Description of a Mass-Timber System

Mass timber is a category of framing styles typically characterized by the use of large solid-wood panels for wall, floor, and roof construction. Building with mass timber offers a reduced carbon footprint, construction efficiency, fire and life safety, and occupant well-being. We will discuss these performance benefits in more detail later in the course, but let’s first review the primary types of mass-timber construction.

Glulam

Glue-laminated timber (glulam) is a structural engineered wood element commonly used for beams and columns in residential and commercial applications. To form a glulam component, dimension lumber wood laminations are positioned according to their stress-rated performance characteristics. In most cases, the strongest laminations sandwich the beam in order to absorb stress proportionally and ensure the member’s longevity. The laminations are jointed end to end, allowing for long spans, and are bonded with a durable, moisture-resistant adhesive. The laminations’ grains run parallel with the member’s length to improve its strength.

Glulam is stronger than steel at comparable weights, and it is stronger and stiffer than dimension lumber, according to APA – The Engineered Wood Association. That makes the material a cost-effective choice for long, structural spans and tall columns with minimal need for additional support. Glulam is a highly visible form of mass timber in contemporary projects, with long spans framing signature designs that have been left exposed to take advantage of wood’s natural aesthetic. Glulam can be used in interior and exterior applications, as several manufacturers sell glulam products with adhesives that can withstand moisture and wear from use outdoors.

NLT

Nail-laminated timber (NLT or nail-lam) is a century-old construction method that is undergoing a design renaissance. It can be found today in many historical buildings as well as compelling new projects of all sizes, where its structural performance and design elegance come together to create inspiring spaces.

NLT gets its strength and durability from the nails that fasten individual dimension lumber, stacked on edge into a single structural element. Applications for NLT include flooring, decking, roofing, and walls as well as elevator and stair shafts. Because NLT is made of wood, it offers a consistent and attractive appearance for decorative or exposed-to-view applications.

Photo courtesy of Think Wood

NLT gets its strength and durability from the nails that fasten individual dimension lumber, stacked on edge, into a single structural element.

CLT

Cross-laminated timber (CLT) is a relatively new structural engineered wood-panel system that is gaining popularity in the United States after being widely adopted in Europe. CLT panels are made of layers of lumber boards (usually three, five, or seven) stacked crosswise at 90-degree angles and glued into place. The panels can be manufactured at custom dimensions, though transportation restrictions dictate their length.

Applications for CLT include floors, walls, and roofing. The panels’ ability to resist high racking and compressive forces makes them especially cost-effective for multistory and long-span diaphragm applications. CLT is the basis of the tall wood movement, as the material’s high strength, dimensional stability, and rigidity allow it to be used in mid- and high-rise construction.

Photo courtesy of Structurlam

CLT is the basis of the tall-wood movement, as the material’s high strength, dimensional stability, and rigidity allow it to be used in mid- and high-rise construction.

DLT

Dowel-laminated timber (DLT) is common in Europe and is gaining traction in the United States for its ease of use with computer-controlled (CNC) machinery, such as lathes, routers, and mills, and its all-wood composition. DLT is similar to NLT, but instead of nails or screws, DLT uses wood dowels to join laminations. To form DLT members, softwood lumber panels are stacked like NLT and are friction fit together with hardwood dowels. Friction fit, achieved by the differing moisture content of the softwood panels and the hardwood dowels, affords additional dimensional stability. The dowels can be inserted diagonally, offering additional resistance. Because DLT does not use nails or screws, it is easier and safer to mill and route, and the lack of an adhesive is also attractive for projects looking to maximize the use of wood.

In application, DLT performs similarly to glulam and NLT. Because its grains run in one direction, DLT is a next-generation mass-timber product that allows for significant architectural flexibility and is well-suited for horizontal spans, such as flooring and roofing applications.

Photo courtesy of Ryan Gobuty

MOTO | Gensler

Buildings with wood podiums have a lower mass, meaning less material is needed for the foundation and shear walls. Consistent structural material use throughout the building also reduces the need for different trades on-site, streamlining labor costs and construction timelines.

MASS-TIMBER CASE STUDY

Photos courtesy of Ema Peter

T3 | Michael Green Architects + DLR Group

T3 is a successful example of a mass-timber structure that is cost-competitive with steel and concrete. With its success, there has been incredible interest across the United States in reviving mass timber as a primary building material. T3 is made chiefly with NLT. The exterior cladding is weathering corrugated steel, which is changing color as the building ages.

T3: Wood Provides a Cool, Tech-Friendly Commercial Space

T3 has gained wide national attention as the nation’s largest mass-timber modern office building. The $24.5-million Minneapolis project successfully fuses mass-timber construction with buzzworthy beauty and amenities. The big surprise? This breakthrough project is also code compliant and mainstream.

T3 may be one of the most closely watched architectural projects in recent years, but often lost in the public fanfare is the fact that the seven-story, 220,000-square-foot, Class A office and retail center “is not pushing any boundaries,” says project lead Candice Nichol, AIBC, NCARB, of Vancouver-based MGA | Michael Green Architecture. Her firm designed T3 (short for timber, transit, technology) in partnership with the DLR Group of Minneapolis.

“What makes T3 special is the way it’s getting the industry to think about building with mass timber again,” Nichol explains. “T3 is an incredibly beautiful building that’s also economical and responsible.” Located in the booming North Loop neighborhood of downtown Minneapolis, T3 “puts a modern spin on an old idea,” she adds.

A great example of that “old idea” is just a few blocks from T3 at Butler Square, a nine-story, 367,717-square-foot brick and heavy-timber building built in 1906 and substantially renovated in 1974. “T3 shows what a new generation of office buildings can look like,” Nichol says. “We’re not used to seeing buildings like this because we’ve gotten away from mass-timber construction. The fact is we didn’t do anything outside of code. T3 is a Type IV building. It’s a concrete podium topped by six floors of mass timber. There was no need for alternative means and methods.”

Code compliance wasn’t the only advantage; construction speed was another. The timber structure for T3 was completed in just nine weeks. “We finished a floor about every nine days,” says Lucas Epp, engineering and 3-D manager at StructureCraft. That work was comparatively quiet too, without the collateral noise associated with other building materials.

Mass-timber structures are 30 percent lighter than the steel equivalent and 60 percent lighter than post-tensioned concrete so the project benefits from a smaller foundation requirement and lower seismic loads. Since many of the components are assembled off-site, the need for skilled construction labor is minimized. “Those savings bleed into every aspect of the project,” Nichol says. For owner/developer Hines, a global real estate investment, development and management firm, T3 has proven to be a critical differentiator in a hot market. Wood’s natural warmth and beauty offers leasing agents a competitive edge. Coupled with T3’s leading-edge technology, LEED Gold certification, and extensive workplace amenities, T3’s leasing story is a powerful one for millennial, tech-focused tenants (Amazon is T3’s lead tenant, occupying two floors).

T3 is a successful example of a mass-timber structure that is cost-competitive with steel and concrete. With its success, there has been incredible interest across the United States in reviving mass timber as a primary building material. “I know some people have been shocked by the scale and composition of T3,” Nichol says. “They shouldn’t be. We’re not doing anything that hasn’t been done before. T3 helps everyone get comfortable with mass timber again. It’s inviting. It’s sustainable. A mixed-use building like T3 makes so much sense for the owner in terms of construction speed, labor, budget, and result. T3 is a very mainstream project.” As an endorsement, Hines recently announced plans to build T3’s twin in downtown Atlanta.