Neuenschwander, Knobloch and Fontanta 2017
This article discusses and tests concrete filled steel tube composite columns with a solid steel core, in order to achieve a longer fire- resistant time, as compared to composite columns without a steel core. As the steel exterior of the column heats up much faster than the steel core interior, each component loses its stiffness at different rates, and the column will eventually fail.
Experimental Study, Results, and Discussion
Two concrete-filled steel tubes with solid steel core of varying diameter were tested; the column with the smaller diameter was tested for thermomechanical properties and was pinned at both ends, whereas three equal columns with the larger diameter were tested with varying axial loads, and were all pinned at one end, and fixed at the other. The fire tests were performed in the Material Test Establishment of the Technical University of Brunswick in Germany, where the hydraulic jack with a capacity of 3MN was installed under where the test specimen would be placed vertically. Three thermometers were placed on each side of the column,with intervals of a quarter, ranging from the bottom to the top and the columns were covered with insulation material to insulate the heat. During the tests, lateral deflection at a height of .44, axial displacement on the left side of the bottom end plate, as well as that on the right side were all recorded by use of LVDTs on the outside of the furnace chamber. The fire tests, compliant with EN 1363-1 consisted of preloading of specimen, followed by ISO fire exposure, until ultimate failure of the specimen. The first stage consisted of 10% P0, followed by 40% P0, 70% P0, and finally 100% P0,which was held for 15 minutes. The second stage consisted of igniting the furnace burners, which followed the ISO fire curve, and load (P0) was held constant. One final experiment was conducted in ETH Zurich to assess material properties of the concrete and steel used. Tensile tests at temperatures of 20, 400, 550, 700, and 900oC, and the displacement was measured. Furthermore, one additional steady-state strain rate- controlled cyclic compression test was performed on the concrete at higher temperatures of 300-800oC, with ranges of 100oC. The failure of all four specimens can be described in three stages: primarily, the outer steel tube expands, followed by expansion of the concrete interior, and finally expansion of the steel tube. Specimen 1, which was the thinnest specimen, and had the lowest load ratio (16%) began to expand at a constant rate for the first 15 minutes, local buckling occurred between minutes 27-30, and again began expanding due to the concrete after 35 minutes, before ultimate failure after 92 minutes. Specimens 2 and 3 were both the same size, yet specimen 2 had load ratio of 57%, and specimen 3 had load ratio of 43%. Specimen 2 did not undergo the last two phases because it stabilized as the load was distributed from the much softer concrete to the steel core, and specimen 2 failed at 24 minutes. On the contrary, specimen 3 underwent thermal expansion of concrete during minutes 20 to 43, and also had an increase in axial deformation, along with an increase in lateral deflection. The steel core expanded from minutes 43-70, followed by failure after 169 minutes. Finally, specimen 4, which was the same size as specimens 2 and 3, and had a load ratio of 38%, had a similar deformation curve as that of specimen 3, before failure at 179 minutes.
Reference
Neuenschwander, M., Knobloch, M., and Fontana, M. (2017). “ISO Standard Fire Tests of Concrete-Filled Steel Tube Columns with Solid Steel Core.” Journal of Structural Engineering, 143 (4), April. doi:10.1061/(ASCE)ST.1943-541X.0001695