Behaviour of steel frame structures under localised fire including progressive collapse during cooling

Accounting for the effects of non-standard fire exposures on structures is a key component of performance-based structural fire engineering. Under natural fire (including cooling), steel and concrete members may fail during or after the cooling phase; yet research so far has focused mainly on isolated beams and columns, therefore disregarding the effects of thermally-induced restraint forces in assemblies. To investigate these effects, this paper analyses the mechanisms of load redistribution in structural systems comprising one steel column subject to localised fire, with a focus on the effects of the cooling phase. Numerical simulations show that significant tensile forces can build up in a fire-exposed column part of a framed structure when the temperatures are reduced back to ambient due to reversed thermal expansion and recovery of the properties. These tensile forces overload the adjacent vertical members, possibly leading to failure of members not directly affected by the fire. Therefore steel-framed structures subjected to localised fire attacking one column are vulnerable to progressive collapse occurring during or after the cooling phase. This suggests that the effects of thermally-induced forces associated with a fire event should be taken into account when designing with alternative load path method under column loss.