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Although nothing new, the debate over which is better as a building material—wood or concrete—intensified in December following the preliminary approval of new codes for cross-laminated timber and mass timber in tall structures.

The discussion among industry professionals has been less about CLT’s structural capabilities and more about its perceived flammability, with either side offering decidedly different perspectives. Comparatively new to the United States, CLT and mass timber products are constructed of several layers of pressed lumber board stacked in alternating directions.

In December, the International Code Council released the unofficial voting results on several code change proposals, including passage of the entire package of 14 tall mass timber codes. The proposals were presented by the ICC’s Ad Hoc Committee on Tall Wood Buildings, comprised mostly of engineers, architects, building and fire code officials, fire service, materials and testing lab representatives. 

The new codes will be included in the 2021 edition of the International Building Code, pending the announcement of official results in the first quarter of 2019. They create three new types of construction, which set fire safety requirements and allowable heights, areas and number of stories for tall mass timber buildings.

  • Type IV-A: Maximum 18 stories, with gypsum wallboard on all mass timber elements. 
  • Type IV-B: Maximum 12 stories, with a limited area of exposed mass timber walls and ceilings allowed. 
  • Type IV-C: Maximum nine stories, with all exposed mass timber designed for two-hour fire resistance.

The wood industry has been ecstatic about the outcome. “Mass timber has captured the imagination of architects and developers, and the ICC result means they can now turn sketches into reality,” says American Wood Council President & CEO Robert Glowinski in a statement. “ICC’s rigorous study, testing and voting process now recognizes a strong, low-carbon alternative to traditional tall building materials used by the building and construction industry.” 

On the other hand, some question the logic behind using CLT in tall structures given its combustible nature. “Human life and devastating economic consequences are at stake when combustible wooden high-rise buildings are on the table, and it deserves the highest level of care and educated consideration,” says Kevin Lawler, spokesperson for Build With Strength, a coalition of groups spearheaded by the National Ready Mixed Concrete Association (NRMCA). Notably, the Steel Framing Industry Association is also a member.

“Where the ICC has failed to protect citizens, fire safety professionals and local economies from the devastating impact of fires, Build with Strength stands committed to fighting for safer communities,” Lawler states.

So, What Now?

The impact of the new codes could be major or minor, depending on location.

Doug Sweeney, associate vice president of McVeigh & Mangum Engineering Inc. in Charlotte, North Carolina, says some regions of the country won’t use CLT for tall wood structures, at least for now, simply because of logistics. His multi-disciplined firm performs projects from coast to coast. “We haven’t seen a lot of CLT in the Southeast,” Sweeney says. “We’ve researched it, but it’s never really panned out for one reason or another, usually because of lack of availability and cost.” 

However, the supply side of the equation could be changing. According to the AWC, there are now three U.S. CLT manufacturers—D.R. Johnson Wood Innovations in Riddle, Oregon; SmartLam LLC in Columbia Falls, Montana; and IB X-Lam USA LLC in Dothan, Alabama—and four others are under development in Washington and Maine. 

Sweeney says a lack of local industry experience with CLT is another drag on the market, but adds that the product could become more attractive as factors change. “The efficiency of manufacturing and construction improves [with CLT] because it goes up more quickly. You’ve got fewer pieces to contend with, and less time in the field,” he says.

In the Northwest, supply has never been a problem, and the prominence of the timber industry there virtually assures strong local support. Various efforts by local and state governments aim to incorporate the product in myriad ways. Joe Mayo, a project architect at Mahlum Architects Inc. in Seattle, recently designed several portable CLT classrooms in Seattle, Sequim and Mount Vernon under a single contract with the Washington Department of Enterprise Services. His 90-architect firm (across two offices) primarily designs K-12 educational facilities, along with various health care and higher educational projects. 

The classroom project was borne out of a desire to identify a project type that could support the increased use of mass timber, and Washington’s growing need for additional classroom space was a perfect fit. “Washington is probably unique in that there’s broad bipartisan support at the state level for new mass timber construction technologies,” Mayo says. “Our challenge was to provide a better learning environment, utilize cross-laminated timber for its prefabricated nature to speed up construction, and provide a warmer, calmer interior environment for learning.”

CLT is gaining a foothold in other areas as well. Lendlease and IHG Army Hotels hosted a Nov. 14 ribbon-cutting ceremony to celebrate the grand opening of the Candlewood Suites on Fort Drum, New York, the second mid-sized hotel in the United States constructed of CLT. The 65,000-square-foot hotel incorporates CLT for the hotel’s floors, roof, exterior walls, shaft walls and select interior walls.

“Implementing CLT increases the safety on our project site during construction, reduces energy consumption and teaches job skills to local veterans,” states Claire Johnston, managing director of Lendlease Communities in a press release. “Partnering with the U.S. Army … has allowed us to push the boundaries of Lendlease’s innovative nature to provide well-built, well-maintained hotels for soldiers for years to come.” 

The Fire Question

The concrete industry argues that using CLT in tall wood buildings puts occupants at unnecessary risk.

Build with Strength has already seen a spike in fire conflagrations in the United States due to an increase in the number of low- to mid-rise buildings built with wood. For his group at least, there’s no question that wood’s flammability makes it a poor choice for tall buildings. “NRMCA and our partners sell a non-combustible product, one that we feel is safer for communities, and for people who live and work and play in these types of buildings,” Lawler says. 

John Norton, a spokesman for the Portland Cement Association in Washington, D.C., says the new codes for tall wood buildings are problematic because CLT is “untested, unproven and unsound. There’s a rush going on here that is not to the benefit of the public.”

In November, the United Kingdom limited the use of CLT in a ban on combustible building materials following a fire in London’s 24-story Grenfell apartment tower that killed 72 occupants. The new government policy severely limits the use of materials to those with a European fire rating of Class A1 or A2, and explicitly states that wood products do not fall under this classification.

Nonetheless, the AWC says the concrete industry’s negative stance is merely about market share, not the flammability of wood. Several states are adopting tall mass timber building codes (Oregon, Washington and California), and others likely will follow as more CLT manufacturing facilities spring up across the country.

Maria Laguarda-Mallo, a BIM/VDC project engineer at Chicago-based VIATechnik LLC, says her research shows that CLT does not pose an unnecessary fire risk. She holds a master’s degree in timber construction and a Ph.D. in bioproducts engineering. “I always ask people: ‘Can you make a log catch fire easily?’ No, you need a lot of time and a lot of combustible material. It’s a long process. It chars first, and then eventually, after hours and hours, it burns.” 

As part of her research, Laguarda-Mallo interviewed architects, engineers, contractors and CLT experts to examine the pros and cons of CLT and other wood products. 

“I discovered that fire marshals like working with wood because it lets them know when it’s going to collapse, whereas steel gets to a certain temperature and collapses abruptly. Wood structures provide them with more time to get people out of a building.”

Glowinski says the ICC Ad Hoc Committee was well aware of flammability concerns, so fire prevention experts played an integral role in the development of the code proposals. He points out that the chairman of the committee was Stephen DiGiovanni, a fire protection engineer for the Clark County Department of Building and Fire Protection in Nevada. 

The Speed and Strength of Construction

Apart from the flammability argument, some in the concrete industry agree that wood could be quicker to install in certain conditions, as it can be pre-assembled and erected onsite. But there are other factors to consider, Lawler says. 

“Concrete is an incredibly sustainable and durable material. It’s very resilient. If you want construction that’s built to last and that has low maintenance issues, concrete is the way to go. It’s one of the oldest building products in the world, whereas CLT is literally something that has been created right here, right now.”

Laguarda-Mallo concedes that CLT doesn’t have the historical track record of concrete, but feels it will become a dominant player once the industry gains awareness. She adds that CLT is highly competitive from a structural standpoint, too, as it is engineered to withstand seismic and other structural forces. 

“In some applications, it’s actually ideal to incorporate CLT with concrete and steel. Wood’s limitations become more apparent the taller the structure, but it can be more effective in a composite application that includes both concrete and steel, using them together to create a more sustainable and environmentally friendly building. When we need the strict structural capabilities of concrete or steel, we can use those elements to support the wood,” Laguarda-Mallo says.

For this and other reasons, there is widespread interest about CLT across the United States, particularly from a design perspective. “This brings another option to the design community, one in which they seem to have embraced,” Glowinski says. “There is a lot of enthusiasm among architects and designers to use mass timber.” 

Photo courtesy of AWC

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