Behaviour of composite columns made of totally encased steel sections in fire

• Experimental research on composite columns made of totally encased steel sections in fire
• The EN 1994-1-2 tabulated method is extremely conservative for these types of columns.
• The critical times of the columns were higher than 180 min showing its good fire performance.
• A big influence of the load level and concrete cover on the critical times was observed.
• The increase of the load level reduces the critical time of the columns.

Composite structures are nowadays used, principally in high-rise buildings, due to its high load-bearing capacity and seismic and fire resistance. There are already several research works on the fire behaviour of composite steel and concrete columns with restrained thermal elongation, mainly of numerical nature, but a few were carried out on composite columns made of totally encased steel sections. This paper presents the results of an experimental research carried out in this last type of columns subjected to fire. Several parameters that might have influence on its behaviour were tested, such as the initial applied load level, the stiffness of the surrounding structure (i.e. axial and rotational restraining to thermal elongation) and the dimensions of the cross-section that resulted in different slenderness values of the columns tested. The tabulated data of EN 1994-1-2 (2005), for these type of columns, showed to be very conservative and on the safe side, when compared with those resulted from the experimental tests of the present investigation, because the ones of this code may have an empiric base and resulted from columns tested without thermal restraining. The load level and concrete cover showed to have a big influence on the critical time of the columns. Increasing the first reduced while increasing the second increased the critical time of the tested columns.