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Concrete-encased concrete-filled steel tube (CFST) composite columns provide high bearing capacity, good seismic performance and an easier connection with arbitrary angle beams, which are widely used in high-rise buildings. Considering the high frequency of building fires, experimental research investigated the axial compressive behavior of the composite columns' exposure to high temperature in this paper. Fourteen specimens after exposure to high temperatures with different parameters, including the heating temperature, steel tube diameter and concrete cover thickness, were fabricated to test under axial compressive loading. The failure pattern, load-displacement curve, bearing capacity, initial stiffness, deformation performance and damage rule of the specimens were discussed. The test results showed obvious differences in damage of specimens subjected to various high temperatures. The failure of the specimens began with the spalling and crushing of the concrete at the edge and ends in a lantern shape. The load-displacement curves of the specimens were significantly affected by high temperature, while the influence the of steel tube diameter and concrete cover thickness was relatively weak. A method of calculating axially loaded capacity for the composite column exposure to high temperature is proposed considering the effects of the main parameters of heating temperature and steel tube position, and the calculated results are in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

This paper mainly focused on the seismic performance and shear calculation method of steel reinforced high-strength concrete (SRHC) columns with rectangular helical hoops. An experimental investigation was performed in this paper. Eleven SRHC columns with rectangular helical hoops and one with ordinary hoops were constructed at the laboratory of Guangxi university. The failure modes, hysteresis loops, envelope curves, characteristic loads and displacements and cumulative damage analysis are presented and investigated. It can be seen from the test results that the failure modes of SRHC columns can be divided into three types with the shear span ratio increased, namely, shear baroclinic failure mode, flexure-shear failure mode and flexure failure mode. In addition, the specimens with rectangular helical hoops have plumper hysteretic loops. Shear span ratio is the main influencing factor of characteristic load; the axial compression ratio and concrete strength have less influence on characteristic load, while stirrup ratio has little effect on the characteristic load. Finally, a calculation method for shear capacity of SRHC columns under shear baroclinic failure and flexure-shear failure mode is proposed. [ABSTRACT FROM AUTHOR]

• Axial compressive behavior of RAC-encased RACFST composite columns was investigated. • The whole load-deformation curves and mechanical performance of specimens were obtained. • The calculation methods of their bearing capacity are presented and compared. A new type of composite column, recycled aggregate concrete (RAC)-encased recycled aggregate concrete-filled steel tube (RACFST) composite columns, promotes the RAC extensive utilization considering synergistic effect between the RAC and the steel tube, which is an attempt to develop environmentally sustainable concrete in high-rise buildings. In this paper, experimental research investigated the axial compressive behavior of RAC-encased RACFST composite columns. Twenty 1:3-scaled specimens were fabricated to test until failure under axial compressive loading. The main variable parameters affecting the behaviour of specimens were recycled coarse aggregate replacement ratio (range from 0% to 100% with 10% increase), stirrup spacing (s = 50 mm, 70 mm, 100 mm, 120 mm), diameter of steel tube (D = 89 mm, 114 mm), concrete strength grade (RC40, RC60) and longitudinal reinforcement ratio (4 12, 8 12). The failure modes, the whole load-deformation curves and the mechanical performance of specimens were also obtained based on static loading experiments and theoretical analysis. It was found that RAC-encased RACFST composite columns, which were subjected to identical axial loads, exhibited more favorable bearing capacity and deformation than those of a comparative reinforced recycled aggregate concrete columns (RACC). The replacement ratio of RCA had a slight effect on the mechanical performance of RAC-encased RACFST composite columns, the optimal substitution ratio of RCA was 50–60%. Based on the previous theory of superposition, the concrete cover is considered, and a calculation formula with double reduction coefficients of the axial load-bearing capacity of composite columns is proposed. Compared to the simplified superimposition method and the Chinese code Technical specification for steel tube-reinforced concrete column structure (CECS188:2005), the calculated results of axial compression bearing capacity by the proposed formula are in more agreement with the experimental values, with errors no greater than 15%. [ABSTRACT FROM AUTHOR]