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American Wood Council
Mass Timber and Tall Wood Buildings

Construction of the 18-story
Brock Commons
Student Residence at the
University of British Columbia

In New York City, it's 10 stories.

In Portland, a pair of them will top-out at 12 and eight stories.

Minneapolis has already moved forward with one at seven stories. Vancouver? It's 18 stories, already under construction. And overseas, there are an assortment of them ranging from eight to 20 stories.

The "it," of course, is tall timber, a growing mid-rise building category that uses mass timber such as cross-laminated timber (CLT) panels as primary building elements. A CLT panel is a relatively new product manufactured with wood laminations, such as from sawn lumber or structural composite lumber. Laminations are aligned to form layers, oriented at right angles to one another. Today, most panels range from three to nine layers in dimension and are up to 10-feet wide by 64-feet long. Panel thicknesses rarely would exceed 20 inches.

These dimensionally-stable, exceptionally-strong mass timber panels are then cut to size to form columns, beams, walls, floors, and roofs. The precut panels, shaped and routed to accommodate utilities, are then erected Lego-like in a precisely-engineered scheme at the jobsite. Other mass timber products, including both glued-laminated or nail-laminated timber, are also used depending on design.

Growing Interest

Today an increasing chorus of owners, architects, general contractors, and building product manufacturers are hailing these advances in tall wood as a powerful new way to construct mid-rise buildings. They cite structural stability, speed, economy, remarkable sustainability, beauty, and proven fire-resistance in office, residential, mixed-use, and commercial development.

In coming months building code professionals are likely to see and hear a lot more about tall wood. This report briefly outlines the current regulatory review being undertaken along with issues and opportunities for this revolutionary design methodology.

Code Approved CLT

The 2015 International Building Code (IBC) already officially recognizes CLT as a building element (IBC 602.4) and approves it for Type IV construction in sprinklered buildings up to 85 feet in height. CLT is also permitted for use in Type III construction for interior building elements and in Type V construction for any building element. The IBC does require that CLT conform to the Standard for Performance-Rated Cross-Laminated Timber (IBC 2303.1.4). As a result, building projects relying on these mass timber products are becoming significantly more numerous across North America.

For example, earlier this year, Lendlease, a property and infrastructure development and management company, opened the doors to a 92-room, 62,688 square foot Candlewood Suites hotel in Alabama built entirely with CLT.

Fire Safety

As news of mass timber projects like this spreads, it's expected owners and architects will choose to advance these building practices to larger-scale, mid-rise development. One factor that is helping to drive acceptance of mass timber is a growing body of fire safety test data.

"There's still confusion between how lightweight, low-rise wood framing reacts to fire versus how mass timber such as CLT reacts to fire," observes David Barber, a principal and fire-protection engineer for Arup, a global engineering company. "They're quite different components."

A team of scientists at the National Research Council of Canada, lead by Dr. Joseph Su, principal research officer, studied the fire performance of CLT versus a lightweight steel-frame system (LSF). The findings are published in a report titled Fire safety summary: Fire research conducted for the project on mid-rise wood construction. Among the findings:

"The CLT test structure performed better than the LSF structure in limiting the involvement of the structural materials in fire and in limiting the contribution of the structural materials to the growth and spread of fire."

"Wood has a very predictable fire behavior," Su explains. Contributing to that predictability is mass timber's inherent resistance to fire. Char, created on the wood surface as it burns, helps protect and insulate unburnt wood, preserving structural strength. Further, inclusion of gypsum wallboard provides additional protection and extends the fire resistance of the CLT even longer.

ICC Tall Wood Building Ad Hoc Committee

The International Code Council (ICC) has recognized the rising interest among many industry sectors in the tall timber discussion. In October 2015, the ICC Board of Directors solicited feedback on creation of an ad hoc committee on tall wood construction. As a result, the Tall Wood Building Ad Hoc Committee (TWB), was formally approved in December 2015.

TWB membership is comprised of a broad array of public- and private-sector officials. The Committee first met in July 2016. Among the proposals the group is exploring is the creation of new "Types of Construction" that would allow for up to 12- and 20-story mass timber buildings.

Next Steps

Building code professionals seeking additional information are encouraged to:

John Catlett, retired building official and now manager of code development for the AWC, says, "The voice of building code professionals has never been more important to the code development process. Building officials have the opportunity right now to be at the forefront of one of the most exciting developments in building regulation in a generation. The tall wood issue before us is the greatest advancement in building technology I've seen in my 30-year career."


The views and opinions expressed in this article are those of the sponsor and do not necessarily reflect those of the International Code Council, or Hanley Wood.


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