Buckling behavior of cold-formed steel columns at both ambient temperature and simulated fire conditions

Cold-formed steel (CFS) profiles with a wide range of cross-section shapes are commonly used in building construction industry. Nowadays several cross-sections can be built using the available standard single sections (C, U, Σ, etc.), namely open built-up and closed built-up cross-sections. This paper reports an extensive experimental investigation on the behavior of single and built-up cold-formed steel columns at both ambient and simulated fire conditions considering the effect of restraint to thermal elongation. The buckling behavior, ultimate loads and failure modes, of different types of CFS columns at both ambient and simulated fire conditions with restraint to thermal elongation, are presented and compared. Regarding the buckling tests at ambient temperature it was observed that the use of built-up cross-sections ensures significantly higher values of buckling loads. Especially for the built-up cross-sections the failure modes were characterized by the interaction of individual buckling modes, namely flexural about the minor axis, distortional and local buckling. Regarding the fire tests, it is clear that the same levels of restraint used in the experimental investigation induce different rates in the generated restraining forces due to thermal elongation of the columns. Another conclusion that can be drawn from the results is that by increasing the level of restraint to thermal elongation the failure of the columns is controlled by the generated restraining forces, whereas for lower levels of restraint the temperature plays a more important role. Hence, higher levels of imposed restraint to thermal elongation will lead to higher values of generated restraining forces and eventually to lower values of critical temperature and time.