Nov. 3, 2021

025 - Structural fire engineering with engineered timber with Felix Wiesner

025 - Structural fire engineering with engineered timber with Felix Wiesner

In Episode 18  we have touched on the important topic of fire performance of engineered wood and its wide use in the modern built environment. Today,  we follow up on this subject with Dr Felix Wiesner from the University of Queensland. We leave the (important) topic of compartment fire dynamics and focus on what happens inside the wood in the fire. And there is much more going on than I have initially thought... The transport of moisture, weakening bonds at the glue line and connections, complex thermodynamics of char layers... I will be honest - this is not an easy episode. But that is how it is supposed to be. Because this topic is not easy. You cannot proxy all of these considerations with simple fire resistance. You cannot pretend these issues do not exist.

So the next time someone writes on LinkedIn that "wood in fire is stronger than steel" please send them a link to this episode, with a comment that it is not that simple...

You should definitely follow Felix on Twitter, where you can learn a lot about timber in fire and be updated with the progress on the National Centre for Timber Durability and Design Life.

To arm you up with some resources, please check Felixs' PhD Thesis on "Structural behaviour of cross-laminated timber elements in fires".

In terms of papers interesting ones to read on the structural reductions in the cooling phase are: 

 In terms of testing of structural wall elements, there is the following paper, which kind of then kickstarted a lot of Felixs' PhD research. He says "I was very fortunate to have worked with great Masters students on this and other projects" Wiesner, Randmael, Wang, Bisby, and Hadden Structural response of cross-laminated timber compression elements exposed to fire)

 Also, if your listeners are interested in outcomes from sitting hours in front of a Universal Test machine and waiting for the wood to heat up to specified temperatures before crushing it then they might be interested in the following paper:  (Wiesner, Thomson and Bisby, The effect of adhesive type and ply number on the compressive strength retention of CLT at elevated temperatures). Again we found that adhesive is an important factor in determining failure temperatures in CLT. Also a big difference between transient and steady-state heating.


[00:00:00] Wojciech Wegrzynski: Hello, and welcome to Fire Science Show session 25. Nice to have you here again. When I've started this podcast, I've set myself a goal to reach episode 25 and see what happens. And, yeah, what's happened is 10,000 downloads, a huge feedback from all over the world and a lot of happy listeners and a growing audience that seems very healthy.

[00:00:21] Wojciech Wegrzynski: So, I guess there's no other option, but continue to go with this. I hope my today's episode does not discourage you from following the podcast because. today I have some real fire science, without taking hostages. We're going to talk about some serious structural fire engineering, but on the topic that's, seems to be the most interesting uh, because it's structural fire resistance of engineered timber

[00:00:47] Wojciech Wegrzynski: so, a word of warning.. If you're not structural fire engineer, it's gonna be probably a bit tough. And, I had the same it's, some difficult concepts being presented, but, precisely, that was the [00:01:00] point of making these episodes, to show you the complexity of the problem to show you what things go into engineering a m as timber structure. So it's truly resists fire. So the next time you see a stupid LinkedIn posts about a wood being stronger than steel, or the fact that, layer of char is better at protecting timber than, mineral wool or something, you, you will know how to react and you will know that it's not a whole story.

[00:01:30] Wojciech Wegrzynski: There's many more aspects to this story that are very important and that these details may get true difference when it comes to the structure, sustaining a huge fire. This is also in a way a follow up to episode with Danny Hopkin, , who shown us to the world of mass timber and engineered timber. And we've talked a lot about compartment fire dynamics and overall aspects of why we should build with timber and, how to make timber [00:02:00] fire safe.

[00:02:01] Wojciech Wegrzynski: Today we go more into the structural fire resistance of the engineered timber. Today, we go a bit into the chemistry of adhesives and, yeah, I think you're gonna like it a lot, even if you're not a structural engineer, I've certainly enjoyed it. My today's guest, has done his PhD on the subject.

[00:02:20] Wojciech Wegrzynski: He's burned a lot of timber structures first at University of Edinburgh now in the University of Queensland and, he's now involved in building the national center for timber durability and design life in Australia, which is super interesting project. And you're going to hear a bit about that so, yeah, please help me welcoming Dr. Felix Wiesner from University of Queensland and yeah, let's spend the intro and jump into the episode. [00:03:00] Hello and welcome to Fire Science Show. Today, I'm with Felix Wiesner from university of Queensland. Hi Felix.

[00:03:17] Felix Wiesner: I thank you for inviting me. Happy to be here.

[00:03:20] Wojciech Wegrzynski: I'm really happy to have you here. It seems saying you're researching the subject that's most interesting for my podcast audience based on statistics. And that is a timber in fire or structural fire resistance of wood. As you know, some weeks ago I had Danny Hopkin when we've discussed a lot of aspects of fire safety in timber buildings, and how this could be introduced. But I have so many more questions, especially to the structural side. So I'm really happy to have you today to answer me all of this pending issues. before we jump into the structural fire resistance, I have a cheesy [00:04:00] question to ask. How sustainable is a building that burned down completely?

[00:04:05] Felix Wiesner: Depends when does it burn down? If you say 40 years into its design life you, probably hit a point where you hit, sustainability is zero and from then you can only gain

[00:04:16] Felix Wiesner: onwards, but, one issue with timber building system, most of the

[00:04:20] Felix Wiesner: accidents we have seen happen in the construction phase. Um, I think it was university of Nottingham that built down new, uh, chemistry building, that had some big soft stick frame and mass timber. And it burned down during construction and it's still one of sustainability of what, when it was finished. So question that has maybe how often can it burn down and still be sustainable?

[00:04:41] Wojciech Wegrzynski: I've asked that because timber is obviously the technology marketed as green, as sustainable as friendly, planet friendly. Which obviously is true as, as long as the building, is built, completed, sustains, its whole life. And, I think, with, [00:05:00] mass timber buildings or engineered timber buildings, not really questioning the structure capability.

[00:05:06] Wojciech Wegrzynski: Maybe we are in a bit, but not very hardly. We're not questioning the ability to build these buildings, but we're worried about will they survive their lifetime? And if a fiery event happens in such a building, what will be the consequences?

[00:05:19] Felix Wiesner: Yeah. And I mean, you would have to same issue for any sort of building, because even in steel or concrete building, if you have a fire that you cannot control and then moves from compartment to compartment, eventually you will lose so much of the building that it might be too expensive to to reconstruct it.

[00:05:36] Felix Wiesner: I think it also boils down a bit to the concept of fire resistance and how it's applied because a lot of people interpret fire resisstance I'd say 90 minutes. And they say, okay, after 90 minutes we can wash our hands and whatever happens happens. But obviously the original concept of fire resistance was maintained a structural capacity until you get burned out of the original fuel load.

[00:05:57] Felix Wiesner: And then the fire goes out by its own [00:06:00] and the building is safe. Maybe the concept, how it's applied now that a lot of structure engineers, when you talk to them, they seem to have the idea that okay, after 90 minutes, as long as everyone out it can collapse and that's all good.

[00:06:12] Felix Wiesner: But if you think about the timber buildings we're building, even if you think about five stories, would it be acceptable to have a five story building in a Metro city collapse from a fire?

[00:06:23] Felix Wiesner: I mean, even if everyone gets out even if we can perfectly anticipate it and control fire spread to the neighbor and buildings and jobs or property protection, that's, that should be a big deal.

[00:06:35] Wojciech Wegrzynski: We had a fire of a car park in Warsaw year ago, like almost exactly a year ago. very large fire that literally, uh, led to significant structural damage to the car park. And it was a residential building, I think, 80 something apartments in the building.

[00:06:54] Wojciech Wegrzynski: , these people have not came back to that building ever yet. Maybe they will because they are [00:07:00] trying to save the building, but it's, horrible. A fire that takes down. a living place. you don't want that. think it's acceptable that the fire, you know, and I'm not talking about the fire where someone would have a barrel of oil in his compartment and, caused an accident. I'm talking about a very common fire, a car fire in a car park is a common fire. That's a common scenario, which you would very, very expect, uh, same like a kitchen fire in a house or a, some electrical appliance fire in your house.

[00:07:32] Wojciech Wegrzynski: uh, these are scenarios that are normal to be considered, yet, in case of timber or mass timber buildings, especially if you have a lot of exposed wood products, compartment fire dynamics of that fire could be completely different than the compartment fire dynamics from let's say a non-combustible, , compartment.

[00:07:53] Wojciech Wegrzynski: And what you've said that structural material will add to the fuel load is now a new factor that you [00:08:00] have to consider.

[00:08:00] Felix Wiesner: Yeah, I mean,

[00:08:01] Felix Wiesner: it, again goes back to the burnout question. I think when Danny was on your show, he discussed that quite a bit and

[00:08:08] Wojciech Wegrzynski: Um,

[00:08:08] Felix Wiesner: highlighted very much the problems with that. But I think also just in what you just said, touches on one of, the issues in fire safety is the economic factor. When you said yeah, you had 80 families living in that building and they cannot return. Looking at some of the origins of fire safety regulation, there were often driven by economics. If you look at the great fire of London afterwards, they changed a lot of the building regulations in the United Kingdom. That, people died in that fire, but the main driver for these changes was that basically the British economy or at that point, the English economy, went, uh, bust for, for a couple of months or even years and the enormous cost to recover that.

[00:08:49] Felix Wiesner: That's an important aspect. And also in terms of, buildings after fires and the effect of fire, sometimes you talk to people and they make that point that actually not timber building. When you have a fire, [00:09:00] it's easier to rebuild because everything turns to ash. I'm not sure if they're being serious or if they're being flippant. it's obviously a serious topic that we should discuss. And, and I think maybe for. One story or two story house in the middle of nowhere. That makes sense. But again, once you get people's, living space, people's property involved. It has real life consequences beyond the fire and people's livelihoods.

[00:09:25] Wojciech Wegrzynski: Cost to recover. That's a nice phrase. I I've written it down. That's I think a great measure of resilience of a building.

[00:09:35] Wojciech Wegrzynski: So let's, let's move on to the structural aspects of, timber. First, timber has always been there. Like it's always been used as a structural material. However, it, comes into this new phase where it's as the material of the future. And, tell me, how is it different today than let's say 50 years ago or a hundred years ago? Why a hundred years ago, we didn't build this large [00:10:00] buildings of wood. We obviously did some in Japan. There's massive infrastructure and castles and stuff like that built with timber, but it's, it has not been, let's say mainstream or you don't see them in your old town districts, this huge, huge wooden buildings. Tell me how it's different today.

[00:10:18] Felix Wiesner: Yeah, I mean that why is probably that at some point you're just running out of large timber because in the past, before we used engineer timber, you were limited in your structural member size by the trees you could find. And obviously there's only so many massive trees. You can chop down to build your buildings. And now we have engineered timber, which has the advantage of, you can take a lot of smaller trees and combine them into a building a structural element that can be as big as you want, um, or any shape you can imagine almost now with looking at modern CNC and how we can shape buildings in a manufacturing process.

[00:10:54] Felix Wiesner: So that has really opened a lot of opportunities of going higher and bigger with timber [00:11:00] buildings,

[00:11:00] Wojciech Wegrzynski: And in terms of structural performance, what are the benefits of this and engineered timber?

[00:11:05] Felix Wiesner: I think, I mean, you can control better again, if you think about, if you look for a big tree and then you're you have to work with what you have there. While if you, take a tree and you chop it up into smaller pieces, and then you can combine those and you have, more reliability and confidence in your structural properties of the end product. So that helps helps you with the design.

[00:11:26] Felix Wiesner: another thing that is changing now, that's not necessarily related to engineered timber, but also, looking into the forestry sector, people are improving what the raw base material and the tree through selective breeding.

[00:11:40] Felix Wiesner: After a few cycles of growth and felling trees that pick the best ones. And eventually they improve the elastic modulus by 10% in 20 years or something like that.

[00:11:52] Felix Wiesner: Don't quote me on those numbers, but it's remarkably high. So that's obviously something where the base material changes and [00:12:00] also if engineered timber is engineered so we can select what we need.

[00:12:04] Felix Wiesner: And now people are also looking to, to hardwoods to be included in engineered timber products, which again, opens new opportunities to, to design your structure element as you need them for your building.

[00:12:16] Wojciech Wegrzynski: There's um, one colleague in my office, Dr. Paweł Sulik he's, researching, the temperature response of, CLT samples built from combinations of like all species available in Poland, you know. mixing them together, hardwood, soft woods all types of to figure out, what are the differences between this and, combinations. So we don't necessarily rely on, on this one type of wood that comes from abroad. that is the base of the CLT industry. And you could do it more locally, which is always also something that improves , the sustainability of, these, products. So I think that's a great direction.

[00:12:54] Felix Wiesner: Absolutely, it's a really great and exciting direction because it helps you to utilize local [00:13:00] products that previously might've been under utilized and you can use now species in CLT that previously wouldn't maybe not have been considered, for structural applications. So it's really about making the most of what we have.

[00:13:13] Felix Wiesner: And that's one of the key aspects we need to consider. If we want to increase the sustainability.

[00:13:18] Wojciech Wegrzynski: And when you build this complex shapes of this, engineered wood, to what extent you would engineer do you, is it just the product and you cut it into that, or you would pick particular layer sizes? I don't know. Adhesives is that also a part of this engineering and and that's done by the manufacturer or who, who decides to build the CLT slab?

[00:13:41] Felix Wiesner: I think the main input is probably from the manufacturer and they will have some idea of what the market wants. I mean, obviously if you look at a building, you could go into real depth and say, okay, I want this member to be a free layer from this species, with this adhesive. And here I want something different, but actually you, [00:14:00] at the moment, the way the timber industry is set up, you would probably increase the cost because. Timbre of the way it's or at least if you look at engineer timber and the most popular product is probably cross laminated timber and at least in terms of volume, the cost is really about reconfiguring the factory where it's produced. So they really want like a standard product that comes out. Like they don't want to have too many changes and, and it would really save money if you have like a few standard sizes,

[00:14:30] Felix Wiesner: Then the next consideration you can use your species of wood, which might be locally availability or, desirable properties. You can change the thickness of the different lamellas that make up your wood. Then you can decide on the adhesive, , which there are certain cost considerations there, health considerations from the, gases that are offset from the adhesive as it cures and in the lifetime of the timber.

[00:14:53] Felix Wiesner: And then there's obviously the fire safety, which, um, in the last few years has become quite prominent for adhesives.

[00:14:59] Felix Wiesner: And all of [00:15:00] these things combined to, to form the product and we'll have a certain fire performance. And one of the thing I've done for my PhD is look at how do these factors influence the end product.

[00:15:11] Felix Wiesner: And it is quite interesting that, in my PhD, when, when we changed from our, from a three-ply to a five ply, and we also changed the adhesive from a polyurethane to melamine urea formaldehyde adhesive, we increased a load bearing capacity at other same heat exposure by 100% for some samples. So, And, under the same equivalent load.

[00:15:32] Wojciech Wegrzynski: and the same width of the elements

[00:15:34] Felix Wiesner: uh, the same overall with, so it was, it was hundred millimeter thickness, the three-ply of a 40, 20, 40, and the five ply were five times 20. So both a hundred millimeters.

[00:15:47] Felix Wiesner: and in theory, the 40, 20, 40 is the one with the higher structural capacity, because it has all the parallel strong layers on the outside, which if you know a bit about structure engineering, that's where you want your material.

[00:15:59] Felix Wiesner: That's why an [00:16:00] I-beam looks like an i-beam. But then in, fire, the five plies perform much better because they have less sheer forces. You don't put all your eggs in one basket because once you you, burn through the first load bearing layer, you hit that first crossway late after 20 mil. And that buys you some time before you get to the next load bearing layer of our under 40 mil.

[00:16:21] Felix Wiesner: You just keep burning through half your load bearing capacity. And if you consider how the neutral axis changes, , you're really rapidly, eating away at your, bearing capacity.

[00:16:31] Wojciech Wegrzynski: When I'm designing a steel structure, it's usually a very simple analysis to find, you know, encapsulation that will give me this temperature on the steel when I'm, when you design concrete, that's the magical layer that, uh, will, prevent my rebar reaching a certain temperature.

[00:16:48] Wojciech Wegrzynski: but here it seems fairly complicated to have this.

[00:16:51] Felix Wiesner: It's interesting that you mentioned a magical number for concrete because we got the same thing for timber and it's the zero strength layer concept, which basically. [00:17:00] Means you, you take your char depth that you get from your charring rate, which you, you don't measure, after test, or you assume certain chairing rate than we have from empirical observation and measurements.

[00:17:11] Felix Wiesner: And then, um, in the eighties Schaffer in the U S , did a test and a lot of simulations on glulam beams, and he'd determined that the load bearing capacity bending can then be back calculated by taking off an extra layer of seven millimeter, which basically lumps all the heat losses from heat to timber below the char layer into seven millimeter to basically assume it's zero and the seven millimeter.

[00:17:34] Felix Wiesner: And then everything below that is ultimate capacity. You also have that magic number. The problem with that is that was for glulam for a limited set of glulam beams and likely just discussed. We now have all these exciting products with different pieces of timber, different adhesives. There's a lot of innovation and the problem with that seven millimeters that we found with CLT, it doesn't really apply.

[00:17:56] Felix Wiesner: You need much more than that. And obviously there is a [00:18:00] point to have these simple values that give you simple solutions, but you shouldn't apply to a complex problem. If you have a glulam beam and a simple structure, fine.

[00:18:09] Felix Wiesner: If you decide the largest timber building in the world, that is by definition on novel and complex structure, you probably shouldn't use that simple method. You should try to understand the behavior of the system and then come up with engineering solutions. And these are like two different ways.

[00:18:27] Felix Wiesner: This is one thing where I struggle a bit with fire resistance testing, because yes, it gives you an answer. it gives your a certain safety, but, I guess Danny touched upon that as well.

[00:18:39] Felix Wiesner: The time fire assistance in our timber building. If you consider the fire dynamics doesn't mean that much. And also it hinder us understanding. So again, if, if you look for example at your colleague that you mentioned, and they're working on different CLT, if they put out on your product, they need to put it for a fire assistance test and a fire assistance test is very expensive.

[00:18:58] Felix Wiesner: so instead we [00:19:00] need to compliment these tests with research to understand how the systems got together and what your ultimate outcome for the structure, fire assistances, because then we can, tweak the product to a point, , where we can say, okay, now I'm confident that I have a safe product.

[00:19:15] Wojciech Wegrzynski: We have one of the biggest fire labs in the Europe. So let's say we're cheating in the department of costs of fire testing our side. But I, I understand that, one lab is not enough to, solve the issues , of the future. You've mentioned the research on the chairing layer or the protective layer was done on glulam beams. But a stupid question. Does it extend also to like columns or slabs or walls? Like, can you extrapolate that

[00:19:47] Felix Wiesner: Yeah, I mean the chairing rate of wood shouldn't change that much. I mean, obviously the fire dynamics will change a bit, whether you have a timber piece that's horizontal or vertical, just in terms of buoyancy and a laminar flame, but [00:20:00] still the chairing rate shouldn't change that much. But one thing that changes the stress distribution that you see and the effect of heat on that stress.

[00:20:09] Felix Wiesner: And to be honest, I don't really care that much about the charring rate. There's so much research out there. There's a lot of numbers. We were getting quite good at getting that right. And once timbers charred it has nearly tripled, strength and stiffness. So you can, if you take just a piece of charcoal, I can easily squeeze that in my hand.

[00:20:28] Felix Wiesner: And that shows your there's There's left there to resist loads. What's interesting is what happens to below the char layer. And that is we had a seven millimeter zero strength layer, or now we're working on, adjusted zero strength, layers come into play. And, to answer your question about the difference between a beam and a column is that timber is affected differently in compression, that it has in tension.

[00:20:49] Felix Wiesner: So in beam we have bending and the tensile side is what is affected in our column where you have mainly compression. One thing that changes is that in compression, the failure mode is, , [00:21:00] compressive yielding. So it's a plastic defamation and, uh, the heat induced changes are more severe. That is not only a thing we need to consider jumps off to temperature.

[00:21:12] Felix Wiesner: Another very important point is the moisture. Cause if you, if you have perfectly dry timber and you can heat it to 70, 80 degrees, it won't change much in terms of the structural capacity. But if you take a piece of timber, that's, very wet and moist. Even at ambient, temperatures will be much weaker than, dry piece of timber.

[00:21:31] Felix Wiesner: And what happens is in a fire, it's that, I mean, most timber in a building situation we'll have about 10% moisture content. So that's water that's in the timber. So then in a fire that was some of the water will go into your compartment and, evaporate but part of the water, the drive deeper into the timber, away from the heat, because you're creating a pressure, so that water can then accumulate and can really weaken the timber deeper inside

[00:21:56] Felix Wiesner: And so, for example, if you think about a column that's exposed from four sides [00:22:00] somehow the water like drives in and, gather somewhere. You don't end a core, or if you have connections, we're accumulated the connections because it can turn into vapor and then travel through the timber and will condense on surfaces.

[00:22:12] Felix Wiesner: That's , if you really work, if you get timber and you're loaded and then you start heating it, you'll see that the, water will travel to where you have, metal connections and where eventually it accumulates and can scape. And when accumulated at a, timber gets really soggy and then it's really easy to actually break it.

[00:22:31] Wojciech Wegrzynski: It's kind of, you know, very abstract concept that exposing something to fire creates this water moving around. would say it's very difficult to imagine, but, then I remember when we're in the lab doing, concrete slab tests. And whenever you do a concrete slab fire resistance test and you heat up the concrete, you end up with a puddle of water on the top of it. So I have this image in my head [00:23:00] that I can relate to. And it's something really interesting that, this let's say ecosystem within your structural element would have this, this big impact.

[00:23:10] Wojciech Wegrzynski: And in terms of engineered products like CLT will this, , also be important for, let's say, into layer bonds, uh, or just the connections to the other structures?

[00:23:22] Felix Wiesner: Measuring the movement of moisture is actually really tricky. You can infer the moisture content from the, from the connectivity and the, resistivity,

[00:23:31] Felix Wiesner: but because it works on all logarithmic scale, you run into trouble at very high moisture contents.

[00:23:36] Felix Wiesner: Really, if you think about a waterfall moving, you're talking about a hundred percent, and it is definitely an issue for the inter layer bonds, because we know that adhesive is weakened by elevated temperatures, but a lot of adhesive types are also, weakened by elevated moisture contents.

[00:23:51] Felix Wiesner: Some tests in the past have also shown. that the biggest reduction comes from hot steam in on adhesive.[00:24:00] , so basically the water that is vaporized and hot enough to travel. And when that hits your bond layer, that will weaken your composite action. I think Danny touched upon the concept of char fall off and the bond and integrity, and, and that is often considered in terms of the fire dynamics. And that's very important because if we get that and we cannot extinguish the fire, the fire does not extinguish itself. Then we don't need to talk about the structural capacity because if we have a fire that never goes out,

[00:24:27] Felix Wiesner: then we can decide the strongest structure in the world. Eventually you burned through.

[00:24:32] Felix Wiesner: But obviously if you get to the point where you design your fire dynamics, right, and you get the fire out. You still have to consider that in that timeframe, your structure needs to remain standing and afterwards it needs to remain standing. And to do that, we need to consider the effect of temperature and moisture movements on the timber and on the glue that holds it together, because we said at the beginning that you have all these layers that you create, this beautiful engineer timber. If you take away the glue that hold it [00:25:00] together, you suddenly have a bunch of timber boards. So they behave differently because they don't have to composite action.

[00:25:05] Wojciech Wegrzynski: And, there a weak spot, like, would that, adhesive line would be the weakest spot? I had this episode with Ali Ashrafi and he was talking about resilience and an important part of that, was, not just the fact that structure will not collapse it may collapse, but to give a fairly long warning and, do not have this, collapse as a very sudden action because that can trap people. Is there a sudden failure mechanism for engineered timber products in fire or it's more like degrading over time?

[00:25:35] Felix Wiesner: I would say it's degrading over time. Timber itself is brittle at least in tension and it can fail quite explosively. Um, I had a timber piece tested in the lab and it snapped in half due to buckling which was very loud and frankly, a bit scary. but in fire, because you slowly get the deterioration, the slow increase in temperature, you will see [00:26:00] deflections that increase hopefully before you get to failure. So you, have some warning.

[00:26:05] Felix Wiesner: The individual failure mode of the adhesive depends on the adhesive type, depends on the temperature. But I wouldn't imagine that you suddenly see a piece of timber, a sudden all the adhesive is gone. And even if you slowly lose the adhesive over time, that will then slowly reduce your composite action, which you reduces your overall second area of, models.

[00:26:27] Felix Wiesner: And that will increase deflections. So , you do get some warning, I think. But then ultimately it also depends on the house, your structure decide, how are your connections decide to give you some redundancy there, because if you just have simply supported elements that are screwed together, then when you get to the point of failure, you either get the tensile failure or the yield failure after compression, which if you don't have redundancy, you will progress very quickly to collapse.

[00:26:54] Wojciech Wegrzynski: I must say that, image of this, uh, say glue line, failure painted in my head [00:27:00] was there will be like whole planks or wood falling down from the ceiling. As soon as the glue is heated up. And that was what I thought it will be. And how I imagine this, because it was brought as a very serious issue in the community.

[00:27:16] Wojciech Wegrzynski: And then I've done this experiments with OFR Danny, and the Structural Timber Alliance. And, we've pushed a slab to mode where it was heated for. Long long period of time. So we had these glue line failure multiple times actually, and it was not really this spectacular failure of the whole timber element.

[00:27:37] Wojciech Wegrzynski: was not that a plank would fall down and then I would have virgin wood exposed , but I could describe it best as rain of charcoal.

[00:27:45] Wojciech Wegrzynski: These small charcoal elements, falling down. Sometimes there was a slightly larger element, but I've never seen a single plank full down at one time

[00:27:56] Wojciech Wegrzynski: For me, it was surprising how well this Product and [00:28:00] remains like, it's integrity in terms of, having these planks attached to each other only through the glue.

[00:28:06] Wojciech Wegrzynski: And I must say it was also a loaded elements, it was not just a slap that was standing there. What's your experience because you've tested so many of these adhesive types, what's your experience with this glue line failure

[00:28:17] Felix Wiesner: I mean, people discuss about the different terminology. Some people call

[00:28:21] Wojciech Wegrzynski: Yeah.

[00:28:21] Felix Wiesner: it char fall off. Some people say delaminations, that people say they're debonding. So there's still a lot of discussion about the correct terminology. Usually what you see, it depends what,

[00:28:33] Felix Wiesner: when this sort of crew line integrity failure happens is if your outer layer, the first layer to the glue line has already completely charred then what happens is the char will fall off. And as we said earlier, that doesn't have structural consequences because you've already lost all the strength and stiffness in that.

[00:28:50] Wojciech Wegrzynski: Okay.

[00:28:51] Felix Wiesner: The other thing that can happen is when you still have, , a bit of timber and that starts to delaminate , So basically you have timber piece that's partially charred [00:29:00] but at the bottom line it's still timber and it starts to weaken there. But the problem there is that not that the whole plank will fall off. Um, in my opinion, the problem happens much earlier because if you get to that stage where it separates, you get to a stage where, the bonding quality reduces.

[00:29:18] Felix Wiesner: And actually tests that, we have, back at the university of Edinburgh, we loaded beams and we heated not to fire temperatures, but, you know, heating chamber up to 150 degrees and we saw like one adhesive type deflected, much more, but we couldn't see it visually. So we couldn't make out any falling off or any separation of the layers.

[00:29:38] Felix Wiesner: But there was clearly a difference because the only difference in these two products was the adhesive. And that's really interesting because it means that deterioration of the after glue happens much earlier before you see these problems of char fall off and delamination. And it has structural consequences. I mean, eventually we saw [00:30:00] something in terms of looking at it from the digital image correlation, where you look at like each image every few seconds and you measure the defamation.

[00:30:08] Felix Wiesner: So you will see,

[00:30:09] Wojciech Wegrzynski: Mm,

[00:30:09] Felix Wiesner: can use that to calculate your sheer strains and you'll see. Yeah, in an adhesive that weakens the strains will be concentrated in the glue line instead of the timber, because you're, you'll see some defamation that you might not see visually, but you catch it by really slowing down the process with, by going image by image, frame, by frame.

[00:30:29] Wojciech Wegrzynski: and it was done on the, on what type of element beams slabs?

[00:30:33] Felix Wiesner: I mean, we've seen that for beams there that was beams that were constantly loaded and then heated. But similarly we have done, wall strips where we used device with higher temperature. So we use proper fire temperature. So we used a radiant panel with 50 kilowatts per meter squared on the surface of the timber.

[00:30:51] Felix Wiesner: And in that case, we also saw that, the deflections that we measured increased for one type of adhesive way before we even saw char [00:31:00] fall off. If we tested that with a reduced heat flux, we eventually got char fall off before we reached the structural failure. But, um, at a high heat flux we actually saw a structural failure before the char falls off.

[00:31:16] Felix Wiesner: That is an issue because, even if you can prevent the char fall off and therefore solve your fire dynamics problem, the glue might still be weakened. That's the case where I said earlier, actually, , by thinking about that, you can increase your structure, fire performance and thereby your fire resistance simply by the right, making the right choices in your product design.

[00:31:36] Felix Wiesner: Obviously it's informed by more than fire safety. If one of them is a hundred times more expensive, that's not point of choosing that because you would make the product non-competitive. If you put a chemical on the adhesive that turns out causes cancer. So these are all considerations that need to be considered holistically in the design of both the product of timber and the building [00:32:00] itself.

[00:32:00] Wojciech Wegrzynski: So when this structure element is exposed to fire, first, it will lose some of its structural by simply reducing the cross section because you lose some wood then. you would have this moisture traveling through the element that could impact the virgin wood inside the element or softer spots, then you start getting heat, not only moisture, but also heat near the glue line, where it might affect the ability of that glue to transfer loads and in overall impact that a load bearing of the whole element. and, then there's, process or char fall off debonding, whatever you call it it seems like the final act, You know, where everything has already happened to your, , elements and it, in this particular location of your cross section, it does not, have any load bearing capacity anyway, [00:33:00] and it just, falls off us as last suicidal act, in this, uh, theater.

[00:33:05] Wojciech Wegrzynski: Did I get it right? .

[00:33:06] Felix Wiesner: Yeah, kind of, like I said, the important bit is that the structural capacity of the glue line and therefore the loss of composite action yield happens much earlier. Like as soon as you start heating that glue line, you might already see losses. I'd like you said, you had a chair fall office, then the final symptom of that diesease. So that's, when your patient dies, but obviously, illness started much earlier and various symptoms that leads to

[00:33:31] Wojciech Wegrzynski: We're in the show business, you have to intro some fancy and metaphors here fire show business,

[00:33:37] Wojciech Wegrzynski: Uh, where are we with our modeling capabilities? like I'm a structural engineer, I would like to model my structure. I all of these things into account or my models today are not, fit to, to do that.

[00:33:50] Felix Wiesner: I mean, it depends on how comfortable you are with using standards because the standards gives you some calculation methods, like to see your strength layer, but[00:34:00] they don't really account for the glue effect on the structure yet. So then you have to make a choice. What do I know about adhesives? Do I know that it weakens in fire? Does it show us that if yes, I have to decide how much, if I want to be able to safe site, I can say like, okay, when it heatenns I can bounce it and say, okay I say at this point all composite actions is lost, and the idea or other cases are glued that doesn't lose any capacity and behaves like solid timber of the same thickness. And the reality is probably somewhere in between. So right now we probably just have , these two pounds.

[00:34:37] Wojciech Wegrzynski: in practical engineering, is this something that, for example, a structural engineering designing the biggest, uh, world timber building, should really play, or they should just rely on the manufacturer data with some margin of safety And, they will be good because it seems, if you, they can look at the history of, fire safety engineering, there were many great models, [00:35:00] methods, software's introduced, but in the end it's always the magic numbers that make the greatest career of them all.

[00:35:07] Wojciech Wegrzynski: So, , to what extent can a simple structural engineer, design or a fact that if in the end they just get the product from the factory. That's very hard to changed the way, how it builds the product.

[00:35:19] Felix Wiesner: I think one of the key aspects of any engineering or structural engineering, civil engineering education should be engineering ethics. And part of that is knowing your limits. So if I'm I'm not a mechanical engineer. And if someone tells me design this elevator , I will probably say, this is not my expertise. And I will go and find a mechanical engineer to help me with that. And so the, question is, if you are a structural engineer and you've work on a simple building somewhere, you're probably okay. Using standardized guidance, or following the manufacturer's recommendations because the consequences in a fire, if it's a one or two stories, CLT a [00:36:00] house somewhere, as long as you can guarantee that the flame dont spread to the neighboring properties, you get everyone out.

[00:36:07] Felix Wiesner: It's probably fine. But then if you work on the biggest timber building in the world and your structure engineer you're saying like, oh, he has the simple formulation. Should I use this? You should probably think again and, ask the building developer, can we get some money to hire a structure, fire engineer who has this expertise? And then, and then they can work with, with models that can question the magic numbers that can maybe the metric number will work. But, um, it's really about if you design a extraordinary building, it's about, demonstrating understanding. And really also, I assume if you work on a fancy prestige building, you get paid big monies, and then that has to be justified on some, on the engineering

[00:36:48] Wojciech Wegrzynski: Let's try now working out something, brainstorming something useful to the listeners. Let's say someone approaches you in your elevator rides to the [00:37:00] 50th floor of your headquarters. And they tell you that a wood is stronger than steel in fire, or they drop this one of these cliche arguments that you see, wherever a wooden building is being built at that charring layer magic, the fire resistance period is enough. wood is stronger than concrete, lighter than concrete. You know, all these arguments that, that come online all the time. What would you say to such a person cut the discussion? Give, a healthy dose of, of, doubt about that, but not destroy the enthusiasm because I ultimately think, I don't want to be seen, or I don't want us fire engineers to be seen as, uh, the guys who ruined the party and everyone is excited about wood and people would like living in a wooden skyscrapers. I would like to live in it's beautiful. It's nice. It smells nice. My laboratory after delivery of CLT smells [00:38:00] beautiful, which is not very often for a fire laboratory.

[00:38:02] Felix Wiesner: Yeah,

[00:38:03] Wojciech Wegrzynski: So I don't want to ruin the party, but also I don't want to push people into another Greenfield type of scenario where we realize there is an issue after multiple people die or the losses are so tremendous. It becomes obvious, you know, so let's brainstorm a, a good 60 second answer to someone.

[00:38:21] Wojciech Wegrzynski: What do you think?

[00:38:22] Felix Wiesner: well, I'm not sure if I could do it in 60 seconds, but, um, it's funny that

[00:38:27] Felix Wiesner: you I'll come back to the 60 seconds, but, just, it's funny that you mentioned the elevator, right? Because that's scenario a happens to be a lot when I read students' work. Um, obviously they also pick up the excitement of our timber and it doesn't even start on fire they will say timber is stronger than steel.

[00:38:47] Felix Wiesner: And then I forget to say, its strength to weight ratio might be close to what you can give you or even better, but that doesn't mean it's stronger than steel because strong is a material point on steel is [00:39:00] stronger than timber. We don't have to argue about that. So that's one thing people tend to forget and that, um, going back to the 60 seconds and, , the example of fire is really the main argument people say is that like fire affects our materials that say timber buckles, concrete spalls and timber burns.

[00:39:17] Felix Wiesner: They kind of try to create this false equivalency between these. Cause buckling is structural response, and I've seen a lot of timber structure buckle, so that isn't an argument to rule one out or the other. And really you have to treat all three materials not the same because they post different challenges and they're built environmental with timber. This is something that you discussed with Danny. The first issue is to find dynamics. And then in terms of structural capacity does different things because we have steel on concrete. we know much more about it because we've had big fires , in massive steel and concrete buildings.

[00:39:55] Felix Wiesner: And some, in some instances we have learned very painful lessons from that the World Trade Center [00:40:00] comes to mind. So yes, these can fail on fire, but we have learned these lessons. We have, , protective measures we can take, therefore we can build them maybe with a bit more confidence. we probably had big fires in timber buildings because you set, we have big timber buildings in Japan, but if you actually go to Japan and you look at these timber buildings always be a little information booklet next to it.

[00:40:23] Felix Wiesner: And it will say it burned down in 1789 and rebuilt. So these things happen, so that tend to destroy the whole building. So, yeah. And it's really about understanding material, knowing the limitations and dealing with it in a responsible way. I love working with timber. I've helped some of my colleagues do some concrete casts for their PhD and it's terrible.

[00:40:44] Felix Wiesner: You need so many people. It's messy with timber. , I mean, ideally you have a good relationship with the manufacturer. You get a pre-sawn to your site and all you have to do is put it in your rig and start. So as a researcher, I left him, but, and I love seeing it in buildings, but I also want people to treat [00:41:00] it with the respect it deserves. And the thing that annoys me most is when I see these articles in the newspaper where people just gloss over it with these arguments, that timber is slow to ignite. That's not the point, it's not the condition. That's the issue. The issue is when it starts to burn and then wont stop to burn. So all these things that, that we are not currently considering, I mean, another thing that we haven't even fully touched upon us is what happens after the fire.

[00:41:26] Felix Wiesner: We don't even fully know in terms of how much strength is recovered when the timber cools, how much more strength does it lose after the fire? Um, either the temperatures continue or the moisture content increases again, because it is a material that has an incredibly promotional content. So we'll lose, you get adsorption and loss of moisture with our changing temperature. there's all these unanswered questions

[00:41:53] Wojciech Wegrzynski: So maybe a good answer would be that, fires in, compartments for all types of structure would [00:42:00] look the more or less the same would have very similar challenges. But the difference is that after the contents burned down, the fire of timber structure may continue and it posts a completely new world of challenges, which were not explored in large buildings, built with non-combustible materials yet. And I guess these are the ones that we worry the most about,

[00:42:22] Felix Wiesner: yes, exactly. Also I guess to the way we assess the suitability of building materials because, I mean, I noticed there's a lot of discussion about the fire resistance framework. I don't really want to go into detail on that because that's not the topic of this episode. but in the end that timber, when you test them in fire resistance in a furnace, and I know you have done some work on that, fuel consumption is completely different.

[00:42:48] Felix Wiesner: So are you subjecting them to the same conditions? The same temperature conditions, but is it a fair test to use different fuel between them? And what does that mean for the application in real life?

[00:42:59] Wojciech Wegrzynski: Yeah, we [00:43:00] have raised at the concern also in the paper at workshop that John Gales organized, , like two or three years ago. We've took a statistics from like, I dunno, 300 fire tests done with different types of materials, like in combustible, combustible materials, the most interesting were fire doors, you know, because fire doors are fairly small elements.

[00:43:19] Wojciech Wegrzynski: possibly three technologies that you can build it, either steel with some sandwich material inside installation, or it's either aluminum frame and, fire resistant glazing or some boards inside them, or they're wooden that's, preventive technologies you'd have for doors, and even on a, such a small sample, you could observe the change in five dynamics of your furnace.

[00:43:38] Wojciech Wegrzynski: In short words, the fire of the tested element. when It caught fire, that's the element. It was almost enough to sustain the standard fire conditions in the furnace.

[00:43:50] Wojciech Wegrzynski: the fire in the furnace is self-sustaining, which is kind of worrisome. When you think about the purpose of fire testing and you get into this discussion [00:44:00] with, fundamental fire, , testing, , researchers, who focus on the equivalency of fire resistance tests and stuff like that.

[00:44:06] Wojciech Wegrzynski: And, it's only based on temperature comparison at your plate thermometers. That's pretty much equivalency that, the exposure conditions to which your sample was exposed to were equivalent among these tests. And if you consider only that measure. Yeah, that's true. The temperatures in the furnace were very similar between these tests. But I'm not very comfortable with the fact that my furnace behaved in a completely different manner between one and another.

[00:44:38] Wojciech Wegrzynski: And I would love to open this discussion in the fire community, because I think this is a discussion that needs to be taken the fact that we fire engineers understand this nuance difference between these tests. Maybe not even all a fire engineers may be just a group of fire engineers and the research just understands that, but it's a small nuance in the test, but in my [00:45:00] opinion, makes it a large difference. But to an outsider, to an architect, the structural engineer, who's not a fire safety engineer. says two hours, fire resistance, period. And, my has two hours fire resistance. My steel has two hours, fire resistance. timber element has two hours resistance. To me, they're all the same terms of how will they behave in fire? Because the number is the same. And this, is dangerous for us fire people, because if there is a huge fire in the timber building and there will be a huge fire in a timber building, that's, not a matter of if, but when and the timber was designed to have two hours of fire resistance, it will be on us.

[00:45:45] Wojciech Wegrzynski: Why it didn't resist fire for two hours, , because you fire people told us it resists fire for two hours.

[00:45:50] Felix Wiesner: Yeah, And. I think, you'll pick up on a very important point there. So you have these numbers and that's, you always see that 90 minutes, 120 minutes. And that's where people that are not be from the [00:46:00] fire community, like structural engineers or architects, they hold onto these numbers. Are that just for them, the justification that like, okay, if , that's the way that works, fire resistance does this the same number.

[00:46:10] Felix Wiesner: So I might as well use the timber, but then obviously, like you said, you have to fuel a contribution even from a small door. So it are scaled up to a whole compartment clad in timber. , Then that's the other issue is that if you test, for example, a CLT slab, things like the char fall off delamination that can all happen in a fire resistance test it doesn't change anything for the outcome. Or the chairing rate will increase. But you you do? not capture the effects of the longer fire dynamics that you would see in a real fire.

[00:46:38] Wojciech Wegrzynski: Not every, fire. test is load bearing as well. But, that's why I've invited you to the show, you know, to show the complexity of the problem and where the engineering in that truly is beyond just the seven mil safety layer, because, there is so much more to that.

[00:46:56] Wojciech Wegrzynski: I think obviously people will not, know how to engineer [00:47:00] timber from just this podcast, but I hope they will have, , a little better understandment where the problem in engineering is and what can be done, what cannot be done, what are the choices? So I'm really happy to have had this discussion with you.

[00:47:17] Wojciech Wegrzynski: Where are you heading now? What are you gonna do? Because I know you have some really exciting projects in this regard, in the future. So what people should look forward from you you know, there's high expectancies from your side because, Luke Bisby once said that if you want to read a good exemplary research paper, read the one from, from Wiezmo.

[00:47:38] Wojciech Wegrzynski: So I there's high expectations from the fire community towards, you now.

[00:47:44] Felix Wiesner: Yeah, well, I guess he had to say that seeing that he works a lot with me, and I think he was referring mostly to two graphics or plots that I do, so that's a bit, the actual papers are good, but,

[00:47:54] Wojciech Wegrzynski: fantastic.

[00:47:54] Felix Wiesner: uh, in terms of, , what I do right now, I mean, I'm actually still working on some of the [00:48:00] analysis from my doctoral studies.

[00:48:02] Felix Wiesner: So that's on the other structure, fire. Then actually here in Australia at the university of Queensland, my research is going, slightly different direction. And I'm looking a lot about the interaction of durability at design life and fire safety of timber.

[00:48:17] Felix Wiesner: That means what's how can we use fire retardants, , to increase, not only structural performance, but fire performance in general of timber, but also, one of the things we're looking at is, so you can use treatments to increase the durability.

[00:48:32] Felix Wiesner: How do they interact with the fire safety? of course your silver bullet would be a treatment, or, or the assign method that both increases fire safety and durability, but that's obviously needs rich research. we're looking at, , flammability and suitability of, , new, uh, timber species in terms of how can we use them?

[00:48:52] Felix Wiesner: which timber species can we use in wildfire prone regions? yeah, so I'm definitely looking a bit more on the [00:49:00] flammability side because during my doctoral studies, I really focused on the structure and the kind of, I measured the temperatures, but I didn't care that much about, the dynamics and the flammability.

[00:49:10] Felix Wiesner: And now I'm looking more into that and I'm actually learning a lot, which is very interesting and, , turning into into a proper fire dynamics, nerd and flammability flammability guy. Yeah.

[00:49:19] Wojciech Wegrzynski: That's a lot of but it's sounds really excitement.

[00:49:24] Felix Wiesner: it is so overwhelming because one thing I realized as a PhD student, you have a good, because you have your topic and you're focus on that and you might pick up a thing or two other side, but once you get into a board or off of a research phase where someone pays you a lot of money to come up with ideas and research, you're kind of managing a lot. Like, nothing is really in depth. Oh, you work with your students , and they are the creators and they really bring all the creativity and then you kind of try to guide them.

[00:49:50] Felix Wiesner: And, some days I still get to work in the laboratory, which is nice. yeah, because, I though, if I spent like a week in the laboratory, I'm happy to just sit on my desk with a [00:50:00] tea or coffee. And if I spent two weeks at the office, I just want to be in the laboratory again.

[00:50:05] Wojciech Wegrzynski: I'm really happy that this shift in your career, uh, happens for you. And there is so much more you can do now having these resources, students , and great collaborations. And this, complexity of your future research plan is a great testament to the complexity of the, of the problem.

[00:50:23] Wojciech Wegrzynski: so yeah, Felix, thanks So much for taking the invite to the show yeah, man. Cheers. We'll pick up this talk in the future for sure.

[00:50:33] Felix Wiesner: I'm sure it will, it will stay with us. And, and I think that's a lot of exciting research and development happening across the world. I mean, there's so many working groups that are working on this, so I think we're hopefully heading in the right direction. And like you said, we, we have to make sure that we take people along with us in that conversation. Architects, structural engineers. We have to have patients, I guess, not always easy, but yeah. , thank you very much for having me on here and hopefully it [00:51:00] will be helpful.

[00:51:00] Wojciech Wegrzynski: Thanks Felix. And that was a really great chat. And I'm very happy to had you on the show with this non easy subject of structural fire resistance of timber structures. I really enjoyed. Felix, explain to me the role of moisture transport in wooden products or in timber in general. And that's something I have never thought about that it may pay such a critical role.

[00:51:25] Wojciech Wegrzynski: And then again, when you think about connections or the glue lines, Parts of the structure that you could consider weakest link. Uh,weakening them through this mechanism seems like a potentially quite a dangerous problem. And I'm happy that someone is working on solving this. I also hope you've enjoyed how Felix, showing us the role of the structure of the cross laminated timber elements, the role of, uh, using, propper sizes of lamella the influence of different adhesives.

[00:51:56] Wojciech Wegrzynski: That's really fascinating that you can engineer this product to such a [00:52:00] degree and. Yet on the market, people just drop it down into a mass timber or CLT, and I will not lie. I did as well. Whereas this is a family of solutions, not just one of them. And, if there are so many things you can change to improve the product, it means we got some tools to work with.

[00:52:21] Wojciech Wegrzynski: And yeah, I really hope Felix's research goes into developing new solutions for CLT safety. And it will truly allow us , to make this leap towards sustainable development and use of mass timber in built environment.

[00:52:37] Wojciech Wegrzynski: So once again, thank you for listening to this podcast episode. I hope you've enjoyed it for the next week. I'm going to experiment a bit. I have recently been on a very interesting conference, uh, International Water Mist Association conference that was held in Warsaw. And I learned a lot about what a mist and, I've [00:53:00] actually chased some people with microphone. It was my first conference coverage. As a journalist, if I may.

[00:53:08] Wojciech Wegrzynski: And it was quite fun, I really enjoyed the conference. I really enjoyed talking to people on the site I have some thoughts about the research and technologies shown on the conference and I will bring it all to you next Wednesday. So, yeah, I hope you're looking forward to that Anyway.

[00:53:25] Wojciech Wegrzynski: Thanks for being here. Your hands. It's usually see you next Wednesday. Cheers.