Cross Laminated Timber (CLT) Design + Construction
A-cross the board.
What is CLT?
How It Works
CLT panels consist of layered lumber boards (usually three, five, or seven) stacked crosswise at 90-degree angles and glued into place. Finger joints and structural adhesive connect the boards. Board thickness typically varies between 5/8 inch to 2 inches, with board width ranging from 2.4 to 9.5 inches.The panels can be manufactured at custom dimensions, though transportation restrictions dictate their length.
Alternating grains improve CLT panels’ dimensional stability. This strength affords designers a host of new uses for wood, including wide prefabricated floor slabs, single-level walls, and taller floor plate heights. As with other mass timber products, CLT can be left exposed in building interiors — up to 8 stories in the 2021 IBC, offering additional aesthetic attributes.
Like other mass timber products, CLT can be used in hybrid applications with materials such as concrete and steel. It can also be used as a prefabricated building component, accelerating construction timelines.
How It's Used
Applications for CLT include floors, walls and roofing. The panels’ ability to resist high racking and compressive forces makes them especially cost-effective for multistory and long-span diaphragm applications.
In structural systems, such as walls, floors, and roofs, CLT panels serve as load-bearing elements. As such, in wall applications, the lumber used in the outer layers of a CLT panel is typically oriented vertically so its fibers run parallel to gravity loads, maximizing the wall’s vertical load capacity. In floor and roof applications, the lumber used in the outer layers is oriented so its fibers are parallel to the direction of the span.
The 2015 International Building Code (IBC) and 2015 International Residential Code recognize CLT products manufactured according to the ANSI/APA PRG-320: Standard for Performance Rated Cross-Laminated Timber. Under the 2015 IBC, CLT at the required size is specifically stated for prescribed use in Type IV buildings. However, CLT can be used in all types of combustible construction—i.e., wherever combustible framing or heavy timber materials are allowed. The National Design Specification (NDS ) for Wood Construction is referenced throughout the IBC as the standard for structural wood design, including CLT. The 2012 IBC does not explicitly recognize CLT, but the 2015 IBC provisions for CLT can be a basis for its use under alternative method provisions. The 2021 IBC approves CLT for buildings up to 18 stories in Type IV-A, IV-B, and IV-C.
Case in Point
Albina Yard
Albina Yard, a four-story office building in Portland, Oregon, was the first U.S. building to use a domestically-made CLT structural system when it opened in September 2016.
The project team chose CLT for its ability to store carbon and the smaller environmental footprint the material afforded the project. Check out the project page for images of the building and to learn how CLT and glued-laminated timber (glulam) were used in the design.
