STUDY ON RESTORING FORCE MODEL OF STEEL REINFORCED ULTRA HIGH STRENGTH CONCRETE FRAME JOINTS

In order to study the restoring force model of steel reinforced ultra high strength concrete frame joints, twelve specimens were fabricated and tested under low cyclic loading. The loading process was observed and failure modes were recorded. Three-line models of skeleton curves and restoring force models are presented based on typical characteristics of specimens, and compared with experimental results. The results show that loading process of specimens can be summarized with three stages; first cracking, full cracking and failure. Loading and unloading stiffness degenerates obviously in the stages of full cracking and failure. Residual deformation appears in the hysteretic curve after unloading. The presented three-line models of skeleton curves agree well with the experimental ones, which clearly indicate the failure process of specimens. Additionally, the hysteretic curves from the presented restoring force model agree well with experimental ones, and their trends are approximately identical to experimental ones. The hysteretic behavior of steel reinforced ultra high strength concrete frame joint is described better by the three-line restoring force model. The presented restoring force model can be adopted to analyze nonlinear seismic response of steel reinforced ultra high strength concrete frame.