CLT’s state of the union

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    Promega in Madison, Wis. Promega, a global leader in life sciences, applied the same innovative philosophy they use to manufacture biotech products to the design of the client center for their new production facility. The Crossroads features a mix of exposed glulam and CLT to create a distinctive, warm aesthetic for clients, guests and staff.
    Photo by Structurlam Products

    There has been a lot of speculation about the potential for cross laminated timber (CLT) to change the building landscape. Internationally, CLT has expanded wood design possibilities with groundbreaking projects that range from the 96,875-square-foot Norwich Academy in the UK to a 10-story apartment building in Australia.

    Now produced in North America and soon to be recognized in the International Building Code, CLT is the latest mass timber product to offer a viable, carbon-friendly building alternative to concrete or steel. It is appropriate for mid-rise urban infill, industrial, educational and civic applications (among others), and compares particularly well to concrete tilt-up construction. It also offers quicker installation than other materials (in one case, eight stories of CLT were erected in just eight weeks), as well as reduced waste, improved thermal performance and design versatility.

    This article provides a summary of information on CLT relevant to structural engineers. For more information, WoodWorks (www.woodworks.org) provides technical resources, including free one-on-one project support for design professionals. A detailed list of resources including design tools, a list of North American CLT manufacturers and an extensive FAQ is available at www.woodworks.org/design-with-wood/building-systems-clt/ Additional information on CLT as well as other mass timber products can also be found at www.masstimber.com

    About CLT
    CLT is made from layers of dimension lumber, each stacked at right angles to the adjacent layers and glued to form a solid panel. By varying the number of layers as well as the lumber species, grade and thickness, CLT panels can be used in any assembly type, including walls, floors, roofs, elevator shafts, stairways and others. Lumber is kiln dried prior to lamination, which adds to the dimensional stability of the panels.

    Bridport House in the Borough of Hackney, London. When it was constructed in 2011, this five- and eight-story structure formed the largest timber-built apartment block in the world. The site required a lightweight structure because of the building’s location over a large storm sewer, making CLT an ideal solution. Photo by Karakusevic Carson Architects.

    CLT is currently available in North America with dimensions up to 19-1⁄2 inches thick, 18 feet wide and 98 feet long. Manufacturers use CNC equipment to pre-cut panels and openings to exact specifications, often to meet very tight tolerances (within millimeters).

    Cost-competitive
    CLT is typically most cost effective in large, regular shaped structures or in situations where fabrication is repetitive; it is particularly competitive compared to concrete in these applications. CLT is unlike most wood products on the market in that it is not a commodity product and therefore cannot easily be quoted on a per-square-foot basis. Panel costs are dependent on many factors; it is best to check with a CLT manufacturer for specific pricing.

    Code approvals
    Code-allowed height for combustible buildings varies around the world. Currently, U.S. and Canadian building codes do not explicitly recognize mass timber systems, but their use is allowed in Type V Construction and through alternative method provisions.

    Recently-approved changes to the 2015 IBC will streamline the acceptance of CLT buildings, in part by recognizing CLT products when they are manufactured according to the ANSI/APA PRG 320 2012 standard, which covers manufacturing, qualification and quality assurance requirements. In addition, CLT walls and floors will be explicitly recognized for use in Type IV buildings and permitted in all types of combustible construction.

    Fire performance
    CLT’s thick cross-section provides valuable fire resistance because once formed, char protects the wood from further degradation. Other life safety benefits of CLT include low heat conductance (e.g., a fire in one room would create very little temperature differential in the next), self-extinguishing properties (after the external fuel and flame have been removed), and reduced smoke and fire propagation (when use includes partition walls).

    Recent fire resistance testing conducted by the American Wood Council (AWC) found that CLT exterior walls exceed the requirements for heavy timber construction. AWC conducted a successful ASTM E119 fire endurance test on a CLT wall at NGC Testing Services in Buffalo, N.Y. The wall, a five-ply CLT specimen (approximately 7 in. thick), was covered on each side with a single layer of 5/8-in. Type X gypsum wallboard. The wall was loaded to the maximum attainable by the test equipment, although it remained significantly below the full design strength of the CLT. It was then exposed to a standard fire that reached over 1,800 degrees Fahrenheit in the first 90 minutes of exposure. While AWC was only seeking a two-hour rating as required by building code provisions, the test specimen lasted 3 hours, 6 minutes.

    Seismic performance
    Because of their dimensional stability and rigidity, CLT panels create an effective lateral load resisting system. Extensive seismic testing has found CLT panels to perform exceptionally well, with no residual deformation and good ductile behavior and energy dissipation, particularly in multi-story applications.

    For example, full-scale models of three- and seven-story CLT structures were tested in 2009 on the world’s largest shake table in Japan. The buildings performed extremely well, even when subjected to motions comparable to the 1995 Kobe earthquake (magnitude of 7.2 and accelerations of 0.8 to 1.2 g). After 14 consecutive simulated earthquakes, the seven-story building showed no residual deformation; maximum inter-story drift was 1.57 inches and maximum lateral deformation at the top of the building was 11.3 inches.

    The Long Hall in Whitefish, Mont. Completed in 2011, The Long Hall was the first commercial building project in the U.S. constructed with CLT. Builders completed the two-story, 4,863-square-foot commercial mixed-use building in just four days. CLT was left exposed on the building’s interior to take advantage of wood’s natural aesthetic. Photo by Pete Kobelt.

    A seismic response factor for CLT cannot yet be found in ASCE 7; it is still going through the code acceptance process. Designers rely on Section 12.2.1 in ASCE 7-10 for use under today’s code. The U.S. CLT Handbook provides suggested conservative seismic R values and supporting test data for engineers and building officials to determine whether requirements are met.

    Acoustic performance
    CLT building systems provide adequate noise control for both airborne and impact sound transmission. Numerous assemblies have been tested; the maximum STC rating in tested floor assemblies is 67, and 60 in tested wall assemblies. Certain assemblies can reach an IIC of 65.

    Thermal performance
    CLT’s thermal performance is determined by its U-value, or coefficient of heat transfer, which relates to panel thickness. Thicker panels have lower U-values; they are better insulators and therefore require less insulation.

    Natural advantages
    Manufactured using wood from sustainably managed forests, CLT provides a number of environmental benefits in addition to its excellent thermal performance. Wood is the only major building material that grows naturally and is renewable. Lifecycle assessment studies consistently show that wood outperforms steel and concrete in terms of embodied energy, air pollution and water pollution. It also has a lighter carbon footprint. Wood products continue to store carbon absorbed by the trees while growing, and wood manufacturing requires less energy and results in less greenhouse gas emissions.

    CLT is a new generation building product for a new generation of building.

    New U.S. CLT handbook now available
    The new U.S. CLT Handbook, developed with input from U.S., Canadian and European building experts, is now available. This valuable tool contains details on U.S. codes and standards along with information that engineers can use to achieve building permit approvals under an alternate means approach. With more than 500 pages, the comprehensive handbook also contains structural design calculations and connection details, along with information about vibration, sound and fire performance and more. The Handbook is available as a free download from www.masstimber.com Hard copies may also be purchased.

    Key structural characteristics of CLT

    • CLT systems offer a number of beneficial structural characteristics, including:
    • High dimensional stability and static strength in all directions
    • High stiffness/strength-to-mass ratio
    • High axial load capacity for walls due to large bearing area of the solid panels
    • High shear strength to resist lateral loads coupled with good ductility
    • Wall systems that are rigid around openings, which allows fewer hold-downs
    • Failure modes that are ductile and occur at connections
    • Low probability of in-plane buckling
    • Great span-to-depth ratio for floor panels, which allows shallow floors

    Lisa Podesto, P.E., is the senior technical director and lead for building systems for WoodWorks, an initiative of the Wood Products Council established to provide free education and technical support to design and building professionals using wood in non-residential buildings.