A method for measuring the sensitivity of building structural members to fire decay phases

Anotace: Firefighters face a major threat when intervening in a building during a fire: the possibility of structural collapse during the cooling phase of the fire, or soon thereafter. At present, this threat is amplified by the fact that the behaviour of structures after the time of peak gas temperature is not well understood, and is not taken into account in the design. This work presents an analysis of the behaviour of different structural members under natural fires, and develops a method for characterizing their sensitivity to fire decay phases. Thermo-mechanical numerical simulations based on the non-linear finite element method are conducted using the parametric fire model of the Eurocode to represent natural fires. The results show that, for all the members (a column, a beam) and materials (reinforced concrete, steel and timber) that are studied here, structural failure during or after the cooling phase of a fire is a possible event. The major factors that promote delayed structural failure are thermal inertia and the constituting material of the member. A method, based on a new indicator, is proposed for quantifying the propensity to delayed failure for structural members under natural fire. This work enhances the understanding of the behaviour of structures under natural fires, and has important implications for the safety of fire brigades and of people responsible for making a building inspection after a fire.