Dowel-Laminated Timber (DLT)

Dowel-laminated timber panels are a next-generation mass timber product commonly used in Europe. Panels are made from softwood lumber boards (2-by-4, 2-by-6, 2-by-8, etc.) stacked like the boards of NLT and friction-fit together with dowels. Typically made from hardwood lumber, the dowels hold each board side-by-side, similar to how nails work in an NLT panel, and the friction fit lends some dimensional stability to the panel.

There isn’t a prescriptive code path for the use of DLT under the current IBC, and the NDS doesn’t provide published design values or equations for calculating capacities of wood dowel joints. To calculate capacities, the Timber Framers Guild provides some information. However, because nothing is referenced in the code, the use of DLT would require approval by the Authority Having Jurisdiction on a case-by-case basis.

Among the advantages of DLT, acoustic strips can be integrated directly into the bottom surface of a panel. This can help a designer achieve acoustic objectives, while keeping the wood exposed and allowing for a wide variety of surface finishes.

With growing interest in DLT, continued product innovation is likely, along with increased availability to U.S. building designers.

Photo courtesy of StructureCraft

Pictured is a dowel-laminated timber panel with an acoustic profile integrated into the exposed surface.

Structural Composite Lumber (SCL)

SCL is a family of wood products created by layering dried and graded wood veneers, strands, or flakes with moisture-resistant adhesive into blocks of material, which are subsequently re-sawn into specified sizes. Two SCL products—laminated veneer lumber (LVL) and laminated strand lumber (LSL)—are relevant to the mass timber category, as they can be manufactured as panels in sizes up to 8 feet wide, with varying thicknesses and lengths, depending on the product and manufacturer. Parallel strand lumber (PSL) columns are also commonly used in conjunction with other mass timber products.

The manufacture of SCL is standardized. However, while SCL is included in the NDS, design values are provided by the manufacturers. International Code Council Evaluation Service (ICC-ES) evaluation reports and APA product reports are available to assist with structural design capacities and specifications.

Wood-Concrete Composites

Mass timber systems vary widely, and hybrids are an option for wood high-rises, very long spans, or other project-specific requirements. No material is perfect for every job, and it’s important for designers to choose a combination of materials that effectively meets the performance objectives.

At a product level, most of the panels described above can be made into a wood-concrete composite by applying a concrete topping in such a way that the two materials act as one.

One example is the University of Massachusetts Design Building described later in this course (see Schools), which includes CLT/concrete composite floors. According to architect Tom Chung of Leers Weinzapfel Associates, the team relied on the CLT panels for the building load requirements. However, the composite action between the CLT and concrete provided extreme stiffness and minimal deflection which, along with an insulation layer between the materials, provided good acoustic separation between floors.

When is Mass Timber Appropriate?

Because of its strength and dimensional stability, mass timber offers a low-carbon alternative to steel, concrete, and masonry for many applications. A complement to other wood framing systems, it can be used on its own, in conjunction with other wood systems such as post-and-beam, or in hybrid structures with steel or concrete. Mass timber is not necessarily a good substitute for light wood-frame construction, only because dimension lumber framing offers such a compelling combination of performance and cost where permitted by code. For this reason, building types where designers typically default to forms of construction other than light wood-frame—including offices, public/institutional buildings, schools, and taller mixed-use occupancies—may offer greater appeal for mass timber than low-rise commercial or residential buildings, though examples of the latter do exist.

“We’re seeing a lot of interest in mass timber for midrise buildings such as hotels and high-end offices that would have typically used concrete or steel,” said Lucas Epp of StructureCraft, a specialty timber engineering and construction company known for innovative wood structures. “In addition to the warmth of exposed wood, people are discovering that it’s a viable option for creating high-performing and cost-competitive structures.”

Thomas Robinson, whose firm is designing three mass timber projects in addition to Albina Yard and Framework, sees particular potential in multifamily housing and other building types that lend themselves to modular prefabrication. “The time spent upfront designing and perfecting a building system can be leveraged in projects where you have repeatable elements,” he says.