{"id":26626,"date":"2017-02-01T14:56:45","date_gmt":"2017-02-01T19:56:45","guid":{"rendered":"http:\/\/www.constructionspecifier.com\/?p=26626"},"modified":"2017-02-15T15:52:59","modified_gmt":"2017-02-15T20:52:59","slug":"cross-laminated-timber-new-projects-show-how-the-material-is-fulfilling-tall-orders","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/cross-laminated-timber-new-projects-show-how-the-material-is-fulfilling-tall-orders\/","title":{"rendered":"Cross-laminated Timber: New projects show how the material is fulfilling ‘tall’ orders"},"content":{"rendered":"
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Images courtesy LEVER Architecture <\/span><\/em><\/figcaption><\/figure>\n

by Kenneth E. Bland, PE
\n<\/strong>Modern-day technologies and product delivery methods have created a bold new opportunity for wood products, which in turn has given rise to a new category of structural framing systems using mass timber. The possibility of tall wood buildings represents the first significant challenge to concrete and steel structures since their inception in high-rise building design more than a century ago. Cross-laminated timber (CLT) is among the products helping drive this trend.<\/p>\n

CLT is a prefabricated engineered wood product made by gluing layers of solid-sawn lumber or structural composite lumber. Each layer can consist of one or more laminations of wood overlaid in perpendicular fashion. This cross-lamination provides dimensional stability, strength, and durability equal to or greater than competing materials in many applications. Size varies by manufacturer, but panels typically include three, five, or seven layers; they rarely exceed 508 mm (20 in.) in thickness.<\/p>\n

CLT can be connected in various ways in wall-to-foundation, wall-to-wall (straight or junction), floor-to-floor, wall-to-floor, and wall-to-roof applications. Panels may be connected to each other with half-lapped joints or splines made from engineered wood products to construct large surfaces of CLT. Metal brackets, hold-downs, and plates are often used to transfer forces between horizontal and vertical diaphragms.<\/p>\n

Using CLT in combination with other mass timber elements\u2014such as glued-laminated timbers (glulam) and structural composite lumber (SCL)\u2014offers several advantages, including use of a renewable resource, greater environmental performance, cost-effectiveness, reusability, and aesthetics. However, one of the biggest advantages is improved speed and efficiency of installation, given the panels can be prefabricated offsite according to the end-use application, with pre-cut openings for windows, doors, stairs, service channels, and ducts. On arrival from the manufacturing facility, the panels are ready for quick installation, shaving weeks or even months off the construction schedule.<\/p>\n

Additionally, CLT meets stringent fire and seismic building code requirements.\u00a0<\/sup>(For more on the topic, see this author\u2019s previous article for The Construction Specifier<\/em> [September 2016], \u201cUpdating Code-conforming Wood Designs.\u201d It can be read online by visiting www.constructionspecifier.com\/updating-code-conforming-wood-designs<\/a>.) As with heavy timber, CLT chars at a slow and predictable rate, maintaining strength longer and giving occupants more time to exit the building. Structures using CLT for wall and floor assemblies also tend not to have as many concealed spaces within the floor and wall assemblies, further reducing the risk a fire will spread unnoticed.<\/p>\n

When seismic performance is considered, CLT structures with fasteners designed in yielding mode offer many of the same favorable characteristics
\nas lightweight wood-frame construction. Yielding fasteners offer ductility, helping absorb energy from seismic ground motions and distributing forces throughout the seismic-resisting system.<\/p>\n

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Albina Yard (Portland, Oregon) was designed by LEVER Architecture and engineered by KRFF Consulting Engineers. It is the country’s first structure to use domestically fabricated mass timber for a building-wide structural system.<\/span><\/figcaption><\/figure>\n

State of the art: Recent CLT projects
\n<\/strong>Spanning from Oregon to Massachusetts, several U.S. projects are already harnessing the potential of CLT. These projects are not only demonstrating what is \u2018permissible\u2019 when using wood, but also what is \u2018possible.\u2019<\/p>\n

Albina Yard
\n<\/em>Designed by LEVER Architecture and engineered by KPFF Consulting Engineers, Albina Yard (Portland, Oregon) is the first structure in the United States to use domestically fabricated mass timber for a building-wide structural system. From the beginning, the design and construction teams collaborated closely on digital fabrication to improve the efficiency of construction while minimizing wasted building materials and maximizing the speed of onsite panel erection. Completed in early fall of 2016, the 1485-m2<\/sup> (16,000-sf) creative office\/retail mixed-use building utilizes a glulam frame with CLT structural panels.<\/p>\n

Framework
\n<\/em>Another Portland project designed by LEVER Architecture, Framework is the West Coast winner of the U.S. Tall Wood Building Prize Competition, which was funded in part by the U.S. Department of Agriculture (USDA), Softwood Lumber Board, and the Binational Softwood Lumber Council to support the research and development of tall wood demonstration projects in the United States.<\/p>\n

Set to be the tallest mass timber structure in the country, Framework is a 12-story mixed-use building featuring a unique CLT lateral-force-resisting system with CLT floors and glulam beams and columns. It will be the first post-tensioned rocking CLT wall system in the United States, and the tallest post-tensioned rocking wall project in the world.<\/p>\n

Framework is well on its way to demonstrating the full potential of CLT. Last fall, LEVER successfully demonstrated a mass timber assembly using CLT and glulam members can pass code-required fire-resistive testing for structures of the same height and number of stories. The project team also conducted tests on local CLT in Oregon to confirm the feasibility of locally sourcing wood, fulfilling the USDA grant mission to leverage CLT to propel rural economies. In December, the team completed structural testing to determine the stress-strain properties of the CLT wall panels to be used in the project. It is now beginning testing of the floor assembly to confirm the acoustic performance.<\/p>\n

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Another LEVER Portland project, Framework is set to be the tallest mass timber structure in the country, the 12-story mixed-use building will feature a cross-laminated timber (CLT) lateral-force-resisting system with CLT floors and glulam beams and columns. <\/span><\/figcaption><\/figure>\n

UMass Design Building
\n<\/em>Designed collaboratively by Leers Weinzapfel Associates and Equilibrium Consulting, the new four-story, 8100-m2<\/sup> (87,200-sf) University of Massachusetts Amherst (UMass) Design Building is informed by the school\u2019s current research in building technology. It features an exposed glulam timber post and beam structural system with CLT shear wall cores and roof decking. A CLT zipper truss spans the building commons space at the second level. Construction is estimated to be completed in early 2017. The design is targeting Gold certification under the Leadership in Energy and Environmental Design (LEED) program.<\/p>\n

The new facility will house the university\u2019s multidisciplinary design program\u2014fostering collaboration across architecture, building technology, landscape architecture, and regional planning\u2014and will serve as a highly visible demonstration of sustainable design practice and leading-edge timber engineering. Other sustainable features include automated lighting, an energy-efficient ventilation system, low-flow faucets, nearby access to alternative transportation, and bioswales integrated into the exterior landscaping around the building.<\/p>\n

CLT modular classrooms
\n<\/em>Another example of CLT application can be seen in Washington state, where five school districts are participating in a $5.5 million pilot program to incorporate CLT modular construction in K\u22123 elementary classrooms. From both an economic and environmental standpoint, supporters are increasingly interested in growing the CLT industry in Washington, believing it will create local jobs, and make it profitable to thin forests, reducing the danger of wildfires. The CLT used in these projects will likely come from the millions of \u2018beetle-killed\u2019 fir and pine trees that pose a huge forest fire risk and are already marked for removal. (For a deeper look at possibilities with beetle-killed wood, see the October 2014 edition of The Construction Specifier<\/em> for \u201cSeeing the Urban Forests for the Trees: Secondary benefits of our cities\u2019 wood\u201d by J. Gerard Capell, FCSI, AIA, CCS.
\nVisit www.constructionspecifier.com\/seeing-the-urban-forests-for-the-trees-secondary-benefits-of-our-cities-wood-2<\/a>.)<\/p>\n

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LEVER successfully demonstrated a mass timber assembly using CLT and glulam members passes fire-resistive testing for structures of the same height and number of stories. Structural testing determined the stress-strain properties of the wall panels to be used.<\/span><\/figcaption><\/figure>\n

Resources available
\n<\/strong>There are many resources available to those who are interested in learning more about CLT and mass timber. The 2015 National Design Specification <\/em>(NDS) for Wood Construction<\/em> added a new chapter with design provisions for CLT. (For more on NDS<\/em> [along with other codes and standards, including the International Building Code (IBC)]<\/em>, see our web-exclusive four-part series, \u201cWhat the 2015 International Building Code<\/em> Means for Wood Construction\u201d by Buddy Showalter, PE. Part 1 begins with www.constructionspecifier.com\/what-the-2015-international-building-code-means-for-wood-construction-part-i<\/a>.) CLT-specific changes reflected in the 2015 NDS<\/em> include:<\/p>\n