Fire Resistance of Stiffened Circular Concrete-Filled Steel Tubular (CFT) Columns

The concrete-filled steel tube (CFT) column is a composite system that is made with concrete as inner core and thin walled steel as outer tube. The enhancement of load capacity, increase in lateral stiffness, inherent ductility, use in high rise and long span buildings and fire resistance, are some benefits of CFT columns [1].Columns are the most parts of buildings in fire that carry loads of structure. When fire happens in a structure with CFT columns, the steel tube is the first part of these columns, subjected to fire. Therefore, the steel tube expands and concrete core carries remaining loads. It's a main disadvantage of CFT columns [2]. In the present study, finite element modeling of stiffened circular CFT columns is presented. The main characteristic of the stiffened CFT column is internal longitudinal symmetric stiffeners its internal stiffeners that are longitudinal and symmetric. For predicting the load capacity of the proposed sections of CFT columns, the time – temperature curve is applied to the model.2. Methodology2.1. Fire exposure: The temperature of the column rises up (Eq. (1)) and cools down (Eq. (2)) according to the ISO-834 fire curve (Fig. 1) [3-5]. The temperature is applied to nodes of the columns.where T is the fire temperature in ◦C; t is fire exposure time in minute; ‘‘B–C–C’’ is the heating phase; ‘‘C–D’’ is the cooling phase; ‘‘C’’ is the starting point of cooling; To is the ambient temperature in ◦C; th is the fire duration time in min; Th is the maximum fire temperature in ◦C and tp is the total fire exposure time in min.2.2. Heat transfer analysis: The heat transfer process in concrete filled steel tubes includes radiation, convection and heat conduction, all of them are varying with time. There are four heat transfer processes, from fire to outer steel tube surface, from outer to inner steel tube surface, from inner steel tube surface to outer concrete surface and heat transfer into core of concrete from concrete surface [6,7].2.3. Suggested sections: One of the main disadvantages of CFT columns is the expansion of the steel tube when subjected into the fire. Therefore, the confinement of core and finally, the load capacity of CFT column will be decreased. In order to improve the performance of CFT columns, subjected to the fire, a stiffened steel section is suggested in CFT columns as shown in Fig. 2. The stiffeners enhance the confinement of columns by increasing the contact area between steel and concrete. Hence, the confinement of CFT columns is preserved in heating and cooling stage of the fire exposure. Therefore, it is expected that the load carrying capacity of CFT column to be increased.2.4. Finite element modeling: In the present study, finite element modeling of stiffened circular CFT columns is presented. These models have been analyzed using ANSYS (Ver. 11) [8]. In order to investigate into the behavior of the stiffened CFT columns under axial and thermal loadings,FE analyses should be undertaken involving the geometric and material nonlinearities. Three-dimensional solid element (Solid 65) has been used for modeling of the concrete core, shell element (Shell 43) has been used for modeling of the steel tube. Three-dimensional node-to-node contact element (Contact 178) has been used to model the contact between steel wall and concrete core. Modeling of separation, sliding and contact between two nodes during the loading process are the capabilities of this element. Fig. 3 shows the CFT column, modeled in ANSYS.2.5. Verification of finite element modeling: The material properties of concrete and steel are changing when subjected to the fire. Therefore, some researchers define material properties in ambient temperature, heating and cooling phases. In order to verify the accuracy and validity of the finite element modeling, the numerical results, obtained from nonlinear static analysis, have been compared with the experimental results of the CFT columns. Regarding the axial loading, an experimental CFT column with circular section has been used according to the specifications undertaken by Schneider [9] and in order to verify the accuracy and validity of the equations used for applying fire to the CFT column, model specifications are according to Han [10]. Consequently, it has been found that the finite element model is reliable enough to be used to undertake nonlinear analyses for comparative investigation into the behavior of the sections of CFT columns with and without stiffeners.2.6. Material heat coefficients: The thermal coefficients in CFT columns have been investigated by many researchers. In the present paper, the coefficients for concrete and steel tube, provided by Lie and Stringer [11], are adopted. The thermal properties such as conductivity coefficient (k), specific heat(c), density (ρ) and coefficient of expansion (α) have been defined in these equations. Also, they considered the water evaporation of core concrete after sbeing exposed to the fire. In the evaluation of temperature, k and α decrease, and c increases after a few minutes of fire exposure. They also considered these specifications of concrete and steel in their relations.3. A total of 54 CFT stub columns, in three types, including simple, 2 and 4 stiffened circular CFT columns, have been analyzed after subjected to fire. The specimens are divided into two groups (with the same wall thickness and with the same diameter). All of the specimens, have the same wall thickness and the D/t ratio increased in three steps. All of the specimens are subjected to real fire in ANSYS in three time durations, namely 10 minutes (680oC), 20 minutes (780oC) and 30 minutes (840oC) according to standard fire curve, shown in Fig. 2. In suggested sections the area of stiffeners, is 30% of total area of CFT column and L=3810mm. For steel, we have: Fy= 293MPa and Es= 2.01e5MPa. For concrete, we have: fc= 39.6MPa and Ec= 27800MPa.4. In the present study, finite element modeling of stiffened circular CFT columns is presented. The main characteristic of the stiffened CFT column is its internal stiffeners that are longitudinal and symmetric. These models have been analyzed using ANSYS. Having verified the finite element modeling, several different analyses have been undertaken. Based on the results of analyses, the main conclusions are as follows:-Increasing the tube diameter in simple and stiffened CFT columns has increased the load carrying capacity of columns. The increase rate of the load capacity in CFT columns during fire exposure is more considerable than ambient temperature.-Decrease in the wall thickness of CFT columns, in simple and stiffened columns, decreases the load carrying capacity of columns. The decrease rate of the load capacity in CFT columns during fire exposure is more considerable than ambient temperature.-Increasing the number of stiffeners has enhanced the load carrying capacity of CFT columns. However, increasing the number of stiffeners from 2 to 4 dose not have considerable effect on the load carrying capacity of CFT sections (only 2%).Therefore, the use of 2 stiffeners is more economic, and strongly recommended for CFT columns subjected to fire.-The residual strength index (RSI) is defined to quantify the strength of the CFT columns to standard fire. RSI is the ratio of ultimate strength corresponding to the fire duration time into the ultimate strength at ambient temperature [10]. According to the analysis results, decreasing the number of stiffeners and increasing the tube diameter, has enhanced the RSI of CFT columns.