INSIGHTS

2023.03.07

R&D DISCUSSION Vol 45

Building fire-resistant wooden buildings [Part 2]

Noboru Yasui, Architect and Chairman of NPO team Timberize

R&D DISCUSSION TOP

Q: While there are growing expectations for the future of wooden structures, there have been tragic fire accidents in Japan in recent years. Please tell us about the ingenuity in designing wooden structures to withstand fires.

A: There have been a series of accidents in recent years, such as the burning of Shuri Castle in Okinawa Prefecture, the complete destruction of a semi-fireproof building called Inabe Nursery School in Mie Prefecture, and two large-scale fires at the Kitakyushu Tanga Market in Fukuoka Prefecture. Shuri Castle and Inabe Nursery School were arranged in a U-shape. This made it difficult for firefighters to enter the courtyard. There were areas where water could not be poured, so the fire spread rapidly. At Tanga Market, only the area where the fire prevention wall was located remained burned, and the fire spread mostly through the windows. When designing a building with openings facing each other or connected to each other, I think it is necessary to consider fire prevention measures. Fire spreads upwards, so making the ceiling fireproof is effective. As in the previous experiment on interior restrictions, if the ceiling is fireproof with plasterboard, the fire will spread horizontally and burn slowly. Installing sprinklers and fire doors is one way to do this. If you can buy time before a flashover occurs, which involves rapid combustion, you can also buy time to escape. By designing with fire prevention and fire extinguishing in mind, and by arranging materials cleverly, it is possible to create wooden buildings and wooden spaces that can withstand fire.

According to the Fire and Disaster Management Agency's Fire and Disaster Management Agency's "2018 Fire and Disaster Management Agency White Paper," about 8,000 buildings without fire protection measures, also known as "bare wood construction," and wooden buildings built before the 1950 Building Standards Act were burned in one year [Slide 1]. On the other hand, about 6,000 fire-resistant steel-framed and reinforced concrete buildings have also burned. The numbers are not that different. However, fire news only reports on wooden buildings. This is because the scale of fires is large due to the lack of fire protection measures, while in the case of fire-resistant structures, even if a fire breaks out, it does not burn through and ends up being a small fire. Even with wooden buildings, fireproof or semi-fireproof structures can be created by covering pillars and other parts with fireproof materials. Recently, it has become possible to cover fireproof buildings with fire-retardant treated wood, and technology is improving every day. I hope that designers, builders, and clients will work together to aim for buildings that are "non-flammable" and do not spread fire easily [Slide 2].

[Slide 1] Damage situation by structure of the building where the fire started, based on the Fire and Disaster Management Agency of the Ministry of Internal Affairs and Communications' 2018 Fire and Disaster Management Agency White Paper.
[Slide 2] How a fire burns and the necessary design responses.

Q: The key to further popularizing wooden construction seems to be its expansion into mid- to high-rise buildings and non-residential buildings. What challenges and measures do you think need to be addressed?

A: As I mentioned earlier, the technology of wooden construction and wood use is rapidly improving. In addition to traditional construction methods, wooden and wood-based buildings using laminated timber and LVL (Laminated Veneer Lumber) have been around for a long time, but what is attracting attention now is "CLT (Closs Laminated Timber)", which is made by laminating and gluing sawn boards (laminas) so that the fiber direction is perpendicular to the surface. It has developed mainly in Europe since around 1995, and has become common in Europe and the United States in recent years, with the construction of high-rise buildings. In Japan, the manufacturing standard JAS (Japan Agricultural Standards) was established in December 2013, and the Building Standards Act related to CLT was implemented in April 2016. CLT is strong and can be used as a wall-like structure. It has a high degree of design freedom, and since CLT panels processed in a factory are simply assembled on site, it has the advantage of shortening the construction period.

Although the number of cases has been gradually increasing in Japan [Photos 1 and 2], it is not as widespread as in other countries. The main reason is that CLT panels are large panels, and in Japan, where construction is often done on narrow land, non-wooden construction ends up being cheaper. In 2018, a three-story CLT prefectural housing building for disaster recovery was built in Iwaki City, Fukushima Prefecture, but problems such as poor construction arose. If the construction team continues to build two or three buildings, they will become more "skilled" and find a solution, and construction costs will go down, but before that, they will give up because it is too expensive. Housing materials are mass-produced to ready-made dimensions and construction is well-established, so costs are low. Currently, Japan has machines that can manufacture about 100,000 m3 of CLT every year, but only 15,000 m3 is produced, so material costs are not going down. With various types of large-scale wooden and wood-based buildings, it is difficult to know what to standardize and how to make them more skilled in order to reduce costs, but I think we all need to think about this together.

Aside from the cost aspect, various other issues remain. Wood is said to be a "sustainable material" that contributes to "decarbonization," but for example, gypsum boards, which are commonly used to prevent fires from spreading in wooden buildings, emit a lot of CO2, and even though many trees are cut down for mid- to high-rise buildings, only about 50% of them are actually reforested, so it is not completely "decarbonized" or "sustainable." In the future, we need to think about how to fix CO2 and how to circulate resources.

Our "perception" of wooden construction may also be one of the issues. When we hear the word "wooden construction," in addition to "prone to fire," we imagine old buildings with thin walls and cold drafts. Today's wooden and wood-based buildings are different from the wooden constructions of the past, including in terms of insulation and airtightness. With the understanding that "wooden construction" is a very broad term, it may be easier to move the discussion forward if we share the same goal of building a wooden structure, including the client, whether that be a traditional wooden structure or a new wooden structure.

Finally, what I want everyone to remember today is that October is Wooden Construction Promotion Month, and October 8th is Tree Day. Apparently, this is because the kanji characters for ten and eight combine to form the character for tree, but I ended up asking the Forestry Agency, "Is that really how it was decided?" (laughs). But I think it's good to raise awareness little by little through things like this.

[Photo 1] "CLT PARK HARUMI" was built as a temporary structure to showcase the potential of CLT both domestically and internationally.
(2019, designed Mitsubishi Jisho Design Sekkei and Kengo Kuma and Associates).
After being used as an exhibition facility in Harumi, Tokyo for a year, it was moved to Okayama Prefecture.
[Photo 2] "PARK WOOD office iwamotocho" uses domestic CLT for the floor structure
(2020, design: Mitsubishi Jisho Design).
This is the first case in Japan of an office building with six or more floors that uses CLT as a structural material.

[Slides 1 and 2: Provided by Sakura Architectural Group]

Back to Part 1

PROFILE

Architect, Chairman of NPO team Timberize

Noboru Yasui

Easy to climb

Born in Kyoto City, Kyoto Prefecture in 1968. After completing a Master's course in Architecture at the Tokyo University of Science Graduate School of Science and Engineering in 1993, he worked at Sekisui House Co., Ltd. before opening Sakura Design Group First Class Architect Office in 1999. He completed a doctorate in Construction Engineering at the Waseda University Graduate School of Science and Engineering in 2004. He is currently a part-time lecturer at the Gifu Prefectural Academy of Forest Culture, a director of the NPO Wood Architecture Forum, a specially appointed professor at Kochi Prefectural Forestry College, a researcher at the Waseda University Institute of Science and Engineering, chairman of the NPO team Timberize, and a research fellow at the University of Tokyo Institute of Industrial Science.

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