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A Simplified Assessment of Thermal Feedback in CLT Compartments After Burnout

Mass timber structures based on engineered timber such as Cross Laminated Timber (CLT) and Glue-laminated timber (Glulam) have emerged as renewable construction technology. Despite multiple advantages of timber construction, fire safety hazards emerge in building design due to their combustible nature. The use of exposed CLT challenges the fire resistance framework since the timber structure may continue to burn after burnout of the movable fuel load, thus challenging integral principles of compartmentation and structural integrity. Research on the self-extinction of CLT in small compartment fires by (Gorska et al., 2017) has shown that, for a fixed opening factor, a critical ratio of exposed timber is required for self-extinction. Under this critical ratio, the thermal feedback within the compartment (incident heat flux) after burnout drops such that CLT extinguishes gradually. This phenomenon was also demonstrated on a larger scale (Emberley et al., 2017, Hadden et al., 2017). Despite the progress towards understanding the conditions for CLT self-extinction in compartment fires, there is a lack of engineering tools validated for the prediction of CLT self-extinction. FDS v 6.1.0 is the model used, using the LES turbulence model and sensitivity studies of mesh size and radiation solid angles. To model the boundary condition for burning CLT, three simplified approaches is followed by (Sahoo, 2020) based on the thermal condition and the heat release rate per unit area (HRRPUA). The three approaches are as follows:
  • Approach 1: CLT wall with a constant temperature and null HRRPUA 
  • Approach 2: CLT wall with an adiabatic condition and a constant HRRPUA 

  • Approach 3: CLT wall with a constant temperature and a constant HRRPUA 

The results from the parametric numerical studies are benchmarked with the experimental results by (Gorska et al., 2017).

Figure 1. Comparison of thermal feedback on CLT at different surface temperatures (different marker sizes) and 63 kW/m2 HRRPUA.

Figure 1 shows the results for the third approach, where a constant temperature and constant HRRPUA are prescribed. It is shown that as more CLT is exposed, the thermal feedback increases to values above the critical heat flux for extinction, with a plateau reached after significant CLT exposure. Further, it is shown that the temperature value assumed for CLT plays a major role in the thermal feedback.

From the three approaches, the third approach reaches conclusions regarding self-extinction that approximate better the experimental findings by (Gorska, 2020). The first approach appears to underestimate the thermal feedback significantly for large CLT exposure, whereas the third approach provides a significant overestimation for all scenarios. This confirms that a combustion model is necessary. Therefore, it was found that a specific case where the CLT wall with a constant temperature of 700°C and HRRPUA of 63 kW/m2 was the most suitable approach to assess self-extinction. However, it is discussed that a better approach could be considered based on constant net heat flux to the timber and a constant HRRPUA. When developing this analysis, the time scales to achieving burnout are ignored such that delamination can be neglected in the analysis. The approach developed by Sahoo (Sahoo, 2020) – who is a key member of Sotera’s Gold Coast fire engineering team – has the potential to be used as a conservative tool to design the CLT compartment for self-extinction.

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Bibliography

Emberley, R., Do, T., Yim, J., & Torero, J. L. (2017). Critical heat flux and mass loss rate for extinction of flaming combustion of timber. Fire Safety Journal, 91, 252-258.doi:10.1016/j.firesaf.2017.03.008

Hadden, R. M., Bartlett, A. I., Hidalgo, J. P., Santamaria, S., Wiesner, F., Bisby, L. A., … & Lane, B. (2017). Effects of exposed cross laminated timber on compartment fire dynamics. Fire Safety Journal, 91, 480-489.

Gorska, C., Hidalgo, J. P., & Torero, J. L. (2017). An experimental study of medium-scale compartment fire tests with exposed cross laminated timber.

Gorska Putynska, C. (2020). Fire dynamics in multi-scale timber compartments. Sahoo, S. (2020). A simplified assessment of thermal feedback in CLT compartment after burnout.