The construction and design sector is making steady progress when it comes to mass timber and taller wood buildings. Rigorous fire, seismic, and load bearing tests have led to changes in building codes that will soon go into effect in both Canada and the US that permit up to 12 stories and 18 stories respectively.
In 2019, the International Code Council (ICC) approved 14 code changes as part of the 2021 International Building Code® (IBC), expanding the opportunities to build taller with wood.
Included in the approved code changes are three new construction types:
- IV-A allows for the construction of wood structures up to 18 stories with fully protected—encased in non-flammable materials—mass timber elements and fire-resistance ratings of 3 hours for bearing walls and structural frame construction, 2 hours for floor construction, and 1.5 for roof construction.
- IV-B allows for construction up to 12 stories with fully protected mass timber elements and fire ratings of 2 hours for bearing walls, structural frame and floor construction, and 1 hour for roof construction.
- IV-C allows for 9 stories, protected exterior and exposed mass timber exterior with fire-resistance ratings of 2 hours for bearing walls, structural frame and floor construction, and 1 hour for roof construction.
With these updates to code, builders are increasingly turning to wood as a viable option that can meet their structural and aesthetic needs.
Heavy and Mass Timber Construction
In Type IV wood construction, the terms used to reference the use of lumber are heavy or mass timber. Defined by the American Wood Council, heavy timber is either sawn lumber or structural glue-laminated timber. Mass timber falls under the umbrella of heavy timber and is typically characterized by the use of large, solid wood panels for wall, floor and roof construction, which can include:
- sawn lumbera
- structural glue-laminated timber
- cross-laminated timber (CLT)
- dowel-laminated timber (DLT)
- nail-laminated timber (NLT)
Why Build Taller with Wood?
While the aesthetic and structural benefits of timber construction are a contributing factor, there is a growing list of additional reasons why builders and designers are choosing to build taller with wood.
Light-Weight Structure and Efficient Footprints
Timber structural systems have high building-volume-to-surface-area ratios, allowing for spacious interiors even with space constraints that typically require tall, compact designs. This means spacious interiors, even when the footprint of a building is constricted, which is frequently the case with tall, compact structures like high-rise buildings. Additionally, mass timber buildings weigh only one-fifth of traditional concrete buildings, which reduces foundation requirements. There is also opportunity for application of wood construction in projects to increase the height of existing buildings. The lighter weight of wood can allow additions to building height without foundation reinforcement that might be required if other building materials were used.
Structural and Seismic Performance
Because of wood’s strength-to-weight ratio, foundation loads and seismic forces are reduced. Testing conducted by the Natural Hazards Research Infrastructure in 2017 stated that the structural integrity for a wood building 8 to 20 stories tall was maintained both during and after an earthquake. In some cases, a lighter weight structure not only saves on foundation costs but allows a taller structure to be built in challenging soil conditions that would not be possible with concrete and steel.
Tight Envelopes and Thermal Performance
Mass timber components are fabricated with high levels of precision to ensure a tight fit using Building Information Modeling (BIM) and CNC machining. Together with wood’s natural insulating properties, mass timber construction offers strong thermal performance, which is critical for high energy demands of tall buildings. This is because wood-frame construction is inherently more thermally efficient due to the lower conductivity of wood compared to concrete, steel-frame and masonry construction. For tall wood projects targeting net-zero energy or other stringent energy performance criteria, mass timber can store solar heat energy during the day and release it at night, reducing energy loads.
Mass timber enables inherent fire resistance through the insulation of inner layers. When wood is exposed to fire, the exposed surface burns, creating a natural protective charred layer. Char acts as insulation, delaying the onset of heating of the core of wood below. Due to the solid block makeup of mass timber, air and fire are inhibited in their travel. Additionally, as the code prescribes, timber can be encapsulated to provide further fire resistance.
Faster and Safer On-Site Construction
When it comes to taller wood, prefabricated sections can be manufactured off-site, shipped to the project and then assembled on site, significantly shortening project timelines and improving safety and accuracy. Decreasing the construction time and workers needed has more advantages than just decreasing cost: a shorter timeline with fewer people means less room for error and often a safer construction site.
Occupant Comfort and Well-Being
There is a growing interest in biophilic design (materials derived from or appearing to be derived from nature) as well as health- and wellness-focused spaces, particularly in places where research suggests people are experiencing increased levels of stress—such as high-rise structures and urban environments. There is viable evidence that the physiological responses to natural elements, such as exposed timber, can lower stress levels, making the use of mass timber in high rise construction particularly beneficial.
Build Taller Sustainably
Public policies on climate change and green building are increasingly calling for more sustainable ways to build up and increase density within urban environments, something taller wood construction is well suited to address. Governments, developers and clients are beginning to see the emerging economic advantages of mass timber design and construction due in part to sustainable forest management, a shift in manufacturing and supply chains and new code legislation that “now render engineered wood as cost competitive with more conventional types of construction such as concrete and steel,” according to the case study Tall with Timber: Seattle Mass Timber Tower Case Study (2018) by DLR Group.
Market Distinction and Overall Value
Prefabricated mass timber building systems increasingly offer added value, including environmental benefits, cost/schedule savings, higher quality and more precise construction, and in some instances, better lease rates. In a feasibility study for a 12-story mass timber mixed use building in Seattle, Washington, experts predicted that leases could potentially increase by 5% while the design could create a 15% reduction in operational costs, and a predicted 45% cut in greenhouse gases emitted by the building. Over the lifetime of the building those costs add up to create an overall increased value of the property and a 0.5% savings in build and upkeep compared to the cost of concrete.
All of this points to the fact that building taller with wood is not only gaining traction as an effective building method, it’s also proving to be cost effective, contribute to well-being, and reduce carbon emissions. And an ever-expanding roster of tall wood projects in North America and around the world signals a new generation of architectural design and construction practices that takes full advantage of the capabilities of mass timber and helps address the urgent need for more eco-minded ways to build our cities.
For more information about tall wood construction, Think Wood has a CEU on the use of Tall Wood.