Experimental study on hysteretic behavior of composite frames with concrete-encased CFST columns

• An innovative composite frame structure is proposed for engineering practice.
• Perfect seismic behavior of the composite frame structure is observed.
• The initial cracks could be proponed due to the prestressing effects.
• Mechanism of energy dissipation for beam plastic hinges is expected.

This paper proposed an innovative frame structure composed of concrete-encased steel beams and concrete-encased concrete filled steel tube (CFST) columns for long span building, and two one-bay and one-storey specimens were tested under lateral low cyclic loading. The testing process, such as cracks developments and failure patterns were observed, and the seismic performance such as hysteretic behavior, skeleton curves, rigidity degradation, energy dissipation, and residential deformation were investigated. Additionally, the strain variations for longitudinal bars, flanges of H-shaped steel and steel tube in potential zone of the plastic hinges of beams and columns were analyzed. The test results indicated that two specimens both behaved perfect energy dissipation and ductility even prestressed; and the initial cracks of prestressed specimen was restrained and appeared later than that of unprestressed one due to the prestressing effects; otherwise, during the whole loading the circular strains of steel tubes in the bottom of columns varies unevenly, and steel tubes in compressive zone could supply effective constraint on corresponding concrete. According to the strain variations, the sequence of plastic hinges of the two specimens presented that they occurred on the beam ends firstly and then on the bottoms of columns and mechanism of energy dissipation for beam plastic hinges and a delayed occurring of plastic hinges on column bottom were expected.