Thermo-Mechanical Behavior of Steel-Concrete Composite Columns under Natural Fire Including Heating and Cooling Phases

The fire behavior of concrete filled hollow steel sections has been studied extensively in various countries. Almost all essential parameters influencing their resistance have been identified: section shape and dimensions, concrete filling, reinforcement ratio, steel tube thickness, column slenderness, thermal and mechanical properties of steel and concrete, and even the contact problem at the steel-concrete interface. Most of these works were done under standard fire conditions (ISO), which are represented by a continuously increasing temperature over time. It is thus not really a curve reflecting a natural fire which includes not only a heating phase but also a cooling phase during which the temperature of the fire is decreasing back to ambient temperature.In this paper, the behavior of axially loaded concrete filled square hollow section columns subjected to natural fire conditions has been studied. The main objectives of this study are: first, to demonstrate the phenomenon of delayed collapse of this type of columns during or after the cooling phase of a fire, and then study the influence of certain determinant parameters, such as section size, tube thickness, reinforcement ratio, concrete cover and column length.The results show that delayed failures occur for massive sections, small values of the thickness of the steel tube and for the low-slendernes.

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