Determination of fire resistance of concrete filled hollow steel sections

By combining steel and concrete in one structural component, the advantages of each material can be better exploited. The aim of this research project was to find a method of analyzing the behaviour of centrally loaded concrete-filled hollow section columns in a fire situation. The column sections were restricted to square tubes with or wishout reinforcement. The analysis work can be divided in two parts: the determination of the temperature field of a section and the calculation of a failure load. The temperatures of the section were calculated by the TASEF computer program, which is based on the finite element method. The load-bearing capacity was calculated by a simplified calculation method (summation method). First, different thermal material models (the thermal conductivity and the specific heat capacity of steel and concrete) as well as mcchanical material models (the yield point and the elastic modulus of steel as well as the ultimate compressive strength and the tangent Young's modulus of concrete) were compared by calculating the temperatures of a column section and the load-bearing capacity of a column. The results of temperatures and capacity were also compared with the experimental values. On the basis of previous comparisons calculating the temperatures of the column sections, the thermal material properties given in the National Building Code of Finland were used in the sequel taking into account however, the high water content of concrete in the curve of the specific heat capacity. Correspondingly, when calculating the load-bearing capacities of the composite hollow sections, the mechanical material properties of steel given in the National Building Code of Finland and mechanical material properties of concrete according to the harmonized model were used in the following calculations. Concrete field steel tubes without reinforcement were analyzed first with the material models mentioned above. The size of the column sections varied from 140 mm x 140 mm to 300 mm x 300 mm. The calculated temperatures are generally on the safe side compared with the test results. For the 300 mm x 300 mm columns the calculation method gives load-bearing capacity values which are too high compared with the measured ones, whereas the calculated load-bearing capacities for other columns without reinforcement are principally lower than the test results. It should be noted that the variation in measured temperatures as well as in failure loads is great. The dependency of the utilization factor of the failure load in unreinforced columns on fire resistance was found to be under 30 % when the fire resistance time is over 60 minutes, and at the most 50 % when the fire resistance time is between 30 and 60 minutes. The reinforced composite hollow sections were analyzed in the same way as the unreinforced sections above. The sizes of the column sections varied from 160 mm x 160 mm to 3S0 mm x 350 mm and the degree of reinforcement from 1.0 % to 4.0 %. The agreement between the calculated and experimental temperatures is quite good in concrete, but the calculated temperatures of the longitudinal reinforcing bars are significandy higher than the temperatures measured in the fire test. For that reason the calculated load-bearing capacities are always much lower than the measured ones, when using the calculated temperatures. If the measured temperatures for the reinforcement are wed, the agreement is much better. On the basis of this study the following three general conclusions concerning centrally compressed composite hollow sections in fire can be drawn: 1. The scatter in the results is too high to develop accurate and reliable general calculation methods. 2. The results are on the safe side for reinforced composite hollow sections when the calculation method of this study is used. 3. If the calculation method of this study is used for unreinforced composite hollow sections, the load-bearing capacity must be multiplied by a factor of about 0.65. It should be remembered that for practical applications the tlexural capacity of unreinforced composite hollow sections in fire is close to zero.