What happens when the flames die down? It's a question rarely addressed in fire engineering, yet the decay and cooling phases of fires can be more dangerous than peak fire conditions. In this deep-dive conversation with Dr. Andrea Lucherini from Frisbee at ZAG in Slovenia, we uncover why these overlooked phases matter profoundly for structural safety.

Most engineers focus on protecting structures during the fully developed fire phase, but as Dr. Lucherini reveals, catastrophic failures can actually occur during cooling. We discuss a tragic case where seven firefighters died when a concrete structure collapsed, not during the fire's peak, but while they were extinguishing what appeared to be a dying fire. This sobering reality highlights how current testing methods fail to capture real-world risks—standard fire curves never decrease, creating a dangerous blind spot in our understanding.

The physics of cooling creates unique challenges for different building materials. Reinforced concrete might reach maximum temperatures in the steel reinforcement during decay rather than during peak fire. Steel structures face destructive tensile forces during contraction that can exceed the compressive forces experienced during heating. Mass timber presents particularly complex behaviour that may never truly enter a cooling phase without proper design considerations.

Perhaps most fascinating is how thermal boundary conditions transform as fires decay. When dense smoke thins, radiation patterns change dramatically, creating heat transfer scenarios that standard models fail to capture. These insights aren't just academic—they're essential for performance-based engineering approaches that prioritise realistic structural behaviour throughout a fire's entire timeline.

Andrea was kind enough to share these papers with me:

- Defining the fire decay and the cooling phase of post-flashover compartment fires: https://doi.org/10.1016/j.firesaf.2023.103965
- ⁠Thermal characterisation of the cooling phase of post-flashover compartment fires: https://doi.org/10.1016/j.ijthermalsci.2024.108933
- ⁠More information about FRISSBE project and team: https://www.frissbe.eu/ 

And I can shamelessly plug one of our own: https://onlinelibrary.wiley.com/doi/abs/10.1002/fam.2735

Cover image from experiments with Danny Hopkin that we have discussed here: https://www.firescienceshow.com/172-lessons-from-mass-timber-experiments-with-danny-hopkin/

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