Mass Timber Construction: Friend or Foe?
Updated: Jul 31, 2022
Sustainable design meets traditional construction
It should come as no shock to the engineering community that mass timber has grown increasingly popular among building materials. While it still isn’t as heavily used as its counterparts steel and concrete, it is capable of a lot more than you think. When you hear of wood structures you usually picture a residential home with 2x4 plywood and stick framing, not a massive 18-storey building.
Well, Canada, Norway, and now the United States are a few of the many places pioneering this movement toward mass timber construction.
What is Mass Timber?
Mass Timber is a type of engineered wood product that is stronger than regular wood. It is usually made out of thin sheets of wood that are laminated together. Using various combinations and sizes, mass timber products can serve as beams, columns, floors, roofs, and walls taking into consideration the directional strength of each wood product. Mass timber is also very lightweight, making it ideal for buildings that need to be extremely energy efficient. It is also more sustainable than other types of building materials because it doesn’t require any fossil fuels to produce.
Mass Timber is a generic term that covers all types of wood construction materials such as cross-laminated timber (CLT), nail-laminated timber (NLT), dowel-laminated timber (DLT), glued-laminated timber (glu-lam), and mass plywood panels (see below). Of all the products, cross-laminated timber is the most popular and familiar.
To make CLT, you need to cut lumber into long planks called lumber boards. They then must be trimmed, kiln-dried, and glued one on top of the other in layers, crosswise, with the grain of each layer facing against the grain of the adjacent layer. This technique of stacking boards can create large slabs 0.3 meters thick and on average 3 meters long by 12 meters wide. The size of the lumber is dictated more by transportation limitations than manufacturing ones.
While there are many pros to mass timber there are also a few cons. Let's dive into the positives first.
Benefits of Mass Timber Fire resistance, structural integrity, and environmental attributes make new tall wood buildings among the most innovative structures in the world. — Think Wood
Reduced Carbon Emissions
In 2013, researchers at the University of British Columbia found that mass timber buildings could reduce greenhouse gas emissions by up to 30%. Not only does mass timber require less energy to create than other building materials, but mass timber could absorb carbon from the atmosphere through natural processes. Journal of Green Building (2019) did a study and found that one cubic meter of CLT wood sequesters roughly one tonne (1.1 US tons) of CO2. And because mass timber panels can be made from young or damaged trees, their production moves the needle toward more sustainable forestry.
“Globally, both enough extra wood can be harvested sustainably and enough infrastructure of buildings and bridges needs to be built to reduce annual CO2 emissions by 14 to 31% and FF consumption by 12 to 19% if part of this infrastructure were made of wood.” The biggest drop in CO2 emissions came, it said, from “avoiding the excess [fossil fuel] energy used to make steel and concrete structures.” — Journal of Sustainable Forestry (2014)
“Mass timber buildings are roughly 25% faster to construct than concrete buildings and require 90% less construction traffic.” — Think Wood
Similar to precast concrete, the labor and fabrication for CLT buildings are done at a factory except “computer numerical control” (CNC) machines are responsible for creating precision cuts of wood. This negates the need for materials to be ordered in mass quantities, cut to size on site, and assembled.
If architects and designers provide detailed plans, a factory can create something like a CLT wall exactly according to specifications. There are no wasted materials, as doors and windows are not cut out of the walls. Computer-guided fabrication means that the wood is placed only where needed which reduces waste and saves time and money.
Prefabricated buildings can be assembled quickly and easily, making them ideal for construction sites. These prefabricated pieces are shipped directly to the construction site in small batches, allowing for minimal on-site disruption. Additionally, prefabricated buildings can fit into tight, distinctive spaces, such as those found in cities.
Increased Protection Against Fire (shocking right?)
A 5-ply cross-laminated timber (CLT) panel wall was subjected to temperatures exceeding 982 degrees Celsius (1,800 Farenheit) during a fire resistance test and lasted 3 hours and 6 minutes which exceeds the 2-hour rating that building codes typically require (Vox Media, 2020).
The thickness of compressed, solid mass timber is quite difficult to burn. If there is a fire, exposed mass timber will char on the outside creating an insulating layer protecting the interior wood from damage. This allows the material to retain structural integrity for several hours in even the most intense fire.
Further reports on fire testing of CLT can be found from the US Forest Service, the International Code Council, and the Fire Protection Research Foundation.
Concerns about Mass Timber
Environmentalists worry that North American forests are not sufficiently protected to handle a stark uptick in demand. The Natural Resources Defense Council put out a report stating that the number of greenhouse gases being released by clearcutting the Boreal forest in Canada might be incredibly undercounted.
Numerous environmental groups, led by the Sierra Club, said in an open letter to California state officials that “CLT cannot be climate-smart unless it comes from climate-smart forestry.” The letter provides a detailed list of rules and best practices that should guide climate-smart forestry, including: “Logging of the world’s remaining mature and primary forests, as well as unroaded/undeveloped and other intact forest landscapes, should cease.” And: “Tree plantations should not be established at the expense of natural forests.” (Vox Media 2020).
If we are not careful about sustainable forestry we may be causing more harm than we are doing good. It is essential for the future of mass timber that the proper regulations and specifications are in place so that forests are still maintaining a bio-diverse ecosystem that serves as not only a place for carbon to be stored but also for animals and plants to live and thrive and nature to be admired and appreciated by all.
So now that we know a little more about what mass timber is, here are a few examples to show what it is capable of.
Standing at 84.5 meters tall and 18 storeys high, Mjøstårnet is one of the tallest timber buildings in the world. Mjøstårnet was built four storeys at a time in five construction stages and was completed in 2019. Glulam columns, beams, and diagonals were used for the primary load bearing system, and CLT was used for elevator shafts and balconies. The pre-fabricated sections and floor slabs were hoisted into place with just internal scaffolding and a large crane. The material for the building was sourced locally from the Brumunddal area in Norway given their major forestry and wood processing industry.
The tower has received numerous awards and recognitions, such as the New York Design Awards, Norwegian Tech Awards, and CTBUH’s Award of Excellence.
Brock Commons Tallwood House
Brock Commons Tallwood House is a unique 18-storey hybrid mass timber residence at the University of British Columbia (UBC). The wood structure was built less than 70 days after the prefabricated components were delivered to the site (approximately four months faster than a typical project of this size and scope).
The building is made up of 17 stories of mass timber construction above a concrete podium and two concrete stair cores. The floor structure consists of 5-ply cross-laminated timber (CLT) panels supported on glue laminated timber (glulam) columns. The roof is made of prefabricated sections of steel beams and metal decking.
You can actually see the timelapse of the building being constructed in the video below!
While mass timber is still not even close to being a mainstream material like steel and concrete, it is growing increasingly popular globally. There is a lot of potential for mass timber but we have to remember the associated risks involved. As long as processes are put in place to ensure the safety of our forests worldwide, mass timber could really pave the way for a more sustainable future in construction.
When it comes to designing a mass timber structure, a program like civils.ai could adapt to include mass timber design in its software thanks to its open-source nature. We could bring on an expert in timber design and work on building out a solution that could eventually include timber material catalogues.
Our already existing and popular beam calculator is available to use for steel members. Additionally, our tunneling and geotechnical calculators are growing each day with new and exciting features. While we are still a work in progress, we make sure we are up to date on the latest innovative and evolving practices in the engineering and construction industry and hope to keep you all informed as well.