Behavior of a new steel-concrete hybrid frame system

Plastic mechanism of a structural frame system should be predetermined during design stage, and should be selected such as to achieve a desirable displacement ductility with smallest rotation demands in the plastic hinges. Development of plastic hinges in frame columns are usually associated with very high rotation demand and may result in a total structural instability. Formation of plastic hinges in beams is more favorable. However, formation of a plastic hinge at the face of a column may result in bond deterioration between the reinforcing bars and the surrounding concrete within the beam column joint. This paper introduces a new innovative steel-concrete composite frame system with controlled plastic mechanism. This frame system consists of steel tubed reinforced concrete (STRC) columns, and ordinary reinforced concrete beams with relocated plastic hinges. Beam plastic hinges are relocated by the use of straight headed bars. The STRC column is an ordinary reinforced concrete column but, transversely reinforced with light ordinary ties and a thin steel tube. Compared to concrete filled tube column (CFT), the steel tube of STRC column transfers no axial load, provides better confinement, and consequently, increases column ductility. In this paper, experimental investigation of two full scale STRC columns and two beams with and without headed bars are presented. Test results suggest that STRC columns and beams with relocated plastic hinge regions could offer a more ductile structural frame system for medium and high rise buildings in zones of high seismicity.