An Experimental Investigation into the Impact of Temperature Rise on the Performance of CFDST Columns with Prismatic Geometry Subjected to Cyclic Loading

In recent years, the construction of high - security buildings against the conditions of explosion and fire is a very important issue in the world. Therefore, one of the appropriate strategies in this field is to use steel - concrete composite columns to be used in the core of the concrete column to maintain its durability and stability against high temperatures. This paper studies the impact of temperature rise on the performance of concrete-filled double skin tubular steel columns with prismatic geometry. In doing so, a number of columns whose interior core was square, diamond and circularly-shaped and their exterior core was prismatic with a square cross-sectional area increasing with the slope of 2/1 degrees from top to the bottom, were constructed and exposed to the temperatures of 25, 250, 500 and 700 °c. Accordingly, the temperature rates up in compliance with ISO-834. Afterwards, all column specimens were subjected to cyclic loads adopted from the ATC-24 loading protocol which proceeded until the specimens failed. Firstly, the 28 - day compressive strength of concrete, measured, and steel used is tested under tensile test and the stress and strain characteristics were obtained. The results indicate that although the failure mode of the columns with interior core of square or diamond shape resembles each other, the columns whose interior core was circularly-shaped, experienced more intensive damages compared to the latter specimens. Failure mode of the columns base was developed as a diagonally with degree of 450 by temperature of 500 °c but at the temperature of 700 °c, the damages have occurred horizontally at the level of 10 cm from the column base. Moreover, initial stiffness and ductility ratio of the columns with diamond-shaped interior core was approximately 2 times greater than the other columns. in all columns, the maximum damage in the leg area is caused by the maximum anchor in that area caused by 25 to 700 °C, and with increasing temperature, the central and middle regions are also completely destroyed, so that at 700 °C in all specimens, concrete is completely crushed in such a way that it has virtually no role in the load - bearing capacity of the column. The destruction of concrete in columns is almost identical in all columns of the inner circle of the circle, squares and diamond. In general, the load - bearing capacity of the columns decreased by about 50 % after 15 cycles at a temperature of 700 °C after 9 or 12 cycles, and the loading process was halted. In the meantime, the columns of the inner circle of the circle had broken out earlier than the other columns and did not have proper functioning. also in the diamond - shaped columns, the rupture process and the severe reduction of the carrier capacity are initiated at 500 °C, indicating that this column does not indicate that this column is due to heterogeneous geometrical distribution at the height of the column.