Wood technology is advancing quickly, and our view of what’s possible with timber in the built world is shifting along with it. Traditional post and beam wood construction is now a dated, limiting construct that no longer represents the maturation of the timber industry. So, if not traditional post and beam wood construction, what are we talking about? Mass timber.
The Canadian Wood Council defines “mass timber” in the following way: Mass timber products can be formed by mechanically fastening and/or bonding with adhesive smaller wood components such as dimension lumber or wood veneers, strands or fibres to form large pre-fabricated wood elements used as beams, columns, arches, walls, floors and roofs. Products such as cross-laminated timber (CLT), nailed-laminated timber (NLT), glued-laminated timber (GLT), laminated strand lumber (LSL), laminated veneer lumber (LVL) and other large-dimensioned structural composite lumber (SCL) products are part of a bigger classification known as ‘mass timber’.”
Europe and Canada are ahead of the curve in embracing mass timber, vetting it through research, applicable building codes, and breaking ground on new construction. Mass timber is still in its infancy in the U.S., and the challenges here are two-fold.
First, the coding requirements in the U.S. don’t currently support mass timber’s features and capabilities. Right now, a mass timber project would have to be classified as “heavy timber”, type 4 construction. But mass timber is not heavy timber; it doesn’t have the height limitations of heavy timber, and so doesn’t fit inside that category.
At the moment, the height limit for mass timber is 85 feet, approximately 5-6 floors. In 2021, coding will change to more specifically address that limitation. Beginning in that year, mass timber structures can be built to six stories completely exposed; 12 stories partly exposed, and 18 stories completely covered.
The second challenge has to do with educating both the A&D community and clients about mass timber. Mass timber advocates are beginning to succeed in shaking up the concept of what people picture when they think of a timber building, but there’s still much more work to be done.
Two of the most obvious downsides people imagine when they hear mass timber deal with safety and durability. Contrary to the popular belief that wood structures burn more easily than those made from other materials, mass timber structure meet and generally exceeds fire resistance requirements.
From timber industry advocate Think Wood: “During a fire resistance test of a 5-ply cross-laminated timber (CLT) panel wall, the panel was subjected to temperatures exceeding 1,800 Fahrenheit and lasted 3 hours and 6 minutes, far more than the two-hour rating that building codes require. During fires, exposed mass timber chars on the outside, which forms an insulating layer protecting interior wood from damage. Additionally, when the code requires mass timber to be protected with gypsum wall board, the mass timber can achieve nearly damage-free performance during a contents-fire burnout event.”
Mass timber is also incredibly strong, but isn’t weighed down like concrete and steel.
“Recent mass timber buildings weigh approximately 1/5th that of comparable concrete buildings, which in turn reduces their foundation size, inertial seismic forces and embodied energy,” notes Think Wood’s mass timber resource page. “High strength-to-weight ratios enable mass timber to perform well during seismic activity.”
Another barrier mass timber advocates must eliminate is the idea that mass timber is a danger to forestry.
“In sustainable forestry, growth exceeds harvest by more than 40% in North America,” says Sean McTaggart, a proponent of mass timber and a project architect at Hickok Cole. “There are more than enough trees as long as those trees are harvested responsibly.”
“The concrete and steel industries are highly inefficient and can be highly destructive to our environment, and there’s a finite number of resources,” says Mr. McTaggart. “Forestry definitely has its challenges, but the harvesting of these other materials is way more detrimental than sustainable forestry.”
More research is also being gathered on mass timber’s ability to remove CO2emissions – a quality other materials can’t offer. From Think Wood:
“Replacing steel with mass timber would reduce carbon dioxide emissions by between 15% and 20%. By some estimates, the near-term use of CLT and other emerging wood technologies in buildings 7-15 stories could have the same emissions control effect as taking more than 2 million cars off the road for one year. The fire tests confirmed that mass timber structures meet and generally exceed the fire resistance requirements in the current code. Studies have shown that building with wood produces fewer greenhouse gas emissions than building with other materials.”
The case for mass timber also applies to wellbeing. Wood’s positive biophilic impact on human wellbeing is clear; research on the subject has consistently shown that people inherently prefer natural materials – conveying warmth, nature and comfort – over synthetic ones. In the workplace, that translates to reduced absenteeism, improved productivity, and improved overall worker happiness and wellbeing.
In terms of cost-analysis, mass timber is still a premium material. But its sustainable qualities (noted above) are catching the attention of more developers and clients.
Mass timber advocates also point to the material’s ease and speed of construction; pre-engineered wood takes less time to install, thus cutting down on construction costs. An earlier project completion date also translates to additional revenue from being able to open doors for business sooner.
DLR Group’s mass timber multi-family housing project sits atop a historical theater building in San Francisco’s Mission District.
“We were very interested in figuring out how to build atop our existing cities, in ways that wouldn’t destroy the existing city fabric, but helps solve these really pressing density issues,” says Jose Brunner, a designer at DLR Groupand advocate for mass timber architecture. “We were able to design something that would be built atop an existing building, with a lightweight product that wouldn’t detract the integrity of the existing building.”
Mass timber structures weigh significantly less than other construction types, resulting in lighter weight and less expensive foundations.
And we’re just beginning to scratch the surface in exploring the aesthetic options of mass timber.
“Many designers are trying to design a timber building in a way that it looks like traditional wood, and that mentality needs to change,” notes Mr. Brunner. You’re designing with a product that’s very flexible. There are so many options with exterior cladding systems that are going to enable us to move outside that paradigm.”
Below, peruse the concepts for four different mass timber projects found in specific, real-life locations across the U.S., and gathered here for us by Think Wood. Included in each project description are links to additional information about the project – we encourage you to follow the links to learn more.
>80-story River Beech Tower in Chicago
“Architects at Perkins+Willand engineers at Thornton Tomasetti have conceptualized the 80-story River Beech Tower, a mass timber building in Chicago that would have 300 duplex units using an exterior diagrid system that leverages the natural axial strength of timber. This concept has been informed by the recent Skidmore, Owings and Merrill Timber Towers research that shows that timber-concrete hybrid systems can support loads of 82,000 pounds (8 times the required load) along with some steel hybrid systems that could be marketable in terms of bay sizes and floor openings.”
>Timber Towers in Philadelphia
“D.C. Designers led by Sean McTaggart from Hickok Cole were shortlisted for the Skyhive Skyscraper challenge where they are trying to demonstrate the viability of a mass timber high-rise with the Philly Timber Towers project as an alternative to steel and concrete. In the process of designing a new landmark on Philadelphia’s skyline. This group recently received a grant to work on a mass timber Ranger Station in D.C.“
>Mass Timber Multifamily in San Francisco
“Jose Brunner, a designer at DLR Group’s San Francisco office, designed a timber tower atop a landmarked building in San Francisco’s Mission District for the Mission: Housing design competition, and is a strong proponent of mass timber for multi-family architecture. The current building code in San Francisco isn’t very accommodating for timber construction methods but did mention that there will be a vote to change those local codes in the next few months.”
>Timber Bridge at LongPoint Corridor in Brooklyn, NYC
“For the past few years, architect Jun Aizaki of CRÈME has been working on the Timber Bridge at LongPoint Corridor, a floating passageway he proposes should be made of wood. Between Brooklyn and Queens, there is an opportunity to improve pedestrian transit, create green spaces, and connect the communities of Long Island City and Greenpoint into a new neighborhood that has been coined LongPoint. This one recently launched a crowdfunded Kickstarter campaign to try and show community support.”