Fire Protection Engineers and Cross-Laminated Timber
Cross-laminated timber buildings are cropping throughout the globe: Are Fire Protection Engineers leading the way?
By Joseph Hough and Justin Biller, P.E., FSFPE
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Cross-laminated timber buildings keep gaining traction in many places throughout the world. These cross-laminated timber buildings — as the name indicates - are made of sawn lumber, with the layers (or laminae) of timber crisscrossing over each other. This cross-crossing of timber provide endless dimensions and endless shapes.
Cross-laminated timber (CLT) buildings do attract a wide range of authorities, architects and builders. Why? Mainly, because of two reasons: A) The aesthetics of timber along with its "green" capacity to sequestrate carbon dioxide; B) The desire of the governments to find new added-value use of their natural raw forest products.
However, for most engineers, the first reaction when it comes to using timber in building is: "What?! That’s impossible?!"
Obviously, the entrenched "programming" of dividing the building materials into combustible or non-combustible still has its massive influence. Timber can ignite but will withstand as much as 7232 °F/4000 °C before it reignites (Emmons H.W. & Atreya A., 1982). Steel, when compared, on the other hand, to solid timber, and owing to its homogenous molecular structure; steel will transfer heat quickly, and start losing 50% of its stiffness and strength at 932 °F/600 °C. At 1291 °F/700 °C, steel retains only 20% of its strength and stiffness. And at 2192 °F/1200 °C, steel loses almost all its strength and stiffness (Gewain R. G. et al, 2006). At the same time, concrete can withstand up to about 1112 °F/600 °C. However, when the 20 percent water content of concrete evaporates in high-temperature conditions, concrete spalls (Noumowé et al, 2006; Ko et al, 2011).
A study of 40,000 fires in Switzerland concluded that combustible buildings with active fire protection measures and complete compartmentation resulted in no loss greater than 100,000 Swiss Franc (approximately US$ 111,000) between 1985 to 1995 (Fontana et al, 1999).
Definitely, the “combustible/non-combustible" dichotomy is well-worth a good revision. And yes, most buildings are hybrid and use a combination of these aforementioned building materials!
Now timber withstands such high temperatures because of its ability to char on the surface. The char itself is nothing but the carbon content of the organic material that burn. Carbon — the main component of char — is very hard to reignite. The char layer also acts as a protective shield that hinders the combustion process from extending to the timber core.
When timber chars, cracks are produced. These cracks act as gateways through which heat can escape to the outside. Heat finds it easier to get out through the cracks of the charred layer than to continue "digging" into the virgin timber that has not been consumed by fire. In simple terms, heat (and combustion products) will take the path of least resistance out through the cracked char; while leaving the non-ignited bulk of timber intact (Emmons H.W. & Atreya A., 1982).
Regarding CLT buildings, charring of timber prevents further progression of the fire, but is associated with the initial production of heat and combustion products. At the same time, there are some effective fire safety measures to consider:
Sprinkler systems:
Any tall building design, let alone timber building, that does not include a well-designed and well-maintained sprinkler systems renders the building highly vulnerable to the negative consequences of fire. The main advantage of concrete and steel, and hence their popularity, is their ability to go as high as possible. Sprinkler systems allowed concrete and steel buildings to go that high. Timber building structures on the other hand — until very recently — were restricted to six stories in height.
Now with mass timber — and governmental authorities - pushing the envelope when it comes to tall timber buildings, it becomes obvious that ensuring a well-designed, well-maintained sprinkler system is necessary. The building design team for timber structures should include a fire protection engineer with the requisite knowledge of sprinkler systems and water supplies, and the important role of the sprinkler systems in mitigating fires in mass timber buildings.
The 2018 United States Department of Agriculture (along with the American Wood Council) fire testing for two-story mass timber building had two tests that included sprinkler systems: One where the sprinkler system was activated normally, and another in which the sprinkler activation was delayed (to mimic a malfunctioning sprinkler system). In both of these tests, the CLT walls and ceilings were left fully exposed. The sprinkler system managed to suppress the fire quickly once activated in both of these tests (Baldasarra & Brand, 2017; Zelinla et al, 2018).
More fire testing is currently underway in Sweden with fully exposed CLT ceiling, fully exposed beams, and between 72-90% of exposed walls and exposed columns in one-story mass timber buildings. All other mass timber surfaces were covered with three layers of gypsum. The preliminary results of this testing show that timber fires decayed constantly post-flashover until four hours after ignition and reached radiation temperatures that were significantly below 300 ̊C without the use of any sprinkler system (Brandon et al, 2021).
The ability of the sprinkler system to limit ordinary (and smoldering) fires were well-documented, in the Kemano fire studies of the National Research Council in Canada (Su, J.Z. et al. 2002). However, this is not the full picture. When a sprinkler system is activated in a building, the heat release rate just under the activated sprinkler system decreases; and with it the buoyancy of smoke decreases – leading to the dispersion of the smoke layer to spaces other than where the fire started (Zhang et al, 2005). Further practical research, thus, is needed in this area.
Draftstopping; especially at CLT assembly joints:
Draftstopping is also one of the important aspects in CLT building fire protection; especially between the CLT-panel wall-to-wall joints, CLT-panel wall-to-ceiling joints and CLT wall-to floor joints. These are the points where combustion products can move between one compartment to the other.
Usually the joints between the CLT panels take the form of timber splines (exposed or buried in-between the edges of the panels i.e. concealed), or metal brackets (exposed or concealed), half-lapped joints or other proprietary systems. Screws and nails are used through these latter exposed or concealed joints. Actually, there are specialty tools that grip into two adjacent CLT panels and pull the two panels together; ensuring a tight joint between these two CLT panels (Gagnon, S. & Karacabeyli, E, 2019).
Werther et al (2016) showed that the use of intumescent or elastomeric sealant between the CLT joints resulted in an almost absolute stoppage of passage of combustion products through them. For plumbing, electrical or mechanical penetrations, Werther et al recommends the application of intumescent or elastomeric sealants on both sides of the penetration. It's worth mentioning that intumescent sealants produce their fire stopping mechanism by increasing in volume and charring. The elastomeric fire sealant produces a protective layer of char without increasing in volume.
Compartmentation of a building: The essence of compartmentation in a building is dividing the building into separate smaller fire areas; so that if fire occurs in one of these smaller areas, the whole building remains isolated from where the fire started. This is achieved through fire resistance rated construction and fire protection of openings. There is nothing new about compartmentation in CLT buildings, apart from the fact that careful separation — horizontally or vertically; between compartments needs good detailing and thorough execution.
Timber connections concealment: Linked to compartmentation are connections in timber. In laboratory settings, the shear-stress-slippage behavior of connection represents the primary indicator of the strength of these connections. Most failures of connections under fire result from insufficient timber thickness in which the connections are embedded or from insufficient thickness of these metal connections themselves.
An insufficient timber thickness leads to less protective charring layer around these connections in case of fire. Had enough charring around these connections occurred, the connections would have remained resistant to the shear-stress-slippage behavior in case of fires. Another factor that leads to failure of connections is the use of large diameter bolts. Large diameter bolts allow, again, more heat transmission than usual; leading to the connections’ failure. Believe it or not, dowels instead of metal connectors between timber structures can be more resistant to this shear-stress-slippage behavior (Maraveas et al, 2015).
References
- Baldassarra, C. & Bland, K (2017). Fire Testing on Full-Scale Mass Timber Building Will Inform Code Changes. FPE Extra. Issue 22. Retrieved on June 9th, 2021 from https://www.awc.org/pdf/education/des/AWC-DES603A-FireTesting-1901.pdf
- Brandon, D., Sjöström, J., Hallberg, E., Temple, A. & Kahl F. Fire safe implementation of visible mass timber in tall buildings – compartment fire testing (summary report) (2021). Research Institute of Sweden. RISE Report 2020:94. Retrieved on June 9th, 2021 from https://www.ri.se/sites/default/files/2021-03/Report%202%20Summary%20report%20FINALcorrected_0.pdf.
- Emmons, H. W., & Atreya, A. (1982). The science of wood combustion. Proceedings of the Indian Academy of Sciences Section C: Engineering Sciences, 5(4), 259.
- Fontana, M., Favre, J. P., & Fetz, C. (1999). A survey of 40,000 building fires in Switzerland. Fire safety journal, 32(2), 137-158.
- Gagnon, S. and Karacabeyli, E. (Eds.). (2019). CLT handbook: Cross-laminated timber. FP Innovations.
- Gewain, R. G., Iwankiw, N. R., Alfawakhiri, F., & Frater, G. (2009). Fire Facts For Steel Buildings. Canadian Institute of Steel Construction. Retrieved May 10th, 2021 from https://www.cisc-icca.ca/wp-content/uploads/2017/03/FireFacts1E4P.pdf
- Ko, J., Ryu, D., & Noguchi, T. (2011). The spalling mechanism of high-strength concrete under fire.Magazine of Concrete Research, 63(5), 357-370.
- Maraveas, C., Miamis, K., & Matthaiou, C. E. (2015). Performance of timber connections exposed to fire: a review.Fire Technology, 51(6), 1401-1432.
- Noumowé, A., Carré, H., Daoud, A., & Toutanji, H. (2006). High-strength self-compacting concrete exposed to fire test. Journal of materials in civil engineering, 18(6), 754-758.
- Su, J. Z., Crampton, G. P., Carpenter, D. W., McCartney, C., & Leroux, P. (2002). Kemano fire studies–part 2: response of a residential sprinkler system.
- Werther, N., Denzler, J. K., Stein, R., & Winter, S. (2016). Detailing of CLT with respect to fire resistance. InProceedings of the Joint Conference of COST Actions FP1402 & FP1404(pp. 125-135).
- Zelinka, S. L., Hasburgh, L. E., Bourne, K. J., Tucholski, D. R., Ouellette, J. P., Kochkin, V. & Lebow, S. T. (2018). Compartment fire testing of a two-story mass timber building. United States Department of Agriculture, Forest Service, Forest Products Laboratory.
- Zhang, C. F., Huo, R., Li, Y. Z., & Chow, W. K. (2005). Stability of smoke layer under sprinkler water spray. InHeat Transfer Summer Conference (Vol. 47314, pp. 689-693).