079 - Timber columns failure in the decay phase with Thomas Gernay and Jochen Zehfuss
12/07/22 • 47 min
When the flaming combustion stops and the raging inferno disappears, the environment is still far away from a stable, stationary state. The heat emitted by the fire and accumulated by the structural elements is still on the move, travelling through the members until it gets eventually dissipated. As parts of the structure get heated, some processes will occur, that may influence their load-bearing capacity and other properties. This is nothing new, we recognize this as an obvious process within the so-called "decay" phase of the fire.
What is new, though, are some recent observations related to the behaviour of timber elements in this phase of the fire. Today's guests Thomas Gernay and Jochen Zehfuss (along with a team that I call EU Fire All-Star Team) have performed a very precise study in which they have shown on one example the exact conditions in which the load-bearing capacity is lost in the decay phase by a column. If you missed that, they made quite an impression on LinkedIn (check the post and discussion here). In their experiments carried out within a well-controlled furnace environment, the variable they played with was the duration of the heating phase. It allowed them to find out two separate behaviours - one in which the column collapses in the decay phase, and one (not very different) in which the collapse does not happen. To learn more, please join us in the episode, and for sure - read the research paper provided in here.
If you would like a quick insight, I will also steal some text from Thomas's post on LinkedIn, as he did a great job summarizing their research. So here is his short comment:
"Two of the columns were subjected to ISO 834 heating until failure. They failed after 55 and 58 min (-> standard fire resistance).
Two other columns were subjected to 15 min of ISO 834 heating followed by controlled cooling. Flames self-extinguished after 40 min. But the columns still failed during the cooling phase, respectively after 98 and 153 min.
The load on the timber columns was constant throughout the tests. What changes between 15 min (end of heating) and 153 min (failure)? Heat transfer. The temperature of the inner parts of the column section continues increasing. Hence the strength continues decreasing.
Flaming and charring are not necessary for this inner temperature increase. And the absence of flaming is not a good predictor that the column is safe (see video).
By better understanding these phenomena, we can design to account for them - and achieve safe and resilient timber designs, including for burnout resistance when needed. Numerical modelling can support this objective. But simple methods based on charring rate fall short because they don't account for the slow heat transfer processes during the cooling phase."
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The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.
When the flaming combustion stops and the raging inferno disappears, the environment is still far away from a stable, stationary state. The heat emitted by the fire and accumulated by the structural elements is still on the move, travelling through the members until it gets eventually dissipated. As parts of the structure get heated, some processes will occur, that may influence their load-bearing capacity and other properties. This is nothing new, we recognize this as an obvious process within the so-called "decay" phase of the fire.
What is new, though, are some recent observations related to the behaviour of timber elements in this phase of the fire. Today's guests Thomas Gernay and Jochen Zehfuss (along with a team that I call EU Fire All-Star Team) have performed a very precise study in which they have shown on one example the exact conditions in which the load-bearing capacity is lost in the decay phase by a column. If you missed that, they made quite an impression on LinkedIn (check the post and discussion here). In their experiments carried out within a well-controlled furnace environment, the variable they played with was the duration of the heating phase. It allowed them to find out two separate behaviours - one in which the column collapses in the decay phase, and one (not very different) in which the collapse does not happen. To learn more, please join us in the episode, and for sure - read the research paper provided in here.
If you would like a quick insight, I will also steal some text from Thomas's post on LinkedIn, as he did a great job summarizing their research. So here is his short comment:
"Two of the columns were subjected to ISO 834 heating until failure. They failed after 55 and 58 min (-> standard fire resistance).
Two other columns were subjected to 15 min of ISO 834 heating followed by controlled cooling. Flames self-extinguished after 40 min. But the columns still failed during the cooling phase, respectively after 98 and 153 min.
The load on the timber columns was constant throughout the tests. What changes between 15 min (end of heating) and 153 min (failure)? Heat transfer. The temperature of the inner parts of the column section continues increasing. Hence the strength continues decreasing.
Flaming and charring are not necessary for this inner temperature increase. And the absence of flaming is not a good predictor that the column is safe (see video).
By better understanding these phenomena, we can design to account for them - and achieve safe and resilient timber designs, including for burnout resistance when needed. Numerical modelling can support this objective. But simple methods based on charring rate fall short because they don't account for the slow heat transfer processes during the cooling phase."
----
The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.
Previous Episode
QA#1 - November 2022
Welcome to Questions & Answers session 01 covering the topics brought up in November 2022.
In this session you can find answers to the following questions:
- Fire resistance of joints asked by Millie Wan (answered by Piotr Turkowski) - jump to 1:41
- Fire detection in car parks asked by Elena Funk - jump to 11:10
- Balancing safety and architectural beauty asked by Ekonudim Friday - jump to 15:51
- Comment on driving fire safety in Iran by Neda Farhoudi - jump to 21:34
- Smoke control strategies for boutique shops in malls asked by Szymek Matkowski - jump to 27:00
Please join my 2022 Listener Experience Survey - the link will work till Christmas 2022 :)
Episodes mentioned in this one:
073 - Smoke control in shopping malls - uncommon aspects that make or break the system
070 - Fire resistance is whatever you want it to be with Piotr Turkowski
015 - Global view on the fire safety from a starchitect perpective with Benjamin Ralph
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The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.
Next Episode
080 - Adaptive Fire Testing: A new foundation stone for fire safety (ERC StG Grant) with Ruben van Coile
Today is a great day to celebrate with Prof Ruben van Coile of Ghent University, who is most likely the first representative of Fire Safety Engineering to receive a grant within the European Research Councill Starting Grant scheme. It is not common to celebrate a grant award this much - usually, we would wait till the work gets done and we see the effects... But not here. ERC is something else. ERC is a place for the bravest proposals brought by the brightest minds of science. And even that does not guarantee success when you have to pass 9-12 independent reviewers and a multiple-stage recruitment process... But it seems to be worth it. A five-year funding scheme that allows a truly grand design to be pursued.
And this exactly is the case with the framework proposed by prof. van Coile. He is not the first one to recognize we need a new foundation stone for fire safety, but he surely is one of those who give the clearest and most achievable pathway on how to get there. I highly recommend this episode to all fire safety engineers, not to just learn about the grant Ruben has just obtained, but to view the current state of FSE through the lens of this proposal. As it does, in an excellent way, highlight the shortcomings and failures of modern fire engineering.
Join us in this talk, and if you would like to read more about the grant, here is its official abstract (and near future will bring material for sure!).
Proposal Summary
Adaptive Fire Testing: A new foundation stone for fire safety (AFireTest) The current fire safety paradigm is based on a set of standardized tests which have been developed as part of a prescriptive design framework, and do not provide in-depth understanding of construction products’ fire performance. The resulting incomplete fire performance characterization hampers the much needed innovation in the built environment. The current fire safety paradigm also places tremendous emphasis on the expertise of controlling bodies (AHJ), making them responsible both for the specification of detailed prescriptive rules, and for the acceptance of performance based designs. This is unsustainable in the face of innovation.
AFireTest strives to induce a paradigm shift in fire safety science and engineering (FSSE). The core of AFireTest is the development of Adaptive Fire Testing whereby optimum fire tests are determined from the infinite number of possible test specifications through the maximum expected net information gain (Value of Information, VoI). This will be developed using modern glazing and load bearing glass as innovative case study, resulting in breakthroughs in fire performance understanding. Secondly, a framework for advanced ‘grey’ surrogate modelling will be developed, combining the pattern identification strengths of machine learning with fundamental FSSE constraints. This will introduce a powerful new tool to FSSE and enable the VoI optimization. A grey modelling approach will also be developed for quasiinstantaneous building specific risk evaluations, allowing a new approach to the AHJ acceptance of fire designs. The future operationalization of the new framework for fire design acceptance will require large follow-up investments. Thus, stakeholder buy-in is crucial. Therefore, AFireTest will develop a methodology for the cost-benefit evaluation of fire safety frameworks. For the first time, fire safety approaches will be evaluated from the perspective of Law and Economics, laying the groundwork for an entirely new field of study.
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The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.
Fire Science Show - 079 - Timber columns failure in the decay phase with Thomas Gernay and Jochen Zehfuss
Transcript
Hello, everybody. You're welcome to the fire science show. And other episodes related to timber. So I guess that's getting everyone excited. I usually see. More interest in timber'y episodes than any other type of topic in, in fire science. I really wonder why, why is that? I guess it's one of the most exciting things that happen around and one of the least well understood problems in fire science for people outside of our core group of
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