Research On Fire Resistance Of Concrete-encased CFST Column On All Sides

With the extensive utilization of concrete-encased CFST columns inhigh-rise and super high-rise buildings, the fire resistance of this new type ofcomposite structures in fire is drawing more and more attention of researchers.Both the domestic and foreign research on the mechanical behavior of thestructure exposed to the heating and cooling phases of a fire has found that thestructure reaches the limit state not only in the heating phase, but also in thecooling phase due to the material deterioration caused by temperature hysteresisand structural internal force, the research on the mechanical property of thestructure in the cooling phase cannot be ignored. Therefore, a thorough study onthe mechanical properties of concrete-encased CFST columns during the heatingand cooling fire is conducted in this paper. The main research contents are asfollows:(1) After determining the thermal properties of steel and concrete in theheating and cooling phases, the thermal analysis model of the concrete-encasedCFST column during the heating and cooling fire was established. Existingtemperature experiments on concrete-encased CFST columns were used toverify this model, and the verification showed that thermal analysis model has ahigh accuracy.(2) After determining the mechanical properties of steel and concrete inambient, heating, cooling and post-fire phases, subroutines including concretethermal creep strain, concrete instantaneous thermal strain and steelhigh-temperature creep were used in the finite element analysis softwareABAQUS, the structural analysis model of the concrete-encased CFST columnduring the heating and cooling fire with initial load was established. Therationality and feasibility of the structural analysis model were verified by theexisting test results.(3) Based on the finite element analysis (FEA) model, a deep research wasconducted on the mechanical property and failure mechanism of the concreteencased CFST column under combined initial load and fire including heating and cooling phases. The thermal and mechanical properties of theconcrete-encased CFST column were analyzed such as temperature distribution,failure mode, deformation, internal force redistribution, stress and straindistributions, etc.(4) A parametric analysis was carried out to analyze the influence ofgeometric, loading and physical parameters on the fire resistance ofconcrete-encased CFST columns during the heating and cooling fire, whichincludes sectional dimension, slenderness ratio, steel ratio, reinforcement ratio,heating time ratio, load ratio, load eccentricity ratio, load bearing capacitycoefficient of core column, as well as the strength of steel and concrete. It turnsout that sectional dimension, slenderness ratio, load bearing capacity coefficientof core column, load ratio and heating time ratio are the primary parameters thatinfluence the fire resistance of concrete encased CFST columns. On the basis ofparametric analysis, practical calculation methods for the fire resistances in theheating or cooling phase of concrete-encased CFST columns exposed to theheating and cooling fire were proposed.