Finite-Element Analysis of GFRP-Reinforced Concrete Frame Knee Joints under Closing Loads

This paper presents an analytical investigation of glass fiber–reinforced polymer (GFRP) reinforced concrete knee joints subjected to closing moments, with a focus on reinforcement ratio, joint geometry, and confinement stirrups. The study uses the finite-element method with a commercially available code to verify experimental tests and examines the behavior of the knee joints through a parametric study. The concrete damaged plasticity model was used to define the material behavior of concrete, and the effects of different dilation angles were also studied. The average test-to-predicted ratios between finite-element analyses and test results fall within a range of 2% for moment capacities and 5% for deflections at peak. The results indicate that the dilation angle significantly influences the moment capacity and the ultimate deflection of specimens. Proper reinforcement detailing and concrete confinement are found to be important in knee joints, as unconfined specimens failed at significantly lower strengths than their confined counterparts. Incorporating diagonal stirrups in the knee joints is shown to be more effective than utilizing lateral stirrups in enhancing both moment capacity and deflections of the specimens under closing moments. However, placement of lateral stirrups is more practical than diagonal stirrups and it is shown that lateral stirrups increase the capacity of the joints to acceptable levels.