Last month, a Japanese company named Sumitomo Forestry announced conceptual plans to build the world’s tallest wooden building in Tokyo. The 70-story, 350-meter mixed-use skyscraper would use about 185,000 cubic meters of timber and cost an estimated $5.6 billion to build, according to the company, which is targeting a 2041 completion date.
While architects and environmentalists tout wood’s strength, versatility, and sustainability, some worry how these structures withstand fire. The concerns are that the timber construction could increase the fire load, impact fire growth rate, and could possibly overwhelm suppression systems.
For Phase 2, researchers from the National Research Council of Canada and the National Institute of Standards and Technology teamed up to find out how exposed mass timber in a residential dwellings might impact fire behavior. The researchers built essentially six simulated studio apartments—each 30 feet long, 15 feet wide, and nine feet high—with four walls and a ceiling. Each was filled with typical modern furnishings.
The rooms, or compartments as their known, were made of 175-mm thick five-ply cross-laminated timber panels (CLT) panels. CLT, perhaps the most popular material used in mass timber construction, generally consists of three to seven layers of timber boards crisscrossed and bonded together with glue for maximum strength. In most tall wooden structures, interior CLT panels are covered in gypsum board to add a level of fire protection, however design trends could eventually lead architects to leave the timbers partially exposed. Fire protection researchers wanted to know what those exposed boards could mean for fire growth, heat release, toxicity, and other factors.
“We have limited information on compartment fires in these types of buildings, so it was a knowledge gap that we were looking to fill,” said Amanda Kimball, the research director at the FPRF.
Each of the simulated studio apartments in the burn tests had varying levels of exposed wood—two rooms had one wall with exposed boards, one had a ceiling exposed, and one had both. Two of the rooms were fully encapsulated in gypsum board, and were burned to form a baseline measurement. In two of the four tests, researchers adjusted the amount of ventilation in the room to see how that would impact the fire.
Ultimately, researchers found that the exposed timber did influence the way the fire behaved.
“In all tests with exposed CLT surface(s), flashover occurred approximately three to five minutes earlier than the two baseline tests (i.e., ≈ 15 min),” the report concludes. “The peak compartment temperatures were similar to the baseline. However, the heat release rates and heat fluxes to the exterior facade were higher than the baseline.”
Kimball noted that the tests did not include fire sprinkler systems, which would be required in any tall wooden residential building. The data collected from the tests, however, will inform “the fire service, codes and standards bodies, designers, and insurers about possible risks with these structures,” Kimball said.
There are no plans currently for a phase three study, but several knowledge gaps still remain. Two of those include how the connectors that hold the timber elements together perform in fire; there are also questions about how holes in the wooden panels—cut to allow for passage of cables, HVAC, and other systems—might affect fire behavior. As tall wooden buildings become more popular, studies to answer these and other questions are underway around the world, Kimball said.
NFPA Journal published a feature on the fire and life safety considerations of tall wooden buildings, and other novel types of building design and construction in a 2017 feature, “Different by Design.” In addition to tall wooden buildings, the article details a slew of other designs including pencil buildings, shipping container buildings, tiny homes, and a skyscraper in Miami with a built-in car elevator.