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Fall After Fire
Why Timber Buildings May Collapse Even After the Flames Die
Professor Grunde Jomaas, Akash Jyothivas & Andrea Lucherini
October 07, 2024
Growing demand for timber
In recent years, the global demand for timber-based construction has seen significant growth. This increase in demand can be attributed to a sustainability-focused society, improved properties of timber products, technological advancements in timber processing and construction techniques, and its aesthetic appeal (Ramage et al., 2017). However, as all other construction materials, the integrity and stability of timber structures must be ensured also during and after a fire. In addition, its combustibility increases the concerns over building fire safety and requires specific fire design considerations.
Incidents where the structure collapsed after extinguishment
On 18th June 2022 in Philadelphia, a firefighter was killed in a building collapse after a fire (Gernay et al., 2022). A similar incident was also reported in Switzerland in 2004 when the collapse of an underground car park killed seven members of the fire brigade, who were in the car park after successfully extinguishing the fire and cooling down the structural elements (Gernay et al., 2022). What went wrong and why did the structures collapse even after the fire, when it was cooling down?
Figure 2: Fire engulfs a timber frame residential block. Source: Timber-frame flats under spotlight as fire engulfs block, 2019.
Thermal wave propagation
One of the factors that could have contributed to this is the thermal wave propagation within the structural elements during the cooling phase of a fire, as depicted in Figure 3. In a structural element like a column, the internal temperature continues to rise as the heat continues to distribute and penetrate within the element, even when the temperature on the surface is decreasing (Gernay and Franssen, 2015). As a consequence, the capacity of the structural element to resist load further decreases as it starts to lose its mechanical properties due to the temperature increase.
Figure 3: Thermal wave propagation in a structural element. The red curve depicts the thermal gradient during the heating phase, the orange curve depicts the thermal gradient during the decay phase, and the yellow curve depicts the thermal gradient during the cooling phase.
Timber: The reduction in mechanical properties is larger than for steel/concrete
The thermal wave penetration is particularly concerning for timber structures since timber loses its mechanical properties irreversibly at temperatures as low as 65 °C (Gernay et al., 2022). Figure 4 illustrates how the mechanical properties of timber - strength (left) and stiffness (right) – decrease significantly for lower temperatures when compared to traditional construction materials like concrete and steel.
Figure 4: Comparison of the reduction factor for capacity strength (left) and stiffness (right) of steel, concrete and wood at elevated temperatures. Source: Lucherini and Colic (2024) based on (Eurocode 2 (2023), Eurocode 3 (2024), Eurocode 5 (2009).
Understanding the characteristics of timber at elevated temperatures