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Abstract: In the present article, the effect of fire on the axial compressive strength of reinforced concrete (RC) columns was studied. Method of research was laboratory investigation. The axial compressive strength of RC columns was measured by applying variable heat in different time intervals in the process of experiment. Thirty two samples of fine-scale RC columns with square and circular cross sections, the gross cross section area of 225 cm² (for both sections), longitudinal reinforcement area of 3.1 cm², general height of 30 cm for all samples and other uniform structural characteristics ( 25 c f   MPa , 300 y f  MPa ) were exposed to fire and different temperatures (300°C to 700°C) were applied in the time intervals of 30, 60 and 90 minutes. Then they were exposed to axial loading by hydraulic jack with 200 ton capacity, and their strengths were measured. It is worth noting that, in the heating stage of samples, caps with thermal insulation were used in order to prevent instantaneous strain at the two ends of the samples. By carrying out the experiments, it became clear that at the time of fire, two factors, i.e., the "period of fire" and "temperature of fire" resulted in the loss of strength in RC columns. Of course, the "period of fire" factor was more effective than temperature of fire. It also became clear that RC columns with circular cross section fail faster in comparison with the columns with square cross sections at the time of fire. In order to evaluate and measure the results obtained, a relative factor called "Fire Factor" was utilized. The results indicated that the amount of axial compressive strength loss resulting from fire is approximately 3-5% greater in circular RC columns in comparison with square columns.

Keywords: Fire, Axial strength loss, Thermal insulation, Fire factor, Square and Circular cross section columns

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