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Cross-laminated timber (CLT) is a wood panel system that is gaining in popularity in the U.S. after being widely adopted in Europe. CLT is the basis of the tall wood movement, as the material’s high strength, dimensional stability and rigidity allow it to be used in mid- and high-rise construction.
CLT panels are made of layers of lumber boards (usually three, five or seven) stacked crosswise at 90-degree angles and glued into place. The panels can be manufactured at custom dimensions, though transportation restrictions dictate their length.
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.
Some specifiers view CLT as interchangeable with other wood products and building systems. 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.
Several factors contribute to a growing market for CLT and tall wood construction: advances in wood connectors, the development of hybrid materials and building systems, the commercialization of CLT and growth in off-site fabrication.
Alternating grains improve CLT panels’ dimensional stability. The lumber boards typically vary in thickness from 5/8 inch to 2 inches and in width from 2.4 inches to 9.5 inches. Finger joints and structural adhesive connect the boards.
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.
CLT’s ability to resist high racking and compressive forces makes it a cost-effective solution for multistory and long-span diaphragm applications.
CLT’s shear strength affords designers a host of new uses for wood. Those include 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, offering additional aesthetic attributes.
Currently, U.S. building codes do not explicitly recognize mass timber systems, but this doesn’t prohibit their use under alternative method provisions. The 2015 International Building Code (IBC) streamlines the acceptance of CLT buildings, recognizing CLT products when they are manufactured according to the Standard, ANSI/APA PRG-320: Standard for Performance Rated Cross Laminated Timber. In addition, CLT walls and floors may be permitted in all types of combustible construction, including Type IV buildings.
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 as well as glue-laminated timber (glulam) were used in the design. Read more.
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