Performance Based Fire Design for Mass Timber
Foreword by Ian Thompson, Editor
Today, we’re discussing fire design considerations in mass timber construction. One of the biggest misconceptions about mass timber buildings is that they do not perform well in fire. Although mass timber elements can be encapsulated in gypsum wallboard or a similar material to achieve the required fire-resistance rating like traditional timber framing, many designers prefer to leave the structural mass timber elements exposed because of their visual appeal.
Due to their inherent size, mass timber elements can in many cases be engineered to achieve the required fire resistance by using the initial charred region to provide insulation to the timber beneath. I believe we could term this a sacrificial layer.
While building codes generally recognize at least two prescriptive means of demonstrating fire resistance of exposed mass timber elements (char calculations and fire-tested assemblies), a third option not often considered in the US is performance-based fire engineering. This video includes a discussion with Matt Smith of Entuitive where they explore the design aspects of this approach and how it can be implemented on mass timber projects.
Performance Based Fire Design in Mass Timber Video Transcript:
Well, hey folks! Welcome back to another Two-Minute Tuesday. I’m Ricky McLean with WoodWorks. Today’s video is going to take a look at performance-based fire design for mass timber structures.
There are certain projects and certain instances where performance-based fire design could be advantageous to a mass timber project. So, for this, I’m going to bring in a short conversation that I had with Matt Smith with Intuitive. Matt is a fire protection engineer who works on many mass timber projects and has done many of these performance-based fire design analyses. Let’s jump over to that conversation and hear what he has to say.
All right, well, I’m joined today by Matt Smith, an associate with Intuitive. Matt is a fire protection engineer and structural engineer who works on many mass timber projects. And Matt, I wanted to bring you into today’s video to discuss performance-based fire design for mass timber projects. Maybe, for those who aren’t familiar, if you could give us a brief introduction as to what is performance-based fire design and how does it differ from a more prescriptive code-based approach?
Right, yeah, thanks, Ricky. Performance-based fire design essentially what we’re doing is we’re looking at the mass timber building and trying to actually demonstrate how does that perform in the context of fire. That’s looking at, can everybody safely evacuate? Can the building stand such that they can evacuate? How quickly does smoke and temperature accumulate such that we might have an unsafe condition? Is the building safe for first responders? So, it’s really looking at all of those high-level performance areas and actually demonstrating that we meet those for the building.
How that differs from prescriptive, if you look at the IBC or NBC here in Canada, the building code is really a set of prescriptive limits which tells us how might we meet fire safety objectives in a prescriptive manner, so what has worked historically to give us that level of performance. Whereas performance-based design, we’re not following prescriptive limits, we’re actually demonstrating how the building performs and proving that it is safe for every fire scenario.
When you’re proving how a building performs, what are the criteria that go into evaluating performance-based design? Is it timber exposure, size of building, occupancy, something else?
Yeah, I think it’s all of those. The actual process that we go through for this is called the alternate materials and methods process, which is in the IBC. But what the code asks us to do is it says if you want to deviate from the code, you just have to prove your performance is the same as what would have been code compliant. So, for the examples you mentioned, if we have a larger area, if we have a taller building, if we have more exposed timber, then we would be evaluating that specific timber building and showing that the level of safety or the performance is the same as something that would have been code compliant. If we use the story limit, for example, before we had the IBC 2021, whenever we were beyond six stories, that would be the alternate materials and methods. And so what we would do there is say we have an eight-story mass timber building, we would analyze that eight-story mass timber building and prove that it performs as good if not better than an eight-story code-compliant building, and that could be a concrete building with timber finishes in it.
And you mentioned the code path as far as an alternative designs approach, so I imagine there could be some potential risk within a with an approach like this, additional fees, also the building official needing to accept this. So, are those kind of things that have to be weighed on each project?
Yeah, definitely. And I think with more mass timber projects making their way through the approvals process, I think we’re seeing more building officials get comfortable with it, and that whole concept of the risk of not being approved, I think that’s sort of decreasing the more we see these work their way through approvals. And also, if we’re doing a mass timber building in a city where the official is not familiar with it, there’s always the option of a peer review, and the peer review is a very viable path to bring another engineer in, get a second set of eyes on the project, and help it go through that approvals process.
For sure, last question for you, Matt. Have you seen certain types of projects that tend to benefit more from performance-based fire design?
We’ve seen it quite often in academic buildings. The reason for that is the prescriptive height limit on assembly occupancy is quite low, and so if you want to do any sort of sizeable academic building, that is a performance-based design. Even in IBC 2021, if you’re doing a Type 4B building, that’s encapsulated mass timber up to 12 stories, but there’s only a certain amount of timber you can see, about 20% of the ceiling or 40% of a wall. Even now, we’re seeing that new code have performance-based design done to try and maximize how much timber can we see, and so we’re pushing the envelope on the amount of exposed timber in those spaces as well.
Got it, got it. Great. Well, Matt, thank you very much. This has been super informative. So, thank you.
Great. Thanks, Ricky.
Well, there you have it, folks. I hope you enjoyed that short conversation with Matt, looking into the nuances, the unique aspects of a performance-based fire design for mass timber structures. Comment down below, have you looked into a performance-based fire design? Have you done one for a mass timber building? I thank you so much for watching today’s video, and as always, we’ll see you back here on the next one.
Related mass timber posts:
What is Mass Timber’s sweet spot
How to Start a Mass Timber Building Design
Getting to Yes: Making Effective Use
of the Alternate Means Process
