Disproportionate collapse of steel-framed gravity buildings under travelling fires

Observations from realistic fires have revealed that a fire tends to "travel" across the floors rather than burning simultaneously for the duration. However, the collapse mechanism and mitigation measures of structures under travelling fires are still not well understood. This paper investigates the disproportionate collapse behaviour of three-dimensional steel-framed gravity buildings subjected to travelling fires. The effect of fire curves (burning rate) and fire spreading speeds on the collapse behaviour is first studied. The effectiveness of using fire protections and bracing systems in preventing collapse is examined. It is found that the duration of the heating phase has a great effect on whether a building collapses or not, and the building may collapse during the cooling phase. A "long-cool" fire curve (slow burnig rate) is more dangerous than mild (average burning rate) and "short-hot" fire curves (fast burning rate) since the former causes a higher temperature in the structure. The fire travelling speed significantly affects the failure sequence of columns, range of damage and collapse mode of structures. For the structure analysed, a slow travelling fire (e.g. 2.5 mm/s) may lead to a partial collapse mode confined around the fire-affected region, but a global collapse in a wider range may occur in a fast travelling fire (e.g. 10 mm/s). A higher level of fire protection may prevent the collapse of structures, but may also lead to collapse in the cooling phase due to the delayed increment of temperatures in the heated members. The collapse time of a protected structure under a travelling fire is longer than the fire-resistance rating of its components (up to 130 min). Application of bracing systems (horizontal, vertical or combined) can prevent collapse of structures under a slow travelling fire, but not for a fast travelling fire. It is recommended to consider the possible fire scenarios when determining the fire-resistance rating, and increase the fire-resistance rating of components to 3 h to prevent collapse under a travelling fire with a "long-cool" fire curve. The beam-to-column connections should possess a sufficient tension and compression resistance, which may be equal to 100% and 50% of the yielding capacity of beams, respectively.