Multi-scale Fire Behavior Of Concrete Filled Steel Tubular Columns Under Fire Conditions

Compared to the reinforced concrete columns,concrete filled steel tubular columns have more advantage and widely used in the tall buildings and large structures.It is important to research the behavior especially the fire performance of concrete filled steel tubular columns.This paper investigates the mechanism of concrete filled steel tubular columns under fire conditions(standard fire /natural fire conditions)and analyzes the multi–scale fire behavior ranges from material-interaction-member.Some fire design methods for concrete filled steel tubular columns are proposed based on the engineering practice.The main research work includes:1.Fire developments in the compartment.Based on the full-scale fire tests in Cardington and the room-fire in Maryland University,the fire model in the limit compartment was determined.The temperature and oxygen consumption in the compartment were simulated according to the FDS software.The temperature field of concrete filled steel tubular columns can be calculated with the fire curve of natural fire.2.The fire performance of concrete filled steel tubular columns under ISO-834 or ESTM-E119 standard fire was investigated.Based on the ABAQUS software,a sequent FEM model coupled with temperature and stress was developed,which can be used to predict the ultimate capacity and fire performance of concrete filled steel tubular columns.A modified Rankine method considering the shear bond was developed in order to design for the application of engineering practice.The results of calculation and simulation were verified by the experimental data and have good agreement.3.Based on the fire development model,the natural fire curve can be obtained and imported as the fire conditions to calculate the temperature field of concrete filled steel tubular columns.A sequent FEM model on the ABAQUS software coupled with temperature and stress was developed.With the field variable subroutine USDFLD and constitutive models under different temperature fields,the entire process of concrete filled steel tubular columns under heating and cooling phases was established.A simplified fire resistance design method for the residual strength of concrete filled steel tubular columns was proposed and the comparisons with tests results showed the good agreement.4.The constitutive model of concrete has a large influence on the prediction of fire resistance under fire conditions.Due to the complexity of the concrete mechanical behavior at elevated temperatures,the establishment of applicable concrete model remains a challenging task in structural engineering.A fully three-dimensional model is proposed by combination of elasto-plasticity and continuum damage theories based on the thermodynamic framework.The pressure-dependent damage evolution is considered to characterize the behavior of concrete at high confined stress states.At high temperatures,a transient creep strain and thermal strain are included in the developed model to take account the thermal response.Based on the UMAT subroutine of ABAQUS software,a series of numerical simulations are conducted to validate the constitutive model.The predictions show good accuracy compared with the experimental data.