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Mass Timber and Tall Wood Buildings

Mass Timber and Tall Wood Buildings

In September of 2019, the Sustainable Futures Committee presented information regarding the newest trend in construction, mass timber.

“Mass timber” is an umbrella term for any wood member, generally ~6”x6” or larger. “Heavy timber” generally refers to large solid hewn wood members. Types of mass timber include the following:

  • Glulams consist of layers of wood glued together parallel to one another, often used for beams and columns.
  • Cross-laminated/CLT products are made similarly to glulams, but the layers are glued together perpendicular to one another to form a sheet product that can be used for floor slabs or shear walls.
  • Nail-laminated timber consists typically of nominal 2x members nailed together on their broad sides to form a solid surface, often used for floor slabs.
  • Dowel-laminated timber is similar to nail-laminated, but is held together with wood dowels, making it the only all-wood built-up product.

Code officials are in the process of updating codes to allow for taller wood structures. The current code (2018 IBC) allows for wood buildings up to 6 stories and 85’ in height. These codes were intended to apply to older types of wood structures constructed of solid hewn heavy timber; new technologies that produce stronger wood products with wider applications open the technical possibilities to much taller structures. Stronger products combined with heavy timber’s natural fire resistance and modern fire protection and suppression make wood skyscrapers a possibility with a much lower carbon footprint than steel and concrete.

A revised code has been approved for inclusion in the 2021 IBC that will allow for buildings up to 18 stories tall, greatly increasing the use case for mass timber products. These code revisions have been adopted by the State of Oregon as a “State Alternative Method,” which allowed for the construction of an 8-story wood building here in Portland, currently the tallest such structure in the US.

References: Authors: Scott Breneman, PhD, SE, WoodWorks – Wood Products Council • Matt Timmers, SE, John A. Martin & Associates • Dennis Richardson, PE, CBO, CASp, American Wood Council Author: Brian Libby, published Sep 4, 2018

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