David J. Petersen | Tonkon Torp. | September 6, 2018
On August 8, the Oregon Building Codes Division approved a new state building code called a Statewide Alternate Method. The new code authorizes the construction of wood buildings taller than six stories, which was the previous limit. Taller wood buildings have been made possible by technological advances with cross-laminated timber. CLT, as it is known, is constructed by layering perpendicular sheets of solid lumber and adhering them together. It is similar to plywood, but much thicker, creating the necessary structural support for a high rise building.
CLT has numerous advantages over steel, masonry and concrete. It is lighter and more flexible, which provides excellent seismic resilience, reduces the need for deep foundations and shortens construction time. The base material is easily adaptable to different uses by adding or removing layers to create the desired thickness and strength. As a wood product it sequesters carbon and can be sourced from a renewable, sustainable resource. Materials can be prefabricated off site, potentially lowering construction costs. Disadvantages include current higher production costs and weak sound insulation properties. While an increased risk of fire may seem logical, in fact CLT has been shown to have equal or better fire resistance than other non-carbon based construction materials.
CLT is in common use in Europe, and the largest CLT building currently in existence is Dalston Lane, a mixed use complex in Hackney in the U.K., with towers as tall as 10 stories. The Framework Building, a mixed retail, office, and residential tower to be constructed in Portland’s Pearl District, would have topped Dalston Lane at 12 stories, but due to development challenges the project is currently on hold. Portland also hosts the USA’s first CLT building, the four-story Albina Yard.
Despite the temporary setback of the Framework project and the manufacturing defects recently found in CLT used in a project at Oregon State University, CLT has loads of promise as the high-rise construction material of the future. By using sustainably-sourced wood or wood that would otherwise go to waste (much CLT in the market today is made from pine beetle-infested trees), the carbon footprint of new high-rises can be reduced significantly. As costs come down, as they do with all successful new technologies, expect to see CLT structures rise near you.
Before skyscrapers first touched the sky in New York and other cities over 100 years ago, most buildings were built of wood. The structural limitations and fire risk of wood kept buildings short, to perhaps five or six stories at most. Steel and concrete allowed architects to blow past those limitations. Now, with the advances made possible by CLT, our built environment is on the verge of completing a full circle to the space age wood structure of the future.