The American Wood Council defines mass timber as a “category of framing styles characterized by the use of large, solid wood panels for wall, floor and roof construction.” Also included in the definition are sculptural and non-building structures formed from solid wood panels or framing systems measuring six feet or more in width or depth.
Several products fall within the mass timber family, including cross laminated timber (CLT), nail-laminated timber (NLT), glued-laminated timber (glulam), dowel-laminated timber (DLT), structural composite lumber (SCL) and wood-concrete composites.
Mass timber can be produced in slabs as large as 18-feet-long by 90-feet wide and up to a foot thick. These large panels are created by layering and laminating wood boards such as pine, fir, spruce, birch, ash and beech together so the grain of each layer faces against the grain of the adjacent layer. This helps to make the panels strong.
Cross laminated timber was first developed in Austria in the early 1990s as a more sustainable solution for residential construction. American architects gave it a big boost as a building material once they saw the possibilities of using CLT for bigger, taller commercial buildings. Today, CLT is the most familiar and widely used form of mass timber in the U.S.
Appropriate Applications for Mass Timber
So, what’s the criteria for a mass timber project? If the load-bearing structure is constructed using mass timber, then the project qualifies as a mass timber project. If mass timber is used, but not as the primary structural element, then the project can’t be defined as a mass timber project. In short, if it holds the building up, it’s a mass timber project. If it’s used for decorative effect, it’s not.
Mass timber construction can be used for most every type of application, and results in high-performing and cost-competitive structures. Some applications for mass timber include high-end office buildings, hotels, multi-use buildings, public and institutional buildings, school and academic buildings, and multi-family residences. Here are a few examples.
The Pacific Northwest was one of the first areas in the country to embrace mass timber. The Radiator in Portland, Oregon, a 36,000-square-foot, five-story, Type IIIA project, was completed in 2015.
T3 Minneapolis, a seven-story, 220,000-square-foot, mixed-use building, was completed in 2016. At the time, it was the tallest mass timber building in the U.S. The T3 building features a mix of glulam columns and beams, NLT floors and a concrete core. It took an average of nine days to erect the 30,000-square foot floor and 9-1/2 weeks to frame. The project team estimated that the structure’s mass timber construction made T3 Minneapolis 30 percent lighter than it would have been if it had been made with steel and 60 percent lighter than post-tension concrete construction.
The award-winning Chicago Horizon, a Type IV public pavilion, features a CLT two-way, slab roof supported by glulam columns. The pavilion is the first use of exposed CLT in Chicago, paving the way for future mass timber structures in the city.
The four-story, 62,688-square-foot, Candlewood Suites hotel at Redstone Arsenal Army base in Alabama was quickly erected, even though it had to meet Anti-Terrorism and Force Protection standards. Compared to a hotel using traditional construction materials, this CLT project took 37 percent less time and enjoyed a 44 percent reduction in structural manpower hours.
Why the Hype?
Wonder why everyone’s getting so excited about a building material that’s been around since prehistoric times? There are lots of reasons architects, engineers and developers are eager to design buildings with mass timber.
As we’ve seen, projects using mass timber can be completed much faster and easier. The large panels can be bolted together or slipped into place with a minimum of welding, making it possible for an all-timber building to be built at a rate of nearly an entire floor per week. And less time equates to more savings.
But one of the most compelling reasons people are interested in mass timber is that it’s a renewable resource that can reduce a building’s carbon footprint. A recent study found that using wood products for construction projects could save 14 to 31 percent of global CO2 emissions and 12 to 19 percent of global fossil fuel consumption. During production at the factory, wood products don’t need to heated to more than 2,700 F. like steel or concrete.
Buildings constructed with wood have a warmth and natural beauty that is aesthetically pleasing, while promoting a healthier indoor environment. Developers are counting on this to attract tenants. Other pluses include mass timber’s thermal qualities that can reduce energy costs, its resiliency to seismic events and its performance in fire events.
Mass timber is fire resistant. It doesn’t “go up in flames” like light-frame construction. Instead, it chars on the outside, creating an insulating layer that protects the interior of the wood, adding to its inherent slow-burning properties.
Recent research backs up mass timber’s structural performance during a fire. The American Wood Council sponsored several fire tests of engineered timber, both exposed and clad and with and without sprinklers. Mass timber performed well in all of these scenarios, forming a protective char and “essentially self-extinguishing” after three to four hours of flame exposure, exceeding ASTM E 119 requirements. By contrast, steel melts in about an hour under the same conditions, threatening structural integrity.
Will We See Mass Timber Taking More Share?
The early 1900s was the heyday of large building framing with heavy timber, but all indications are that it’s gearing up for timber to become mainstream once again.
Before 2011, the U.S. didn’t have a single dedicated CLT plant. Mass timber products had to be imported from Canada and Europe. A big boon for mass timber construction came in 2015 when CLT was incorporated into the National Design Specification for wood construction. That led to it being recognized as a building timber product in the 2015 International Building Code (ICC).
In 2019, Katerra, Vaagen Timbers and SmartLam opened mass timber plants, doubling CLT production in the U.S. In mid-2021 Structurlam will open a 288,000-square-foot CLT plant in Arkansas. And, SmartLam plans to open three additional plants by 2022.
Currently, it appears the main thing holding up mass timber’s steady progress in gaining market share in the U.S. has to do with the building codes. Not every state has changed its code to acknowledge and set up regulations for mass timber product construction. And in some states, codes vary from region to region and municipality to municipality.
The ICC has proposed allowing certain mass timber buildings to be up to 18 stories high and 270 feet tall. Once the new codes are passed and kick in, there are at least 70 tall wood building projects in the works that can then get underway.
As with any new trend, there’s always some resistance and skepticism, especially when it comes to heavy timber building construction projects. Some people aren’t sure mass timber will be able to meet its expectations. But for now, it looks like it’s full steam ahead!
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