Most large structures comprise steel and concrete, which have been the materials of choice for large building construction for well over a century. A few builders, however, are trying to set a new trend: creating large structures made primarily out of structural wood. This trend has become known as known as “mass timber” construction. Mass timber’s proponents claim that using structural wood is better for the environment and it leaves less of a carbon footprint. They also like the aesthetic it offers and its potential cost-savings. In some cases, mass timber may even be safer for disaster events, such as building fires. Sounds intriguing, right? So with all of these advantages, why haven’t we seen more mass timber construction projects?
The reason is fairly simple: there are both engineering and legal challenges that need to be overcome first before the method becomes more common in use. For one thing, the building type is largely outside the scope of U.S. building codes. Every state in the U.S. has some version of the International Building Code (IBC) in effect. The IBC imposes stringent restrictions on which building materials are suitable for which uses. Proponents of mass timber have been critical of the International Code Council’s reticence to endorse mass timber for use in a wider variety of applications. All this means that at least to date, projects often are required to present evidence in the form of expensive preliminary testing that the materials will perform equal or better than its traditional equivalent materials.
What exactly is mass timber?
Mass timber is a category of framing design typically featuring the use of large solid wood panels for walls, floors, and roofs. These construction projects are characterized by having their load-bearing structure entirely either solid or engineered wood, contrasted with a building that uses wood as an accent. This type of construction uses a material known as cross-laminated timber, which consists of layers of dimensional lumber oriented at right angles and bound with commercial adhesives to create a building material with sufficient strength, stability, and rigidity for use in large scale projects. Other methods of creating wood-based structural supports sufficient for large-scale construction are nail-laminated timber, dowel-laminated timber, and wood-concrete composites.
Over the past 10 years, there has been a growing interest in tall buildings constructed from mass timber materials. Around the world there are now dozens of mass timber buildings. Some of the most well-known mass timber projects in the world include a 7-story office building in Minneapolis, Minnesota (https://structurecraft.com/projects/t3-minneapolis), an 18-story building at the University of British Columbia in Vancouver, British Columbia (https://www.architectmagazine.com/technology/the-university-of-british-columbias-brock-commons-takes-the-title-of-tallest-wood-tower_o), and a 24-story office tower in Vienna, Austria (https://lightwood.org/worlds-tallest-timber-building-hoho-tower-in-vienna/).
Perhaps the most obvious environmental benefit mass timber as a construction material is that wood is a renewable resource. Much of the United States is rich in available lumber. There are fewer embedded greenhouse gasses in the production and transportation of these materials than there are in concrete and steel.
Mass timber as a building material contributes to cost savings in a number of different ways. For one thing, it is much lighter than steel or concrete, allowing builders to save money on transportation of components. This also presents engineering advantages, allowing for the possibility of constructing buildings on bedrock and soil combinations that might not be able to support heaver steel and concrete structures. Proponents report that modular wooden components are inexpensive to remove and replace, and that wood, when properly integrated and protected, contributes to maintaining operations budgets over the life of a facility.
It may seem counterintuitive, but there is evidence that mass timber buildings can outperform similarly constructed steel buildings in fire events. This is allegedly due to the fact that steel structural supports can heat up as the fire-protectant material around them burns, thus resulting in the interior supports quickly failing once the flames reach it. If properly designed, mass timber structural supports will merely start slowly burning once exterior fire-protectant material is destroyed. Professionals who work with mass timber compare the effect to setting a burning log next to a tree – the tree will not burst into flames, but slowly char.
The Future Is Now
In January 2019, International Code Council (ICC) approved a set of proposals to allow tall wood buildings as part of the 2021 International Building Code (IBC). Based on these proposals, the 2021 IBC will include three new construction types for the use of mass timber. The code will include provisions for up to 18 stories of Type IV-A construction for Business and Residential Occupancies. Based on this code guidance, and various country’s carbon footprint concerns, it is reasonable to assume that within the next decade, we will see many more mass timber constructions.