Critical temperatures of concrete-filled steel tubular columns for early-warning of fire-induced failure

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The collapses of building structures in fire have occurred several times during the last two decades. To save lives from the collapsed building, a reasonable method is to predict the collapse of the structure in advance, for being able to evacuate the persons on site, such as fire-fighters, before the deadly collapse will occur. Therefore, in fire condition, the early-warning of structural collapse or member failure based on accurate on-site data collection and refined failure state determination is becoming a key issue in the fire research field. A parameter measured easily - the temperature - is taken as the indicator of the fire-induced failure, and an investigation on the refined critical temperatures of the concrete filled steel tubular (CFST) columns will be described in this paper. A series of fire resistance tests on CFST columns were conducted and are reported in this paper, and the influence of different test parameters, including column load ratio (0.3, 0.4 and 0.6), section type (circular and square sections) and fire scenario (fire exposure on two surfaces and all around the column), on the critical temperatures of CFST columns are discussed. Based on the test data and validated finite element analysis (FEA) models, refined critical temperatures (RCTs) for CFST columns were defined and are discussed. Compared with the traditional critical temperature, the RCTs clearly defined the positions and values of critical temperature points, which makes it applicable to be used as the early-warning of fire-induced failure of CFST columns. Early-warning of fire-induced structural collapse or member failure is a new research topic, which should be studied based on accurate on-site data collection and refined determination of the failure state. For a column subjected to fire, three indicators shown in Fig. 1 can generally be used to identify its failure state, including residual load bearing capacity, deformation (or deformation rate) and temperature. Among the three early-warning indictors, the temperature is a type of easy-measured parameter, which can be obtained easily in fire condition. This paper selected the temperature as the early-warning failure indicator, and describes an investigation on the refined critical temperatures of concrete-filled steel tubular (CFST) columns. A series of fire resistance tests on CFST columns shown in Fig. 2 were conducted, and the influence of different test parameters, including column load ratio (0.3, 0.4 and 0.6), section type (circular and square sections) and fire scenario (fire exposure on two surfaces and all around the column), on the critical temperatures of CFST columns are discussed. Based on the test data and validated finite element analysis (FEA) models, refined critical temperatures (RCTs) of CFST columns as shown in Fig. 3 were defined and discussed. Compared with the traditional critical temperature, the RCTs clearly defined the positions and values of critical temperature points, which makes it applicable to be used as the early-warning indicator of fire-induced failures for the CFST columns. • Early-warning of fire-induced failure for concrete filled steel tubular (CFST) columns was discussed. • Influence of column load ratio and fire scenario on critical temperature of CFST columns was discussed and analyzed. • Refined critical temperatures based on experimental and theoretical studies were defined and studied.