ASET/RSET is the fire safety engineering idea that sounds almost too clean to argue with: the building must give you more time to escape than you need to get out. But once you try to prove it in a real project, the simplicity could and and the hard questions start. Where does “time zero” actually begin: ignition, detection, or alarm? What does it mean for tenability to fail in a place where nobody is anymore? And are we designing safer buildings, or just winning a seconds-on-a-spreadsheet contest?

I break down the Available Safe Egress Time and Required Safe Egress Time concept from both sides: the human evacuation model and the fire model. On the RSET side, we talk about detection and alarm strategy, pre-movement time distributions, and why congestion, stair capacity, and occupant behavior can dominate the result. On the ASET side, we get into CFD fire modeling, tenability criteria like visibility, heat, and toxicity, and the huge responsibility that comes with choosing a design fire that is both plausible and fail-safe. I also explain why fire location can completely change outcomes even when the heat release rate curve is identical.

We also tackle the famous criticism of ASET/RSET, where it truly breaks down (especially residential scenarios), and why I still find it useful in complex spaces like public venues and tunnels. To move beyond one “magic number,” I share practical alternatives like scenario-by-scenario comparisons, probabilistic thinking, ASET/RSET maps, and agent-based tenability approaches, plus a look at digital twin possibilities for smarter evacuation guidance.

If this helped you rethink how you use ASET/RSET in performance-based fire engineering, subscribe, share the episode with a colleague, and leave a review so more engineers can find it. What’s the toughest ASET/RSET argument you’ve had to defend?

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Three mentioned links in the episode:

Rino's pre evacuation time distributions: https://www.sciencedirect.com/science/article/pii/S0379711218302212
Vyto's paper here: https://onlinelibrary.wiley.com/doi/abs/10.1002/fam.1025
Tenability survey paper here: https://www.scopus.com/pages/publications/105046523596?origin=resultslist

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The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.

In this episode I take you into practicality of introducing wind into fire safety engineering considerations, and focus on it's implementation as a part of fire oriented CFD analyses. In here, we dig into why wind is tied to so many catastrophic fires, why firefighter safety is often shaped by sudden wind shifts, and why designing for calm air can quietly turn fire safety into a coin toss.

I walk through what wind actually is from a fire engineering perspective: an atmospheric, turbulent phenomenon full of vortices and gusts, not a tidy uniform flow. We talk about the trap of modeling only the average wind speed, why gusts matter so much for smoke movement and vent performance, and how the wind profile changes with height and terrain roughness. From there, I connect the dots to computational wind engineering and the messy reality of cities, where nearby buildings, parks, towers, and “urban canyons” can completely reshape the local wind field right at your façade openings.

Then we get practical about CFD. I share the domain and mesh decisions that keep results honest (including blockage and downstream wake space), how to handle wind direction, and what turbulence modeling choices mean when you’re trying to couple wind and fire. Finally, I lay out when the hassle is worth it: constrained historic roofs, atria with limited openings, exposed natural smoke ventilators, high-rise mechanical floors, complex tunnel portals, architected natural vents with uncertain discharge coefficients, and roof features where vortex shedding can damage photovoltaics.

The key resource to implement what has been said in the episode:

- Wind and Fire Coupled Modelling—Part I: Literature Review

- Wind and Fire Coupled Modelling—Part II: Good Practice Guidelines

- Fire and Smoke Modelling Chapter

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The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.

Smoke does not just slow people down, it changes what they believe is even possible. That simple idea is why we get so excited about pyFDS-Evac, an open source workflow that connects Fire Dynamics Simulator (FDS) results directly to modern evacuation modeling with JuPedSim. When the fire environment can remove an exit from view, reduce walking speed through extinction and soot exposure, or even incapacitate an agent, you stop “post-processing evacuation” and start modeling decisions that respond to conditions.

We sit down with Mohcine Chraibi from Forschungszentrum Jülich to unpack how their team builds pedestrian dynamics software that researchers can actually extend. We talk candidly about why legacy tools like FDS-Evac faded, why commercial evacuation software can feel like a black box when you need to publish and validate new ideas, and what it takes to keep an academic codebase alive. Mohcine walks us through the modular pipeline: run FDS as normal, read outputs through FDS Reader, compute sight lines and smoke effects with FDS VisMap, and drive agents with JuPedSim while pyFDS-Evac applies fire-driven behavior changes.

Then we go deeper into routing and wayfinding. You will hear how graphs and A-star pathfinding evolve toward flow-field methods, why “familiarity” and cognitive maps matter when occupants do not know the building, and what JuPedSim 2.0 aims to deliver next, including stairs, 2.5D to 3D buildings, and switching movement models by location.

If you care about fire safety engineering, evacuation simulation, pedestrian dynamics, or visibility in smoke, subscribe, share this with a colleague, and leave a review so more researchers and practitioners can find the tools and the ideas. What would you test first with an open FDS to evacuation pipeline?

Links to the good stuff:

Jupedsim the engine: https://www.jupedsim.org/
its webapp: https://app.jupedsim.org/

Youtube-playlist for jupedsim: https://www.youtube.com/watch?v=ekbvCdy1P3w&list=PLYiH62IDwy5fdFJRiGky0Hdq88xKwjayL

pyFDS-Evac: https://pedestriandynamics.org/pyFDS-Evac/

pyFDS-evac presentation from the visbility seminar https://pedestriandynamics.org/pyFDS-Evac/talks/visibility-seminar-2026/#/1

Cover image comes from the 1st video on the JuPedSim playlist - the Galton Board Walked by Pedestrians in JuPedSim, by Pedestrian and Fire Dynamics Channel on youtube.

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The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.