Visible hardwood is very popular in many new builds. The use of wood materials provides good climate accounting, and wood is beautiful and nice to have in such buildings.
Additionally, the building process goes quickly when large, prefabricated items can only be lifted into place, almost like Lego pieces.
Hardwoods are large elements of wood used as load-bearing structures in walls and floor partitions.
The elements consist of several layers of plates with each layer lying across the next, so-called cross plates. The layers are glued together and are used, among other things, in housing, commercial buildings, schools, healthcare buildings, nurseries and high-rise buildings.
Andreas Satter-Boe from NTNU believes there are some myths about fire safety in solid wood buildings.
– It is said that hardwood cannot be ignited or there is no cause for concern because a protective carbon layer forms on the outside of the hardwood. These statements contain some truth, but they do not tell the whole story. Wood is a combustible material, and the reality is much more complex, Bowie says.
He is a civil engineer at NTNU, worked for six years as a fire researcher at RISE Fire Research in Trondheim and is now a researcher. He has conducted two large-scale fire tests using hardwood.
He says that in the past 10 to 15 years, a number of tests have been carried out internationally to investigate how fires occur in hardwood. Many of them are made in small rooms and with small window openings.
But modern buildings tend to have large open spaces and large windows. I wanted to study how fire would behave in a large-scale room with exposed hardwood, as much like a modern office building as possible.
The room was fire tested
For the experiments, a room was built of solid wood, 18.9 meters long, 5 meters wide, and a ceiling height of 2.5 metres.
The floor area was 95 square meters and the room had large window openings on one wall. Along the entire length of the floor, 5 x 5 centimeter wooden slats were stacked crosswise, with air sandwiched between them.
A log bed, as fire researchers call it, corresponds to the amount of energy found in a typical office building.
The fire was started by setting fire to one end of the wooden bed.
The fire dynamics changed
Bøe led two tries. In the first experiment, the testing room had hardwood visible in the ceiling, while the back wall was covered with noncombustible plaster.
– There was a clear change in the fire dynamics before and after the solid wood roof ignited. Since the roof started burning, it took thirteen minutes for the fire to spread to the entire roof surface. Compared to fire tests without exposed wood materials, this is fast. At the same time, it is slower than other open-top experiences.
The fire in the ceiling led to strong heat radiation on the floor. This caused the fire to also spread rapidly at ground level.
In Experiment 2, both the ceiling and the back wall are made of exposed hardwood. The fire progressed much faster than on the first attempt. From the ceiling igniting to the entire room being engulfed in flames, it only took 91 seconds.
The way the fire spread was the same as in the first test. But it was clear that the flammable wall contributed to a significantly faster development. The burning wall increased heat radiation towards the ceiling and wooden bed, causing the ceiling and floor to burn more strongly.
– After over-ignition, large flames appeared through the window openings. This increases the risk of spreading to upper floors and neighboring buildings, Boye explains.
It turns off very quickly
In both tests, the hardwood fire was out within 15 minutes after ignition. This happens because a carbon layer forms on the surface.
The hardwood then depends on providing it with sufficient heat from the outside to maintain the fire. If the fire brigade begins extinguishing quickly, fire outbreaks usually do not pose a significant threat.
The fact that the fire initially extinguishes itself is a positive result for solid wooden buildings in terms of the stability of the building and the possibility of confining the fire to the fire cell where it started.
I would not advise
– Have experiments shown that it is not necessary to use bare hardwood?
– no. Hardwood has a number of advantages and is a great material to continue to use. Meanwhile, experiments clearly show that exposed hardwood surfaces can influence how a fire develops. Anyone designing such buildings must take this into account.
He explains that in small rooms, fire can develop very quickly, regardless of building materials. Only in large rooms do the differences really appear.
Not just hardwood
He points out that it may take longer before the ceiling catches fire in large rooms with high ceilings.
– Previous research has also shown that the size, number, and distance between exposed surfaces are of great importance in how a fire develops. It should also be noted that large combustible surfaces, for example wood cladding, in non-combustible buildings, can have an equally significant impact on the development of a fire in the early stage.
Evaluate sprinkler systems often
Typically, sprinkler systems will prevent rapid fire development. In many building types, these facilities are required, but not all.
– Where this is not mandatory, it must be assessed on a case-by-case basis whether there is a possibility of rapid fire development – and whether sprinkler systems or other measures should nonetheless be installed to ensure safe escape and rescue, says Bui.
Want to understand more
The cost of the two attempts ranges between five and six million kroner. The fact that fire testing is expensive is one reason we still have knowledge gaps about hardwood and fire, a fire researcher believes.
– Historically, regulations are often tightened or modified only after major fires. By conducting trial fire tests, we will overcome the problems to a greater extent, so that small modifications along the way provide a safer way to build.
Worldwide, numerous fire tests have been performed using different combinations of visible hardwood in rooms of different sizes.
-We still do not have a complete picture of the importance of visible hardwood surfaces in the development of fires. The results of the experiments and further work at the FRIC research center are important contributions to understanding how fires develop in such buildings, says Andreas Satter-Boe.
Andreas Satter-Boe et al: Fire spread in a large room with exposed timbers and open ventilation conditions: #FRIC-01 – exposed ceiling. Fire Safety Magazine2023. Doi.org/10.1016/j.firesaf.2023.103869
Andreas Satter-Boe et al: Fire spread in a large room with exposed timbers and open ventilation conditions: #FRIC-02 – exposed ceiling. Fire Safety Magazine2023. Doi.org/10.1016/j.firesaf.2023.103986
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