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Sept. 15, 2021

018 - Engineered timber with Danny Hopkin

018 - Engineered timber with Danny Hopkin

Engineered timber is on a trajectory to become the construction material of the future. However, on that pathway there stands the fire issue. Wood burns, it is inevitable. This is something we must accept, and learn to work around. Common approach – determination of a char profile and the “healthy” section has its limitations, especially when applied to CLT products in which one could expect the glue line failure. And all of this is the topic of my todays discussion with Dr Danny Hopkin of OFR Consultants. Danny did his PhD on the subject of timber in fire and has participated in multiple interesting research projects on this topic. He is currently working with the Structural Timber Association on a STA-SIG CLT project on unravelling the fire behaviour of CLT, with its experimental part carried at the ITB in Poland.

If you would like to learn more about the STA-SIG CLT project, please visit the project website at:
https://www.structuraltimber.co.uk/sectors/clt-special-interest-group

Or read the introduction to the project by Dr Danny Hopkin.

Very quick and critical reading material:
STA-SIG CLT group - Compliance Road-map for the Structural Fire Safety Design of Mass Timber Buildings in England
A. Law - Burnout means burnout

Connect with Dr Danny Hopkin at:
https://twitter.com/DannyHopkin
https://www.linkedin.com/in/dannyjhopkin/
Danny.Hopkin@OFRConsultants.com

--- Useful links ---

This talk was a goldmine of resources, and here are all of them (I hope I did not miss out on something!)

Ronquillo G., Hopkin D., Spearpoint M., Review of large-scale fire tests on cross-laminated timber
Hopkin D., Anatsasov S., Brandon D., Reviewing the veracity of a zone-model-based-approach for the assessment of enclosures formed of exposed CLT
Hopkin D., Schmid J., Friquin K.L. Timber structures subject to non-standard fire exposure - Advances & challenges

Wegrzynski W. et al., The discrepancies in energy balance in furnace testing, a bug or a feature?

CROSS UK Cross-laminated timber (CLT) in multi-storey buildings
CROSS UK The risk of collapse of multi-storey CLT buildings during a fire
Angus Law and Luke Bisby, The rise and rise of fire resistance, Fire Safety Journal
We need to talk about timber - Angus Law gives the lecture for IStructE
Angus Law and Rory Hadden - We need to talk about timber

Wiesner F. et al. Structural capacity in fire of laminated timber elements in compartments with exposed timber surfaces


Transcript

Wojciech Wegrzynski:

"Get used to wooden skyscrapers. They're stronger, cleaner and fire resistant." That's one of the headlines. I, so in the past years, treating about the benefits of timber structures used for, very challenging engineering projects. And as most of them do they give you this hint that the timber is the material of the future. Because we have soft of the mobility problem and all of his other benefits are kind of obvious this kind of lines. And not, not because they are plain wrong because in some aspects, the wood can really outperform seal. It's just this cherry picking of a single metric and building the whole narrative around it. Feels like building something really not safe and not sustainable. And certainly it's not doing much good for the timber industry. If you want to learn, what do we need to do? You probably should start with the landmark paper by Angus Law and Rory Hadden. We need to talk about timber and I agree with them. .We truly need to talk about timber and we need to talk about it with competent people and has the kind of person I got. Today is a brilliant young structural fire engineer about to be editor of structural fire engineering handbook by SFPE um, he's done his PhD in timber in fire. So he's more than competent to talk about this subject. Today is also probably the best, um, rugby player among fire engineers and the best fire engineer among rugby players. So, yeah, please help me welcome Dr. Danny Hopkin from OFR Consultants. And let's talk about timber with Danny. Hello everybody. Welcome. I'm here today with Dr. Danny Hopkin from OFR consultants. Hi, Danny.

Danny Hopkin:

good morning. Hi Wojciech.

Wojciech Wegrzynski:

Great to great to have you here. So, Danny, how did you become a fire engineer? I heard that's a good story.

Danny Hopkin:

Perhaps I've oversold the quality of the story. If I'm honest. But as a teenager, my only real interest in the world was chasing a rugby ball around the field. But, uh, I grew up about five miles away from from silverstone circuit in the UK, which is sort of the home of British motor sport. My hometown is actually home to Mercedes GP. Growing up, the engineering thing for me was all about automotive and an aeronautics. I was quite interested in sort of moving into formula one, but through the wisdom of my dad, he suggested look, okay, you could go and study civil engineering instead because they might give you some kind of studentship because it's not as popular. And so that combined with the interest in rugby. Let me to Loughborough university, cause I could double up on sport and an engineering and one go. So I actually worked my way through a civil engineering degree over what was four years and did absolutely nothing to do with fire. I was completely intent on becoming a sort of a run of the mill if you like. And probably a, quite an average structural engineer. But my personal tutor was a guy called Jamala Romare and he was one of Ian Burgess and Roger Planks, first PhD students. And he worked on what was kind of the early version of the Vulcan finite elements, software that so many people now use for modeling frames and tensile membrane action and all that good stuff. So he got me to do a dissertation. It was the effect of heating, concrete structures. And that sort of led me down the path of, uh, yeah. Touch on a topic like that without seeing everything that Bre had done in that space at particularly a Cardington. And so I kind of last minute having had some offers to go into structure engineering, applied to Bre just on the off chance that they might take me. And they said, yes, and not only that they would they would fund some further research on me. So that's how I ended up doing my PhD, which was on the fire performance engineer, timber. And that PhD was completely a matter of convenience. I wanted to do something on steel and concrete but we had funding at the time to look at timber from some government research projects that we had. So sort of upset for the convenient option of being able to research something, but with a healthy pot of money to burn lots of different compartments. And so it was a series of almost accidental decisions that have gotten me to the point of being a fire engineer, which is probably a familiar story for many.

Wojciech Wegrzynski:

Oh, it's you would not believe how familiar it is. And it also confirms the theory of Kees Both that once you entered the fire lab, there is no, no way out.

Danny Hopkin:

Exactly. Yeah. , once I was at BRE I was kind of on this path to become a fire safety engineer. Albeit I've, I've kind of stayed mainly within the arena of structural fire safety, which is where I operate today at OFR.

Wojciech Wegrzynski:

Okay. So yeah, if you kept the rugby career, we would probably be on a different podcast,

Danny Hopkin:

I'd probably have, have quite serious concussions and would currently be I would be at the point of retiring if not having done so already looking for another job Right. But yeah, I think ultimately it was a good choice.

Wojciech Wegrzynski:

No. I'm very happy that you went into fire performance of timber. And and because of that, I can ask you some very important questions today. Like can jet fuel melt timber beams?

Danny Hopkin:

well, I'm not sure. I'm not sure that the metal thing is the problem. Yeah.

Wojciech Wegrzynski:

No, but yeah lets drop the conspiracy theories. Let's go into proper fire engineering. So, structural timber. It seems we've went the whole circle as civilization in a way turning away from wood when during the war, it was extremely easy to destroy whole cities by setting them on fire, and then some mass conflagrations that devoured whole cities a movement towards incombustible materials, modern materials of the time concrete and. And then the Renaissance of concrete in fifties steel structures, modern buildings build as extremely complex arrays of glass, concrete, and steel. And now all I see is can we build new skyscraper from timber? Can we go back to this? Uh, let's say old ways. It's not old ways because the timber is now engineered, but it's this in a way, a full circle. So what's the hype about? And why are we ending up in this point now and why it's inevitable. It's only going to increasingly in the future,

Danny Hopkin:

Yeah. It's I think it's for a few different reasons. The complexity of buildings that we're now talking about is substantially different to what we were originally building with solid timber back in that sort of pre-war period. Uh, I think the obvious motivation is the climate change. Yeah. We have a climate emergency. Everybody appreciates it. It's it's a massive challenge for humanity and the built environment is one of the biggest contributors to that carbon. And so when you're dealing with a steel or a concrete building, what you're actually you've got a huge amount of embodied carbon in those materials and actually manufacturing them. But with timber, you can tend towards net zero if no sort of negative carbon values because trees absorb carbon from the atmosphere as they grow. If you cut them down, you have to plant more, more trees grow, they absorb more carbon and you get into this, this nice cycle of of addressing this sort of ever increasing amount of carbon in the built environment. So there's the obvious sort of climate based motivations to build more with timber. And that's what I hear first and foremost as being a driver, um, but also kind of allied to that, particularly in the UK, we have, we have quite an old building stock, I suppose. And we have the option sometimes of knocking a building down and building a new one, or sometimes we can extend those buildings or we can build over the top of stations, train stations and things like that. And there's a quite a lightweight material. Adding extra stories to buildings with the timber is becomes a really good option for making better use of the space you have available to you. And not necessarily having to knock buildings down, you can extend them and refurbish them without necessarily paying massive additional forces on the foundations that you've inherited. Reuse and repurpose in buildings along with the climate aspect is definitely what's driving it. And then you've got the technological advances which have been huge when you've got a solid piece of wood you inherit a huge amount of imperfections. Is it's a natural material. You've got knots, you've got variations in how the grain develops, et cetera. And with engineered products, what you can do is you can kind of take the best strongest bits of wood you can glue them or bond them together. And you get a far more stable homogeneous material that, that is stronger and more predictable in terms of what it does. And that allows you to go to places that you can be previously. Things are cross laminated, timber, glulam, LVL they're all products that allow you to think about building tall buildings out of wood. It wouldn't have been feasible if you were just using solid sections of timber.

Wojciech Wegrzynski:

I'm, let's say large, um, advocate for the change in the climate policies and the way how we consume the resources of the planet, regarding timber seriously, I'm not sure if I, you know, building the next headquarters of a global mega corporation with timber, instead of concrete is the way to save the planet. Probably like getting rid of the corporations with maybe, but yeah. It is a way to, to test and promote new technology, which eventually going into mass use could make this a major difference in what we consume while building the buildings. But the second thing you mentioned, lightweightness of it, the ability to adapt the existing buildings, improve their features. That's really interesting because re adaptation of the buildings in changing the way they are used, improving them is much more cost-effective than building a second building next to it, to have to fill the same roles. I see there is a future in that, and this engineered timber products may, may really be an answer to the needs.

Danny Hopkin:

Yeah, absolutely. And you go back to the sort of embodied carbon discussion demolishing a building is not a particularly sustainable way to construct. You effectively have to replace a lot of that material. That's sort of already had a huge amount of embodied carbon. So, yeah, you've got some RIBA campaigns, Royal Institute of British Architects that are asking people to prioritize refurbishment and reuse over demolition. And if you can extend your building within the constraints of your existing foundations or whatever, it might be using something like mass timber, that's, that's obviously preferential to knocking something down. And to just to touch on that sort of global corporations point yes. There are a lot of organizations like Google, for example, that their headquarters in London is there's a hybrid building that skirts some CLT in it but actually mass timber can make a pretty big difference in, in sort of low to medium rise. Residential buildings where we're actually, we have a housing shortage, so we are going to have to build something and potentially using more mass timber in that less sort of glamorous space of buildings is where you'd make a true difference in, in terms of climate

Wojciech Wegrzynski:

There's also some improvements in the whole technology, how buildings are built. I mean the timber comes from a factory. I know that because I'm receiving a mass shipment of CLT next week, I know the logistics being behind unpacking 30 tons of wood now. But for a construction yard, that may be actually easier than unpacking 30 tons of concrete and pouring it upwards. The process seems streamlined, and since a lot of this comes preassembled, you get these benefits of factory accuracy in building the elements right in crafting.

Danny Hopkin:

Yeah, that's right. So something like CLT is arriving from an offsite manufacturing facility is quite often cut to them. The exact dimensions that you specify for your building. If you're dealing with some, I like London actually were sort of blocking the roads with lorries to unload stuff, is really problematic. If you're really constrained sites, then fewer deliveries with quicker unloading of those trucks is obviously a huge advantage that comes with, with building with timber. For sure.

Wojciech Wegrzynski:

Obviously there are a lot of pros of building building with timber. Now, the obvious cons is that a, it burns. That's quite a tricky challenge every now and then you see these articles in papers that wood is more fire resistant than steel or stuff like that, which you reach, I personally hate. But, I mean there, there is something, that wood burns. That's the point. And, we have to create, reasonable future with engineered timber products as a material for our buildings. We cannot pretend it does not burn because it burns. We need to find a way how to manage the risks around it. What's your point on achieving the acceptable risk. What is the path forward to have an reasonable risk, estimation and acceptance criteria that will let a person which is not engineered wood or risk expert, take a reasonable decision that, okay, this is fit for my project.

Danny Hopkin:

And that's a, it's a really huge question. I would start by saying we started building buildings that we couldn't necessarily evidence the performance of with mass timber week. We kind of ran before we could walk with, with some of the things that. very cellularized CLT apartment buildings have lots of exposed surface area are kind of the perfect storm for for a fire. This is not necessarily going to extinguish before we put for your structure potentially fails. So the first challenge I guess, is in understanding the remit of the common codes and standards that we apply in design. And I think a real challenge is the competency of the people designing buildings. So we have codes and standards that in their origin of come from non-combustible buildings that they were designed to deliver an adequate level of safety for concrete enclosures, for steel frames, for brick work, for other forms of masonry. And so a lot of concepts we have in building design things like fire resistance, as you mentioned are premised on the structure not becoming involved as a source of fuel. The very early mass timber buildings, particularly in the UK kind of assumed that those rules and guidance that apply for structures that don't burn can simply be extrapolated to structures where they're contributing as a source of fuel. And we know that's absolutely not true, and there's been some great papers have been written on it. You've got the "We need to talk about timber" a lecture by Angus Law and the corresponding paper in the structural engineer. So the first challenge has been in educating people and understanding. When they're applying a code or a standard, what they're getting and where the scope of that ultimately run runs its course and where timber fits into that discussion. and that's not been as easy or challenge to address as you might think. And I think that's for a couple of reasons. Structural or fire engineers to start with to my mind at least not actually that profficient at dealing with combustion problems. So a lot of fire engineering consultants at least kind of operate in this little bubble of I think Angus, Lou and Graham Spinardi, defined it as sort of code speak there, you develop an expertise being able to read back what codes and standards are telling you to do. So there was almost like a memorizing and interpretation of a series of rules and regurgitating those bats and design teams Which doesn't help you design mass timber buildings. You have to understand the fire dynamics quite well. And you have to have definitely a good understanding of the combustion processes, which unfortunately what's a lot of phone engineers are initially educated to understand this stuff. Industry can quite quickly almost turn them away from it and push them more towards codes and standards and becoming proficient in their application. So we've almost dumbed down a lot of fire engineers which has made it problematic to, to design these buildings. And then the other part of it is from a structural engineering perspective. Normally it's very hard to get structural engineers, to engage with the topic of fire. If you ask a structural engineer to, to specify the fire protection to a steel structure, you'll get some quite blank looks at time. The sort of principle with limiting temperatures and fire resistance periods are pretty alien to them, but actually timber is a space that historically structured engineers have felt quite comfortable meddling and I say meddling because it's sort of playing around the edges of a lot of the problems, because what we do historically is we kind of resolve the fire problem in timber structures to one, if a charring rate and a time and a bit of multiplication, and then ultimately designing what's left as if the char is to eat in a way some of your sections.

Wojciech Wegrzynski:

In a way, similar to how we would design a steel structure by giving it adequate protection. So the interior is safe from temperature and we'll have the load bearing capacity, right?

Danny Hopkin:

Exactly it says,

Wojciech Wegrzynski:

Or in concrete where you would calculate the depth at which your rebar is to make sure it does not receive a thermal shock that will make it lose its little bearing capacity. So in the way this simplified the problem to the same approach, to all types of structure, because eventually you need to understand what is the depth of your thermal protection being that external elements, char or mortar or anything else. And then your let's say healthy structured that has the capacity.

Danny Hopkin:

Yeah. You oversize your section and in principle, I guess it doesn't necessarily always need oversizing, but you made sure that there's enough material left at the end, after the char has eroded some of that section to make sure it can support the loads. And actually conceptually, a lot of structural engineers are on board with that and are quite happy to apply those methods that you see in things like Eurocode five. But the problem with that is, is kind of the charring process and the application of the fire resistance period. It masks an awful lot of complicated stuff that's going on. Charring is obviously a by-product of the combustion process and when the structure is burning, that's changing the enclosure fire. And that in turn brings into question, the very relevance of a fire resistance periods when you applied them to combustible structures. So, we've got sort of structure engineers thinking they have a huge amount of confidence in this space, but actually their kind of knowledge knowledges is pretty minimal. And so I liken it to sort of the, the Dunning-Kruger analogy, wherever it's a peak confidence, but also peak ignorance, somewhere applying charring rates to timber structures. And so a challenge has been to, to understand when can we apply that really simplistic approach. To designing timber structures, charring rates in fire resistance periods. And when do we need to do something more clever? And I'm really, that comes down to what your objectives for your structure up. So, we somewhat confusingly all under the umbrella of fire resistance, where we contain a myriad of different functions and objectives that we're trying to achieve. And when we've got tall buildings in particular where the consequences of failure are huge, what that fire resistance rating is doing is it's trying to be a proxy for the structure surviving the full duration of the fire or burnout as the term is commonly used. But in lower consequence buildings. So maybe your sort of single story, maybe two story buildings that fire resistance period is serving as a proxy of of giving you enough time, enough time to get out enough time for the fire rescue service to instigate some kind of intervention. And so under these fire resistance ratings you have different objectives. And objective of surviving long enough and objective of having an adequate likelihood of surviving indefinitely. And it's quite challenging to tease out what type of buildings need to survive burnout and what type of buildings need to survive long enough. And actually it's where the post-war building studies start to elucidate that problem. And they give you some different types of buildings that historically, at least surviving the fire has been an objective. And for some types of buildings, it hasn't been an objective. And what we've done in a project we'll talk about a bit later is set some guidance out that tries to steer people towards the kind of evidence they need to generate for their design, depending upon what their underlying objectives are. And that's just dealing with in the umbrella of life safety and health and safety. The discussion, around resilience and losses and insureability is almost entirely separate to, to that in terms of reinstatement and potential or estimated maximum losses.

Wojciech Wegrzynski:

For me, the whole concept of fire resistance as the proxy of structural fire safety is kind of difficult. Me as someone who's who was a deputy chief of a fire laboratory and we do fire resistance testing. And, we do hundreds of these tests every year. You take a look on how the structures behave in fire tests and you immediately realized that there is no equivalency between the results of these tests. And the approach is obviously the standard time temperature curve. And we know where it came from. And if you don't know where it came from, there was a podcast episode about that with John Gales. And you should listen to that to get some background on why it's so ridiculous to, to use it today as a proxy of structural fire engineering. But in the end, the assumption that the structure made of combustible material and noncombustible material that they have, the same fire resistance is fault because these structures will lead to completely different fire. And it's very difficult to quantify them with one set temperature profile. In fact, when you do that for combustible structures and engineered timber is certainly a combustible structure. Yes. Exposure is very unrealistic because sometimes the combustion of the specimen itself is sufficient to create the conditions within the furnace. That for example, for concrete wall, you would have to use burners to create. And this whole fire dynamics within the furnace, I feel it is something that was not necessarily thought out when the method was devised. And today okay. The test is passed. You have the same number behind your rating for one and another structure, but in consequence, their behavior in a fire will be completely different and there's quite a good chance. The fire will be completely different because of the material being used. So the problem we are discussing here is certainly not the problem is fire resistance measured as the standardized test outcome. Is the fire resistance of timber structures sufficient to replace steel or concrete or any other material, because it is. This is not the question. The question is how the fire environment of the building will change with the, with this change of a material. And for me, this change is quite a profound because you're completely turning around the fire dynamics in your, in your building with this movement, right?

Danny Hopkin:

Yeah, I think that the question becomes what relevance does far resistance have for combustible structure and when? to sort of elaborate on this a bit, there's a couple of things to consider. One is there's the fire resistance period itself, which we go back to Inberg in 1928, and I'm sure John Gales spoke about this. The original concept of a fire resistance period was this equating or there's very crude equating of energy. There's a principle of area under the curve and this, this principle that that your fire resistance period would to some extent, correlate with the fuel load density in a room. And so to apply a fire resistance periods, you have to in advance of the fire, know what your fire load is. And once that fire load is gone, then you can use the fire resistance rating as a proxy for surviving the fire. So, the timber structures where they're exposed is problematic because once they become involved as a source of fuel, are you no longer know what your fire load is. because it structure itself is a contributor. And then you've got the test. The test ceases to be a very good comparative across materials when the fuel supply, whether it's propane or oil or whatever, it might be, becomes a variable depending upon the material that's being tested. And so as you mentioned, now, you put a piece of wood in a furnace. It starts burning that burning is going to change the gas temperature in the furnace. And the response of the furnaces is to turn down the fuel supply to achieve the time temperature curve. And so in real terms, what you're, what you're doing is changing the incident heat flux from the contents of the room to the structural element as a consequence of turning that burner down. And so really you can only say with any great confidence. That fire resistance is relevant for mass timber. When you prevent it from contributing as a source of fuel. And that's where there's this term encapsulation starts to emerge of this idea that if you put enough plaster board or whatever, the product might be on the CLT or whatever the timber substrate might be to prevent it from pyrolizing, then you can be confident that you're working in a space where the only fire load driving the fire dynamics is the contents of the room. It's not the structural elements themselves.

Wojciech Wegrzynski:

From my experience with encapsulated timber structures in my furnaces, they behave exactly in the same way or very similar how would a concrete or steel with installation behave. It's the moment when the structural,element becomes the contributor to the fire when it all changes. You also mentioned is going to be the same but there are many outcomes of this compartment, fire dynamics. There's a chance that it, even though the wood will pyrolize and adds to the fuel, it will not have sufficient oxygen to burn in the room. So in essence, you just add more fuel, you have more fuel rich mixture but, the amount of energy released within the compartment is the exact same as it was before. But what happens is that you have much bigger fire outside, that's one voice in the discussion. On the other hand, you can have this fuel to increase the size of the fire within the compartment and change the temperatures inside, increased them and accelerate the fire in a way, because then the pyrolysis will be quicker. The destruction of the elements will be quicker. So it can also lead into this loop of, you know, accelerating the fire quite quickly. And on top of that, the wood is a very good insulator as well. And one thing that you have with concrete with gypsum plasterboards for example, they take a lot of heat out of the fire. Because of the processes within the materials because of their bulkness because of specific heat that these materials have, you need a lot of energy to heat them up. So they basically take the energy out of the compartment into the structure With wood you have the similar effect as with any well-insulated material that you don't need that much heat to heat up the surface. The heat is very slowly transported to the middle of the elements. So in the end, you don't take that much heat of the compartment as you would with the concrete. And this is also something very rarely quantified in the analysis and the differences can be quite profound in hundreds of degrees.

Danny Hopkin:

Yeah, I think what you identify there is a myriad of things that, that make building, with timber far more complicated than just addressing sort of structural stability in the event of fire. It can be common, can touch on virtually every facet of your fire strategy. You talk about the sort of the global equivalence ratio aspect of the problem where you have the external flaming cause you've got the access, pyrolysis gas is burning outside the opening. If you're wanting to build a tool building out of timber and you've changed your flame Heights, and as a result, you've changed your heat fluxes to the stories above that has implications for external fire spread, whether that's to a surrounding property or from story to story, fire spread. So you might have to put some other interventions in place to at least achieve some level of equivalence in the delay you're likely to have in story-to-story, fire spread, for example, and that's something that's often neglected. I think we've, we focused an awful lot on the structural part of the problem. What we're missing is all the various other fire strategy implications. So the fire spread part there that you spoke about if you're ceiling ignites, for example, that's going to change the the fire spread characteristics in the compartment. And that might have an impact on the occupants who are escaping and the extent of burning in the compartment could also make for a far more challenging fire for the fire rescue service to intervene with. And so we might have to think more carefully about the facilities we provide for intervention. When you move towards a combustible framing solution, you have to do a very thorough review across all facets of your fire strategy in terms of the implications and show that you've addressed them rigorously.

Wojciech Wegrzynski:

So it's what you say this complexity of the thing, how big the changes are. Does that mean that, you just cannot put it under the simple compliance of ADB, right?

Danny Hopkin:

Yeah. In general terms, I think you're right. I think what we've produced and we have a project with the Structural Timber Association, which is funded by the, kind of the big three CLT suppliers KLH, Storaenson and Binderholtz and an output from that project is to try and help designers early on. From a structural perspective, not from a full fire strategy perspective to understand what evidence they need to produce as a designer to satisfy everybody that the level of safety they're achieving is appropriate in the circumstances. So we call this the sort of compliance roadmap. And you go back to the post-war building studies and the origins of ADB. And as I said, at the beginning, you've got fire resistance as a principal is spanning multiple objectives. So while you're, whilst you're in the domain, a fire resistance being a proxy for buying you some time to get out allow firefigther intervention, then there may be fire resistance testing and fire-resistance periods. Isn't a bad starting point for that objective. Obviously it says it's going to change the fire dynamics and it's not going to be the same for a steel structure versus a concrete structure versus a timber structure in terms of what you're ultimately getting. But what we've shown over the years and an Angus Law and Luke Bisby got that great paper on the rise and rise of fire resistance. Is it generally we've started from the point of view of what do we need needs to achieve or to survive burnout. And all we've really done is just keep ramping the fire resistance up. So in a lot of these numbers in guidance, we have a fair amount of meat on the bone in terms of, where that uncertainty can go in terms of the conservatives and in those fire resistance periods. I think where it becomes really problematic is where you're dealing with a taller building where your objective is surviving the fire and surviving the full duration of the farm. And what that means is if you want to apply Approved Document B, as you referenced, then you've got to put a huge amount of plasterboard on your structure to prevent it from contributing as a source of fuel. That's the principle that has to underpin applying that fire resistance period. If you want to do anything else. If you want to expose your structure, if you want to partially protect it. So your plasterboard is potentially only gonna survive partway through the fire. What you're accepting is your structure is going to become involved as a source of fuel. You invalidate the fire resistance period in terms of it being a proxy for surviving burnout. And you have to show a, when all is said and done, when the fire load in the room is consumed, that the structure self extinguishes or undergoes also extinction. And that what's left in terms of the residual section can support the loads. So the structure is still standing after the contents has gone in the structure itself has stopped burning and that's where you'll find some, some great work on auto extinction by the likes of Alistair Bartlet from the university of Edinburgh Rick Emberley from a UQ and Carmen Gorska have all been working in this space over the last sort of fiveor so years.

Wojciech Wegrzynski:

Surviving of the structure is not that it did not collapse within the certain amount of time. It's also means that it can be fixed and used once again, after the fire. Right?

Danny Hopkin:

Well, I mean, fix and reuse is where you kind of, you move from life safety, into resilience and recoverability. So I would say purely from a life safety objective, and that's not to say is societaly accepted or to have to demolish a building after a fire, but from a life safety purview, it standing up at the end. After the fire brigade of intervened is arguably enough to satisfy that very narrow focus of where the regulations currently put their attention.

Wojciech Wegrzynski:

Okay. Understood. Now coming back to the physics, you mentioned the autoignition, and previously you have also mentioned the we'll use of char as in a way thermal insulation for the healthy timber inside the structural element. But now the engineered timber comes into the market. And the difference between engineered timber and the natural mass timber is that It's structure is much more complex. You have layers, which you may lose and lose the chart insullation with them in the process we call delamination. And I assume for technologies where you use nails or other doubles or something, similar things can happen where you lose chunks of all the structure, and then suddenly expose that.

Danny Hopkin:

Yeah. Let's call it glue line integrity failure overdelamination. So I think delamination has this sort of bad image of huge surface areas, peeling from the underside of the slab. And I don't think that's what we're really talking about here. What we're talking about is sort of the char losing its adhesion and, and ultimately detaching. To my mind, at least there's the glue line integrity failure part of the problem is secondary to the fire dynamics part and that you have combustible surface area, and that changes the fire dynamics and what the glue line unpredictability doing , is just introducing some complexity into an already complex problem. I think to some extent we're developing a bit of an adhesive obsession, or almost an, a decent apathy where, if we think we can prevent glue line integrity, failure, then we've solved. And I don't think that's true. I think even if you, avoid, that integrity failure, you can still have a bad outcome in terms of your compartment, not self extinguishing. And if you go to something like Carmen Gorska's PhD, you see that she, she found in her bench scale experiments, a tipping point where, over a certain percentage surface area exposed, the compartment can continue to burn, even if delamination or glue line integrity failure happened. it's a real challenge in terms of the unpredictability and what that detachment does, because as you mentioned, a key thing in the self extinction part is is that kind of energy balance or the pyrolysis from, so you have an external heat flux that's received at the surface of the word and you have some losses and convection and ranges of losses at the surfaces. There's not all of it as absorbed and in a certain amount of that heat flux makes it through the char layer to the pyrolysis front and then some of that is absorbed or conducted deeper into the section. And what we know from these PhDs I mentioned previously is there's kind of a critical mass loss rate for that flaming combustion to continue . And what we know is that timber heat source, but if you lose the char layer, then what you're losing is a fairly significant term in that energy balance in terms of how much energy is absorbed and how much is ultimately making it to the pyrolysis front to continue to produce those combustible gases. So, I think the importance of the char fall off very much depends on the configuration of your compartment. The energy balance dictates, I think, and I think we've proven it in large-scale experiments that people will get to read about hopefully in the next few months that if you limit yourself to a single exposed surface, particularly a ceiling and you have a glue line integrity failure, you can still have a good outcome in terms of if the structure self extinguishing in terms of flaming combustion, smoldering is a more complex matter. But in terms of the flaming combustion you can have self extinction or two extinction after delamination has happened, even with some quite extensive delamination but you can, if you want to, you can remove some of that uncertainty or say remove, you can mitigate some of that uncertainty about the glue line integrity failure by using different types of adhesive. Traditionally, or almost exclusively up until fairly recently as adopted a polyurethane based adhesive, which softens at relatively low temperatures. And actually you can get that detachment of the char layer before the char has reached the glue line. We're talking sort of anything, 150, 200 degrees where without glue starts to degrade. So some attention has gone on to improving the, the sort of thermal characteristics of that adhesive. There's a few different options. You can not use a polyurethane based adhesive there's alternatives like melamine, urea formaldehyde, or PRF which don't degrade to the same extent at high temperature. So they, they kind of perform in terms of stickability terms, very similarly to the char layer itself, but they don't prevent a char fall off. Delay it or increase it at the temperature at which it happens or reduce the extent to which it happens.

Wojciech Wegrzynski:

But that also means that by delaying it, you're also creating a situation where what you exposed after all is not, like, fresh wood that's, about to ignite, but you would expose something between a paralyzed wood to a char.

Danny Hopkin:

yeah, th there's that component and that's the time component that if you, if you delay the char fall off, then hopefully you're at a point in the fire where the heat flux is to the surface substantially reduced.

Wojciech Wegrzynski:

Okay.

Danny Hopkin:

So there's that, and then there's been attention on modifying polyurethane based adhesives. The perceived problem with the, MUS and the PRS is the formaldehyde component. You've got a nasty chemical, which I'm kind of selling that as a solution in buildings, in which people spend an awful lot of time is can be quite challenging. So, it also comes with complexities in the manufacturing process of the CO2 itself. You often have to heat curate and so polyurethane is attractive for lots of reasons it's here. So it's a nicer material for human beings to be around. And also you can produce your CLT more quickly, which is important given the demand that you currently have on the product. And so, for the U S market, there was, modified polyurethane, develops is called HBX is a proprietary product by Henkel and what that has shown in some experiments that we've done and experiments by others. It reduces the susceptibility for that glue line integrity failure. It doesn't remove it altogether, but it definitely delays it. And it seems to have some interesting qualities in terms of what it does to the temperature profile, through the wood which we're quite keen to explore.

Wojciech Wegrzynski:

And what about the encapsulation strategy? Because you've mentioned for the compliance with ADB would need a whole lot of, encapsulating boards to, to cover the destruction, prevented completely from taking an active part in the fire. And is there some middle ground where I didn't know, we can delay the moment where the timber becomes a part of the fire and that's sufficient?

Danny Hopkin:

It's a similar sort of principle and what the encapsulation of the lining is doing in that case is hopefully. staying intact long enough to get you to the point in the fire where they, the contents of the room and the heat flux and the contents of the room is significantly reduced. Your strategy would be to impose that delay in terms of when the timber is involved and ultimately, hopefully by providing that encapsulation mitigate the prospect of that glue line, integrity failure as well, because you've insulated the surface until ultimately the timber becomes exposed. And hopefully by the time the contents of the room and the structures potentially self extinguished that critical temperature of such a thing exists the adhesive line hasn't been attained. So it's another strategy in delaying the propagation at temperature through the wood.

Wojciech Wegrzynski:

From, from this discussion, I really enjoyed the fact that there is some true engineering in that. And we often said that, we need to engineer this timber for buildings. I didn't really understand to what extent are we really able to engineer this because timber is timber. And from here, there's a lot of sound solutions that actually change this dynamics or the consequences of such a fire. Now let's, let's forget Danny the scientists let's talk with Danny, the consultant and how do you, how do you sell that to an architect who would like to have an exposed wood surface? Because it's beautiful. Is it even possible to maintain aesthetics of a wood and create it into something safe?

Danny Hopkin:

I would argue it is provided you work within a pretty tight envelope. So we as OFR particularly dealing with taller buildings we have a series of, of ground rules or, or kind of an envelope that we're comfortable working within, where we feel like the, the kind of the auto extinction problem is, is simplified to the most simple version you can get in a building, which is a single surface, typically a ceiling. And what, I'm a great believer in, I, I feel like when we talk about we're going to build the world's tallest timber building, it's going to be the biggest, the largest. It's going to have the most volume that they're kind of vanity projects. They're not necessarily projects that are a good engineering. Um, and the good engineering comes from using different materials where they work best. So, we are very supportive of hybrid structures. Whereby you use things like steel for your vertical load paths. If you want long spans, you use, steel beams, and actually you make use of the CLT where you might have a concrete slab. So a lot of our buildings will be a mixture of materials. So it might be a steel frame with a CLT floor maybe even a concrete core. And then what we've got is a, is in a nerve combustible frame that can demonstrably survive the full duration of fire. It's not contributing as a source of fuel. And you've got a single surface where in most cases, as we've evidenced for our large scale testing, even if you have a glue line integrity failure, you can have that auto extinction outcome. And that's kind of where we stay. That's that's we identify where the big uncertainties exist and as consultants, we don't go there until somebody has generated that knowledge and has helped solve that problem. What we do separately to that is we work hand in hand with a lot of academic and research institutions to try and understand things that might open up more opportunities in the consultancies part of the business. So I know you've had Colleen Wade on who who's spoken about our collaboration on B risk and how we've started to build a zone model where that combustion of the enclosure is a source term in enclosure energy balance. And we can start to use that and understand that in terms of how it might enable us to understand, enclosure fire dynamics and also the external flaming part of the problem. So that's one sort of investment in time that we've made. Outside of that, we've currently who funded two PhDs. We've got one at the University of Edinburgh, really bright, a nice student Antonela oli who is looking at the adhesive behavior specifically, and actually wanting to understand the degradation of the adhesives that we commonly use in CLT. So she's been supervised by Luke and this is where it's at the moment having a placement and we've, also got a student at the University of Sheffield. Who's kind of looking at it from a more structural mechanics perspective rather than the sort of five dynamics perspectives. They're more in terms of how can we model the structural response of CLT and in hybridized systems and in particularly with a view to almost like how you might incorporate that into a tool like Vulcan potentially, as a additional functionality. So, we accept, we don't, we don't have all the answers to build all the buildings. Every architect could possibly want. But we are quite robust in our positions in terms of where we think they should go and where we'll support them and going. I think that's an important part of why we kind of get the projects that we do is, we're very transparent and honest upfront about what, what you're doing and what you're selling.

Wojciech Wegrzynski:

I think, here you, you show the pathways, you know, if you want, a net negative carbon building with the whole structure made of engineered timber elements, there's a way to encapsulate them and make sure they do not participate in the fire as a source term. If you own the aesthetics, you can settle on the exposed ceiling and with a single surface and the knowledge we have, it's possible to create this as a safe solution. If you want a combination of both, maybe there are ways to compensate for the loss of safety due to the, do you do these choices by altering? I didn't know the whole building strategy. There are tools that, that we have, so. It truly, is an engineer's way, you know, to create a safe building. You just have to follow that and not fall into the equivalency of fire resistance solves the issue. And we don't care anymore because the number behind my letter is, as the law says it has to be. So, so that's truly promising because that's what engineering should be. Yeah.

Danny Hopkin:

Yeah. You have a huge amount of architectural parameters that you can play with that kind of drive that extinction outcomes. You want huge openings. You want to maximize your regulative and convective losses from your enclosure, but you have to balance that against the kind of the exposure hazard, the space separation hazard that might impose on your neighbor and the benefit of a big ventilation area is also your external flaming is significantly reduced because you're unlikely to have a ventilation control fire, or it's going to be substantially less ventilation control. So that's one parameter that is within the control of the design team. You have the amount of combustible surface areas and their proximity to one another is another important parameter that you can control. Or if you if you don't want to. Or if you're concerned about that extinction outcome, you might fully encapsulate some walls and prevent them from becoming involved in and strategically exposed parts or some of other surfaces, depending upon how close they are together. So we're definitely getting a handle on the things that drive the problems. But I can't say that we're all the way there. And I think there is a duty on those that want to use the materials and know selling the materials to make a pretty big investment in research, and fair play and credit where it's due to the CLT suppliers. They've, they've made that first step. And they're allowing us to start to develop this envelope that we can work within. But I think if people genuinely want to expand that envelope, then the investment needs to be made on, on far more research than is currently being undertaken.

Wojciech Wegrzynski:

And I think it's really worth investing because the engineered timber is just the tip of the iceberg. We are entering a promising world of engineered bamboo products. There's engineered hemp products that can be used as a structure, elements may be in the future. This race for sustainability and finding new materials, for our buildings is really accelerating. And it seems that all the new materials have the same let's say flammability issues. It's like when, when you discover a magical material that solves all of your problems, it's suddenly it's either killing you biologically or burns for us. For some reason, we were not having great luck in there, but it's great that people like you are finding solutions. And hopefully what we learn now about engineer timber, because you know, this compartment, fire dynamics discussions, these opening factors and these, participation within the fire and at what stage of the fire, the material participates, it's not just engineered timber it's absolutely universal for any material you would have in the building. And I think this fundamental discussions are maybe that we should have started at that point before we started running, as you said, we've run before we walked, but I'm happy that we can now contribute to this fundamental knowledge and with every experiment, with every PhD, with every paper, we know a bit better and we gain confidence in how to deliver safety in such buildings.

Danny Hopkin:

yeah, sustainability. Absolutely, can't be at the expense of safety. And this is the common argument that's made that the fire engineers are kind of becoming an obstacle to this sort of sustainable utopia but a disaster because we got a design wrong and ultimately sort of the legislative landscape being what it is, probably a ban on that material as a consequence of such a disaster. That's not sustainable. If you want to use these materials and for them to have some kind of longevity, then you've got to understand what you understand. You've got to understand what you don't understand most importantly, and you've got to start to fill those knowledge gaps with research. And if you're not willing to do with that, then you've got to operate within a quite strict bubble of what we currently understand, which might mean lots of plasterboard, or it might mean quite severe limitations on how much you can have exposed.

Wojciech Wegrzynski:

Actually, a lot of plasterboard is not a bad outcome because the, at least to safety, I'm more worried about forgetting about the issues because the tests said it's okay. If you judge the fish by its ability to climb a tree, it's not a really good climber. Right. And I have a feeling we're using fire resistance test to test, firewood engineered products. And it may be not the best measure to actually quantify complexity of the fire behavior of that. Okay, man, For some parting words where are you heading now with your research on engineered wood? Because I know it's, not finished and I know there are exciting things on the horizon.

Danny Hopkin:

Yeah. So this Structural Timber Association project that I mentioned has been running for a good 18 months, at least. So we started out with this compliance roadmap, trying to give people good information about what evidence they need when and why. And working with ITB we've just completed a program of what was for large scale, kind of office type compartments was what we're reigning for looking at extinction and the relationship with glue line integrity failure. And actually getting some really good data in terms of heat fluxes to different surfaces, heat fluxes, to the floor in advance of the fire and starting to understand how fires might spread through large compartments without necessarily having to undertake absolutely massive compartment tests. We've got really good data in terms of, heat fluxes that will allow us to make some estimates of what spread rates are likely to be three compartments. And also to do that with different adhesive types. Because it, I think it's all well and good sort of advocating particular types of decent, but if they're not commercially available or, the supply rate is fairly limited at this point in time, then we perhaps have to look at different ways of approaching the problem with lamella thickness and perhaps not so heat resistant adhesives. So that's allowed us to sort of look at that those different options to attacking that problem. And then the say names or the in effect ban has done in England, for residential buildings, it's pretty much killed a market where you can build residential buildings out of mass timber or anything combustible, frankly. And that's quite a reasonable position. I mean, there's, there's a huge lack of trust in people generally to use combustible materials in buildings where people live and sleep. But as I said at the beginning, I think the timber in the residential space is potentially quite important. Cause it's where the significant amount of buildings stock resides. So if you really want to use the material to impact embodied carbon that's where you'll get the most bang for your buck. So we want to do some experiments and again, they're going to be with ITB. We've got nine, enclosure experiments planned, where we were actually going to start to, to try and understand and predict. Firstly that the sort of the plasterboard failure part of the problem, because we talk about encapsulation, just riding out the fire and it'll be there at the end and the timber will be unaffected underneath, but that kind of understanding plasterboard failure is sort of the fire safety engineering, holy grail in many respects, it's not a problem. That's that's well understood. I tried it a bit with my PhD and it's a real minefield in terms of understanding how that degrades and how it detaches. So we need some data in that respect. And also we need to understand if allowing that plasterboard to fail when you've got pretty much every surface in your enclosure formed of a combustible material, whether that's even a realistic proposition or whether we accept that, actually, if you're going to have multiple combustible surfaces and the solution is always going to converge on something with an awful lot of encapsulated surface area. Or can we optimize that to some extent, whereby actually some of our plasterboard is failing during the fire. We're exposing some of the wood, but it's not becoming exposed too early. So it was to have this sort of a never-ending cyclical jar fall off re-ignition and ultimately the structure

Wojciech Wegrzynski:

until it burns down. Yeah.

Danny Hopkin:

Exactly. So that's where we're going with that. And there'll be some publications in the pipeline. I'm sure when all is said and done.

Wojciech Wegrzynski:

And I'm more than excited to burn these nine buildings for you. I'm little less excited about unpacking 30 tons of CLT. You have kindly shipped to my laboratory, but yeah, first let's learn the logistics of working with CLT and then let's learn how to build the buildings with CLT. And then we're gonna learn how to safely burn them in a way that does not lead to severe structural failure and I really hope to to see how it goes. And I think we'll come back to this topic some time later when did all the data is processed, when the conclusions are formed and even the clearer pathway is available. And for now, if an engineer seeks guidance where they should look for it what would you recommend?

Danny Hopkin:

So Cross UK is publishing really good articles on from, from a sort of legislative and guidance standpoint on where timber sits in that sort of definition of a common building situation. So I definitely recommend the Cross reports. There is a summary article that we wrote in SFPE Europe on the compliance roadmap. If you don't want to read a fairly lengthy report on the STA website. But there's no there's no specific sort of design guidance or tools available at the moment. We're in that period where the current Eurocodes don't cover such things. The future Eurocodes might cover them to some extent, but really it's going back to what we understood about timber many decades ago, you've got sort of the Rasbash fire point theory. You've got the various stuff in Drysdale that is there, and we've seemingly ignored for an awful long time about the fundamentals of how timber burns. And we've, I bet we just seem to either ignore its existence or just forgot it was there and forgot the lesson. And went for the convenience of chairing rates and fire resistance periods. So I definitely definitely advocate those and the theses and the publications of the few people I mentioned

Wojciech Wegrzynski:

I will link all of them in the show notes. So people know where to go from. And for the final thing, congratulations on the Structural Fire Engineering Handbook. That's, about to reach the surface, man. That's a, that's such a huge project and maybe even separate podcast episode.

Danny Hopkin:

Oh yeah. You should say you can get Kevin and I, to give you the, the lowdown on what's

Wojciech Wegrzynski:

How, to write the handbook,

Danny Hopkin:

how to write, well, maybe how not to write a handbook would be, we, we can give advice on all the mistakes we made.

Wojciech Wegrzynski:

The bad practices in the handbook writing that. So that will be a popular episode.

Danny Hopkin:

Exactly. Yeah. It's, it's something we embarked on. I think we officially started about two months before the Grenfell Tower fire and then you combine the outcomes of that for, fire engineers around the world with COVID mixed in there as well. And yeah it's been a long time coming.

Wojciech Wegrzynski:

Well I'm glad it's reaching us. And I cannot wait to get my hands on that. So, congratulations once again, and, thank you so much for joining Fire Science Show. And, it was a pleasure to have you here, Danny.

Danny Hopkin:

no problem. Thanks for having me, Cheers

Wojciech Wegrzynski:

This talk was really rejuvinating for me, not because the issues are solved or I feel much safer about timber buildings today, but Danny showed me clear pathways on how to solve engineering problems. And no, every time we hear that, we, you need to hire a competent fire engineers to design infrastructures that you should look holistically, timber buildings and fire and stuff. But it's really difficult to understand what people mean with that. Where should we go? What kinds of holistic approach should we actually implement when there is the flammability issue, which is obvious, and you somehow have to deal with that. And I'm really happy they showed this. Approaches with encapsulation, partial encapsulation, some technical solutions to prevent a glue line, , failure, some aspects of compartment file dynamics that can be taken into account. To, to really understand the scale of a problem. You're creating by making a building a compartment, part of a building, uh, from engineered timber. And once you understand the risks, you can compensate for them or find solutions, or maybe just agree to the risk that you have and, and live well with your building. That's, that's engineering again. That's why I became one of engineers actually. So, I'm really happy we had this talk and, uh, Danny also mentioned a lot of resources that are there for you. We certainly don't have all the problems worked out. We certainly need more knowledge. We need more fundamental research. We need more applied research. We need more case studies and understand and, uh, impact the structural timber can have on the built industry. But there are resources. There are resources. All the PhDs mentions. All the papers mentioned are linked in the show notes. There are some great videos for you to watch, and I hope it becomes a nice resource hub for people looking for some safety advice. And, uh, yeah. Also look forward to the, uh, Handbook of Structural Fire Engineering, Danny and Kevin LaMalva, and the editors of this handbook. It's going to be live soon ish. And it's this one I've been told is quite comprehensive. So I cannot wait to have my copy and dig into that. And so that's it for today's episode. I hope you enjoyed our little chat about Denver and fire and yeah, as usual. Thank you very much for listening. Make sure to share the knowledge about the podcast with your colleagues and, as usual, see you next Wednesday. Cheers.