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Hello everybody, welcome to the Fire Science Show.
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In today's episode we're diving a little bit into structural fire engineering and, thankfully, a bit of compartment fire dynamics along the structural fire engineering, so I feel a little bit more comfortable in there.
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I have invited Dr Andrea Lucherini from Frisbee at ZAG in Slovenia to discuss some research of his that was presented a few years ago at IAFSS in Japan and some other papers followed, and namely that research touches the decay and cooling phases of fires.
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I found this a very interesting, intriguing discussion.
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You perhaps remember some episodes ago I had an episode with Thomas Jeunet and Professor Zephus on failure of timber columns in the cooling phase.
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Today we talk more generally about what cooling and decay phases of fires are, and it's an interesting concept.
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If you use fire resistance as a proxy for your structural fire safety, you probably don't even need them.
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If you over design your structure, there's a good chance those considerations are not important to you.
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But if you're doing anything related to performance based engineering of the structural fire safety, the decay phase is something that you most likely need to account for, Because it's not necessarily the peak heat release of the fire when your structure is most thermally stressed, if I am allowed to say that In this episode we discuss how to define those decay and cooling phases, which on its own is quite a challenge.
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We discuss what happens at different types of structure reinforced concrete, steel, timber In the cooling phase.
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We talk about some physics, how the heat transfers through the structure, and we also discuss the challenges with setting up the correct boundary conditions for those phases, because the boundary conditions we use for fully developed fires may not really be the ones that you would like to use in the decay phase.
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So a lot of physics, a lot of structural fire engineering, but, given in a very approachable way, I've enjoyed it.
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I don't know much about structural fire engineering, so I am sure that you'll also like and enjoy that.
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And if you know a lot about structural fire engineering, you will definitely enjoy it.
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I mean, if you like fires and that's your part of the world of fire, you will enjoy it.
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Anyway, that's way more talking than did it.
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Let's spin the intro and jump into the episode.
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Welcome to the Firesize Show.
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My name is Wojciech Wigrzynski and I will be your host.
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The FireSense Show is into its third year of continued support from its sponsor, OFR Consultants, who are an independent, multi-award-winning fire engineering consultancy with a reputation for delivering innovative safety-driven solutions.
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Established in the UK in 2016 as a startup business by two highly experienced fire engineering consultants, the business continues to grow at a phenomenal rate, with offices across the country in eight locations, from Edinburgh to Bath, and plans for future expansions.
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If you're keen to find out more or join OFR Consultants during this exciting period of growth, visit their website at ofrconsultantscom.
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And now back to the episode.
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Hello everybody.
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I am joined today by Andrea Luccherini from Frisbee at Zag in Slovenia.
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Hello Andrea.
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Hello Wojciech, Nice to see you.
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Nice to see you again.
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We've just seen each other in Slovenia at the European Symposium for Fire Safety Science.
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Are you alive after it?
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That was intense for the organizers.
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I can only imagine.
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That was very intense indeed.
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It took a little bit of time to prepare it because we started last year after the fourth edition.
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The fourth edition last year was in Barcelona, in Spain, and you know after that we started preparing the fifth one.
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Last week we had it.
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There were three very intense days plus a pre-day for heavy career researchers, but I don't know.
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You told me everything went smoothly, we were happy, everything went well.
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So I have not seen a single mess up really, so that's like that's excellent and I've enjoyed it thoroughly.
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Like you know it all, this ESFS conference always has this nice atmosphere of very like early career researcher oriented but I don't mean like first year PhD students, I mean like everyone between their start and seniority.
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It's just like a nice place you know, to stand up on the stage, give your presentation, get constructive feedback, get some experience with that, especially that we're like IFSS submissions in two weeks Exactly.
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It's a good moment to get your final, final feedback on your ideas, really right.
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No, I think it's the ESFSS, the European Symposium for City Science.
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They found with time a good recipe of, you know, having two and a half days of conference with a single track, as you said, mainly with every career, researchers, phd postdocs talking a bit of a bit, a big range of topics related to first city science and, you know, keep it on a decent and manageable numbers.
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So usually we talk about, you know, 150, 200 people.
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So what I'd heard so far, and myself since I was a phd student, is always a nice, pleasant conference.
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People live happy I would recommend this uh to to everyone.
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The next one is not next year.
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Next year we have ifss part here, so 2027 so next year is international symposium for psychic science, which will be in june in france, and the idea of the european symposium will be every year but the year of the international symp.
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The European Symposium will be every year but the year of the International Symposium.
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So the next one will be in 2027, I think around the same period, so September, and will be in Prague, in Czech.
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Republic.
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Very nice, very lovely Looking forward to that and also excellent keynote and they will end up in the Farcense show eventually, sooner than later.
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You have to grab them.
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It already has been discussed, it's already in the making, but anyway, I have not invited you to talk about conferences.
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We can spend a lot of time discussing conference experiences, but there are more interesting things in the realm of fire science than our troubles.
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I always wanted to talk with you about the decay phase of fires.
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You know the stuff that happens when the fire dies and and it does not magically disappear in our world.
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You are a structural engineer.
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I think it's a very interesting topic from the perspective of structural engineering and, in general, how fire safety engineering in terms of structures is performed worldwide.
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So you already agreed to talk to K-Face, so first let's, like you, maybe tell me why, from your perspective, it's worth discussing what importance that carries for fire safety engineers, I mean the whole motivation behind my research that started, you know, more or less after my PhD about this topic, in the fact that I've seen over the years there has been more and more papers published and presentation related to fire decay, cooling, fire dynamics, related to, you know, the fire decay and cooling phase of compartment fires.
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And what you see is that there are more and more publications.
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But what I found, at least from my side, that I usually see it's between you know the solids or the structure, the compartment boundaries, and the fire dynamics, so the gases, the flames and the hot temperatures, so how these two aspects they work together.
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Because what you see is that there is definitely, and there was definitely and there is still on some aspects, an inconsistency in why and how we treat these aspects, because usually there is confusion between you know what is the fire decay, what is cooling, how do you apply a boundary condition to a structure goes into discussion of okay, if you're trying to do a performance-based design once you have completed the fully developed phase of the fire, what happens afterwards.
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And people have also seen you have people in the past in podcasts discussing, for example, about different columns suffering this problem in different materials.
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So there was the whole motivation trying to shed some light, to try to give some definitions related to this aspect, more on the fire dynamics point of view, that of course apply to solid, so structural, materials, but as well as compartment boundaries.
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So the same concept even applied to a compartmentation wall, a door, if you want, want and so on.
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I appreciate your touching into compartment fire dynamics.
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I am anything but structural fire engineer, so that's probably the least comfortable part of fire science for me to talk about.
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But I'm very happy to talk about compartment fire physics, so you gave me some area of familiarity in there.
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Maybe a rough question, but do we have a history of building failures in decay cooling phase that you can think of or like?
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Is this problem really relevant for real-world fires?
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You know, what you see is that for these big tragedies and now today we are talking on the 11th of September, which is a quite hot day the idea is that you know you usually don't have lots of tragedies, but usually in first-party science, the structure for engineering, you have a few.
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That usually creates a lot of movement, questions, and you know they challenge the status quo of what happened, why it happened.
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And this is the case, for example, for the failures or structural failures or tragedies during the fire, decay or cooling.
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And there have been a few examples outside.
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You know the research world, but the most famous one was the one in Switzerland.
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There was I don't remember exactly the year, but it was about 15 years ago where there was an underground car park oh yeah, made of reinforced concrete, where mainly there was a fire in the underground car park oh yeah, made of reinforced concrete, where mainly there was a fire in, again, their ground car park.
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The firefighters get in, they start fighting the fire and then when you know they were extinguishing the fire, they were cooling down the structure.
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Mainly the fire looked extinguished, so they, you know, can be in danger and you think that it was part of the past.
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Then the whole structure collapsed with having, if I'm not mistaken, seven firefighters trapped and killed in the underground structure that collapsed on Danang.
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So this is an example of you know, possibly the danger and the problems are not always in the big flames and the big smoke and in very high temperatures.
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So you have to think that sometimes you can have a local failure that can trigger different failures that can possibly hand up to something very serious.
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Yeah, I remember the case study.
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We're dealing a lot with car parks, so that case study is brought a lot because it's also one of very few real-world incidents where actually we had fatalities in a car park fire and that one triggered, if I'm not wrong, a lot of development in the world of steel, structural steel and fire protection of steel.
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But regardless, I also think you know a lot of time when we think about fire safety, engineering is, how do we protect our structures from the fire?
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So your first goal is that structure has to survive the fire.
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Fire resistance framework, uh, being proxy of that the best example, but I I think it's it's the cooling phase and being able to uh, you know, account for the stuff that's happening back then it's.
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It's also pretty important when you have to fix the structure after fire.
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You know like you had a fire somewhere and now how badly it was for the structure and unless you capture the whole image till the very end, when it's ambient, you probably cannot judge how.
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I'm not sure how relevant that is.
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You have to think I mean something that I always tell students when I teach this part is that you have to remember that.
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You know, in order to counteract the threat of fires so means mainly usually related to thermal effects so a structure that possibly start heating up and start losing strength and stiffness or start having local or global failures.
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You know, we usually use low thermal inertia materials, somehow thermal insulation, and this works very well when the fire is big and we have the fully developed face.
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But you don't have to forget that the same material works in the opposite way when you're trying to cool down the structure.
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Okay, so sometimes and this is something we can come back to later the fact that if you have an insulation material that insulates really well during the fully developed phase of the fire, also it doesn't enable fast cooling during the fire decay and cooling because the whole effect is a sort of lagged, everything is delayed.
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So possibly this is the whole concept, that if you have something that heats up very slowly, possibly all the effects are seen at a later stage, not when you have the big flames but, for example, when you have a bit later than what would happen in the fire.
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This is the case of, again, reinforced concrete structures.
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For example, reinforced concrete structures, they have the magic concrete cover that they're always trying to protect the inside of steel reinforcement from the effect of the fire.
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So you try to have a very thick concrete cover because, in a way, concrete is relatively good for elevated temperatures but steel is not.
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So what you're trying to do do you're trying to create a thermal barrier for a steel reinforcement not to heat up, but we know the concrete intention is not that good.
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So as soon as the tensile reinforcement, the concrete heat up, then loses capacity and you can imagine, since concrete is very high density yeah, so it's a high thermal inertia the whole effect on the bars, which usually sits a bit in, is not seen when the fire is the biggest.
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It's actually seen a bit later than that because the energy takes some time to actually go through the concrete and reach the steel.
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So this is one of the examples of some of the research that is going on this topic that focuses on really the delayed failure or the delayed critical situation in some low thermal inertia materials where you have reinforced concrete structure, where the maximum temperature and the steel reinforcement is actually achieved a bit later, so during the fire decay, during the cooling, rather than during the fully developed fire.
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And this happens the same for possibly, you know, steel structures protected with insulation material.
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Okay, because the energy has to penetrate.
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What changes if you have a low-density insulation material like mineral wool?
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It will not accumulate that much heat, but still it will make it very difficult for heat to penetrate.
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Yeah, I mean you have to think also, you know what is the ratio and all of that.
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But the lower the thermal inertia material, the faster sort of response to what affects in the fire.
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So the idea is that you know if you protect the best.
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The substrate structure is always the best, because we don't have to forget that the heat fluxes that you achieve during the fully developed phase of the fire are the highest and sometimes depending we can discuss about it later during fire decay and cooling, this heat flux can be negligible compared to what we actually achieve during the fully developed phase when we have flashover, high temperature and smoke burning and all of this.
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Are decay cooling phase considerations relevant for all types of structures, or there are types of structures for which it's uh, yeah, definitely definitely.
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You know.
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I mentioned to you the reinforced concrete structures and I mentioned the key importance of this concrete cover and the possible delayed highest temperature in the steel reinforcements or the tensile reinforcement.
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This thing happens in steel structures, for example, when they are highly and heavily protected.
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But for example in steel structures what you see also for example the Cardington fire test in the 90s that you see that after the full phase of the fire, let's say the high temperatures, steel particularly suffers from large thermal deformations.
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What happens when the materials start cooling down and start trying to take the original shape and the original length in particular, but now it's deformed.
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That what happens?
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Mainly you start having very big forces at the connections.
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So possibly you know, at the connection, during the fully developed phase of the fire the steel element is trying to expand, so it's pushing sort of the connection to receive compressive stresses.
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But then imagine when it's cooling down the depth force changes because now the element is trying to go back to the original dimension.
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So now the connections bolted well that they could have big thin cell forces.
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This thin cell force is big when it's trying to contract, but it's even bigger when you have a deformed shape and a force applied on it that you have some dead ends.
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So that is the case, and for that you don't really need to go to like 500 degrees temperature to create some changes in the steel.
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This can happen already when you heat it up to a few hundred degrees, because it's just thermal response expansion and compression right, exactly.
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And you could have a very big concentrated force at the specific ball.
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So the part of the connection that can trigger some failure and then you know, to cover the last big construction material is timber and we can discuss later about fire decay cooling construction and everything.
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But the main topic related to timber.
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Timber loses mechanical properties, so strength and stiffness, at relatively low temperatures.
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So we know that by the time that still you know, loses you know strength and stiffness.
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So we usually say you know 500, 600 degrees as critical temperatures.
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Concrete, we usually say you know we typically the compressive strength is okay below 500 degrees In timber.
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We know that by 300 degrees, which is our assumed charring temperature, the strength and stiffness of timber is zero.
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So we have a very steep reduction of mechanical properties at relatively low temperatures.
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Relatively means relatively low temperature that we usually see achieving compartment fires.
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This becomes relevant because everything you know, the fully developed phase, but also the fire decay and cooling that has a big effect on the structure.
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Yeah, thank you.
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I think this is a very good summary of the response of materials to these decay phases.
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Perhaps we should clean up the phases of fires as well, if you would like to talk about that in the episode.
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Traditionally, we usually have the growth phase, the fully developed phase, and then we usually discuss about fire, decay and cooling in a mixed manner.
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I tried to break the two apart and I think we can go through them one by one.
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What are the relevant parts of that?
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And of course let's frame it into the thermal boundaries in the compartment and the fire dynamics happening in between the structure and the fire itself.
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So let's go gold phase.
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So the whole concept of first disclaimer before discussing this.
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I think and I'm talking about these phases as a typical compartment fire framework where we have a compartment of relatively small dimensions where we can assume some sort of generalized flashover over the compartment, because as soon as you have a huge compartment then we can start talking about traveling fire, growing fires and localized flashover and all these things, with this disclaimer if we take a small compartment, which is a typical design framework for any compartment in typical buildings.
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So we start with the growth phase.
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The growth phase, you know you do a lot of safety calculations.
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What we usually use are apothecary squares or some sort of like curves of heat release rate of the fire that they know as a function of a quadratic function or exponential, something like that.
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But in general you have the fire, that you have ignition, the fire start growing.
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But usually you can have typically two zone, two zones within the compartment.
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So we have the cold zone and the hot zone, where in the cold zone we have an ambient temperature.
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In the hot zone we usually have a smoke layer that is building up and usually we assume that the smoke layer is not very dense yet.
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So we can discuss about optical thickness.
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But in general, you know we have a smoke layer that is building up, is collecting within the.
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You know the volume of the compartment and typically we know that the curve of the temperature evolution of smoke layer is related to the heat risk rate of the compartment and typically we know that the curve of the, the temperature evolution of smoke layer, is related to the heat risk rate of the fire.
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So the faster the fire grows, the faster the smoke layer heats up.
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But then for related to the temperature conditions, usually we see, you see most of the for structural calculations and the growth phase is usually disregarded because the heat fluxes are relatively negligible unless you have an impinging flame on a very low soffit or your ceiling.
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That you can consider that.
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But the actual big heat fluxes that started happening in the fluid phase.
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Yeah, let's move there.
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If you have sufficient amount of heat in your structure, sufficient amount of radiation, you encounter flashover.
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I guess our friends fire safety engineers know what it is.
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Well, feel free to summarize the flashover phase and the fully developed phase.
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How does it work?
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certain point the fire would lead to a fully developed phase.
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Means is fully developed in terms of it tends to a steady state condition in terms of release of heat, because the heat release rate will be controlled by two different aspects it can be controlled by the ventilation, so the opening, so the amount of oxygen that actually is able to enter the compartment, or the fuel itself.
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So if the fuel is somehow limited in an area or from geometry, then of course can be limited.
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But then if we think about the most critical condition that you usually look at as a critical case, which is the fully developed ventilation control fire, then we can assume that we have a spill plume, so the compartment doesn't have enough oxygen to fill the fire.
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So what happens if the pyrolysis gases, the combustible gases, they come out of the opening trying to reach fresh oxygen in order to fill the fire?
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And what we typically see?
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We see the spill plumes, so these flames that come out of windows or doors where we have an environment full of oxygen, so usually unlit conditions what was the heat, what's the dominant heat transfer mode and and what is the structure exposed to?
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I mean you have to think that, since the heat release rate becomes very large.
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So what happens is the compartment quickly collects all the smoke and fills up with smoke, where the smoke is usually very dense, so we say that it's optic and thick.
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So this means that if, for example, we look at the ceiling or we look at the beam at the ceiling, what the beam is seeing is seeing only the smoke around it.
00:23:21.154 --> 00:23:36.084
So when you're trying to characterize the boundary condition of the thermal boundary condition to the structural element, you usually consider a convective term and a radiative term that is directly connected to the evolution of the gas phase.
00:23:36.084 --> 00:23:44.932
Yeah, so since the smoke is very dense and very dark, very black, so the energy, so both the radiation and convection is coming from the smoke.
00:23:44.932 --> 00:23:46.993
So the energy, so both the radiation and convection, is coming from the smoke.
00:23:46.993 --> 00:24:01.134
And here you achieve, of course, very high heat fluxes, which can go up to very high values 100 to 100 kilowatts per square meter, where actually the structure is very challenged at this stage.
00:24:01.153 --> 00:24:02.944
But in this phase the structure is not yet at the gas temperature.
00:24:02.944 --> 00:24:04.050
It's a continuous like.
00:24:04.050 --> 00:24:13.801
It's a sort of thermal equilibrium in terms of continuous heat transfer from the flame to the structure, to the deep parts of the structure.
00:24:13.821 --> 00:24:15.390
You know right, I mean that for sure.
00:24:15.390 --> 00:24:23.971
You know you have to think always on an energy balance, because what I was focusing now is the energy that comes from the fire and goes into the solid.
00:24:23.971 --> 00:24:26.092
Then what happens to the solid?
00:24:26.092 --> 00:24:38.976
You know you could have material that takes a lot of time to heat up, like you know, a thick steel beam that has a very high inertia, because steel is well known as a very, very high density.
00:24:38.976 --> 00:24:41.753
So it takes a lot of time to actually heat up.
00:24:42.246 --> 00:24:46.192
Of course, depending on the boundary condition, depending on the shape, depending on many things.
00:24:46.192 --> 00:25:00.634
But also you have to think that possibly the energy can also go somewhere else, can be conducted somewhere else, or if you have a thin wall, then the wall has a lot of energy coming from one side, but then it's cooling down from the other side.
00:25:00.634 --> 00:25:02.708
Then you have also losses from the other side.
00:25:02.708 --> 00:25:24.037
So it's an energy balance and there is a lot of fancy software and calculation tools to actually see when the energy comes from the fire, where it goes within the structure and also how the structure is challenged I always like it was fascinated by observing those surface thermocouples and in-depth thermocouples during my fire resistance tests.
00:25:24.076 --> 00:25:39.309
You know it's always like interesting to see how different things happen at different times and sometimes how long it takes for a structure to heat up, and sometimes like it's seconds, and then already you're getting the full power of fire in somewhere where you don't really want it to be.
00:25:39.309 --> 00:25:42.436
But okay, those are like the first phases.
00:25:42.717 --> 00:25:54.958
The fire was fully developed and the main concept I want to say is that for these two phases, if you characterize well the smoke layer, then you get a quite good quantification of the thermal boundary conditions.
00:25:55.605 --> 00:25:57.050
That the structure will be exposed to.
00:25:57.050 --> 00:26:02.536
Yes, exactly, and, like you could say, it's fire safety engineers' bread and butter.
00:26:02.536 --> 00:26:13.946
This is what we do normally in the design, with the caveat proxy of fire resistance testing, which we'll come back to, Because basically those are the phases that fire.
00:26:13.946 --> 00:26:19.807
Maybe this is the moment to talk about fire resistance testing, because this is pretty much it for fire resistance testing.
00:26:19.807 --> 00:26:23.071
The ISO curve never goes down, it can only go up.
00:26:23.664 --> 00:26:29.034
Exactly no, but this is the full concept, that is, the standard fire framework.
00:26:29.034 --> 00:26:45.212
Yeah, you know, the idea is that you disregard the growth phase, because if you look at the standard temperature curve or any other post-wishover curve that is implemented in a furnace, you have a huge heating rate at the beginning because, mainly, you want to achieve high temperature in very fast time.
00:26:46.246 --> 00:26:46.606
Have you ever?
00:26:46.606 --> 00:26:51.606
You have a furnace in Zagat have you ever looked into the fuel consumption in the early phase?
00:26:51.606 --> 00:26:57.165
It's really funny because, like we've done this exercise on the furnaces, I have a paper on it.
00:26:57.165 --> 00:26:57.747
It's published.
00:26:57.747 --> 00:27:00.432
I'll shamelessly plug it in the show notes.
00:27:01.035 --> 00:27:13.226
So we looked into that and in the first like minute or two minutes of of a furnace test you, for example, go to like three megawatt heat release rate in the furnace and then you get to like 600, 700 degrees on your sample.
00:27:13.226 --> 00:27:20.746
You go down, you take it down like by a half at least, like to one megawatt, 1.2 megawatt, very low.
00:27:20.746 --> 00:27:21.267
So.
00:27:21.267 --> 00:27:25.471
So you start the fire by insane peak for a few minutes and then you go down.
00:27:25.471 --> 00:27:36.435
And what's even funnier, we've done an exercise some time ago with Piotr Tafiło, a colleague from Polish Fire Academy, and he'd written a reverse zone model.
00:27:36.435 --> 00:27:48.944
So you gave a time-temperature relation to the zone model and it's trying to backtrack what the heat release rate would have to be in the compartment to create that condition, what the heat release rate would have to be in the compartment to create that condition.
00:27:48.944 --> 00:27:54.305
It was funny as hell because again we got this massive peak and then it's much lower to continue with this standard curve.
00:27:54.305 --> 00:27:55.851
Standard curve is really funny.
00:27:56.384 --> 00:27:59.773
Also because you have to consider that the furnace itself has an inertia.
00:27:59.773 --> 00:28:11.570
Yeah, yeah, yeah, and we are used to have these bricks-based furnaces and bricks and refectory materials as a usually very high density to be, you know, durable.
00:28:11.570 --> 00:28:23.191
But this means they have to pump in a huge amount of energy to heat them up, and heat them up very fast according to the curve, but then the whole framework of what you think about it.
00:28:23.191 --> 00:28:40.266
So, if you disregard so what you do, you do an engineering analysis or you need to engineer a simplification and say, okay, I don't care about the growth phase, so I immediately try to go into the fluid level phase, which means the time zero of my standard temperature time curve is flash over, pretty much.
00:28:40.266 --> 00:28:41.432
Flash over, yeah, exactly.
00:28:41.432 --> 00:28:42.877
And then what I focus?
00:28:42.877 --> 00:28:50.294
I focus on capturing the right duration of the fully developed fire, because this is what happens in a furnace.
00:28:50.294 --> 00:28:56.959
So you test for half an hour or one hour or two hours, or we can go to very high requirements.
00:28:58.027 --> 00:28:58.810
Welcome to Poland.
00:28:58.810 --> 00:29:01.493
Four hours for super tall buildings?
00:29:01.493 --> 00:29:02.750
Oh, for God's sake.
00:29:03.806 --> 00:29:05.112
But the idea is what you're trying to do.
00:29:05.112 --> 00:29:11.862
You try to test the material for the fully developed phase and you know traditionally you were trying to.
00:29:11.862 --> 00:29:19.548
You created this standard temperature curve because it was representative of the worst case file you could have in a building, all of these things.
00:29:19.548 --> 00:29:29.576
But then in general, you focus on the duration in the fully developed phase and then if during that fully developed phase, none of the failure criteria are actually achieved, that'd be days.
00:29:29.938 --> 00:29:30.680
Yep, you're done.
00:29:30.680 --> 00:29:42.565
And now in the lab, as you say, we end the assessment at the end of the fire test, plus the uncertainty, but still a lot of things happen to the sample afterwards.
00:29:42.565 --> 00:29:51.065
The cooling phase in the furnace is also very different from the real-world cooling phase and it's actually very difficult to enforce a cooling phase on a furnace.
00:29:51.065 --> 00:30:02.569
We've actually attempted that and we also have a paper on Interflam this year about enforcing furnace to do that and we were not extremely successful because of the thermal inertia and everything you've mentioned before.
00:30:02.569 --> 00:30:05.281
So this is the reasons why we were not successful.
00:30:05.281 --> 00:30:08.614
But in the real compartment it doesn't magically disappear.
