The Big Picture: Building with Mass Timber
Mass Timber continues to gain ground as an alternative building material. It is engineered for high strengths ratings like concrete and steel and can also be prefabricated as a "kit of parts" for ease of installation.
“Building with mass timber is a manufactured assembly process, not a construction process,” said DPR’s Scott Brown. "In a manufactured assembly process, we can develop a playbook that outlines daily activities including when certain trade partners need to be on-site, what engagement we need to have with them, when inspections are scheduled, and so on. We can develop this playbook before the project even starts."
Benefits of Mass Timber
Mass timber structures can include a variety of engineered wood products like large panels, columns, or beams. These members can be incorporated into structural supports, interior finishes, or as a complement to light frame structures. There are a multitude of benefits to building with mass timber.
Mass timber products are made of thick, compressed layers of wood laminated and fortified with strong adhesives and/or fasteners. This lamination process creates strong, load-bearing beams and panels. Two of the most common types of mass timber products are:
- Cross Laminated Timber (CLT): Panels which are made of layers of kiln-dried lumber oriented at right angles to one another and glued together with non-toxic adhesives.
- Glue Laminated Timber (GLT): Columns, Beams, Purlins which are made up of layers of dimensional lumber bonded together in a parallel orientation.
The manufacturing process of mass timber products gives them high tensile strength ratings despite being only a fraction of the density of concrete and steel. In fact, mass timber buildings weigh approximately one-fifth that of comparable concrete buildings. These buildings perform well during seismic activity because of their high strength-to-weight ratios. Additionally, new building codes now allow these structures to be built to 18-stories high.
Using prefabricated mass timber elements can reduce labor costs by up to 50% and can speed up the erection of a structural frame by up to 25%. These can result in significant overall cost savings on a project.
While wood is combustible, mass timber is fire resistant due to its natural bulk. When mass timber is exposed to fire, the outer layer burns, creating a natural, protective char layer on the surface, leaving the interior undamaged and structurally sound. The lack of free air within the solid block of wood also prevents internal combustion. The mass of the timber itself provides an inherent fire rating for the building. In some cases, protection far exceeds the two-hour rating that building codes require.
During photosynthesis, trees absorb carbon dioxide from the atmosphere and convert it to sugars that feed the tree. In this way, trees are not only a renewable resource, but they also act as a carbon sink. Mass timber structures can reduce the carbon footprint of a building by up to 40% and can reduce global carbon dioxide emissions by up to 31%.
Biophilia is the practice of incorporating aspects of nature into built environments. With timber features openly exposed, the “biophilia effect” evokes the positive sensory experience of being in nature.
Why Mass Timber?
DPR is a leader in building with mass timber, with an ever-growing portfolio. DPR was the first to incorporate mass timber into a lab space during the construction of a project in San Diego in 2018. At the Discovery Meadows Child Development Center in Gaithersburg, MD, DPR was one of the first to introduce CLT as an aspect of biophilia. DPR’s own office space in Sacramento, CA, was the first application of CLT shear walls in California. At the time of design review, Sacramento did not recognize CLT panels for use as lateral force-resisting elements. Using case studies, research and testing from different organizations, DPR’s design team was able to demonstrate to the city its equivalent or superior performance. In total, DPR has built 480,122 sq. ft. of space with mass timber across the U.S., equaling $16.6 million in revenue, $3.1 million of which was self-performed by DPR’s own craft employees. There are currently six active projects incorporating mass timber, and nine scheduled to be constructed in 2023, meaning DPR could create more than 1 million sq. ft. of mass timber space in the next two years.
The use of mass timber is accelerating. In 2020, the North American Mass Timber Report projected that there would be about 1,300 mass timber projects in the U.S. by 2024, but that number has been revised to around 3,000 projects. If this rate of construction continues, it means that by 2034, more carbon will be sequestered through mass timber buildings than is emitted by the construction industry.
There are, however, some concerns about building with mass timber, especially with regards to price, demand, and possible deforestation. The prices of lumber are returning to pre-COVID levels, making a stronger case for mass timber structures. That being said, mass timber buildings are expensive to build, but cost savings can be found in time and labor efficiencies. Concerning deforestation, forest growth is approximately 18% higher than the rate of harvesting thanks to responsible forest management practices. If sustainable forestry efforts continue to expand, increased use of mass timber should be feasible.
“Wood has long been considered one of the most sustainable building materials due to its renewability and carbon sequestration during growth,” said Ryan Poole, DPR’s global sustainability leader. Poole advises that the use of mass timber still needs to be incorporated with other building materials. “Mass timber buildings still consist of concrete and steel, and it is paramount we continue to find the correct balance that helps provide sustainable growth as we build a healthier future for people and the planet.”
Posted on January 25, 2023