Tall timber buildings are a hot topic these days. There was even a TED talk on why we should build wooden skyscrapers. A quick search for tall timber buildings shows quite a few in the design phase both in the United States and in other parts of the world. Before getting into the pros and cons of tall timber buildings, it is important to understand the type of timber that would be used.
The timber would not be light weight wood frame construction usually used in one- and two-family homes, but rather mass timber. There are many types of mass timber, including cross-laminated timber (CLT), nail laminated timber (NLT), and glue laminated timber (Glulam). Most of the discussions I've been involved in center around CLT. For those who are new to the mass timber world, CLT is usually made of three, five, or seven layers of wood laid in a crisscross pattern that are then glued together. For more information on the types of mass timber this link is a good resource.
Many people are asking, why mass timber? As the green initiative grows, mass timber becomes more appealing. Many proponents argue that it is a renewable resource and has a significantly smaller carbon footprint as compared to steel or concrete buildings. Prefabricated CLT panels have also been shown to decrease construction time. The time lapse video below shows the construction of a nine story building in nine weeks.
While mass timber sounds great from a building material standpoint, there are definitely some concerns. It was touched on briefly in the TED talk, but the fire concern is larger than it was described. We know that wood burns, and we know that it chars. The char layer does form a sort of protective layer to the rest of the wood, but can we rely on the char layer to remain in place on the loaded structural components? There have been many discussions about removing the gypsum board coverings and allowing exposed CLT. How will exposed CLT contribute to fire growth? We already know that today's furnishings burn faster and hotter than the furnishings of decades ago. Now we are talking about adding a combustible building material into the mix. Structural stability after a fire is also a concern. In a fire event, we need to not only think about getting everyone safely out, but also about the firefighters who go into the burning building to rescue those who couldn't get out on their own or to simply carryout firefighting operations. There are additional concerns regarding the exterior exposure of mass timber buildings. Over the years, we have seen numerous exterior facade fires. If a wood building were nearby, how can we protect it?
There are numerous studies underway to look at the fire risk of mass timber buildings. The Fire Protection Research Foundation is working on phase two of their study. Phase one, a literature review can be found here. NEWBuildS of Canada has also done research into mass timber buildings. They have a lot of information available on their website.
This is an exciting time for building construction, but before making the jump to a prescriptive allowance for tall timber buildings, we need to gather all the information. Building codes are meant to protect both the occupants and the firefighters, so we owe it to them to do the research.