What you should know about Mass Timber

By TJ McCann, PE – Structural Engineering Project Manager | LA Office

Mass timber has been a popular construction method in Europe for many years; however, it is relatively new in North America. TJ McCann, PE, a structural project manager in Coffman’s LA office, breaks down what you should know and why there’s increased interest in its use.

What is Mass Timber, and why is it growing in popularity?

Mass timber, unlike Light Wood-Frame or Heavy Timber, uses a combination of smaller sections of wood and adhesives or fasteners to create larger sections to provide strength, dimensional stability, and fire resistance. The mass timber family of products includes cross-laminated timber (CLT), nail-laminated timber (NLT), dowel-laminated timber (DLT), glue-laminated timber (glulam or GLT), and some types of structural composite lumber (SCL). In addition to being strong and visually appealing, mass timber can achieve a better fire resistance rating than traditional construction and provide a sustainable alternative to fossil-fuel intensive materials, such as steel or concrete. Improvements in the production of engineered wood products, the desire to create more sustainable structures, and recent code changes have pushed mass timber to the forefront of the construction frontier.

How is the construction type and fire rating classified?

Before a mass timber project can begin, the building occupancy and construction type must be determined. The construction type determines what type of wood system can be used and plays a major role in the cost of the project. Wood buildings will fall into Types III-A, III-B, IV-A, IV-B, IV-C, IV-HT, V-A, or V-B construction.  Type V construction requires the lowest fire resistance, whereas Type III requires a higher level of fire resistance. Consequently, Type V construction will be limited in size compared to a Type III building.

With the amendments to the 2021 International Building Code (IBC) allowing up to 18 stories of mass timber, we expect to see high-rise mass timber projects continue to flourish. However, with taller buildings come increased seismic and wind hazards. One method of accounting for these hazards is utilizing hybrid timber construction. Hybrid timber construction is the combination of mass timber with traditional building materials such as steel or concrete. Hybrid timber buildings may incorporate steel frames or concrete shear walls with a mass timber gravity system. By combining these materials, wood construction can quite literally reach new heights.

Contrary to common belief, wood can provide significant fire resistance. The fire resistance is directly correlated to the size of the wood members. As a wood member burns, a layer of protective char forms on the surface while the interior retains its strength. By increasing the size of a member to account for this char layer, mass timber construction can take advantage of wood’s natural properties and avoid fireproofing while still maintaining an appropriate fire rating.  Despite the members having inherent fire resistance capabilities, it is important to note that connections between members must also have sufficient fire protection to provide the same fire-rating. Options for protecting connections include concealed connections protected by wood, fire-rated gypsum board, or intumescent paints.

In designing for mass timber construction, it is essential for Structural Engineers and Fire Protection Engineers to understand the required fire ratings because the size of the members will be directly impacted.

What makes Mass Timber sustainable?

Mass timber has become increasingly favorable on projects placing an emphasis on sustainability.  Conventional construction methods, such as steel and concrete, produce greenhouse emissions that drive significant contributions to global warming. Steel and cement production combined are responsible for 16% of global carbon emissions, with the building industry as a whole accounting for 40% of global emissions. On the other hand, wood stores carbon dioxide captured from the atmosphere during photosynthesis and is a renewable resource that can be replanted and replenished in about 20 years.

What is the architectural and construction appeal?

The appeal of mass timber extends far beyond its aesthetic and sustainability. Mass timber can be a less expensive alternative to concrete and steel due to its shorter construction sequence and indirect material costs. Mass timber beams, panels, and columns are prefabricated in shops which makes design and coordination paramount on a successful project. Prefabrication can accelerate construction schedules leading to significant labor cost savings. Because of the ability to leave wood structural members exposed, material costs such as fireproofing and architectural finishes can be reduced. Wood is also much lighter than concrete or steel, which means less load on the foundation, further reducing material costs.

Coffman’s Experience with Mass Timber

As the technology grows in popularity among owners, architects, contractors, and engineers, Coffman’s portfolio of mass timber projects has steadily grown. Coffman’s structural and fire protection engineering teams understand Mass Timber performance, its advantages, and the design challenges that come along with it. We acknowledge these unique issues, including production lead times, supplier limitations, and constructability, and plan accordingly while engaging the greater design and construction teams to develop the best solutions.

Our mass timber experience includes educational campuses, commercial buildings, industrial warehouses, multi-family residential buildings, and hotels. Most recently, Coffman was brought on to provide structural engineering services for a 67,500 sf warehouse with CLT mass timber-bearing wall panels in Spokane, Washington.

The momentum in this space is invigorating, and we look forward to discussing the possibilities for your project.