Mass timber has been on the move in Europe and Canada for some time, where the intersection of energy savings and renewable resources propels the industry forward. Though the technology has found a market in places like Bergen, Norway; Vancouver, British Columbia; and Stockholm, perhaps the greatest expression of confidence in engineered timber to date is the new 18-story dormitory at the University of British Columbia—the tallest mass timber structure in the world.

And yet, mass timber projects are scarce in the United States. While there has been considerable growth in the domestic production of mass timber products like cross-laminated (CLT) and glue-laminated timber (glulam), there are but a handful of demonstration projects—none, so far, exceeding the International Building Code’s (IBC) six-story limit for Type IV heavy timber construction.

One new office building in Minneapolis by British Columbia–based StructureCraft Builders, T3 (for Timber, Technology, and Transit), reaches only seven stories and includes a concrete podium. Other projects never make it off the boards, despite early buzz and backing. Regardless of this halting progress, innovation is happening with the scale and application of engineered timber.

The University of Massachusetts Amherst’s Design Building opened in January as the country’s largest mass timber framed academic building. Its novel and expressive use of exposed timber in stairwells and in zipper trusses knotted high above the atrium, in lateral and seismic bracing, renders it a key demonstration project.

As with any building code, wide local and regional disparity exists in the recognition of and reception to mass timber. In this regulatory vacuum, states can act as boosters.

The Massachusetts legislature opened funding channels and wrote letters to inspectors in support of the UMass Design Building. “We were not exempted from any code requirements,” says Tom Chung, AIA, principal at Boston-based Leers Weinzapfel Associates, the building’s designer. “Crucially, we engaged inspectors and fire marshals early in the design process.” Chung recommends this course of action for any would-be designer looking to smooth over the approval process. Mass timber is an architect-driven movement based around an unfamiliar technology.

A Lean Green Machine
A major boon to engineered timber lies in the technology’s superior energy performance, innate sustainability, and seamless assembly. An all-timber building can be built at a rate of nearly an entire floor per week. If a structure includes a concrete core or podium, the construction timetable may double or triple. T3, for example, is five times lighter than an equivalent concrete-and-steel building, with savings rippling through the jobsite and the supply chain, and was framed in nine-and-a-half weeks. With conventional construction, you’re building the structure three times: once for the scaffolding, once for the shell and framing, and again in taking down the scaffolding.

“All timber building components are either bolted together or slip right into place, with minimal welding,” Chung says. “This also makes for a relatively quiet construction site—great for urban settings.”

Mass timber buildings are inherently green. Other material and system inputs may compromise this initial state, but a highly sustainable completed project, often certified at LEED-Gold or higher, remains in easy reach. “Most timber frame buildings are carbon neutral or net at the outset,” says Lucas Epp, head of engineering at StructureCraft. “They trap embedded carbon rather than release it like steel.” (Epp discussed timber innovation in a recent episode of ArchitectChats.)

StructureCraft lays claim to myriad tall wood projects, including T3 and the 12-story Framework mixed-use building currently in design for Portland, Ore., which will feature the world’s first use of a post-tensioned rocking wall CLT core as the lateral seismic system.

Leers Weinzapfel also has a huge academic residence hall project—at 200,000 square feet with 700 beds—on the boards for the University of Arkansas. It’s not certain that it will be mass timber, but that option is being explored in the initial design phase. If timber is selected, the structure will be more than twice the size of the Design Building. “We’re trying to prove that any type of building can be done sustainably and cost-effectively in mass timber,” Chung says.

Though the concrete industry has lobbied against engineered timber as a threat on the jobsite, that industry does need to adapt to a greater diversity of building materials or else find ways to go green. As The Economist writes, energy-intensive cement production is one of the leading sources of pollution worldwide, and thus far the industry has exhibited little appetite for change.

Chung, for his part, doesn’t think the concrete industry should be too concerned. “We’ll always find uses for concrete, steel, and wood,” he says. “Concrete remains the best for foundations and cores, and steel for cantilevers and transfer beams.”

What about Fire?
For years, international testing has shown the impressive fire resistance of mass timber products, and advancements continue apace. CLT and sister products like nail-laminated timber can withstand fire for hours without the aid of a sprinkler system. Steel, meanwhile, melts in about an hour under the same conditions, threatening structural integrity.

Recently, the American Wood Council sponsored several fire tests of engineered timber in exposed and clad states, with and without sprinklers. The International Code Council’s (ICC) Ad Hoc Committee on Tall Wood Buildings created the test parameters. In good news for fans of tall wood, a press release reports that the CLT—which is required to have a minimum thickness of 8 inches plus another inch as the char layer—performed well in multiple scenarios, forming a protective char and “essentially self-extinguishing” after three to four hours of flame exposure.

Epp brings it home: “The [IBC’s] six-story height limit is based on an archaic standard that says a fire department ladder has to reach the top floor,” he says. “This doesn’t account for modern sprinkler systems or the improved fire resistance of mass timber.”

Epp and StructureCraft see a potential for wood to rise again 100 years after its heyday in large building framing. Heavy-timber warehouses like Minneapolis’ nine-story Butler Building, built in 1906, and others in places like Portland and Seattle which have deep economic ties to the resource, are testaments to mass timber’s strength and longevity.

Going to Committee
Codes move slowly, but there is reason to believe a new embrace of tall wood is on the horizon. One reasonable outcome, according to Epp, would be the creation of a tiered schedule of code requirements based on height, incorporated into the Type IV heavy timber building guidelines in the 2021 edition of the IBC. For instance, to build above six stories, designers may have to encase timber in stairwells in drywall; above 12 stories, all mass timber framing may require additional fire protection. These are just some possibilities, subject to a few more years of wrangling.

Mike Pfeiffer, senior vice president of technical services at the ICC, says professionals from the timber, steel, concrete, and masonry industries are represented on the Ad Hoc Committee on Tall Wood Buildings, setting the table for conflict and compromise—the very nature of code development.

The committee was formed in 2015 and meets periodically to review research and tall wood case studies. Working groups evaluate the science and built performance of tall wood, reporting findings to the committee. “I can’t speculate on what will or won’t be adopted in 2021, or even what the committee will recommend at the start of 2018,” Pfeiffer says, “but I will say that we’re very focused on gathering all the supporting research and documentation for the ICC’s consideration.”

The ICC’s educational apparatus would support a rollout and encourage local adoption of new timber codes. A modified Type IV code would constitute a substantial change, so it is crucial to orient policy makers, code professionals, builders, and designers to the implications and opportunities for their practices and in their cities and towns.

“I suspect the sustainability and cost-saving prospects of mass timber alone will inspire many jurisdictions to adopt,” Pfeiffer says.

Looking Forward
Advancements in digital technology for fabrication have opened up previously unthinkable applications for timber, supporting complex building geometries and ever-taller wood structures. Wood buildings of 30 to 40 stories have been proposed in Europe, and Skidmore, Owings & Merrill’s Timber Tower Research Project has prototyped a 42-story tower using a “concrete jointed timber frame” that relies on mass timber members with reinforced concrete applied at the stress points.

Most provocative are near-twin concepts for 80-story timber towers in London and Chicago. Perkins+Will and Thornton Tomasetti, architect and engineer of the Chicago proposal River Beech Tower, believe it can be built in the not-too-distant future.

They may be on to something.