Behaviour of continuous concrete filled steel tubular columns loaded concentrically in fire

Design recommendations for concrete filled steel tubular (CFST) columns in fire are based on the results of experimental standard fire testing of CFST members where the same temperature is applied to the column over the full column height. However, this is not representative of a CFST column in a typical building, which is continuous between floors and which, in fire, is subjected to severe fire conditions on one floor at a time while the floors above and below remain cooler. In the experimental tests described in this paper, the columns are of 3.2 m height with the fire applied only to the central 2 m. Significant differences are observed between these tests and those previously conducted due to the partial length heating. In total, ten tests are conducted; the tests cover three different types of infill: plain concrete; bar reinforced concrete; and steel fibre reinforced concrete. End restraint conditions of fixed-fixed (F-F) and pinned-fixed (P-F) are considered; the axial load levels are between 0.33 to 0.38 of the squash load. The longitudinal elongation of the steel tube was less than 3 mm. Using the experimentally measured structural fire resistance (R), the axial capacity in fire was calculated in accordance with the codes of practice and are Compared with the experimentally tested structural fire resistance, showing that in some instances current design practice can be un-conservative.