Structural behaviour of built-up cold-formed steel column under ambient temperature and standard fire

Abstract

Cold-formed steel (CFS) is a popular material with various advantages. Its easy production and assembly allow engineers to speed up the construction process. However, CFS is susceptible to buckling because of its natural thinness, especially when dealing with high temperatures. The unprotected CFS behaviour under fire is expected to have little strength compared to the hot-rolled steel. At present, CFS column subjected to standard fire has been done by few researchers. The study included the difference in shape and size of the column, differences in thickness, steel grade, column length, and restraint or unrestraint in thermal elongation. However, the larger size is more reliable to be used as the column but has never been tested in fire. The test for unrestrained thermal elongation columns is also unavailable. As such, the purpose of this study was to evaluate the structural behaviour of CFS columns under ambient and fire conditions. Based on these behaviours, the experimental findings were compared with the prediction from BS EN 1993-1-2: 2005 to evaluate the suitability of the code for CFS column fire design. In achieving the objectives, an investigation into fire resistance subjected to the ISO 834 fire standard was conducted on the CFS column. The variables involved were the CFS sections with various cross-section types and service loadings known as the degree of utilization. Three types of cross-sections, known as channel, back-to-back (BTB), and box-up (BU) sections, were studied. First, the column was preloaded at 30%, 50%, and 70% of its ultimate strength to simulate the real fire situation. Then, the column was exposed to the ISO 834 fire standard condition. The temperature at the column surface and the time were recorded until the CFS column failed. The temperature increase on the column surface were monitored using thermocouple Type K, and the analyses of these thermocouple readings were used to evaluate the mean temperature of the column. The results showed that the shape had no significant effects on the critical temperature of the CFS columns. The temperature behaviours of a BTB column for all degrees of utilization showed that the web had a lower temperature compared to the flange due to the greater thickness of the web. Meanwhile, the failure temperature of the CFS could reach up to 600⁰C for 30% degree of utilization. The critical temperature and time for the column could be used in proposing a fire safety design rule for the CFS column. The modified design curve for the CFS column was also proposed. It is concluded that the BS EN 1993-1-2: 2005 could be used to evaluate the safe buckling load under fire by including a modification factor due to buckling behaviour for built-up and channel CFS columns.