079 - Timber columns failure in the decay phase with Thomas Gernay and Jochen Zehfuss

Hello, everybody. You're welcome to the fire science show. And other episodes related to timber. So I guess that's getting everyone excited. I usually see. More interest in timber'y episodes than any other type of topic in, in fire science. I really wonder why, why is that? I guess it's one of the most exciting things that happen around and one of the least well understood problems in fire science for people outside of our core group of fire science engineers. And, My today's guests have certainly stirred the attention to the problem and the issues and some features of it. For sure. Uh, with their post on LinkedIn, that went absolutely crazy. They've posted a video of a collapse of a column under loads. The column was heated for fairly short amount of time, like 15 minutes, and then left all alone. Still. Underload. And after 90 or so minutes, it just cracked. And that was one of the. Shoulders. And the easiest visual. Representations of what the threat looks like. The fact that fire has ended doesn't mean. the, the structural performance is from now unhindered by the consequences of the fire. The fact that you have completely different material that behaves completely different. At certain temperatures at certain. Points of heat transfer into the element. That's beautiful. fire science. Absolutely beautiful. So I, after I saw this poster, I had to invite them immediately to the podcast episode to discuss the decay face collapse and all the stuff around that. It's still an early research. I think they've been pointed something. Explicitly important for the fire science and the design of timber buildings. And it seems they're looking further into that. So. More is expected to come, but I'm very happy. We are able to share these results with you at this point. Okay. My today's guests are professor Thomas Gernay from Johns Hopkins university. Uh, he was already in the podcast talking about the structural fire engineering. So view, we're very welcome to check that episode as well. And the second guest is professor Jochen Zehfuss from Technical University in Braunschweig Who has carried their research that we are talking about in here, the collaboration was, was much larger than you learn about it. From the episode, but these two, these two guys. And invited to talk and the talk with it. So let's not prolong this anymore. Let's spin the intro and jump into the episode. Hello everybody. Welcome to Fire Science Show. I'm here today with, two guests who did quite, a mess on LinkedIn a few weeks ago by their, their post on, on failure of timber element. Uh, that is, professor Thomas Gernay hey, Thomas. Great to have you back in the podcast.

hello was, yes. Thank you very much for having me.

And, uh, a new guest, professor Johan. Zeus. hello Johan. Very nice to have you in the.

Hello everybody.

Fantastic. Uh, thank, thank you for, um, taking my invite. You, you seem to be superstars now with, your research. Uh, actually I'll describe if, if someone missed what happened. you have performed an interesting research into timber column failure during the cooling phase, and, posted a post on LinkedIn with a video of the failure and. For engineering standards. It, it went quite viral and I found it super interesting because you've captured in a very visual way. One of the things we really struggle to discuss, it's super difficult to explain someone why decay phase would be important, and you, with the ten second video, have, nailed it, perfectly. But, let's not have this, uh, episode about the video. Let, let's have it, about the research that that led. So, uh, how about we discuss the decay phase overall? Like what, what is the decay phase and, why would we care about it at all?

indeed it starts with looking into how real fire develop and grow and then decay and understanding what effects that can have on, on structural stability and on structures and in the. structural fire community, there's been a shift to going to those performance based fire design approach. So we want to really understand on the real fire, um, how is tructure going to respond. And so in the community, we start to understand that when the gas temperature in a components start to decrease, that's not the end. Uh, for the structure far. Because, uh, for one, there is going to be a thermal lag. There is going to be these heat waves that continues, penetrating, deeper in the sections. And so you have the core of the section of your structural members which temperatures continues increasing. As a result, their strength and mechanical properties continue decreasing, and you can have delayed failure. This. Simple physics. This is distance true for all material. You always have this know, this heat wave comes from the differential equations of, uh, heat condition. so it's independent of whether material is combustible, such as timber or non combustible, such as steel and concrete. And so, Many people in the community have been, becoming more and more interested in understanding that, quantifying that, threat of delayed collapse. We have also seen some real fire accidents where there were delayed failure during cooling that can be particularly threatening to firefighters and first responders who might be inside. So it's very important for us to, for engineers to, to understand that. And from there we started working with modeling and then now we moved to experiments and eventually to these, experiments on timber columns.

But this delay phase, this, late failure or maybe even post fire failure within our paradigm of, fire resistance and, ISO curve and, REI classes, It's not there, I mean, the, the standard curve goes, uh, up all the way, it never goes down. So it's not a part of our paradigm today of, of testing structural members, right.

Yes, that's right. Yeah. the, resistance class approach is, let's say a very simple approach. We use all over the world with the ISO 8 34 curve. That is also a very simple model of the realistic fire. But, as it is a very simple model. you can subject all, elements for this, uh, ISO curve. and um, of course, we neglect the cooling phase. And as Thomas said, we have this delay problem. This is also a problem for concrete elements as well. Not so much for steel elements because they have a very high heat connectivity and they are not so massive in the cross sections. But the difference for the timber between timber and concrete is then, of course, that timber is a burning material. So that we have, not only the delayed heating, um, where the core is heated later after, or in the cooling phase as well. And we have also, uh, fire or, yeah, a reducing cross section, caused by the burning. as we know, the burning, happens, until the fire is nearly extinguished. We, we have also burning in our cross sections, uh, with temperatures beyond 300 degrees in the cooling phase. And then, uh, another point is, uh, as mentioned also in the paper, the degradation of mechanical properties of, timber, for example, which degrade, um, until temperature of 100 degrees and even lower where we loss, capacity or strength.

I just wanted to point out that I actually had an podcast episode with Felix Wiesner about the moisture transport within timber elements and how the, the tur wave propagates. He has tortured me with very scientific terms and, uh, , but it was a popular episode for some reason. Maybe it's, it's the magic of, of Felix, but, but indeed, a lot happens within. timber, uh, at, quite low temperatures because let's face it, a hundred degrees, for a fire temperatures, that's not a very, very high temperature. so, you can probably expect this be reached at very deep into your, structural element. So now, now let's, uh, talk about when have you realized that. Such a research is necessary, and the differences may be so big that it, it really requires calls for additional investigation.

So on my hand, it has been quite a, a long process of wrapping my head around this, this issue of delayed failure. I can trace it back to 2010, 2011 when I was working on my PhD and I was developing, um, a material model for concrete in fire. And, um, I was focusing also on the, on the cooling phase, trying to get those. Material properties and irreversibility, right. And transient Cris trying and, and all of that. And so as part of this, I, I was doing some numerical modeling with columns, loaded columns. And when I was applying natural fires with the cooling phase, I, I could observe in some cases delayed failure. So that was 2010, 2011. And I thought, well that's, that's interesting. And as I mentioned, there had been a few. Accidents such as the, and bus parking in Switzerland where there had been a failure in cooling. So I thought that's something to, to work further on. So a little bit later, I started, trying to systematize this study and I had a paper with Jean-Marc Franssens where we. Proposed an, uh, standard indicator to try to approach the problem of burnout, resistance, or resistance to, to full burnout in a systematic way with numerical modeling. And when we applied this approach to different types of structural members, We identified that, um, it was possible to show that timber was possibly more at risk for the reasons mentioned by by Jochen and really most of the reasons are the fact that you have this very low conductivity. So the heat transfer takes a long, a long time coupled with reduction of strength at low temperature. So even if four, five hours after the fire you get. 80 degrees C, a hundred degrees C in the core, you lose a lot of of strength just to put number. If you trust the Euro code, Annex B on advanced calculation methods at 100 degrees C reduction factor for the strength is 25% for compression and, and 65% for tension. So you've lost actually, most of your strengths, right? and so from there, I did more numerical modeling, studying reinforc concrete column, uh, timber collumns and so on. and, and we could indeed show that the burnout resistance for timber was significantly lower than, its fire resistance, meaning there was a risk of failure on the relatively short fire. Uh, but there were still no experiments that had specifically been designed to, to quantify this. so testing members loaded under heating and cooling to, to, to see the failure. So finally there was this, great consortium with lots of, lots of institutions and colleagues who had all been working on this issue, you know, independently. I was like, oh, we should, we should do something together. So, uh, Tim experiments, of course, uh, Johan who's here, who, who conducted the experiments. But in the consortium there is also CERIB. Who's Fabian Robert, Jean-Marc Franssens at University of Liege, uh, Robert McNamee Rise. Patrick Baumonte at in Milan. And I, I think I, I'm not forgetting anyone, but that, that's a great team. So we work together to devise and experiments to be able to study this and try to be as systematic as possible and show what the behavior would be.

From my point of view, I think in the last 10 years we made in our institute several projects, on the fire behavior of. Timber structures, and most of them, of course were protected, with gypsum boards. but then I think in most other European countries, we have a trend, to applying more timber buildings or, uh, there were, there's a trend to build more timber buildings. Yeah. Due to, uh, for example, sustainable sustainability or. decarbonization reasons and so, and so building your regulations are changing and they need, uh, research. And so we had, uh, in several projects the question, what will happen when we have. Timber elements which are not protected. massive timber elements, for example, CLTs and so on, and how many percent is allowed, of the enclosure, to be unprotected. And then there was also the question. Do we have a self extinction of the timber elements when the so-called mobile fire load, the furnishings And, others is burn. Um, but on the other hand we have, uh, the timber, which is also burning, which is called structural fire load. We see in, several projects. It's not so easy to predict self extinction. And it depends of course on the geometry of the fire compartment, on, the oxygen, um, which oxygen flow and some other parameters. And, so, self exigence is, uh, I would say, very seldom case, yeah. In some circumstances it could happen. And then of course there's the question, what will happen in the cooling phase when the cross section, is reducing, reducing, reducing? And then of course, there could. be a failure. And this is, uh, our motivation to go deeper in this subject. And so, and then we find we find ourself in this group, uh, as you, said, the European. Champions, dunno if this is the right word,

All Stars, European All Stars.

but some of the leading researchers from the European countries. And, I think it's a good thing that we find together and, uh, do a systematical research investigating also the approach, of, uh, Thomas. that we, calibrate a method which, is originally used for concrete elements and to adopt that yet now, um, for timber elements as well.

Beautiful. So, tell me more about, the research. And I already see there is so many, I'm noting the factors that, that you've mentioned. there was different losses in, in compression in tension, uh, I guess, utilization factor of the element, would, play a big role. Johan mentioned technology, so I guess when we get come into like, uh, glued elements like clt, it must be super Interesting. So, so tell, tell me how, how did you approach this research to really like narrow it down to the most interesting. And, and where did you find the most interesting, thing to, go for?

The the key idea. Is to be systematic and to find a way to, I would say almost standardize the, quantification of the behavior of structural elements under fires that include the decay phase. So the approach, that we proposed consistent subjecting structural members to fire of varying duration of heating

Okay.

Identifying the shortest fire that, would lead to failure, or the longest fire that could be survived indefinitely to burnout if you want. So this, this threshold, in order to do that and to be systematic, what we propose is to use the heating according to ISO834, since everybody's familiar with this heating, but then to stop at some point. And to have a linear cooling phase that is in accordance with the Euro code parametric fire model. So why a linear cooling phase? Because we could take any cooling phase, but the only natural fire that is in the Euro code that is codified is the parametric fire. So we may as well take that one and everybody can take that one.

Sorry Thomas. And it's important to note that is the cooling of the furnace itself. So it is the, external boundary condition on imposed on your element. And what happens in the element is, is the physics of the element, and that's what you're looking at, right?

Exactly. So we are talking here about temperature in the furnace, but once the temperature starts decreasing in the furnace, of course in parts of the elements in the interior, the temperature keeps on increasing. So this is the idea. Find the burnout resistance, the shortest fire that leads to failure. and then from there we devised. An experimental, program, a test metrics where we built identical specimens. So we had, for the timber, we had eight glulam columns identical. We, defined the load on these currents, assuming that they were in a typical, uh, buildings and doing the euro code design. So they had, utilization factor in terms of holding that would be typical in the first situation according to the codes. And the only thing that vari. Between the experiments on each of those eight columns was the fire, the definition of the, I would say more gas temperature time curve in the furnace to be more, uh, specific. So we repeated all tests twice. We had tests, to measure the standard fair resistance. R so for R obtained, uh, 55 and 58 minutes in the two tests, then informed by the numerical modeling by finite, models conducted Prior to the test, we had a test with the heating for 15 minutes, followed by the linear cooling phase. And we did it twice and the two columns failed during cooling quite late in the cooling phase. So they failed after 98 minutes for one and 153 minutes for the other. So again, that's a column that's heated for 15 minutes. then then cool. Then the F cooling and it fails at after two and a half hour.

what, what, what was the approximate temperature after 15 600

Yeah. 700.

That's not much for, for fire. Uh, that, that's, heat flu of like 35 kilowatts per square. that's not, not huge in terms of, of, uh, exposure, to be honest. Yeah.

Right. And when it failed, so again, after two hours and a half, the temperature was in the furnace almost back to around 100 degrees because it plateaus, it doesn't go exactly to 20, but it was, it was cool, right? And then two columns under, 10 minutes heating followed by the, and these two survived. And we measured the, the strengths. But so we reached our objective in this experimental campaign to test identical members. Only vari the heating exposure and have the three outcomes that you wanted to have. So, measure the standard for resistance, find a short fire, but that results in failure, and then find the slightly shorter fire that would be, survived. So we, bounded this burnout resistance, this fire that's the threshold between failure and cooling and, and survivability to full burnout.

okay. I guess the most interesting thing is what different in the 10 versus 15 minute, fire. So how much did the temperature profiles inside vary and, how big the difference, uh, was in, in what you've observed between these two tests?

Yeah. it seems to be, only five minutes difference in this, uh, DHP time. 10 or 15 minutes, but. It means when you see, um, on the, uh, ISAC curve in the ISAC curve is increasing, uh, very high in the first minutes. And there we have a difference about 100, 120 degrees, about, between the 10th and 15th minutes. And, uh, that means, um, that also in the cooling phase because, the cooling phase, um, the temperature time, causes in the cooling phase is parallel. Now going from the, uh, starting from the 10th minute, or from the 15th minute. And that means that we have, even higher temperatures in the cross section, which are approximately this 100 degrees and more. And, then we have, in the, columns which survive the fire with the d h P of 10 limits. We, uh, remain the main parts of the cross section under the 300 degrees in the core, we had lower or very lower temperatures, but, in the, cases of the dhp of 15 minutes, we have higher temperatures in the cores so the cross section. Which remains, under the 300 degrees is very much lower.

So, To put it into perspective, the amount of heat that this column got from this additional five minutes of exposure, obviously in higher temperatures at the higher exposure. So the sum of heat just, could penetrate so much deeper into the column that it, pretty much damaged or weakened. Such a significantly larger part of the cross section that, under this load it was not, not enough anymore. that's superb difference. But we were talking here about something that is heated from four sides. Let's, uh, I hope you're comfortable with, uh, hypothe, but what if this was a slab? I mean, then we would be talking about one side exposure.

I would say that we can generalize that, of course, not the numbers, the quantification depends members, the applied load, et, etc. And in a real building, or these will be slightly different. But it's important to understand that what we showed in those tests is not the fluke. It's not something, Particular that happened because of denomination or because of this specific column being foresight. It's really a demonstration of the physics of the fact that heat transfer continues during the cooling phase and even thereafter. As a result, the interior part of the sections get. Hotter than at the time of the peak gas temperature. And so they lose their strengths. And if the loss of strength in the core is, is sufficiently severe, then it gets to failure. So this is true regardless of the, the boundary condition in terms of the number of heated, uh, sides, it's true sore regardless of the type of member. And again, it's true, but to a different extent because material loads are different, but it's also for concrete. And we had, uh, papers with numer modeling that show, that showed that for concrete also wanted to add that. So this phenomenon, we, we, we understand it, we can relate it to physics and again, we were able to model it with finite treatment models, including before conducting the experiment. So it's. Calibrating, you know, but app remodeling. So we had the paper published before and the agreement is, is actually quite close, which as a side note is also interesting because finite element models of timber elements in cooling at this time. They rely on, properties, mechanical thermal properties that are provided in the Euro code, um, Euro Code five part one, two, and xb, which have been derived based on ISO exposure for heating. so there, there was a little bit of an unknown whether. Predictions, you extrapolating to modeling the cooling would be appropriate. And through those experiments we see that although, inputs can always be improved, overall, we can, uh, predict the behavior. So really these models, finite element models can be very useful in understanding and predicting whether they will be, failure in cooling, including with our current knowledge of properties, which again, I hope we improve. I hope we get that even more accurate, but it's already working quite well.

Were there any other things observed during the test? Like maybe the, the failure mode have, shocked you in, in a way? Uh, how, how did the failure mode actually look compared to the failure of the same element without any heating? Because I know you also like crushed one of them, right? Uh,

Yes. We also crossed one, um, element without heating. to be sure. What, what load degree we assumed as to set, we, defined the load degree in a way which is usual for, uh, let's say flat buildings, following the codes. but in our tests and our cold tests, um, we state at a very, higher, um, capacity as we zoomed with the, um, formulas of the code. So, One reason might be that there is a very large spread, due to the material timber, which you cannot compare with steel or with concrete, and, Yeah. Um, the failure mode was, for, for the, um, fire expos, columns. , due to the reduced cross section of course. And then, . Let's say it was a, I think more or less, the failure on the tension side, uh, where we had the failure of the finger joints, that seems to be the merge in the tests. Um, We had for, isof fire and also for the tests in the cooling phase.

So you mean the connections where the Ellas are glutes together?

Yes. so we have to go deeper in this, subject. but, these, well, the first, uh, insights we had, yeah.

Interesting, interesting. And, uh, the timber was, much stronger than eurocodes. I guess you've ordered the sample. As a laboratory, they always send you the strongest samples They have, you know, ruining science by, by sending

maybe, maybe.

okay. Okay. What stresses me about, this, experiment and the visuals, you know, when I do a large, clt, Experiments with, with the whole structures. You can usually see after the, flames self extinguish at some point if they do. But if they do, you can observe how the, all the charts whitens the, on the structures. And there's this ation eating out your, your column. And you know, you see this giant glulam columns like slowly but, but very steadily disappear in front of your eyes. Especially, uh, we, we had one test. We had, uh, there was a massive steel joint at the bottom and you could like literally see more and more of this still joint coming out of the column. That was really stressful, but, it was stressful looking at it without knowing your experi. Now knowing your experiments, it's exponentially more stressful because I do not see what's happening inside. and now when we don't have combustion, but there is char oxidation I guess this is also exothermic process. So again, this, for one, it, it generates new heat. So, so the heat propagates again into the element. And the other problem is what, what Carmen Górska has shown in her PhD. It's. So the heat goes, uh, from hotter to colder. If the surface is hot, it, it's not gonna go that way, right.

Absolutely. So again, for all materials, even non-combustible, the heat travels from hot to cool, so it's going to keep on traveling toward the core with timber there. Lots of additional complex physics going on because it's a combintion material. You can have, you know, combintion can continue and Johan has talked about self extinction and how that's a challenge to, to, to understand exactly and to know what's going to happen with respect to that. You have smoldering and you have drying Deion and so on. Paralysis, right? So, Again, it's a, it's a combination of all of that. Timber is even more complex because it's combustible. But even if we could control and make sure we have self extinction, we don't have smoldering, there is a heat wave and the thermal lag and the thermal deity is very low in timber. So it takes time and. if we are dealing with a building for which we want burnout, resistance, or for which we want firefighters, you know, to go inside to fight the building, we need to better understand an account for, for this delayed phenomena. We are not saying that it's. Needed necessarily for all types of buildings that the fire resistance approach, you know, is, is not valid, should be thrown away? Absolutely not. It depends on the performance subjective. So it depends what type of building we are dealing with. But you want to point that if firefighters are going to go inside, if a building is, is expected to, you know, build a component and then stay stable, then you need engineers who take account and demonstrate that these effects are taken into.

How simple can it be? Like, do you need finite element modeling to solve that? Or maybe we can come up with, some, I dunno, general rule or, or rule of a thumb that, that could help you assess, okay, if uh, your column was, uh, meter by meter and the fire has ended, you can expect this wave propagate. Three hours, which means it will reach the half of the column. I, I don't know, just speeding numbers from my head, but, I, I think we need especially on the subject, that on of timber where people get bored very quickly when you start going very technical cuz they want simple answers, you know, like iso for, for concrete, they, they want as simple as that. So can we make it as simple as that?

Yeah, it, uh, it is a very complex process and so, using finite element, modeling, it's also more complex as for concrete elements, for example, where whereas also complex for a, uh, usual design engineer, I would say. Um, because we have all these phenomenas, a small ring, osis and so on. Um, but on the other, We need, uh, some simplified methods for design, but they are still, Not there. I would say, considering the cooling phase, there are some approaches also in the new generation of Euro code five. but I think there is a lot of work to do, research, work to do, cause it depends on so much parameters. when you look to the real fire. So, this approach, the DHP approach, from my point of view, it's a very good approach to to simplify the fire side. Yeah. But to find the response of the element, it's not so easy. So I think there we need more work to find, a simplified model, which could be. Used for a daily design, for example.

do you think we can simplify it to just consider the thermal layer or we need to model a structural response of the timber, the changes on the performance of timber as the temperature increases in various cross sections? Or, or maybe we can just simplify it to one ISO toine and just be done with it.

Yeah. Um, you mean like we, we have the approach, now in Euro code, which is only valid for, for heating phase where we reduce the cross section.

a simple approximation of, okay, the heating stopped at this moment. How much further can the ISO travel?

so

What I can say is that, We need, if we are interested in burnout, instance in cooling phase, we need to go past, charring and charring rates. That's the first thing that it's a big change in paradigms in the way we talk about it and t it because the resistance of the member is not going to depend only on the position of the 300 degree iso. If I want to simplify,

So Char is, used to be the, whole thing we would care about. Now, we consider this one of the array of, of things we need

correct. Because what's behind char is going to be to become very important. And whether you have a heating that's, you know, uh, more, more progressive but goes much deeper and you get to 100, 100 degrees C in the whole. Core at some point during the cooling, then you have a behavior very different compared to if you have a massive section and the core remains cool, even if, I mean the Charing is, is equivalent. So that's the first thing. But then can we develop these simple design methods? We are not there yet, but that we are working toward that and I believe with some simplifications and so on. But there will, it'll be possible to have pragmatic approach. And our idea conceptually is that we would still have the fire resistance and we keep it and, or is it, it is useful, but we would add a second indicator, which is this. Dhp as we call for duration of heating phase. So to quantify, ability to survive, to burnout, and we would be able to play with the two indicators and hopefully to have simple methods for both. And then depending on the performance objectives, you would pick or require, what is, meaningful, what is necessary for both fire resistance and uh, burnout. Resistance. And maybe for some buildings, there is no requirement at all for burnout resistance.

it's not that I, I vote for super simple methods, or, I want one, I'm, I'm quite happy with, leaving engineering stuff into hands of competent engineers. It's just, you know, observing the history of fire science and, understanding which, uh, ways have. Historically worked for a long time. Unfortunately, uh, as someone said, we will never have enough competent fire engineers to solve all the fire problems of the world. So we, we need to put some tools into hands of, of people who, who may not be as experts and. Conservative, simple rules usually work well with within this hands. However, already see an issue in here given my experience in fire testing iso term 100 if we care about a hundred degrees. Uh, That's a tricky thing. One, because you may have a quite a large part of your cross section at a hundred degrees because of the heat of operation of water. So it's, it's not like a, a narrow line that travels like the charring 300, that's, that's very sharp interface, but a hundred degrees you can have a quite a significant part of your cross section, at a hundred degrees. Um, now, now, I, I would also like to, To hypothesize a little bit. so, you were talking about this, duration of heating phase as your approach to test, and, uh, you were focused on heating quickly and then having decay. How about, heating slowly and for long, period of time? I mean, that's my immediate thought that, uh, it's a little reversed, but you would have safe phenomena. What would you say to that, Thomas?

I'm glad you bring it up because now it's, my opportu mentioned that we are still working on that with the same consortium. We are very happy that we can continue this research. So we are now in a phase two where we are actually studying this behavior, but under natural fires now, really the approach was to be systematic. So we wanted to have a phase one. The experiments were as controlled, reproducible as possible. So it was done in, in, in furnaces with a controlled cooling phase. And everybody can see we did the, each experiment twice. The results align very well. Others can reproduce. Um, but of course it raises a lot of questions on the. Type of thermal exposure and how this behavior would be different if the thermal exposure the fire is, is different both in heating and in cooling. So we have been working on the phase two and we already have some of these experiments. I cannot tell you more at this stage, but we, this is work, work in progress. Um, so especially to your question with the slow heating, that's, it's very interesting, right? Because. I presented this, this research at, um, Congress for Firefighters in, in France in in May. And, uh, one of the firefighter asked me, what about if, if a colonist, maybe not even in the is very far from the fire, but it gets heated at 100, 150 degrees for very long, and it doesn't even char, but, but, honesty, I mean, we, we've not really studied that yet, but. Brings a lot of question because if we, if you trust again those, reduction of strengths that you. In the eurocode and that are from, you know, day duration, drying, all those phenomena that other guests have explained on your very well on your post podcast, Felix Whiner, Danny Hopkin and so on. you could have significant reduction of capacity from moderate heating and if it's long because it has to penetrate right in the section. We are not saying that to hot summer day is going to bring timber buildings down, but if it's from a fire last hours and you have. Thermal wave. that's something to, to investigate. Yes.

And in terms of, localized fires and the damage to like, uh, ceilings. Let's imagine we didn't have a fully fleshed over fire. We just had, let's, let's say a vehicle burning or, or, or something burning in, in proximity of timber. Well, in that case, I guess we, we should consider the timber fla bilities. So the scenario may look not as, uh, as, uh, as pretty as I just described,

yeah, I would be less concerned about that in terms of the burn resistance specifically, because what is, Critical in this concept of bone res is a total amount of heat that penetrates the section, right? And it's the idea that even if it's a relatively short time or if it's a long time, but with no very high peak, if the total amount of heat is, you know, is significant, you'll get. Your heat, also the, the core of the section. But if it's a localized fire, I mean every case is different of course, but, you don't have that much heat that, that p section in the first place, I would assume.

I have a question that may, may be stupid. Excuse me if it is, but, and what, what if firefighters, let's imagine it is a really structure. What if firefighters apply on the, the water on it and, and quickly cool. the external surface of the wall. I mean, the heat is still inside, I guess this, this time. It travels both ways, but you'd probably still see the, the propagation in inside and the damage would not be stopped by just sprinkling water on on.

It's an excellent question actually, because that's something we, we are studying. Yes. No, that's something we have identified as possibly an important parameter and, and we are studying, and we want to quantify to, to what extent it, it helps. But, uh, if you can reduce the, the heat that is transfer still in the cooling phase as well, uh, that

yeah, we did some experiments on, massive timber elements, uh, ct, um, elements. Where the ceiling and, some parts of the walls were unprotected. And then we had one test, where, the firefighters fighted the fire, extinguished the fire and they used not so much border, it was about 3000 liters. So it was only one experiment and it was a compartment of about 40 square meters, but they were surprised not to need so much water. when they cooled down the, the propagation of the penetration into the element stops very early. So I think that's not so a big problem. Of course, it's a bigger problem when you have timber framework element, uh, where you have a hidden fire or something like that.

And, uh, and the final one also to you, Johan. if you have a timber structure protected by, let's say a layer of gypsum plaster board, just to extend the, uh, the defy resistance class of it, you would expect that at some, at some point, uh, the heat goes through the board and can penetrate the. The column, uh, and, and it would not cause a immediate structural failure of, of, of the r criterion within the fire test. If you omitted the decay phase in such scenarios, you would also be, worried about the heat transfer inside. Is it the same scenario for you, or, or how do you view that

Now, of course, um, due to the job sum elements, you have a delay. In the heating and then delaying in the ignition. And, the velocity of, the, py of the showering process is different, is lower as we know. Due to the lack of oxygen. But then after a while, the protection material on the boards will fail, and then you have nearly in the same situation, as you have, for an unprotected element.

so we, we shouldn't consider this as, as a complete solution. It's just a delay to the, to the processes. It may be solution for the structure you are considering. It may solve the problem for the one that, but, but you need to understand and, and measure that. Fantastic. okay, Thank you very much. This, this was very insightful. I need to get one more thing out of you. I think that we can, clearly share with our colleagues, architects, structural engineers who are not Very, um, keen in fire. So let's try and nail the, the one most important thing that you, you've learned here that differentiates why this would be different from, let's say, steel structure or a concrete structure. Thomas, you wanna start?

So timber burns, that's something obvious we know. And because of that, a lot of the focus on ensuring fire safety of timber tends to be on understanding the charring speed and how long it takes for this combu to to proceed and so on. And what we showed with those experiments is, Of course all disability issues, self extinction are important, but they are not the whole story. If you're interested in stability to full burn out because timber also is affected by heat at temperatures lower than pyrosis or condition temperature. There is lot going on the duration, drying lots of processes, so. Everybody needs to remember that even that relatively low temperatures, timber is affected. And because of that, a timber structural member continues to see its its strengths, its capacity, decrease for hours after a fire, during and after a fire. So we should take that into account if we expect firefighters to go inside building, or if we expect building to remain stable, remain standing under fire.

Johan, you want to add something to that? Maybe how to use.

yeah, I could emphasize, I think one very important issue is, uh, uh, for the firefighting strategy when they go into a timber building where we have a fully developed fire, um, they have to be more careful even after hours. Because we have a higher risk for failure in the cooling phase, as I think we can say this as, uh, we have, uh, when we have a concrete structure. But, uh, on the other hand, um, as we said, we are not against timber. Uh, not at all. Um, timber can have a very high fire rating now. Uh, it's a question of design. It's a question of. But, especially in the firefighters when they come late, when they are, uh, very, in a very early stage, of the fire, uh, starting with the firefighting process, the problem is minor, I would think. Um, but when we have a very long time starting the extinction, the risk is higher that we have a failure in the cooling. Then when we need other, when we use other materials,

Thank you. Thank you very much. Very much appreciated message to this, part of my audience, and for structural engineers who would like to approach this process. Recognizing the issue is enough and they will know what to do.

So in this research, as always, we want to first observe, understand the behavior, and then provide the tools to have solutions. said is very important. So it's certainly not being against, you know, one type of solution or another. There are solutions to build fire safe structures, you know, with, with e, every type of. Structural systems materials. Uh, so we are working on the solutions that structural engineers today can use Finite element modeling or research suggests even with the effective properties from your code five, you get really good agreement with those experiments. And we are working on providing the simple design methods for dhp hp

Fantastic. And when you do that, make sure to come to the podcast and explain them. you're already invited. guys, once again, congratulations on, on your research. I, think, What we talked today may not be that easy from understanding it perspective, but it really is very visual when you see the, the failure of your element and you immediately realize, what happens there. I think you've shown a very important distinction why there is such a movement in the fire community on researching timber. I mean, you've really captured one of these differences that, that really Easter and, uh, may, uh, have a critical impact and, uh, As Johan said, especially impact on the, the firefighters in the, in the phase that they would usually consider safe based on their experience. So, thank you very much for that and, I'm very looking forward to the secret project. Uh, you've, uh, you've teased us without using the word traveling at all actually. Thanks. Uh, Thank you guys.

you

you very much West.

And that's it. Super nice people being super nice research European all-star team. Researching. Timber. For you. I really love how we start getting all the puzzles together too. Be able to clearly say what makes timber and fire different than other materials in fire. It seems that we're on a great route to, to finally identify all the issues. Hopefully all at least most of the issues. And be able to actually communicate them and be able to work around them and, uh, you know, By understanding that that's the first step to finding solutions. If we don't understand what's happening, if we don't understand what's different. We are unable to act. And if we do understand what's different, we can work around that. That's beautiful. The research was carried on single type of an element, single size of an element. So definitely we need more. We need to understand better what's happening. At the columns that are larger cross sections at columns The other heated at different rates of heat exposure. But that's all to come. What we have now is the first indication that the numerical models. We're showing something that really happens in reality. They demonstrated the physics that. This collapsed can, can take place and, uh, Yeah, we know more today. We know more today and, we can already act on this knowledge. So thank you, Thomas. Thank you Jochen for coming. Thank you for leading these beautiful research. And I must say I'm really, really looking forward to four more coming from this collaboration on, on the collapsing, the decay phase. It's really exciting research to it, to look at. Uh, listeners to us. Thank you very much for staying here with me and, hopefully see here next week. Cheers. Bye.