Reliability of Mass Timber Buildings in Fire: A performance-based design method and its application to office buildings

Wood is making a quiet but powerful return to the skyline of modern cities. For decades, the construction industry favored steel and concrete, materials that do not burn, while wood was relegated to smaller structures. Today, however, engineers are increasingly turning to mass timber, a material made by layering solid wood panels to create beams and walls strong enough to support tall buildings. This shift is driven by a desire to lower the carbon footprint of construction, as wood stores carbon rather than emitting it during production. Yet, a fundamental question remains: if a building made of wood catches fire, how does it behave? Unlike steel, which weakens when heated, wood burns in a predictable way, forming a charred layer on its surface that can insulate the inner core and allow the structure to stand for a long time. The challenge for safety regulators is to determine exactly how much of this wood can be left exposed to the air inside a room without compromising the safety of the people inside or the buildings nearby.

A team of researchers from France, Canada, and Sweden has tackled this question by developing a new way to calculate fire safety for these timber structures. Instead of relying on rigid, one-size-fits-all rules that often require covering all wood in fire-resistant plaster, they used a computer simulation to model thousands of different fire scenarios. They treated fire not as a single event, but as a random occurrence that depends on many variables: how much furniture and paper are in the room, how big the windows are, whether the fire brigade arrives quickly, and how much of the wood is left bare versus covered. By running these simulations over and over again, they could calculate the statistical likelihood of a building surviving a fire without collapsing or spreading flames to the outside. Their work focuses specifically on office buildings, a common typology where the balance between safety and the desire for exposed wood aesthetics is most debated.

The researchers built a digital tool that mimics the physics of a fire in a room. They started by acknowledging that fire is unpredictable. In their simulations, they varied the amount of fuel, the size of the room, and the time it took for firefighters to arrive. They also accounted for the unique behavior of mass timber. When wood burns, it chars, and this char layer acts as a shield. However, if the wood is covered in protective boards like gypsum, that protection can fail if the boards fall off due to heat. The team's model tracks this carefully, calculating when the protective layer might fail and how much the exposed wood would then contribute to the fire's intensity. They also included the role of sprinklers, which can control the fire before it grows too large, and the actions of the fire brigade, whose ability to put out a fire decreases as the fire gets hotter and more intense.

To test their method, the team applied it to a standard office compartment. They ran 15,000 separate simulations for each scenario, effectively asking the computer to imagine 15,000 different fires happening in that room over the lifetime of the building. They set a strict safety goal: the chance of the building failing must be lower than a specific threshold that is considered acceptable for public safety. This threshold corresponds to a safety level used across Europe for buildings where people gather. They tested three different levels of structural fire resistance, which represent how long the building needs to stay standing during a fire: 60 minutes, 90 minutes, and 120 minutes. For each level, they tested different amounts of exposed wood, ranging from 30 percent to 90 percent of the wall surface, with and without sprinklers.

The results offered a clear path forward for designers. For buildings requiring a 60-minute fire resistance, the simulations showed that leaving 30 percent of the wall surface as exposed wood was safe, even without sprinklers. If sprinklers were added, this percentage could be increased to 60 percent. For buildings requiring 90 minutes of resistance, the safe limit for exposed wood rose to 60 percent without sprinklers and 75 percent with them. The most complex case was the 120-minute requirement, typical for very tall buildings. Here, the researchers found that leaving 75 percent of the wall surface exposed was safe, provided sprinklers were present. Without sprinklers, the risk of the fire spreading outside the building became a concern, but with sprinklers, the system held up to the safety targets.

A critical finding of the study was that the risk of fire spreading to neighboring buildings or up the side of the building was generally low, provided the amount of exposed wood stayed within these calculated limits. In the simulations for the tallest buildings, the heat radiating from the windows occasionally reached a level that could ignite nearby structures, but this only happened in extreme, rare scenarios where the fire was exceptionally large and the sprinklers failed to control it. When the researchers adjusted the distance to a more realistic 15 meters for tall buildings, the risk dropped to an acceptable level. This suggests that the current strict rules, which often demand that all wood be covered, might be more conservative than necessary. The study indicates that with the right combination of sprinklers and a calculated amount of exposed wood, tall timber buildings can meet the same high safety standards as traditional structures.

The researchers were careful to note that their work is based on computer models, not physical fire tests in every single scenario. They validated their tool against 54 real-world fire experiments conducted around the world, and the computer predictions matched the real-world outcomes closely, often erring on the side of caution. This gives confidence in their results, but they also identified areas for future improvement. For instance, they noted that the behavior of the protective boards and the way heat moves through the wood during the cooling phase of a fire could be modeled even more precisely. They also suggested that future rules should consider factors like wind speed and the specific location of the fire within a building, which could affect how quickly firefighters can intervene.

Ultimately, this study provides a scientific foundation for a more flexible approach to building with wood. It demonstrates that safety does not require hiding the material; rather, it requires understanding how the material behaves under stress. By using probabilistic methods, engineers can now design buildings that expose more of the beautiful, sustainable wood while still guaranteeing that the structure will not fail and the fire will not spread. The work suggests that for office buildings, it is possible to leave a significant portion of the timber walls and ceilings exposed, as long as active safety measures like sprinklers are in place and the design follows the specific limits calculated by this new method. This opens the door for architects to create spaces that feel warmer and more natural, without sacrificing the rigorous safety standards that protect the public.