Written by Ann Nordby
The very latest thing in high-rise building construction is… wood?
Since 2013, thousands of high-rise buildings have been built using timber, some of them 18-25 stories tall. You may have seen them and not realized their secret though, because, like concrete and steel buildings, the structure is not always visible. But these structures are not "stick built." They are made with super strong, stable and light Cross-Laminated Timber (CLT).
Like plywood, CLT is made from layers of wood set 90 degrees to each other. Unlike thin plywood, however, CLT uses 2x4 or 2x6 lumber for its layers. The result is a strong and stable structural material. Because of its large size, it's sometimes called mass timber. Prof. Omar Espinoza, a wood scientist, has been researching CLT at the University of Minnesota since 2011.
CLT has many benefits:
Sustainable
Timber is carbon negative – one cubic meter of wood stores about one metric ton of carbon dioxide (CO2). On the other hand, half a ton of CO2 is emitted to make 1 ton of cement. One ton of steel puts 1.8 tons of CO2 into the atmosphere.
Lightweight
Compared with steel and concrete, cross-laminated timber is lightweight. As a result, the foundation needed to support it can be smaller. That means less budget for materials and foundation works. "That's attractive for contractors because the cash cycle accelerates," Espinoza said.
Quick assembly
CLT walls and floors are custom-built in a factory, with windows and other openings already cut out. "You then move it to the construction site and put it together like a lego set," Espinoza said. "You only need a very small crew to erect the structure. For a nine-story building you might need just 4-5 carpenters and one crane operator." He estimates that assembly takes about half the time of and an equivalent concrete and steel structure.
Cheaper over time
Over its life a CLT building will cost less than a comparable concrete one. The materials cost more up front, but they are projected to have a longer lifespan and high end-of-life salvage value than concrete ones. Espinoza noted that most buildings are not decommissioned for safety concerns, but rather because someone wants to build something else in its place. BBE
Barriers to adoption in the USA
CLT is not new. It originated in Austria and Germany in the early 1990s. There, mass timber buildings are much more common than in the USA. Espinoza, whose education was in wood science, forest products and business, saw the benefits of CLT, but also the business potential for it. In Canada, Australia and Europe, the market for CLT was growing fast. He and other scientists and construction professionals took on the challenge of making CLT mainstream in the USA.
Despite its many benefits and broad use, CLT has faced headwinds to adoption here. The first was just lack of familiarity. Contractors, developers and lenders are reluctant to try new materials and techniques until they are proven safe and reliable.
"There needed to be a lot of research done to convince architects and engineers to adopt it," Espinoza said. So in 2011 when he joined the University of Minnesota, he started doing tht research. "My work started on the perceptions. No matter how good a product is if people don't perceive it that way they are hesitant to adopt it," he said. "I asked what are the factors that needed to be addressed."
The other big barrier to adoption was the building code. A 2012 article, "Outlook for Cross Laminated Timber in the United States" addressed the fact that CLT was not even mentioned in the International Building Code (IBC). As a result, developers and builders knew they would face an uphill battle to get building projects approved, so they didn't try. Much more research and testing followed, tackling different aspects about CLT. The IBC included CLT in 2015.
One of the biggest misconceptions about CLT was, and is, fire safety. "Wood passes fire safety tests better than steel, Espinoza said. "A very big section of wood will char on the outer layer, which will protect the core. Unprotected assemblies of CLT have passed 2-hour fire tests.On the other hand, steel can fail quickly and unpredictably when exposed to fire."
The next step was to amass the work of others and make it easily accessible. Espinoza knew that he wasn't the only advocate for these products, so he began compiling the research of others. In 2019, he created a repository of research about CLT and other forms of mass-timber construction, such as dowel-laminated timber (DLT) and glue-laminated timber (glulam).
The audience for the CLT Knowledge Database is broad – architects and structural engineers, construction professionals and other researchers on this topic. The US Forest Service has been a strong supporter.
"Different professionals are looking for different things," he said. "A structural engineer is looking for structural performance, for example." The database allows users to search by category, such as fire safety or costs. Users may also select for peer-reviewed publications, product sheets, reports and more.
The CLT Knowledge Database now has more than 4,000 peer-reviewed journal articles, conference papers and presentations, standards, reports, news notes, projects, manufacturers, suppliers, and much more.
New frontiers
Espinoza is now researching a different wood product: thermally modified wood (TMW). High temperatures can make wood more stable, moisture resistant and durable. Graduate and undergraduate students working with him are looking into the mechanical and durability performance of TWM from lesser-used wood species. Espinoza is also collaborating with the Natural Resources Research Institute, a research unit of the University of Minnesota, on projects that combine different wood technologies, such as CLT and TMW, to develop new, more sustainable, building materials. These wood scientists hope to develop products that are more sustainable and serve new markets worldwide.