SENSITIVITY OF STRUCTURES TO FIRE DECAY PHASES, Quantitative comparison of structural components made of different materials

Abstract

This work presents an analysis of the behaviour of different structural members under natural fires, with the aim to characterize their sensitivity to the fire decay phase. Thermo-mechanical numerical simulations based on the non-linear finite element method are conducted using the parametric fire model of Eurocode to represent the natural fires. Results show that, for all the studied members (column, beam) and materials (reinforced concrete, steel, timber), structural failure during or after the cooling phase of a fire is a possible event. The major factors that promote delayed structural failure are the thermal inertia and the constituting material of the member. An indicator is proposed to quantify the propensity to delayed failure for structural members under natural fire. This work enhances the understanding of the structural behaviour under natural fires and has implications for the safety of the fire brigades and people proceeding to a building inspection after a fire.

Response to “Commentary on DHP concept”

“…The effect of thermal inertia is physically well understood; can easily be modeled; and has been measured in countless experiments. The concept of fire resistance rating (FRR) that currently underpins prescriptive regulations cannot capture these thermomechanical effects developing during the cooling phase, because the FRR is based on a fire curve with a heating phase only. Although it is well understood that the FRR does not directly measure in‐situ performance, it is nevertheless at the base of most prescriptive fire regulations, with the implicit assumption of a positive correlation between the FRR and the performance level during a real fire 10 . The expectation is that increasing the FRR of a structural member from 60 to 90 min will increase the performance during a real fire that may develop, and will thus give a “better” fire safety. Yet, studies have shown that structural members of different types with the same FRR may respond very differently during the cooling phase, due to differences in thermal inertia and material behavior 2–5,11,12 . As a result, two different members with the same FRR may yield different levels of performance during a real fire.…”

“…To propose a systematic method in response to this issue, the DHP concept has been introduced, 1 with its implementation through a burnout resistance rating. The concept has been described and applied in numerical studies of different materials and structural members, 2,3,9,11,12 has been validated experimentally in furnace tests which aligned with a priori numerical simulations, 4,5 and has further been corroborated by compartment fire tests ("natural fires"). 23 The DHP concept has also been adopted by other scholars who have independently shown consistent findings 27 and by members of the fire service who are directly concerned with the issue of stability until burnout.…”

“…Yet, studies have shown that structural members of different types with the same FRR may respond very differently during the cooling phase, due to differences in thermal inertia and material behavior. [2][3][4][5]11,12 As a result, two different members with the same FRR may yield different levels of performance during a real fire. In fact, it is possible that a certain member that has an FRR of 60 min survives a heating-cooling exposure until burnout while another member that has an FRR of 90 min fails under the same exposure.…”