full/partial replacement for the conventional Grade 420 MPa deformed bars as beam longitudinal reinforcement. The study also developed a special hooked anchorage detail to anchor the steel strands in the joint region. The test specimens failed by beam plastic hinge formation near the column face as intended in design, with drift capacities between 5.12% and 7.57%. However, the full (100%) and partial (50%) replacement of deformed bars by strands led to a 58% and 28% decrease in average relative energy dissipation. Despite this, the full/partial replacement of deformed bars by strands as beam longitudinal reinforcement resulted in enhanced reentering capabilities. Fracture of longitudinal steel strands observed in the beam plastic hinge region at the end of the test indicated the effectiveness of the special hooked anchorage detail in allowing steel strands to develop their capacity. Further, the flexural strength of the beams of test specimens was evaluated through a detailed moment-curvature analysis, which predicted the beam flexural strength with high accuracy and less variation (mean = 1.01, standard deviation = 0.02). After that, beam flexural strengths were evaluated following the ACI 318–19 procedure modified to include the proposed bilinear strand model, which produced conservative estimates of beam flexural strength (mean = 1.15) suitable for design. Also, the procedure to estimate the beam's probable flexural strength is presented. Finally, required modifications to the conventional seismic design procedure were proposed to incorporate the use of strands as beam longitudinal reinforcement.